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[8] Microstructure and Mechanical Properties of Hypo Hyper-eutectic Al-Si Alloys Synthesized Using a Near-net Shape Forming Technique

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Microstructure and mechanical properties of hypo hyper-eutectic Al-Si alloys synthesized using a near-net shape forming technique
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  L Journal 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. 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 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 of an 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 andThe 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 of the 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 highThe 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 usingthem. In order to circumvent the disadvantages associatedwith these techniques, a relatively new technique common- *Corresponding author. Tel.:  1 65-874-6358; fax:  1 65-779-1459.  E  - mail address :   mpegm@nus.edu.sg (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   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 silicon subsequent deposition on the metallic substrate. Thedynamic disintegration and deposition steps enables thisQuantitative 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 measurement  the disintegrated melt deposited Al–Si alloys (both inhypo- and hypereutectic composition range) in order toThe densities of the as-processed and extruded Al–Siassess the feasibility of the disintegrated melt depositionsamples 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 effectdensity measurements involved weighing polished cubes of of secondary processing on the microstructure and me-the extruded samples in air and when immersed in distilledchanical properties of the hypereutectic (Al–19Si) alloywater. 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 procedure balance to an accuracy of   6 0.0001 g. 2.1.  Materials 2.6.  Aging studies In 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- andpeak hardness time for the as-processed and extrudedhypereutectic Al–Si alloys.Al–Si samples. Specimens (10 mm diameter 3 7 mmheight) were solutionized for 1 h at 529 8 C, quenched in 2.2.  Processing cold water and aged at 160 8 C 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 kgtic Al–Si alloys with starting weight percentages of 7, 10load using a GNEHM HORGEN digital hardness testerand 20 wt % of Si was carried out using the DMDfollowing 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 950 6 10 8 C in graphite crucible, impellerassisted stirring to ensure complete mixing of elemental  2.7.  Microstructural characterization materials 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 thediameter 3 75 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 primarilyaccomplished using an optical microscope and a JEOL 2.3.  Secondary processing  scanning electron microscope equipped with EDS. Thesamples were metallographically polished prior to exami-Al–Si alloy ingot with starting weight percentage of nation. Microstructural characterization of the samples was20% silicon was machined to a diameter of 35 mm and conducted in both etched and unetched conditions. Etchingthen hot extruded at 350 8 C 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  286  M  .  Gupta ,  S  .  Ling  /   Journal of Alloys and Compounds  287 (1999) 284  – 294  Table 1 2.8.  Mechanical behavior  Results of the density and porosity determinationAlloy Processing Wt % Si Density Porosity Vickers microhardness of the matrix of as-processed and 2 3 designation condition (g cm ) (vol %) extruded Al–Si samples was determined on a MatsuzawaMXT50 Automatic Digital microhardness tester using an  Al–7Si As-processed 7 2.64 6 0.01 1.07Al–10Si As-processed 10 2.62 6 0.02 1.51 indentation load of 100 g.Vickers microhardness measure- Al–19Si As-processed 19 2.55 6 0.01 2.65 ments were made in order to provide insight into the Al–19Si(Ext) Extruded 19 2.60 6 0.06 0.65 ability of secondary phases to strengthen the metallicmatrix.Smooth bar tensile properties were determined on the  3.3.  Quantitative assessment of silicon as-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 siliconweight percentages of 20) revealed that approximately 7,10, 19 and 19 wt % Si was retained, respectively, follow- 2.9.  Fracture behavior  ing 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 provideinsight into the various fracture mechanisms operative 3.4.  Density measurement  during tensile loading of the peak aged samples. Fracturesurface characterization studies were primarily accom-The results of density measurements conducted on theplished using a JEOL scanning electron microscopeAl–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 studies 3.1.  Processing The results of aging studies conducted on the as-pro-The deposited yield of the Al–Si alloys with startingcessed and extruded samples are shown in Fig. 1. Theweight percentages of 7, 10, and 20 wt % of silicon wasresults exhibit the presence of a hardness peak at 9 h for allfound out to be 89, 88 and 86%, respectively. Thethe samples. Both the as-solutionized and peak hardnesspreforms in all the three cases were associated with a smallvalues were found to increase with an increase in theshrinkage cavity on the top. After defacing the ingots so assilicon content in aluminum and from the as-processed toto remove the shrinkage cavity, the final yield wasextruded condition in the case of hypereutectic Al–19Sidetermined to be 85, 84 and 80%, respectively. The overallalloy. The results also reveal an increase in the magnitudedimensions of the disintegrated melt deposited preformsof age hardening with an increase in the weight percentagefollowing defacing were approximately 35 mm in heightof 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.   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 features optical 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 of microstructural 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 features found 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 behavior  extruded 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 studiesAlloy As-solutionized Peak hardness Peak aging Magnitude of agehardness (HR15T) (HR15T) time (h) hardening (HR15T)Al–7Si 41.2 6 1.2 44.8 6 0.9 9 3.6Al–10Si 43.7 6 1.5 52.4 6 1.4 9 8.7Al–19Si 44.8 6 1.3 58.2 6 1.8 9 13.4Al–19Si(Ext) 56.6 6 0.3 59.8 6 0.8 9 3.2
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