Qizhen Li

Equal Channel Angular Pressing (ECAP) technique is widely used to refine grains in metallic materials, including magnesium and its alloys. This paper reviews the grain refinement data for various magnesium alloys achieved through ECAP processing. The data are analyzed to explore the effect of ECAP parameters on grain refinement effectiveness and provide us with some guidelines for selecting appropriate values for the processing parameters. The parameters studied include the internal channel angle ϕ, the outer arc angle Ψ, the pressing temperature T, the number of ECAP passes N, and the type of ECAP processing Route. Some key observations and general trends were obtained based on the analysis: (i) route Bc is generally the most effective processing route to generate finer grains and it is the most commonly used pressing route in the literature; (ii) the most commonly selected ECAP temperature falls between 200 °C and 300 °C; (iii) ECAP processing often refines grains the most during the first pass; and (iv) the most commonly used number of ECAP passes is less than 8.

This study focused on the analysis of geometrically necessary dislocation (GND) densities for five selected fine-grained magnesium samples. Among the samples, three were tested under different fatigue-loading conditions at 0 °C, one experienced quasi-static tensile loading at 0 °C, and one represented the as-rolled state. The fatigue-tested samples were chosen according to the relationship between the maximum loading stress of a test and the material’s yield strength. This study provides new insights on the deformation mechanism of fine-grained magnesium at 0 °C. It is observed that the average GND densities were increased by 95~111% for the tested samples when compared with the as-rolled sample. It is especially interesting that there is a significant increase in the average GND density for the sample that experienced the fatigue loading with a low-maximum applied stress, and the maximum applied stress was lower than the material’s yield strength. This observation implies that the grain boundary mediated the dislocation-emission mechanism.

Powder metallurgy is a popular method of making raw powders into specific shaped samples. However, the pressure distribution and the microhardness difference within the sample are nonnegligible and unclear when the sample is long or exceeds a specific size. In this study, the long magnesium blocks, with a ratio of about 2.8 between the sample height and the sample side length, are successfully synthesized under three uniaxial and two biaxial conditions. Then, the sample hardness values on the outer surface and the center plane are tested to study the microhardness distribution. The modified analytical expression indicates that the normal pressure exponentially decreases along the compression direction, which is consistent with the hardness distribution trend. Because higher pressure leads to a more compact arrangement of the powders, more metal bonds are formed after sintering. During the first pressing, the sidewall pressure makes the surface hardness higher. The secondary reverse compression mainly improves the bottom and core hardness due to the re-orientation and re-location of the powders. The obtained relationship between the applied pressure and the hardness distribution is instructive in predicting and improving the sample quality.