Mixed grain structure and mechanical property anisotropy of AZ40 magnesium alloy bar with diameter of 160 mm

The mixed grain structure and mechanical property anisotropy of AZ40 magnesium alloy bar with a diameter of 160 mm manufactured by “multi-direction forging (MDF) + extrusion + online cooling” technique were investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction macro-texture measurement and room temperature (RT) tensile test. The results show that mixed grain structure is caused by the micro-segregation of Al in semi-continuous casting ingot. Homogenization of (380 °C, 8 h) + (410 °C, 12 h) cannot totally eliminate such micro-segregation. During MDF and extrusion, the dendrite interiors with 3%-4% Al (mass fraction) transform to fine grain zones, yet the dendrite edges with about 6% Al transform to coarse grain zones. XRD macro-textures of the outer, R/2 and center show typical fiber texture characteristics and the intensity of [0001]//RaD orientation in the outer (11.245) is about twice as big as those in the R/2 (6.026) and center (6.979). The as-extruded AZ40 magnesium alloy bar has high elongation (A) and moderate ultimate tensile strength (Rm) in both extrusion direction (ED) and radius direction (RaD), i.e., A of 19%-25% and Rm of 256-264 MPa; however, yield strength (Rp0.2) shows anisotropy and heterogeneity, i.e., 103 MPa in RaD, 137 MPa in ED-C (the center) and 161 MPa in ED-O (the outer), which are mainly caused by the texture. (155 °C, 7 h) + (170 °C, 24 h) aging has no influence on strength and elongation of AZ40 magnesium bar.