Semisolid processing characteristics of AM series Mg alloys by rheo-diecasting
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I. INTRODUCTION
THE development of the semisolid metal (SSM) processing technique was initiated immediately following the discovery of the pseudoplastic and thixotropic properties of metallic alloys at temperatures between the liquidus and the solidus.[1] Through more than three decades of development, SSM processing has been successfully established as a unique technique for production of metallic components with high integrity and improved mechanical properties.[2,3] One of the latest technical developments in SSM processing is the rheodiecasting (RDC) process,[4,5] where high-quality SSM slurry containing fine and spheroidal primary particles is produced via a twin-screw shearing mechanism, and subsequently cast for near-net shape forming by conventional high pressure die casting (HPDC). The technique is applicable to a wide variety of metallic materials, particularly light metals. Owing to their high strength-to-weight ratio, ease of casting, and superior machinability, magnesium alloys are receiving increased attention as structural materials. The AM series alloys are among the most widely used magnesium alloys because of their excellent combination of ductility and strength. In these alloys, aluminum (Al) and manganese (Mn) are two primary alloying elements. The Al improves strength and hardness and lowers the melting point for easy casting, and Mn mainly improves the seawater corrosion resistance, by removing iron and possibly other heavy metal elements into relatively harmless intermetallic compounds.[6] It has been found that an Al content of 6 wt pct gives the optimum combination of ductility and strength to Mg-Al–based alloys when processed by conventional means.[6] Further increasS. JI, Chief Engineer, MA QIAN, Lecturer, and Z. FAN, Professor, are with the Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University, Uxbridge, Middlesex, UB8 3PH, United Kingdom. Contact e-mail: [email protected] Manuscript submitted July 22, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
ing the Al content will degrade the elongation to failure of Mg-Al–based alloys. This has been well demonstrated for AZ series Mg alloys. For instance, the elongation of the HPDC-processed AZ91D (Mg-9 pct Al-1 pct Zn) alloy is typically just around 3 pct.[6] The widely used HPDCprocessed AM50 and AM60B are based on this principle. No higher Al content AM alloys have been developed for HPDC yet. However, how the Al content affects the microstructure and mechanical properties of SSM-processed AM series alloys is not clear. In fact, there have been contradictory reports about the advantages of SSM processing over HPDC for AM alloys. Koren et al.[7] conducted both rheocasting and thixocasting trials on AM50 under different casting conditions, and concluded that no obvious advantages were observed for SSM processing of AM50, in terms of both rheological and mechanical properties. On the other hand, improved mechanical properties have been reported for thixomolded AM50 and AM60B alloys. The ultimate tensile streng
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