Evaluation of mechanical and metallurgical properties of AZ91 seamless tubes produced by radial-forward extrusion method

Radial forward extrusion (RFE) is a suitable method for producing large diameter seamless tubes with superior properties from smaller cylindrical billets. Material flow along the radial and forward channels causes large effective strains and consequently improved mechanical and metallurgical properties of the final tube. In this study, isothermal RFE process was applied to a commercial AZ91 magnesium alloy at 300 °C, and the mechanical properties and microstructure evolution were examined. Due to severe plastic deformation at 300 °C, dynamic recrystallization occurs, and equiaxed grains were formed. The grain size was reduced to ~3 µm from the initial value of ~150 µm. Yield and ultimate strength were increased by 3.2 and 2.6 times compared to the initial values, respectively. Though, the resulting properties from RFE are similar to those of hot extruded AZ91. Besides, elongation was increased up to 2 times after the RFE process. Microhardness was also increased to 88 Hv from the initial value of 52 HV. Good homogeneity of effective strain and microhardness in the longitudinal section was observed while force requirement is dramatically reduced compared to the conventional extrusion. The RFE process seems to be a very promising extrusion process for producing fine-grained seamless tubes.