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Materials Science and Engineering A 535 (2012) 108 114

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering A

journa l h o me pa ge: www.elsev ier .com/ locate /msea

orosity reduction and mechanical properties improvement in die cast enginelocks

.A. Irfana,,1, D. Schwamb,2, A. Karvec, R. Ryderc

Department of Mechanical Engineering, Qassim University, Saudi ArabiaDepartment of Material Science and Engineering, Case Western Reserve University, Cleveland, OH, USANemak USA High Pressure Die Casting Operations, Sylacauga, AL, USA

r t i c l e i n f o

rticle history:eceived 26 July 2011eceived in revised form 2 December 2011ccepted 3 December 2011vailable online 19 December 2011

a b s t r a c t

This paper presents an experimental investigation into the reduction of porosity and improvement ofmechanical properties of die cast engine blocks. The focus of this research was to seek improvement inmechanical properties of aluminum die cast engine blocks by a reduction in the secondary dendrite armspacing, modification of the acicular Si microstructure in the eutectic phase and reduction of porosity.eywords:echanical characterizationluminum alloys

Introducing additional cooling in the thick sections of the casting resulted in reduction of secondarydendrite arm spacing and consequent improvement of mechanical properties. However the effect ofincreased cooling faded with increasing distance from the core. 3D porosity measurements using CT Scanshowed significant reduction in porosity in sections incorporating a cooling core. Eutectic modificationwas achieved by modification with Sr. A target modification of 140 ppm led to significant improvementsastingin elongation.

. Introduction

Aluminum alloys are a prime choice for the manufacture ofngine blocks because of their reduced weight leading to lesseruel consumption [13]. Different manufacturers are using variousasting processes for the manufacture of engine blocks. Some of theost popular processes are die casting, precision sand casting and

ost foam casting. While die casting is very competitive for high vol-me production, porosity can affect mechanical properties unlessarefully controlled by optimizing the process parameters.

Aluminumsilicon alloys are widely used in automotive anderospace applications. They provide good fluidity, strength anductility as well wear and corrosion resistance. Further improve-ent of mechanical properties is a high priority for the metalasting industry. In the as cast state the microstructure oflSi alloys commonly displays a dendritic network, intermetallichases, Si particles as well as undesirable porosity and inclusions.n addition there are grains and grain boundaries which are knowno affect the yield strength of most metals. However for the castetals, yield strength is not significantly affected by grain sizeeduction [4]. The improvement in mechanical properties can bebtained primarily by reduction of dendrite arm spacing (DAS);articularly secondary dendrite arm spacing (SDAS), reduction of

Corresponding author. Tel.: +966 5933 79840.E-mail address: mairfan@qec.edu.sa (M.A. Irfan).

1 Country of origin: Pakistan.2 Country of origin: USA.

921-5093/$ see front matter 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2011.12.049 2011 Elsevier B.V. All rights reserved.

porosity and grain size and improvement of microstructure homo-geneity of the final product.

Thick-wall aluminum castings are often a viable alternative toheavier iron castings in structural applications. Engine blocks areamong the most common automotive components to have beenconverted from ferrous alloys to aluminum. Product design loadingrequires these castings to provide acceptable mechanical proper-ties in the thick sections. Die casting competes with Low Pressure,Precision Sand and PRC/VRC in delivering these high integrity alu-minum components at competitive prices. This paper presents anexperimental study to improve the mechanical properties of diecast ADC 12 aluminum blocks. ADC 12 is a die cast alloy as perJapan JIS5032 composition: Cu 1.53.5, Si 9.612.0, Mg 0.3 max, Zn1.0 max, Fe 0.9 max, Mn 0.5 max, Sn 0.3 max, Al remainder. ADC12 is primarily used for pressure die casting. It is similar to LM2 asper BS 1490, Aluminum Association 384.0, ISO AlSi10Cu2Fe, and AC46100 as per EN 1706. The engine blocks were heat treated to T5.

Production constraints limited the variables in the study to ther-mal control, heat treatment and alloy modification. The qualityimprovements were documented with detailed mechanical prop-erty evaluation including tensile, yield, elongation and 3D porositymeasurements.

1.1. The effect of cooling rates on dendrite arm spacing of

aluminum alloys

A number of researchers have associated mechanical propertyimprovements with reduction in dendrite arm spacing. Goulart

dx.doi.org/10.1016/j.msea.2011.12.049http://www.sciencedirect.com/science/journal/09215093http://www.elsevier.com/locate/mseamailto:mairfan@qec.edu.sadx.doi.org/10.1016/j.msea.2011.12.049

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M.A. Irfan et al. / Materials Science

t al. [4] developed experimental expressions to correlate theTS with secondary DAS for AlSi hypoeutectic alloys. They havehown that the dendrite fineness can be of more importance inmprovement of mechanical properties than grain size. Improvinghe DAS leads to a shorter periodicity of microsegregation. Theyemonstrated an almost linear relationship between DAS and solid-fication time. The DAS increased from 20 m to 50 m when theolidification time was increased from 10 to 100 s for an Al9 wt.%i. The UTS increased in a linear fashion with the decrease in DAS.owever the Yield Strength did not show a significant improve-ent with the reduction in DAS.Bamberger et al. [5] measured minimum solidification times of

0 s with a DAS of 50 m for Al9.55 wt.% Si alloy. They also showedhat the following relation holds for AlSiMg die castings.

= a t0.43s (1)here is the secondary dendrite arm spacing, ts is the local solid-

fication time and a is a material constant. The values of a werehown to be 15.5 for a Al5 wt.% Si and 11.5 for Al9 wt.% Si. Usinghe above mathematical model on the variation of DAS with solid-fication time for the Al9 wt.% Si casting, it can be estimated thatolidification times of 6 s and 2 s correspond to DAS of 30 and 18 mespectively. This relatively simple formulation enables the calcula-ion of fineness of microstructure directly from solidification times.hey further illustrated that heat flux from the casting to the chillepends on the initial temperature of the chill and the thermaliffusivities of the cast and chill material.Zhang et al. [6,7] demonstrated a faster cooling technology using

copper mold cooled by a phase transition medium (Sn) for solidi-cation of cast aluminum 356 alloy. They demonstrated that DAS isainly affected by cooling rate. A higher cooling rate and a shorter

olidification time leads to a more refined microstructure. Theirxperiments measured a DAS of 18 m at a cooling rate of 100 K/snd a very fine DAS of 8 m at a cooling rate of 650 K/s. They furtherlaborated that the coarser eutectic fibrous Si phase that surroundshe -Al greatly deteriorates the ultimate strength and elongationf the alloy. However with faster cooling rates the microstructurebtained is more refined leading to improved UTS.

.2. Factors affecting solidification rate

Solidification rate during casting depends on the rate of heatissipation. A number of factors influence heat transfer duringolidification, which include (1) thermal diffusivity of the liquidetal, (2) thermal diffusivity of the solidified metal, (3) the mold-etal interface heat transfer coefficient, (4) thermal conductivity

diffusivity of the mold material (depends on the efficiency ofxternally cooled mold), and (5) convection and radiation of theold material to the atmosphere [8]. Although all of the above

isted factors contribute towards the rate of solidification, it is theold-metal interface that has the dominant thermal resistance.By using equations of heat transfer the solidification time can

e modeled as [8]

= Lh(Tf To)

.(V

A

)(2)

here and L are the density and latent heat of solidification ofhe metal, h is the mold-metal interface heat transfer coefficient,f and To are the solidification temperature and temperature of theold respectively. (V/A) is the ratio of volume to surface area and

epresents the thickness solidified. It can be seen that for a givenetal-mold interface, the only variable that can be controlled ishe mold temperature To. Reduction of mold temperature To is theost feasible way to reduce solidification time.It should be noted that the mold-material interface is highly

ynamic (so is h); it starts with contact at a number of solidifiedngineering A 535 (2012) 108 114 109

points, which decrease as the metal contracts during shrinkage.Because of this volumetric shrinkage the mold-metal contact willdiminish during the later stages of cooling. The above equation istherefore not valid throughout the solidification stages, but canonly be used as a rough theoretical estimate of solidification time.In pressure assisted casting processes including die casting, themold-metal contact is improved.

The above literature confirms the effect of cooling rates on DASof aluminum alloys. High cooling rates lead to low solidificationtimes which in turn lead to smaller DAS. Smaller DAS have a bene-ficial effect on UTS of die castings. In the die casting process, coolingrates are mainly controlled by the amount of water flowing throughthe die cooling channels. Further experimentation/study is neededto establish the correlation between efficient heat transfer mecha-nisms of die cooling and solidification times of die castings. Willthere be a cap on the maximum cooling rate that is physicallypossible in a die casting system, is a question that needs furtherinvestigation.

1.3. Grain refinement and eutectic modification

There are two approaches for improving the microstructure ofdie cast alloys. First is grain refinement which reduces the grainsize. The second is eutectic modification which reduces the nee-dle shaped Si particles into fine fibrous and lamellar morphology.The sharp Si needles are preferential sites for crack initiation andpropagation, thus leading to early failure of the material. Lozanoand Pena [9] studied the roles of Ti as a refiner and Sr as a modi-fier in Al12 % Si. Their studies indicated reduction in size of -Aldendrites from 553 m2 to 204 m2 by an addition of 0.024% of Tito the commercial alloy. Addition of 0.02% Sr led to a reduction inSi cuboid size from 3.7 m to 2.2 m. Addition of both Ti (0.04%)and Sr (0.04%) led to a grain size of 307 m2 and Si cuboid sizeof 2.4 m. In another study Lozano and Pena [10] illustrated thataddition of both Ti (0.05%) and Sr (0.05%) led to an improvement inyield strength from 90 MPa in the control alloys to 128 MPa in therefined and modified alloy. The yield strength improvement wascomplimented by an increase in elongation from 1.77% in the con-trol alloys to 4.31% in the refined and modified alloy. Eidhed [11]demonstrated that the addition of Sr also leads to reduction of -Al5FeSi phase. The -Al5FeSi phase normally occurs as plates andis known to obstruct inter-dendritic feeding which in turn leads toformation of porosity.

Finer grain structures, with equiaxed grain size, fibrous Si nee-dles and smaller iron rich plates lead to many benefits, includingimproved metal feeding and uniform distribution of second phasesand microporosity. All these factor have a beneficial effect on theyield strength, tensile strength and percentage elongation of diecastings. Ti as a grain refiner and Sr as a microstructure modifierhave shown to substantially improve the mechanical properties ofcastings.

2. Initial mechanical testing, porosity and microstructuralanalysis

2.1. Mechanical testing

Tensile samples were extracted from V-6 engine block saddlesas shown in Figs. 1 and 2. These tensile tests were carried out as perASTM E-8 on round, threaded samples. The mechanical properties

of samples taken from five engine blocks are summarized in Table 1.The average Ultimate Tensile Strength (UTS), average Yield Strength(YS) and average percentage elongation was 200 MPa, 158 MPa and0.8% respectively. The average is based on a sample size of 30 tests.

110 M.A. Irfan et al. / Materials Science and Engineering A 535 (2012) 108 114

Fig. 1. Front (a) and bottom (b) views of a die cast engine block.

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SDAS is basically a function of cooling rate. Different casting pro-cesses having different cooling rates will yield a range of SDAS. Aliterature survey was carried out to find out the SDAS obtainedFig. 2. (a) Location of tensile sample take

.2. Area porosity measurements

Tensile samples returned from the mechanical tests wereectioned, mounted for metallography, ground and polished suc-essively to 0.5 m finish. For each sample 10 micrographs wereaken across the diameters. Pores greater than 20 m were mea-ured using an image analysis software. The results of percentageorosity and their correlation with mechanical properties arehown in Fig. 3. Due to the scatter in data it is hard to make anyeaningful conclusions based on this data. The porosity measure-ents are taken on one particular section of the tensile sample thatay not representative of the whole volume of the tensile sample.echanical deformation is a volumetric process (3D) and may notorrelate well to area measurements (2D). It might be noted thathe scatter in much less when the UTS is measured against SDAS ashown in Fig. 4.able 1echanical properties of V6 engine blocks.

TS (MPa) YS (MPa) % Elong.

Ave 200 158 0.80Max 246 187 1.03Min 146 145 0.38SD 20 9 0.18 the saddle. (b) A broken tensile sample.

2.3. Secondary dendrite arm spacing (SDAS)

Secondary dendrite arm spacing (SDAS) was a measured on thepolished samples using image analysis software. Five micrographswere taken for each tensile sample and three measurements weretaken on each micrograph, thus each data point in Fig. 4 is an aver-age of 15 measurements. SDAS correlates relatively well with theUltimate Tensile Strength (UTS) of the samples. Similar results wereobtained for the Yield Strength (YS) and percentage elongation. Itshould be noted that a reduction of 5 m in SDAS can lead to animprovement of 50 MPa in UTS.Fig. 3. Variation of ultimate tensile strength with the percentage pore area.

M.A. Irfan et al. / Materials Science and E

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4. Effect of eutectic modificationFig. 4. Variation of UTS with SDAS.

hrough various casting processes [6,1214]. Fig. 5 illustrates SDASeported for different casting processes. The second bar corre-ponds to the values obtained in the experimental measurements ofhis project. Die casting stands out among various casting processesor its low SDAS and hence better mechanical properties.

.4. Microstructure

In order to gain a better understanding of the material response,canning Electron Microscopy and Energy Dispersive Spectroscopyere carried out on samples obtained from the engine block. Fig. 6aepicts a typical microstructure obtained from a die cast ADC12ample. There are three features to be noted. First, the aciculartructure of the Si known to be detrimental to mechanical prop-rties. Second, the iron rich phase (Fig. 6b) that appears as platesnd poses an obstruction to inter-dendritic metal flow during solid-fication, leading to micro-shrinkage porosity. Finally, the irregularhaped Cu-Mg that forms a discontinuous interface giving rise tohrinkage porosity.

.5. Fractography

Fractography was carried out on the broken tensile samplessing Scanning Electron Microscopy. It can be seen in Fig. 7, thathe fracture surface is generally brittle, with limited evidence of

uctility. The fracture surface also reveals a few relatively largeores, indicating volumetric growth of pre-existing pores duringhe tensile test.

Fig. 5. Comparison of SDAS reported for various casting processes (Irfan [1ngineering A 535 (2012) 108 114 111

3. Effect of cooling rates on SDAS

3.1. Introduction of cooling core

Having established an improvement in mechanical propertiesdue to reduction in SDAS (Fig. 4), the next logical step was to seekreduction of SDAS in the thicker section of the castings (saddles) bythe substitution of the solid steel core with a water-cooled core asshown in Fig. 8.

Saddles from blocks cast with the water-cooled H13 steel corewere sampled for testing. The sectioned saddle was cut into threepieces and the pieces were ground and polished for microscopy.Fig. 9 shows the variation of SDAS in the middle piece of the sad-dle measured at 3 O clock position. The SDAS in the cooled corewas a few microns smaller than measured in the solid core. Even areduction of a few microns in SDAS can lead to an improvement of1020 MPa in the UTS. However, the cooling effect fades at increas-ing distance form the core. The mechanical properties of the engineblocks with un-cooled core and with a cooled core are summarizedin Table 2. The advantages of using a cooled core can clearly be seenfrom the improved mechanical properties.

Fig. 10 shows a comparison of SDAS obtained in different engineblocks studied in this research. Three different types of engineblocks are represented in this test. First is the 4-cylinder (I-4) engineblock with a cooling core, second is the 4-cylinder (I-4) engine blockwithout a cooling core and third is a V6 engine block also without acooling core. For each of the tensile samples taken from these blocksthe average SDAS was measured. The minimum SDAS and hencethe maximum mechanical properties are obtained in the smallerengine block with the water-cooled core.

3.2. Volumetric porosity measurements around cooling core

Volumetric porosity was measured by using a 3D ComputedTomography Scan. The volumetric porosity data is given in Table 3.It is clearly evident that a cooled core reduces the volumetric poros-ity almost by half as compared to the porosity in an un-cooledcore.Having studied the effects of cooling core on mechanical prop-erty improvements it was decided to further study effect of eutectic

3], Oswalt and Misra [12], Radhakrishna et al. [14], Zhang et al. [6]).

112 M.A. Irfan et al. / Materials Science and Engineering A 535 (2012) 108 114

Fig. 6. (a) SEM micrograph of ADC 12 die cast microstructure. (b) -Phase iron plates.

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same mixing technique was carried out. However the metal flowfrom the launder was blocked in this case to achieve controlledmodification. Ten pounds of master alloy was added to 7000 lbsof holding furnace. Alu-Delta readings confirmed successfulFig. 7. SEM microgra

odification on mechanical properties and finally compare the costersus benefit of the above mentioned interventions.

.1. Trials at production facility

After conducting some initial trials at Case it was decided tory eutectic modification in die cast engine blocks at the produc-ion facility in Nemak. The first modification was attempted on I-4ngine blocks. Five pounds of Master alloy Al15Sr was added to000 pound holding furnace for a target modification of 140 ppmf Sr. The well geometry was not conducive to efficient mixing andhe flow from the lauder was not stopped. On-line measurement

f eutectic modification was carried out by using Alu-Delta appa-atus. The chemistry sample confirmed 101 ppm of Sr which wasess than the target modification. The mixing technique developedt Case was carefully followed.

Fig. 8. Section of a saddle showing location of the cooling core. the fracture surface.

Table 4 shows the comparison of mechanical properties of I-4 engine blocks with and without Sr addition. The base line forcomparison is the previous data [13] from Design of Experimentsis summarized in the bottom row. It might be noted that the targetmodification of 140 ppm could not be achieved and the resultantSr in the melt was about 100 ppm. The improvement in UTS andelongation was not substantial in this case.

Next modification was attempted on V-6 engine blocks. TheFig. 9. Variation of SDAS with distance from the cooling core.

M.A. Irfan et al. / Materials Science and Engineering A 535 (2012) 108 114 113

Table 2Mechanical properties of I-4 engine blocks with and without a cooling core.

Engine block with un-cooled core

ID-06 Tensile (MPa) Yield (MPa) % Elong.

J1-F 190 0.7J1-R 186 0.0J2-F 285 269* 0.5J2-R 212 J4-F 115** 0.6J4-R 195 0.5J5-F 257 J5-R 218 0.5Ave. 220 0.47

Engine block with water cooled core

ID-07 Tensile (MPa) Yield (MPa) % Elong.

J1-F 230 0.5J1-R 276 207 1.7J2-F 247 241 BOGMJ2-R 266 215 1.2J4-F 207 0.5J4-R 227 209 0.5J5-F 272 220 0.5J5-R 252 210 0.5Ave. 247 217 0.77

* Only piece with good yield, all others broke prior to 0.2% offset.** 115 MPa, an outlier, not included in finding the average.

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Table 3Volumetric porosity data.

38406: Un-cooled core 38407: Cooled core

Material volume (mm3) 39,358 39,414Pore volume (mm3) 482 266Volumetric porosity (%) 1.21 0.67

Table 4Summary of mechanical properties with and without modification of I-4 blocks.

Specimen ID Modification(Target 0.014%)

UTS (MPa) YS (MPa) % Elong.

Block # 08 w/o Sr 259 187 1.5Block # 26 w/ Sr (0.0101%) 270 214 1.75Previous DOE data:average from 4engine blocks

Nil 268 218 1.22

Table 5Summary of mechanical properties with and without modification of V-6 blocks.

Specimen ID Modification(Target 0.014%)

UTS (MPa) YS (MPa) % Elong.

Block # 30 w/o Sr 219 180 0.5ig. 10. A comparison of SDAS and UTS obtained with different engine blocks.

odification and the chemistry samples indicated 155 ppm Sr

odification. With a more controlled modification in the V-6 blocks

he improvement in elongation was three times as shown in Table 5.ere the target modification of 140 ppm was well achieved. Much

Fig. 11. (a) Microstructure of ADC-12 without the modificatBlock # 35 w/ Sr (0.0155%) 224 163 1.5Previous data: Table 1 w/o Sr 200 158 0.8

of this improvement can be attributed to breaking down of acicularSi needles into fibrous structure as shown in Fig. 11.

5. Conclusions

The water cooled core reduces SDAS and porosity thus improv-ing mechanical properties. However the cooling effect diminisheswith increasing distance from the core. Higher cooling rates anddeeper penetration can be achieved by using cores made of higherthermal conductivity alloys and/or higher cooling water flow rates.SDAS can effectively be used as a predictor of mechanical prop-erties. Percent Area Porosity (2D) is not a reliable predictor ofmechanical properties due to the probabilistic and random natureof porosity at the section under observation. For a better cor-relation with mechanical properties (which are volumetric innature) it is recommended to measure volumetric porosity (3D)instead.

Addition of Sr is effective in die casting for modifying the aci-cular Si needles into a fibrous structure. Addition of 0.0155% Sr

(target 0.014%) showed an improvement in elongation from 0.5% to1.5%. Among the two possible directions for mechanical propertyimprovements, i.e. vent core cooling and Sr addition, the former is

ion. (b) Microstructure of ADC-12 with Sr refinement.

1 and E

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ore suitable for a production process as it reduces cycle time andoes not add cost.

cknowledgements

The authors wish to acknowledge the 3D porosity CT scan andnalysis done by Varian Inc. Financial support by North Ameri-an Die Casting Association (NADCA) and Fulbright Organizations gratefully acknowledged.

eferences[1] R. Donahue, SAE 200 (2000).[2] H. Kurita, H. Yamagata, H. Arai, T. Nakamura, Hypereutectic Al20% Si Alloy

Engine Block Using High Pressure Die Casting, SAE International Congress 2004-01-1028, 2004.

[[[

[

ngineering A 535 (2012) 108 114

[3] P. Labelle, E. Baril, A. Bunk, AJ (MgAlSr) Alloy System Used for New EngineBlock, SAE International Congress 2004-01-0659, 2004.

[4] P.R. Goulart, J.E. Spinelli, W.R. Osorio, A. Garcia, Materials Science and Engi-neering A 421 (2006) 245253.

[5] M. Bamberger, I. Minkoff, M.M. Stupel, Journal of Material Science 21 (1986)27812786.

[6] B. Zhang, M. Garro, A. Giglio, C. Tagliano, Effect of Dendrite Arm Spacing onMechanical Properties of Aluminum Alloy Cylinder Heads and Engine Blocks,SAE World Congress, 2005.

[7] L.Y. Zhang, Y.H. Jiang, Z. Ma, S.F. Shan, Y.Z. Jia, C.Z. Fan, W.K. Wang, Journal ofMaterial Processing Technology 207 (2008) 107111.

[8] R. Asthana, A. Kumar, N. Dahotre, Material Science in Manufacturing, AcademicPress, 2006.

[9] J.A. Lozano, B.S. Pena., Materials Characterization 56 (2006) 169177.10] J.A. Lozano, B.S. Pena., Materials Characterization 57 (2006) 218226.

11] W. Eidhed, Journal of Material Science Technology 24 (1) (2008) 4547.12] K.J. Oswalt, M.S. Misra, AFS Transactions 88 (1980) 845862.13] M. Irfan, D. Schwam, A. Karve, R. Ryder, Mechanical Property Improvements in

Die Cast Engine Blocks, NADCA Technical Report. Project 146, 2009.14] K. Radhakrishna, S. Seshan, M.R. Seshadri, AFS Transactions 8 (1980) 8087.

Porosity reduction and mechanical properties improvement in die cast engine blocks1 Introduction1.1 The effect of cooling rates on dendrite arm spacing of aluminum alloys1.2 Factors affecting solidification rate1.3 Grain refinement and eutectic modification

2 Initial mechanical testing, porosity and microstructural analysis2.1 Mechanical testing2.2 Area porosity measurements2.3 Secondary dendrite arm spacing (SDAS)2.4 Microstructure2.5 Fractography

3 Effect of cooling rates on SDAS3.1 Introduction of cooling core3.2 Volumetric porosity measurements around cooling core

4 Effect of eutectic modification4.1 Trials at production facility

5 ConclusionsAcknowledgementsReferences