Study on Thixotropic Plastic Forming of Magnesium Matrix Composites

Abstract

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°C. 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].