INFLUENCE OF EXTRUSION TEMPERATURE AND COOLING RATE ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF PRE-AGED EXTRUDED ZK61 Mg ALLOY

Deformation temperature and cooling rate have a large influence on the microstructure and mechanical properties. In this paper, the effects of pre-ageing extrusion temperatures (240, 320) °C and subsequent cooling modes (air cooled, 27 °C water cooled) on the microstructure and mechanical properties of extruded ZK61 alloy were investigated. The microstructure evolution mechanisms as well as dynamic recrystallization (DRX) principle were explored, and the strength-plasticity mechanism was analyzed. The higher the extrusion temperature, the higher the DRX ratio, and the larger the grain size. A higher temperature also provides energy for grain-boundary migration. The faster the cooling rate, the fewer the precipitated phases, the lower the DRX fraction, and the finer the grain size. A faster cooling rate did not allow enough time for the precipitates to nucleate, and DRX grains did not have the time to grow. The 240 °C extruded and air-cooled (LA) sample showed the best strength-plasticity synergy, with ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) of 359.7 MPa, 281.8 MPa, and 18.8 %, respectively. This was mainly the result of a combined effect of fine-grain strengthening, dislocation strengthening, and texture strengthening.