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L Journal of Alloys and Compounds 287 (1999) 284–294 Microstructure and mechanical properties of hypo / hyper-eutectic Al–Si alloys synthesized using a near-net shape forming technique * M. Gupta , S. Ling Department of Mechanical and Production Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 30 May 1998; received in revised form 30 January 1999 Abstract In the present study, three aluminum–silicon alloys containing 7, 10 and 19 wt % silicon were synthesized using a novel technique commonly known as disintegrated melt deposition technique. The results following processing revealed that a yield of at least 80% can be achieved after defacing the shrinkage cavity from the as-processed ingots. Microstructural characterization studies conducted on the as-processed samples revealed an increase in the volume fraction of porosity with an increase in silicon content. Porosity levels of 1.07, 1.51 and 2.65% attained in the case of Al–7Si, Al–10Si, and Al–19Si alloys indicates the near-net shape forming capability of the disintegrated melt deposition technique. The results of aging studies conducted on the aluminum–silicon alloys revealed similar aging kinetics irrespective of different silicon content. Results of ambient temperature mechanical tests demonstrate an increase in matrix microhardness and 0.2% yield stress and decrease in ductility with an increase in silicon content in aluminum. Furthermore, the results of an attempt to investigate the effect of extrusion on Al–19Si alloy revealed that the extrusion process significantly assists in reducing porosity and improving microstructural uniformity, 0.2% yield strength, ultimate tensile strength and ductility when compared to the as-processed Al–19Si alloy. The results of microstructural characterization and mechanical properties of aluminum–silicon alloys were finally correlated with the amount of silicon in aluminum and secondary processing technique. 1999 Elsevier Science S.A. All rights reserved. Keywords: Disintegrated melt deposition; Microstructure; Mechanical behavior; Aluminum–silicon alloys 1. Introduction depends on the level of microstructurally governed end properties, cost effectiveness, industrial adaptability and The ability of silicon to reduce the density and coeffi- reproducibility in terms of microstructure and properties cient of thermal expansion and to improve the hardness, (such as physical, electrical, magnetic, mechanical etc.) ambient temperature mechanical properties such as [10]. For example, liquid phase processes such as conven- modulus and strength, thermal stability and wear resistance tional casting are cost effective but can not be used to of aluminum had been catalytic in engendering consider- make components for critical applications since the prop- able interest in the materials science community to explore erties level that can be obtained are inferior as a result of the Al–Si family of alloys for possible applications in coarser microstructural features commonly associated with automotive, electrical and aerospace industries [1–4]. The conventionally cast materials. The solid phase processes, addition of silicon is made in both the hypoeutectic and such as powder based techniques, helps in realizing hypereutectic range depending primarily on the end appli- superior properties but have limitations related to the cation [1–6]. dimensions of the component and in addition involves high The existing literature survey indicates that the synthesis cost. Two phase processes, on the other hand, are techni- of Al–Si alloys is carried out principally by liquid phase cally innovative and hold the promise to synthesize bulk [7], liquid–solid phase [2–4], solid phase [1], and rapid materials with superior properties, however, very limited solidification [8,9] techniques. The selection of processing information is available regarding the processing, micro- technique for a given constitutional formulation, however, structure and properties of materials synthesized using them. In order to circumvent the disadvantages associated with these techniques, a relatively new technique common- *Corresponding author. Tel.: 165-874-6358; fax: 165-779-1459. E-mail address: [email protected] (M. Gupta) ly known as disintegrated melt deposition (DMD) is used 0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388(99)00062-6

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Page 1: Microstructure and mechanical properties of hypo/hyper ......Microstructure and mechanical properties of hypo/hyper-eutectic Al–Si ... of secondary processing on the microstructure

LJournal of Alloys and Compounds 287 (1999) 284–294

Microstructure and mechanical properties of hypo/hyper-eutectic Al–Sialloys synthesized using a near-net shape forming technique

*M. Gupta , S. LingDepartment of Mechanical and Production Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore

Received 30 May 1998; received in revised form 30 January 1999

Abstract

In the present study, three aluminum–silicon alloys containing 7, 10 and 19 wt % silicon were synthesized using a novel techniquecommonly known as disintegrated melt deposition technique. The results following processing revealed that a yield of at least 80% can beachieved after defacing the shrinkage cavity from the as-processed ingots. Microstructural characterization studies conducted on theas-processed samples revealed an increase in the volume fraction of porosity with an increase in silicon content. Porosity levels of 1.07,1.51 and 2.65% attained in the case of Al–7Si, Al–10Si, and Al–19Si alloys indicates the near-net shape forming capability of thedisintegrated melt deposition technique. The results of aging studies conducted on the aluminum–silicon alloys revealed similar agingkinetics irrespective of different silicon content. Results of ambient temperature mechanical tests demonstrate an increase in matrixmicrohardness and 0.2% yield stress and decrease in ductility with an increase in silicon content in aluminum. Furthermore, the results ofan attempt to investigate the effect of extrusion on Al–19Si alloy revealed that the extrusion process significantly assists in reducingporosity and improving microstructural uniformity, 0.2% yield strength, ultimate tensile strength and ductility when compared to theas-processed Al–19Si alloy. The results of microstructural characterization and mechanical properties of aluminum–silicon alloys werefinally correlated with the amount of silicon in aluminum and secondary processing technique. 1999 Elsevier Science S.A. All rightsreserved.

Keywords: Disintegrated melt deposition; Microstructure; Mechanical behavior; Aluminum–silicon alloys

1. Introduction depends on the level of microstructurally governed endproperties, cost effectiveness, industrial adaptability and

The ability of silicon to reduce the density and coeffi- reproducibility in terms of microstructure and propertiescient of thermal expansion and to improve the hardness, (such as physical, electrical, magnetic, mechanical etc.)ambient temperature mechanical properties such as [10]. For example, liquid phase processes such as conven-modulus and strength, thermal stability and wear resistance tional casting are cost effective but can not be used toof aluminum had been catalytic in engendering consider- make components for critical applications since the prop-able interest in the materials science community to explore erties level that can be obtained are inferior as a result ofthe Al–Si family of alloys for possible applications in coarser microstructural features commonly associated withautomotive, electrical and aerospace industries [1–4]. The conventionally cast materials. The solid phase processes,addition of silicon is made in both the hypoeutectic and such as powder based techniques, helps in realizinghypereutectic range depending primarily on the end appli- superior properties but have limitations related to thecation [1–6]. dimensions of the component and in addition involves high

The existing literature survey indicates that the synthesis cost. Two phase processes, on the other hand, are techni-of Al–Si alloys is carried out principally by liquid phase cally innovative and hold the promise to synthesize bulk[7], liquid–solid phase [2–4], solid phase [1], and rapid materials with superior properties, however, very limitedsolidification [8,9] techniques. The selection of processing information is available regarding the processing, micro-technique for a given constitutional formulation, however, structure and properties of materials synthesized using

them. In order to circumvent the disadvantages associatedwith these techniques, a relatively new technique common-*Corresponding author. Tel.: 165-874-6358; fax: 165-779-1459.

E-mail address: [email protected] (M. Gupta) ly known as disintegrated melt deposition (DMD) is used

0925-8388/99/$ – see front matter 1999 Elsevier Science S.A. All rights reserved.PI I : S0925-8388( 99 )00062-6

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M. Gupta, S. Ling / Journal of Alloys and Compounds 287 (1999) 284 –294 285

in the present study to synthesize Al–Si alloys in both as the lubricant. Extrusion was conducted in order to studyhypo- and hypereutectic composition range. This tech- the effect of secondary processing on the microstructuralnique, in the past, has been successfully utilized to and mechanical properties variation of as-processed Al–Sisynthesize monolithic and reinforced materials [11,12] and alloy.involves, in principal, the disintegration of superheatedmolten metal slurry using inert gas jets followed by its

2.4. Quantitative assessment of siliconsubsequent deposition on the metallic substrate. Thedynamic disintegration and deposition steps enables this

Quantitative assessment of Si in the as-processed andtechnique to synthesize bulk materials with improvedextruded Al–Si samples was carried out using standardizedmicrostructural homogeneity when compared to conven-energy dispersive spectroscopy (EDS) method.tional casting techniques [11,12].

Accordingly, the objective of the present study was toinvestigate the microstructure and mechanical properties of

2.5. Density measurementthe disintegrated melt deposited Al–Si alloys (both inhypo- and hypereutectic composition range) in order to

The densities of the as-processed and extruded Al–Siassess the feasibility of the disintegrated melt deposition

samples were measured by Archimedes’ principle totechnique to synthesize the Al–Si family of alloys. Par-

quantify the volume fraction of porosity [6,11,12]. Theticular emphasis was placed, in addition, to study the effect

density measurements involved weighing polished cubes ofof secondary processing on the microstructure and me-

the extruded samples in air and when immersed in distilledchanical properties of the hypereutectic (Al–19Si) alloy

water. The densities, derived from the recorded weights,synthesized in the present study.

were then compared to the theoretical densities from whichthe volume fractions of porosity were calculated. Thesamples were weighed using an A&D ER-182A electronic

2. Experimental procedurebalance to an accuracy of 60.0001 g.

2.1. Materials

2.6. Aging studiesIn this study, an aluminum alloy AA1050 ($99.5 wt %

Al) was used as the base alloy and silicon ($98.5 wt % Si) Aging studies were carried out in order to obtain thewas used as an addition element to synthesize hypo- and peak hardness time for the as-processed and extrudedhypereutectic Al–Si alloys. Al–Si samples. Specimens (10 mm diameter37 mm

height) were solutionized for 1 h at 5298C, quenched in2.2. Processing cold water and aged at 1608C for various intervals of time.

Rockwell superficial hardness measurements were madeIn the present study, synthesis of hypo- and hypereutec- using a 1.58 mm diameter steel ball indenter with a 15 kg

tic Al–Si alloys with starting weight percentages of 7, 10 load using a GNEHM HORGEN digital hardness testerand 20 wt % of Si was carried out using the DMD following ASTM standard E18-92. A minimum of threetechnique. The synthesizing procedure involved: super- hardness readings were taken for each specimen.heating of properly cleaned elemental materials to atemperature of 9506108C in graphite crucible, impellerassisted stirring to ensure complete mixing of elemental 2.7. Microstructural characterizationmaterials followed by argon gas-assisted melt disinte-gration at 0.18 m from the melt pouring point and Microstructural characterization studies were conductedsubsequent deposition in a metallic mould (55 mm on the as-processed and extruded Al–Si samples in thediameter375 mm long) located at 0.25 m from the gas peak aged condition to investigate the grain morphology,disintegration point. The experiment was carried out under presence of porosity, morphological characteristics andcontrolled atmospheric conditions. The Al–Si alloy ingots distribution of the secondary phases, and Si–Al interfacialobtained following processing were weighed in order to characteristics.determine the deposited yield of the starting raw materials. Microstructural characterization studies were primarily

accomplished using an optical microscope and a JEOL2.3. Secondary processing scanning electron microscope equipped with EDS. The

samples were metallographically polished prior to exami-Al–Si alloy ingot with starting weight percentage of nation. Microstructural characterization of the samples was

20% silicon was machined to a diameter of 35 mm and conducted in both etched and unetched conditions. Etchingthen hot extruded at 3508C employing a reduction ratio of was accomplished using Keller’s reagent [0.5 HF–1.513:1 on a 150 ton hydraulic press using colloidal graphite HCl–2.5 HNO –95.5 H 0].3 2

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286 M. Gupta, S. Ling / Journal of Alloys and Compounds 287 (1999) 284 –294

Table 12.8. Mechanical behaviorResults of the density and porosity determination

Alloy Processing Wt % Si Density PorosityVickers microhardness of the matrix of as-processed and23designation condition (g cm ) (vol %)extruded Al–Si samples was determined on a Matsuzawa

MXT50 Automatic Digital microhardness tester using an Al–7Si As-processed 7 2.6460.01 1.07Al–10Si As-processed 10 2.6260.02 1.51indentation load of 100 g. Vickers microhardness measure-Al–19Si As-processed 19 2.5560.01 2.65ments were made in order to provide insight into theAl–19Si(Ext) Extruded 19 2.6060.06 0.65

ability of secondary phases to strengthen the metallicmatrix.

Smooth bar tensile properties were determined on the 3.3. Quantitative assessment of siliconas-processed and extruded samples in the peak agedcondition following ASTM standard E8M-91. Tensile tests The results of standardized EDS chemical analysiswere conducted using an automated servohydraulic Instron conducted for Si element determination in the as-processed8501 testing machine on 4 mm diameter specimens using a Al–Si alloys with starting silicon weight percentages of 7,crosshead speed of 0.254 mm per minute. 10 and 20 and extruded Al–Si alloy (with starting silicon

weight percentages of 20) revealed that approximately 7,10, 19 and 19 wt % Si was retained, respectively, follow-2.9. Fracture behavioring DMD processing (see Table 1). Accordingly, thesematerials will now be referred as Al–7Si, Al–10Si, Al–Fracture surface characterization studies were carried19Si, and Al–19Si(Ext) in the forthcoming sections.out on the tensile fractured samples in order to provide

insight into the various fracture mechanisms operative3.4. Density measurementduring tensile loading of the peak aged samples. Fracture

surface characterization studies were primarily accom-The results of density measurements conducted on theplished using a JEOL scanning electron microscope

Al–7Si, Al–10Si, Al–19Si, and Al–19Si(Ext) samples andequipped with EDS.the volume percent of the porosity computed using theexperimentally determined density values are shown inTable 1.

3. Results

3.5. Aging studies3.1. Processing

The results of aging studies conducted on the as-pro-The deposited yield of the Al–Si alloys with starting cessed and extruded samples are shown in Fig. 1. The

weight percentages of 7, 10, and 20 wt % of silicon was results exhibit the presence of a hardness peak at 9 h for allfound out to be 89, 88 and 86%, respectively. The the samples. Both the as-solutionized and peak hardnesspreforms in all the three cases were associated with a small values were found to increase with an increase in theshrinkage cavity on the top. After defacing the ingots so as silicon content in aluminum and from the as-processed toto remove the shrinkage cavity, the final yield was extruded condition in the case of hypereutectic Al–19Sidetermined to be 85, 84 and 80%, respectively. The overall alloy. The results also reveal an increase in the magnitudedimensions of the disintegrated melt deposited preforms of age hardening with an increase in the weight percentagefollowing defacing were approximately 35 mm in height of silicon in the case of as-processed samples. Theand 55 mm in diameter. The preform of the Al–Si alloywith starting weight percentage of 20 wt % Si wassubsequently machined to a diameter of 35 mm so as to fitin the extrusion container. The specimens for heat treat-ment, microstructural analysis and mechanical propertiescharacterization were removed randomly from the as-pro-cessed and extruded rods.

3.2. Macrostructure

Macrostructural characterization conducted on the ma-chined and polished surfaces of as-DMD processed sam-ples did not reveal the presence of either macropores or themacrosegregation of silicon across the vertical andhorizontal sections. Fig. 1. Aging curves of as-processed and extruded Al–Si samples.

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M. Gupta, S. Ling / Journal of Alloys and Compounds 287 (1999) 284 –294 287

percentage increase in hardness of the peak aged sampleswhen compared to that in the as-solutionized condition, forexample, was found to be 8.74, 19.91 and 29.91 forAl–7Si, Al–10Si and Al–19Si samples, respectively. Themagnitude of age hardening, however, was found to beminimum (5.65%) in the case of Al–19Si(Ext) samples(see Table 2).

3.6. Microstructural characterization

The results of optical and scanning electron microscopyrevealed the presence of a-Al dendrites and eutectic siliconphase in the case of Al–7Si and Al–10Si samples. Thepresence of dendritic structure precluded the determinationof matrix grain size. Figs. 2 and 3 show the representative Fig. 2. Optical micrograph showing the salient microstructural featuresoptical micrographs showing the salient microstructural exhibited by DMD processed Al–7Si samples.

features exhibited by Al–7Si and Al–10Si samples, re-spectively. In the case of hypereutectic as-processed Al–19Si alloy, the results of microstructural characterization(see Fig. 4) revealed the presence of primary silicon (Si)and eutectic silicon phases. The primary Si exhibited theblocky morphology while the eutectic silicon exhibitedneedle shape morphology. The size of the eutectic Si was,however, found to be comparatively larger when comparedto the hypoeutectic (Al–7Si and Al–10Si) alloys (seeTable 3). For the extruded Al–19Si samples, the results ofmicrostructural characterization studies revealed an in-crease in the volume fraction of the primary and eutecticsilicon phases and a reduction in their size when comparedto the as-processed Al–19Si samples (see Fig. 5 and Table3). Microstructural characterization studies, in addition,also revealed the presence of nearly equiaxed, randomlydistributed, non-connected micron size porosity in all thesamples investigated in the present study. The interfacialintegrity between primary Si and the aluminum matrix was Fig. 3. Optical micrograph showing the salient microstructural featuresfound to be good and only in some instances interfacially exhibited by DMD processed Al–10Si samples.

located voids were observed. The results of EDS pointanalyses conducted in the near-vicinity of primary Siparticles in the case of as-processed Al–19Si samples and 3.7. Mechanical behaviorextruded Al–19Si samples revealed the presence of segre-gation of silicon. One such representative variation in the The results of ambient temperature microhardness andamount of silicon with increasing distance from primary tensile testing on the as-processed Al–Si samples, aged toSi–Al interface observed in the case of Al–19Si(Ext) peak hardness, are summarized in Table 4. The results insamples is shown in Fig. 6. Table 4 reveal an increase in microhardness and 0.2%

Table 2Results of the aging studies

Alloy As-solutionized Peak hardness Peak aging Magnitude of agehardness (HR15T) (HR15T) time (h) hardening (HR15T)

Al–7Si 41.261.2 44.860.9 9 3.6Al–10Si 43.761.5 52.461.4 9 8.7Al–19Si 44.861.3 58.261.8 9 13.4Al–19Si(Ext) 56.660.3 59.860.8 9 3.2

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288 M. Gupta, S. Ling / Journal of Alloys and Compounds 287 (1999) 284 –294

Fig. 5. Optical micrograph showing the salient microstructural featuresFig. 4. Optical micrograph showing the presence of primary silicon andexhibited by DMD processed Al–19Si(Ext) samples.eutectic silicon in the case of DMD processed Al–19Si samples.

yield stress (0.2% YS) and decrease in ductility with anincrease in the amount of silicon in the aluminum matrix.The ultimate tensile strength (UTS), however, increasedwith an increase in silicon content from 7 to 10 wt % andwas found to be minimum in the case of as-processedAl–19Si samples. Following extrusion, Al–19Si samplesexhibited the highest 0.2% YS, UTS, ductility and mi-crohardness when compared to all the as-processed Al–Sisamples investigated in the present study (see Table 4).

3.8. Fracture behavior

Fig. 6. Graphical representation of variation in silicon weight percent inThe tensile fracture surfaces of the as-processed andthe Si–Al interfacial region in the case of Al–19Si(Ext) samples.

extruded samples are shown in Figs. 7–10. The fracturedsurfaces of the as-processed Al–Si samples revealed an

Table 4increase in the degree of brittleness with an increase in theResults of tensile properties measurements made on peak aged samplessilicon content in the matrix (see Figs. 7–9). In the case ofMaterial 0.2% YS UTS Ductility Microhardnessas-processed Al–19Si samples (see Fig. 9), fracture surface

(MPa) (MPa) (%) (HV)revealed the presence of cracked primary Si particles andminimal evidence of matrix undergoing plastic deforma- Al–7Si 55.362.1 141.762.1 12.260.5 38.561.2

Al–10Si 75.461.6 154.763.4 10.360.8 39.260.4tion. For the Al–19Si(Ext) samples, the fracture surfaceAl–19Si 80.863.2 129.668.7 2.361.9 43.462.1revealed the presence of broken primary Si particlesAl–19Si(Ext) 82.763.1 189.0612.1 21.468.8 59.260.5

similar to that observed in as-processed Al–19Si samples

Table 3Results of microstructural characterization

aMaterial Microstructural feature V Parameter Roundnessf

bEq. size (mm) l (mm)

Al–7Si Eutectic silicon 0.065 2.6 10.2 N.DAl–10Si Eutectic silicon 0.162 2.7 6.7 N.DAl–19Si Primary silicon 0.093 77.5 254.0 7.34

Eutectic silicon 0.074 7.5 27.5 3.21Al–19Si(Ext) Primary silicon 0.131 65.5 181.0 5.13

Eutectic silicon 0.125 3.7 10.5 2.64a Computed using image analysis.b 1 / 2Computed using the formula suggested by Nardone and Prewo [22]: l5(lt /V ) where l is the interparticle spacing and t, l and V are the thickness,f f

length and volume fraction of the secondary phases, respectively.

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M. Gupta, S. Ling / Journal of Alloys and Compounds 287 (1999) 284 –294 289

Fig. 7. SEM fractograph showing the fracture surface features in the caseof Al–7Si samples.

and the evidence of significant matrix plastic deformation,in contrast to the predominantly brittle behavior exhibitedby the as-processed Al–19Si samples (see Figs. 9 and 10).

4. Discussion

4.1. Processing

The results of the disintegrated melt deposition process-ing revealed three salient features in the as-processedcondition:

• high yield of the hypo- and hypereutectic alloys• low volume fraction of porosity• complete retention of elemental silicon in aluminum in Fig. 9. SEM fractographs showing: (a) general fracture surface features

the case of hypoeutectic formulations and 95% re- and (b) presence of cracked primary silicon particles in the case ofAl–19Si samples.

tention by weight in the case of hypereutectic formula-tion

In the present study, DMD processed Al–Si alloysrevealed high values of yield in both the as-processed($86%) and finally machined ($80%) conditions irre-spective of the lower volume of starting elemental materi-als. In the as-processed condition, high yield of alloys canbe attributed to the low gas flow rate associated dis-integration of molten stream of alloy resulting in thecomplete absence of overspray powders which are normal-ly associated with conventional spray processing tech-niques adopted by other investigators [13–15]. In ma-chined condition, high yield of Al–Si alloys can beattributed to the formation of a shallow shrinkage cavity asFig. 8. SEM fractograph showing the fracture surface features in the casea result of enhanced solidification of the molten alloy onof Al–10Si samples.

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phases predicted by binary Al–Si phase diagram [17]. Theeutectic phase exhibited divorced morphology as a resultof high interfacial energy between the two componentphases and is consistent with the similar observationsreported elsewhere [18].

In the case of hypereutectic Al–19Si alloy, microstruc-ture characterization results revealed the presence ofprimary silicon and acicular eutectic silicon phases in boththe as-processed and extruded conditions. The presence ofthese phases along with the microstructural characteristicsexhibited by hypoeutectic alloys establishes the existenceof predominantly equilibrium solidification conditionsduring disintegrated melt deposition processing of materi-als.

The results of quantitative microstructural characteriza-tion conducted on hypoeutectic Al–Si alloys revealed an

Fig. 10. SEM fractograph showing fracture surface features in the case of increase in the volume fraction of eutectic silicon phaseAl–19Si(Ext) samples. and a marginal increase in its size with an increase in

silicon content (see Table 3). The increase in volumethe deposition surface. The enhanced solidification can be fraction of eutectic silicon phase with an increase in siliconattributed to the convectional heat transfer associated with content is in accordance with the Lever’s rule [19] while athe disintegration step of the DMD processing when marginal increase in the size of eutectic silicon can becompared to the conventional casting techniques. The attributed to an increase in the probability of silicon atomsresults also revealed that the yield of the alloys in both the to attach themselves on the growing eutectic silicon phase.as-processed and finally machined conditions decreases The higher probability of silicon atoms to attach them-with an increase in the silicon content. selves to the growing eutectic silicon phase can be

Another characteristic feature associated with the DMD attributed to their higher number and shorter travel dis-processed Al–Si alloys was the presence of low ($2.65%) tances as a result of higher weight percentage of silicon involume fraction of porosity (see Table 1) indicating the the aluminum melt [18]. This is further supported by thenear-net shape forming capabilities of the DMD technique. fact that the room temperature solid solubility of silicon inThe presence of lower volume fraction of porosity in the aluminum will be the same in the case of both the Al–7Sias-processed condition ensures the realization of near- and Al–10Si alloys due to the same processing parametersoptimum properties from the material precluding the and the near-equilibrium nature of the DMD processingnecessity to employ secondary processing techniques at technique.least for conventional engineering applications [16]. It may Regarding the hypereutectic Al–19Si alloy in the as-further be noted that the volume fractions of porosity processed and extruded condition, the results of quantita-revealed by Al–Si alloys synthesized in the present study tive microstructural characterization revealed that theare similar to the porosity levels reported in cases of other extrusion process leads to an increase in microstructuralnear-net shape forming techniques [13]. uniformity by decreasing the average size, interparticle

Finally, the complete retention of silicon in the case of spacing and roundness of the primary and eutectic siliconhypoeutectic alloys and 95% by weight in the case of phases (see Table 3). The decrease in average size andAl–19Si alloy can be attributed to the coupled effects of roundness of the primary and eutectic silicon phases can bethe ability of aluminum to dissolve silicon completely at attributed to the partial dissolution of these phases during9508C [17] and the kinetics of dissolution accomplished by hot extrusion and subsequent reprecipitation followingstirring conditions used in the present study. The stirring extrusion. It may further be noted that the dissolution ofprocedure that involved stirring at 596 rpm for a time not silicon at the sharp tips and edges can be attributed to theexceeding 10 min also helped to ensure uniform dis- high solute concentration gradients in these regions intribution of silicon in the ingot following solidification. accordance with the Freundlich–Thomson equation [20,21]

leading to the reduction in roundness. The decrease in4.2. Microstructure interparticle spacing of primary and eutectic silicon phases

can primarily be attributed to the decrease in their average1 / 2The results of microstructural characterization conducted size in accordance with the formula, l 5 (lt /V ) , pro-f

on hypoeutectic Al–7Si and Al–10Si alloys revealed, in posed by Nardone and Prewo [22]. Finally, the increase incommon, the presence of a-Al dendrites and eutectic the volume fraction of primary and eutectic silicon phasessilicon (see Figs. 2 and 3). The presence of these phases (studied at magnification levels of up to 20003; see Tableare in accordance with the equilibrium microstructural 3) following extrusion may be attributed to the coalescence

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M. Gupta, S. Ling / Journal of Alloys and Compounds 287 (1999) 284 –294 291

of finer distribution of these phases at the extrusion elsewhere [27]. It may further be noted that the increasedtemperature leading to their increased presence at magnifi- precipitation of the secondary phases and hence thecations up to 20003. It may be noted that the coalescence increase in magnitude of age hardening may be attributedof finer phases assists in reducing the particle /matrix to the ability of the defect structure to serve as a heteroge-interfacial area and the relaxation of misfit strains in the neous nucleation site during aging treatment. In relatedmatrix [23]. Further work is continuing to establish the studies, for example, investigators have shown usingvariation in distribution pattern of these phases (especially transmission electron microscopy the precipitation ofin the finer size range) as a function of extrusion step using strengthening phases on the lattice defects such as disloca-high magnification metallography techniques. tions [28]. The progressive increase in the peak hardness

Another interesting feature observed in the case of when compared to as-solutionized hardness with an in-hypereutectic Al–19Si samples was the presence of silicon crease in weight percentage of silicon also indicates thatsegregation in the immediate vicinity of primary Si much superior mechanical properties can be realizedparticles (see Fig. 6). This can be attributed to the localized following aging heat treatment. This is also consistent withpresence of point defects and line defects in the interfacial the work reported elsewhere [16] which suggest a clearregion of primary Si particles during solid state quenching correlation between hardness and strength. The results areas a result of a difference in the coefficient of thermal also consistent with the similar findings made on anexpansion between the aluminum matrix and Si particles Al–7Si based alloy supplied by Duralcan, USA [6].(CTE (Al) /CTE (Si) : 3.1 [24]). The presence of an Regarding the influence of silicon content on the agingincreased number of defects assists in promoting the kinetic, the results of the aging studies suggest that thediffusion of alloying elements from the adjacent region variation in silicon content in the range of 7–19 wt % wasleading to the segregation in the interfacial region. These not sufficient to bring the microstructural changes capableresults are also consistent with the similar studies con- of altering the aging kinetics of the aluminum matrix. Thisducted on the conventionally cast A390 alloy [4]. is consistent with the work of other investigators who

suggested that the aging kinetics of the metallic matrix4.3. Aging studies containing the secondary phases with different CTE can

only be influenced if the variation in microstructure as aThe results of aging studies conducted on as-processed result of their presence is significant [29].

Al–Si alloys revealed three salient features: Regarding the effect of extrusion, the results show thatthe hot extrusion step used in the present study increases

• an increase in as-solutionized and peak hardness with the as-solutionized hardness significantly while maintain-an increase in the weight percent of silicon, ing the peak hardness similar to that observed in the case

• an increase in the magnitude of age hardening with an of as-processed Al–19Si samples (see Fig. 1). The increaseincrease in the weight percent of silicon, and in as-solutionized hardness of the extruded samples may be

• an aging kinetics independent of weight percent of attributed to the reduction in the volume fraction ofsilicon porosity as a result of the extrusion (see Table 1) and a

minimal amount of age hardening suggests that the hotThe increase in the as-solutionized and peak hardness of extrusion step assists in establishing a uniform distribution

Al–Si alloys with an increase in the weight percent of pattern of silicon based phases as a result of high tempera-silicon can be attributed to an increase in the volume ture exposure (3508C) during extrusion and subsequentfraction of harder silicon based phases in the aluminum cooling to room temperature resulting into partial dissolu-matrix (see Table 3). The silicon based phases refer to tion and reprecipitation of silicon based phases in theeutectic silicon in the case of hypoeutectic Al–Si alloys matrix. As a result of this, the effect of age hardening areand primary and eutectic silicon in the case of hypereutec- minimized as reflected in only a 5.65% increase intic Al–Si alloys. It may be noted that the hardness of hardness of the Al–19Si(Ext) samples when compared to

9 22silicon (10 kg m ) [25] is significantly higher when 29.9% in the case of the as-processed Al–19Si samples in6 22compared to that of aluminum (19310 kg m for 99.6% the peak aged condition.

Al) [26].The increase in the magnitude of age hardening with 4.4. Mechanical behavior

silicon content may be attributed to the capability ofincreasing volume fraction of silicon based phases to The results of the mechanical properties characterizationgenerate increasing volume fraction of the defect structure revealed an increase in the matrix microhardness with anin the matrix (see Table 3). The formation of defect increase in the weight percentage of silicon in the as-structure can be attributed to a significant difference in processed Al–Si alloys (see Table 4). This is consistentcoefficients of thermal expansion of aluminum and silicon with the increase in the cumulative volume fraction of thephases (CTE (Al) /CTE (Si) : 3.1 [24]). Such a correlation harder silicon based phases (see Table 3). It may be notedhas been convincingly established by the researchers that an increase in the volume fraction of the harder silicon

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292 M. Gupta, S. Ling / Journal of Alloys and Compounds 287 (1999) 284 –294

based phases and the associated defect structure will lead [16,34]. Bocchini [16] and Payne et al. [34], for example,to higher constraint in the localized deformation of softer asserted that the presence of pores lead to weakening of amatrix under the application of indentational load. In the material by reducing the amount of stress bearing area andcase of Al–19Si(Ext) samples, however, a significant therefore lowers the amount of stress the material is able toincrease in microhardness when compared to the Al–19Si withstand. Similarly, the presence of plastically incompat-samples can be attributed to the significant microstructural ible phases lead to the stress accumulation at the interfacerefinement and enhanced microstructural uniformity com- with the ductile matrix and under conventional tensilemonly achieved following extrusion process [30]. loading microcracks /microvoids may appear either at the

The results of tensile properties characterization re- pole or equator locations depending on the mechanicalvealed an increase in 0.2% YS and a decrease in ductility properties of the strengthening phase [35], thus preventingwith an increase in silicon content. An increase in 0.2% the optimum realization of UTS by the material.YS with an increase in the amount of silicon can be The results of the tensile properties characterizationattributed to the relatively higher constraint exerted by an conducted on Al–19Si(Ext) samples revealing an increaseincreasing volume fraction of the silicon based phases in in ultimate tensile strength and ductility when compared tothe aluminum matrix for the onset of the slip process. It the as-processed Al–19Si samples are consistent with themay be noted that an increase in the dislocation pinning dependence of these properties on the morphology ofsites (such as silicon based phases and the associated plastically incompatible phases and the volume fraction ofdefect structure) delays the onset of plastic deformation porosity. Since the constitution of Al–19Si and Al–which is reflected in the values of the 0.2% YS obtained in 19Si(Ext) samples is the same, the volume fraction ofthis study for Al–Si alloys [31]. In a related work [26], for silicon based phases will also be the same since theexample, it has been shown that general yield stress (s ) identical near-equilibrium primary processing technique iso

can be expressed as: used to synthesize them [17]. The only difference may bein the shift in the distribution pattern of the silicon based

s 5 s 1 s (1)o s i phases towards the coarser side due to the hot extrusionwhere s is the stress to operate the dislocation sources and step used in the case of Al–19Si(Ext) samples. This iss

s is the friction stress representing the combined effect of consistent with the results of the microstructural characteri-i

all the obstacles to the motion of dislocations arising from zation studies (Table 3) which show an increase in thethe sources. In the present study, based on microstructural volume fraction of micron-size silicon based phases andcharacterization results (see Table 3), it is evident that an more interestingly show the lower roundness (indicatingincrease in the amount of silicon leads to an increase in the increasing equiaxed nature) values. The decrease in thes as a result of an increase in the volume fraction of roundness value of the silicon based phases will bei

silicon based phases. This is also consistent with the instrumental in increasing the resistance of the aluminumsimilar 0.2% YS values obtained in the case of Al–19Si matrix to microcracking as a result of the decrease in theand Al–19Si(Ext) samples (see Table 4). The same stress concentration at the sharp edges, thus leading toamount of silicon in these two different category of superior values of UTS and ductility. Similarly, a reductionsamples is indicative of the similar volume fraction of in porosity from 2.65 to 0.65% (see Table 1) may also besilicon based phases [17] and hence the similar values of attributed to an increase in UTS and ductility values of0.2% YS (within each other’s standard deviation) arising Al–19Si(Ext) samples when compared to Al–19Si sam-from similar s . ples. This is consistent with the work of other investigatorsi

The decrease in ductility with the increasing amount of [25] who reported an increase in the value of UTS bysilicon may be attributed to the increasing volume fraction about 51 MPa as a result of reduction in porosity by aboutof plastically incompatible silicon based phases in the soft 2% in the case of Al–5Si samples heat treated to T6and ductile aluminum matrix. It has been, for example, condition. It may be noted that in the present study anestablished that the increasing volume fraction of plastical- increase in UTS by about 51 MPa was realized as a resultly incompatible phases decreases the cavitation resistance of a reduction in porosity by about 2% in the case of T6of the matrix leading to early microcracking and hence the heat treated Al–19Si samples. The tensile testing resultsreduced ductility under the application of tensile loads thus obtained are consistent with the microstructural[31–33]. characterization results obtained in the present study and

The inferior ultimate tensile strength exhibited by Al– the work of other investigators [25].19Si samples when compared to Al–7Si and Al–10Sisamples may be primarily attributed to the coupled effects 4.5. Fracture behaviorof the presence of primary Si and an increase in theporosity and volume fraction of plastically incompatible The results of fractographic studies conducted on thesilicon based phases. The porosity associated reduction in hypoeutectic Al–7Si and Al–10Si samples revealed similarstrength has been previously established by other inves- fracture surface features exhibiting ample evidence oftigators for steels, copper and aluminum based alloys matrix plastic deformation (see Figs. 7 and 8). The fracture

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M. Gupta, S. Ling / Journal of Alloys and Compounds 287 (1999) 284 –294 293

surfaces, in common, also showed the presence of rela- cant increase in the degree of brittleness in the hy-tively flat and featureless zones which may be indicative of pereutectic samples when compared to the hypoeutecticcrack propagation along the embrittled boundaries of a-Al samples. The results also revealed that the fracturedendrites /grains. This may be attributed to the presence of mode can be changed from predominantly brittle tothe hard and brittle eutectic silicon phase as shown in Figs. predominantly ductile following extrusion.2 and 3. The fracture surface of Al–19Si samples wassignificantly different when compared to Al–7Si and Al–10Si samples and exhibited a high degree of brittleness and

Acknowledgementscracked primary silicon particles (see Fig. 9). This may beattributed to the reduced ability of metallic matrix to

The authors would like to thank Mr. Tham Leung Mundeform due to the increased number of crack nucleation

and Mr. Tung Siew Kong (National University of Singa-sites such as increased volume fraction of hard and brittle

pore, Singapore) for their valuable experimental assistancesilicon based phases and porosity. On the contrary, the

and for many useful discussions and to Ms Neerja Guptafracture surface of Al–19Si(Ext) samples revealed evi-

for improving the readability of this manuscript.dence of significant matrix plastic deformation (see Fig.10) and the presence of cracked and/or partially debondedprimary silicon particles. The plastic deformation ability

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