Microstructural Development of Aluminum A201 Alloy Semi-Solid Processing Feedstock Produced by Different Routes

Abstract

Thixoforming or Semi-Solid Metal (SSM) forming is a manufacturing route that relies on a non-dendritic microstructure, a structure that is retained after processing resulting in components of fine and uniform microstructures with enhanced mechanical properties as compared to conventional cast products. In this project, an investigation is conducted on the microstructural development of Aluminium A201 alloy in the semi-solid range, through a combination of variable time and temperature conditions. Using equilibrium and non-equilibrium (Scheil) curves, the solidus and the liquidus temperatures as well as the melting behaviour of this alloy are estimated. The temperature range for the thixoforming process typically lies within the 40-60% fraction liquid and this is found to be between 620oC to 637oC for this particular alloy. The aim is to heat treat specimens of this alloy within the semi-solid range (the process window) and observe the resulting microstructures to establish if they comprise of near-spheroidal solid phase particles surrounded by a liquid phase. This microstructure gives the material its thixotropic properties; i.e. the material flows like a liquid when sheared but thickens and behaves like a solid when it is allowed to stand. Three different routes for obtaining the non-dendritic microstructure necessary for thixoforming are investigated in this work: Magneto-Hydrodynamic (MHD) stirring, Strain Induced Melt Activated (SIMA) and Semi-Solid Rheocasting via Cooling Slope (SSR+CS) route. These routes are compared and their potential for future thixoforming applications studied.