A study on local thermal and strain phenomena of high-speed friction stir-processed Mg AZ31


The increasing demand for low-weight structural materials yields a growing interest in Mg alloys. The aim of this work is to investigate the microstructural, thermal, and mechanical development of high-speed friction stir-processed Mg AZ31. The process parameters were developed with underlying preset to develop a fine-grained microstructure. Microstructural as well as hardness investigations were conducted. The energy input per unit length as well as the temperature evolution around the tool was predicted using finite element methods calibrated and backed up by temperature measurements. Local strain evolution analysis was conducted using digital image correlation (DIC) on tensile test. Process zone investigations have been conducted using microtensile tests. This study has shown that while maintaining constant weld quality, the energy input per unit length continuously decreased with increasing processing speed between 1 and 5 m/min, reached a threshold at higher speeds. Thermal analysis revealed temperatures between 470 and 300 °C directly under the tool depending on processing speed accompanied by a strong asymmetry between the advancing side (AS) and the retreating side. Hardness measurements showed an increase with increasing processing speed as well as a hardness peak development on the AS. DIC on tensile test confirmed the asymmetric strength distribution within the process zone. Microtensile test display a ductility increase of up to 85 % compared to the base material within the process zone.
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