Objective Universal demands to reduce energy consumption and emissions have elevated the role of lightweight design in the fields of aviation, ship, and automobile. These demands can be satisfied using Mg/steel dissimilar materials welding; however, many problems must be considered when attempting to realize a high-quality connection between Mg alloy and steel. The physical, chemical, and mechanical properties of Mg alloy and Fe are quite different. In addition, they are immiscible, and no intermetallic compounds are produced between Fe and Mg. Currently, to solve this problem, the formation of interfacial compounds is promoted by adding or coating an Ni, Cu, Ag, Zn, Sn, Cu-Zn, and Zn-xAl interlayer. Applying alternating magnetic field during laser welding process mainly brings two effects, including skin effect and electromagnetic force. When an alternating magnetic field is applied, liquid metal will produce an alternating induced current, which has a skin effect. In addition, the magnetic field and induced current produce an electromagnetic force; the alternating electromagnetic force promotes the positive and negative rotation of the molten pool, and convection in the molten pool is intensified. Previous studies have shown that the external alternating magnetic field can improve the weakness of joint compounds in the welding process of Mg/steel dissimilar materials. Ni can form intermetallic compounds and a solid solution with Mg and Fe, respectively. An alternating magnetic field is expected to regulate the distribution of compounds, which will improve the mechanical properties of joints. Therefore, laser welding-brazing technology assisted by alternating magnetic field is adapted to Mg/steel dissimilar materials welding with Ni interlayer. Microstructure and mechanical properties of the joint with and without alternating magnetic field are compared. The Mg/steel dissimilar metal connection with the Ni interlayer technology is further improved by applying an alternating magnetic field, and relevant data support is provided.

Methods Experimental materials included AZ31B Mg alloy and Q235 low carbon steel. The dimensions of Mg alloy and steel plates were 120 mm×60 mm×1 mm. A 0.1 mm thick pure Ni foil (99.9%)was used as the interlayer. The dimensions of the Ni foil were 60 mm×10 mm×0.1 mm. A 6 kW fiber laser (IPG YLS-6000CUT) was used for welding, and the laser beam was focused as a spot with a 0 mm diameter on the plates. During welding, the top surface of the plates was protected by argon with 99% purity at a flow of 15 L/min. AZ31B Mg alloy was lapped on Q235 low carbon steel with a clamp, and a 0.1 mm Ni foil was sandwiched between Mg alloy and the Q235 low carbon steel. After welding, the sample was cut along the direction perpendicular to the weld seam using a wire cutting machine, and the inlay was made using an XQ-1 hot mosaic machine and then polished. According to a preliminary test, the optimum welding parameters were P=1250 W, v=20 mm/s; the alternating magnetic field parameters were excitation current, I_E=0.8--2.0 A (every 0.2 A is an increasing unit) and excitation frequency, f=15--55 Hz (every 10 Hz is an increasing unit). Scanning electron microscope and energy dispersion spectrometer were used to study the cross-section morphology, Mg side weld zone, steel side weld zone, and intermetallic compound (IMC) layer of the joint under longitudinal alternating magnetic field. A WDW-100 electronic universal tensile testing machine was used to conduct tensile and shear tests on the joint. The tensile rate was 0.5 mm/min. Three groups of samples were stretched with each parameter. The average value of the three groups of parameters was calculated to determine the corresponding tensile and shear strength.

Results and Discussion The diffusion and reaction of Mg, Ni, and Fe elements in weld metal were promoted by strong electromagnetic stirring after applying the alternating magnetic field (Figs.3 and 4). The appearce of zonal structures of Fe-Ni solid solution in the weld pool of Mg side weld zone and the serrated Mg₂Ni in the brazing zone of the IMC layer enhanced the mechanical bite effect. A continuous nanoscale AlNi layer was formed in the fusion welding zone, and this strengthened the joint (Figs.5 and 6). With increased excitation current I_E and excitation frequency f, the tensile and shear strength σ_b of the joint initially increased and then decreased. For P=1250 W, v=20 mm/s, I_E= 1.2 A, and f=35 Hz, the maximum σ_b of the joint reached 228 MPa, which was approximately 15% higher than that without the magnetic field (Fig.8).

Conclusions It is expected that the compound distribution of Mg/steel joints can be controlled by applying an alternating magnetic field, and the mechanical properties of the joints can be improved. The diffusion and reaction of Mg, Ni, and Fe elements in weld metal were promoted via strong electromagnetic stirring after applying the alternating magnetic field. The appearance of zonal structures of the Fe-Ni solid solution in the weld pool of the Mg side weld zone and the serrated Mg₂Ni in the brazing zone of the IMC layer enhanced the mechanical bite effect. A continuous nanoscale AlNi layer was formed in the fusion welding zone, and it strengthened the joint. With increased excitation current I_E and excitation frequency f, the tensile and shear strength σ_b of the joint increased initially and then decreased. For P=1250 W, v=20 mm/s, I_E=1.2 A, and f=35 Hz, the maximum σ_b of the joint reached 228 MPa, which is approximately 15% higher than that without the magnetic field.