Viscosity-shear rate relationship during thixoforming of HP9/4/30 Steel

Thixoforming involves shaping metal components in the semi-solid state. Work on the thixoforming of high temperature materials, such as steel, is still at its initial stage; this is mainly due to the high processing temperatures involved and the potential for oxidation. For thixoforming to be possible, it is preferable for an alloy to have an appreciable melting range and before forming the microstructure must ideally consist of solid metal spheroids in a liquid matrix. This paper discusses the thixoforming load versus displacement curves of HP9/4/30 steel semi-solid slugs under compression. The data from the corresponding load-displacement curves is converted into viscosity against shear rate adopting Stefan’s equation for flow between two parallel planes. The viscosity at processing conditions, i.e. at processing temperatures in the range of 1470 to 1480°C and zero to two minutes soak times, showed a rapid decrease initially, which corresponds to a rapid breakdown in the structure, followed by a steady decrease to a near constant value with increasing rate. The work shows pseudoplasticity (or shear-thinning) behaviour of the HP9/4/30 semi-solid slurries. This data would be required for modeling the die fill with these slurries.