Development of a super ductile diecast Al-Mg-Si alloy

The super ductile diecast aluminium alloys have been developed particularly for application in automotive body structure. On the basis of the reviewing aluminium alloys currently available, the requirement of diecast aluminium alloys is summarized and the Al-Mg-Si system is focused in the development. The effect of various alloying elements on the microstructure and the mechanical properties, such as yield strength, ultimate tensile strength and elongation is assessed. The optimized composition of the super ductile Al-Mg-Si alloy has been found to be at 5.0-5.5 wt% Mg, 1.5-2.0 wt% Si, 0.5-0.7 wt% Mn, 0.15-0.2 wt% Ti with Fe < 0.25 wt% for the best combination of strength and ductility, which shows 150 MPa of yield strength, 300 MPa of ultimate tensile strength, and 15% of elongation under as-cast condition. The paint baking hardenability of the optimized alloy is found to be insignificant. Less than a 10% increase in the yield strength was achieved, with a slight decrease in the elongation after aging at 180 degrees C for 30 min, which is a simulated process of paint baking. Cu is found to slightly increase the yield strength under the as-cast condition and after the heat treatment, but with a significant reduction in the ductility. Therefore, Cu should be limited in the super ductile aluminium alloy. The microstructure of diecast aluminium alloys at the optimized composition consists of the primary alpha-Al phase, the alpha-AlFeMnSi intermetallics and the Al-Mg2Si eutectics. There are two types of primary alpha-Al phase: dendritic or fragmented dendritic alpha-Al phase solidified in the shot sleeve and globular alpha-Al particles solidified in the die cavity. The alpha-AlFeMnSi intermetallics is in the form of compact morphology and with a size of less than 3 mu m. The eutectic cells are at size of 10 mu m with a typical lamellar morphology of alpha-Al phase and Mg2Si phase. Crown Copyright (c) 2012 Published by Elsevier B.V. All rights reserved.