Heat Treatment of High-Pressure Die Castings
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HIGH-PRESSURE die casting (HPDC) is widely used as a cost-effective way to mass produce metal components that have close dimensional tolerances and smooth surface finishes. Worldwide, HPDC accounts for approximately 50 pct of all Al castings and is commonly used for manufacturing automotive components and other consumer products. Two features of the HPDC process are the extreme turbulence experienced by the shot of molten metal as it is forced at high speed into a die and the very rapid solidification rate experienced by the metal. For these reasons, the castings usually contain internal pores that arise due to the presence of entrapped gases such as air, steam, hydrogen, or vapors formed by the decomposition of organic die-wall lubricants. Metal shrinkage during solidification and planar defects, such as oxide skins or cold shuts, also result in porosity. The gas quantity inside high-pressure die castings varies with the part complexity, venting efficiency, die spray, and other factors. The gas content inside high-pressure die-cast parts has been shown experimentally to vary between 1 and 12 cc/ 100 g (normalized to ambient temperature and pressure),[4] and the gas volume is compressed up to 1000 times during intensification under a typical metal pressure of 100 MPa during the casting process. Therefore, not only do conventionally produced HPDCs contain substantial gas contents, these gases are also entrapped under high pressure. Although some porosity in die-cast components is normally accepted, the presence of these gaseous phases under high pressure does have the disadvantage that conventionally produced HPDC components cannot subsequently be heat treated at high temperatures.[4–6] This follows because the R.N. LUMLEY, R.G. O’DONNELL, and D.R. GUNASEGARAM, Senior Research Scientist, and M. GIVORD, Diecasting Engineer, are with the Light Metals Flagship, CSIRO Manufacturing and Materials Technology, Clayton South MDC, Victoria 3169, Australia. Contact e-mail: [email protected] Manuscript submitted February 18, 2007. Article published online August 29, 2007. 2564—VOLUME 38A, OCTOBER 2007
entrapped gases expand, resulting in unacceptable surface blistering and dimensional instability when the pressure inside the pores causes the skin of the component to plastically deform at elevated temperatures. Aluminum alloys used for HPDC are primarily those based on the alloys 360 (Al-9.5Si-0.5Mg) and 380 (Al-8.5Si-3.5Cu) and their close variants, which are listed with their specified compositions in Table I. These alloys are based on the Al-Si system and have microstructures containing primary a-Al grains in a matrix that consists mainly of plates of the Al-Si eutectic. Both have potential for strengthening by an age-hardening heat treatment, because they contain Cu and Mg as alloying elements. In alloy 360, the level of Cu is low (maximum 0.6 pct by weight), although there is a requirement for 0.45 to 0.6 pct Mg. Alloy 380 has a relatively high Cu content (3.0 to 4.0 pct), which should promote a good response to age h
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