Al alloy/steel composite structures combine the advantage of a lightweight Al alloy and high-strength steel and are widely used in new energy vehicles, solar photovoltaic, and other fields. The main problems with the connection of an Al alloy and steel are poor weld formation and difficulty in controlling the thickness of the intermetallic compounds (IMCs) at the interface of the Al alloy and steel, which deteriorates the mechanical properties and corrosion resistance of the Al alloy/steel joints. Therefore, experiments on Al alloy/steel CMT (cold metal transfer, CMT) welding brazing were conducted by using AlSi5 and AlSi12 flux-cored welding wires as filler metals. The macro morphology, microstructure composition, tensile strength, and corrosion resistance of the Al alloy/steel joints were then analyzed. The mechanism of the Noclock flux on the wettability and spreadability of the Al–Si welding wire to a low-carbon steel surface was discussed and the formation behavior of the IMCs at the interface layer of the Al alloy/steel joints was clarified. The results showed that the NH₄F and NH₄AlF₄ of the Noclock flux induced and accelerated the removal of oxide films on the surface of the Al alloy and Al–Si welding wire at a high temperature. It promoted the wettability and spreadability of the Al–Si welding wire, which resulted in the improvement of the Al alloy/steel joint formation. Under the CMT arc heat source, the Al–Si welding wire melted, and then a chemical metallurgical reaction occurred among the Al, Si, and Fe elements. The τ₅-Al_7.2Fe_1.8Si phase formed preferentially near the Al alloy fusion zone while the θ-Fe (Al, Si)₃ phase formed near the steel side. Actually, the interface reaction layer was composed of a double-layer compound including the τ₅-Al_7.2Fe_1.8Si phase and θ-Fe (Al, Si)₃ phase. Additionally, the IMC thickness of the Al alloy/steel joint with the AlSi12 flux-cored welding wire was 3.01 μm, which was less than that with the AlSi5 flux-cored welding wire, so its tensile strength was less but its corrosion resistance was superior. The main reason for the corrosion resistance of Al alloy/steel joints was the presence of a large amount of Al₂O₃, FeO, and Fe₂O₃ in the passive film.

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.

Al-xSi-35Ge (x = 4, 6, 8, 10, 12, wt.%) filler metals were prepared to vacuum braze 6061 aluminum alloy. The wettability of filler metals was studied. A thermodynamics model of the Al-Si-Ge ternary alloy was established to analyze the mechanism and impact of Si in the microstructure of the brazed joint. The findings indicated that Si addition had a slight effect on the melting point of Al-xSi-35Ge filler metals. Great molten temperature region of fillers was responsible for the loss of Ge during the wetting process, making residual filler metal difficult to melt. The microstructure of the joint was characterized by a multilayer structure that was primarily composed of three zones: two transition regions (Zone I) and a filler residual region (Zone II). There was liquidation of filler metal for Al-Si-35Ge filler metals during brazing, resulting in holes and cracks in joints. Increasing the Si content in fillers could alleviate the liquidation of filler metal, owing to diminishing difference of chemical potential of Ge (μ_Ge) in fillers and 6061 substrates, hindering the diffusion of Ge from filler metal to substrates.

The properties of flux-cored Zn-Al filler metals are prone to deteriorating due to corrosion, making filler metals unusable. In this study, flux-cored Zn-2Al-xGe (x = 0, 0.3, 0.5, and 0.8 wt.%) filler metals are prepared to explore the effect of minor Ge on the corrosion resistance of Zn-2Al filler metals. The salt spray test is carried out on filler metals. A scanning transmission electron microscope is used to identify the phases in filler metals. The electrochemical performance of filler metals was tested by a workstation. The findings indicate that the microstructure of the Zn-2Al filler metal is composed of α-Al and η-Zn. Diamond-Ge forms in the microstructure of the Zn-2Al filler metal due to the introduction of Ge. Zn-2Al-xGe filler metals exhibit pitting corrosion characterized by intergranular corrosion (IGC) in the salt spray environment. Ge improves the IGC resistance of filler metals by changing the distribution of α-Al in the filler metal. The Zn-2Al-0.3Ge filler metal demonstrates the most excellent corrosion resistance. It has 16% elongation after 15 d of corrosion, which is higher than that of Zn-2Al by 13.6%.