Laser–arc hybrid welding (LAHW) is an advanced welding technology that integrates both laser and arc heat sources within a single molten pool, achieving synergistic benefits that surpass the sum of their individual contributions. This method enhances the welding speed and depth of the fusion, stabilizes the process, and minimizes welding defects. Numerous studies have investigated the principles, synergistic effects, keyhole dynamics, joint performance, and various factors influencing the parameters of laser–arc hybrid welding. This paper begins with an introduction to the classification of LAHW, followed by a discussion of the characteristics of gas-shielded welding, argon arc welding, and plasma hybrid welding. Subsequently, the welding principles underlying laser–arc hybrid welding will be elucidated. To enhance weld integrity and quality, this paper will analyze keyhole behavior, droplet transfer dynamics, welding quality performance, and the generation and prevention of welding defects that affect laser–arc hybrid welding. Additionally, a detailed analysis of the effects of residual stress on the shape, microstructure, and phase composition of the weld will be provided, along with an exploration of the influences of various welding parameters on post-weld deformation and mechanical properties.

A pure Ni interlayer with a thickness of 0.1 mm was introduced between high-strength interstitial free steel and Al–Mg–Si alloy, which were friction stir lap welded, producing an excellent welded joint. The interface layer consisted of a γ-Ni solid solution, and the mixed stirring zone contained alternate lamellae of γ-Ni and α-Fe solid solutions. The addition of a Ni interlayer strongly suppressed the reaction between Al and Fe because of the atomic arrangement of Ni. Furthermore, the insertion depth of the stirring pin has a significant influence on the Al/steel interfacial reaction. Under shallow insertion depth, the intermetallic compounds of both FeAl and Fe₂Al₅ were observed at the interface layer. A maximum tensile-shear fracture load of 4.3 kN was achieved, with fractures being present in the steel substrate far away from the Al/steel weld.

The microstructural characteristics and fracture behavior of a magnetic pole screw were investigated here. The screw threads were produced by cold thread rolling. Microstructural analysis (OM, SEM, EBSD), mechanical testing (tensile, hardness, fastening), and fracture morphology observation were conducted. The results indicate that work hardening and microstructural deformation were introduced by the gradient plastic deformation in the screw thread. The elongated microstructure of ferrite and pearlite was obtained in the deformation zones, resulting in increased hardness and decreased plasticity. The thread root subsurface experienced severe localized indentation deformation and exhibited the highest hardness. The distinct forming stress states led to a notable difference in the hardened layer depth between the thread crest and root. The torsional overload fracture was initiated at the stress-concentrated thread root, where the work-hardened microstructure exhibited a limited capacity to accommodate large plastic deformation. The crack propagation was influenced by the gradient microstructure, following three primary propagation paths: transgranular through ferrite, along the ferrite–pearlite phase interface, and cracking through lamellar pearlite. The results provide theoretical support for material design and process optimization to achieve the production of high-performance screws with high strength and hardness at the thread surface and high plasticity in the center.

Coarse grains and blocky M-A constituents are often generated in the heat-affected zone (HAZ) of fusion welded pipeline steel joints owing to high heat input, causing a significant deterioration of toughness. This study demonstrated the effect of heat input in friction stir welding (FSW) on the microstructure and toughness of the nugget zone (NZ), elucidating the microstructure evolution and toughening mechanism. The results revealed a marked reduction in effective grain size within the NZ at low heat input (LHI) and a significant increase in the ratio of the refined M-A constituent. Furthermore, the decreased heat input leads to weak texture components (D1, D2, and F) accompanied by a decrease in the kernel average misorientation (KAM) value. This microstructural optimization clearly enhances toughness, and an excellent toughness value of 200 J, representing 95.3% of the basal metal (BM), was achieved in the NZ at LHI. The primary reason for this improvement is the refinement of effective grains and M-A constituents resulting from reduced heat input. During crack propagation, the high proportion of effective grain boundaries and fine M-A constituents acts as a barrier, arresting and deflecting cracks and thereby enhancing toughness.

Fusion welding easily causes microstructural coarsening and tempering softening in the heat-affected zone (HAZ) of high-strength pipeline steel joints, which considerably deteriorates the strength and toughness. Here, X80 pipeline steel was subjected to friction stir welding (FSW), and external cooling was used to tailor the microstructure to optimize the strength–ductility combination of the nugget zone (NZ). Coarse granular bainite (GB) appeared at air cooling, whereas a fine ferrite/martensite microstructure was achieved at solid CO₂ cooling. The highest ratio of high-angle boundaries was obtained at solid CO₂ cooling because the variants were evenly distributed within the four close-packed (CP) groups. The low yield strength (YS) of 595 MPa was obtained in the NZ under air cooling, whereas a high YS of 755 MPa was achieved in the NZ under solid CO₂ cooling due to dislocation strengthening and fine-grain strengthening. Furthermore, an ultra-high tensile strength of 910 MPa and utilizable elongation of 15% were obtained in the NZ under solid CO₂ cooling, which was attributed to the fine effective grains and ferrite/martensite microstructure facilitating a ductile fracture.