Semi-Solid Metal Processing - A Processing Method Under Low Flow Loads

Semi-solid metal processing, also known as thixoforming, is a forming process that shapes metal components in their semisolid state. For this to be possible, it is preferable for an alloy to have an appreciable melting range and before forming the microstructure must consist of solid metal spheroids in a liquid matrix. There is strong interest in thixoforming high temperature materials such as tool steels. The established solid state processing approach to obtaining a non-dendritic microstructure is through recrystallisation processes of RAP (Recrystallisation And Partial melting) and SIMA (Strain Induced Melt Activated). The first involves cold or warm working below the recrystallisation temperature followed by heating above the solidusofthe material, while the latter involves hot working between recrystallisation and solidus temperatures,followed by cold working so as to produce a critical level of strain, before the material is reheated above the solidus. Here, a commercially produced M2 tool steel alloy, i.e. being hot worked, tempered and annealed, was subjected to a direct partial remelting experiment in a protective atmosphere from room temperature with no prior additional cold working. The microstructures revealed equiaxed solid grains (in liquid matrix) of average sizes between 29 and 31 ^m when reheated to above the solidus of 1320-1360°C. Subsequent thixoforming of the material in the form of a slug of 36 mm diameter and 46 mm height into a simple finger of 66 mm long,43 mm wide and 7 mm thick at 1360°C showed thatthe slurryflowed thixotropically undervery lowflow load oftypically lessthan about 2 kN.