Pin length, pin offset, and reversed metal flow interaction in the improvement of dissimilar friction stir welded T-lap joints

The structural elements of T-joints which include skin and stringer plates are extensively used in many fields due to their excellent properties. The high-strength joints can contribute significantly to effective construction. In this study, the dissimilar joint between 5083 and 7075 aluminum alloys was fabricated by different friction stir welding methods in T-lap shape. Four pins with different pin lengths were used to fabricate the joints in both single- and double-pass welding. In addition, when welding in two passes, the pins were offset toward the advancing side and the pin rotations were reversed. The interaction of pin length, pin offset, and inverse material flow in the interface formation was clarified. The fracture behavior of the T-joins was explored in both skin and stringer tests. Experimental results showed that the pin length made a minor contribution to weld improvement in single-pass welding. In double-pass welding, the interfacial morphology was significantly improved by utilizing a suitable pin length and offsetting the pin toward the advancing side. In the latter case, a notable oxide layer was eliminated along the interface. A solid quality of the joint was observed with small kissing bond defects, and a high tensile and fatigue strength (about 90% joint efficiency in both skin and stringer tensile tests) was found. Notably, the use of a short pin length was ineffective solution in fragmenting the oxide layer during both single- and double-pass welding. To clarify these results, the interfacial formation and crack propagation behavior of the T-joints were analyzed.