Mechanical behavior of as welded and smooth cord titanium welded joints

This work aims to study the fatigue behaviour of hybrid welded joints made of titanium alloy. Butt welded joints with plate thickness of 5 mm are compared to butt welded joints of plate thickness 3 mm smoothed to the cord. All specimens are realized with a hybrid welding technique. The fatigue strength of specimens is analysed in order to plot fatigue curves both in terms of nominal stress and local strain amplitude, according to the WELFARE method. Experimental tests show that the fatigue strength is highly dependent on structural variations, defects and imperfections near to the cord, due to an inadequate welding process. Superficial imperfections as fused material drops, are nevertheless eliminated through filing the cord; in this way an increase of the fatigue strength can be obtained. Smooth specimens with plate thickness of 3 mm result in having longer life duration than specimens with thickness of 5 mm. Moreover, 3 mm thickness hybrid welded joints with a grinded cord seem to have the same behaviour in fatigue resistance as laser welded joints of the same thickness. However, even after the filing, some kinds of defects as internal micro bubbles in the cord, or welding spots, continue to reduce the life of titanium hybrid welded joints.