Anisotropy and Non-Uniformity in Plastic Behavior of AZ31 Magnesium Alloy Plates

Abstract

On the AZ31-O magnesium alloy plates of 20 mm in thickness, basal plane texture was studied for the samples taken from different layers. Compression tests were carried out in the rolling, width and thickness directions at room temperature. Tensile tests were conducted for the specimens taken from the different layers of the plates in different planar directions. The formability in deep drawing and stretch forming was evaluated for the thin sheet specimens taken from different layers of the plates and the results were discussed in relation to the texture and mechanical properties. The severity of the basal plane texture is higher at the surface layer than the inner layers. In tensile tests at room temperature, proof stress is higher for the surface layer than the inner layers, whereas elongation is lower and r-value is higher at the surface layer. In compression tests at room temperature, yield stress in the rolling and width directions is appreciably lower than in the thickness direction. At 573 K, anisotropic and non-uniform deformation behavior disappeared. Thin sheet specimens taken from inner layers of the plates showed higher formability than those from the surface layer in deepdrawing and stretch forming. It is concluded that the formability of magnesium alloy sheets can be improved by decreasing the severity of the basal plane texture. In rolling of magnesium and its alloys, the basal plane of the hexagonal lattice becomes aligned parallel to the sheet plane so that a strong basal plane texture is formed. Such textures are maintained even after recrystallization by annealing. Thus, rolled sheets of magnesium and its alloys generally show a strong anisotropy in plastic deformation behavior at around room temperature. Deformation accom- panying with thickness reduction is always difficult, that is, they show low limits of plastic deformation in cold rolling and in sheet metal forming. This is the main reason why the rolled sheets of magnesium and its alloys are not widely used in practice in spite of the recent high demands for the light- weight materials. However, high plastic anisotropy of magnesium alloy sheets becomes disappeared at tempera- tures above 473 K due to contribution of non-basal plane slip systems. At these temperatures, sheet rolling of magnesium alloys becomes much easier and the formability in deep drawing, 1-3) stretch forming, 4) bending 5) and forming limit curves 6) is appreciably improved. In thick plates of magnesium alloys, basal plane texture is considered to be less developed than in thin sheets due to lower reduction and higher rolling temperature. Additionally, mechanical properties in the thickness direction can be evaluated in thick plates. The anisotropy and non-uniformity of the basal plane texture and mechanical properties can also be discussed by testing specimens from the different layers of the plates. Obtained results are considered to suggest the desirable textures and mechanical properties for higher formability at room temperature. It has been reported in plane strain compression tests of AZ31 magnesium alloy plates that the compressive yield stress in the rolling direction is appreciably lower than in the other directions, which can be attributed to the highly developed basal plane textures. 7) However, very little has been known on the plastic behavior of magnesium plates. It is the purpose of this work to elucidate the anisotropy and non-uniformity of plastic behavior of the magnesium plates. In this work, the basal plane texture was studied for the samples taken from different layers of AZ31-O magnesium alloy plates of 20 mm in thickness. Compression tests were done in the rolling, width and thickness directions at room temperature. Tensile tests were conducted for the specimens taken from the different layers of the plates in different planar directions. Finally, formability in deepdrawing and stretch forming was evaluated for the thin sheet specimens taken from different layers of the plates and the results were discussed in relation to the texture and mechanical properties.