Microstructure Formation in AlSi4MgMn and AlMg5Si2Mn High-Pressure Die Castings
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HIGH-PRESSURE die casting (HPDC) is a key manufacturing process for the global automotive industry. In the past, high-pressure diecast Al-alloy parts were limited to nonstructural applications such as housings and fittings, for which pressure tightness is the main mechanical requirement. However, new alloys have been developed in recent years that target structural automotive applications and are specifically designed for the HPDC process.[1–3] Very high ductility with elongation >15 pct[1] is a key mechanical requirement for structural automotive components such as chassis parts.[3] To improve ductility, the HPDC-specific alloys typically contain a higher volume fraction of primary a-Al than do conventional Al-casting alloys; they also contain a low Fe content (~0.15 wt pct) and a relatively high Mn content (>0.5 wt pct).[1–3] Control of the latter two elements aims to do the following: (1) minimize the volume fraction of brittle Fe-bearing intermetallics, (2) ensure that any Fe-based intermetallics are a type rather than b type, and (3) minimize die soldering.[1] These alloy changes result in compositions that still have good castability by the HPDC process. Two important classes of HPDC-specific Al alloys are based on the Al-Si[4] and Al-Mg-Si[5] systems. When some of the current generation of HPDC-specific Al alloys are combined with high-integrity HPDC S. OTARAWANNA, Postgraduate Student, CAST Cooperative Research Centre, and A.K. DAHLE, Professor, Australian Research Council’s Centre of Excellence for Design in Light Metals, are with Materials Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. Contact e-mail: [email protected] C.M. GOURLAY, Royal Academy of Engineering and EPSRC Research Fellow, is with the Department of Materials, Imperial College, London SW7 2AZ, United Kingdom. H.I. LAUKLI, Head of Department, is with Research and Technology Development, Hydro Aluminium, N-6601 Sunndalsøra, Norway. Manuscript submitted October 21, 2008. Article published online April 28, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
practices, complex automotive components with elongations of more than 10 pct and yield strengths up to 200 MPa can be cast and used in the as-cast condition.[6] Based on these improvements, HPDC is capable of producing castings for structural automotive applications, such as crash-sensitive parts, and suspension components, such as the A-pillar and shock tower.[6] Consequently, use of these ‘‘ductile’’ high-pressure diecast alloys has become widespread in automotive manufacturing, particularly in Europe.[2] However, only limited research is published on the formation of the high-pressure diecast microstructure in these new alloys. In this article, we seek to understand the microstructure formation in AlSi4MgMn and AlMg5Si2Mn under HPDC processing conditions. The HPDC process involves the high-velocity injection of a solidifying alloy into a steel die followed by the application of high pressure to the alloy, once the die is filled. The combination of the following:
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