Mechanical behaviour of hybrid welded-bonded joints of 3.7035 titanium alloy and CFRP in thin-walled tubular trusses.

This study investigates mechanical behaviour and load-bearing capacity of welded, bonded, and hybrid welded-bonded joints made from thin-walled 3.7035 titanium alloy (Grade 2) and carbon fibre-reinforced polymer (CFRP) tubes under quasi-static loading at room temperature. The motivation lies in the aerospace industry, where significant weight savings can be achieved by replacing welded metal frames with bonded or hybrid configurations (by combining metals and composites). A comprehensive experimental program is carried out, including tensile tests on titanium base material, welded butt and T-joints, peel tests on bonded joints, combined peel-tensile tests on hybrid joints, and three-point bending tests on welded butt-joints. The results show that welded butt-joints exhibit ~ 30% lower strength than the base material, while T-joints exhibit ~ 20% lower strength than butt-joints due to non-pure tensile loading. Bonded joints demonstrate ~ 45% lower load-bearing capacity than welded joints, while hybrid welded-bonded joints show comparable strength to the base material with ~ 16% reduction relative to welded joints. Bending tests confirm the adequacy of the welding process, as no cracks or fractures are observed. The tested specimens exhibited excellent repeatability (CoV ≤ 0.10) in terms of initial stiffness and load-bearing capacity (i.e., ultimate strength and peel strength). Microscopic and macroscopic examinations, as well as hardness measurements, are conducted on welded Y-joints revealing grain coarsening and softening in the heat-affected zone, explaining the observed failure locations in welded specimens.