A novel forming process of AZ31 magnesium alloy square tube based on numerical simulations and experiments

In this work, a hot extrusion forming process for magnesium (Mg) alloy square tubes, namely, tube-extrusion-continuous expending shear (TECES), was proposed. The thin-walled hollow square tube of Mg alloy with a thickness of 3 mm was successfully prepared by hot extrusion experiment. The evolution rules of effective stress, effective strain, and flow velocity in the process of tube forming were analyzed by numerical simulation, and the influence of different expanding ratios on tube forming was compared. The results show that the effective stress distribution of the tube is not uniform during the forming process, and the maximum stress appears at the first shear corner. In the shear deformation (SD) zone, there is a high strain zone at the corner of the square tube and outside the tube wall. As the tube is extruded, the uniformity of strain distribution in the deformation zone is improved. In the whole forming process, the flow rate of the tube increases first and then decreases, and the whole remains stable. As the expansion ratio increases when the expansion ratio is greater than 1.6, the resistance of plastic deformation increases, the fluidity of the metal becomes worse, and the formed tube cracks at the corner or is difficult to fill. When the expansion ratio is less than 1.6, the damage value area appears on the inner side of the tube wall, and the distribution uniformity is poor, which may lead to cracks or uneven thickness of the tube wall. The microstructure characterization of different deformation zones of the tube was carried out, and the results show that the TECES process can effectively refine the grain, and the average grain size of the formed tube is 10.1 μm.