Adding alloy elements to develop excellent properties in Al alloy/steel welding–brazing joints to realize lightweight vehicle structures has become a feasible task. Here, CMT welding–brazing was used to realize the reliable connection of Al/steel with a Cu coating. Al alloy/steel welding–brazing joints with different thicknesses of Cu coating were designed to study the influence of Cu on microstructure composition, mechanical properties and corrosion resistance. Then, the action rule of the Cu element on the interfacial microstructure evolution was clarified. The results showed that the appropriate Cu coating could effectively inhibit the reaction of Al and Fe and reduce the thickness of interfacial brittle intermetallic compounds (IMCs). Moreover, Cu could react with Al, Fe and Si to promote the formation of the τ₅-Al_7.2(Cu,Fe)_1.8Si phase and control the formation of the brittle θ-Fe(Al,Si)₃ phase, which improved the toughness of IMCs. When the thickness of the Cu coating on the surface of steel was 10 μm, the tensile strength of the Al alloy/steel CMT welding–brazing joint reached a maximum of 138.7 MPa and the corrosion resistance of the joint reached the optimum level. However, when the thickness of the Cu coating increased to 20 μm, the IMCs’ thickness at the interface of the joint reached the minimum, but Cu reacted with Al to form a brittle Al₂Cu phase under the higher Cu content, which deteriorated the tensile strength and corrosion resistance of joints. This work will provide an experimental and theoretical basis for developing good properties in Al/Cu-coating/steel welding joints.

This study analyzes the influence of different ultrasonic amplitudes on the microstructure composition, microhardness, tensile strength, and corrosion resistance of Al alloy/steel laser welding-brazing joints assisted by ultrasonic vibration. The application of ultrasonic vibration did not change the microstructure composition of the joints but refined them. The joints were all composed of θ-Fe(Al, Si)₃ and τ₅-Al_7.2Fe_1.8Si formed at the interface reaction zone, as well as an α-Al solid solution and Al-Si eutectic phase generated in the weld seam zone. Meanwhile, the thickness of the IMCs at the interface decreased with an increase in the ultrasonic amplitude. When the ultrasonic amplitude was 8 μm, the IMCs thickness was a minimum of 1.62 μm. In this condition, the reduction of the IMCs thickness and the refined grain of joints made the microhardness and tensile strength reach the maximum. The fracture of joints with ultrasonic amplitudes of 0 and 4.8 μm began at the weld seam and extended to the interface reaction zone at the steel side, while the fracture of joints was located in the heat-affected zone (HAZ) of the Al alloy side when the ultrasonic amplitude was 8.0 and 11.2 μm. The fracture mode of the former presented a typical mixed fracture with cleavage steps and tearing edges, and that of the latter showed ductile fracture with uniform and fine ductile dimples. The corrosion resistance of the joints was improved by adding ultrasonic vibration. When the ultrasonic amplitude was 8 μm, its corrosion resistance was optimum; it was ascribed to a dense oxide film formed on the surface of the metal under the action of ultrasonic vibration.

Plasma arc welding (PAW)-cable-type seven-wire GMAW (gas metal arc welding) hybrid welding is known as a high-efficiency welding combining plasma arc, GMAW arc and cable-type welding wire. In this study, numerical simulation via Fluent of the molten pool temperature field and flow field and experimental verification were conducted on Q235 thin plate hybrid welding with cable-type wire to explore molten pool fluid behavior. The simulation results show that keyholes form in the molten pool due to the strong penetration ability of a plasma arc and then the evolved pores by the surface tension float out of the molten pool. When the GMAW welding current increases, both the length and width of the weld pool enlarge, the weld reinforcement increases and the flow rate of molten metal in the weld pool also speeds up. While the PAW current increases, the weld pool length also increases and the molten metal in the weld pool significantly flows faster, but the weld reinforcement decreases. When the welding speed increases, the weld pool length and fusion depth decrease, but the reinforcement will first increase and then decrease. The experimental results are in strong agreement with the simulation results. It shows that the numerical analysis model established in this paper is accurate, laying a certain theoretical foundation for the popularization of PAW-cable-type seven-wire GMAW hybrid welding.

The cold metal transfer (CMT) welding-brazing process was chosen to join Al alloy and Ni-coated steel using AlSi12 as the filler wire. The macrostructure and microstructure of the joints were tested by using an optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometry (EDS), and X-ray diffraction (XRD). The tensile properties and corrosion properties of the joints were also tested. The results showed that Ni coating could improve the wettability and spreadability of molten AlSi12 filler metal on the steel surface, resulting in a good appearance for the Al alloy/steel joint. Ni coating could hinder the chemical metallurgical reaction between Al atom and Fe atoms to inhibit the formation of brittle Fe-Al intermetallic compounds (IMCs) and reduce the thickness of the IMCs layer. Meanwhile, the Ni atom reacted with the Fe and Al atoms to form Al₃Ni₂, (Fe, Ni) Al₃ and (Fe, Ni)₂Al₃, which improved the tensile strength of the joints. All joints with Ni coating cracked near the Al alloy. When the Ni-coating thickness was 5 μm, the tensile strength of the joint reached a maximum of 202.5 MPa. The addition of Ni could also improve the corrosion resistance of the joints. Significantly, when the Ni-coating thickness was 10 μm, most of the Ni coating was still solid, and the interface reaction layer was mainly composed of α-Ni solid solution and some (Fe, Ni)₂Al₃.