3D Printing as an Alternative Manufacturing Method for the Micro­gas Turbine Heat Exchanger

Wolfgang Seiya and Sherry Zhang July 2015

Department of Energy Technology Royal Institute of Technology

Stockholm, Sweden

Pratt School of Engineering, Smarthome Program Duke University

Durham, North Carolina, USA

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Acknowledgements We would like to thank InnoEnergy, Compower, and the ‘‘STandUP for Energy’’ project for providing resourceful background information for this study. We owe our deepest gratitude to our advisor, Anders Malmquist for his continuous support for this study throughout the summer. His guidance, motivation, and expertise were invaluable assets in all areas of the study. Our sincere thanks goes to Joachim Claesson at KTH for his time and knowledge on the subject of heat exchangers. We take this opportunity to thank all the company correspondents that took their time and interest to help us with the vast information needed for this study. Lastly, we are immensely grateful to Duke Smart Home Program and its director Jim Gaston for providing us with the opportunity and necessary funds to live in Sweden while conducting this study.

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I. Table of Contents List of Figures …………………………………………………………………………….… 6 List of Tables ……………………………………………………………………………….. 7 Abbreviations and Equation Nomenclature ………………………………………………… 8 Abstract …………………………………………………………………………………...… 9 Introduction …………………………………………………………………………………. 9 Methodology ……………………………………………………………………………….. 10

Materials ……………………………………………………………………………….. 10 Manufacturing ………………………………………………………………………….. 13

Materials ………………………………………………………………………………….... 14 Material Overview ……………………………………………………………………... 15 Silicon Carbide ……………………………………………………………………….... 18

i. Creep and Oxidation ……………………………………………………………... 18 ii. Fatigue Toughness ………………………………………………………………. 18 iii. Crack Healing …………………………………………………………………... 18 iv. Compatibility with 3D Printing ………………………………………………… 19 v. Sustainability ……………………………………………………………………. 19

Inconel alloys ………………………………………………………………………….. 19 i. Impact Strength ………………………………………………………………….. 19 ii. Fatigue Strength ………………………………………………………………… 20 iii. Creep and Rupture Properties ………………………………………………….. 20 iv. Corrosion resistance ……………………………………………………………. 20 v. Compatibility with 3D printing …………………………………………………. 21 vi. Sustainability …………………………………………………………………… 21

Haynes 214 Alloy ……………………………………………………………………… 21 i. Oxidation and Creep ……………………………………………………….…….. 21 ii. Cracking ………………………………………………………………….……… 21 iii. Compatibility with 3D Printing ……………………………………………....… 22 iv. Sustainability ………………………………………………………………….... 22

Stainless Steel 304 ……………………………………………………………………... 22 i. Creep and Creep­Fatigue ………………………………………………………… 22 ii. Oxidation ………………………………………………………………………... 23 iii. Compatibility with 3D Printing ………………………………………………… 23 iv. Sustainability …………………………………………………………………… 23

Evaluating high­temperature thermal and tensile properties …………………………... 24 Ranking results ……………………………………………………………………….... 27

3D Printing / Additive Manufacturing …………………………………………………….. 28 Manufacturing Techniques Overview ……………………………………………….… 29

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Powder­Based Fusion Processes (PBF) ……………………………………………….. 30 i. Solid­State Sintering …………………………………………………….………. 31 ii. Chemically­Induced Sintering ………………………………………………….. 32 iii. Liquid­Phase Sintering (LPS) ………………………………………………….. 32 iv. Full Melting …………………………………………………………………….. 32 v. Electron Beam / Electron Beam Melting (EB / EBM) ………………………….. 32 vi. Sustainability …………………………………………………………………… 33

Binder Jetting Additive Manufacturing (BJAM) …………………………………….... 33 Laminated Object Manufacturing (LOM) ……………………………………………... 33 Electron Beam Additive Manufacturing (EBAM) …………………………………..… 34 Ultrasonic Consolidation (UC) ……………………………………………………….... 35 Ranking Techniques …………………………………………………………………… 36 Companies / Researchers ………………………………………………………………. 37

Arcam AB …………………………………………………………………………. 38 Aurora Labs 3D ……………………………………………………………………. 38 Ceralink ……………………………………………………………………………. 39 EOS ….…………………………………………………………………………….. 39 ExOne ……………………………………………………………………....……… 39 Fabrisonic ………………………………………………………………………….. 40 Renishaw ……………………………………………………………………...…… 40 Sciaky …………………………………………………………………………….... 40 Other Companies ………………………………………………………………..…. 40

Growth ……………………………………………………………………………….… 42 Heat Exchanger Design ………………………………………………………………….… 43

Classification of Heat Exchangers …………………………………………………….. 43 i. From Construction ………………………………………………………………. 43 ii. Based on the Heat Transfer Process ……………………………………………. 45 iii. Based on Surface Compactness ……………………………………………...… 45 iv. Based on Flow Arrangements …………………………………………….….… 45

Thermal Hydraulic Performance of Heat Exchangers ………………………………… 46 i. Heat Transfer Mechanisms in Heat Exchangers ………………………………… 46 ii. Conduction …………………………………………………………………….... 46 iii. Convection …………………………………………………………………...… 46 iv. Heat Transfer and Temperature Difference ………………………………….… 47

Pressure Drops ………………………………………………………………………… 49 Effectiveness and Rating …………………………………………………………….... 50 Design Recommendations …………………………………………………………….. 52

Economics ……………………………………………………………………………….... 54 Barriers ……………………………………………………………………………………. 57

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Recycling ………………………………….…………………………………………….… 57 Discussion and Recommendations …………………………………………………...…… 58 Sensitivity Analysis ……………………………………………………………………….. 59 Conclusion ………………………………………………………………………………… 61 References ………………………………………………………………………………… 62 Appendix ………………………………………………………………………………..… 69

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II. List of Figures

Figure 1 ­ AspenPlus model of Compower’s ET10 …………………………………...….. Figure 2 ­ Ashby’s Material and Selection Chart ……………………………………….... Figure 3 ­ Internal oxidation attack in Inconel 601 ………………………………………. Figure 4 ­ Thermal conductivities of the top five materials with temperature ……….…... Figure 5 ­ Specific heat capacities measured from a reference of 20°C ………………….. Figure 6 ­ Coefficient of linear expansion of the top five materials with temperature ….... Figure 7 ­ Yield strengths of top 5 materials with temperature …………………………... Figure 8 ­ Elastic modulus of top five materials with temperature ………………………. Figure 9 ­ Schematic of selective­laser sintering …………………………………………. Figure 10 ­ Schematic of solid­state sintering ……………………………………………. Figure 11 ­ The laminated object manufacturing process ………………………………... Figure 12 ­ Schematic of the ultrasonic consolidation process …………………………... Figure 13 ­ Different types of tubular heat exchangers …………………………………... Figure 14a ­ Different types of plate heat exchangers ……………………………………. Figure 14b ­ Different types of plate heat exchangers ……………………………………. Figure 15 ­ Parallel, counter and cross flow configurations …………………………….... Figure 16 ­ LMTD Method for parallel and counterflow heat exchangers ………………. Figure 17 ­ Temperature profile along a cross­flow heat exchanger …………………….. Figure 18 ­ Brayton cycle ………………………………………………………………... Figure 19 ­ Plot of overall performance coefficient P vs. relevance factor α …………….. Figure 20 ­ Cost in US dollars of commercial computers from 1950 to 1985 ……………. Figure 21 ­ Price index of personal computer and peripheral equipment ………………....

pg. 10 pg. 15 pg. 21 pg. 24 pg. 24 pg. 25 pg. 26 pg. 26 pg. 31 pg. 32 pg. 34 pg. 35 pg. 44 pg. 44 pg. 45 pg. 45 pg. 48 pg. 48 pg. 49 pg. 52 pg. 54 pg. 55

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III. List of Tables

Table 1 ­ Weights for each criteria used to rank the materials …………………………... Table 2 ­ Ranking criteria for top five materials and their respective weights ………….. Table 3 ­ Weights for each criteria used to rank manufacturing techniques …………….. Table 4 ­ All the materials initially considered for the heat exchanger design ………….. Table 5 ­ The top 22 materials and their material properties ……………………………. Table 6 ­ Ranking results for the top five materials …………………………………….. Table 7 ­ Overview of additive manufacturing techniques compatible with plastics, metals, and ceramics …………………………………………………………………….. Table 8 ­ Ranking and totals for each manufacturing technique ………………………... Table 9 ­ Machine data from various companies ………………………………………..

pg. 11 pg. 12 pg. 14 pg. 15 pg. 16­17 pg. 28 pg. 30 pg. 37 pg. 41

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IV. Abbreviations and Equation Nomenclature

Abbreviations ABC­SiC = Aluminum­Boron­Carbon­Silicon Carbide AM = Additive Manufacturing BJAM = Binder Jetting Additive Manufacturing BJT = Binder Jetting Technology CAD = Computer­Aided Design CAGR = Compound Annual Growth Rate CNC = Computer Numerical Control DMLM = Direct Metal Laser Melting DMLS = Direct Metal Laser Sintering EBAM = Electron Beam Additive Manufacturing EB / EBM = Electron Beam / Electron Beam Melting HX= Heat Exchanger LMTD = Log Mean Temperature Difference LOM = Laminated Object Manufacturing LPS = Liquid­Phase Sintering PBF = Powder Based Fusion Processes R&D = research and development SiC = Silicon Carbide SLA = Stereolithography SLM = Selective Laser Melting SLS = Selective Laser Sintering SS = Stainless Steel UAM = Ultrasonic Additive Manufacturing UC = Ultrasonic Consolidation XaaS = everything­as­a­service

Equation Nomenclature A = Area (m2) C = Heat capacity (J/kg.K) dT/dx = thermal gradient (K/m) D = Hydraulic diameter (m) f = Friction factor h = Specific enthalpy (J/kg) hc = Convective heat transfer coefficient (W/m2K) k = Material’s thermal conductivity constant (W/mK) L = Length (m) m = Mass (kg) m’ = Mass flow (kg/s) Ns = Number of entropy production units Nu = Nusselt number Q’ = Rate of heat transfer (J/s) ΔP = Pressure drop (%) Pr = Prandtl number Re = Reynold’s number St = Stanton number T = Temperature (K) V = Flow velocity (m/s) = Density (kg/m3)ρ = Viscosity (N.s/m2)μ

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V. Abstract A variety of materials for high temperature applications were studied. Best materials for constructing heat exchangers were selected using models based on preferential weights. Current additive manufacturing techniques and industries were also studied and rated to determine the best material­printing­technique combination. Although the rating models do not include every important criterion, the results were expected to be the same if the state of the 3D manufacturing industries and user preferences do not change. Design recommendations for a compact air­to­air heat exchanger were made without considering manufacturing limitations. An economical assessment of 3D manufacturing techniques was made to determine whether 3D manufacturing could be a better alternative for heat exchangers. Although very promising, the choice to print heat exchangers with 3D techniques would not be economical at the moment. Future predictions of the additive manufacturing industry were made having studied related industries.

VI. Introduction 3D printing technologies are new, growing, and disruptive to the manufacturing industries. Their inherent design freedom gives them an immediate competitive edge over traditional manufacturing techniques. Their use in industries has, however, been a limited one due to other factors such as cost. Polymer­based printing processes have been more popular than in the case of printing with metals and ceramics. The study aims to evaluate the feasibility of 3D printing as an alternative method for manufacturing heat exchangers that are used in Compower’s ET10 microturbine. More information about the microturbine can be found from a Master of Science thesis by Loshan Palalayangoda (Palalayangoda 2010). As seen from the model in Figure 1, the heat exchanger under investigation will be operational in Compower’s externally fired micro­gas­turbine for current cogeneration purposes (electricity and domestic heating). The company is aiming to integrate other energy sources to the microturbine system in the future (Compower 2015). The heat exchanger in consideration passes hot exhaust gases from the burner on the hot side while having ambient air from the compressor to the turbine in its cold side. As the heat exchanger replaces the combustion process in an idealized Brayton cycle for the turbine system, its performance will greatly affect the overall performance of the system. The study will investigate the best materials to be used for the heat exchangers and available 3D printing technologies for high­temperature materials. An in­depth look into other industrial aspects like growth over time, economics, and barriers will assist further in evaluating the feasibility question.

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Figure 1: AspenPlus model of Compower’s ET10 (Palalayangoda 2010)

The study will also approach heat exchanger design in the light of the design freedom that comes with 3D printing. The main focus will be on the design that improves the thermal performance of the heat exchanger while prioritizing the aim of achieving minimum pressure losses in operation.

VII. Methodology In order to assess the feasibility of using 3D printing as an alternative method of manufacturing for the heat exchanger, the problem was broken down into two smaller problems: material and 3D manufacturing technique. After analyzing the two subproblems, a final conclusion was made about whether or not 3D printing is a viable option while simultaneously keeping design and costs in mind.

A. Materials The material selection process began with extensive research on past and present materials used for high­temperature heat exchangers (HX). The preliminary list of potential materials (Table 4) contains all materials capable of being used regardless of price. The preliminary list of 38 materials was narrowed down to 22 materials based on eliminating materials with exorbitant prices and limited research. Using a decision matrix (Table 5), the 22 materials were ranked and five materials were analyzed in detail. The criteria used to judge these materials were: maximum service temperature, density, yield strength, yield strength at 600°C, thermal conductivity at 1000°C, and price per pound.

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Each criterion was given a weight between one and ten ­­ one being the least important and ten being the most important. Each criterion was also given a target value in order to assess how the materials’ natural properties measured up to the necessary requirements. Table 1 summarizes the weights and targets for each criterion. Maximum service temperature is critical to the efficiency of the complete microturbine and therefore was given a weight of nine. In order for the material to be considered a good viable option, it needs to have a maximum service temperature above 850°C. Light parts are preferred to heavier ones for mechanical design and logistical reasons. Lighter parts require less support system and are easier to carry and transport. For any desired HX volume, materials with lower densities are therefore better choices than those with higher densities. Density was given a weight of three (negative) because low­density material is preferred, but not required for the success of the heat exchanger. Yield strength is very important in determining the life cycle of the heat exchanger. Higher yield strength can tolerate higher system stresses, which will correlate to less fractures and failures. Ideally, material properties for yield strength would be found at 600°C. However, due to the lack of information, another method was used to determine the material’s qualification at 600°C. The yield strength at room temperature was taken as one aspect of the overall yield strength. Knowing that material properties change at high temperatures, a second aspect was taken into consideration. Each material was given a score of one or two depending on if it passed 200 MPa at 600°C. If the material passed, then it was given a two; if the material did not pass, then it was given a one. Ones and twos were used to fit the format of the sum equation used for the final score shown in equation 1. Similarly, thermal conductivity and cost were evaluated for all the materials with thermal conductivity having a weight of seven and cost having a weight of negative ten.

Max service temp Density

Yield Strength @ Room

Does it pass 200 MPa @ 600°C

Thermal Conductivity Cost/lb

Weight 9 ­3 5 10 7 ­10

Target 850°C 8 g/cm3 300 MPa 1 27 W/m­°C $2

Table 1: Weights for each criteria used to rank the materials

core1 SUM[Weight ] S = * Target(V alue−Target)

core1 9 0 0S = * 850

(V alue−850) − 3 * 8(V alue−8) + 5 * 300

(V alue−300) + 1 * 1(V alue−1) + 7 * 27

(V alue−27) − 1 * 2(V alue−2)

(Eqn. 1)

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A second equation (Eqn. 2), based on ratios between material values to target values, was used to verify the results from the first equation. Only score 1 is shown in Table 5.

core2 SUM[Weight ]S = * Target(V alue) (Eqn. 2)

After determining the top five materials, further analysis on their high temperature mechanical and thermal properties was done to determine the best material to use. The analysis focused on high temperature failure mechanisms and corrosion. Sustainability and current printability with 3D printing techniques were also added as additional factors in ranking the top five materials. The ranking system was similar to the one discussed above, the main difference is that the weights were multiplied by relative scores, scaled from 1 to 5. Table 2 shows the ranking criteria used for the top five materials and their respective weights.

core SUM[Weight Relative score in each criterion] S = * (Eqn. 3)

Criterion for high temperature applications Weight (1­10)

Printability (with current machines & possible methods) 7

Density 3

Thermal conductivity 8

Expansivity 7

Yield Strength 6

Ductility 4

Oxidation resistance 9

Corrosion from fuel combustion products 7

Creep & rupture strengths 5

Fatigue 4

Sustainability 10

Cost 10

Table 2: Ranking criteria for top five materials and their respective weights

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High temperature mechanical failure criteria (creep, rupture, yield, and fatigue) had lower weights because the top five materials displayed good overall mechanical properties at high temperatures. Most of the materials could withstand exposures to significant stresses for a long time. This is seen in the material property discussion.

B. Manufacturing In order to select the best and most appropriate manufacturing technique, extensive research was conducted for all current and progressing techniques. There are currently 11 additive manufacturing techniques, but only six methods were found to be relevant to this case. The six methods are laser melting, laser sintering, electron beam melting, binder jetting technology, laminated object manufacturing, and ultrasonic consolidation. Electron beam additive manufacturing was also briefly discussed in the Manufacturing section, but was ultimately excluded from the analysis due to the limited information on the technology. The six techniques were ranked from one to six (one being the worst and six being the best) in six categories: energy input, time required, bond quality / density, design freedom, accuracy, and maintenance cost. Each category was also given a weight for importance from one to ten ­­ one being not very important to ten being very important. Being mindful of cost and the environment, lower energy consumption would be ideal and was given a weight of eight. This criterion was assessed qualitatively by energy input needed to complete each process. For example, melting takes more energy than sintering. Following a similar logic, the lowest energy input system would receive a ranking of six. Time is an essential criterion because it also feeds into cost and was given a weight of six. The weight of six is justified by the rationale that 3D printing will not be a large production volume process and therefore is only somewhat important. However, the less time it takes to manufacture a product, the more product can be produced. Time was also assessed qualitatively with the fastest production time given a ranking of six. The major advantage of 3D printing is the design freedom it gives to engineers so three criteria that affect the product design are density, design freedom, and accuracy. The sturdiness and safety of the structure is highly important and therefore was given a weight of nine. The technique with the best bonding between the materials was given a ranking of six. Since the major advantage of 3D printing is design freedom, the category was given a weight of seven where techniques with virtually no limitation were given a ranking of six. The accuracy to which each technique can produce the finished product affects the mechanical properties of the heat

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exchanger. Although efficiency is important, the main focus of the study is on feasibility and therefore accuracy was given a weight of five. The most precise technique with low surface roughness was given a ranking of six. Lastly, maintenance cost is important in terms of determining economic viability. Techniques were ranked by the perception of how much maintenance would be needed. For example, lasers are highly sensitive and require special maintenance and therefore would cost more than other techniques. Binder jetting involves fluids that may get clogged or contaminated, which would require replacement. A summary of the weights is shown in Table 3. Equation 4 shows how the total score was calculated.

Energy Input Time Bond Quality / Density Design Freedom Accuracy Maintenance

Weight 8 6 9 7 5 5

Table 3: Weights for each criteria used to rank manufacturing techniques

Total = SUM(Weight*Rank) (Eqn. 4)

After all the ranking and weights were appropriated, the technique with the highest score was judged to be the best theoretical method of manufacturing. Further analysis was conducted qualitatively to discuss machine cost in order to analyze everything holistically.

VIII. Materials Material selection is one of the most crucial aspects of this project. The material used for the heat exchanger must withstand a high inlet temperature and must operate above 600°C, although ideally well above 850°C in order to achieve a good overall efficiency. As the temperature is raised, the material may creep, limiting its ability to carry loads. It may degrade or decompose, changing its chemical structure in ways that make it unusable (Granta 2009). The two current models of heat exchangers are built using Stainless Steel 347 and Inconel 718. Using Ashby’s material and selection chart for Strength vs. Maximum Service Temperature shown in Figure 2, the materials were narrowed down to nickel alloys, stainless steels, tungsten alloys, and certain technical ceramics. Certain titanium metals were also considered, but would not likely be the final choice due to the lower maximum service temperature.

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Figure 2: Ashby’s Material and Selection Chart (Granta 2009)

A. Material Overview All the materials listed in Table 4 were initially considered for the heat exchanger, however, only the bolded materials were analyzed further. The items in Table 4 represent materials commonly mentioned in research literature and additive manufacturing. The bolded materials were considered to be superior based on scientific experiments conducted by other researchers as well as common properties such as maximum service temperature, density, and cost.

AL20­25+Nb alloy Incoloy alloy 800HT Nickel 200 / 201 Stainless Steel 316L

Alloy 230 Incoloy alloy 825 Nickel Alloy 333 Stainless Steel SS347

Alloy 242 Inconel alloy HX Nicrofer Alloy 45TM Tantalum

Alloy 602CA Inconel 600 Niobium Ti­64+TiC

Alloy modified 803 Inconel 617 Rhenium Ti­6Al­4V PM

Cobalt Chrome Inconel 625 Silicon Carbide Tungsten

Haynes Alloy 120 Inconel 718 Silicon Nitride TZM Molybdenum

Haynes Alloy 214 Inconel alloy 601 Stainless Steel 17­4PH Yttrium Oxide

Incoloy alloy 800 Iron Chrome Aluminum Stainless Steel 304

Incoloy Alloy 800H Molybdenum Stainless Steel 316

Table 4: All the materials initially considered for the heat exchanger design

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Using a decision matrix explained in the Methodology section above, the 22 materials were narrowed down to 5 top materials in Table 5.

Material

Max service

temp (°C) Density (g/cm3)

Yield Strength (MPa) @ Room

Yield strength

@ 600°C

Does it pass 200 Mpa @

600°C

Thermal Conductivity

@ 1000(W/mC) Cost/lb Score

Silicon Carbide 1 1600 4.60 1600.0 250.0 2.0 41.0 8.00 14.51

Inconel 718 2 875 8.19 1034.0 980.0 2.0 26.7 5.46 5.06

Haynes Alloy 214 3 800 8.05 605.0 2.0 32.7 4.30 4.51

Inconel 601 4 875 8.11 350.0 330.0 2.0 27.8 3.52 3.66

Stainless Steel 304 5 750 7.85 215.0 92.0 1.0 27.8 0.88 3.39

Stainless Steel 316 6 800 7.99 205.0 153.8 1.0 27.8 1.14 2.38

Stainless Steel 316L6 800 7.99 170.0 153.8 1.0 27.8 1.23 1.36

Stainless Steel SS347 7 750 7.96 205.0 176.0 1.0 27.8 1.40 0.58

Alloy modified 803 8 875 7.86 290.0 210.0 2.0 27.4 4.22 ­0.87

Haynes Alloy 120 9 800 8.07 375.0 2.0 26.2 4.31 ­1.07

1 MEMSnet, “Material: Silicon Carbide (SiC), bulk,” MEMSnet, accessed July 26, 2015, https://www.memsnet.org/material/siliconcarbidesicbulk/. 2 Elgin Fastener Group, “Inconel 718,” Elgin Fasteners, accessed July 26, 2015, http://elginfasteners.com/resources/material­properties/inconel­718/. 3 Haynes International, “Haynes 214 Alloy.” Haynes International, accessed July 26, 2015. https://www.haynesintl.com/pdf/h3008.pdf. 4 Special Metals, “Inconel alloy 601.” Special Metals, accessed July 26, 2015. http://www.specialmetals.com/documents/inconel_alloy_601.pdf. 5 AK Steel, “304/304L Stainless Steel.” AK Steel, accessed July 26, 2015. http://www.aksteel.com/pdf/markets_products/stainless/austenitic/304_304l_data_bulletin.pdf. 6 AK Steel 7 Sandmeyer Steel Company, “321 and 347.” Sand Meyer Steel, accessed July 26, 2015. http://www.sandmeyersteel.com/images/321­347­Spec­Sheet.pdf. 8 Special Metals 9 Elgin Fastener Group

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Material

Max service

temp (°C) Density (g/cm3)

Yield Strength (MPa) @ Room

Yield strength

@ 600°C

Does it pass 200 Mpa @

600°C

Thermal Conductivity

@ 1000(W/mC) Cost/lb Score

Silicon Nitride 10 1500 2.81 525.0 500.0 2.0 43.0 8.00 ­3.27

Inconel 6178 875 8.36 320.0 255.0 2.0 28.7 5.54 ­6.82

Incoloy alloy 8008 875 7.94 347.5 205.0 2.0 31.9 6.00 ­7.65

Inconel 6258 875 8.44 586.0 207.0 2.0 25.2 6.42 ­7.72

Nickel Alloy 333 11 900 8.20 324.0 200.8 2.0 28.9 6.00 ­8.65

Nicrofer 6025 HT / Alloy 602CA 12 900 7.90 270.0 385.0 2.0 27.7 6.00 ­9.75

Inconel 6008 875 8.47 255.0 210.0 2.0 27.5 5.91 ­10.11

Inconel alloy HX8 875 8.20 345.0 210.0 2.0 27.9 6.80 ­12.83

Incoloy Alloy 800H8 875 7.94 150.0 205.0 2.0 31.9 6.80 ­14.94

Incoloy alloy 800HT8 875 7.94 150.0 205.0 2.0 31.9 7.00 ­15.94

Nicrofer Alloy 45TM 13 900 8.00 240.0 135.0 1.0 27.0 6.00 ­20.47

Haynes Alloy 230 14 800 8.97 395.0 274.4 2.0 28.4 9.15 ­24.71

Table 5: The top 22 materials and their material properties It can be seen from Table 5 that SiC has outperformed the rest of top five materials by at least a factor of two.

10 Azo Materials, “Silicon Nitride Properties and Applications.” AZOM, accessed July 26, 2015. http://www.azom.com/properties.aspx?ArticleID=53 11 Rolled Alloys, “Data Sheet RA333.” Rolled Alloys, accessed July 26, 2015. http://content.rolledalloys.com/technical­resources/databooks/RA333_DB_US_EN.pdf. 12 ThyssenKrupp Stainless, “Nicrofer 6025 HT ­ alloy 602 CA.” VDM, accessed July 26, 2015. http://www.vdm­metals.com/fileadmin/user_upload/Downloads/Datenblaetter_­_Data_Sheets/Data_Sheet_VDM_Alloy_602_CA.pdf. 13 ThyssenKrupp Stainless 14 High Temp Metals, “Haynes 230 Technical Data.” High Temp Metals, accessed July 26, 2015. http://www.hightempmetals.com/techdata/hitempHaynes230data.php.

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B. Silicon Carbide Silicon carbide (SiC) was accidentally discovered in 1890 by Edward G. Acheson while he was running an experiment on the synthesis of diamonds. Silicon carbide occurs naturally in meteorites, though very rarely and in very small amounts. Today, SiC is produced via a solid state reaction between sand (silicon dioxide) and petroleum coke (carbon) at very high temperatures in an electric arc furnace (Poco 2002). SiC is low­weight and has high strength, hardness, and strong covalency. Combining those properties with low thermal expansion coefficient and high thermal conductivity, SiC is a promising alternative to conventional metals, alloys, and ionic­bonded ceramic oxides (Poco 2002). The most common forms of SiC include powders, fibers, whiskers, coatings, and single crystals. Since these materials will be paired with 3D printing, powder production will mainly be considered (Poco 2002).

i. Creep and Oxidation Creep rates of ceramics possessing a glassy grain­boundary phase are degraded compared with the inherent creep resistance. Silicon carbide ceramics are generally kinetically stable in air to temperatures around 1000°C. Rapid surface oxide layers start to form in the 1000°C­1150°C temperature range after which oxidation becomes passive (Poco 2002). Oxidation rates start to become significant (when oxide layer starts to form) at 1650°C. The presence of impurities, introduced by the sintering additives, often reduces the oxidation resistance of SiC as well. For temperatures of around 1500°C, alpha sintered SiC samples show creep strength values of at least ten times those exhibited by Inconel 601 samples at ~1000°C (Munro 1997).

ii. Fatigue Toughness The low inherent fracture toughness of conventional SiC ceramics can be improved by producing a composite, typically by incorporating continuous fibers, whiskers, or second­phase particles. Recent research has focused on monolithic SiC hot pressed with aluminum metal as well as boron and carbon. This is often referred to as ABC­SiC and it has been shown to have ambient temperature fracture toughness as high as 9 Mpa m1/2 with strengths of 650 MPa (Chen 2000).

iii. Crack Healing An interesting behavior that occurs in sintered ceramics is the crack­healing behavior. One experiment showed that SiC sintered with scandium oxide and aluminum nitride with a surface crack of 100 micrometer fully recovered its strength at room temperature after a heat treatment at 1300°C for one hour in air under no stress and at 1200°C for five hours under an applied stress of 200 MPa (Lee 2005).

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iv. Compatibility with 3D Printing SiC have been tested with powder based fusion processes and new research is being conducted with laminated object manufacturing technology. The major hindrance to SiC’s compatibility with 3D printing is purity. Sintering ceramic powder must be less than 1 micron for certain machines and this would require SiC powdered to be further milled and acid­treated to remove metallic impurities (Poco 2002). Another factor to take into consideration with 3D printing ceramics is the shrinkage that occurs during sintering. The density of SiC sintered at 1750°C is close to the theoretical values (97­98%) and the mass loss and shrinkage ratio are small. SiC ceramics sintered at 2000°C have more shrinkage after sintering and achieve less density (92%) (Liu 2005). This lower density at 2000°C can be explained by the evaporation of SiC and oxide additives. Therefore, the maximum temperature capability of SiC formulation is about 1900°C. Higher temperatures may cause SiC to vaporize in the presence of nitrogen and may dissociate to pure silicon and carbon (Liu 2005).

v. Sustainability Unlike stainless steels, ceramics are not easily recyclable. The best way to reuse ceramics is to smash the product and mill them back into powder (Zero 2015). Ceramics production is very energy intensive even with the EU industry halving its energy consumption over the last 25 years. Dust and gaseous emissions arise during the firing and spray drying of ceramics (European 2015).

C. Inconel alloys The name Inconel is a trademark of Special Metals Corporation and it is for a group of nickel­chromium based alloys that show excellent performance under high temperature and corrosive environments (Special 2015). This is largely due to their compositions and also heat treatment. Age­hardened Inconel 718 and solution­treated Inconel 601 are commonly used in high temperature applications due to their higher strengths (Special 2015). While Inconel 601 has a larger nickel base and more chromium content, Inconel 718 contains several other elements in its matrix. Additional molybdenum in Inconel 718 significantly improves its strength and the higher aluminum content in Inconel 601 achieves a high oxidation resistance (Special 2015).

i. Impact Strength Alloy 601 retains high impact strengths even after long exposure to high temperatures. Solution treated samples of Alloy 601 had an average impact strength values of ~170J at room temperature. These varied from 120­160J after being exposed to temperatures from 540­870°C for over 1000 hours of operation (Special 2015). This shows that the alloy maintained its good

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ductility properties. Aged samples of Inconel 718 had maximum impact strengths of ~60J at room temperature, a significantly lower value compared to that of Inconel 601 (Special 2015).

ii. Fatigue Strength The solution­treated Inconel 601 rod has an endurance limit of ~260 MPa for infinite stress cycles at room temperature. A similar annealed sample had an endurance limit of ~330 MPa for infinite stress cycles (Special 2015). Room temperature endurance limit of an Inconel 718 rod of similar size was ~620 MPa for infinite stress cycles (Special 2015). Low­cycle endurance limits are higher for both samples.

iii. Creep and Rupture Properties For creep rates of 0.01% in an hour at 870°C, the creep limit for samples of solution treated Inconel 601 was ~30 MPa. The limit was 100 MPa at 705°C while that of age­hardened Inconel 718 was ~450 MPa at the same temperature. Rupture strengths at 705°C for 1000 hours were ~100 MPa and ~ 500 MPa for alloys 601 and 718, respectively. Alloy 718 showed better creep­rupture properties (Special 2015).

iv. Corrosion resistance The alloys show great corrosion resistance properties especially due to their nickel­chromium base composition. Nickel contributes to corrosion resistance in many organic and inorganic media (Special 2015). Chromium increases the resistance to oxidation and reactions to sulfur while additional aluminum in alloy 601 helps to combat oxidation. Molybdenum in alloy 718 is known to reduce pitting corrosion (Special 2015). A good way to compare oxidation resistances of the superalloys is by looking at their oxidation rate constants ‘Kp’ at high temperatures. The rate constants are obtained by the slopes from the graphs of the squares of the mass gained per unit surface area versus time. These tend to be linear within a given range of temperatures because oxidation is a chemical reaction that follows Arrhenius behavior (Clark 2013). Open­air oxidation rate constants of Inconel 601 are generally lower than those of Inconel 718 for temperature of around 1000°C (Yang 2002). The rates in Inconel 718 increase by a factor of two per 100°C rise in temperature (Greene 2001). Inconel 601 shows good corrosion resistance around both oxidizing and reducing environments while both the alloys are performing well under hydrogen attacks (Haynes 2015). However, the oxidation rates for both the Inconel alloys are much greater in comparison to Haynes 214 that has excellent corrosion resistance in a wide range of chemical media (Yang 2002). Studies have also shown internal oxidation attacks in Inconel 601 (Special 2015), (Haynes 2015). This is when oxide regions are formed inside the surface as a result of oxygen diffusion. Other alloys have tendencies to form a protective oxide layer at the surface that reduces oxidation rates and prevents further attack inside the alloy base. Figure 3 shows oxidation results in Inconel 601 versus those in Haynes 214 (Haynes 2015). Internal attack is seen in Inconel 601.

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Figure 3: Internal oxidation attack in Inconel 601 (Haynes 2015)

v. Compatibility with 3D printing Standard manufacturing forms for both alloys are pipes, tubes, sheets, strips, plates, round bars, flat bars, forging stocks, hexagons, and wires. Many metal alloy powder companies do produce superalloys in powder forms of different sizes for high­purity purposes (American 2015). Thus both powder­based and laminated­based 3D printing processes can implement the Inconel alloys.

vi. Sustainability Inconel 601 is known for its excellent machinability and formability, relative to both Haynes 214 and Inconel 718 (Yang 2002). Its manufacturing costs, those associated with preparing powders and sheets, are thus expected to be relatively lower. Recycling of Inconel alloys is done by a few companies due to the growing demand of the scrap Inconel alloys in different applications (Monico 2015).

D. Haynes 214 Alloy Haynes 214 alloy is a nickel­chromium­aluminum­iron alloy with excellent high­temperature oxidation resistance. Its intended use is at temperatures above 955°C. Under 955°C, Haynes 214 still provides oxidation resistance equal to the best nickel­based alloys (Haynes 2015).

i. Oxidation and Creep When exposed to air flowing at 213.4 cm/min for 1008 hours at 1150°C, Haynes 214 showed only 8 micrometers of metal affected (Haynes 2015). The most important part of the result was the virtual absence of internal attack for the Haynes 214 (Figure 3). At 870°C, Haynes 214 can withstand 54 MPa after 1000 hours of operation until rupture. The creep­rupture properties of Haynes 214 and Inconel 601 samples are generally similar for up to 10000 hours of high temperature exposures (Haynes 2015).

ii. Cracking Similar to nickel­base alloys, Haynes 214 will exhibit age­hardening as a result of the formation of a second phase, gamma prime (Ni3Al) (Haynes 2015). This causes a significant loss of

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intermediate and low temperature tensile ductility, which results in alloy 214 being susceptible to strain­age cracking when highly­stressed welded components are slowly heated.

iii. Compatibility with 3D Printing Haynes 214 alloy is currently available in the form of plate, sheet, strip, billet, bar, and wire, which makes it compatible with Electron Beam Additive Manufacturing and Ultrasonic Consolidation, but not with the powder­based techniques. There are currently no companies printing with Haynes 214.

iv. Sustainability Haynes alloy 214 scraps are collected by private companies such as Greystone Alloys and Monico Alloys that handle the maintenance and reselling of the material

E. Stainless Steel 304 Stainless Steel 304 (SS304) is one of the most common stainless steels. It is an austenitic steel that is not particularly electrically or thermally conductive, but has a higher corrosion resistance than regular steel. SS304 and SS316 are often compared and going back to Table 5, the low cost of SS304 won out in the end. The main difference between 304 and 316 stainless steel is that SS316 contains 2%­3% molybdenum and 304 has no molybdenum. The molybdenum added improves corrosion resistance to chlorides (Slipnot 2015).

i. Creep and Creep­Fatigue The dependence of minimum creep rate on stress during high­temperature deformation of SS304 follows the power law, , where α and β are dependent on temperature (Arcam 2009).= (σ) ε * α β The stress exponent β determined by one experiment was 5.6, 5.9, and 6.5 for temperatures of 700, 650, and 600°C, respectively. During high­temperature deformation, creep deformations result from not only the activation of normal slip systems, but also the translation of grains relative to one another along their boundaries. At 700°C and 76 MPa of applied stress, fracture occurs (Zhang 2014). Over 482°C, deformation under stress is plastic rather than elastic, so the yield point as determined by the short­time tensile test is higher than the creep or creep­rupture strength (Nickel 2015). A creep­rate curve of annealed SS304 show ~10 MPa of stress at creep rates of 0.0001% / hr at 800°C. The number of cycles to failure decreases linearly with increasing the hold time in double logarithmic coordinate less than 650°C. At high temperature of operation above 700°C, SS304 would fail after about 60 cycles of 30 minute hold time (Zhang 2014).

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ii. Oxidation The oxidation resistance of stainless steel occurs due to its ability to form a protective coating layer on the surface. The coating is a passive film, which resists further oxidation. The non­corrosive property in stainless steel derives from the existence of chromium in the alloy. SS304 is 18% chromium and SS316 is about 16­18% chromium (AJMFG 2015). The kinetics of oxidation of SS304 was found to be 2.3 μg/cm2 of oxide formed at 500°C in six hours. The rate of increase was 0.1 μg/cm2/hr after the first six hours. At 800°C, a transition is observed and the rate of oxidation followed a linear growth rate rather than the previous parabolic rate law. This transition occurred for a weight gain of 9 μg/cm2. At 900°C and for weight gains above 90 μg/cm2, the parabolic rate law was found to hold again. The second transition is found in the kinetics of oxidation at 1150°C (Gulbransen 1962). The maximum temperature to which SS304 can be exposed continuously without appreciable scaling is about 900°C. For intermittent cyclic exposure, the maximum exposure temperature is about 815°C (AK 2015).

iii. Compatibility with 3D Printing Since SS304 comes in powder, plate, wire, and sheet form it is highly compatible with 3D printing. Many companies such as Optomec already have SS304 listed as one of their materials. More companies have explored 3D printing with SS316 & 316L powders.

iv. Sustainability Stainless steel is an infinitely recyclable commodity which lends itself to be highly sustainable. The typical amount of recycled stainless steel scrap is about 65 to 80%. Since stainless steels are widely used in a variety of markets, many green steps are being taken to reduce the carbon footprint of manufacturing. Environmental legislation is also forcing petrochemical and refinery industries to recycle secondary cooling water in closed systems (Advameg 2015).

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F. Evaluating high­temperature thermal and tensile properties

Figure 4: Thermal conductivities of the top five materials with temperature

Figure 5: Specific heat capacities measured from a reference of 20°C

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Thermal performance was evaluated from looking at thermal conductivities and average specific heat capacities of the top five at a range of elevated temperatures. Figure 4 shows the superiority of the conductivity of SiC in applications from cryogenic ranges to 1100°C. While the ceramic shows declining conductivities with rising temperatures, metallic alloys showed the opposite trend ­­ owing to the increasing vibrational kinetic energies with temperature. SS304 shows a slightly better performance relative to the two Inconel alloys. The second best material was Haynes 214, which showed a significantly disproportional rise in its conductivities at temperatures above 600°C. The average specific heat capacities of SiC (Figure 5) were significantly higher than those of the metallic alloys and they increased steeply with temperatures. Despite a better heat transfer that comes with high heat capacity, SiC could be of a slight disadvantage since it would take more thermal energy inputs (than in other materials) for a given rise of temperature. The ceramic is also expected to take more time to warm up or cool down, contributing to a problem of residual heats in operations. The high heat capacity advantage, however, outweighs other penalties and could be of much advantage as discussed in the design section.

Figure 6: Coefficient of linear expansion of the top five materials with temperature

SiC showed the lowest values of linear expansion coefficients while SS304 had the highest. Inconel 718 had the smallest coefficient of the three remaining alloys. Material expansion at elevated temperatures could be detrimental to designs (unless accounted for) and also they add additional and unexpected stress concentration points, especially if the expansion is anisotropic. Volume expansion coefficients could also be estimated from the Figure 6 as γ~3ᵯ for cubes.

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Figure 7: Yield strengths of top 5 materials with temperature

Figure 8: Elastic modulus of top five materials with temperature

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High temperature tensile properties of the materials were compared by using yield strength (Figure 7) and elastic moduli (Figure 8) data obtained from different specimens. These values could vary significantly depending on the method of sintering (for SiC) and temperature working. Solution­treated Inconel 601 (on the plot) is usually used for high temperature applications because of its higher strength values than in annealed samples (Special 2015). Values of tensile strength for sintered SiC could go as high as 300 MPa as well (Poco 2002). It can be seen that Inconel 718 and Haynes 214 show excellent strengths (>250 MPa) up to temperatures around 900°C. SiC shows a constant strength at even higher temperatures. SS304 has the lowest strength values and it is essentially ineffective past 750°C. Inconel 601’s best range of application is seen to be between 600°C and 800°C. Its strength values are significantly lower than those of Inconel 718, Haynes 214, and sintered SiC until around 1000°C. SiC is, however, very brittle as indicated by its high elastic moduli relative to the metallic alloys. Brittleness could pose a challenge to the formability of ceramics despite its high strength. Ceramics are still more likely to have brittle fractures especially during sudden stress loads. SS304 has the best ductility properties at high temperatures.

G. Ranking results Table 6 shows the average scores of each of the top five materials after considering a wider array of properties discussed above. The ranking method is described in the Methodology section. From the ranking method, it can be seen that SiC is the best material to use for heat exchangers despite the high cost, poor sustainability, and limited printability concerns. The excellent mechanical and thermal properties, together with its unparalleled corrosion resistance, gives SiC an edge over the others. Its density that is almost half of other top five materials also adds more benefits that come with the choice of SiC. Inconel 718 comes as a close second best material because of its high strength, extensive printability, and sustainability advantages. Haynes 214’s excellent corrosion resistance puts it in between the top two and bottom two materials. Inconel 601 and SS304 had the lowest scores because of the penalties from poor corrosion resistance and strengths at high temperatures. SS304, despite its excellent sustainability and cost advantage still had the lowest score because it performed poorly in other criteria.

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Property Silicon Carbide Inconel 718 Haynes 214 Inconel 601 SS304

Printability 2 5 1 4 4

Density 5 2 2 2 3

Thermal conductivity 5 2 4 2 3

Expansivity 5 3 2 2 1

Yield Strength 4 5 3 2 1

Ductility 1 4 3 4 5

Oxidation resistance 5 3 4 2 1

Corrosion resistance 5 3 4 2 1

Creep & rupture strengths 5 4 2 2 1

Fatigue 4 3 5 3 2

Sustainability 1 4 3 4 5

Cost 1 2 3 4 5

Total 273 264 243 226 223 Table 6: Ranking results for the top five materials

IX. 3D Printing / Additive Manufacturing 3D printing is the process of making three dimensional objects from a digital CAD file. Interest in 3D printing has boomed within the last five years raising nearly four billion USD in public offering since 2011 (Wheeler 2015). Typically used as a prototyping method to speed up the design process, 3D printing is now being considered as a manufacturing process for the final product. Unlike injection molding and other traditional methods of manufacturing, 3D printing is not limited by production restraints, which opens up for design creativity.

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In February 2015, Australia’s Monash University created the first 3D­printed jet engine (Coxworth 2015). Three months later, in May 2015, GE fired up a simple jet engine made entirely of 3D­printed parts and revved it up to 33,000 RPM (Keller 2015). The aerospace industry has jump­started the race to 3D­print parts because of the reduced lead time, lighter weight of parts, and lower production costs. These same motivations led to research of manufacturing affordable heat exchangers for the micro­gas turbines. The key to how additive manufacturing (AM) works is that parts are made by adding material in layers; each layer is a thin cross­section of the part derived from the original CAD data. The thinner each layer, the closer the final product will be to the original design (Gibson 2015). All commercialized AM machines to date use a layer­based approach and the major difference lies in the materials used, how the layers are created, and how the layers are bonded together. These differences determine the speed of the process, the amount of post­processing required, the size of the AM machine used, and the overall cost (Gibson 2015).

A. Manufacturing Techniques Overview Several methods of 3D printing and additive manufacturing are being researched and are listed in Table 7. The methods highlighted are viable alternatives that will be considered in this report. Many of the methods use the powder bed fusion process, which follows the same basic loading procedure (Noe 2014). Basic Procedure:

1. A designer / engineer designs a part using a CAD software 2. The part is cut into virtual slices on the horizontal plane 3. A chamber is filled with powder 4. A laser / electron beam scans the powder, solidifying a thin layer 5. Another layer of powder is added as the platform moves down 6. Layer by layer, the product is built up until it is finished 7. The leftover powder is reused

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Plastics Metals Ceramics

Selective Laser Sintering (SLS): does not fully melt the powder, but heats it to a point that the powder can fuse together on a molecular level. Used with alloys and porosity can be controlled

Selective Laser Melting (SLM): the material is heated to a full melt and forms one homogenous form

Binder Jetting Technology: spreads a binder over the powder (infuses steel with bronze)

Laminated Object Manufacturing (LOM): uses a continuous sheet (paper, plastic, less common metal) that is cut by a laser and built layer by layer using a heat rollers

Stereolithography (SLA): converts liquid plastic to solid layer by layer using solvent and ultraviolet oven

Direct Metal Laser Sintering (DMLS) is the same as SLS, but specifically for metals

Direct Metal Laser Melting (DMLM) is the same as SLM, but specifically for metals

Electron Beam Melting (EBM) is the same as SLM except it uses an electron beam instead of a laser

Fused Deposition Modeling (FDM): heating and extruding thermoplastic filaments

Electron Beam Additive Manufacturing (EBAM): an electron beam gun deposits metals layer by layer via wire feedstock

Ultrasonic Consolidation (UC): welds metal foils using CNC contour milling

Table 7: Overview of additive manufacturing techniques compatible with plastics, metals, and ceramics

B. Powder­Based Fusion Processes (PBF) Metallic powder­based fusion in AM mimics the polymeric powder­based processes, which were amongst the first commercialized AM processes (Copra 1977). Building layers are formed by fusing thin layers of metallic powder that are deposited on the build platform. The fusion occurs either by solid­state sintering, melting, or a mix of both. All PBF processes have one or more

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thermal sources to induce fusion between powder particles, a method to control that fusion in specific regions, and mechanisms for adding powder layers. Most common thermal energy sources for metal PBF processes are lasers and electron beams. These are usually assisted by preheating to maintain elevated temperatures across the powder layers. Figure 9 shows the schematic of a selective laser­sintering process, a setup that is very similar to many other PBF processes.

Figure 9: Schematic of selective­laser sintering (Gibson 2015)

The roller spreads thin layers of powder across the build platform. Once the powder deposition is complete the laser is directed into the powder bed and moved to thermally fuse the powder to form a slice of the cross section. The surrounding powder is left loose and can be used to support subsequent layers. After completing a slice, the build platform is lowered by one layer thickness and a new powder layer begins. The process repeats until the part is complete. Due to the reactive nature of powders and the elevated temperatures on the bed, sintering and melting techniques require inert conditions to avoid rapid oxidation or reaction with other substances. There are principally four different fusion mechanisms that are associated with PBF processes: solid­state sintering, chemically induced binding, liquid­phase sintering, and full melting (Gibson 2015).

i. Solid­State Sintering This implies fusion of the powder particles without melting. The process usually occurs at temperatures between half of the melting point and the melting point (0.5Tm< Ts < Tm). Solid state sintering occurs to minimize the surface free energy (proportional to the total surface area)

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at elevated temperatures. The particles diffuse to minimize the total surface area causing a decreased porosity and solid part formation shown in Figure 10. The process takes an abundance of time and is usually not the primary fusion mechanism in PBF processes.

Figure 10: Schematic of solid­state sintering (Gibson 2015)

ii. Chemically­Induced Sintering This involves a thermally activated chemical reaction between two types of powders or a powder and a gas to form a by­product that acts as a binder. Chemically induced sintering is mostly applied for ceramic materials. For example, the formation of silicon dioxide that binds silicon carbide in laser processing (Gibson 2015).

iii. Liquid­Phase Sintering (LPS) This process involves the fusion of powder particles when a portion of constituents within the powder is molten while the other remains solid. The molten part acts as a binder that holds the high temperature particles together. The binder material is usually included in the main powder as a coated substance, a composite, or a separate powder. LPS processes are the most rapid and do not require full sintering or melting (Gibson 2015).

iv. Full Melting This is when the powder is fully melted beyond the depth of the layer to create a well­bonded and highly dense structure with a very low porosity (Gibson 2015). This method is commonly used for metal alloys due to strength requirements of the final parts.

v. Electron Beam / Electron Beam Melting (EB / EBM) This technology uses high­energy electron beam to fuse metal powder particles instead of the typical high intensity lasers. Heating is caused by the transfer of kinetic energy from incoming electrons instead of absorption of photons from lasers. EB processes require vacuums (to preserve the kinetic energy) and have achieved higher scan speeds. EB can even be applied to moderate particle sizes (Gibson 2015). They are, however, limited to metals and metal alloys due to the high conductivity requirements. EB processes are also associated with poor surface finishes (Gibson 2015).

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vi. Sustainability Metal PBF are the most energy intensive processes due to their high temperature requirements. Full melting involves remelting part of the already formed solid layer underneath, contributing to more energy costs. Additional energy costs come from the need for more sophisticated finishing (post­print) processes due to low accuracies that are associated with PBF processes (Gibson 2015). Careless powder and laser handling could easily result in serious hazards as many powders are toxic and reactive, especially under energy­intensive lasers. Powder recycling in sintering processes is still a challenge as the heated powders tend to have different properties from normal ones (Gibson 2015).

C. Binder Jetting Additive Manufacturing (BJAM) Binder jetting technology was developed in the early 1990’s by MIT and it has been licensed for more than five companies for commercialization (Gibson 2015). The method is very similar to that the other PBF processes. The difference is that layers are formed at the powder bed with a liquid binder. The final product is built entirely without heating. After the product is built up, post­print processes such as curing, heating in the furnace to vaporize the binder, and sintering take place. This is usually associated with some degree of loss accuracy. Some companies have also applied infiltrants to compensate for the part’s density and strength losses in the post­print heating processes (Gibson 2015). Binder jetting technologies do not require high­energy inputs or dangerous lasers and are relatively fast, depending on the flow rate of the binder (Gibson 2015). There is also a promising research being done on printing ceramics parts by using binder jetting.

D. Laminated Object Manufacturing (LOM) Developed by California­based Helisys Inc., now Cubic Technologies, laminated object manufacturing is a 3D printing technique where layers of a material are fused together using heat and pressure and then cut into the desired shape with a controlled laser or blade. The LOM apparatus uses a continuous sheet of material (usually paper, but is now being experimented with metal and ceramics), which is drawn across a build platform by a system of feed rollers. Figure 11 shows that the material is unwound from feed roll (A) onto the stack and bonded to the previous layer using a heat roller (B). The roller melts a coating on the bottom side of the material to create the bond. The 2D profiles of the desired product are traced by a laser mounted on an XY stage (C). The excess material is cut away and fed to the take­up roll (D). The process generates a lot of smoke and localized flame; therefore, either a chimney or filtration system (E)

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is required (Palermo 2013). After one layer is complete, the build platform lowers about one­sixteenth of an inch, or the thickness of one layer and the process repeats.

Figure 11: The laminated object manufacturing process (Palermo 2013)

LOM is not ideal for complex geometries and cannot create hollow objects. Therefore, this manufacturing technique creates limitations on the design similar to traditional techniques. LOM is not highly accurate and as a result has been limited primarily to conceptual prototyping. Furthermore, the main material used in this type of manufacturing has been paper. LOM is very fast, low cost, and has ease of material handling. LOM is not very popular due to the limited materials compatible with this technology and was considered because Ceralinks is currently using this method to 3D print ceramic objects.

E. Electron Beam Additive Manufacturing (EBAM) Sciaky’s Electron Beam Additive Manufacturing should not be confused with electron beam melting. Electron beam melting is a powder­based method that uses an electron beam to fuse the metal together. EBAM, on the other hand, deposits metal via wire feedstock layer by layer until the product is complete. It has a standard deposition rate ranging from seven to twenty pounds per hour (Sciaky 2015).

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Materials compatible with EBAM include Inconel 718 and Stainless Steels 300 Series. The best material candidate for EBAM are high­value metals with long lead times that can be manufactured with wire feedstock (Sciaky 2015).

F. Ultrasonic Consolidation (UC) Introduced by Solidica Inc. USA in 2000, ultrasonic consolidation implements ultrasonic welding in additive manufacturing of metals (Yang 2009). The process involves a rotating head (sonotrode) that vibrates at frequencies of around 20 kHz moving along a thin metal foil that is placed in intimate contact with a layer or foil below. A constant normal force is usually applied by the head and the amplitudes of vibrations are controlled by the user. The process is repeated by placing foils next to one another (horizontally) and on top of another (vertically) until a layer is formed. A contour computer numerical control (CNC) mill then shapes the formed layer into its slice contours, a subtractive process. Layers are then ultrasonically added until the part is complete (an additive process). It can thus be seen that ultrasonic consolidation is a hybrid process that combines both additive and subtractive manufacturing. Figure 12 is a schematic of the UC process.

Figure 12 : Schematic of the ultrasonic consolidation process (Gibson 2015)

Bond formation between the foils is a pure solid state bonding by either one or a combination of the following processes: mechanical interlocking, interfacial melting, metal diffusion, and atomic forces across nascent contact points (Hunt 2014). Mechanical interlocking involves a liquid­like metal flow across the weld interfaces to form a non­uniform interfacial pattern that locks one

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side to the other. Mechanical interlocking has been frequently reported when ultrasonic consolidation is implemented on dissimilar foils that have significantly different hardness values (Hunt 2014). Melting occurs at the interfaces as a result of high friction rates at the interface during melting. The degree of melting can be aided by processes like preheating and varying welding parameters to increase relative interfacial displacements (Hunt 2014). Diffusion is both temperature and time dependent and it occurs as a result of formation of vacancies above or below the interface due to high strain rates in welding. Although the previous three bonding processes have been reported to occur in ultrasonic consolidation, atomic forces across nascent contact points has been the most dominant of the four (Hunt 2014). The main requirements are clean surfaces and intimate contact between the surfaces. High friction effects due to the vibration of the sonotrode help break oxide layers at the surfaces and the normal forces produced help keep the surfaces intimate. The fully bonded regions in ultrasonic welding have been reported to be oxide­free (Hunt 2014). Ultrasonic consolidation can create geometries more accurately with tighter tolerances due to its hybrid CNC and 3D printing method. CNC machined parts can have tolerances of +/­0.125mm or 0.025mm/mm, whichever is greater (Stratasys 2015). Other advantages of UC include its ability to bond with both similar and dissimilar metal foils, its ability to embed fibers in manufacturing, and most importantly its low thermal energy requirements compared to powder­based processes. Maximum processing temperatures are about half of the metal melting points (Gibson 2015). The current challenge this technology is facing is how to obtain better bond qualities. Oxide layers, cracks, vacancies, and insufficient linear bond densities are still major problems that ultrasonic consolidation has to address (Gibson 2015).

G. Ranking Techniques In order to determine which method was the best option to pursue, another decision matrix was made for each of the additive manufacturing technologies. The methodology is explained in further details in the Methodology section. Each criterion was given a weight out of ten ­­ one being the least important and ten being the most important ­­ and then each manufacturing technique was ranked from 1 to 6 ­­ one being the worst and six being the best. The total score was the sum of the weight times the ranking. Therefore, the technique with the highest score is deemed the best option. Table 3 in the Methodology sections shows the weights given to each criterion and Table 8 shows the totals for each technique as well as the rankings without factoring in machine costs. Electron Beam Additive Manufacturing technique was excluded from the matrices due to the limited knowledge about the technology currently available.

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Technique Energy Input Time

Bond Quality / Density

Design Freedom Accuracy Maintenance Total

Laser Melting 2 2 6 6 4 3 159

Laser Sintering 3 1 4 6 5 3 148

EBM 1 3 6 6 4 1 147

BJAM 4 4 2 6 1 4 141

LOM 5 5 1 2 1 3 113

UC 6 6 3 2 6 6 185 Table 8: Ranking and totals for each manufacturing technique

Pinning down precise cost numbers for each manufacturing technique is difficult with the wide array of companies and machines currently in development. Therefore, costs will be addressed more generally with the existing research. Without considering costs, ultrasonic consolidation is the most promising technique followed by the laser and electron beam techniques. Ultrasonic consolidation is currently limited by the available working material. One company that has been contacted, Fabrisonic, currently works with aluminum, copper, and bilayers of titanium and aluminum. It will be another six months before Inconel and stainless steels can be considered. Laser melting, on the other hand, is one of the most popular powder­based techniques and new machines are constantly being developed. It is already compatible with stainless steels and Inconels and progress is being made to expand to new alloys. Furthermore, discussion with Aurora Labs 3D revealed that laser melting will also be the cheapest option amongst all six by at least tenfold. Another look at Table 8 also reveals that the two lowest ranking techniques, binder jetting additive manufacturing and laminated object manufacturing, are also the two techniques compatible with ceramics. Due to poor density and low accuracy, it can be concluded that additive manufacturing technology is not quite ready for ceramic heat exchangers.

H. Companies / Researchers There are hundreds of companies working with various types of AM technologies in order to cater to a wide array of customers. Listed below are a few companies that work with each of the

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manufacturing techniques described earlier. Furthermore, most of the companies listed are either interested in a potential future partnership or were able to be connected via email.

Arcam AB Arcam AB developed and patented the electron beam melting machines first in 2001 (Arcam 2009). Arcam’s A2X machines can implement a variable beam powers and has high scan speeds. They, however, require vacuum conditions (extremely low pressures) and only achieve accuracies as high as ±0.13mm (Arcam 2015). So far, they have worked with titanium and cobalt­chrome alloys.

Aurora Labs 3D Located in Western Australia, Aurora Labs 3D makes affordable 3D printers for roughly 33000­43000 AUD, according to Aurora Labs 3D CEO David Budge. The mission of the company is to make 3D printers broadly available to the masses by bringing prices down and partnering with universities to enhance innovation and progress. The printing technique involves sophisticated robotic automation with three modes of printing: selective laser sintering, selective laser melting, and directed energy deposition. The acute angle technique is still patent pending, however, large format printers are expected to enter the marketplace by the end of 2016. The printers are compatible with steels and nickel alloys and have currently been tested with Stainless Steel 316 and Inconel 718. Currently, these printers do not seem to be compatible with silicon carbide due to the sintering requirements for ceramics. Sintered metal powder must be less than 10 microns, melting metal powder must be between 10­60 micron, and ceramic powder must be less than 1 micron. According to David Budge, current testing show that printed parts can reach up to 99.5% density with surface roughness comparable to parts sand casted with tolerances about 100 microns. In order to achieve the desired surface roughness for the heat exchanger, post­processing and polishing will be necessary with this technique and should be taken into consideration in terms of design and cost. If the printers are in constant use, they will also need to be serviced and lubricated every one to two weeks. David Budge, CEO and founder of Aurora Labs, predicts to manufacture 1000­5000 units per year by the end of 2016. Within that time, Aurora Labs hopes to obtain a service bureau as well in order to manufacture parts alongside selling the printers. Contact: David Budge | david@auroralabs3d.com

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Ceralink Ceralink is an independent, woman­owned small business founded in 2000 by Dr. Holly S. Shulman that focuses on expanding the capabilities of 3D printers with silicon carbide. The project objective is to design and build a prototype compact high­temperature heat exchanger by leveraging materials, modeling, and additive manufacturing (Ross 2015). The targets for this project are operation above 816°C, 25% turbine thermal cycle efficiency improvement, and 60% weight to volume reduction compared to metal heat exchangers. The additive manufacturing technique being used to create the prototypes is laminated object manufacturing (Ross 2015).

EOS EOS has implemented a lot of tuning and scanning strategies to get the best sintering results for different metal alloys (Gibson 2015). EOS supplies a large variety of powders including cobalt, nickel, aluminum alloys, titanium alloys and materials of interest such as SS316L, Inconel 625, and Inconel 718 (EOS 2015). Sintered samples with EOS machines, for instance Inconel 718, have achieved very close physical properties to the traditionally manufactured ones.

ExOne Starting as a spin­off of Extrude Hone Corporation, ExOne is a global leader in sand printing systems. Using binder jetting technology, ExOne is in various stages of production using silica sand, ceramics, stainless steel, bronze, and glass. So far, ExOne has printed parts with Inconel 625 and SS316 (ExOne 2015). Infiltration of SS316 with bronze has been implemented at furnace temperatures around 1100°C to improve the part’s density, which is usually around 60% before post print (ExOne 2015). The company has also claimed to print parts with Inconel 625 with 99% density without infiltration in 2014 (Gibson 2015). If this is replicated with a wider variety of high­temperature alloys, ExOne’s metal binder jetting technology could significantly change the economics of metal additive manufacturing. One of the limitations of the machine is that holes cannot be smaller than 1 ­ 1,4mm and sharp edges are advised to be rounded if possible. Current machine costs for the S­Max system start around 800k€ although some refurbished systems called the S­15 are around 350­400k€. Direct metal printers start around 150k€ for an Innovent systems or 360k€ for the M­Flex system. The representative from ExOne is very interested in a potential partnership with KTH and is willing to work with budget planning. ExOne will be opening a Production Service Center in cooperation with Swerea Swecast in Jönköping in August 2015 for their 3D sand printing technology. Contact: David Stevenson | david.stevenson@exone.com

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Fabrisonic Fabrisonic is one of the leading researchers in ultrasonic consolidation. Based in Ohio in the United States, Fabrisonic controls nine patents covering all aspects of ultrasonic additive manufacturing (Fabrisonic 2015). Currently, Fabrisonic works mainly with aluminum, copper, and bilayers of titanium and aluminum. The current rotating head used for the process is made of steel and therefore the harder materials tend to bond with the head. According to Hilary John, by the end of 2015, Fabrisonic is hoping to design a ceramic rotating head in order to use the same ultrasonic consolidation procedure with Inconels and stainless steels. This manufacturing process is very promising due to its high strength and high speed of production. Since the process requires strips of metal and pressing, there needs to be more development in support material. Using this process would limit the design flexibility as the process needs to adapt to produce better cavities. Contact: Hilary Johnson | hjohnson@fabrisonic.com

Renishaw A UK­based business, Renishaw dabbles in a variety of industries from jet engines and wind turbines to dental and brain surgery equipment. Their current manufacturing system, AM250, costs 360­400k€ depending on the material used. Renishaw machines work mostly with SS316L, titanium, and nickel alloys and have safety features to help minimize the risk of powder fires (Renishaw 2015). Contact: Bo Eneholm | bo.eneholm@renishaw.com

Sciaky Sciaky began its research on Electron Beam Additive Manufacturing back in 1996 and is one of the only companies working with this technology. Sciaky’s system can make parts as long as 19 feet in length and has a documented deposition rate ranging from 7 to 20 pounds of metal per hour. Sciaky’s machines are very large and range from 400k­1M USD .

Other Companies Other companies of interest include: Additive Industries (Netherlands), MatterFab (USA), Selective Laser Melting Solutions (Germany), 3D Micromac (Germany), and 3D Systems (USA). Table 9 shows a few machine models from each of the companies discussed previously as well as cost, build volume, build speed, and layer thickness abilities. The cost of the machines were converted from AUD or euros and rounded for comparison and the exchange rate used was the

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average for the month of July in 2015. Each machine could either build one heat exchanger or multiple depending on the build volume.

Company Machine Type Cost (SEK) Materials Build volume Build Speed Layer

Thickness

3D Systems SLS Series Sintering ­ 380 x 330 x 435 0.9 ­ 2 L/hr 80­150 µm

3D Systems SLS Series Big Sintering ­ 550 x 550 x 460 3 ­ 5 L/hr 100­150 µm

3D Systems ProX 300 Melting ­ Stainless and

superalloys 250 x 250 x 300

5 ­ 30 µm

3D Systems ProX 400 Melting ­ Stainless and

superalloys 500 x 500 x 500

5 ­ 30 µm

Arcam AB Arcam Q10 EBM ­ 200 x 200 x 180 8000 m/s transition

Arcam AB Arcam Q20 EBM ­ 350D x 380 variable transition

Arcam AB Arcam A2X EBM ­ 55/80 cm3/hr

Aurora Labs 3D S­Titanium

Melting, Sintering 210,000 Inc. 718, SS316 180 x 180 x 500 < 20 for small 50 ­ 200 µm

Aurora Labs 3D S­Titanium Pro

Melting, Sintering 275,000 Inc. 718, SS316 200 x 200 x 500 < 20 for small 50 ­ 200 µm

Ceralink LOM ­ Silicon Carbide

EOS EOSINT µ280 Sintering ­ Inc. 718, SS316 250 x 250 x 325

EOS EOSINT µ290 Sintering ­ Inc. 718, SS316 250 x 250 x 325

ExOne S­Max BJT 11,000,000 Ceramic 1800 x 1000 x 700 65,000 ­ 80,000 cm3/hr 280 ­ 500 µm

ExOne S­Print BJT 7,500,000 Ceramic 800 x 500 x 400 20,000­36,000 cm3/hr 280 ­ 500 µm

ExOne Exerial BJT 1,500,000 Inc. 718, SS316 2200 x 1200 x 700 300 ­ 400 L / hr 280 ­ 500 µm

ExOne M­Print BJT 3,500,000 Inc. 718, SS316 800 x 500 x 400 30 ­ 60 s / layer min 150 µm

ExOne M­Flex BJT 3,500,000 Inc. 718, SS316 400 x 250 x 250 30 ­ 60 s / layer min 150 µm

Fabrisonic UC ­ Work in progress

Renishaw AM250 Melting 3,500,000 Inc. 718, SS316 250 x 250 x 365

Sciaky EBW 50 EBAM 3,500,000 Inc. 718, SS300s 1370 x 1270 x 1370

Sciaky EBW 68 EBAM 7,400,000 Inc. 718, SS300s 1730 x 1730 x 2130

SLM Solutions SLM280 Melting

­ 280 x 280 x 350 20­35 cm3 / hr 20 ­ 75 µm

SLM Solutions SLM500 Melting

­ 500 x 280 x 350 70 cm3 / hr 20 ­ 200 µm

Table 9: Machine data from various companies

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I. Growth Additive manufacturing is a revolutionary and disruptive technology that will change the future of manufacturing. In 2014, metal additive manufacturing grew at an explosive pace, surpassing even the most generous market estimates. Machines sales grew an estimated 89 percent according to SmarTech’s ongoing market assessment in metal 3D printing (Dunham 2015). That number translated to about 790 units sold and about 80 to 90 percent was estimated to have been delivered to the clients and is capable of operation today. Wohlers Associates, Inc. announced the publication of its Wohlers Report 2015, which provides an in­depth review and analysis of the global additive manufacturing industry. In 1995, the AM industry represented a mere 295M USD. In twenty years, the market for AM, including all AM products and services worldwide, grew at a compound annual growth rate (CAGR) of 35.2% to $4.1 billion (Metal 2015). The major advantage of AM is the lead­time savings compared to conventional manufacturing processes. Pratt & Whitney testified up to 15 months of saving and up to 50 percent weight reduction in a single part using AM (Metal 2015). Furthermore, the complexity of the design does not affect production time as dramatically as it would with conventional methods. The streamline process that AM creates is its most valuable asset. As AM continues to save designers and engineers time, the market will continue to grow giving research more time. Comparing each of the manufacturing techniques, it can be seen that powder­based fusion processes are the furthest along in terms of development. PBF research is focused on material database expansion, energy savings, and precision. Binder jetting technology would be next in terms of growth and research is focused on improving density and quality of the product. Lastly, laminated object manufacturing and ultrasonic consolidation technology still need a lot of development in terms of machine accuracy, material compatibility, and cost before being capable of manufacturing the desired heat exchanger. The AM disruption can already be seen in other industries such as aerospace, medicine, and construction. A group of Dutch architects have 3D printed a canal house in the hopes of solving housing problems in third world countries. The next project they are working on is using robotic 3D printers combined with MX3D­Resin to build a metal bridge (Starr 2015). The disruptive nature of AM opens doors for designers to build creatively. However, it is the same disruption that may cause the suspension of growth as it poses a threat to its competitors.

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X. Heat Exchanger Design One of the first considerations in designing a heat exchanger is understanding its purpose. An ideal recuperator will mimic an ideal isobaric combustion process, one that transfers the total useful thermal energy from the exhaust gases into the turbine’s cycle without any losses or pressure drops. The goal of the heat exchanger design is to optimize the shape for maximum heat transfer while minimizing pressure drops on either side. A compact device using the least amount of material would be ideal in reducing space, weight, and cost.

A. Classification of Heat Exchangers Heat exchangers are usually formed by a core that contains heat transfer surface(s) and the fluid distribution elements: headers, nozzles, tanks, and pipes. Both parts are usually stationary except for some regenerators that have a rotary core matrix. The classification of heat exchangers can be based on several things, from geometry (construction) to phases of the working fluids in heat exchangers. The following classification is obtained from Heat exchanger design handbook, 2nd edition (Heat 2015) :

i. From Construction 1. Tubular heat exchangers (double pipe, shell and tube, coiled tube) 2. Plate heat exchangers (gasketed, brazed, welded, panel coil, lamella) 3. Extended surface heat exchangers (tube­fin, plate­fin) 4. Regenerators (fixed matrix, rotary matrix)

Tubular heat exchangers are the most widely used heat exchangers due to their relatively less complex and cheaper manufacturing processes. Figure 13 shows the three main types of tubular heat exchangers. They are the most robust heat exchangers that can be used with any working fluids, from semi­solid to gases. They can also withstand high pressures. Their main disadvantages are the significant pressure losses that are associated with tube bends and pipes and they also occupy large volumes with insufficient heat transfer surfaces.

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Double Pipe (Reynolds 2015) Shell and Tube (Techform 2015)

Coiled Tube (Allbiz 2015)

Figure 13: Different types of tubular heat exchangers Most plate heat exchanger cores consist of a stack of corrugated metal plates in mutual contact and are held together by a common strong support. Figures 14a and 14b show five various types of plate heat exchangers. Their manufacturing has grown with the rise and development of brazing, welding, and stamping technologies. They have been replacing tubular heat exchangers in many applications that are more sensitive to pressure drops and those that require less volume. They also save cost since they offer a large heat transfer surface with smaller volume and less mass of material used. Plate heat exchanger inlet temperatures are limited by the gasket or weld material and many cannot withstand large pressures.

Gasketed (Delta 2015) Brazed (West 2015) Welded (WeldPack 2015)

Figure 14a: Different types of plate heat exchangers

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Panel Coil (Tranter 2015) Lamella (Anthermo 2015)

Figure 14b: Different types of plate heat exchangers

ii. Based on the Heat Transfer Process This classification is based on whether there is mixing of the hot and cold fluid streams in the heat exchangers. Indirect contact heat exchangers keep the fluid streams separated and the heat transfer takes place through a dividing impervious wall. Direct contact heat exchangers allow mixing of the fluid streams to a common temperature.

iii. Based on Surface Compactness Compactness is important when considering restrictions in weight and size. The quickest measure of compactness is the ratio of total heat transfer area to the total volume of the heat exchanger. This ratio is called area density (β). Compact heat exchangers have β >700m2/m3.

iv. Based on Flow Arrangements Flow arrangements can be parallel, counter, or cross (Figure 15). Parallel flow heat exchangers have the fluids in both streams moving parallel to each other in the same direction. These have large thermal stresses in one end of the heat exchanger and this effect tends to limit their performances. Counterflow exchangers have fluids flowing parallel to each other but in opposite directions. Counterflow arrangements have minimum thermal stresses and have the best performance of the three. Crossflow heat exchangers have the two types of fluid streams flowing normal to one another.

Figure 15: Parallel, counter and cross flow configurations

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Currently Compower uses a plate, cross head­counterflow, single pass heat exchanger (Palalayangoda 2010). The flow is largely counterflow except at the inlets and outlets.

B. Thermal Hydraulic Performance of Heat Exchangers

i. Heat Transfer Mechanisms in Heat Exchangers Heat transfer in an indirect contact heat exchanger takes place mainly through two heat transfer mechanisms: conduction at the solid surface and convection in the air passages. There also exists some small degree of solid radiation, but the effects are neglected in analyses (Theodore 2011).

ii. Conduction Conduction happens in solids as a result of transferring vibrational kinetic energy in their constrained molecules. It is well described by Fourier’s law that the rate of heat flow by conduction is . The negative shows that heat flows against the thermal gradient− A(dT /dx) Q′ = k dT/dx, where A is the area and k is the material’s thermal conductivity constant. The differential equation above shows that an increase in conductive heat transfer rate could be a result of increases in thermal conductivity, surface area of the solid, or thermal gradient along the conducting surface. Metal alloys generally have high thermal conductivities that increase linearly with operating temperatures and thus they are the most suitable for high temperature applications. Thermal conductivity of gases, although relatively low, generally increases with temperature and are also sensitive to pressures. Extended surfaces can be used to enhance conductive heat transfer based on Fourier’s equation (Hotz 2015).

iii. Convection Convective heat transfer results from both motion and mixing of macroscopic elements of the material (Theodore 2011). It happens mainly in fluids since solids do not have bulk internal motions. A natural convection happens as a result of temperature difference. That temperature difference causes a density difference. A forced convection happens from an influence of an external force such as a pump or a compressor. Convective transfer between a surface and a fluid in contact is summarized by Newton’s law of cooling.

A(T ) Q′ = hc s − Tm Ts and Tm are the temperatures of the solid and the fluid medium, respectively. is a quantity hc called the convective heat transfer coefficient (the magnitude of which depends on the flow characteristics, geometry, conductivities of materials, surface roughness, and the type of convection process that is happening). hc can be evaluated from analytical methods, integration, or dimension analysis depending on the complexity of the flow and geometry. The values of hc are generally higher for turbulent flows than for laminar cases. Surface roughness effects are

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negligible in laminar flows. Dimensional analysis of a problem where (D(D, k, , , , ) hc = ψ V ρ μ C is the hydraulic diameter, k is the thermal conductivity, V is the flow velocity, is the density, ρ μ is the viscosity, and is the heat capacity) shows that the following dimensionless classes areC more important in evaluating hc:

­ Nusselt number: Nu=(hcD)/k ­ Prandtl number: Pr= (Cµ)/k ­ Stanton number: St= hc/ (CVρ) ­ Reynolds number: Re= (DVρ)/µ.

It should also be noted that the ratio between convective heat transfer and conductive heat transfer at the surface is Nu= (St)(Re)(Pr).

iv. Heat Transfer and Temperature Difference The rate of heat transfer between the two streams of a heat exchanger (hot and cold) can be evaluated from energy relations. Assuming no energy losses.

Q’= m’h( hh1­hh2) = m’c( hc1­hc2) h represent enthalpy values obtained from inlet and outlet temperatures. In case of constant heat capacity applications, heat transfer in the exchanger can be obtained from

Q’= m’cp[(Ts­t1)­ (Ts­T2)] = m’cp(ΔT1­ΔT2) where Ts, T1, and T2 are the temperatures of the separating surface and the two fluid streams, respectively (Theodore 2011). The driving force of the heat transfer is seen to be the temperature difference across the streams. The logarithmic average of this difference is called the Log Mean Temperature Difference (LMTD) and is defined as

ΔTlm= (ΔT1­ΔT2)/ln(ΔT1/ΔT2) LMTD is useful for understanding the weighted average of the temperature differences across the two stream ends. The discussion about average temperature differences as the driving forces for heat transfers in exchangers verifies the advantages of counterflow heat exchangers over parallel flow heat exchangers. Consider the following temperature profiles in Figure 16 for the two flow types.

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Parallel flow temperature profile Counterflow temperature profile

Figure 16: LMTD Method for parallel and counterflow heat exchangers (Theodore 2011) It can be seen that there is more driving force at the inlet of a parallel flow than a counterflow, but the force declines along the surface. Counterflow offers an almost constant driving force and it can be seen that a good heat transfer takes place all along the surface. The temperature profile along a cross flow heat exchanger (Figure 17) resembles that of a counterflow heat exchanger however the latter has the best thermal performance of the three types.

Figure 17: Temperature profile along a cross­flow heat exchanger (Hotz 2015)

An overall heat transfer coefficient for a heat exchanger could also be introduced to combine the effects of both conduction and convection. In this case, the total heat transfer rate could be given by Q’=UAΔT. The quantity U could be used to compare effects of factors like fouling on the overall performance of the heat exchanger.

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C. Pressure Drops Loss of pressure is an important aspect to consider in any heat exchanger design. The efficiency of the Brayton cycle depends on both temperature and pressure ratios. It can be seen from Figure 18 that the net work done (thus efficiency) is proportional to how high the temperature and pressure ratios (between processes 4­1 and 2­3) should be maintained.

Figure 18: Brayton cycle (UWaterloo 2015)

Efficiency = η= 1­ [(rp)(1­k)/k]

The P­v plot show that processes 1­2 and 3­4 are isentropic (constant entropy) and the T­s plot show that processes 2­3 and 4­1 are isobaric (constant pressure). These assumptions are for ideal cycles. Real cycles do have pressure losses and entropy increases. rp = p2/p1 = p3/p4 for isobaric processes (UWaterloo 2015). High pressure ratios (typically above sixteen) have efficiencies that are less sensitive to pressure drops, however, pressure drops could have significant effects on efficiencies in low­pressure applications. Apart from seeking to preserve the pressure of the compressed air in the cold side of the exchanger (for good cycle efficiency values), pressure losses should also be avoided in the hot side of the heat exchanger. This is because the current ET10 model configuration has turbine exhausts being fed into the heat exchanger, in combination with hot gases from the burner. This configuration should maintain the operational pressures until these gases are finally released into the atmosphere after the boiler. Heat exchangers are likely to have pressure drops from the inlet and outlet headers and also from the exchanger core. A majority of pressure losses, or pressure head losses, in fluid flows occur through viscous dissipations along the course of flow. The major head loss is the pressure loss incurred in a steady, fully developed flow through a straight and a constant segment area of a pipe or a duct (Shaughnessy 2005). Minor losses occur when there is a change in the flow direction or geometry along the flow path (Shaughnessy 2005). Sudden expansions, contractions, bends, and

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leaks cause disturbances in the form of turbulence, vortices, and flow separation as a result eddies that increase viscous dissipation.

ΔP = f(L/D)(V2/2). ΔP is the pressure drop along a duct, f is the frictional factor, L is the length of the flow section, D is the hydraulic diameter, and V is the flow velocity. The friction factor f is dependent on the roughness of the surface and the Reynold’s number.

(e/D, Re) f = ψ e is the roughness coefficient of a given surface, D is the hydraulic diameter and Re is the flow’s Reynolds number. Rougher surfaces tend to have high frictional factors (Shaughnessy 2005). It can also be noted that high Reynolds flows (turbulent) tend to have higher pressure drops and lower hydraulic diameters, which results in higher pressure drops. Pressure losses are difficult to model in complex flow systems, but a good design should consider minimizing minor head losses.

D. Effectiveness and Rating The performance of heat exchangers is usually rated by how well it performs the main purpose of heat transfer from its hot side to its cold side. Effectiveness is given by the ratio between the achieved temperature transfer to the maximum possible temperature transfer.

ffectiveness (%) e = Th−Tin Thb−Tin

Th is the temperature of the hot gas to the turbine, Tin is the inlet temperature from the compressor, and Thb is the temperature of the flue gas from the burner (Lagerström, Xie 2002). It is also commonly given through a dimensionless number of heat transfer units (NTU) obtained from the ratio between total heat transfer to the maximum heat transfer having found the overall heat transfer coefficient U (Hotz 2015). The main method that has been used to rate heat transfer performance while also taking into account the pressure drops has been the goodness factor. The goodness factor is the ratio of Colborn factor, j=StPr2/3 to the friction factor f.

Goodness = St*Pr2/3/ f = Nu*Pr­1/3/(f*Re)

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The method is useful when comparing different surface geometries for a fixed pressure drops, but it is less reliable for other comparisons (Stone 1996). Bejan’s novel approach of evaluating the performance of heat exchangers is based on using their rate of irreversibility or entropy production rate (Bejan 1978). Irreversibilities arise from both frictional fluid flows in the heat exchanger and in the heat transfer process as a result of limited transfer area. The entropy production rate corresponds to the total useful power lost as a result of heat exchanger idealities. Bejan came up with a dimensionless number, Ns ­­ number of entropy production units. Larger Ns numbers represented a worse performance. Bejan’s entropy method is more suitable to evaluate the performance of the heat exchangers, especially when considering sustainability. A very useful method of comparing the performance of different designs was put forth by Giulio Lorenzini, Simone Moretti, and Alessandra Conti (Lorenzini 2011). An overall performance coefficient P can be calculated from a modified equation that included design parameters.

Pi,i,k…= αqi,j,k… + (1­α)/Δpi,j,k…, ,

i, j, k… are design parameters, Δp is a dimensionless pressure drop, q is a dimensionless heat transfer with each design, and Pi,i,k… is the performance coefficient for each design. α is a relevant number that varies from 0­1, presenting the weight of heat removal maximization with respect to minimizing pressure drops. Plots of P vs. α could be made for each design and this would simplify the process of finding the optimum design. Figure 19 is an example of such plots. For the designs in the plots, shape D would be superior if more heat transfer is desired and pressure drops were less of a concern. Shape C would be the best design if pressure drops were more of a priority than heat transfer.

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Figure 19: Plot of overall performance coefficient P vs. relevance factor α

E. Design Recommendations 1. Both conduction and convective heat transfers depend on the temperature gradient between

the two streams. A counterflow configuration not only maintains a constant gradient throughout the length of the flow, but also maximizes the average value of temperature difference (LMTD). It is thus the most suitable for an enhanced heat transfer.

2. Transfer surfaces with high specific heat capacities are more suitable for heat transfers on the

hot side as they don’t get heated up quickly. This will have an effect of increasing thermal gradients on the hot side and as a result, a better heat transfer. Those of low heat capacities are most suitable for heat transfers on the cold side as they easily maintain the elevated temperatures and thus maintain a higher thermal gradient with the incoming ambient air. While keeping the surfaces thin is also a requirement, an ideal transfer surface would be composed of very thin fused foils of the two kinds of surfaces, each facing their suitable sides. Multiple­material additive manufacturing technologies could offer a plausible solution to the design of such surfaces.

3. Surfaces with high specific heat capacities and lower thermal conductivities are suitable for

forming the outermost regions of heat exchangers. This is to reduce heat transfers outside the

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exchanger’s core. Some ceramics, for example mullite, forsterite, and zirconia are suitable for these applications (Kyocera 2015).

4. Although the concept of extended surfaces seems to be of less importance in gas­to­gas

compact heat exchangers, the concept could be worth revisiting with new kinds of designs. This could utilize the small difference in average conductivities of gases in the hot and cold streams.

5. Surface roughness is of less importance in convective heat enhancement than other factors.

Smoother surfaces, as opposed to rougher surfaces, are the better choices because the benefit of having less friction outweighs the penalty of having less heat transfer.

6. For open channels, semicircular cross sections offer the minimum flow resistance of all

geometries. Circular channels offer the least flow resistance for closed channels. These are obtained from optimizing minimum perimeter to area ratios (pmin/A) for different geometries (Lorenzini 2011). The two geometries are thus recommended for any channel design to minimize pressure losses. Parabolic and elliptical channels of different aspect ratios have been used in different heat transfer applications to optimize the overall performance of heat exchangers (Khan 2015).

7. Narrow and deeper channels are better than shallow and wider channels for both heat transfer

and minimization of pressure drops (Upadhye 2004). Very small hydraulic diameters could, however, result in high pressure losses as seen in the equations for both minor and major losses. Optimization of channel diameter through simulations is thus necessary in any design.

8. Heat transfer enhancement through induced turbulence and boundary layer destruction, (both

caused by change in flow directions) produce better results than from those having high flow rates in the channels. While the former can remain largely laminar, the latter approach results in high turbulence that is coupled with high pressure losses.

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XI. Economics One of the biggest impediments to scientific revolutions is cost. In order to make reasonable predictions for the future cost of AM, existing data on long­term and short­term trends for personal computers are used to infer potential trends with 3D printers. The personal computer is an example of an innovative technology success. Looking at the trends for a successful model could shed insight on a potential timeline for AM. Furthermore, personal computers and AM share many similar qualities of being disruptive. At one point, the computer was just a glorified calculator, but it is now capable of computing an immense amount of data in a short period of time. It was seen as a threat to the print industry ­­ potentially rendering print obsolete. Likewise, AM is a threat to traditional manufacturing companies and has the possibility to span many different industries. If AM is to be successful, then the personal computer trends can foreshadow a potential timeline. Hewlett­Packard was founded in 1939 in a Palo Alto garage and two years later Konrad Zuse, a German engineer, finished the Z3 computer. After years of computer advancement during World War II, the first commercial computer debuted in 1951 for 1 000 000 USD each (unadjusted for inflation) (Computer 2015). The cost of commercial computers from 1950 to 1985 was extracted from the Computer History Museum and is plotted in Figure 20.

Figure 20: Cost in US dollars of commercial computers from 1950 to 1985 (Computer 2015)

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Figure 20 shows that there was a drastic exponential drop in prices within the first two decades of the “personal computers”. It was not until 1977 when the Commodore PET (Personal Electronic Transactor) began to resemble the personal computer known today. By the end of 1985, computer prices still ranged from Apple’s 2500 USD to NeXT’s 6500 USD (Computer 2015). Nevertheless, computer prices continued to drop between 1985 and 2010 as cost per CPU and cost per RAM decreased significantly. Adjusted for inflation, the Apple iMac in 2009 was 26 percent cheaper (3849 USD) than the 1984 Macintosh (5186 USD). The real dollar cost per CPU (MHz) of the 1984 Apple Macintosh was 662,35 USD compared to that of 0,34 USD for the 2009 Apple iMac. The real dollar cost per RAM (KB) of the 1984 Apple Macintosh was 40,52 USD compared to that of 0,00025 USD for the 2009 iMac (Perry 2010). Figure 21 shows the trend in price index of personal computer and peripheral equipment between 1999 and 2009.

Figure 21: Price index of personal computer and peripheral equipment (Freeby50 2009)

While Figure 20 showed a long­term trend, Figure 21 provides the short­term trend for personal computers. The price of computers dropped significantly within ten years decreasing at a rate of 11­12 percent annually between 2006­2009 (Freeby50 2009). Since it is an index of prices, the figure includes very expensive computers, cheap computers, monitors, printers, etc. and should not be taken to mean that any individual computer part in 2009 was 1/7th the cost of any individual computer in 1999. The first patent application for rapid prototyping technology was filed by Dr. Kodama in Japan in 1980, but it wasn’t until 1986 that the origins of 3D printing can be traced back to Charles Hull, who invented the first SLA apparatus. Several companies were formed in the late 1980 and early 1990s that focused on different processes such as sintering, fused deposition modeling, etc. and new technologies continued to be introduced in the early 2000s (3D 2014). This time of development could be equated to the time of development for personal computers during the 1970s ­ 1980s.

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If 3D printing follows a similar trend to personal computers, then it will be another 15­30 years before the technology is marketable to the industry and the masses. The main driver in reducing that time is the incredible amount of existing technology and knowledge. Progress occurs a lot faster nowadays than it did fifty years ago. Open source models such as RepRap, a site that encourages group collaboration and innovation, may also accelerate the timeline. Looking back at Figures 20 and 21, it is up to the user to decide whether 3D printing is still in the 1980s phase or early 2000s. Those in favor of the technology and those who work in the industry predict only five years before AM makes a strong impact on the market. Competitors, who fear the disruption of the technology, may wishingly predict another 30­50 years. There are two factors determining whether or not 3D printing will be an economically viable option in the next decade: supply and demand, and profit. With any innovation, it is difficult to define demand. What 3D printing does is create a need for demand. As manufacturing techniques improve, the industry must also make consumers and engineers demand the possibilities that 3D printing brings. Currently, the cost is high and therefore not attractive enough to the consumer. According to the law of supply and demand, research and development need to lower the cost in order to increase demand. As seen with personal computers, two ways to reduce the cost are technological progress and reduction in material parts. For most machines, the most expensive parts of the 3D printers are the X,Y, and Z linear axis, the recoater, the print head, and the electronic control system. Any reduction in the cost of the parts will significantly reduce the cost of the overall machine. Another way to reduce cost in the future is to implement a micro­version of economies of scale. Table 9 shows a list of machines with current costs. The larger machines cost more, but also have larger build volumes. An investment in larger machines could be made now in order to produce multiple parts simultaneously. This may reduce time, space, and number of machines. Another path towards economic viability is through profit. Profit is the difference between revenue and expense. One way to increase profit would be through the reduction of expenses as previously discussed. The key to increasing profits with any technology firm is not by cutting costs, but by outpacing competitors in rapidly evolving high­growth areas such as cloud computing, everything­as­a­service (XaaS), and digital content. In order words, 3D printing’s growth will depend on continual innovation (Casey 2015). Over the years, the 3D printing sector started to show signs of distinct diversification into two specific areas that are more clearly defined today. First, there is the high­end 3D printing systems geared towards part production of high value, highly engineered, and complex parts. The other end of the spectrum is focused on improving concept development and functional prototyping for office­friendly, user­friendly, and cost­effective systems (3D 2014). The high­end product development increases profit through continual innovation and the user­friendly development can increase profit by becoming a XaaS

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system. 3D printing’s possibility for significant profit in the future would be strongest argument for potential economic viability. Currently, the ET10 heat exchanger cost 15 000 SEK or 1500€ to manufacture. Powder cost range from 50­650€ per kilogram, with high temperature material being on the upper end. Assuming a 50 kilogram unit, one Inconel 718 heat exchanger would cost 30 000€ ­­ twenty times the current cost in powder alone. Factoring in maintenance and operation costs as well as the machines themselves, 3D printing as an alternative method of manufacturing remains an infeasible option. Over the past few decades, additive manufacturing has become a billion­dollar industry and is expected to grow to a 10 billion USD market by 2020. Improvements in material purity, manufacturing techniques, and technology will need to occur before 3D printing makes its mark.

XII. Barriers Right now, 3D printing is growing really quickly in the hands of tech­savvy innovators, but as additive manufacturing continues to grow, there may be legal and political barriers that can block its progress. As mentioned previously, 3D printing has two distinct paths: highly specialized industrialized manufacturing and consumer friendly domestic use. In the industry, 3D printing progresses will render a lot of existing companies to become obsolete. For example, research on drill presses for heat exchangers. Without the need to assemble parts, 3D printing streamlines the manufacturing process and cuts many middleman companies out of the market. Competitors will try to lower their prices in order to prevent 3D printing from entering the market. In the domestic consumer market, some may appeal to ethics and rule 3D printing as a potential dangerous technology. The argument would center on people printing their own weapons. If these arguments succeed, then 3D printing machines would need to be highly regulated, which would cost additional time.

XIII. Recycling Another concern for the future is the handling of waste 3D printed products. There is currently progress underway to create a recycling coding system for polymers for distributed manufacturing with 3D Printers (Hunt 2014). The premise would be that all 3D printing software would come prepackaged with design labels to embed products in order to streamline the process after the life cycle of the product. A similar process should be created for 3D printed metal and ceramic parts now rather than later. The coding system would help identify the types of material used in each part to expedite the recovery process.

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XIV. Discussion and Recommendations Analysis of the top five materials showed the superiority of silicon carbide and Inconel 718 over other materials with silicon carbide being the best overall. Although the results were obtained having considered the material’s printability (with current machines and possibility with the discussed techniques), the results from ranking the printing techniques and current industrial growth show that printing with superalloys (Inconel 718 in this case) is a better practical choice than with SiC. Laser processes are far more advanced in terms of industrial growth and research and they have successfully worked with superalloys, especially Inconel 718. Industries like EOS have proven to produce sintered superalloy parts with close to similar tensile properties with the traditionally made samples. These achievements have not yet been realized in ceramic­based techniques or even in industries that have tried to work with ceramics. The lowest ranked techniques are also the ones that are working with ceramics and have been seen to be the least accurate with the lowest material qualities. Laser sintering and melting with ceramics, particularly SiC have not yet been pursued due to powder handling issues and more importantly due to its high energy requirements. CIS and LPS could be better options due to their lower energy requirements. There are, however, no companies that have pursued these techniques with ceramics. Ultrasonic consolidation is the most promising printing technology. The least energy demanding, most rapid, accurate, and expectedly low cost technology is currently being handled by only one industry that has yet caught up with other metal and ceramics 3D printing industries. The technique seems to have virtually no limitations other than those associated with design freedom. It can potentially incorporate both metal alloys and ceramics and produce multiple­material parts. Furthermore, it can also incorporate other technologies like fiber embedment to improve part’s strength and other material properties. Thus with UC, the potential future high­temperature applications could favor low­cost stainless steels and their composites instead of expensive superalloys. This dream could, however, be far away from coming true, especially after considering the UC’s industrial growth and current research. A common challenge that metallic and ceramic 3D printing industries have left unaddressed is the study on material properties after printing. As superalloys are known to be formed from additions of specific elements into base matrices in heating processes, techniques like sintering are expected to cause significant changes in the microstructures. Apart from a few studies that have shown changes post­print tensile properties, there are very few studies about other post­print material properties in many industries. This makes the choice to print a functional part with many printing techniques still a gamble.

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Despite the realized growth in 3D printing industries over the years, the industry has not yet set its mark in the current manufacturing markets as seen in the Economics section above. The growth in 3D companies is largely associated with polymer­based industries as they are the leading in research, variety of applications, and technologies. Metal­based printing industries are newer and their technology has been largely mimicking the already established polymer­printing technologies. The growth in ceramics­based industries is the slowest and the industries have not made a mark yet in the 3D printing world. Both metal and ceramic industries still need to invest more in improving their techniques to achieve better speed, accuracy, and surface finish. It is also a challenge to produce high­purity spherical powders at a reasonable cost. The solution to a better heat exchanger than what Compower currently uses could lay in two approaches: an exploration of different 3D designs for effective heat transfer with minimal pressure drop or eliminating the disadvantages of the flaws in traditional manufacturing technologies (welding, brazing, stamping) with more consolidated 3D printed parts. The former could be realized through following the design recommendations (in the above section) and an extensive study on fluid dynamics and heat and mass transfer. The latter is easier to achieve; Heatric’s printed circuit heat exchanger is a plate heat exchanger made of semi­circular channels and can achieve effectiveness of well above 90% with pressure drops of less than 2.6% (even with wavy channels) and can withstand temperatures as high as 900°C (Heatric 2015), (Khan 2015). The key to the excellent performance is the use of diffusion fusion between plates. Sintering and melting can achieve equally strong bonding and therefore be viable alternative manufacturing methods. To reiterate, the only concerns associated with printing the Heatric’s design would be surface roughness and product quality.

XV. Sensitivity Analysis Major conclusions were drawn from the ranking models used to determine the best materials and printing techniques. As seen in the Discussion section, the second best material and the second best printing technique were concluded to be the most practical combination at the moment. With time, these conclusions could change as printing techniques develop and more materials are applied. An interesting case is Haynes 214, which was penalized for printability. If more industries started to print with the superalloy, then it could bump Inconel 718 in the rankings. While the printability criterion is a good safety measure, it also blocked some materials with better properties because the materials are not currently as popular in the industry. The material selection process was done under the assumption that the heat exchanger core is made of just one material. This assumption rippled into the selection criteria, results, and

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conclusions. While the assumption is valid, since most of the current techniques build parts from one material, with time the reevaluation on materials will be worth revisiting. As seen in the Discussion section, with techniques that enable printing multi­material thin surfaces or strengthening through fiber embedment, a wider variety of materials and respective properties could be considered for optimizing the performance of a heat exchanger. Further into the material selection process, the model did not include all necessary criteria that could potentially matter for the construction and performance of the heat exchanger. Properties like thermal shock, fouling rates, powder properties, and specific corrosion rates (i.e. steam and nitrogen) were not included due to limited literature studies and the variation in the presented results. Nonetheless, the criteria used in material selection represent most important factors when considering high­temperature applications. This indicates that the models should only be reliable to present the ranks, not to inform further how well each material performs over others. Apart from SiC, properties of other materials were from standard manufactured bars of similar sizes. Powder­based printed parts are expected to have variations in properties although within some limits. For instance, the same sintering process would not be expected to produce samples of Inconel 601 with better strength values than those of Inconel 718. Thus, the same ranking results, not score values, would be expected to remain the same if properties from sintered samples were used. The results would, however, change if material cost changed as the market fluctuated with supply and demand. The same sensitivity can be made to printing techniques. The results are only informative in terms of rankings and not how well each technique performs over one another. While there might be many day­to­day variations in the criteria, the inherent characteristics of each technique will remain the same. For example, melting processes will always require more energy input than solid­state processes. The models are flexible to preferences; therefore, the results will change when weights change. The Economic discussion focuses more on how long it would take AM to be successful and ways to increase its success. This study does not present an argument for whether or not it will succeed. Economic success depends on market status, advertising, and stickiness of the technology. Consumers and manufacturers must know about the product and the potential opportunities AM provides in order to consider it as a manufacturing technique. How well­received the technology is depends on its stickiness. If AM is to be as successful as the personal computer, the long­term and short­term trends of personal computers suggest that we are anywhere from 15­30 years away from affordable technology. This prediction is adjusted for the beholder and how rapidly research and development occurs.

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Lastly, affordability of the machines may also vary over time as international markets change. Machines are made all over the world (Australia, Germany, USA, etc.) and the prices of these machines can be made more affordable depending on conversion rates. All estimates of comparison have been made with average conversion rates for July 2015.

XVI. Conclusion The best material and printing technique for current additive manufacturing are Inconel 718 and laser melting process, respectively. It is possible to print heat exchangers with the best current material and technique, but the decision would not be economical. It is possible to print a heat exchanger with traditional design by using current 3D printing techniques, but it is not feasible. Apart from improving the current printing techniques, an extensive study in post­print material properties is needed to make the methods more reliable.

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XVIII. Appendix Matlab code for Figures 4­8 clear;clc %%Inco601 Inco601T=[ 20, 100, 200, 300, 400, 500, 600, 700, 800, 900 1000]; Inco601Cond=[ 11.2, 12.7, 14.3, 16.0,17.7, 19.5, 21.0, 22.8, 24.4, 26.1, 27.8]; Inco601SpHeat=[448,469,498,523,548,578,603,632,657,686,712]; Inco601Exp=[6,13.75,14.36,14.58,14.83,15.19,15.62,16.11,16.67,17.24,17.82]; Inco601Modelast=[ 206.5, 202.4, 196.8, 191.2,184.8,178.2,170.8, 161.3,150.2,137.9,124.7]; %%Inco718 T=[70, 100, 200,300,400,500,600,700,800,900,1000,1100,1200,1300,1400,1500,1600,1700,1800,1900,2000]; Inco718T1=(T­32).*(5/9); Inco718Modelast=6.895*[29,28.8,28.4,28,27.6,27.1,26.7,26.2,25.8,25.3,24.8,24.2,23.7,23,22.3,21.3,20.2,18.8,17.4,15.9,14.3]; T2=[70,200,400,600,800,1000,1200,1400,1600,1800,2000]; Inco718T=(T2­32).*(5/9); Inco718ThermCond=0.144227889*[79,87,100,112,124,136,148,161,173,186,199]; Inco718Exp=1.8*[ 5.9,6,7.31,7.53,7.74,7.97,8.09,8.39,8.91, 9,9] ; Inco718Tf=linspace(400,1350,20); Inco718Tff=Inco718Tf­273.15; Inco718HeatC=286+ 2*0.1685.*(Inco718Tf); %%Haynes 214 HaynesT=[20,100,200,300,400,500,600,700,800,900,1000,1100,1200]; HaynesThermCond=[12,12.8,14.2,15.9,18.4,21.1,23.9,26.9,29.7,31.4,32.7,34.0,36.7]; HaynesSpHe=[452,470,493,515,538,561,611,668,705,728,742,749,753]; HaynesElMod=[218,210,204,199,190,184,177,170,162,151,137,120,110]; HaynesExp=[11,12,13.3,13.6,14.1,14.6,15.2,15.8,16.6,17.6,18.6,20.2,22]; %% SS304 SS304T=[300,400,500,600,700,800,900,1000,1100,1200,1300,1400,1500,1600,1672,1727,1800]­273.15;

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SS304Cond=[14.89,16.61,18.28,19.77,21.21,22.59,23.99,25.33,26.58,27.81,29.18,30.34,31.55,32.70,33.53,28.15,28.99]; SS304T2=[ 100, 200,300, 400, 500, 600, 700, 800]; SS304Exp=[ 16.3,16.7,17.1,17.6,18.0,18.3,19.0,20.0]; SS304T3=[20,90,200,320,430,540,650,760,870]; SS304SPHeat=[456,490,532,557,574,586,599,620,645]; SS304T4=[24,90,150,200,260,320,370,430,480,540,590,650,700,760,820,870] SSModel=9.807*[19.9,19.6,19.1,18.7,18.3,18.0,17.4,16.9,16.3,15.8,15.3,14.8,14.3,13.6,12.7,12]; %% SiC SiCT=[280,320,400,460,570,650,750,880,990,1100,1210,1320,1420,1540]­273.15; SiCond=[ 170,142,118,100,84,75,62,55,50,45,40,36,33,30]; SICT3=linspace(0,1150,50); SiCExp=(­5.926*10^­12).*(SICT3.^4) + (1.701*10^­8).*(SICT3.^3) ­(1.824*10^­5).*(SICT3.^2)... +(9.791*10^­3).*(SICT3) + 1.735; SiCT2=[400,500,600,700,800,900,1000,1100,1200,1300,1400]; SicHeatC=[810,950,1057,1130,1180,1220,1252,1257,1300,1300,1300]; SICT5=[20,500,1000,1200,1400,1500]; SICELMod=[415,404,392,387,383,380]; %% figures figure(1); clf plot(Inco601T,Inco601Cond,'­o',Inco718T,Inco718ThermCond,'­.',HaynesT,HaynesThermCond,'­+'... ,SS304T,SS304Cond,':',SiCT,SiCond,'­') ylabel('Thermal Conductivity (W/m\circ))'); xlabel('Temperature (\circ C)') title ( 'Plot of thermal conductivity with temperature'); legend('Inconel 601', 'Inconel 718', 'Haynes 214', 'SS304','Silicon Carbide') figure(2); clf plot(Inco601T,Inco601SpHeat,'­o',Inco718Tff,Inco718HeatC,'­.',HaynesT,HaynesSpHe,'­+'... ,SS304T3,SS304SPHeat,':',SiCT2,SicHeatC,'­') ylabel('Specific Heat Capacity (J/kg\circC)'); xlabel('Temperature (\circ C)') title ( 'Plot of Specific Heat Capacity with Temperature'); legend('Inconel 601','Inconel 718', 'Haynes 214', 'SS304','Silicon Carbide') figure(3);clf

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plot(Inco601T,Inco601Exp,'­o',Inco718T,Inco718Exp,'­.',HaynesT,HaynesExp,'­+'... ,SS304T2,SS304Exp,':',SICT3,SiCExp,'­') ylabel('Coefficient of linear expansion (\circ C)^­1'); xlabel('Temperature (\circ C)') title ( 'Plot of Thermal expansion coefficient with Temperature'); legend('Inconel 601', 'Inconel 718','Haynes 214', 'SS304','Silicon Carbide') figure(4);clf plot(Inco601T,Inco601Modelast,'­o',Inco718T1,Inco718Modelast,'­.',HaynesT,HaynesElMod,'­+'... ,SS304T4,SSModel,':',SICT5,SICELMod,'­') ylabel('Elastic Modulus(GPa)'); xlabel('Temperature (\circ C)') title ( 'Plot of modulus of elasticity with Temperature'); legend('Inconel 601', 'Inconel 718','Haynes 214', 'SS304','Sintered Silicon Carbide') %% tensile Inco718T= ([70,600, 1000, 1200, 1300, 1400, 1500]­32)*(5/9); Inco718YStr= 6.895*[163,156,148,140,135,116,100]% annealed and aged round Inco601T=(5/9).*([200,400,600,800,1000,1200,1300,1400,1600,1800,2000]­32); Inco601YStr=6.895.*[34.5,30,27,24.5,24,27.5,28,27.5,18,11,5];% solution­treated , hot finished Haynes214T=[20,540,650,760,870,980,1095,1150,1205]; Haynes214YStr=[605,545,559,543,310,54,27,12,9]%Cold rolled and solution anealed SS304T=[20,204,316,427,538,649,760]; SS304YStr=[241,159,134,114,97,88,76]; SiCT=[20,500,500, 700, 800, 900,1000, 1200]; SinteredSiC= [250, 250, 250,250, 250,250,250, 250]; %% figure(5);clf plot(Inco601T,Inco601YStr,'­o',Inco718T,Inco718YStr,'­.',Haynes214T,Haynes214YStr,'­+'... ,SS304T,SS304YStr,':',SiCT,SinteredSiC,'­') ylabel('2% offset Yield Strength(MPa)'); xlabel('Temperature (\circ C)') title ( 'Plot of Yield Strength with Temperature'); legend('Inconel 601(annealed)', 'Inconel 718(annealed & aged)','Haynes 214(solution annealed)', 'SS304','Sintered Silicon Carbide')

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