Narrow Results

This paper describes the effect of hot extrusion on the microstructure and mechanical properties of electromagnetic continuous cast (EMC) AZ31 alloy. The microstructure, mechanical properties and fracture surfaces of AZ31 alloy before and after extrusion were investigated. The results demonstrate that extrusion processing gives rise to a strong basal texture. The grains are significantly refined and the average grain size of localized fine grain area is 2μm. Compared with EMC ingots, as-extruded specimens have much finer grain size and more uniform microstructure, and the second phase (Mg₁₇Al₁₂) becomes smaller and distributes more uniformly. The mechanical properties of the deformed AZ31 were improved after hot-extrusion. When the extrusion ratio was 10, the yield strength, ultimate tensile strength and reduction in cross-sectional area of as-extruded AZ31 alloy were 248MPa, 306MPa and 28.44%, which were respectively enhanced by 78.4%, 41% and 45.25%, compared with those of as-cast samples. With the increase of extrusion ratio, the grain refining effect was more significant and the microstructure was more uniform. The yield strength, ultimate tensile strength and reduction in cross-sectional area increased obviously with increasing the extrusion ratio. The observation on fracture surfaces demonstrates that the fracture mode changes from ductile-brittle fracture to ductile fracture after extrusion.

The powder of the micro Al and variant volume fractions of nano Al₂O₃ were milled by a high energy planetary ball-mill. By milling, a homogenous distribution of nano Al₂O₃ particles in the metal matrix were developed. Then the milled powder was cold compressed and sintered at 545°C for one hr. The mold and the sintered sample hold in a furnace until the temperature reached 545°C. Then the hot 27mm diameter sample was extruded to 6mm diameter. From the extruded specimens, tensile, hardness and microstructure of the prepared specimens were determined. By these tests the effect of milling time, the percent of nano-particles and the microstructure were evaluated. The hardness and tensile behaviors of aluminum matrix composites reinforced with nano Al₂O₃ particulate have been found to increase remarkably with the volume fraction of the reinforcement.

Magnesium AZ91D alloys reinforced with fine SiC particles (AZ91/SiCp) were fabricated by mechanical milling, hot pressing and extrusion. The properties of AZ91D alloy composites were generally improved by SiC addition. The Young's modulus and hardness increased correspondingly with SiC addition up to 20-volume percent. However, the UTS and proof stress showed a maximum value at 15-volume percent and decrease at 20-volume percent. Nevertheless, the UTS values of the 20-volume percent SiC composites increased when the milling duration was increased. The influence of the volume fraction of SiC particle and milling term on mechanical properties, microstructure and density will be discussed.