Conventional forming processes for metal products are forging or near net shape casting. In the first process, ingots of the metal are formed in the solid state and in the latter process the products are cast from molten metal. Discoveries in the early seventies at MIT showed that metals can achieve thixotropic properties in the semisolid state. This means that the viscosity of a metal may be considered to be both shear and time dependant. By heating an ingot of thixotropic material into the two-phase region, the metal is allowed to flow like a liquid containing up to 60% solid particles.
The fluid like behaviour of thixotropic metals has resulted in several suggestions for new casting processes, or modifications of conventional methods such as die casting. The most important benefits of semi-solid materials are, first of all, a laminar mould filling process resulting in less entrapped air. Second, the low volume fraction of liquid also contributes to less porosity caused by solidification. These benefits result in a product that can be heat treated along with identifying new opportunities for magnesium castings in highly stressed applications. Concerning magnesium alloys, thixotropic casting could provide the casting industry with new process technology for a “melt-less foundry”.
The paper will discuss a conventional method for production of thixotropic magnesium alloys. The ingots have been reheated and processed in a conventional squeeze casting machine into various products. Structures and properties of these products are presented.
Laboratory tests for characterizing thixotropic structures and establishing the casting parameters for the alloys will be presented.