Thixotropy in Semisolid Steel Slurries under Rapid Compression

Two grades of steel, an M2 tool steel and a high-performance HP9/4/30 steel, were used as raw material for thixoforming. The M2 has a large semisolid range of over 473 K (200 °C), while that for HP9/4/30 is much narrower, i.e., around 70 °C. Thixoforming is easier in alloys with a wide semisolid temperature range. Partial remelting was carried out to produce feedstock material for thixoforming. On partial remelting, the large carbides in M2 seed liquation and help with the breakup of the microstructure to form spheroids as required for thixoforming. In contrast, HP9/4/30 partial remelting produces a coarser microstructure with truncated solid cells, but this has been shown to be suitable for thixoforming. Thixoforming of M2 at 1633 K (1360 °C), which corresponds to about 32 pct liquid volume content, resulted in complete die filling. HP9/4/30 was thixoformed at a higher liquid fraction due to its less suitable microstructure, in the temperature range of 1743 K to 1753 K (1470 °C to 1480 °C) at various holding times and ram speeds. During thixoforming, load-displacement curves were obtained. Viscosities were deduced from these using the analysis developed by Laxmanan and Flemings. Hence, the shear thinning of two high-temperature metallic systems was characterized and the load-displacement signals correlated with thixoforming behavior. For successful thixoforming, the load-displacement curve will have very little initial resistance to flow (the initial peak that characterizes the breakup of the solid skeleton) and a low flow stress.