Evaluation of optimal processing parameters for a Zn-based eutectoid alloy processed by friction-stir welding

Zn-Al structures offer various benefits, such as high mechanical strength, biocompatibility, good resistance to corrosion, low melting point and low cost. In this work, the study of optimal processing parameters of Zn-22Al-2Cu eutectoid alloy for friction-stir welding is undertaken. Firstly, as-cast plates with a thickness of 3.5 mm were processed considering two different tool geometries (flat and grooved surface), a rotary speed from 1100 to 1500 rpm and a welding speed of 15 mm/s. The metallurgical characterization of the joints was analyzed by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction. Tensile tests were performed in as-cast and welded samples, detecting a larger value of 280 MPa for as-cast condition. Moreover, Vickers hardness measurements were recorded for different joints and a maximum value of 150 HV was determined. The results showed that the welded Zn-22Al-2Cu joints with a grooved surface at 1100 rpm present superior mechanical properties and non-appearance of tunneling defects. In addition, the welding process was replicated by applying the finite element method in order to achieve numerical results of temperature, strain and stress in the stir zone. Welding microstructures revealed a grain refinement in the stirring zone due to the movement of α and η grains. The dispersed duplex microstructure in Zn-Al alloys prevented excessive grain growth and the tool characteristics played a substantial role in the weld quality preventing tunneling defects in the joint.