Magnesium alloy parts have the merits of low specific gravity, high specific strength, electromagnetic wave-proof shelter, and recyclability; therefore, it has been extensively applied to 3C and car industries. However, the processing and forming of magnesium is quite difficult to control due to magnesium’s hexagonal close-packed (HCP) structure, making the slipping face of itself less than the FCC material. Currently, common processing methods of magnesium alloys are die casting, semi-solid forming, and plastic forming. In the employment of a fixed-speed method for extrusion, the extruded sheet had serious defects in the forms of cracks on the surface. Hence, in this research, AZ31 magnesium alloy sheet metals were processed by hot extrusion using a variable speed method. The formability of AZ31 sheets under converging dies was investigated. Three converging dies with semi die angle of 20°, 30°, and 40° were used. Experiments were conducted and analyzed utilizing the Taguchi method. L9 orthogonal array was used to design the experiments under extrusion ratio of 35.9. Four important process parameters considered in this research are the heating temperature of the billet (320°C, 340°C and 360°C), the temperature of the container (300°C, 350°C and 400°C), the initial speed of extrusion (2mm/sec, 3mm/sec and 4mm/sec), and the lubricants (boron nitride, molybdenum disulphide and graphite) applied in the extrusion. The influences of these parameters to the extrusion load and the resulting mechanical properties were investigated. Moreover, the microstructure of the extruded sheets was observed to provide better insight of the formability. As a result, the optimal combinations of the process parameters were determined for the maximum tensile strength.
Improving mechanical properties of ZK60 magnesium alloy by cryogenic treatment before hot extrusion
