The effect of Mn on the high temperature flow stress of Al–Mg–Si alloys

Abstract

The demand for Al–Mg–Si extrusion alloys in the transportation sector is increasing rapidly. An important aspect of the process is high temperature extrusion and in particular, the breakthrough force for the extrusion press. The breakthrough force varies as a function of the extrusion ratio, extrusion speed and temperature, the complexity of the profile and the flow stress of the alloy. Further, Mn and Cr are often added to the alloy which form dispersoids during homogenization which precipitation strengthen the alloy at high temperature. This work presents a new approach to calculate the volume fraction of dispersoids using electron microprobe analysis, electrical resistivity measurements and transmission electron microscopy (TEM). The new method substantially increases the accuracy of the volume fraction determination compared with traditional image analysis or TEM since the volume of material sampled is much larger avoiding effects of local segregation. Using the volume fraction and dispersoid size measurements, a high temperature flow stress model based on dislocation theory has been developed. It is shown to describe the high temperature flow stress measurements to within 5% in 95% of the cases. The model was validated by independent experimentation where it predicted the flow stress within 5% accuracy. The flow stress model can be used independently based on experimental quantification of the dispersoids or as part of a through process model which includes a KWN based precipitation model for dispersoids and then implemented as constitutive equation for a FEM based model of the extrusion process.