Rapid compression testing of semi-solid alloy slugs

Thixoforming -or Semi-Solid Metal Processing (SSM)- is the shaping of metal components in the semi-solid state by injection of the slurry into a die. For this to be possible, the alloy microstructure must consist of solid metal spheroids in a liquid matrix. In this state, the alloy is thixotropic: if it is sheared the viscosity falls and it flows like a liquid, but if allowed to stand it thickens again. Thus, a slug of alloy, heated into the semi-solid state can be cut with a knife and spread like butter, provided the microstructure is non-dendritic. Under normal thixoforming conditions, the semi-solid slugs experience a rapid compression before flowing into the die cavity. During a very short time interval, they experience intense shear thinning and their flow behaviour changes from one resembling a solid able to support its own shape, to that of a viscous liquid [1]. Here, a technique of rapid compression testing is outlined, carried out under conditions similar to normal thixoforming, to assess its flow behaviour. The economic benefits of being able to model numerically the filling of dies in semi-solid processing to ensure defect-free products and to avoid the costly and time-consuming trial and error approach, are self-evident. To carry this out properly will eventually require both the computation of slurry flow and of heat transfer into the die. However at the present stage of progress, there appears to be disagreement about the appropriate rheological models to be employed even under isothermal conditions, which clearly must be resolved first before faking the matter further. Many of the rheological investigations on alloy slurries to date have been concerned with fraction solids less than 0.4 and with steady-state viscosities achieved after several minutes of constant shearing. Since commercial thixocasting is carried out on slugs having 0.5 fraction solid or above and die filling takes around 0.1s, steady-state conditions can hardly apply and the results of these studies are of limited value for industrial processing. Recently, Barkhudarov et al. [2] have modified a commercial CFD program (FLOW-3D code, Version 7.1) to include thixotropic behaviour during flow, and this has been used to analyse the results of rapid compression tests carried out at Sheffield on A357 aluminium alloy at fractions solid and shear rate conditions close to those experienced at the initial stages of commercial thixocasting.[3].