Effect of Mg Content and Cooling Rate After Solidification on the Mechanical Properties of T5-treated Al-Si-Mg Alloy

The effects of Mg content and cooling rate on the T5 treatment of Al-Si-Mg alloys have been investigated using Avrami kinetics and transmission electron microscopy. The alloys were super-heated at 800 oC for 1 hr followed by degassing treatment at 720 oC for 15 min, and poured into a metallic mold preheated to 250 oC. The as-cast alloys were then T5-treated at a temperature of 190 oC. As the Mg content and/or cooling rate increased, the strength of the T5-treated alloys increased while the elongation decreased over the entire aging time. However, some T5 treatment conditions were effective to at enhancinge elongation as well as strength. The alloy with low Mg content showed excellent elongation in the early stage of aging. B, but as the aging time increased, h. However, the elongation decreased rapidly so that there was showed no significant difference from with the alloy with high Mg content in the over-aging stage. This was due to a coarse and agglomerated precipitate structure, which were likely to be formed by the rapid precipitation and agglomeration of the β′ phase. The alloy with a high cooling rate exhibited superior strength with similar elongation over the entire aging time compared to the slowly cooled alloy. The reason for the decrease in strength for in the slowly cooled alloy was attributed to the coarsening of precipitates and suppression of the formation of fine precipitates during cooling.