Effect of Alloy Composition on the Hot Deformation Behavior, Extrudability and Mechanical Properties of AA6XXX Aluminum Alloys

Abstract

As automotive companies adjust to stricter environmental legislations, there is increased interest in the application of high specific strength alloys such as the AA6xxx aluminum alloys. In the near term, light weighting of vehicles through the use of high strength aluminum alloys is one solution to improve fuel efficiency. This research was performed to help understand the extrudability of some Al-Mg-Si alloys as well as the effect of extrusion conditions on final mechanical properties. The alloys studied were designed by General Motors (GM) and include variations in the Mg, Si and Cr contents. The research is primarily experimental in nature and consisted of measuring and modelling the hot flow stress behaviour of these alloys, performing both laboratory scale and industrial extrusion trials as well as mechanical and microstructural characterization of the extruded materials. The well-known Sellars-Tegart model was applied in development of constitutive equations for predicting the hot flow stress behavior of Al-Mg-Si alloys. The influence of hold time on Mg-Si solute content level during pre-deformation heating of an as-cast and homogenized Al-Mg-Si alloy was determined and the Mg-Si solute content (X) integrated into a constitutive equation for predicting the alloy’s flow stress behavior during hot compression testing. The constitutive model predictions were observed to agree well with experimental data for the aluminum alloys tested in this research. The effect of extrusion processing condition, alloy composition and aging time on the extrudability, mechanical properties and fracture morphology as well as microstructure of Al-Mg-Si alloys was studied. The extrudability of the Al-Mg-Si alloys was observed to reduce with an increase in the Mg-Si content, presence of 0.2 wt % Cr and reduction in extrusion temperature. For laboratory extrusion trials, peripheral coarse grains were not observed in alloys containing Cr and Mn. The strength property in the T4, T5 and T6 conditions was observed to increase with Mg-Si content while 0.2 wt % Cr addition favors improved ductility. The alloys containing Cr were observed to exhibit ductile dimple fracture and a transgranular morphology while an intergranular fracture was observed in the Al-Mg-Si alloys with no Cr content. The strain rate sensitivity and anisotropic behaviors of an Al-Mg-Si alloy were determined by performing quasi-static tensile test at 0.001, 0.01 and 0.1 s-1 on heat treated (T4 and T6) profiles extruded in industrial scale extrusion equipment. The T4 alloy samples were observed to exhibit reduced anisotropic behavior in comparison with T6 samples. In this thesis, a more effective constitutive model for predicting the hot flow stress behavior of Al-Mg-Si alloys has been developed. The developed constitutive relation can be an effective tool for modeling forming processes such as extrusion and rolling, Also, a systematic through process (from extrusion processing to aging) investigation of the effect of alloy composition, extrusion and aging conditions on the final mechanical properties of Al-Mg-Si alloys has been performed.