Microstructure And Mechanical Properties Of Al/Steel Resistance Welded Joint With Ni-Zn Interlayer

As a large country in the production and consumption of aluminum alloy,the anode conductive device,an important conductive part of the aluminum electrolytic cell,involves the connection of Al/steel dissimilar metals in the manufacturing process.Hence,a new type of resistance welding technology（RWT）was proposed in this paper,the principle of which was based on the Joule heating effect generated by high-resistivity graphite as a thermo-compensated auxiliary electrode to obtain a certain section Al/steel RWT joint.The interfacial microstructure and mechanical properties of the Al/steel RWT joints were studied.The Ni-Zn interlayer and an asymmetric heating method formed by the auxiliary current were introduced to dynamically control the growth mechanism of intermetallic compounds（IMCs）at the interface to obtain the best mechanical properties of the welded joints.Combined with finite element simulation and thermodynamic analysis,the growth mechanism of the interfacial reaction layer and the formation mechanism of the final joint were discussed in detail.For Al/steel RWT joints,the Joule heat generated by the auxiliary electrode was higher than the joint in the welding process because of the high-resistivity feature of graphite.Therefore,the Joule heat generated by the graphite electrodes on both sides of the joint was directly related to the interfacial structure and mechanical properties of the final joints.The results indicated that surface temperature fields of graphite electrodes with different welding parameters were pre-generated from one point and gradually diffused to the whole electrode during the welding process.The maximum value of the temperature field improved with the increase of the welding current and welding time.By comparison,the welding current had a greater influence on the temperature field.Due to the low melting point of Al alloy,a certain strength of Al/steel joints without deformation could only be obtained within a narrow welding parameter range.Combined with mechanical properties and fracture morphology of the joint with different welding parameters,the optimized parameters were determined as follows:welding current of 1.5 kA,welding time of 90 s,and welding pressure of 0.2 MPa.Based on the idea that the interlayer improved the joint load,Zn foil,Ni foil,and“Ni+Zn”composite foils were inserted between Al alloy and stainless steel（ss）.The results depicted that the introduction of Zn foil improved the temperature field of the joint.An obvious Al（s.s）reaction layer was formed at the interface,and the load increased by 25.7%compared to the joint without an interlayer.Due to the high melting point of Ni element,when Ni foil and“Ni+Zn”composite foils were chosen as the interlayer,the Joule heat generated during the welding process could not realize the connection between Ni and steel.On this basis,defect-free Al/Ni/steel and Al/Ni+Zn/steel joints were successfully obtained by consolidating Ni foil and“Ni+Zn”composite foil to stainless steel based on resistance seam welding as an interlayer.Ni foil effectively inhibited the formation of Fe-Al IMCs.For Al/Ni/steel joints,the interface structure was transformed into Al alloy,Al（s.s）diffusion layer,Ni interlayer,and stainless steel.The tensile-shear load of the joint reached 1793.3 N.However,due to the addition of Zn,the different thicknesses of reaction layers with the main components of Ni-Al IMCs were formed between Al alloy and Ni interlayer in the Al/Ni+Zn/steel joint,the joint load reached 2202.3 N,which was 3.7 times that of the joint obtained by single direct RWT welding.Zn and Ni-Zn coatings were prepared on the surface of the stainless steel substrate by cold spray technology as an interlayer.Hence,the contact reaction of Fe-Al atoms at the joint interface was effectively blocked,and Al/Zn/steel and Al/Ni+Zn/steel RWT joints were successfully obtained.Because of the low melting point of Al alloy,the maximum thickness of different types of coatings was 500μm.The joint interface structure with the maximum thickness Zn coating as an interlayer was Al alloy,Al（s.s）,Al（s.s）+Al₂O₃,and stainless steel,and the joint load was 2087.6 N.The joint interface structure with the maximum thickness Ni-Zn coating as interlayers was Al alloy,NiAl₃ layer,Ni-Zn coating,and stainless steel,and the joint load was 2480 N.Combined with the fracture mechanism of the joints,the 500μm Ni-Zn coating was chosen as the best coating.Introducing the auxiliary current from the stainless steel to the lower welding electrode,formed an asymmetric heating method,effectively reconstructing the temperature field distribution of the Al/steel joint.The temperature of the stainless steel side was increased,which regulated the growth of the interface compounds.An obvious Al（s.s）diffusion layer and Fe₂Al₅ layer were generated at the Al/steel interface.When the auxiliary current was 0.3 kA,the thickness of Al（s.s）diffusion layer and Fe₂Al₅ layer reached 5.25μm and 0.86μm,respectively.The tensile-shear load of the joint reached 1789.7 N,which was three times that of the joint without auxiliary current.Meanwhile,when the best coating was used as the interlayer,an asymmetric heating method was also introduced to promote the metallurgical bonding between the Al alloy and the Ni-Zn coating.The interface structure was transformed into Al,NiAl₃,Ni₂Al₃,and Ni-Zn coating.The thickness of the NiAl₃and Ni₂Al₃IMCs increased with the improvement of the auxiliary current.When the auxiliary current was 0.3 kA,the thickness of the IMCs layer reached the maximum at the interface,and the tensile-shear load of the joint reached 3914 N.Based on the temperature field simulation and thermodynamic analysis,the temperature field distribution of the different positions was clarified during the welding process.The formation sequence of the IMCs at the interface was determined.The evolution process of the interface structure and the formation mechanism of the IMCs layer were revealed under the action of the asymmetric heating method.