Weld Quality and Microstructure Development in Ultrasonically Welded Titanium Joints

Joints of 0.5 mm thick sheets of commercially pure titanium have been processed by ultrasonic spot welding with the amplitude of vibrations 20 mu m and welding time 2 and 3 seconds under the clamping force varied from 5 to 7 kN. It is shown that an increase in the clamping force and welding time results in an increase of the dimensions of defect-free bonded area and decrease of the thickness of the sheets. Ultrasonic welding results in the formation of thermomechanically affected zone (TMAZ), in which significant changes in the microstructure and microtexture occur. The alpha grain size in the central region of the TMAZ increases from 7 up to 130 mu m, the fraction of low angle boundaries increases and sharp peaks near 60 deg and 90 deg appear in the misorientation angle distribution of alpha grain boundaries. The ultrasonically welded samples exhibit high lap shear strengths similar to those obtained by resistance spot welding.