Influence of composition, Sr modification, and annealing treatment on the structure and properties of cast Al-4 pct Mg alloys

The presence of various amounts of strontium and iron in aluminum alloys of the type used in automotive sheet applications and can-end stock contributes to the formation of coarse constituents, including Mg2Si and Fe intermetallics. As these intermetallics influence the alloy sheet formability, it is always a goal to reduce their number employing different means. The role of strontium addition and heat treatment in controlling the size and distribution of the intermetallics present in three Al-4 pct Mg 5XXX type alloys (one industrial and two experimental alloys) was studied, in relation to their properties. Particular attention was paid to the oxidation of magnesium during the process of annealing or homogenization at temperatures as high as 520 degreesC. The phases obtained were analyzed using thermal analysis, optical microscopy, and electron probe microanalysis (EPMA) coupled with energy dispersive X-ray (EDX) analysis. The effect of the phases present on the alloy performance was estimated in terms of the tensile properties. The results show that increasing the Si and Fe contents increases the number of coarse intermetallics, which deteriorate the alloy properties, particularly ductility. The main phases observed are Al-6(Fe,Mn), Al-8(Fe,Mn)(2)Si, Al-15(Fe,Mn)(3)Si-2, and Mg2Si in script form, and traces of a complex ternary eutectic containing Al8Mg5. Addition of Sr (250 to 300 ppm) to low Si-low Fe alloys lowers the properties after annealing at 345 degreesC, due to precipitation of Sr-based intermetallics, but improves the properties in high Si-high Fe alloys annealed at 520 degreesC, through fragmentation of the existing intermetallics, in particular, Mg2Si. Property improvements of 23 pct yield strength (YS), 45 pct ultimate tensile strength (UTS), and 120 pct elongation (El) are observed after 8 hours/520 degreesC. Spinel formation occurs during annealing at 520 degreesC and accelerates with annealing time, leading to lower alloy properties. Addition of 100 to 250 ppm Be is found to be effective in preventing spinel formation, whereas addition of up to 1200 ppm Sr is seen to neither prevent nor minimize the reaction. The main fracture mechanism in these alloys is governed by the formation of large dimples, at the interiors of which fragments of intermetallics are observed. Slip lines on the surface of the dimples are most likely a result of the Portevin-Chatelier effect.