Thermomechanical processing of AA6061 billets for semi-solid forming

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 degrees C and 635 degrees C for extrusion ratios of 14: 1 and 5: 1, respectively.