Effect of Zener-Hollomon Parameter on Microstructure and Mechanical Properties of Copper Subjected to Friction Stir Welding

In this work, the influence of the Zener-Hollomon (Z) parameter on the microstructure and mechanical properties of copper subjected to friction stir welding (FSW) was investigated. Liquid N₂ cooling was conducted to control the cooling rate after the FSW. The obtained results demonstrate that the Z parameter was dependent on the tool rotation rate during the FSW, i.e., a higher tool rotating rate resulted in a lower Z parameter. The grain size in the stir zone decreased with the increase in the Z parameter. The relationship between the yield strength and the Z parameter is established as σ_0.2 = σ₀ + kZⁿ. This relationship exhibited two different plots under the conditions of air cooling and liquid N₂ cooling. Even at a similar Z parameter, a significant yield strength difference occurred because massive dislocations, which were caused by the prevention of the post-annealing effect, were maintained in the stir zone. This study suggests that the influence of the post-annealing effect should not be neglected when analyzing the relationship between the Z parameter, microstructure, and mechanical properties.