Modification of mechanical properties of friction stir welded Cu joint by additional liquid CO2 cooling

•Additional liquid CO2 cooling was applied during the FSW of Cu.•The cooling rate of the joint significantly increased when using liquid CO2 cooling.•Additional liquid CO2 cooling provided ultrafine grains with a high density of dislocations in the stir zone.•The post-annealing effect caused by the friction heat was successfully reduced by additional liquid CO2 cooling.•The additional liquid CO2 cooling produced joints with excellent mechanical properties.

A notable effect of the retained heat after the fiction stir welding (FSW) of Cu was studied by the application of additional liquid CO2 cooling. The adjustment of the welding parameters made it possible to conduct the FSW processes, but the cooling rate of the joint was not sufficiently changed due to the existence of the post-annealing effect. However, the accelerated cooling can reduce the post-annealing effect and provide an ultrafine grain structure with a high dislocation density in the stir zone, and a joint with mechanical properties better than the base metal level was achieved.