Microstructures and microhardness for sheets and TIG welded joints of TA15 alloy using friction stir spot processing

Effects of friction stir spot processing (FSSP) on the microstructures and microhardness of tungsten inert gas (TIG) welded TA15 titanium alloy joints were investigated. The macro/micro structural observation and microhardness evaluation of the TA15 alloy sheets and TA15 TIG welded joints were carried out using optical microscope and microhardness tests. The results show that FSSP effectively improves the microstructure and increases the microhardness of the TA15 sheets. As for the TIG welded joints, FSSP also effectively improves the microstructure of joints. And the average microhardness value in weld nugget zone is improved significantly, while a small increase of this value in heat affected zone is observed. The hardness in stirring zone is significantly higher than that in the base metal. Two peak values of hardness appear along the width direction in stirring zone. After FSSP, the average hardness of the weld zone of TA15 TIG welded joint is significantly higher than that before FSSP. Under the present process parameters, both the surface oxidation in TA15 sheets and in TIG welded joints after FSSP are not evident, while the surface forms the bright white layer, which is composed of a great multitude of fine grains.