Micromechanisms of Medical Mg–Zn alloys during Semi-solid powder forming (SPF)

Mg alloys with advantages of biocompatibility and biodegradability are considered as a new type of medical material with great potential. Compared with casting and additive manufacturing, Semi-solid powder forming (SPF) is a promising new technique to prepare medical Mg alloys due to the characteristics of fine microstructures, a short process cycle and good comprehensive properties. In this study, the micromechanism of Mg–6Zn mixed powder materials under a semi-solid isothermal and deformation station was analyzed and discussed based on the results of SEM, EDS, XRD, synchrotron radiation X-ray computed tomography (SRXCT) and kinetic analysis. The results show all the Zn powders melt and Mg powder dissolves into the liquid Zn to form a Zn–Mg liquid wrapping around the Mg powders the semi-solid isothermal state. The powder particle boundary blurred and transgranular liquation cracking (TLC) occurs under the loading force, making the powders being broken into fragments which spheroidize and coarsen afterward. Simultaneously, the dynamic model of Mg–6Zn mixed powders was deduced and established, which agrees well with the experimental results and can predict the actual liquid fraction during SPF. The pores reconstructed by SRXCT has a low connectivity, near-spherical morphology and flat surface, combined with encapsulated microstructure with finer grains and fewer secondary phases, which are beneficial to slow down the degradation rate. Finally, the micromechanism of SPF was concluded as atomic diffusion, liquid flowing and filling, transgranular liquation cracking (TLC) of particles as well as coarsening and spheroidizing of its fragments, which provides theoretical guidance for optimizing the forming process.