D. G. Sanders, The Boeing Company, Seattle, WA
A development program was performed for friction stir welding (FSW) of titanium alloy 6Al-4V to make butt welds. It was found that the top and bottom surfaces of the FSW welds are profoundly influenced by the key process parameters, which are the spindle speed (rpm), feed rate (mm/sec), the pin tool material/geometry and the amount of heat removed from the process by water cooling the pin tool. The swirling patterned “onion band” marks on the top surface of the weld joint were investigated using metallography, SEM and other methods. The marks, which are caused by the rotating shoulder of the pin tool being pressed against the surface of the part under high temperature and pressure, were found to make very sharp and deep crack-like features. Testing has shown that the “onion bands” have a profound influence on the fatigue performance of the weld joint and that this condition is aggravated by the notch sensitivity of titanium.
Summary: The titanium friction stir welding (FSW) process parameters have a very large effect on the performance of the weld joint. This study looks at the "onion bands" that occur on the top surface of butt welds and their effect on fatigue life.
