Squeeze-Casting and Thixo-Casting of Magnesium Alloys

摘要

Extended use of magnesium in safety relevant parts of car body frames, the drive train, and the chassis requires parts with customized characteristic profiles. Combinations of cast parts, cast knots, sheet mould parts, and extruded profiles comparable to the aluminium space frame realized, are possible. Figure 1 shows a schematic view. The requirements for the materials cannot as yet be entirely fulfilled by magnesium alloys. The biggest problem is their limited ductility at room temperature. Magnesium crystallizes in the hexagonal closest packed structure and is therefore not amenable to satisfactory cold-forming. This is a problem of metal physics that can only be overcome by a change of the crystal structure by alloying (e.g. magnesium/lithium alloys). Besides, the ductility is highly dependent on the grain size and the number of defects in the lattice, such as pores and enclosures. The grain size of both cast and wrought alloys can be reduced. With cast alloys, grain-refining additives are included, but they only work to a certain extent with machine-cast parts like die-cast parts. An adequate fine-grained microstructure can either be obtained by the rapid solidification of thin-walled parts or by applying new casting procedures such as squeeze-casting and thixo-casting.