Abstract
<span><span style="font-family: Times New Roman; font-size: medium;" face="Times New Roman" size="3"> </span> <p><span style="font-family: Times New Roman;" face="Times New Roman"><span style="font-size: medium;" size="3">This paper investigates the influence of natural pre-ageing time on T6 peak hardness of rheo-high pressure die cast (R-HPDC) 6xxx series aluminium alloys. Natural pre-ageing has a negative effect on the 6xxx series Al-Mg-Si alloys that contain higher quantities of Mg</span><span style="font-size: small;" size="2">2</span><span style="font-size: medium;" size="3">Si (typically > 0.90%). However, a positive effect is observed in alloys with lower quantities of Mg</span><span style="font-size: small;" size="2">2</span><span style="font-size: medium;" size="3">Si. The negative effect can be attributed to the clustering of solute atoms that form at room temperature because the precipitates which develop directly from clusters formed at room temperature are coarser than those developed in alloys artificially aged immediately after quenching. The opposite occurs in alloys where a positive effect of natural pre-ageing is found. The effects of the duration of natural pre-ageing on the subsequent T6 properties are of importance. In this study the T6 (190</span><span style="font-size: small;" size="2">o</span><span style="font-size: medium;" size="3">C-4h) hardness values of various R-HPDC 6xxx series alloys were determined after natural pre-ageing times ranging from 0h to 3240 hours. Alloys that show either the positive or the negative effect of natural pre-ageing are used. This paper also compares the influence of natural pre-ageing time on Cu-containing and Cu-free alloys.</span><span style="font-size: medium;" size="3"> </span><span style="font-size: medium;" size="3">The addition of copper to the 6xxx series aluminium alloys lessens the negative effect of natural pre-ageing in the higher strength alloys. </span></span></p> <span style="font-family: Times New Roman; font-size: medium;" face="Times New Roman" size="3"> </span>
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