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  • LJournal 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: 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. PI I : 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 study hypo- and hypereutectic composition range. This tech- the effect of secondary processing on the microstructural nique, in the past, has been successfully utilized to and mechanical properties variation of as-processed Al–Si synthesize monolithic and reinforced materials [11,12] and alloy. involves, in principal, the disintegration of superheated molten metal slurry using inert gas jets followed by its

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

    Quantitative assessment of Si in the as-processed andtechnique to synthesize bulk materials with improved extruded 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 to investigate the microstructure and mechanical properties of

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

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

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

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

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

    the extruded samples in air and when immersed in distilled chanical 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 which the volume fractions of porosity were calculated. The samples were weighed using an A&D ER-182A electronic

    2. Experimental procedure balance to an accuracy of 60.0001 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 the was used as an addition element to synthesize hypo- and peak hardness time for the as-processed and extruded hypereutectic Al–Si alloys. Al–Si samples. Specimens (10 mm diameter37 mm

    height) were solutionized for 1 h at 5298C, quenched in 2.2. Processing cold water and aged at 1608C for various intervals of time.

    Rockwell superficial hardness measurements were made In 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 tester and 20 wt % of Si was carried out using the DMD following ASTM standard E18-92. A minimum of three technique. The synthesizing procedure involved: super- hardness readings were taken for each specimen. heating of properly cleaned elemental materials to a temperature of 9506108C in graphite crucible, impeller assisted 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 conducted subsequent deposition in a metallic mould (55 mm on the as-processed and extruded Al–Si samples in the diameter375 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 and controlled atmospheric conditions. The Al–Si alloy ingots distribution of the secondary phases, and Si–Al interfacial obtained 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 JEOL 2.3. 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. Etching then hot extruded at 3508C employing a reduction ratio of was accomplished using Keller’s reagent [0.5 HF–1.5 13:1 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 12.8. Mechanical behavior Results of the density and porosity determination

    Alloy Processing Wt % Si Density PorosityVickers microhardness of the matrix of as-processed and 23designation condition (g cm ) (vol %)extruded Al–Si samples was dete