T6 heat treatment of semi-solid metal processed alloy A356

Abstract

Semi-solid metal (SSM) processing is a unique manufacturing method to produce near-net shape products for various industrial applications. The aim is to obtain a semi-solid structure free of dendrites (which are formed by conventional liquid casting), with the solid present as nearly a spherical form as possible. This semi-solid mixture (like a gel, or toothpaste) flows homogeneously, behaving as a thixotropic fluid with viscosity depending on shear rate and fraction solid. There are two different SSM processes: thixocasting and rheocasting. With thixocasting, a specially prepared billet of solid material with a globular microstructure is reheated into the semi-solid range and formed. Rheocasting involves preparation of a SSM slurry directly from the liquid, followed by a forming process such as high pressure die casting (HPDC). The increased costs associated with thixocasting (recycling of thixocast scrap and the necessity of an outside manufacturer for billet production) have resulted in rheocasting becoming the preferred semi-solid process.