Effects of silicon, magnesium and strontium content on the qualities of Al-Si-Mg alloys

According to the specification of AA standard, the magnesium content of 356.1 alloy ranges from 0.25 to 0.45%. In producing Al-Si-Mg alloy the strontium content for the modified 356 (Al-7%-Mg) alloy might range from 0.005 to 0.02%. Therefore, 356 alloys might be produced with different percentages of Mg and/or Sr. The effects of changes in Si content (from 6.6% to 10.8% in gravity casting and 6.5% to 10.3% in squeeze casting), Mg content (from 0.36% to 0.48% in gravity casting and 0.3% to 0.44% in squeeze casting), and Sr content (from 0.0007% to 0.0158% in gravity casting and 0.0015% to 0.04% in squeeze casting) on the microstructures, density, mechanical properties and strength of different Al-Si-Mg alloys were fully investigated and discussed. Different melts were poured in the ASTM B108 permanent mould and dies in a vertical squeeze machine to produce bar castings. These bar castings were then machined and the mechanical properties tested. Experimental results showed that if a high strength and a high elongation were desired in a squeeze casting after T6 treatment, an increase in Si and Sr content to 9.9% and 0.019% (Mg at 0.3%) would lead to 280 MPa strength, 12% elongation and Weibull modulus of 34 in reliability of strength. If both strength and reliability were desired in a squeeze casting after T6 treatment, an increase in Si content to 10.3% (Sr at 0.0015% and Mg at 0.3%) would result in 294 MPa strength, 8.7% elongation and a great Weibull modulus of 67. In gravity casting, after T6 treatment, increasing Si content to 10.4-10.55% and Sr content in the range of 0.0007-0.0208% and 0.35% Mg could develop 282-284 MPa strength at about 6.6% elongation with Weibull modulus of around 31. In Al-7Si-Mg alloy, increasing Mg content from 0.26% to 0.48% decreased the tested elongations of both as-cast and heat treated squeeze castings and gravity castings.