Pre-Heat Treatment of Gas Atomized Al 2024 Powder
and its Effects on the Properties of Cold Spray Coatings
A Thesis Presented
By
Lauren Elizabeth Randaccio
to
The Department of Mechanical & Industrial Engineering
in partial fulfillment of the requirements
for the degree of
Master of Science
in the field of
Mechanical Engineering
Northeastern University
Boston, Massachusetts
December 2019
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ABSTRACT
Cold spray (CS) is a solid-state powder consolidation process used to produce
coatings of varying thicknesses and to build free standing parts. Micron-sized particles
are accelerated at supersonic speeds toward a substrate and experience mechanical and
metallurgical bonding upon impact. CS has a wide range of applications, both reparative
and structural. Commercially available CS feedstock includes gas-atomized powders
which possess rapidly solidified dendritic microstructures. Al 2024, Al 6061, and Al 7075
are three such alloys, widely accepted for fabrication and repair of aeronautical
components for their desirable strength-to-weight ratios. However, CS deposits often
lack in ductility and fracture toughness which is unfavorable for load bearing components.
At present, there is a need to develop new procedures and standards for high-
temperature powder processing, which will increase the reproducibility of CS coatings
and allow for manipulation of properties in the as-deposited state. Existing literature on
pre-heat-treating Al 2024 powders to improve ductility of as-sprayed parts is limited.
In this work, a study was conducted to investigate the effects of pre-heat-treating
Al 2024 powder on the properties of cold spray deposits. Three unique powder samples
were studied: (i) as-received, no heat treatment, (ii) solid solution heat-treated (495 °C, 1
hr), and (iii) annealed (415 °C, 2.5 hrs). Particles were analyzed for changes in size,
microstructure, and micro-hardness. Each powder sample was then cold sprayed on to
an Al 2024-T351 substrate using Helium as the processing gas. The deposits were tested
to determine micro-hardness and tensile properties in the as-sprayed condition.
Scanning electron microscopy revealed a dendritic/cellular microstructure within the as-
received powder, which was eliminated with both solution treatment and annealing.
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Particle hardness decreased by 33.4 % after annealing, but showed no significant change
after solution treatment, implying that full solution may not have been achieved.
Particles agglomerated during both high temperature heat-treatments due to sintering
effects, resulting in a measurable increase in size distributions. Solution heat-treated
powder produced a decrease in as-sprayed ductility and no significant change in ultimate
tensile strength (UTS), while annealed powder produced a decrease in as-spayed UTS and
no significant change in ductility. Elastic modulus remained constant across all CS
deposits.
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ACKNOWLEDGMENTS
I would first like to thank my advisor, Sinan Müftü, the Principle Investigator for
this work, for presenting me with the opportunity to join Northeastern’s Cold Spray Team
so early on in my time as a Masters student. Your support and trust in me over the past
two years allowed me the freedom to complete a thesis as well as a degree program that
was dignifying to me, and for that I am forever grateful.
I am equally grateful to Ozan Ozdemir, my colleague, mentor, friend, and interim
advisor whenever Professor Müftü was unavailable. Thank you for teaching me the ins
and outs of all things cold spray. Your patience, guidance and advice throughout this
process has been, and always will be, very much appreciated.
I must also acknowledge Northeastern’s Advanced Materials Processing
Laboratory (AMPL) and Professor Teiichi Ando. Thank you for treating me as if I was one
of your students, for providing your metallurgical expertise, and for allowing me to use
various equipment in your lab. And thank you Xingdong Dan, my peer and fellow Masters-
thesis student, with whom I collaborated in many ways. I am so appreciative of the many
micro-hardness measurements you performed for me in the AMPL.
Thank you Tricia Schwartz, research engineer at our Cold Spray Lab, for assisting
me with any and all things at KRI. Traveling to Burlington, MA sometimes four days a
week to perform experiments was challenging at times, but with your expertise and
generosity of time, I was able to accomplish so much more than I could have on my own.
Your overall support and kind nature made my experience in the lab much more
enjoyable, and it was so nice to have another woman around.
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The technical expertise of Bill Fowle and Wentao Liang from Northeastern’s
Electron Microscopy Core Facilities is also very much appreciated. Thank you Wentao
especially, for spending many hours with me doing SEM at the Kostas Research Institute
(KRI).
Thank you Professor Andrew Gouldstone for your materials science expertise and
overall support of this work in its early stages. Your genuine interest in my present and
future endeavors was always very encouraging.
Thank you Joe Conahan for assisting me with all of the tensile tests performed for
this work.
And lastly, I must acknowledge my colleagues Qiyong, Enqiang, Runyang, Scott,
and Salih, who make up the rest of Northeastern’s Cold Spray Team, as well as our office-
mate Soroush, whom I had a desk next to for the entirety of my time in this group. Your
kindness and willingness to collaborate made my time in this group much more pleasant
from day to day.
This work was financially supported by the United States Army Research
Laboratory through grant number W911NF15-2-0026. Any opinions in this thesis are
those of the author and do not necessarily reflect the viewpoints of the funding agency.
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DEDICATION
This thesis is dedicated to the young women of STEM.
You can do it.
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TABLE OF CONTENTS
ABSTRACT ............................................................................................................................ iii
ACKNOWLEDGMENTS .......................................................................................................... v
DEDICATION ....................................................................................................................... vii
TABLE OF CONTENTS......................................................................................................... viii
LIST OF TABLES ..................................................................................................................... x
LIST OF FIGURES .................................................................................................................. xi
1 INTRODUCTION ........................................................................................................... 1
1.1 Overview .............................................................................................................. 1
1.2 Research motivation ............................................................................................ 2
2 LITERATURE REVIEW .................................................................................................... 4
2.1 Cold spray ............................................................................................................. 4
2.2 Room for improvement ........................................................................................ 5
2.3 Pre-processing CS powders .................................................................................. 7
2.3.1 Solid solution heat treatment ..................................................................... 11
2.3.2 Annealing .................................................................................................... 15
2.4 Summary ............................................................................................................ 17
3 MATERIALS & METHODS ........................................................................................... 18
3.1 Material .............................................................................................................. 18
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3.2 Powder heat-treatments .................................................................................... 18
3.3 Microstructure characterization ........................................................................ 21
3.3.1 Metallographic Preparation & Etching ....................................................... 21
3.3.2 Microscopy .................................................................................................. 22
3.4 Particle size measurement ................................................................................. 22
3.5 Particle velocity measurement .......................................................................... 23
3.6 Micro-hardness testing ...................................................................................... 23
3.7 CS deposition process ........................................................................................ 24
3.8 Tensile samples .................................................................................................. 25
4 RESULTS & DISCUSSION ............................................................................................. 27
4.1 Microstructure characterization ........................................................................ 27
4.1.1 Powder ........................................................................................................ 27
4.1.2 CS deposits .................................................................................................. 29
4.2 Particle size distributions ................................................................................... 31
4.3 Particle velocities ............................................................................................... 34
4.4 Micro-hardness .................................................................................................. 35
4.5 Tensile Testing .................................................................................................... 37
5 CONCLUSIONS............................................................................................................ 41
6 FUTURE WORK ........................................................................................................... 43
APPENDIX .......................................................................................................................... 45
REFERENCES ...................................................................................................................... 49
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LIST OF TABLES
Table 1. Chemical composition of Al 2024 alloy provided by powder manufacturer (Valimet Inc., Stockton, CA). The main alloying element is Copper, at 4.16 wt-%. ................................................................................................................ 12
Table 2. Sample identification codes for Al 2024 powder in three heat-treated conditions. “-CS” is added to indicate a cold spray deposit produced from that particular powder. .................................................................................... 19
Table 3. Spray parameters held constant throughout the study. ................................... 24
Table 4. Statistical analysis of variance (ANOVA) of UTS and elongation-% data. If the P-value is less than 0.05, there is a statistical confidence of 95% that one of the data sets produced a mean that is significantly different from the other. ................................................................................................................ 40
Table 5. Complete list of tensile samples and their corresponding cross-section geometries. ...................................................................................................... 48
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LIST OF FIGURES
Figure 1. Relative temperatures for solid solution and precipitation heat-treatments [1, 2]. ................................................................................................................ 11
Figure 2. Aluminum-rich end of the aluminum-copper equilibrium diagram. Line AB represents the increase in solubility of Cu in solid aluminum as temperature increases [2-4]. Alloys containing more than 5.65 wt-% Cu (line CD) are incapable of achieving complete solid solution. ......................... 12
Figure 3. Relative temperatures for various annealing treatments. Full annealing was selected for this study. ..................................................................................... 16
Figure 4. Solid solution heat-treatment vessel made from steel hydraulic tubing. Ends of the tube are flanged to mate with compression fittings and create an air-tight mechanical seal when tightened. ................................................. 20
Figure 5. Flow chart of powder life-cycle during the study. ............................................ 20
Figure 6. Standard ASTM E8/E8M-16a tensile specimen geometry [2][4] . All dimensions are in inches. ................................................................................. 25
Figure 7. Modified ASTM E8/E8M-16a tensile specimen geometry. All dimensions are in inches. .......................................................................................................... 25
Figure 8. Cold sprayed tensile specimens prior to EDM removal from substrate. Tensile bars are made up of only cold sprayed material. ................................ 26
Figure 9. SEM images of Al 2024 particles from three heat-treatment conditions: (a, b) as-received, (c, d) solution heat-treated, (e, f) annealed. Cross-sections (left) were polished and etched to reveal microstructure. Images of whole particles (right) are only representative of their cross-sectioned counterparts and are not necessarily the same particle. ................................ 27
Figure 10. Optical micrographs of CS deposits made from three different Al 2024 powders: (a) as-received, (b) solution heat-treated, and (c) annealed.
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Cross-sections were cut parallel to the spray direction, then polished and etched to reveal microstructure. Black spots in images (b) and (c) are most-likely a result of the polish-etch procedure. .................................................... 29
Figure 11. SEM images of CS deposits made from three different Al 2024 powders: (a) as-received, (b) solution heat-treated, and (c) annealed. Cross-sections were cut parallel to the spray direction, then polished and etched to reveal microstructure. ................................................................................................ 30
Figure 12. Volume based cumulative size distribution of powder particles from three heat-treatment conditions. .............................................................................. 31
Figure 13. SEM images of whole powder particles from three heat-treatment conditions: (a) as-received, (b) solution heat-treated, and (c) annealed. ....... 32
Figure 14. ZEISS optical images of whole powder particles from three heat-treatment conditions: (a) as-received, (b) solution heat-treated, and (c) annealed. Particles were placed on a glass slide with a light source positioned below, so particles appear as black blobs. ................................................................... 32
Figure 15. Al 2024 powder from three heat-treatment conditions: (a) as-received, (b) solution heat-treated, and (c) annealed. Images were taken immediately following heat-treatment, prior to any sieving or breaking up of agglomerated particles. ................................................................................... 32
Figure 16. Overall frequency based (left) and volume based (right) particle size distributions of Al 2024 as-received powder. .................................................. 33
Figure 17. Overall frequency based (left) and volume based (right) particle size distributions of Al 2024 solution heat-treated powder. .................................. 33
Figure 18. Overall frequency based (left) and volume based (right) particle size distributions of Al 2024 annealed powder. ..................................................... 33
Figure 19. Particle velocities of Al 2024 as-received powder captured by HiWatch HR1 System. The average particle speed was measured to be 1027.61 m/s with a standard deviation of 237.03 m/s. ........................................................ 34
Figure 20. Micro-hardness of Al 2024 powders from three different heat-treatment conditions and their subsequent CS deposits: (left) as-received, (middle) solid solution heat-treated, and (right) annealed. ........................................... 35
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Figure 21. Representative stress-strain curves from uniaxial tensile testing. Samples were produced from four different materials: wrought Al 2024-T351 substrate (substrate), cold sprayed Al 2024 as-received powder (AR-CS), cold sprayed Al 2024 solution heat-treated powder (SSHT-CS), and cold sprayed Al 2024 annealed powder (AHT-CS). .................................................. 37
Figure 22. Average (a) ultimate tensile strength (UTS), (b) elongation-% to fracture, and (c) elastic modulus from four sets of tensile samples. ............................. 38
Figure 23. Stress-strain curves from uniaxial tensile testing. Samples were produced from four different materials: (a) wrought Al 2024-T351 substrate, (b) cold sprayed Al 2024 as-received powder, (c) cold sprayed Al 2024 solution heat-treated powder, and (d) cold sprayed Al 2024 annealed powder. ......... 39
Figure 24. High magnification SEM images of CS deposits made from three different Al 2024 powders: (a) as-received, (b) solution heat-treated, and (c) annealed. Cross-sections were cut parallel to the spray direction, then polished and etched to reveal microstructure. ............................................... 46
Figure 25. High magnification optical micrographs of CS deposits made from three different Al 2024 powders: (a) as-received, (b) solution heat-treated, and (c) annealed. Cross-sections were cut parallel to the spray direction, then polished and etched to reveal microstructure. Black spots in images (b) and (c) are most-likely a result of the polish-etch procedure. ............................... 47
1
1 INTRODUCTION
1.1 Overview
Cold spray (CS) is an additive manufacturing (AM) technology used to produce
coatings of varying thicknesses as well as to build free standing parts. Research on cold
spray has gained a lot of traction in the past 20 years, as the process has a wide range of
applications, both reparative and structural, and it is currently being implemented in both
industry and in defense-related projects [6, 7]. Cold spray is a solid-state process that
involves high strain rate deformation of particles flowing out of a DeLaval nozzle directed
at a substrate [6]. Particles flow out of the nozzle at super-sonic speeds via an inert gas
stream, remaining well below their melting temperatures the entire time. When critical
velocities are achieved, the particles bond upon impact with the substrate surface and
then with previously deposited particles to form a solid coating or block of material [8].
Helium is often used as the processing gas to achieve high pressures and temperatures,
and therefore higher particle speeds.
Commercially available, CS feedstock materials are metal powders consisting of
fine-sized particles (5-100 µm) that have rapidly cooled and solidified via gas-atomization.
These spherical powders possess flow properties that promote high impact velocities and
result in good mechanical and metallurgical bonding, but the rapid solidification they
experience during manufacture can also produce heterogeneous and metastable
microstructures. This inconsistency among particles can have deleterious effects on the
properties of a CS deposit as well as limit the reproducibility of a coating. Materials used
as CS powders include, but are not limited to, the following metal alloy systems: copper,
aluminum, zinc, nickel, titanium, iron-based alloys, nickel-based superalloys, etc. [9].
Aluminum aerospace alloys of 2000, 6000, 7000 series possess desirable strength-to-
weight ratios and are widely used in CS repair applications for parts subject to severe
conditions and corrosion [8, 10, 11]. The alloy chosen for this study is Al 2024, an age-
hardenable alloy used in aeronautical components exposed to severe conditions, such as
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gearboxes and fuselage parts [11]. Repairing components with cold spray coatings can
extend the life of a part at a much lower cost than traditional methods, and it is often one
of the only options when dealing with thermally sensitive substrate material [8]. Another
advantage of using cold spray as a repair solution is that it has desirable properties in its
as-deposited condition, meaning that no post-processing is necessary. In fact, some
components repaired by cold spray are simply too large to be post-processed.
Cold spray is also often compared to thermal spray and other powder
consolidation methods, over which it has many advantages. Unlike in thermal spray, CS
particles are exposed only to inert gases, instead of combustion gases, preventing certain
phenomena such as powder melting, grain growth, phase changes, and excessive
substrate heating [6, 7]. CS is also a low-temperature deposition process, so it can be
used on a much wider variety of materials than thermal spray [7]. Regardless of the
thermal history of feedstock powder, CS can produce coatings with substantial levels of
adhesion and mechanical strength [12] while retaining the chemical composition of the
feedstock material [6]. Ultimate tensile strength (UTS) and elastic modulus of cold
sprayed aluminum alloys is comparable to that of wrought material, though ductility of
the in the as-deposited condition is often sacrificed [6]. This is most-likely due to the
microstructural evolution of the powder particles that occurs during high strain rate
plastic deformation upon impact [9, 12]. It just so happens that the resulting
microstructure of the CS deposit depends heavily on the initial microstructure of the
powder; consequently, many are turning research efforts to the development of pre-
processing or powder-processing methods. This work investigates pre-heat-treatment of
gas-atomized powders as a pre-processing method for cold spray.
1.2 Research motivation
Metal alloy powders used for cold spray (CS) differ greatly from their wrought
counterparts in both thermal history and microstructure. The properties that feedstock
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particles possess often correlate with the properties of the subsequent cold sprayed
deposit, thus pre-processing powders will allow for manipulation of microstructure and
mechanical properties in a CS deposit. At present, there is a need to develop new
facilities, procedures, and standards for the pre-heat-treatment of CS feedstock powders.
Aluminum alloys fall under the umbrella of age-hardenable, heat-treatable alloys, and
vast amounts of research have gone into developing standard treatments and test
methods for wrought material. Additionally, thanks to efforts by the Army Research Lab
(ARL), aluminum alloys (i.e. 6061, 7075, and 2024) have also been heavily studied and
commercialized as powders for CS, making these materials a good starting point for
powder heat-treatment research [2]. Adapting standard heat-treatment methods to gas-
atomized powders, and developing a simple, repeatable, and cost-effective system for
pre-processing CS feedstock could revolutionize the entire additive manufacturing
process. Since a smaller body of literature exists for pre-processing Al 2024 powders than
it does for Al 6061 and Al 7075, this was the alloy selected for the study.
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2 LITERATURE REVIEW
2.1 Cold spray
Cold spray (CS) is a solid-state consolidation process where powder particles are
accelerated at super-sonic speeds toward a substrate. Raw feedstock materials include
ferrous and non-ferrous alloys and are selected based on specific application
requirements. The spherical particles, ranging 5-100 µm in diameter, are injected into a
high-velocity jet stream generated by the expansion of a pressurized, pre-heated gas
through a converging-diverging, DeLaval nozzle [6, 7]. The particles are initially carried by
a separate, cold gas stream, which meets the hot gas stream at an application site just
before the nozzle throat. Thus, the particle temperatures remain well below their melting
temperature throughout the entire process. Finally, upon exiting the nozzle, the solid
particles impact the substrate and plastically deform, creating a combination of a
metallurgical and a mechanical bond with the surrounding material [7]. However,
successful deposition and bonding is only achievable if the particles reach a critical impact
velocity, which is entirely dependent on feedstock material and gas temperatures. The
resulting deposit takes the form of a solid coating or a freestanding shape [7]. Cold spray
has recently gained a lot of traction in the world of metal additive manufacturing (AM)
technologies and is already being implemented as a repair method in military and
commercial sectors. The technology also has many advantages over thermal spray,
making it very popular in the realm of powder consolidation and other forming techniques
[6, 7].
The advantages of cold spray can be grouped into two categories: (a) as-deposited
material properties and (b) manufacturing capabilities. In its as-deposited condition, a
cold sprayed material possesses desirable properties for a variety of applications. For
example, CS has the ability to produce high-density, low-porosity coatings which is
important for components that require corrosion- and/or wear-resistance in severe
conditions [6, 11, 13-16]. Additionally, CS deposits made from aluminum alloy feedstock
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show high ultimate tensile strength (UTS) values, comparable to their wrought
counterparts, making them useful for AM and structural repair of aeronautical
components [2].
In terms of manufacturing capabilities, perhaps the greatest advantage of cold
spray is its solid-state nature. The low-temperature process prevents changes in
chemistry, phase composition, and oxidation in the as-deposited material, allowing for
deposition of heat-sensitive materials like copper, titanium, and aluminum alloys [6, 7,
11, 14, 16, 17]. Avoiding deleterious high-temperature reactions and oxidation of
substrate and/or feedstock powder makes cold spray a very attractive method for wear
restoration and protective coating applications [2, 14, 18-20]. Even the aerospace
industry, with its strict criteria for material selection and processing technologies for
flight-safety critical components, has adopted cold spray. Research efforts of the U.S.
Army Research Laboratory (ARL), in collaboration with original equipment manufacturers
(OEMs), have resulted in the qualification of cold spray aluminum alloys for use in specific
applications for the repair and dimensional restoration of magnesium and aluminum
aerospace components for Army, Navy, and Air Force aircrafts [2, 18, 19]. Aluminum
2000, 6000, and 7000 series alloys are common aluminum aerospace alloys for their
generally light weight and high strength [2]. Of these, Aluminum 2024, 6061, and 7075
are the most common, and are therefore studied at length as feedstock powders for cold
spray. Aluminum-copper alloys specifically (i.e. Al 2024) are often used for components
that experience extreme conditions (i.e. gear boxes and fuselage parts). They are
reported to have high strength, low density, and somewhat high temperature stability,
but poor corrosion performance [2, 11].
2.2 Room for improvement
Tireless research is being conducted in academic and military labs to reduce the
cost of processing, increase deposition efficiency, expand the number of CS materials
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(powder and substrate), enhance the quality of particle-substrate bonding, enhance
feedstock material microstructures, and improve mechanical properties of CS coatings.
Each of these potential advances brings cold spray one step closer to full
commercialization. Both experimental and model-based research has been published on
the optimization of cold spray processing parameters such as nozzle material and
geometries [21], gas pressure and temperature, gas type, standoff distance, and raster
speed [2]. Since process parameters are what govern particle speed and impact velocity,
specific parameters must be determined for each powder-substrate combination to
ensure good deposition efficiency and optimal mechanical properties of the as-deposited
material [18]. Gas selection and usage is another extremely important parameter and is
one major area for improvement in the cold spray process. Helium is often chosen over
Nitrogen gas, as it has favorable thermal properties allowing for faster particle velocities
and critical impact velocities. That being said, Helium is a finite resource that is very
costly, so significant attention has been shifted to the research and development of
Helium recovery systems [22-24]. Recycling helium during sprays will drive economic
costs down and increase potential for higher volume depositions [24].
Attention has also been focused on developing methods for improving, or
manipulating, mechanical properties of CS coatings in their as-deposited state. Even
when cold spray deposition produces high strength material, the same material often
suffers in ductility and fracture toughness [25]. Many researchers have addressed this
challenge by adding various post-spray heat-treatments (i.e. solution heat-treatments
and annealing treatments) to their process [25-30], but as of late, researchers are also
exploring the possibilities of pre-processing methods. Additionally, a number of
experimental studies have confirmed that the microstructure and grain size of the
feedstock powder, which is largely a result of the gas-atomization process, affects the
properties of the as-deposited CS coating [9, 10, 12, 31]. During gas-atomization, molten
metal is poured through a small opening to meet one or more gas streams [32]. The liquid
metal is rapidly dispersed by the impinging jet(s) and subdivided into fine droplets that
quickly cool and solidify into highly spherical particles [33]. The flow properties that these
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powders possess are advantageous for cold spray because they allow for high impact
velocities, which result in good bonding; however, gas-atomization is extremely chaotic,
and the rapid solidification that occurs can cause irregular grain growth, localized
segregation of alloying elements, and/or formation of metastable phases within
individual powder particles [12, 33].
These non-equilibrium/metastable microstructures, combined with high strain rate
deformation, can cause a CS deposited material to experience any of the following
harmful phenomena: [9]. The non-uniformity of particle microstructures can hinder the
reproducibility of cold spray coatings and inhibit one’s ability to predict or manipulate
properties in the as-deposited condition. Additionally, altering feedstock material
properties prior to CS deposition to manipulate properties in the as-sprayed state has the
potential to eliminate the need for post-processing, again pushing CS toward
commercialization. The Worcester Polytechnic Institute (Worcester, MA) has done a
considerable amount of work on the thermomechanical modeling side of pre-processing
Al 2024 and 6061 feedstock powders [34-38]. Their studies have furthered the
understanding of secondary phase evolution and stability during thermal treatment of CS
feedstock. They have shown that solution heat-treating aluminum powders effective in
dissolving secondary phases and has a significant effect on altering the granular structures
within particles.
2.3 Pre-processing CS powders
Because cold spray is a solid-state process, and the energy transfer from particle
to substrate is largely kinetic [6], the as-deposited microstructure, along with the
mechanical and bonding properties, largely depend on the nature of feedstock powder.
As discussed in the previous sections, achieving desired properties in the as-deposited
condition by means of pre-processing is valuable for reducing production costs and lead-
time. It is also useful for repair applications that: (a) involve components that are too
8
large to be post-processed; or, (b) are time-sensitive and need to be returned to service
as soon as possible [39]. Pre-processing methods to ensure these outcomes might include
mixing, degassing, coating, or heat-treating raw feedstock powder.
Recall that CS powders must be somewhat ductile at high rates of strain to be able
to deform plastically on impact and produce dense coatings with high bond strength [6,
9], but by mixing powders, researchers have been able to successfully spray brittle
materials as well. In addition to the metal alloy systems listed in Section 1.1, both pure
metals and metal matrix composites (MMCs) have been cold sprayed successfully.
Spraying MMCs was introduced to facilitate CS deposition of brittle materials, because
hard particles lack in ductility and are difficult to deposit directly [40]. It was determined
that adding hard particles to a deformable metallic matrix could allow them to be cold
sprayed [40, 41]. This pre-processing method, referred to as powder mixing, involves
blending two or more powders that differ in some physical property. Mixing powders of
varying size-distributions has also been done. Spencer et al. [42] attempted to optimize
stainless steel cold spray coatings using this method. Powder mixing is a widely accepted
powder-treatment strategy incorporated prior to cold spray deposition [40].
Other powder-treatment methods, such as particle coating and degassing, have
been developed to combat moisture, oxidation, and residual gases found on/in gas-
atomized aluminum particles, which pose major problems for powder consolidation [43].
Typically, aluminum powders are surrounded by a thin oxide layer on their outer surface.
Thus, their critical velocities tend to be higher (600-800 m/s), to ensure the oxide layer
ruptures on impact [44, 45]. Unfortunately, oxidation is unavoidable given the conditions
of gas-atomization; it takes place during in-flight rapid solidification and every other
instance particles are exposed to the air prior to cold spraying (i.e. collection and handling,
transportation, and storage) [46]. This thin, amorphous oxide film (2-10 nm thick) can
actually impede particle bonding during cold spray and lead to delamination and/or poor
ductility of a coating [46-48], therefore significant effort has gone in to understanding
how this oxide layer develops and how it, along with other particle impurities, might be
removed or altered before cold spraying [46-49]. Both high-temperature (300-500 °C)
9
and vacuum degassing have proven to be effective surface contaminant removal and gas
desorption procedures for aluminum alloy powders [47, 48]. U.S. Pat. No. 7,141,207
proposes using a fluidized bed to apply a copper coating to aluminum particles, providing
another method for preventing surface oxide contamination during additive
manufacturing [50].
Researchers have also investigated how degassing methods might affect the oxide
layer and/or removal of other contaminants. Degassing is a type of thermal treatment
commonly used as a pre-processing method for aluminum alloy powders to remove any
harmful moisture or gases trapped inside particles during gas atomization [49].
Degassing, similar to other standard heat-treatments, involves heating the powder to a
specified temperature and holding it there for a period of time. Higher temperature
degassing of aluminum powders (350–450°C) has the potential to lower the hardness of
powders, to prevent blistering, and to improve mechanical properties in the as-deposited
condition [47, 48, 51]. Rokni et al. [51] studied the effects of degassing Al 5056 powder
at 490°C for 6 hrs, reporting that pre-processing softened the particles and eliminated
internal porosity, resulting in more uniform deformation and improved micro-tensile
properties (UTS and elongation %).
Degassing is closely related to another powder-processing method: heat-treating.
Extensive research has been done to develop heat-treatments (HTs) that favorably alter
the microstructure and mechanical properties of wrought materials [52]. The
standardized HTs for aluminum alloys, often referred to as basic temper designations (i.e.
T4, T6) exist in traditional manufacturing mainly as a final fabrication step to relieve
residual stresses within the material and to achieve higher or maximum strength [3, 5,
52]. Only in the last five years, have research groups (to be mentioned later in this review)
begun to apply these standard treatments to heat-treatable CS powders. Extensive
research has been done on the post-heat-treatment of CS deposits and how it can alter
or improve material properties, but fewer have taken the powder heat-treatment
approach – attempting to enhance material properties by altering feedstock properties
prior to spray [34]. A pre-heat-treatment approach is more challenging to implement
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than a post-treatment. There are risks of particles sintering together and agglomerating
during heating, which would alter their flow capabilities and slow down impact velocities,
and there are certain safety limitations involved with heating combustible metal powders
that complicate experimental design. U.S. Pat. No. 9,555,474 proposes a promising
furnace-fluidized-bed assembly for high temperature powder treatment [53], but further
investigation is needed in regard to HT procedure, scalability, cost-reduction, and quality-
control.
To determine the proper heat-treatment for a material, it is important to
understand its thermal and mechanical history. For example, conventionally cast
aluminum alloys have often experienced some strain-hardening due to previous forming,
or they have already been heat-treated to a designated temper to improve strength [52].
In the case of CS powders, thermomechanical history depends on the gas-atomization
conditions (detailed in Section 1.1). Certain HTs for wrought aluminum alloys which
involve a precipitation treatment (i.e. natural aging), such as the T3 and T4 tempers, are
used to maximize strength, fracture toughness, and resistance to fatigue. This review
however, will focus on solid solution heat-treating (SSHT) and annealing, and the crucial
aspects of both: temperature, hold time, and cooling rate. These two treatments were
selected for their ability to soften a material, therefore increasing its ductility, and for
their ability to homogenize the microstructure of a material. Applying these HTs to CS
powders would enable particles to deform more readily on impact, potentially increasing
bond strength as well as ductility in the as-deposited material. Some work has been done
on solution heat-treating small batches of Al 6061, 7075, and 2024 powders to investigate
whether or not the effects prove to be advantageous for cold spray deposition [10, 12,
34, 36, 54].
11
2.3.1 Solid solution heat treatment
During solid solution heat-treatment (SSHT), an alloy is heated to a temperature
slightly below its solidus line and held there long enough that its constituents dissolve
into a solid solution. The material is then rapidly cooled, or quenched, to lock those
constituents in their solutionized state [52]. For wrought aluminum alloys, the purpose
of the HT is to put the maximum quantity of hardening solutes (i.e. copper, magnesium,
silicon, or zinc) into solid solution in the aluminum matrix, and then induce a slow
controlled precipitation so the material reaches its maximum hardness and strength [52,
54]. This final step of the SSHT is referred to as a low temperature age or precipitation
HT (Figure 1). Since the aim of solution heat-treating CS powders is to decrease particle
hardness and homogenize microstructures, the precipitation step was removed for this
study.
There are three key components in designing a solution heat-treatment: solution
temperature, hold time, and quench rate. All of these are material dependent as well as
part dependent, meaning that the size and shape of the part must also be factored in.
Solution temperature is determined by the melting temperature of a specific alloy;
therefore, it is imperative to know its chemical compositional and thermomechanical
Figure 1. Relative temperatures for solid solution and precipitation heat-treatments [1, 2].
12
behavior. Some Al alloys are more dilute in terms of maximum solubility [52] and can
tolerate higher solution temperatures, but the temperature alone does not guarantee full
solution. Al 2024 for example, the selected feedstock alloy for this study, has a solution
temperature of 493 °C and contains 4.16% Copper (Table 1). Al 2219 on the other hand,
has an even higher solution temperature of 535 °C, but it contains more than 5.65%
Copper, making complete solution impossible (Figure 2) [3, 5].
Figure 2. Aluminum-rich end of the aluminum-copper equilibrium diagram. Line AB
represents the increase in solubility of Cu in solid aluminum as temperature increases [2-4].
Alloys containing more than 5.65 wt-% Cu (line CD) are incapable of achieving complete solid
solution.
Table 1. Chemical composition of Al 2024 alloy provided by powder manufacturer (Valimet
Inc., Stockton, CA). The main alloying element is Copper, at 4.16 wt-%.
% Al:
Bal.
%C:
--
%Ca:
--
%Cr:
< 0.01
%Cu:
4.16
%Fe:
0.06
%Mg:
1.33
%Mn:
0.69
%N:
< 0.001
%Ni:
--
%O:
0.08
%Pb:
--
%S:
--
%Si:
0.05
%Sn:
--
%Ti:
0.01
%V:
--
%Zn:
< 0.01
Others,
Each:
< 0.05
Others,
Total:
< 0.15
Chemical Analysis: Al 2024 gas-atomized powder
13
As a material is heated or cooled, a series of exo- and endo-thermic reactions (also
referred to as solution and precipitation reactions) occur as elements either dissolve or
precipitate. Temperatures and rates at which these reactions occur depend on the
diffusion rates of different solutes – i.e. copper, magnesium, silicon, and zinc have
relatively high rates of diffusion in aluminum – and they can be measured directly by
Differential Thermal Analysis (DTA) [52]. Walde et al. [34] studied the microstructural
evolution of Al 2024 CS powder during thermal processing using Differential Scanning
Calorimetry (DSC) and material modeling. Due to unique powder precipitation kinetics,
they were not able to achieve successful dissolving of secondary phases via high-
temperature solution heat-treatment, but they were able to transform some phases [34].
They were heat-treating very small batches of powder, as DSC sample pans run on the
order of ~40 µL in volume, and this powder was not cold sprayed after heat-treatment.
Evans et al. [54] investigated at the effects of pre-heat-treating Al 6061 powder, but they
were mainly focused on small batch treatments and single-particle impacts and did not
perform HTs in-house.
Hold time often depends on the size of the part and its pre-existing microstructure [3,
52]. In the case of traditional casting and fabrication processes, grain size often coincides
with part or material thickness. Since CS powders possess fine-grained microstructures,
hold time can be reduced. In their study of Al 2024 powder, Walde et al. stated that due
to the higher percentage of grain boundary area, much shorter solutionization (hold)
times will be needed for powders compared to their wrought counterparts [34].
Quench rate is the dominant parameter in SSHT. Quenching is meant to preserve the solid
solution formed at the solution temperature [52]. For this to happen, the material must
be cooled fast enough so that the homogenous microstructure achieved during
solutionization is retained. A successful quench will keep solute atoms in solution as well
as retain a certain number of vacancies within the crystal lattice structure [52]. The key
is to quench fast enough so that the elements dissolved into their liquid phase during
solution treatment do not re-precipitate [55]. There are numerous strategies and media
used to quench parts and materials, but this review will focus solely on water quenching.
14
A typical water quench can be as simple as transferring a part directly from a furnace into
a container of cool, or room temperature, water, but it is crucial that the lag time between
furnace and water is minimized.
Sabard et al. [12, 31] measured the success of solution heat-treated Al 7075 and
Al 6061 powders by tracking particle hardness and particle grain patterns, by measuring
thickness of CS coatings made with as-received and SSHT powders, and by comparing the
morphology of particle deformation in those CS deposits. They report a significant
decrease in micro-hardness of both powders after solution heat-treatment as well as a
higher degree of particle deformation and fewer voids in the as-sprayed material. Story
et al. [10] solution heat-treated Al 2024, 6061, and 7075 powders based on guidelines
from the ASM Handbook Vol. 4; for reference, Al 2024 powder was heated for 75 min at
498 °C ± 6 °C and stored at -20 °C to prevent natural aging. Data from un-treated powder
was compared to data from solutionized powder, and HT success was measured in terms
of deposition efficiency, microstructural analysis of particles and coatings, and
mechanical properties of CS deposits. The solution HT was reported to decrease particle
hardness, improve deposition efficiency, and homogenize microstructures, but it did not
improve strength or ductility in comparison to as-received powder [10].
It must be noted here that fine-size aluminum powders are combustible when in
the presence of an ignition source and oxygen, so one must remove oxygen from the
equation by heating in an inert environment or vacuum [12, 43]. Aluminum powders can
also react with water to produce flammable hydrogen gas, so one must exercise caution
when performing a water quench. Sabard et al. [12] mitigated these concerns by vacuum
sealing powders inside quartz tubes. The tubes were heat-sealed so that the powder was
never exposed to air or water. SSHTs were conducted in 140 g powder batches, allowing
for easy handling and transfer from furnace to water quench where the quartz tube was
submerged for 2-5 minutes until it reached room temperature. Given the sealed nature
of the tube, it was impossible to record the exact temperature of the powder inside;
therefore, assumptions were made regarding the temperature of the particles
themselves [12, 31].
15
Story et al. [10] took a different approach to powder containment and quenching.
With an aim of increasing formability of the particles, they designed a custom tube
furnace system for SSHT. An air-tight tube was constructed from mild steel to house the
aluminum powders during treatment. To mitigate risk of particle oxidation, the tube was
flooded with helium and then sealed. An array of thermocouples was used to record the
temperature inside the tube during heat-treatments of empty tubes, but since the use of
thermocouples was not feasible during SSHT, some assumptions were made regarding
the temperature of the powders [10]. Their custom design allowed for easy and
immediate transfer of the tube into a brine-water bath, where an average powder cooling
rate of 25 °C/s was reported based on the thermal conductivity of mild steel [10]. As is
evident from these two powder quenching techniques, when the material is not directly
exposed to the water, it is difficult to control the quench rate. Particles are being cooled
by conductive heat transfer from tube material to particle, and quench rate may suffer.
2.3.2 Annealing
The second heat-treatment to address is annealing, or annealing heat-treatment (AHT).
Annealing temperatures tend to be lower than SSHT temperatures, but again, the
temperature and duration of the treatment remains largely dependent on the alloy and
the initial structure and temper of the part [3]. Even for a specific alloy there are several
types of annealing (Figure 3), but this review will focus solely on full annealing. One can
assume from here on that the terms annealing and AHT are always referring to full
annealing, which is a typical HT for wrought aluminum alloys used to render the material
to the “O” temper, producing the softest, most ductile, and most workable condition of
the material [3]. Annealing is an advantageous choice for conventionally-made alloys, as
the reduction or elimination of the effects of cold-working is accomplished by heat-
treating at 260-440°C [3].
16
Typical annealing conditions for wrought Al 2024 are 415°C for 2-3 hrs. Annealing
can be performed in open-air, since only temperatures greater than 415°C would result
in oxidation and unwanted grain growth [3]. Additionally, annealing does not involve a
quench. Both of these facts suggest annealing to be a much more time- and cost-effective
HT for CS powder than SSHT. Larger batches could be treated without increasing safety
risks. A crucial part of annealing, like most HTs, is ensuring that the entire part reaches
the annealing temperature. For wrought parts, this is easily achieved by using a hold time
of at least 1 hr [3], but more care has to be taken when adapting this treatment to gas-
atomized powders. Some sort of in-situ powder agitation is necessary to achieve
homogenous heating and to prevent agglomeration of the powder particles. Again, this
is why US Pat. No. 9,555,474 discloses using a fluidized bed for high temperature powder
treatment [53]. Ning et al. [56] studied the effects of vacuum annealing Copper feedstock
powders at both 390°C and 500°C. They reported an increase in CS deposition efficiency,
a decrease in micro-hardness of powder particles, and a decrease in critical velocity.
Figure 3. Relative temperatures for various annealing treatments. Full annealing was selected
for this study.
17
2.4 Summary
As is evident from the literature, research has only just begun to explore how pre-
processing by means of pre-heat-treating powders can alter and manipulate the
microstructure of a CS deposition. In the last 12 or so years, few have performed
annealing treatments on gas-atomized powders for cold spray, and only in the last 3 years,
performed solid solution heat-treatments on CS powders, so it is clear that this powder
heat-treatment is far from being commercialized. Readily controlling the microstructures
in aluminum alloy feedstock particles by means of heat-treatment could revolutionize the
CS manufacturing process entirely. In the future, powder manufacturers and consumers
will work together to transform gas-atomized powders into novel CS feedstock material
with predetermined, homogeneous microstructures allowing for cold sprayed coatings
with more predictable mechanical properties more reliable reproducibility. That being
said, there is need to develop a simpler, more cost-effective version of this particular pre-
processing method. Due to procedural limitations, researchers have been unable to
produce large volume sprays with heat-treated powders and unable to report a significant
improvement in mechanical properties by means of powder heat-treatment. The
following work will address both of these shortcomings. There is some literature on the
pre-heat-treatment of Al 7075, Al 6061, and Al 2024 powders, but further investigation is
needed on the correlation between powder microstructure and the mechanical
properties of as-sprayed material. Furthermore, if the ultimate goal is to commercialize
these processes, steps need to be taken to scale-up powder-treatment systems and to
incorporate quality control.
18
3 MATERIALS & METHODS
The objective of this study is to (a) investigate if the properties of CS powders, both
microstructural and mechanical, correlate with the properties of their subsequent CS
depositions, and then (b) determine if the properties of a CS deposit can be manipulated
by altering the properties of the feedstock powder prior to spraying. The following
characterization and test procedures make up a broad, yet systematic, investigation using
powder heat-treatment as the main pre-processing method and Al 2024 as the reference
material.
3.1 Material
The feedstock material used in this study was gas-atomized Al 2024 powder
(Table 1): batch no. 49-1/18-9004S (Valimet, Stockton, CA, USA). According to the
manufacturer, no heat-treatments were performed on the powder prior to shipment, and
the powder from this batch is a “modified Mil-spec” with D10: 20.15 µm, D50: 35.30 µm,
D90: 53.85 µm sizing. Throughout the study, the powder was stored and handled inside
a glovebox chamber purged with pure N2 gas (LC Technology Solutions Inc., Salisbury, MA,
USA), which maintains an inert, moisture-free, and oxygen-free environment to prevent
excess oxidation of particle surfaces as well as any other powder contamination.
3.2 Powder heat-treatments
Feedstock powder was heat-treated to three unique conditions prior to cold
spraying: as-received (AR), solid solution heat-treated (SSHT), and annealed (AHT). All
heat-treatments (HTs) were selected based on standard treatments for wrought Al 2024
found in ASTM B918/B918M-17a and ASM Metals Handbook Vol 4: Heat Treating [3, 5]
and were performed in an 1100 °C open-air box furnace (Thermo Fisher Scientific,
19
Waltham, MA, USA). Each powder sample was given a letter code (Table 2) and stored in
the glovebox between HT and cold spray. A “-CS” is added to the letter code to denote
the cold spray deposit produced from that particular powder (i.e. AR-CS is a cold spray
deposit made from as-received powder). This notation will appear throughout this
document.
A custom in-house procedure for heating and quenching combustible aluminum
powders was designed for the solid solution heat-treatment (Figure 5). While inside the
inert glovebox, 20-25 g batches of powder were mechanically sealed inside a vessel
consisting of a hydraulic steel tube (K&M Hose Services, Woburn, MA) with two flanged
ends mated with threaded compression fittings (Figure 4). The tube was then removed
from the glovebox and tightened to 88-115 Nm (65-85 ft-lb) with a torque wrench to
ensure the air-tight seal would hold during heating and quenching. A series of torque
tests were conducted on empty tubes, prior to HT, to determine the optimal torque range
necessary to maintain a seal during water quenching. After 1 hr of heating, the tubes
were removed from the furnace and immediately submerged in a room temperature
water bath. The SSHT was deemed successful if the powder came out of the tube dry
after quenching, implying that the compression fittings held their seal for the duration of
Table 2. Sample identification codes for Al 2024 powder in three heat-treated conditions. “-CS”
is added to indicate a cold spray deposit produced from that particular powder.
Temperature Time
ARRaw powder that remains “as-
received” from manufacturerRoom temp. 5-10 months
SSHTRaw powder that has been solid
solution heat-treated 495 °C 1 h
AHT Raw powder that has been annealed 415 °C 2.5 h
Sample
CodeDescription
Heat Treatment
20
the HT. Due to the constraints of the tube dimensions, 10 discrete SSHTs were performed
to produce approximately 200 g of solution heat-treated powder. All storage and
handling of powder not sealed in a steel tube was done inside the glovebox, so powder
was never exposed to oxygen or moisture prior to cold spraying. This treatment was
created with the aim of homogenizing the microstructure of the powder and decreasing
the hardness of the particles. Such an outcome would aid in particle deformation during
cold spray and potentially improve elongation properties in the subsequent CS deposit.
Figure 4. Solid solution heat-treatment vessel made from steel hydraulic tubing. Ends of the
tube are flanged to mate with compression fittings and create an air-tight mechanical seal when
tightened.
Figure 5. Flow chart of powder life-cycle during the study.
21
Since annealing is a more commonly used heat-treatment for aluminum powders,
a simpler, more conventional approach was used for the AHT. Two batches of Al 2024
powder were placed inside aluminum pans and wrapped in aluminum foil to prevent
excess oxidation. Both pans were then placed in the same open-air furnace mentioned
above. After 2.5 hours of heating, the furnace was shut off and powder was left to air
cool, yielding approximately 400 g of AHT powder. Significant agglomeration occurred
during both high-temperature heat-treatments, so large chunks of powder had to be
eliminated with gentle powder mixing and crushing, as well as sieving, prior to cold spray.
3.3 Microstructure characterization
3.3.1 Metallographic Preparation & Etching
Powder samples were taken from each heat-treatment condition (AR, SSHT, and
AHT) (Table 2). Standard metallographic preparation techniques were used to create
mirror-finish, scratch-free cross-sections for microscopic imaging and micro-hardness
testing [57]. Samples were first mounted in high edge-retention epoxy resin (Pace
Technologies, Tuscan, AZ, USA). Once the epoxy set, a 4-5 step grinding procedure was
carried out with SiC paper increasing in fineness from 320-grit to 1200-grit (Pace
Technologies). The final wet polish was conducted using 0.05 µm alumina suspension
(Pace Technologies). Both grinding and polishing were done by hand on an EXTEC® Labpol
Duo 8 Twin Grinding/Polishing Machine (Extec Corporation, Enfield, CT) at speeds ranging
from 500-700 rpm. Additionally, half of each sample was etched using Keller’s Reagent.
Swabbing the surfaces 5-6 times was sufficient to reveal the grain structure of the
powders [58].
The same grind-polish-and-etch procedure was used to produce cross-section
samples of each cold spray deposit (AR-CS, SSHT-CS, and AHT-CS). These samples were
subject to microscopic imaging, micro-hardness testing, and porosity measurements.
22
3.3.2 Microscopy
SEM analysis was conducted, with the assistance of Wentao Liang, at Northeastern
University’s Kostas Research Institute (KRI) using a Thermo Scientific™ Scios™ DualBeam™
ultra-high-resolution system operated at 5.0 kV. Since powder samples were mounted in
a non-conductive epoxy resin, the surfaces were coated with a 3-5 nm conductive
platinum layer using a Cressington 208HR High Resolution Sputter Coater prior to SEM to
prevent surface charging defects while imaging. SEM images of the powders were used
to compare grain structure of individual particles from each HT condition (Table 2).
Optical microscopy (OM) was conducted both at KRI, using a ZEISS Axioscope 7 microscope
(Carl Zeiss AG, Oberkochen, Germany), and in Northeastern’s Advanced Materials
Processing Laboratory using an Olympus-AH-2. OM and SEM images of the CS deposits
were used to compare morphology of deformed particles, area percentage porosity, and
general microstructure resulting from the as-received, solid solution heat-treated, and
annealed powder conditions.
3.4 Particle size measurement
Particle size distributions of each powder (AR, SSHT, and AHT) were measured using
digital image processing. Twelve micrographs were taken from each powder sample using
a ZEISS Axioscope 7 microscope by spreading particles onto a glass slide with a light source
positioned below. A MATLAB program was written to threshold and contrast the images
so that individual particles appeared as blobs of dark pixels on a white background. Blob
sizes were then measured and grouped into 10 µm size ranges, so that frequency-based
and volume-based size distributions could be determined. The method was developed
based on techniques found in ASTM E2109-01 standard for determining area percentage
porosity in thermal spray coatings [59].
23
3.5 Particle velocity measurement
Particle velocity during spray operation was measured using a HiWatch HR1 System
(Oseir Oy, Tampere, Finland) specifically designed for research and development of cold
spray processes. The system was mounted inside the cold spray booth so that the nozzle
could be situated within the measuring area. This is not an in-situ measurement system,
so processing conditions (Table 3) were replicated to determine the average particle
impact velocity during CS deposition. Due to limited powder supply, measurements were
taken only from the as-received Al 2024 powder sample.
3.6 Micro-hardness testing
Vickers micro-hardness testing was performed on the mirror-polished cross-
sections described in Section 3.3.1. All measurements were conducted, with the
assistance of Mr. Xingdong Dan, on a Shimadzu HMV Micro Vickers Hardness Tester in
Northeastern’s Advanced Materials Processing Laboratory. Indentations were made on
the powder samples using the minimum loading available: 98.07 mN. This loading
condition resulted in indentations that were observed repeatedly to be approximately
1/4 the diameter of a particle cross-section, implying that the results may have a larger
error than is reported here. Indentations were made on the as-deposited CS samples with
a load of 245.2 mN.
24
3.7 CS deposition process
All cold spray deposits were produced with a VRC Gen III high-pressure cold spray
system (VRC Metal Systems, Rapid City, SD, USA) in Northeastern’s Cold Spray Laboratory
at KRI. Helium was used as the carrier gas to achieve high impact velocities. Wrought Al
2024-T351 was used as the substrate material. Powders from each HT condition
(Table 2) were sieved through a -270 mesh and then deposited onto a 0.375 inch thick
substrate that was grit blasted prior to spray to enhance particle-substrate bonding. A
simple rectangular raster pattern of the robot arm was used to produce three separate
CS deposits ranging 0.11 – 0.25 inches in thickness. Each CS block was then machined in
its as-sprayed state to produce tensile bars and microstructure samples. Table 3 details
the specific spray parameters that were held constant for each of the three CS deposits.
Table 3. Spray parameters held constant throughout the study.
CS Parameter
Carrier gas Helium
Gas pressure 510 psi
Gas temperature 415 °C
Nozzle material PBI
Nozzle type VRC #71
Deposition angle 90 °
Standoff distance 25 mm
Nozzle speed 254 mm/s
Powder feed rate 0.247 kg/hr
Layer Height 0.1 mm
25
3.8 Tensile samples
Tensile samples were machined from the as-deposited cold spray material with
standard ASTM E8/E8M-16 dimensions shown in Figure 6 and Figure 7 [4]. The
geometries were machined first in Northeastern’s machine shop with the help of Mr. Ben
Macalister. The resulting dogbone samples (Figure 8) were then sent out to be cut from
the substrate via wire EDM (United Tool & Machine, Wilmington, MA). Specimens were
machined such that the tensile loading axis was oriented perpendicular to the spray
direction. A total of 19 samples were tested including 4-5 each from the three CS deposits
(AR-CS, SSHT-CS, and AHT-CS), as well as 5 samples machined from the wrought Al 2024-
T351 substrate material. A complete list of tensile samples and their cross-sectional areas
is given in Table 6.
Figure 6. Standard ASTM E8/E8M-16a tensile specimen geometry [2, 4] . All dimensions are
in inches.
Figure 7. Modified ASTM E8/E8M-16a tensile specimen geometry. All dimensions are
in inches.
26
Uniaxial tensile testing was performed at room temperature on an Instron Model
5582 static 100 kN load frame (Instron, Norwood, MA, USA). Specimens were pulled in
the direction perpendicular to the direction of spray at an extension rate of 0.03 in/min,
and strain was measured using a 1 inch gauge length extensometer (Instron). It is
important to note that some of the cold sprayed samples fractured outside the range of
the extensometer clips, but because these classify as a brittle fractures, the elongation
data was still included in results of this study (Section 4.5).
Figure 8. Cold sprayed tensile specimens prior to EDM removal from substrate. Tensile bars
are made up of only cold sprayed material.
27
4 RESULTS & DISCUSSION
4.1 Microstructure characterization
4.1.1 Powder
Figure 9. SEM images of Al 2024 particles from three heat-treatment conditions: (a, b) as-
received, (c, d) solution heat-treated, (e, f) annealed. Cross-sections (left) were polished and
etched to reveal microstructure. Images of whole particles (right) are only representative of
their cross-sectioned counterparts and are not necessarily the same particle.
(a)
(c)
(e)
(b)
(d)
(f)
28
Figure 9 a and b reveal a typical microstructure and surface structure of gas-
atomized aluminum alloy powder. The dendritic cell structure exhibited in the cross-
sectioned particle is a direct result of the rapid solidification that occurs during gas-
atomization. These cellular boundaries are also visible in the image of the whole particle,
which appears to have bumps and grooves instead of a flat surface. This analysis of the
as-received Al 2024 powder was used as the baseline for comparison to particles from the
other two HT conditions (solutionized and annealed).
Figure 9 c and d reveal the microstructure produced by means of a solid-solution
heat-treatment (490 °C for 1 hr). The intercellular grain structure is no longer observable;
thus, it was concluded that the SSHT homogenized the microstructure. Figure 9 e and f
reveal the microstructure produced by means of a full annealing treatment (415 °C for 2.5
hr). Like the SSHT, the annealing treatment has eliminated the cellular grain structure
and homogenized the particles.
29
4.1.2 CS deposits
Figure 10 and Figure 11 display the microstructure of cold spray deposits
produced from the three different Al 2024 powders. It is evident from both SEM and
optical images of the as-received (AR) powder, that the as-sprayed material possesses the
same intercellular structure that the individual particles possessed prior to CS deposition
(Figure 9a). The intercellular boundaries remain clearly visible even in their deformed
state, making it easy to distinguish deposition layers and individual particles (Figure 10a).
This is not the case, however, for the deposits made from solutionized and annealed
powders (Figure 10 b, c). Since the intercellular precipitates in the particles have been
Figure 10. Optical micrographs of CS deposits made from three different Al 2024 powders:
(a) as-received, (b) solution heat-treated, and (c) annealed. Cross-sections were cut parallel to
the spray direction, then polished and etched to reveal microstructure. Black spots in images (b)
and (c) are most-likely a result of the polish-etch procedure.
(a) (b)
(c)
100 µm 100 µm
100 µm
30
dissolved during high temperature heat-treatments, individual particles and layers are
much less discernible. The black spots in the optical images of as-deposited solutionized
powder and as-deposited annealed powder (Figure 10 b, c) may be a result of softened
phases being ripped out during a polish and etch procedure that is perhaps too aggressive
for the material. We believe that these black spots do not necessarily correspond to
porosity within the CS deposit and that further investigation is necessary.
Figure 11. SEM images of CS deposits made from three different Al 2024 powders: (a) as-
received, (b) solution heat-treated, and (c) annealed. Cross-sections were cut parallel to the
spray direction, then polished and etched to reveal microstructure.
(a) (b)
(c)
31
4.2 Particle size distributions
It appears from Figure 12 that powder size distribution increased significantly with
high-temperature heat-treatment, but further consideration suggests that this may not a
true representation of individual particles increasing in size. SEM images, shown in
Figure 13, show little no observable effect on overall particle size or shape. However, it
does look as though the annealed and solution treated powders have larger numbers of
agglomerated particles and satellite particulate than the as-received powder (Figure 15).
This is most-likely a result of sintering occurring during heat-treatment and is probably
the reason for the increase in measured size distribution. This effect was addressed post-
heat-treatment with gentle powder mixing/crushing and sieving, but it was impossible to
separate all of the fine-sized “moon” particulate that had adhered to the surfaces of larger
particles. Agglomerated particles are indistinguishable to the image processing MATLAB
program and therefore register as larger particles (Figure 14). This is ultimately why there
appears to be a higher percentage of large particles in the solution heat-treated and
annealed powders in Figure 12. However, regardless of whether or not these size ranges
Figure 12. Volume based cumulative size distribution of powder particles from three heat-
treatment conditions.
32
represent those of individual spherical particles, the material traveling through the cold
spray system has clearly increased in size with heat-treatment, which significantly alters
flow properties and reduces particle velocities during deposition. Figures 16-18 further
supplement the curves plotted in Figure 12 providing bar charts of both frequency based
and volume based size distributions.
Figure 13. SEM images of whole powder particles from three heat-treatment conditions: (a)
as-received, (b) solution heat-treated, and (c) annealed.
(c)(b)(a)
Figure 14. ZEISS optical images of whole powder particles from three heat-treatment
conditions: (a) as-received, (b) solution heat-treated, and (c) annealed. Particles were placed on
a glass slide with a light source positioned below, so particles appear as black blobs.
(c)(a) (b)
Figure 15. Al 2024 powder from three heat-treatment conditions: (a) as-received, (b) solution
heat-treated, and (c) annealed. Images were taken immediately following heat-treatment, prior
to any sieving or breaking up of agglomerated particles.
(c)(a) (b)
33
Figure 16. Overall frequency based (left) and volume based (right) particle size distributions
of Al 2024 as-received powder.
Figure 17. Overall frequency based (left) and volume based (right) particle size distributions
of Al 2024 solution heat-treated powder.
Figure 18. Overall frequency based (left) and volume based (right) particle size distributions
of Al 2024 annealed powder.
34
4.3 Particle velocities
A mean particle velocity of 1027.15 m/s was recorded for the Al 2024 as received
powder. Processing conditions were mimicked during this test, so we assume that during
deposition, particles impacted the substrate and surrounding material at a mean rate of
1027.61 m/s (Figure 19). Such high velocities were achieved by using Helium as the
processing gas. Due to powder supply limitations, particle velocities were not recorded
for the solution heat-treated powder or the annealed powder. However, due to particle
agglomeration and increase in particle size distribution discussed in Section 4.2, we have
assumed that the solution-treated and the annealed particle velocities were lower than
that of the as-received powder during deposition. Further HiWatch testing is necessary
to validate this assumption, but in the future, any discrepancies should be accounted for
by adjusting spray parameters (Table 3) for the heat-treated powders so that impact
velocity remains constant across all three sprays.
Figure 19. Particle velocities of Al 2024 as-received powder captured by HiWatch HR1
System. The average particle speed was measured to be 1027.61 m/s with a standard deviation
of 237.03 m/s.
35
4.4 Micro-hardness
Figure 20 illustrates how two high temperature heat-treatments affected the
micro-hardness of Al 2024 powder and corresponding cold sprayed deposits. One
observation is the correlation between hardness of powder particles and hardness of
subsequently sprayed material. If the powder increased in hardness after heat-treatment
(i.e. solution HT), the CS deposit made with that powder also showed an increase in
hardness. Similarly, if the powder decreased in hardness after heat-treatment (i.e.
annealing), the CS deposit made with that powder also showed a decrease in hardness.
Additionally, the increase in hardness from powder particles to as-sprayed material
indicates the strain hardening occurring during CS deposition.
Figure 20. Micro-hardness of Al 2024 powders from three different heat-treatment conditions
and their subsequent CS deposits: (left) as-received, (middle) solid solution heat-treated, and
(right) annealed.
36
After solid solution heat-treatment (SSHT) the average hardness of the powder
increased by 9.3 % compared to the as-received powder. Although the AR powder column
and the SSHT column (Figure 20) appear to fall with standard deviations of each other, a
statistical analysis of variance produced a p-value of 7.8E-06, confirming that the two data
sets are in fact significantly different from one another. That being said, the solution heat-
treatment was intended to decrease the hardness of the powder; therefore, this result
was not expected and eludes to a few possibilities:
(i) The particles were not quenched at a high enough rate to retain the solid
solution formed during HT, implying that elements began to reprecipitate
immediately. This age-hardening effect would explain the significant
increase in micro-hardness compared to the as-received powder.
(ii) The particles experienced some amount of solid solution strengthening
during HT, resulting in an increase in micro-hardness.
(iii) The particles were not fully solutionized during HT, implying that the
intercellular precipitates did not fully dissolve into the aluminum matrix.
Further elemental analysis is necessary.
After the annealing treatment (AHT) the average hardness of the powder decreased
by 33.4 % compared to the as-received powder, and the microhardness of as-sprayed
material decreased by 8.0 %. This result was anticipated, as full annealing of aluminum
alloys is intended to render the softest, most ductile, and most workable condition of the
material [3].
37
4.5 Tensile Testing
Five tensile samples were tested from substrate, SSHT-CS, and AHT-CS material, and
four samples were tested from the AR-CS material. Figure 21 presents one stress-strain
curve from each of the four materials, that is only representative of its larger data set.
The four curves are presented on the same axes for visual comparison.
Figure 22 displays the average values for UTS, elongation-% to fracture, and elastic
modulus. CS deposits made from the three heat-treated powders were not able to
achieve tensile strength or ductility greater than that of the wrought Al 2024-T351
material. Comparing mechanical properties of the three CS deposits, solution heat-
treating the powder produced a minor decrease in as-sprayed elongation-% but little to
Figure 21. Representative stress-strain curves from uniaxial tensile testing. Samples were
produced from four different materials: wrought Al 2024-T351 substrate (substrate), cold
sprayed Al 2024 as-received powder (AR-CS), cold sprayed Al 2024 solution heat-treated
powder (SSHT-CS), and cold sprayed Al 2024 annealed powder (AHT-CS).
38
no change in as-sprayed UTS (confirmed with ANOVA in Table 4). This was not surprising,
as the solution treatment was unsuccessful in softening powder particles.
The annealed powder, on the other hand, produced a decrease in as-sprayed UTS
but little to no change in as-sprayed elongation-% when compared to samples made from
as-received powder (confirmed with ANOVA in Table 4). More tensile samples must be
tested to clear up the discrepancies apparent from statistical analysis of variance.
Figure 22. Average (a) ultimate tensile strength (UTS), (b) elongation-% to fracture, and (c)
elastic modulus from four sets of tensile samples.
39
Additionally, Figure 22c shows that the elastic modulus remained constant across
all four Al 2024 materials. Average values ranged from 6.70E04 to 6.86E04 MPa, but all
fall within standard deviations of one another. This result was anticipated as behavior in
the elastic region is dominated by the bulk material’s response to tensile stress, which is
not being altered in this study. Aluminum is the bulk material in each of the four tested
samples. The stress-strain curves in Figure 21 reveal the brittle nature of each of the
three fractured cold sprayed materials in comparison to the ductile fracture of wrought
substrate material.
Figure 23. Stress-strain curves from uniaxial tensile testing. Samples were produced from four
different materials: (a) wrought Al 2024-T351 substrate, (b) cold sprayed Al 2024 as-received
powder, (c) cold sprayed Al 2024 solution heat-treated powder, and (d) cold sprayed Al 2024
annealed powder.
40
Table 4. Statistical analysis of variance (ANOVA) of UTS and elongation-% data. If the P-
value is less than 0.05, there is a statistical confidence of 95% that one of the data sets produced
a mean that is significantly different from the other.
41
5 CONCLUSIONS
A number of conclusions were drawn from the findings of this study regarding the
effects of pre-heat-treating Al 2024 powder on particle characteristics and on mechanical
properties of cold sprayed material. First of all, it was determined that annealing is a
much simpler pre-heat-treatment for combustible aluminum powders than solid solution
heat-treating. There is no quench step involved, allowing for larger volumes of powder
to be treated at once. It was also determined that heating Al 2024 powders to
temperatures of 415 °C and higher results in significant particle sintering and
agglomeration which should be mitigated in future works. Additionally, both annealing
and solution heat-treatments homogenized particle microstructure by eliminating the
dendritic intercellular boundaries found in as-received powder. Though the three powder
samples polished and etched quite differently, this affect was revealed with scanning
electron microscopy (SEM).
Solution heat-treating raw Al 2024 powder resulted in a 9.3 % increase in Vickers
hardness. This unanticipated result indicates that some hardening occurred within the
material, age-hardening or solution hardening, or that the quench was unsuccessful in
retaining the solid solution formed during heating. The increase in particle hardness
resulted in a minor decrease in as-sprayed elongation-%, but it did not yield any
measurable change in as-sprayed ultimate tensile strength (UTS).
Conversely, annealing raw Al 2024 powder produced a 33.4 % decrease in Vickers
hardness. This significantly softened powder yielded a decrease in as-sprayed UTS, but it
produced no significant change in elongation-%. It was concluded that annealing still has
the potential to improve material ductility in the as-deposited state, but more data is
necessary to confirm with statistical confidence.
As anticipated from micro-hardness results, the CS deposits made with solution heat-
treated powder also showed an increase in hardness, and those made with annealed
powder also showed a decrease in hardness. The increase in hardness from powder to
42
subsequent cold sprayed deposit does not correlate with powder heat-treatment but
indicates the strain hardening that occurs during CS deposition. The three CS deposits
also polished and etched very differently; therefore, the optical and SEM images of CS
cross-sections were inconclusive.
Lastly, by comparing properties of substrate material (Al 2024-T351) to the as-
received powder deposit (AR-CS), it was confirmed that cold sprayed Al 2024 produces
UTS comparable to wrought but suffers significantly in ductility. However, during this
study, neither solution heat-treating nor annealing powder was able to significantly
improve ductility in the as-deposited state.
43
6 FUTURE WORK
This study presents a correlation between the microstructure of feedstock powders
and cold sprayed deposits. Further, it presents that the microstructures of gas-atomized
aluminum alloy powder can be significantly altered via high temperature heat-treatment.
The work is limited predominantly by the size of the data set that was collected; thus, it
can be supplemented and expanded as follows:
1. To achieve full commercialization, the powder heat-treatment process needs further
refinement to increase efficiency and implement quality control. This may be
achieved by:
a. Performing the same study using an alternative HT system custom designed by
Northeastern capstone students. A prototype is currently located in
Northeastern’s Cold Spray Lab and consists of a rotating tube furnace with a built
in N2 gas quench. A rotating furnace would provide gentle agitation of the
powders, promoting uniform heating and preventing agglomeration and/or
sintering of particles. Quenching with inert gas instead of water would also
eliminate safety concerns, cutting down production time and allowing for higher
volume HTs.
b. Developing a technique to make in-situ temperature measurements of the
powders during heating and quenching to ensure particles reach desired solution
or annealing temperature.
c. Continuing to refine the design of powder heat-treatment tubes or vessels by
experimenting with various materials, heat-treating larger batches of powder, or
designing a system to HT multiple tubes at one time.
2. There is a need for investigating the effect of pre-processing additional heat-treatable
alloys (i.e. steels, tantalum, and nonferrous alloys: titanium, copper, lead, magnesium,
nickel) used as cold spray feedstock powders.
44
3. Particle deformation behavior, parametrized by the flattening parameter, can provide
valuable information regarding the bonding process in a CS deposit. An alternative
polish/etch procedure should be investigated to try and reveal individual particle
boundaries in micrographs of CS cross-sections. If these boundaries were apparent,
flattening ratios could be measured and compared across heat-treatment conditions.
4. Particle-particle and particle-substrate bonding and/or adhesion is another
measurable property that will affect mechanical properties of a CS deposit. It would
be beneficial to deposit powders onto substrate material that differs from the
feedstock material (i.e. spray Al 2024 onto Al 6061) so that the interface has greater
contrast in the micrographs. Particle-particle bonding can also be investigated by
creating FIB samples at specific locations within the deposit.
45
APPENDIX
Table 5. Standard heat-treatments for wrought alloys used to determine temperature and
hold/soak time for solid solution heat-treatment of Al 2024 powder [3, 5].
Material Temp TemperMaterial
Thickness [in]Min Max
Max quench
delay
ASTM standard 493°C (488-499°C) 0.25 - 0.50 55-65 65-75 15s
ASM Metals
Handbook495°C
each additional
0.50+30 +30 15s
ASTM standard 466°C (460-499°C) 0.25 - 0.50 60 70
ASM Metals
Handbook465, 480, 490°C 0.50 - 1.0 90 100
1.0 - 1.5 120 130
ASTM standard 529°C (516-579°C) 1.5 - 2.0 150 160
ASM Metals
Handbook530°C 2.0 - 2.5 180 190
Solid Solution Heat Treatment Soak Time [min]
Al 2024 T4
Al 7075 W, W51
Al 6061 T4
Metals
Handbook
Vol. 4
15sASTM
standard
Material Temp TemperMaterial
Thickness [in]Min Max
Max quench
delay
ASTM standard 493°C (488-499°C) 0.25 - 0.50 55-65 65-75 15s
ASM Metals
Handbook495°C
each additional
0.50+30 +30 15s
ASTM standard 466°C (460-499°C) 0.25 - 0.50 60 70
ASM Metals
Handbook465, 480, 490°C 0.50 - 1.0 90 100
1.0 - 1.5 120 130
ASTM standard 529°C (516-579°C) 1.5 - 2.0 150 160
ASM Metals
Handbook530°C 2.0 - 2.5 180 190
Solid Solution Heat Treatment Soak Time [min]
Al 2024 T4
Al 7075 W, W51
Al 6061 T4
Metals
Handbook
Vol. 4
15sASTM
standard
46
Figure 24. High magnification SEM images of CS deposits made from three different Al 2024
powders: (a) as-received, (b) solution heat-treated, and (c) annealed. Cross-sections were cut
parallel to the spray direction, then polished and etched to reveal microstructure.
(a) (b)
(c)
47
Figure 25. High magnification optical micrographs of CS deposits made from three different
Al 2024 powders: (a) as-received, (b) solution heat-treated, and (c) annealed. Cross-sections
were cut parallel to the spray direction, then polished and etched to reveal microstructure. Black
spots in images (b) and (c) are most-likely a result of the polish-etch procedure.
(a) (b)
(c)
48
Table 6. Complete list of tensile samples and their corresponding cross-section geometries.
49
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