Thixoforming of non-dendritic AA6061 feedstock produced by low superheat casting with and without a cooling slope

Abstract

Abstract Low superheat casting with and without a cooling slope was employed to produce AA6061 thixoforming feedstock. The dendritic features were more frequent in the latter, suggesting that the degeneration of the dendritic structure was promoted by the cooling slope casting process. Minor differences in the primary phase morphology in the as-cast state were no longer evident after partial melting, with nearly equiaxed polygonal grains in all ingots cast with and without a cooling slope. 5 min soaking at 640 °C was identified as the best reheating practice as it provided the smallest grains and the most complete grain boundary wetting. The slugs machined from AA6061 ingots cast from a pouring temperature of 680 °C with and without a cooling slope were both successfully thixoformed into sound parts with no signs of porosity and segregation. The T6 heat treatment produced hardness values as high as 107 HB which compare very favourably with those of the extruded products and are even better than that obtained in the thermomechanically processed feedstock.