Forming at the semi-solid state - Rheoforming and thixoforming

The shaping of alloys from the liquid state can be carried out using a variety of processes, all of which have in common the fact that the alloy must be cast in liquid form at a sufficiently high temperature to enable the mold to be filled correctly. This often results in acute problems of segregation, porosity, cracking and shrinkage. Shaping at a lower temperature, from an already partially solidified state, seems, a priori, very interesting; however, it runs up against the dendritic nature of solidification, which means that the alloy cannot be cast successfully in a mold as soon as the solidified volume fraction exceeds around 20 to 30%. Nor can this state undergo significant deformation without the appearance of cracks and segregation of the liquid phase. Work carried out at MIT (Massachusetts Institute of Technology) in the 1970s showed that intense mechanical stirring of the solidifying liquid could prevent the formation of this dendritic structure, resulting in a relatively low-viscosity, semi-solid jelly suitable for shaping in this state. The initial work was soon put to good use in applying the technique to ferrous alloys as part of programs supported by the US government. The feasibility of semi-solid forming was thus demonstrated, not only by injection molding the jelly produced by stirring, but also by molding semi-solid billets obtained by partial remelting of bars or ingots previously solidified from the jelly.

Since then, significant progress has been made. Initially, electromagnetic stirring was widely used to obtain the jelly. Then, new processes, based on controlled solidification of the alloy, were proposed to generate the globular structure more economically. At the same time, other semi-solid shaping processes (forging, rolling, extrusion) were developed.

Rheoforming refers to all shaping processes applied to alloys in the semi-solid state obtained during solidification.

Thixoforming refers to all shaping processes applied to alloys in the semi-solid state obtained during partial remelting.