NEW HEAT TREATMENT FOR Al HIGH PRESSURE DIE-CASTINGS
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R.N. Lumley, R.G. O’Donnell, D.R Gunasegaram, and M. Givord CSIRO Manufacturing and Infrastructure Technology Victoria, Australia igh pressure die-casting (HPDC) is a popular, cost-effective method for mass producing metal components where physical dimensions must be accurately replicated and surface finish is important. Approximately half of all castings worldwide made of aluminum alloys are manufactured this way, and are used for a wide range of automotive parts and other consumer goods. Two features of the conventional HPDC process are high turbulence experienced by the shot of molten metal as it is forced at high speed into a die and the very rapid rate at which it solidifies. The castings, therefore, usually contain internal pores comprising entrapped gases such as air, hydrogen, and vapors formed by the decomposition of organic die-wall lubricants. Metal shrinkage during solidification and planar defects such as oxide skins and cold shuts can also result in porosity. While some level of porosity in die castings is normally accepted, a major disadvantage of porosity is that components cannot subsequently be heat treated at high temperatures. The pores expand during solution treatment (e.g., at 540°C, or 1000°F, for 8h) resulting in unacceptable surface blistering. Furthermore, the dimensions of die cast parts can change due to swelling and mechanical properties are adversely affected. Common aluminum alloys for HPDC are based mainly on the Al-Si system, with the most common examples being AA360 (Al-9.5Si-0.5Mg) and AA380 (Al-8.5Si-3.5Cu). Table 1 lists the chemical composition ranges for these alloys. Microstructures of these alloys are similar and comprise aluminum grains in a matrix of Al-Si eutectic. The presence of Cu, Fe, and other elements such as Mn introduce fine intermetallic compounds, which are normally dispersed among the eutectic. Both alloys may contain potent age hardening elements copper and magnesium to enhance age hardening via heat treatment. In both conventional wrought and cast heat-treated products, strengthening precipitate phases such as (Al2Cu), S (Al2CuMg) and (Mg2Si) form within the aluminum grains and provide an impediment to the process of crystallographic slip. Although up to 3% Zn is permissible in some 380 alloy variants, there is insufficient magnesium present to form substantial quantities of Zn-containing precipitates. CSIRO Light Metals Flagship has developed a novel heat treatment whereby conventional high pressure die-castings made of alloys 360 and 380 can be heat treated at high temperatures without incurring blistering problems[1]. The alloys may then respond to age hardening resulting in significant improvements in mechanical properties. In addition, tensile properties are Aluminum diecast samples being loaded into furnace for heat treating