Effect of extrusion speed on properties of the extruded AZ31B magnesium alloy machined chip

The effect of ram speed on surface quality and room temperature mechanical properties of AZ31B alloy machined chips extruded at elevated temperatures has been studied. Hot extrusion was applied to the machined chips as a recycling process for the chips. The extrusion tests were carried out at a ratio (R) of 100 and extrusion temperatures (T _E) of 573K and 623K, while the ram speed (V _R) was varied from 0.5 mm/s to 5 mm/s. Even at a relatively high ratio of 100, a sound extrusion surface without any cracks was obtained at ram speeds less than 3 mm/s by using a die with a round exit (R-die). However, at a ram speed of 5 mm/s, surface cracks were formed almost perpendicular to the extrusion direction on a macro- as well as micro-scale. Furthermore, the ductility of the extrusions decreased remarkably at high ram speeds. With an increase in the ram speed, the surface temperature of the extrusions measured at the die exit increased resulting in grain coarsening at the narrow surface layer of the extrusion cross section. Based on microstructural observations, the grain coarsening was found to originate at the metal layer flowing into the die exit corner along the die surface. Nitrogen gas cooling immediately after the metal layer flowed out of the die was found to be effective in suppressing the grain coarsening at the surface layer. As a result, Nitrogen gas cooling was utilized to obtain good surface quality without cracks and high ductility of the extrusions even at a high ram speed of 5 mm/s.