Stir-modification of primary Si crystals in semisolid metal processed hypereutectic Al-Si (20 wt.%) alloy by fatigue-erosion mechanism

摘要

The aim of the present paper was to investigate stir-modification of primary Si crystals (hereafter referred to as PSCs) in a hypereutectic Al-Si (20 wt.%) alloy produced by semisolid metal (hereafter referred to as SSM) processing. Different parameters affecting SSM processing such as shear rates, time, temperature and their combined effects were selected close to the practical industrial conditions in order to obtain near net shaping route. Based on the present experimental observations, it is proposed that the morphological evolution of long plate-like PSCs into much smaller particles having microfaceted and/or near spherical shape is governed by the combined mechanisms of plastic bending during the initial stage of SSM processing (i.e. bending stage), followed by high temperature erosion-fatigue processes. The results of this research indicate that neither the fracture mechanism nor the increased nucleation rate due to continuous stirring is responsible for the morphological transformation, as it has been reported in the literature. Furthermore, in the stirred slurries, repeated mutual collisions of rapidly moving fractured or fragmented PSCs in the viscous media cause further morphological evolution of mechanically stirred solidified alloys (termed stir-modification in this research). Optical and Scanning Electron Microscopy with EBS and EDS, TEM and AFM were used for micro structural characterization.