Thermomechanical processing of an aluminium casting alloy for thixoforming

Thermomechanical processing of Al-Si casting alloys may be attractive when the material of choice for a large number of small parts to be thixoformed is a higher silicon grade aluminium alloy. With particle dispersion characteristics unique to hot-worked metals and a matrix no longer dendritic owing to extensive plastic flow and welding of α-Al fibrous grains, the microstructure of the extruded AlSi8Cu3Fe stock is typical of particulate-reinforced wrought alloys. Its response to reheating in the semi-solid temperature range is also similar to that of wrought aluminium alloys processed thermomechanically. Of the three different extrusion temperatures employed in the present work, 420 °C produced, after reheating into the semi-solid tempertaure range, the best features for thixoformability, with nearly globular α-Al grains across much of the section. While the anisotropy of growth rates due to the abundance of Si particles aligned in the extrusion axis gives elongated grains in the interior of the slug, high aspect ratios are largely compensated for with increasing soaking time. α-Al grains tend to globularize and the difference in their size between the surface and the interior is also reduced with soaking time owing to the growth rates which become increasingly more uniform throughout the volume.