Microstructural Evolution in Intensively Melt Sheared Direct Chill Cast Al-Alloys

Abstract

The work presented here introduces the novel melt conditioned direct chill casting (MC-DC) technology, where intensive melt shearing is applied to the conventional direct-chill casting process. MC-DC casting can successfully produce high quality Al-alloy billets. The results obtained from 80 mm diameter billets cast at speed of 200 mm/min show that MC-DC casting of Al-alloys, substantially refines the microstructure and reduces macro-segregation. In this paper, we present the preliminary results and discuss microstructural evolution during MC-DC casting of Al-alloys. Introduction The production of wrought alloys starts with direct-chill (DC) casting to produce billets or slabs of convenient shape and size for extrusion or rolling to make semi-fabricated products [1-3]. Unfortunately, problems such as the formation of coarse and non-uniform microstructures with severe chemical segregation [4], porosity and hot tearing are common in DC cast billets and slabs. These compositional heterogeneities and casting defects have a detrimental effect on rollability and extrudability and the mechanical properties of both semi-fabricated and finished products. Technologies such as ultrasonic cavitation, electromagnetic stirring and intensive melt shearing (MC) have been investigated [5-9], to overcome these problems. Our work involves the implementation of intensive melt conditioning during conventional DC casting, which is based on the high shear processing technology [5, 7]. The MC-DC casting process exploits the benefits of solidification under intensive forced convection provided by using a rotor-stator melt shearing system immersed in molten alloy. It has been reported previously that intensive melt shearing results in significant grain refinement of Al alloys [9]. The work demonstrates for the first time, the use of the MC-DC casting technology using a typical DC caster set up. The main focus has been to understand the mechanisms of grain refinement upon following intensive melt shearing during DC casting of Al-alloys.