Thermomechanical processing of AA6061 billets for semi-solid forming

Abstract

Abstract Extruded bars of AA6061 alloy were heat treated above the solidus temperature to produce wrought aluminium feedstock for thixoforming. The extruded material recrystallized during heating to the semi-solid state, producing nearly equiaxed polygonal grains with high-angle boundaries. Melting started at triple junctions during subsequent isothermal heating and progressed along grain boundaries. The low melting point eutectic phase and the high-angle grain boundaries introduced by recrystallization both promoted the grain boundary melting process. The desired feedstock characteristics, comprising complete grain boundary wetting, a perfectly cylindrical slug shape and an average grain size below 100 microns were obtained after 5 min at 640 °C and 635 °C for extrusion ratios of 14: 1 and 5: 1, respectively.