Lightweight cars have become an inevitable trend for coping with global warming. A large number of advanced high-strength steels (AHSSs) have been developed to improve lightweight, safety, and other aspects. Third-generation AHSSs, represented by QP980 steel, are suitable for a variety of automotive parts, such as cross members, stringers, B-pillar reinforcements, base beams, and bumper reinforcements. Welding is an important joining method used in the production of automotive structures. Therefore, it is necessary to develop advanced welding technology for QP980 steel.

A new welding method using laser-TIG hybrid welding technology and a low-melting-point welding wire was developed. The lap contact portion of the substrate was 10 mm without gaps. The laser beam acted vertically on the substrate, and the angle between the TIG welding wire and substrate or laser beam was 45°. The welding wire was placed 25° away from the substrate. Ar (99.99% purity) was used as a shielding gas. The TIG current was used in DC mode. A scanning electron microscope equipped with an energy spectrometer was used to observe the microstructures and fracture surface morphologies of the welded joints. An electron microhardness tester was used to determine the microhardness variations in the joint. The sample tensile parameters were tested three times using a DN300 universal testing machine at a constant speed of 1 mm/min.

For the same arc current, the contact angle θ is smaller for laser-TIG welded joints than for TIG welded joints, and molten metal spread length S and brazing zone length W are larger than those for TIG welded joints (Fig.4). The results showed that the wettability of the laser-TIG-welded joint was better than that of the TIG welded joint. The laser improved the arc energy utilization, and the melt-pool temperature was higher. The higher the temperature, the better the wettability of the molten metal. The cracks started from the brazing zone (Fig.10); therefore, the formation of the brazing zone was directly related to the mechanical properties of the joint. The analysis of the joint strengthening mechanism was as follows: the arc current increases from 100 A to 140 A, and the increase in W increased the bearing zone of the brazing zone and the total load of the joint. During the tensile process, the deflection of the joint causes the stress concentration in the brazing zone to crack more easily. A longer W also limits the deflection of the joint. Simultaneously, with an increase in the current, the melting amount of the substrate increased, and the strengthening phase (Fe-rich islands and Fe-rich particles) of the joint increased. Therefore, with the increase of the current, the ultimate tensile shear load of the joint increases. After the laser was introduced, the attraction and compression effects of the laser on the arc concentrated the arc energy and improved the wettability of the joint. Simultaneously, the melting amount of the substrate increased, and the strengthening phases in the fusion welding and brazing zones increased. Therefore, the performance of the laser-TIG hybrid welded joint was better than that of TIG welded joint under the same arc current. With an increase in current, the attraction and compression effects of the laser on the arc are weakened, and the length gap of W decreases under a larger current; thus, the gap in the joint performance decreases. Based on the above analysis, it is important to further improve the performance of the lap welded joint of QP980 steel by using laser-TIG control joint forming.

A new welding method for laser-TIG hybrid welding technology and a low-melting-point wire were developed to investigate the QP980 lap welded joint formation, organization, and strengthening mechanisms. Macroscopic morphology: the joint consists of the fusion welding, brazing, and heat-affected zones; owing to the addition of the laser, the contact angle of the laser-TIG hybrid welded joint is further reduced, and the length of the brazing zone is further increased. The microstructure of the fusion welding zone is dominated by a copper-based organization with Fe-rich islands and Fe-rich particles distributed among them, and the brazing zone is distributed with Fe-rich particles. The tensile shear load for both welding methods increase with the increase of arc current; the laser-TIG hybrid welded joint has a higher tensile shear load for the same arc current. Increasing the arc current and introducing a laser increase the length of the brazing zone and improve joint performance.