Effect of Mg content and solution treatment on the microstructure of Al-Si-Cu-Mg alloys

Three different quaternary alloys Al-6Si-3Cu-xMg (x = 0.59, 3.80 and 6.78 wt.%) were produced using conventional ingot casting metallurgy. The study was focused to investigate the effect of magnesium and solution heat treatment on the microstructure. Results shown variations in composition and morphology for the silicon-rich phases as well as a change of the predominant copper-rich phase Al2Cu (theta) to Al5Cu2Mg8Si6 (Q phase) when magnesium content is increased. The amount of Mg in solid solution was constant for the three different cast-alloys, increasing considerably after solution heat treatment to 2.7 at.% for the alloy with higher Mg content This fact allowed to obtain Cu:Mg ratios (in at.%) in solid solution lower than 1.0 for the alloys with 3.80 and 6.78 Mg wt.%, impossible to reach for the alloy with low Mg content. During dissolution process, Al2Cu phase was observed to be more suitable to dissolve than Q phase. Fragmentation, spheroidization and coarsening of Q and silicon-rich phases were observed. Solution time required for these processes occurrence was longer for Q phase. Solution heat treatments at 480 degrees C for 12 h were found to be appropriate for the studied alloys.