Rapid Heat Treatment of Aluminum High-Pressure Diecastings
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INTRODUCTION
ALUMINUM alloys used for high-pressure diecasting (HPDC) are mostly those based on the systems Al-Si-Cu and Al-Si-Mg, each of which has the capacity to respond to age hardening. Earlier work[1–10] has revealed that HPDC components made from these alloys can be successfully heat treated without causing surface blistering or dimensional instability if the solution treatment stage is carried out at lower than normal temperatures (e.g., 440 C to 490 C), and for much shorter times. Due in part to the unique microstructure generated by the HPDC process, this procedure is sufficient to attain an adequate supersaturated solid solution of the solute elements. Following solution treatment and quenching, a T6 temper is then used, which involves artificial aging at an intermediate temperature (e.g., 24 hours at 150 C) to achieve peak strength. As a result, the ability to age harden the HPDC alloys enables 0.2 pct proof stress levels to be approximately doubled for T6 tempers with little effect on ductility, although these parameters can vary depending on the precise aging treatments used.[2,8] Furthermore, the heat-treated HPDCs show improved fatigue strength and higher thermal conductivity, and may also be more resistant to fracture when specifically heat treated for this purpose.[9–11] The composition ranges of some common Al-Si-Cu and Al-Si-Mg-(Cu) alloys used worldwide that are heat R.N. LUMLEY and P.R. CURTIS, Principal Research Scientists, and I.J. POLMEAR, Professor Emeritus (Monash University, Melbourne, Australia), are with CSIRO Light Metals Flagship, Clayton South MDC, Victoria, Australia 3169. Contact e-mail: [email protected] Manuscript submitted May 14, 2008. Article published online May 2, 2009 1716—VOLUME 40A, JULY 2009
treatable are shown in Table I. The presence of Cu, Fe, and other elements such as Mn introduces intermetallic compounds that are usually dispersed within the solidified eutectic regions. With respect to the potential for age hardening by heat treatment, these alloys contain Cu, Si, and Mg, which are all potent strengthening elements and may be expected to form a range of precipitates such as h¢ (Al2Cu), S (Al2CuMg), b¢¢*, r *This was previously believed to be Mg2Si or MgSi, now shown to be Al3(MgSi) with a variable Mg:Si ratio.[12]
(Al5Cu6Mg2), and Q¢ (Al5Cu2Mg8Si6)**, as well as
**The composition of the Q¢ (and Q) phase has also been suggested to be Al4CuMg5Si4, Al4Cu2Mg8Si7, and Al3Cu2Mg9Si7.[13]
several precursor phases. Although up to 3 pct Zn is permissible in some HPDC alloys, the Mg content is usually insufficient to form substantial quantities of g¢ (MgZn2) precipitates that could also contribute to age hardening. Earlier studies have also shown that, to achieve peak strength, alloys such as A380 or C380 may require 24 hours aging at 150 C, whereas alloys such as A360 may need only 2 to 3 hours at the higher temperature of 180 C.[1,2] The opportunity to obtain substantial increases in the strength of aluminum alloy HPDCs by heat treatment will incur some capital and pr
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