Investigation on the strength-ductility synergy in the laser-welded titanium/steel bimetallic sheets used for pressure vessels

The welding of titanium/steel bimetallic sheets exhibits a great challenge owing to the formation of Fe-Ti intermetallic compounds, which can severely degrade the mechanical properties of the welded joint. In this paper, a FeCrNiCu filler metal was used for laser welding TA2/Q235 bimetallic sheets. The strength-ductility synergy and fracture behavior of the welded joint were investigated. A noteworthy finding was that the FeCrNiCu filler metal could generate a high mixing entropy value in the weld seam, which promoted the formation of a primarily face-centered cubic (FCC) phase and coarse grains within the weld seam. The negative enthalpy variation in the transition zones (TZs) on the TA2 layer resulted in phase structures predominantly composed of Fe₂Ti, FCC, and α-Ti phases, accompanied by fine grains. Consequently, extremely high hardness values, ranging from 600 to 764 HV_0.2 were observed in the TZ. The fragile zones of the welded joint shifted from the weld seam to the TZ, which played a significant role in promoting crack initiation and propagation in the welded joint during mechanical testing. The welded joint fabricated with the FeCrNiCu filler metal exhibited a favorable strength-ductility synergy. The strength coefficient of the welded joint was up to 92.5 %, with a fracture elongation of 6.9 %. Additionally, the welded joint demonstrated promising bending properties. A bending angle of 180° was achieved with no surface cracks observed on the weld seam during root bending tests (compressive stress on the TA2 layer).