Research On Interface Joining Behavior Of Al/Steel Additieve Manufacturing Structure And The Effect Mechanism Of Magnetic Field

Due to the light weight, high strength and good thermal conductivity, aluminium/steel joints have a lot of potential applications in the aerospace, automotive and other industrial fields. However, current methods on Al/steel joining are confined to the plate structures with regular shape, it is difficult to join Al/steel components with complex structure in the practical application. In this study, a method of arc welding additive manufacturing（AM） which can achieve the direct fabrication of Al/steel structure with arbitrary shape was studied. Moreover, the interface joining behavior of AM structure with assisted external magnetic field and different metal interlayers were systematicly investigated.The construction of arc welding AM set-up system was first put forward. Cold metal transfer welding source and 6-axis robot arm were selected as the hardware actuator, and a suitable program for arc welding AM method on slicing algorithm and trajectory planning was written based on the MATLAB software, meanwhile, the conversion and communication between the program language and the robot assembly language were also achieved. By means of manufacturing a specified practical model structure, the arc welding AM set-up system was verified to well realize the function of AM process, the surface and outer contour of the manufactured specimen were flat and continuous.According to investigations of the forming characteristics of single-weld, the optimum welding parameter ranges of arc welding AM process for aluminium and stainless steel were obtained. Moreover, in order to further improve the shaping accuracy in the AM process, the mathematical model of adjacent welds of the multi-pass joints were developed. By means of accurately extracting the contour shape function of the weld in the specified welding parameter, the optimal Toffset（transverse offset） value between the adjacent welds and optimal Hoffset（hight offset） value between layers in AM process were calculated, which provided an accurate and effective data support for the slicing and filling module. For the sake of characterizing the microstructure and mechanical properties of AM specimens, AM wall-like specimens of Al and steel were fabricated in the subsequent experiments. The results showed that the AM specimens were uniform, dense and without any pore or crack. However, there is a slight nonuniformity of tensile property found in different shaping direction of wall-like specimen.In the subsequent experiment, Al/steel joining structure was fabricated by arc welding AM method using pure nickel and copper as the transition layers, and the interface joining behavior and mechanical properties of Al/Ni, Al/Cu in the cold metal transfer welding mode were investigated, respectively. The results showed that the interface structure of Ni/Al mainly consisted of Ni substrate/ Ni₃ Al, Ni_0.9A_l1.1, Ni₂Al₃, Ni Al₃/Al alloy, the maximum shear strength can reach 42 MPa. The interface structure of Cu/Al mainly consisted of Cu substrate / Cu₉Al₄, Cu₃Al₂, Cu Al₂/ Al alloy, the joint shear strength can reach 98 MPa. When nickel and copper was used as interlayers to fabricate the Al/steel AM structure, the interface of A l/Ni and Al/Cu was the weak part of the specimen. The tensile strength of Al/steel AM specimen with nickel layers can reach 54 MPa, while 36 MPa with the copper layers. In order to improve the joining strength of dissimilar metal joint, external magnetic field（EMF） was applied in the welding process. The results reveal ed that the weld arc made rotate movement with application of EMF, and the contact area of weld arc on base plate was increased. Meanwhile, the transfer behavior of molten drop changed from spherical in normal weld process to tabular shape in EMF weld process. Besides that, the wettability of Al alloy on the surface of steel, nickel and copper were significantly improved with the application of EMF, which increased the effective contact area of the joining interface. In the aspect of in terface microstructure, it showed that the EMF could inhibit the growth of brittleness intermetallic compounds（IMCs） layer and enhance the joining strength of interface. With the magnetic induction intensity increased, the joining strength of specimens showed an increasing trend. When the EMF was applied into the fabrication of steel-sandwich-Al AM structure, the maximum tensile strength can reach 78 MPa.Finally, taking Al/steel joining as an example, a simulation model of welding process with EMF was established using ANSYS software. Combining the materials thermodynamics and interface reaction dynamics, the effect mechanism of EMF on the growth behavior of IMCs at interface of dissimilar metal joining was analyzed. The simulation results showed that the EMF could change the movement of arc plasma, and ultimately affect the arc shape as well as the heat conduction behavior from weld arc to the molten pool and base plate. With ap plication of EMF, the high-temperature zone moved from the center of molten pool to the lateral, which decreased the reaction temperature and retention time of high-temperature at the front interface of Al/steel joint, thus reduced the concentration of iron atoms in the frontier liquid of Al. Meanwhile, the application of EMF also changed the flow and mass transfer behavior of molten pool. The flow behavior inside the molten pool changed from the single circulation into a double circulation flow characteristics by application of EMF. On the one hand, the outer circulation driven by the surface tension prompted spreading of the molten pool, which increased the effective contact area of molten pool on the steel plate and the heat transfer process. On the other hand, the way of double circulation flow behavior accelerated heat and mass transfer process between the front interface and inside of molten pool, which increased the supersaturated formation time of iron atoms in the front liquid of Al, and eventually inhibited the growth behavior of Fe/Al IMCs.