Microstructure analysis of rheoformed AZ91 alloy produced by rotating magnetic fields

AZ91 magnesium alloy was rheoformed using rotating magnetic fields (RMFs) after isothermal near-liquidus treatment. Thixotropic microstructures obtained at different temperatures were characterized in detail and linked to the corresponding volume solid fraction, and serial sectioning experiments were performed to construct three-dimensional morphologies of clusters consisting of conglutinated primary grains. The experimental results indicated that incompact structure agglomerating a few primary solid particles was observed within structures of low solid fraction, while a reduction in the castings’ forming temperature and so increasing the fraction of primary solid phase brought out interaction among the primary particles within vigorously stirring melt, and thus caused slide and plastic deformation between or among the neighboring solid particles. In contrast with structure with low solid fraction, this slide and plastic deformation resulted in differently morphological 3D structures welded together with much more solid particles. Moreover, the increase in solid fraction mainly lied on the formation of new particles.