Study on Thixotropic Plastic Forming of Wrought Magnesium Alloy

Abstract

Semi-sold material (SSM) forming was proposed by Flemings et al. from Massachusetts Institute of Technology (MIT) in 1972. SSM forming is one of the near-net-shape-forming processes which manufacture the final part by loading the materials at a temperature between liquidus and solidus. This way of forming difficult-to-machine materials is useful because there is much less strain resistance compared with conventional forging processes. Furthermore, energy savings and compact forming machinery become possible because of forming at smaller loads. Therefore, SSM forming is taken for one of the near-net-shapeforming having the best future in the 21th century (Kang et al., 1999). Compared with casting magnesium alloys, wrought magnesium alloys have higher mechanical strength, better ductility and more varied mechanical properties. But the magnesium element is so lively that it is liable to be oxidated or burned during smelting and working. Simultaneously, the lower strength under high temperature and creep properties restrict the application of magnesium alloys in high temperature occasion. In addition, influenced by the microstructure, magnesium alloys have low deforming property and poor plasticity. Start from pursuing to the net forming technology and lighter production, the semi-solid thixo-forming technology becomes a preferred hot forming technique for wrought magnesium alloys (Flemings, 1991; Yan & Xia, 2005). At high solid volume fraction (≥50%~60%), the behaviour of semi-solid material mechanics in the thixotropic plastic deformation process can be solved only when the relationship between stress and strain is obtained. Recently there have been a lot of studies on this field. A stress-strain relationship was investigated through considering the separate coefficient for semi-solid materials (Kang et al., 1998). The mathematical models of yield criterion and constitutive equations of mushy/semi-solid alloys were proposed (Kiuchi et al., 1998). The viscoplastic constitutive equations of SSM were presented by analyzing viscoplastic behaviour of material (Martin et al., 1997). On the basis of analysis of the thixotropic plastic deformation behaviour of semi-solid wrought magnesium alloy in compression process (Yan & Zhou, 2006), its constitutive model was established (Yan & Zhou, 2006). Semi-solid processing technology is a new metal working technology in recent years, in which the alloys in semi-solid state have the excellent thixotropic and rheological properties. The deformation resistance in semi-solid state is influenced by not only solid volume