Fatigue Threshold of Friction Welded Ti-6Al-4V

The α/β alloy Ti-6Al-4V (Ti-64) is extensively used in aerospace industry due to its good balance of properties, and being one of the most developed and tested titanium alloys.  The application of linear friction welding (LFW) technology to join Ti-64 to itself has necessitated the mechanical assessments of fatigue and fatigue threshold of the welded joints.  Fatigue threshold tests were carried out on linear friction welded Ti-64 testpieces under various R ratios at both room temperature and an elevated temperature of 300°C, using a single edge notched bending testpiece geometry with the weld plane located in the centre of the gauge section and being perpendicular to the mode I crack opening stress.  The crack growth resistance and threshold value were found to be sensitive to fatigue crack direction relative to the welding oscillation direction with smaller threshold values measured with a crack growth direction parallel to the welding oscillation direction.  This is attributed to texture developed during LFW process. It is evident that stronger texture results in larger departure in threshold values between two crack growth directions. At a temperature of 300°C, the influence of texture on fatigue threshold is much reduced. The observed trends are interpreted with the aid of metallographic and fractographic observations, together with TEM analysis of deformation characteristics ahead of the crack tip.