Objective Currently, the development of lightweight materials has become the primary objective of manufacturing, and the application of high-strength steel has emerged. Compared with traditional lightweight materials such as aluminum and magnesium alloys, 1000 MPa ultrahigh-strength steel exhibits good strength and toughness, high safety performance, and low application costs. It also exhibits good economic benefits. However, the application of high-strength steel requires reliable welding joints. Recently, the welding joints obtained via traditional welding methods are softened, the heat-affected zone is embrittled, the impact toughness of welding joints is low, the joint strength is lower than that of the base metal, the welding deformation is large, and the welding efficiency is low. As a new type of heat source welding method, laser-arc hybrid welding can effectively restrain the softening of the heat-affected zone, reduce damage to the base material, and exhibits a high welding efficiency and small deformation, etc. Furthermore, the performance of welding joints of 1000-MPa-grade tempered ultrahigh-strength steel increasingly deteriorates with the increase of strength, limiting the popularization and application of this type of high-strength steel. Therefore, it is considerably important to select appropriate filling materials and adopt laser-arc hybrid welding for studying the performance of welding joints of this type of high-strength steel.

Methods Equal-strength matching welding wire MG90-G and low-strength matching welding wire ER80YM were selected to perform laser-arc hybrid single-pass welding on the 1000-MPa-grade quenched and tempered ultrahigh-strength steel BS960E to well solve the deterioration of the welded joint performance of this type of high-strength steel. The welding equipment is manufactured using a 10 kW fiber laser (TRUMPF LASER TruDisk 10002) and a welding machine. The laser-guided hybrid welding method is adopted. In addition, the groove form is type I, and the butt gap is 1 mm. The welding process is optimized through the single factor variable method, and the welding quality is evaluated via non-destructive flaw detection. The heat input changes are collected based on the thermal cycle. The tensile strength, hardness, and low-temperature impact toughness of the welding joints for two types of welding materials were tested, and the fracture was analyzed via microscope and scanning.

Results and Discussions The mechanical properties of the BS960E laser-arc hybrid welded joints were analyzed when using different welding speeds and welding materials, and the following results were obtained. 1) Welding quality: The weld formation was good when the welding speeds were 1.32 m/min and 0.72 m/min. With the increase of welding speed, the porosity of welding seam decreased significantly. Thus, a stable molten pool at high welding speed can inhibit the generation of porosity. At a high speed, the fusion ratio of welding joints increases, the beneficial elements of welding wire transition to welding seam are reduced, and the impact property of welding seam deteriorates. 2) Hardness and microstructure: Under the condition of high-speed welding, the hardness of welds increases by 9.7% compared with that observed under low welding speeds. Laser-arc hybrid welding can effectively improve and restrain the softening of welding joints, and the hardness of the softening zone is reduced by 8.8% compared with that of the base material. The microstructure of the weld with a welding speed of 1.32 m/min includes bainite and M-A component, and the microstructure in the heat-affected zone is lath martensite. 3) Tensile properties: The tensile strengths of 90 wire and 80 wire welded joints are 1129, 1117, 1145, and 1084 MPa, respectively. The tensile strength is equivalent to that of the base metal, and the elongations observed in the aforementioned cases are 93%, 98%, 104%, and 102% of the base metal, respectively. Thus, low-strength matching has better ductility. 4) Impact performance and fracture: The impact performances of the two welds at low welding speeds increased by 28.5% and 15.7%, respectively, compared with those observed at high welding speeds. The impact fracture of the welding seam of the two types of welding wires exhibits the characteristics of ductile fracture, and the impact fracture of the heat-affected zone is mainly manifested as a brittle fracture. With the decrease of welding speed, the brittleness of the MG90-G welding joint in the heat-affected zone increases, but the impact performance deteriorates less obviously because of the small difference in the welding input energy. However, the impact toughness of the ER80YM wire welded joint in the heat-affected zone increases with the decrease of welding speed.

Conclusions Research results show that laser-arc hybrid welding can effectively improve the softening of joints and reduce the width of heat-affected zone of joints; therefore, the tensile fracture can be observed in the base metal. Under the action of the laser heat source, laser-arc hybrid welding can effectively improve defects such as undercuts caused by traditional welding. Further, under the conditions of fast cooling and heating, the weld porosity can be effectively reduced by increasing the welding speed. From the matching analysis of welding materials, the laser-arc hybrid welded joint with low-strength matching wires exhibits better ductility than that obtained in case of equal-strength matching wires. Further, the tensile strength is equivalent to that of the base metal. Under the same welding process parameters, the impact performance of different welding wire welded joints is small, which can be attributed to the unevenness of the weld metal composition and the influence of fusion ratio. Currently, the impact performance of the welded joints of this type of high-strength steel is a weak link, which must be further improved. Therefore, under the condition of laser-arc hybrid welding, the selection of appropriate heat input and low-strength matching welding wires can effectively improve the low-temperature impact performance of welding joints.