Pre-strain assisted low heat-input friction stir processing to achieve ultrafine-grained copper

In this study, the effects of pre-rolling and friction stir processing (FSP) on the microstructure and mechanical behavior of pure copper were investigated. With increasing the tool rotational speed from 400 to 800 rpm, the average grain size increased for both 0% (annealed) and 60% (pre-rolled) samples. Also, pre-rolling of copper led to a reduction in the average grain size of the FSPed samples. It was found that homogeneously distributed fine grains structure in the 400–100 (0%) (400 rpm–100 mm/min – with 0% pre-rolling), 400–100 (60%), and 800–100 (60%) samples were observed due to the large heat transfer caused by the copper backing plate. The application of rolling and FSP caused an improvement in hardness, yield strength, and ultimate tensile strength values due to the grain refinement mechanism. Among the FSPed samples, the 600–100 (60%) sample showed a better strength-ductility balance than the other samples. In the initial and FSPed samples, many dimples were observed, revealing the ductile fracture. With increasing the rotational speed, the dimples morphology became more homogeneous. Also, the pre-rolled samples had dimples smaller than the annealed samples.