Studies of texture and mechanical properties of 6xxx automotive alloy extrusions

Abstract

The use of aluminium in the automotive industry has increased during the last decades, since aluminium alloys show some advantages compared to more traditional structural materials. Their light weight can result in a total vehicle weight reduction and enhance the reduction of CO2 emissions and fuel consumption. Extrusion is a leading process for wrought aluminium alloys, capably to provide finished or semi-finished components. The effect of preferred crystallographic orientation (texture) on rolled aluminium alloys has been widely studied, but there is limited literature on extruded materials. Moreover, since extrusion is a complex procedure, involving various parameters, there is still a lack of understanding of how these parameters affect texture and final mechanical properties. In this thesis a deeper understanding of this effect is attempted. An industrial high strength 6xxx aluminium alloy, provided by Constellium, was extruded and investigated. Microstructure and texture were studied with the use of EBSD, OIM and XRD. Mechanical properties were obtained by standard tensile testing. Experimental results showed the role of extrusion speed and extrusion exit temperature. It was shown that the increase of speed, and furthermore the increase of exit temperature, increased the average grain size. The overall texture was not affected by the speed, but the changes of the amount of various texture component was noticed. The decrease of Cube and increase of S component resulted in an increase of 13 MPa in yield strength. The effect of extrusion geometry on microstructure, texture and mechanical properties was found to be significant. Four profiles were chosen, a flat bar, a hollow rectangle, a round bar and a tow nut, based on their different extrusion ratios (ER). It was found that both microstructure and texture were grouped into two profiles with low ER (round bar and tow nut) and the two profiles with high ER (flat bar and hollow rectangle). The texture of the higher ER profiles resembled rolling texture with a combination of Brass, S and Cube components while for the lower ER profiles, the texture was reported Cu- Brass-Cube. Moreover, the mechanical properties followed the same division, with the strength of round bar and tow nut being higher than the other two. To decouple the effect of geometry from that of ER, two investigations were introduced. An investigation of same geometry with various ER using different diameter of round bar was performed. Another investigation of similar ER with various geometries using small/large round bar, flat bar and tow nut was performed. For the first investigation, it was discovered that the effect of ER on the three round bars was limited to small microstructural changes, while the overall texture remained the same. On the other hand, the comparison of different geometries with similar ER resulted in completely different microstructure and texture for the profiles with high ER. These differences were also reflected on the mechanical properties. The investigation of a complex profile, a hollow rectangle with fins at the bottom, showed that observed strength differences from side to side are related to differences in crystallographic texture and grains morphology. The effect of further thermomechanical treatment (aDA) after extrusion on texture was found to be limited and it was mainly resulted from the deformation part of the aDA.