Observation of interface of two kinds of bi-metal composite parts prepared by thixo-forging

Two kinds of bi-metal composite parts （Sn-15%Pb and Pb-22%Sn bi-metal system, and Al-7%Si and SiCp/6061 MMC bi-metal system） were prepared by the strain-induced melt activated thixo-forging. The interfaces of the bi-metal composites were observed by OM and SEM. The observations show that the semisolid metals keep independence during thixo-forging. The solid phases in the semisolid slurries maintain their original morphologies after thixo-forging. The liquid phases near the interface mix together and form a thin layer. The interfaces are bonded firmly with the metallurgical bonding. No oxide layers are found at the interfaces. Strengths of the interfaces were investigated by the micro-hardness test. The experimental results show that the composite interfaces have high strength. However, the agglomerated enhancing particles cause fine defect on the interface of the Al-7%Si and SiCr/6061 MMC bi-metal composite.

Grain Size Control of Semisolid A356 Alloy Manufactured by Electromagnetic Stirring

To investigate the possibility of substituting the mechanical stirring system with electromagnetic stirring (EMS) system for aluminum rheo die-casting, the EMS under the different stirring cooling conditions was carried out. It was found that in the early period of solidification, the dendrite breakages led to a fine primary phase. When dendrites grew coarsely, the effect of ripening on grain size overwhelmed that of dendrite breakage. It was also found that the high cooling rate favored large nucleation rate, and led to a fine primary phase. But high cooling rate also made the growth rate of the dendrite arm, which prevented the dendrite arm from being sheared off. Therefore there were a suitable stirring time and suitable cooling rate to obtain the best rheo die-casting structure. Qualified semisolid A356 aluminum alloy was successfully manufactured with short time EMS.

Microstructural Evolution of Solid-solution-treated Zn-22Al in the Semisolid State

The effect of solid-solution-treatment on the semisolid microstructure of Zn-22Al with developed dendrites was investigated. Forming Zn-22Al products by semisolid metal processing offers significant advantages, such as reductions in macro-segregation, porosity and forming costs. Thermal and rnicrostructural analyses of the formed Zn-22Al alloy were performed by differential scanning calorimetry, scanning electron microscopy and optical microscopy. The changes in the microstructures and phase transformation in response to various solid- solution-treatments were analysed. In this study, as-cast samples were held isothermally at 330 ℃ for 0.5- 5 h and then partially remelted at a semisolid temperature of 438 ℃ for 1 h to produce a solid-globular grain structure in a liquid matrix. A non-dendritic semisolid microstructure could not be obtained when the traditionally cast Zn 22Al alloy with developed dendrites was subjected directly to partial remelting. After solid-solution-treatment at 330 ℃, the black interdendritic eutectics were dissolved, and the dendritic structures gradually transformed into uniform β structures when the treatment time was increased. The coarsened and merged dendrites were separated as a result of penetration by the liquid phase and melting of the residual eutectic at sites along the former grain boundaries. The microstructure of the solid-solution- treated sample transformed into a small globular structure; the best shape factor of 0.9, corresponding to a particle size of 40 ± 16 μm, is achieved when the sample was treated for 3 h followed by direct partial remelting into its semisolid zone.