Influence of in-process cooling on tensile behaviour of friction stir welded joints of AA7039

In this investigation, precipitation hardening Al-Zn-Mg alloy AA7039 was friction stir welded with and without in-process cooling in order to investigate the effect of in-process cooling on microstructure and tensile properties of the developed joints. The in-process cooling was employed using compressed air, liquid nitrogen and normal water. The effect of in-process cooling on the microstructure and mechanical properties of the weld joints was investigated by means of optical microscopy, tensile and microhardness tests. Fracture surfaces of the tensile tested weld joints were studied with a field emission scanning electron microscope (FE-SEM). The in-process cooling decreased the size of heat affected zones (HAZs) and thermo-mechanically affected zones (TMAZs) and also reduced the size of α aluminum grains in weld nugget zone (WNZ), TMAZs and HAZs. The hardness of the HAZ was found higher than WNZ and TMAZ of the joints developed with in-process cooling while reverse trend was observed for normal joints. In-process cooling with water was found to be more effective than other in-process cooling methods, and had ultimate tensile strength and percentage elongation equivalent to 73.5% and 86.8% of the base metal. In-process cooling changed the fracture location from heat affected zone to weld nugget zone.