| In this thesis, to promote applications of magnesium alloy as an opportunity, combine stainless steel which has an extremely important role in the automotive industry, focusing on an overview of the welding process and related research status of magnesium alloy and stainless steel dissimilar metals. Transient Liquid Phase diffusion bonding which has low bonding temperature, low residual stress, joints dense and smaller deformation, is widely used to connect dissimilar metals. In this paper,using Cu foil, Ni foil as an interlayer respectively, conduct transient liquid phase diffusion bondingof AZ31 B magnesium alloy and 304 stainless steel. The microstructure and mechanical properties of joints were tested and analyzed by using such as scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy, micro-hardness tester, electronic universal tensile testing machine,etc. Study the microstructure and the micro-hardness of the joints interface zone, and the law of shear strength variation with the process parameters. While the double stage diffusion bonding process was researched using a Ni foil interlayer.Studies shown that: in the transient liquid phase diffusion welding between magnesium alloy and stainless steel using Cu as the interlayer, the reactive of Mg and Cu produce eutectic liquid which have a good wettability and spreadability at the stainless steel substrate, while the microstructure of the joint was dense. At 510℃×3min~90min, 530℃×3min~60min, 550℃×3min~10min, Microstructure of the welded joints from the stainless steel side successively by eutectic reaction layer formed with first eutectic α-Mg and(α-Mg + Mg2Cu)eutectic composition、 the Mg-based solid solution layer by eutectic liquid phase epitaxial growth on the substrate surface and magnesium matrix permeability layer consisting of by Mg17(Cu,Al)12dispersed in the magnesium matrix phase and Mg-Cu-Al ternary compound distributed reticulation in the magnesium matrix grain boundary. With temperature increasing and time prolonging, the magnesium matrix permeable layer increases, the eutectic reaction layer first widens and then decreases until its disappear. At 530℃×90min and 550℃×30min, the joint occurred entirely isothermal solidification, so the microstructure of the joint constitute with magnesium matrix permeability layer and Mg-based solid solution layer due to isothermal solidification. At 530℃×30min, the maximum shear strength of the joint is 57 MPa.At 510℃×3min~90min, 530℃×3min~90min, 550℃×3min~10min, the microstructure of joint in the transient liquid phase diffusion welding between magnesium alloy and stainless steel using Ni as the interlayer, consist of(Mg+Mg2Ni) eutectic composition eutectic reaction layer including AlNi, Mg-Ni-Al compoundsc and the magnesium matrix permeability layer. As the temperature increases and time prolongs, the width of the welded joint showed similar variation law similar with the magnesium alloy/Cu/stainless steel joint. At 550℃×30min, the joint occurred entirely isothermal solidification, microstructure joint constitutes of AlNi, Mg-Ni-Al compound layer, Mg-based solid solution layer and magnesium matrix permeability layer. At 510℃× 30 min, the maximum shear strength of the joint is 40 MPa.For the solid phase-liquid phase double-stage diffusion bonding between magnesium alloy and stainless steel with Ni as the interlayer, the microstrusture of joint interface is dense and joint organizational structure from the stainless steel side successively by inter-diffusion layer of Fe, Cr and Ni, the residual Ni layer, Mg-Ni binary eutectic layer containing Mg-Ni compound and Mg-Al-Ni compound, α-Mg solid solution layer formed by isothermal solidification and magnesium matrix grain boundaries permeable layer.The maximum shear strength of the joint is 50 MPa. The Joint shear fracture occurs in the stainless steel side, showing features of brittle-ductile mix fracture. |
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