Objective In recent years, the aerospace manufacturing industry has proposed the requirements for large-scale and lightweight aircraft, and weight reduction has become the first priority of the aircraft manufacturing industry. Dual-laser-beam bilateral synchronous welding (DLBSW) is an innovative manufacturing process for producing the skin-stringer T-shaped structure, which was first proposed by Airbus. Currently, the study of DLBSW has mainly focused on the skin-stringer T-joints of aluminum alloy, with only a few references to the Ti6Al4V alloy T-joints and their properties. This T-joint forms a molten pool under the action of bilateral laser beams during the process of DLBSW, and the thermal effect is superimposed at the bottom of the molten pool. Because the Ti6Al4V alloy is sensitive to thermal action, the equiaxial grain in the heat-affected zone (HAZ) is easily coarsened during welding, which weakens the mechanical properties and service performance of the T-joint. Therefore, a rational selection of heat input is necessary for the laser welding of the titanium alloy. This study aims to reveal the influence of laser power on the microstructure of the Ti6Al4V alloy T-joint and explore the influence of microstructure on the fracture performance of the joint.
Methods In this study, the DLBSW experiment of the Ti6Al4V alloy T-joints was performed using two KUKA robots and a TruDisk 12003 laser produced by TRUMPF. The laser and the laser welding head are connected by two optical fibers. During the welding process, the laser beam and shielding gas were delivered symmetrically along the center line of the stringer (Fig. 1). Following the welding experiments, two groups of well-formed T-joints were selected for metallographic and tensile specimen preparation. The specimens were cut along the cross-section of T-joints, and the uneven area is avoided. The microstructure of the joint after polishing and corrosion was observed under an MR-5000 metallographic microscope. The tensile experiment was performed using an electromechanical universal testing machine at 25 ℃. The loading rate was set to 0.8 mm/min, and the fracture morphology was observed using a scanning electron microscope.
Results and Discussions When the laser power is 2.1 kW, the HAZ above the upper fusion zone of the joint can be divided into the obvious coarse grain region (CGR), fine grain region (FGR), and transition region (TR) owing to the thermal effect of the molten metal on both sides of specimen. There is a large area of TR below the lower fusion zone without the obvious CGR and FGR (Fig. 6). The fracture surface of the joint is near the HAZ, and the tensile strength exceeds 800 MPa (Fig. 8). When the laser power is 2.3 kW, the α' phase that precipitated inside the columnar crystal in the weld bead zone coarsenes significantly because of the increase in heat input. Additionally, different from the joint adopting laser power of 2.1 kW, the HAZ below the lower fusion zone of joint No.2 can be divided into the CGR, FGR, and TR owing to the increase in laser power. A wide CGR appears below the lower fusion zone, where the grains have severely coarsened and some of the equiaxed grains have increased to 100 μm in size (Fig. 7). The joint is invalid in the weld bead zone, and the tensile strength is only 478 MPa (Fig. 8). The fractographs of the joint are mixed, except for the equiaxed dimples, and the exfoliated lamellar α' phase interface can be found on the fracture surface. It can be concluded that the fracture mode of specimen No.3 is a hybrid fracture, comprising both the cleavage and ductile fractures (Fig. 11).
Conclusions Experimental results indicate that the Ti6Al4V alloy T-joint manufactured by DLBSW is more sensitive to changes in the laser power. When the laser power increases, a large area of CGR is observed below the lower fusion zone, with some grain size reaching 100 μm. However, the region below the lower fusion zone of the joint adopting the laser power of 2.1 kW is a wider TR, while those of the CGR and FGR are not obvious. The joint under the laser power of 2.1 kW failed in the HAZ on the skin when a higher tensile strength exceeded 90% of the strength of the base material. When the laser power is increased to 2.3 kW, the joint is fractured in the weld bead zone, and its strength is significantly reduced compared with the specimen adopting a laser power of 2.1 kW. According to the fractographs of the joint, the fracture mode of the joint under lower laser power is a microporous aggregation fracture, which is classified as a ductile fracture. When the laser power is increased to 2.3 kW, the fracture mode of the joint is a combination of the ductile and cleavage fractures, with a mixed morphology comprising equiaxed dimples and exfoliated lamellar α' phase interface.
