The influence of process factors on the production of semi solid feedstock.

Abstract

Semi-solid manufacturing is a near net shape forming process that takes advantage of an alloy's thixotropic behaviour. However, in order to obtain the desired thixotropic properties from an alloy in the semi -solid state, the microstructure of the as-cast feedstock metal needs to display a fine grained, equiaxed primary phase prior to reheating for the forming operation. Various methods are currently in use to obtain the required microstructure of which the MagnetoHydroDynamic (MHD) process is predominant. Two fundamental factors, namely shear rate and cooling rate, influence the formation of the fine grained, equiaxed primary phase during the MHD process. The aim of this research was to produce semi solid billets and in so doing, determine how the influence of the combination of the two fundamental factors contribute towards the final formation of the primary phase and to determine an optimal level ofthese factors' settings to deliver the desired microstructure. An MHD apparatus was constructed and the Taguchi method was used to design an experiment to investigate the influence ofthe fundamental factors involved in casting semi solid feedstock of aluminium A356.2. The issues ofthe formation of a fine eutectic phase and solidification shrinkage were also investigated. An experimental method was designed to investigate the significance ofthe fundamental factors' influence towards the appearance of the primary phase; the latter was evaluated using an image analysis system. The shear rate was controlled by varying the line frequency and the base frequency supplied to the electromagnetic stirrer and the cooling rate was controlled by initiation of a fixed, fast cooling rate at a certain melt temperature (TJ Results showed that a fine grained, equiaxed primary phase, with an average grain size of 55 /lm, was achieved after casting, prior to reheating for forming. The contribution of the base frequency and the line frequency were 8 % and 3.5 % respectively and the contribution ofTi was 86.5 % towards the outcome ofthe result. The cooling rate changed from approximately 0.3 QC/sec to 4.5 QC/sec at Ti. A fine textured eutectic phase was achieved with the fast cooling rate. The solidification shrinkage was accounted for by incorporating a riser on the mould. The feedstock produced in this research was compared, on a microstructural basis, to commercially available Semi Solid Metal (SSM) feedstock from Pechiney and SAG. The research feedstock had a larger, average primary grain size, however, it was more discrete and round grained than the commercial alloys which were finer and more rosette grained. Upon reheating to the semi solid state, ready for forming, the final, evolved grain sizes and shapes were almost identical between the research and commercial feedstock, despite the initial differences in grain sizes and shapes. However, the commercial alloys showed primary grains with trapped eutectic whereas in the research alloy, the primary grains were largely free of trapped eutectic.