**1. Introduction**

252 Mechanical Engineering

Zerbst, U., Madler, K., Hintze, H., (2005). Fracture mechanics in railway applications-an

Magnesium alloys have a lot of advantages in mechanical and physical properties such as lightness, high specific strength, good thermal conductivity and damping. They are widely developed for automobile, spaceflight, electron, light instrument and so on. The damping capacity of materials plays a critical role in regulating the vibration of structure, decreasing the noise pollution and improving the fatigue properties of workpiece under circulating loading. Magnesium and its alloy have higher thermodynamically stability and aging stability as well as better damping capacity, whose applications are limited because of their poor mechanical properties. Creep is an important characteristic of mechanics behavior of metal at high temperature, which is a phenomenon for plastic deformation taken place slowly on the condition of constant temperature of long time and constant load. For the industrial application fields such as automobile industry and aviation industry, the creep is an important index to measure the good or bad property of a material at high temperature. The strength and creep resistance of magnesium alloys (AZ91D and AM60 alloys) are rapid decreased when the temperature is beyond 150℃. There are the disadvantages such as poor strength and toughness and poor creep resistance in magnesium alloy applied process, which limits its farther application. So it is an important to develop the magnesium matrix composites (MMCs) of high strength and toughness and good creep resistance and its forming technology. Specially, particle-reinforced magnesium matrix composites are characterized by low cost and simple process, which is a research focus of MMCs fields [Hai et al., 2004]. However, magnesium possesses low melting point, high chemical activity and ease of flammability, so preparing magnesium matrix composites is difficult in some extent. As a result, it is important to seek a better fabrication method for magnesium matrix composites [Zhou et al., 1997]. Powder metallurgy (PM) and casting are common methods for obtaining these composite materials. PM process needs complex equipments with higher expense, and can't fabricate large sized and complicated MMCs components. It has hazards such as powder burning and exploding. In contrast, casting method can produce large sized composites (up to 500kg) in industry at mass production levels with its simple process and convenient operation because of few investing equipment and low cost. So MMCs fabricated by casting process are now investigated by many researchers [Kang et al., 1999].

The plastic formability of MMCs is poor, which need to introduce an advanced forming method. With the growing development of semi-solid forming technology, the thixoforming

Study on Thixotropic Plastic Forming of Magnesium Matrix Composites 255

Fig. 1. Schematic diagram of SiCp/AZ61 composites fabricated in stirring casting process 1. thermocouple 2. resistance thread 3. crucible 4. BT608 (artificial aptitude modulator) 5. resistance furnace 6. strring lamina 7. vacuum jar 8. pressure meter 9. pipette 10. vacuum

The microstructures of SiCp/AZ61 composites in three casting processes were shown in Fig.2. The variations of influence of three casting processes on the microstructures of SiCp/AZ61 composites were shown in Fig.2- Fig.4. The distribution of SiC particles was a little uniform in the fully-liquid casting process where a lot of gas cavities and slacks were presented, and SiC particles were easy to sink and float. There were a few gas cavities in the semi-solid casting process where the distribution of SiC particles was inhomogeneous. SiCp/AZ61 composites fabricated by the stirring-melt casting method possessed not only

The existence of gas cavities and slacks was attributed to the following factors: (1) Gas was involved in the molten during the mechanical stirring process. (2) Their non-uniform volume shrinkages presented in the composites solidification process due to the differences of their thermal expansion coefficient and heat conduction between matrix and reinforcement. (3) Hydrogen produced in the chemical reactions between Mg and H2O was dissolved in the molten and formed gas cavities during solidification. (4) The formation of

The main problem in the stirring casting process was the inhomogeneous distribution of reinforcement phase. The major reasons were followed as. (1) Due to having the different densities between matrix and reinforcement, SiC particles were settled down. (2) The higher surface tension and poor wettability between SiC and matrix presented, a few SiC particles

SiC particles were introduced at the semi-solid state during the semi-solid stirring casting process where the high viscosity semi-solid alloy can help withstand SiC particles from sinking and floating, but the uniform distribution can not be solved (shown in Fig.3). During the stirring-melt casting process the reinforcement were added at the semi-solid state, and the composites were poured immediately after reached 690℃ (liquidus). The quite uniform

SiCp/AZ61 composites can be obtained in this method (shown in Fig.4).

pump 11. guiding windpipe 12. timing electrical machine 13. stirring bar

few gas cavities but fairly uniform distribution.

gas cavities was resulted from the particles clustering.

were floated on the surface of the molten.

technology of magnesium matrix composites is a new method. The semi-solid material forming technology has advantages such as lower deformation resistance, good material mobility and so on [Flemings 1991, Yan et al., 2005]. It was composed of three processes such as: semi-solid billet fabrication [Yan et al., 2005], partial remelting [Yan et al., 2006] and thixoforming [Yan et al., 2008]. For this reason, the research on the basic theory of semi-solid stirring melting fabrication method and thixoforming process for the advanced MMCs is studied in this item. The works include the study of semi-solid stirring melting fabrication method [Yan & Fu et al., 2007; Yan & Lin et al., 2008] and reheating process [Yan & Zhang et al., 2008; Zhang et al., 2011] for the particle-reinforced MMCs. The material constitutive relation will be proposed [Yan & Wang et al., 2011]. Then the finite element model coupled with multi-physical fields will be built. The simulation will be gone based on the developed analytical program. The forming performances and deformed laws in the thixoforming for the particle-reinforced MMCs will be studied by the way of combining theoretical analysis with experimental method [Yan & Huang, 2011]. The results will play an important function to bulid the theoretical and technological fundament for the thixoforming process of the particle-reinforced MMCs applied the industry area.
