Laser–Arc Hybrid Welding of 10-mm-thick Q690E High-Strength Steel: Formation, Microstructure, and Mechanical Properties

Q690E high-strength low-alloy (HSLA) steel, with excellent low-temperature toughness and impact resistance, is an ideal material for building long-span bridges. HSLA steel arc/laser welding faces the weld heat-affected zone (HAZ) softening and porosity/spatter defects, respectively. Using response surface methodology combined with multi-objective optimization of process parameters, this study successfully fabricated welded joints in 10-mm-thick Q690E HSLA steel by laser–arc hybrid welding. The microstructure and mechanical properties of the welded joints were systematically investigated. The results indicate that the welded joint consisted of the weld metal zone and the HAZ. The weld metal zone was subdivided into the upper fusion zone (FZ) and the lower laser zone (LZ). The HAZ consisted of the coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), and intercritical HAZ (ICHAZ). The microstructures of the FZ, LZ, CGHAZ, and FGHAZ were dominated by lath martensite. The microstructure of the ICHAZ consisted of martensite and bainite. Differences in microstructural distribution led to nonuniform microhardness profiles. Test results showed that the CGHAZ exhibited the highest microhardness, followed by the LZ and FZ. Tensile tests showed that all welded joints fractured in the base material (BM) away from the weld seam. In room-temperature three-point bending tests, the welded joints exhibited flexural strength higher than that of the BM. Low-temperature Charpy impact tests indicated brittle fracture in the welded joints, with the impact absorption energy reaching 79.8% of that of the BM, exceeding the industrial testing standard. These results provide theoretical support for the industrial welding of 10-mm-thick Q690E HSLA steel.