Advances in the Welding and Joining of Alloys and Steel

1. Introduction and Scope

Welding and joining technologies, as well as metal forming processes, are the foundation of modern manufacturing industries and are widely used in fields such as automotive, aerospace, pipeline engineering, and electronic packaging. With the increasing demand for high-performance, lightweight, and durable metal components, the optimization of process parameters, the control of microstructural evolution, and the improvement of mechanical properties have become key research priorities. The performance of metal components, whether serving under extreme conditions (such as high temperature, corrosion, or cyclic loading) or applied in precision scenarios, is closely related to their processing history, such as welding heat input, aging treatment, rolling schedule, and joining methods. Therefore, in-depth research on the thermal, mechanical, and electrochemical behavior of advanced metallic materials during processing and application is crucial for promoting technological innovation and ensuring structural safety and reliability.

This Special Issue focuses on the latest research advances in the fields of welding and joining technologies, microstructure, and the properties of advanced metallic materials, covering a wide range of research topics aimed at addressing key challenges in this field. The included contributions involve numerical simulation and experimental studies on laser welding, cold metal transfer (CMT) welding, friction plug welding, refilling friction stir spot welding, and laser-arc hybrid welding, with various metallic materials as research objects, such as 2195 Al-Li alloys, 6061-T6 and 6082 aluminum alloys, 347H heat-resistant steel, high-strength pipeline steel, high-strength steel, 70S-6 welding wire steel, and Sn1.0Ag lead-free solder. Specifically, the research content includes the thermal and flow behaviors during welding, microstructural evolution mechanisms (such as martensitic packet refinement and dynamic recrystallization), the effects of process parameters (such as heat input, groove angle, plunge depth, and rolling speed) on material properties, and the improvement of key performance indicators (such as low-temperature impact toughness, tensile strength, corrosion resistance, and electrochemical stability).

The scope of this Special Issue is intended to gather cutting-edge research results and build a bridge between basic metallurgical theory and industrial applications. Original research papers, review articles, and technical notes are welcome, covering research directions including but not limited to the welding and joining of metallic materials, numerical analysis, in situ observation of microstructural evolution, optimization of processing technologies, and the relationship between microstructure and mechanical/electrochemical properties. By integrating multi-disciplinary methods, such as finite element simulation, advanced characterization techniques (such as electron backscatter diffraction and in situ observation), and machine learning (such as a GA-BP neural network for welding process modeling), this Special Issue aims to provide a platform for researchers and engineers to exchange ideas and share innovative achievements, promote the development and application of advanced metallic material processing technologies, and ultimately contribute to the sustainable development and competitiveness improvement of the manufacturing industry.

2. Contributions

Liu et al. (Contribution 1) combined laser filler wire welding experiments and CFD numerical simulations to explore the effects of heat transfer and molten pool flow on the weld structure of 2195 Al-Li alloys. They find that the welding speed and wire feeding rate regulate the weld cooling rate, and the filler wire absorbs 6–16% of the total energy input. The formation of the equiaxed grain zone (EQZ) is critical for weld performance, and excessive cooling rates inhibit EQZ formation, providing guidance for optimizing welding parameters.

Zhou et al. (Contribution 2) investigate the electrochemical migration (ECM) behavior of low-silver Sn1.0Ag lead-free solder using the water droplet method, SEM, and XPS. ECM occurs in 0.01 M NaCl solution, with dendritic Sn and precipitates of Sn(OH)₂/Sn(OH)₄; higher NaCl concentrations intensify corrosion but hinder dendrite formation, offering new insights for improving solder joint reliability.

Xiao et al. (Contribution 3) systematically study the microstructural evolution and second-phase precipitation of 347H heat-resistant steel welded joints during aging. Aging at 700 °C promotes M₂₃C₆ and MX phase precipitation, while temperatures above 800 °C enhance MX phase stability; MX phase particle size follows W > HAZ > BM at 900 °C, providing a basis for aging process optimization.

Wu et al. (Contribution 4) propose a GA-optimized BP neural network adaptive model for the laser-arc hybrid welding of Q345 steel, addressing inconsistent pre-welding groove conditions. With an MSE of 0.75%, the model predicts optimal welding parameters for target morphologies, supporting the intelligent welding of complex components.

Li et al. (Contribution 5) develop a concave backing hole technique for the SAH-FPW of 6082-T6 aluminum alloy, eliminating conventional “filling zone” defects. The joint achieves a maximum tensile strength of 278.10 MPa (89.71% of base metal) with ductile fracture, enhancing load-bearing capacity and structural integrity.

Zhang et al. (Contribution 6) use RFSSW to repair FSW keyhole defects in 6082 aluminum alloy, finding that a 1.5 mm sleeve plunge depth yields a repaired joint tensile strength of 210 MPa (equivalent to FSW seam strength), demonstrating the feasibility of the repair technique.

Ito et al. (Contribution 7) develop a high-magnification in situ observation system to visualize residual liquid morphology during stainless steel laser welding solidification, correlating it with solidification cracking susceptibility and advancing our understanding of cracking mechanisms.

Xian et al. (Contribution 8) investigate the CMT welding of Al-6061-T6 for automotive air suspension components, revealing the coupling effect of liquation cracking and metal flow lines on crack propagation; the 90-L parameter set yields superior mechanical properties, supporting process optimization.

Gu et al. (Contribution 9) study high-strength pipeline steels, finding that HAZ softening mainly occurs in FGHAZ (900~1000 °C). Steel strength is linearly related to cooling phase transformation temperature and carbon equivalent (Ceq), with controlled Ceq mitigating softening.

Zhang et al. (Contribution 10) find that high-Ni high-strength steel maintains higher CGHAZ impact toughness (59.6 J at 20 kJ/cm) due to Ni-driven martensitic packet refinement, challenging the conventional view that lath bainite outperforms martensite in toughness.

Zhou et al. (Contribution 11) use ABAQUS to simulate 70S-6 welding wire steel multi-pass finish rolling, identifying the optimal process (12 passes, 880–900 °C, 1.0–1.5 Vp) to reduce peak rolling force and homogenize stress/temperature.

3. Conclusions and Outlook

Advanced welding, joining, and metal processing technologies are essential to support the demand for high-performance metallic components in modern manufacturing industries. This Special Issue contributes with outstanding works in this field, bringing new insights into the process optimization, microstructure regulation, and performance improvement of metallic materials. Future Special Issues and works should focus on the challenge of transforming the knowledge of laboratory-scale experiments and simulations into pilot and industrial-scale applications worldwide, promoting the practical application of advanced metal processing technologies and contributing to the sustainability and competitiveness of the manufacturing industry.