Microstructure Evolution of Semi-Solid 6061 Aluminium Alloy Billet during Partial Remelting

The 6061 semi-solid aluminium alloy feedstock was prepared by nearby-liquidus semi-continuous casting technology.The feedstock was re-heated and partially remelted at 610 ℃.The microstructure and size characteristics of the alloy heated for different times were analyzed by optical microscopy and image analyzer.The liquid formation,microstructure evolution and growth of grains during holding time were observed and analyzed.It is found that the volume fraction of liquid phase increased first and reached equilibrium then with the prolongation of holding time due to the effects of phase transition and diffusion.The rosette-like non-dendritic structure characteristic was gradually weakened and the microstructure transformed to near-spherical structure.The grains grew and the size distributed heterogeneously with the increase of the holding time.The grains were round and the size distributed homogeneously after holding for 480 s,which was suitable for thixocasting.