Secondary solidification behaviour of the Al–Si–Mg alloy prepared by the rheo-diecasting process

Solidification behaviour of the liquid in semisolid Al–7Si–0.6Mg slurry in the rheo-diecasting process has been investigated. It was found that the intensive shearing not only influences the solidification of the primary α-Al globules formed inside the slurry maker, but also the solidification process of the remaining liquid. Due to the prior shearing, the liquid is extremely uniform in terms of composition fields and thermal conditions, so that the nucleation of the α-Al occurs throughout the entire volume of the liquid in the die. More importantly, the nuclei can survive and contribute to the final microstructure. Analysis based on the Mullins–Sekerka stability criterion reveals that the solidification of the α-Al is completed before the nuclei start to lose their growth stability. The eutectic reaction involving the formation of the eutectic Si is confined to small intergranular areas. The high local cooling rate allows the eutectic Si to solidify in fine fibrous or well-branched platelet-like morphologies.