Welding copper (Cu) and aluminum (Al) is highly demanded for lightweight and cost-effective manufacturing. However, it faces significant challenges. First, substantial differences in physical properties may lead to high residual stresses and distortion. Second, brittle intermetallic compounds (IMCs) readily form at the interface, severely compromising the joint’s mechanical properties and electrical conductivity. Third, the native oxide film on Al impedes effective wetting and bonding. Therefore, effective control over the interfacial microstructure of the welded joint is essential. This review provides a critical analysis and comparison of several typical welding techniques, including laser welding (LW), friction stir welding (FSW), ultrasonic welding (UW), brazing and soldering, and welding–brazing. These analyses focus on their process characteristics, joint microstructures, and corresponding formation mechanisms. Furthermore, this review synthesizes key strategies for enhancing joint quality, including process parameter optimization, introduction of functional interlayers, and external assistance, aimed at optimizing joint microstructure and minimizing defects. Based on the analysis, this work provides comparative insights into process selection and microstructure control, and highlights future directions: advancing novel methods such as magnetic pulse welding and transient liquid phase bonding; developing intelligent real-time process control to suppress brittle IMCs and associated defects; promoting sustainable practices and establishing standardized performance evaluation; and systematically investigating long-term reliability to support the industrial application of robust Cu/Al joints.

A WC-10Ni/NiCrBSi coating was prepared and applied to the surface of Q235 steel through vacuum brazing. Using a self-developed dry sand abrasion test machine, the effects of the abrasive sand’s type, load, and sliding speed on the dry sand abrasion property of the coating were analysed. The wear mechanism of dry sand abrasion was also investigated. The results indicated that the coating cross-section comprised three layers: the substrate, the interface layer, and the surface layer. The hard layer served as the main distribution area of WC hard particles, which directly determined the hardness and wear resistance of the coating. WC particles, fortified by a γ-Ni solid solution, enhanced the wear resistance and hardness of the coating. In the friction and wear test, when ceramic abrasives were employed, the coating sample exhibited a loss of only 23 mg, constituting only 7.9% of that observed with quartz sand abrasives. Under low loading conditions, the wear mass loss exhibited a linear relationship with the applied load. During these low-load scenarios, the abrasive particles operated through a rolling motion, thereby entailing an abrasive wear mechanism. Conversely, when the load exceeded 0.05 MPa, the primary mode of abrasive particle motion transitioned into sliding with burial, resulting in a combination of fatigue wear and abrasive wear mechanisms. Therefore, the dry sand abrasion mechanism inherent to composite coatings can be attributed to the protective shielding role played by WC particles on the substrate. This shielding function effectively mitigates and counteracts the abrasive cutting effects induced by abrasive particles.

The third generation of advanced ultra-high-strength steel PHS1500, is widely used in automobile manufacturing due to its excellent performance. Still, severe liquid metal embrittlement (LME) cracks will occur when joined by resistance spot welding (RSW). This study aims to elucidate the formation mechanism of LME cracks and optimize welding process parameters to mitigate the severity of LME cracks in PHS1500 steel. Conventional double-pulse resistance spot welding was used to join PHS1500 steel, and the welded joints were characterized using optical microscopy and scanning electron microscopy to analyze the crack morphology, elemental distribution, and the LME formation mechanism. The results revealed that LME cracks were caused by the diffusion of liquid Zn into the grain boundaries of the base metal to form the low-melting-point Γ phase and α-Fe(Zn) phase, which fractured under tensile stress. After optimizing welding parameters, the maximum crack length at the heat-affected zone slope (HAZ slope) decreased from 117.59 μm to 32.76 μm (72.14% reduction), while that in the outward region of the HAZ (HAZ outward) decreased from 80.88 μm to 21.65 μm (73.23% reduction), with most cracks exhibiting a single dendritic morphology.

Composite coatings were prepared on the surface of TC4 titanium alloy using a flexible ‘coating cloth’ and vacuum brazing technology with WC-10Ni particles as the hard phase. In the process of vacuum brazing for the preparation of composite coatings, the vacuum brazing temperature was 1160 °C, the holding time was 40 min, and the composition ratios of the hard phase WC-10Ni were in the order of 30%,50% and 70%. With a view to studying the effects of different ratios of WC-10Ni on the microscopic morphology, elemental distribution, hardness and interfacial bonding strength of the composite coatings. The composite coatings were prepared on the surface of TC4 titanium alloy by using flexible ‘coating cloth’ and vacuum brazing technology, with WC-10Ni particles as the hard phase at 1160 °C, with a view to studying the effects of different ratios of WC-10Ni on the microscopic morphology, elemental distribution, hardness and interfacial bonding strength of the composite coatings. The study revealed the presence of metallurgical bonding between the nickel-based WC-10Ni composite coating, and the substrate, as well as between the hard phase and the hard phase within the composite coating. The interfacial bonding was found to be satisfactory, the surface of the prepared composite coating is flat and smooth, without obvious defects such as cracks and pores. The hard layer is closely combined with the substrate. The composite coating consists of a hard layer, an interfacial layer and a matrix, with the hard layer consisting of spherical TiC and WC in the white region and Ti₅Si₃ as well as α-Ti with W in the black block; The interfacial layer is a mesh of Ti₂Ni and Ti₅Si₃. The hardness of the interface layer of the coating with a 50%WC-10Ni ratio was 514–583.8HV_0.3, and the hardness of the hard layer was 694–734.1HV_0.3, which were 1.6 times and 2.1 times that of the substrate respectively. It can be seen that the preparation of WC-10Ni/BNi-5 composite coatings on the surface of TC4 substrate can significantly improve the hardness of the material surface.the shear strength of 50% WC-10Ni is 285.7MPa, and the shear mechanisms are all brittle quasi-dissociative fracture.