New Welding Materials and Green Joint Technology

1. Introduction

Welding technology, recognized as a critical process in materials engineering, is widely applied across various industrial sectors, including aerospace, energy, transportation, chemicals, defense, machinery, electronics, and the manufacturing of diverse metal structures [1,2,3]. Significant progress has been driven in welding science and technology by the continuous emergence of advanced technologies and new materials [4,5]. New opportunities are provided by the rapid development of new materials, digital technologies (encompassing information technology and computer science), and robotics technology for the in-depth exploration of welding mechanisms and process innovation. However, increasingly stringent requirements are imposed on welding processes and the performance of welding materials by the widespread application of new materials, complex components, and precision equipment [6,7]. To address these requirements, traditional connection technologies urgently need to achieve breakthrough innovations in connecting new or special materials. This is required to enhance the precision of reliability testing and service life assessment for complex welded components and to accelerate the research, development, and application of new welding processes, along with green and sustainable joint technologies [8,9,10]. To tackle these key challenges, this Special Issue is focused on the field of new materials science. Nine original research papers were selected, primarily exploring three central themes: innovative joint technologies, novel welding materials, and welding quality monitoring. Through these investigations, a deeper understanding of the behavioral characteristics of materials under different processing and service conditions has been achieved, providing valuable insights for both academic research and industrial applications.

2. An Overview of the Published Articles

The papers included in this Special Issue cover a diverse range of topics, reflecting the multidisciplinary nature of the welding process. They can be broadly categorized into three main themes: innovative joint technologies, novel welding materials, and welding quality monitoring:

2.1. Innovative Joint Technologies

Joint metals and optimizing welding processes are critical for enhancing component performance and reducing weight in engineering applications. Yamashita et al. (Contribution 1) investigated the impact joint of copper (C1100) and aluminum alloy (A6061-T6) via high-speed sliding with compression, revealing that adjusting the tip width of copper plates and applying longitudinal emery paper finishing significantly improved joint efficiency, with a maximum joint efficiency of 100% achieved. This study provides a novel approach for joint dissimilar metals in electromobility and aerospace applications. Livieri and Tovo (Contribution 2) proposed an implicit gradient method to optimize welded joints under fatigue loading, successfully applying it to load-carrying cruciform joints and spot welds. This method enables accurate prediction of effective stress and fatigue life, facilitating the design of fatigue-resistant welded structures.

2.2. Novel Welding Materials

Lead-free solders and filler material modifications are essential for optimizing welding processes and ensuring reliable joints. Kotlarski et al. (Contribution 3) explored electron-beam welding (EBW) of commercially pure titanium (CP-Ti) and Ti6Al4V using magnetron-sputtered Nb, V, and Cu fillers. Their results showed that Cu fillers refined the weld structure, improved ductility, and reduced residual stress gradients, addressing the common issue of low ductility in EBW joints. Dong et al. (Contribution 4) studied the effect of oxygen (a surface-active element) on laser welding of 304 stainless steel and nickel, revealing that oxygen-enriched atmospheres enhanced molten pool depth, promoted uniform element dilution, and improved microhardness distribution at the joint interface. This work sheds light on the role of gas atmospheres in laser dissimilar welding, guiding process optimization for high-quality joints. Han et al. (Contribution 5) modified Sn-0.7Cu-10Bi solder with Cr, demonstrating that 0.2 wt.% Cr reduced the melting point, improved wettability (minimum wetting angle of 25.84°), and refined the microstructure by inhibiting intermetallic compound (IMC) growth. Liu et al. (Contribution 6) explored the effect of Ni-coated carbon nanotubes (CNTs) on Sn-0.7Cu solder, finding that 0.05 wt.% Ni-coated CNTs transformed rod-shaped Cu6Sn5 into fine dot-shaped particles, enhanced wettability, and slightly reduced the melting point, providing a new strategy for reinforcing solder materials.

2.3. Welding Quality Monitoring

Ensuring welding quality and joint integrity is critical. Huang et al. (Contribution 7) focused on vacuum-brazed Ti-6Al-4V joints, investigating fatigue crack growth behavior under variable amplitude loading. They found that the Willenborg model, considering residual stress modifications, provided the most accurate predictions, offering valuable guidance for fatigue life assessment in brazed titanium structures. Lu et al. (Contribution 8) investigated the corrosion behavior of Sn-0.7Cu solder modified with Ni-MOF derivatives, showing that 0.08 wt.% Ni@C minimized the corrosion rate (0.205 mm/y) by refining corrosion products (Sn₃O(OH)₂Cl₂) and reducing galvanic potential differences between IMCs and the β-Sn matrix. This work highlights the potential of MOF derivatives in improving solder durability. Yu et al. (Contribution 9) developed a welding penetration sensing method for variable groove weldments using infrared sensors and artificial neural networks (ANN). By extracting temperature field features via Gaussian fitting and thermal cycle parameters, the ANN model achieved high accuracy in identifying non-penetration, full penetration, and excessive penetration, overcoming the interference of arc light in traditional visual sensing.

3. Conclusions and Outlook

This Special Issue showcases the latest advancements in innovative joint technologies, novel welding materials, and welding quality monitoring. The studies included here not only advance fundamental knowledge—such as the role of oxygen in laser welding, the mechanism of IMC refinement in solders, and fatigue crack growth in brazed joints—but also provide practical solutions for industrial challenges, including dissimilar metal joints, solder reliability, and online quality monitoring. These contributions reflect the growing trend of integrating experimental characterization with numerical modeling and advanced characterization techniques. Future research may further explore multi-scale modeling, intelligent process control, and sustainable materials to meet the evolving demands of high-performance and eco-friendly metal-based products.

We express our sincere gratitude to all authors for their valuable contributions, reviewers for their rigorous feedback, and the editorial team of Metals for their support in compiling this Special Issue. We hope these studies will inspire further innovation in materials science and engineering.