| Lightweighting has become the inevitable trend in the development of automobiles,especially in that of new energy automobiles,under the general background of global participation in energy-saving and emission reduction.As the lightest structural material,Mg alloys have many advantages,such as low density,high specific strength,good damping capacity,and so on.Steel is the most widely used metal material with high strength,good plasticity and toughness,and simple structure manufacturing.The combination of Mg alloy and steel to form heterogeneous metal parts is an important direction to realize the automotive lightweight,and the welding of Mg alloy and steel is still the key to restrict its design and application.Immiscible Mg alloy/steel is still the focus of research at home and abroad to promote the interface metallurgical reaction,improve the interfacial structure,and the matching between the interface layer and the matrix.This paper took AZ31B Mg alloy/Q235 steel dissimilar metals as the research object and increasing interface reaction temperature as the pointcuts,adopting the method of laser-induced arc hybrid heat source directly butt fusion welding,butt fusion welding with AZ61 Mg alloy filler wire,butt fusion welding with Ni interlayer to study the weld seam formation characteristics,regulation mechanisms of interface layer and enhancement mechanism of joint.The main research conclusions of this paper were as follows:(1)The technology of laser-induced arc direct butt fusion welding of Mg alloy/steel and the influences of welding parameters and base metal thickness on the joint forming and properties were studied.The experimental result indicated that when the laser power was 100 W and the arc current was 35 A,the tensile strength of 2.0 mm Mg alloy/0.8 mm steel joint was about 186 MPa in this study,which was 73.8%of the Mg alloy base metal.However,a 1.6 mm Mg alloy/1.0 mm steel joint was hard to form due to the serious burning loss of Mg alloy.The steel was melted by pulse laser at the interface during the laser-induced arc welding process,which was conducive to improve the interface reaction temperature.The Mg alloy was melted by arc heat causes and spread to the steel surface,which improves the joint forming ability while making up for the burning loss of Mg alloy.The melted steel at the interface reacts with the alloy elements in the Mg alloy to realize interface metallurgical bonding at the high interface reaction temperature.(2)The technology of laser-induced arc butt fusion welding of Mg alloy/steel with filler wire and the influences of laser power and laser offset on the joint forming and properties were studied.The experimental result indicated that when the laser power was 850 W and the arc current was 45 A,AZ61 Mg alloy filler wire,1.6 mm Mg alloy/1.0 mm steel joint was about 238 MPa in this study,which was 94.4%of the Mg alloy base metal.The filler wire can effectively compensate for the burning loss of Mg alloy during the butt fusion welding with filler wire process,promoting the spread of Mg alloy to the steel surface,and realizing good forming.The melted steel reacts with the alloy elements in the filler wire at the high interface reaction temperature to achieve the metallurgical bonding interface and then obtained high strength Mg alloy/steel butt joint with filler wire.(3)The technology of laser-induced arc butt fusion welding of Mg alloy/steel with Ni interlayer and the influences of the single side welding and double-side welding on the joint forming and properties were studied.The experimental result indicated that Mg alloy/steel single side welding joint was only 84 MPa due to a high melting point Ni interlayer was incompletely melted and crack often detected in the joint.The front and back of the Mg alloy/steel joints were formed well as double side welding used and realized good forming without filler wire.When the laser power was 360 W and the arc current was 35 A,the tensile strength of 1.6 mm Mg alloy/1.0 mm steel double side welding joint was about 241 MPa,which was 95.6%of the Mg alloy base metal.The Ni was reacted with steel at the high interface reaction temperature,and the coarse structure in the weld seam was eliminated and realizing the metallurgical bonding interface during welding with Ni interlayer.(4)Two kinds of laser-induced arc welding with filler wire and Ni high strength Mg alloy/steel joints were selected to analyze the interfacial structure and established interface bonding mode.The analysis results stated that the interface layer was comprised of HT-Al11(Mn,Fe)4 intermetallic-based and ?-Fe(Al)solid solution in the Mg alloy/steel welding with filler wire.The interface layer was comprised of(Al,Mg)(Ni,Mn,Fe)intermetallic-based and ?-(Fe,Ni,Al,Mn)solid solution in the Mg alloy/steel welding with Ni interlayer.Double-solid solution interface bonding model for Mg alloy/steel laser-induced arc welding was proposed based on the above analysis,that is,the interface layer was composed of nanoscale Al-X intermetallic-based and Fe-based solid solution.The performance of the Mg/steel butt fusion welding joint was effectively improved with the double-solid solution interface layer.(5)The microstructure between the double-solid solution interface layer of Mg alloy/steel joint with Ni interlayer and it's both adjacent sides were studied,and revealed the matching relationship of the interface of Mg alloy/steel butt fusion joint.The analysis result indicated that the atomic spatial structure position of the B2_AlNi interface layer was as that of BCC_Fe-Ni,and the lattice constant was close,which resulted in B2_AlNi/BCC_Fe-Ni interface completely coherent.The lattice distortion occurs on both sides of B2_AlNi/HCP_Mg interface,and the(0111)Mg plane and(110)AlNi plane are completely coherent.The O-lattice model was utilized to analyze the AlNi/Mg interface mismatch.Mismatch degree of AlNi/Mg interface was shown as follows:(110)AlNi//(0002)Mg<(200)AlNi//(0002)Mg<(211)AlNi//(0002)Mg.(6)The interface strengthening mechanism of Mg alloy/steel laser-induced arc butt fusion welding was proposed based on the above research,which mainly includes the following three aspects:firstly,the coarse structure in the weld eliminated by using high interface reaction temperature and selecting appropriate alloying elements,and then the steel/interface layer/Mg alloy structure joint was obtained to strengthen the weld of Mg alloy side;secondly,the interfacial structure was transformed from Fe-Al intermetallic compound to the double-solid solution interface layer,which was composed of nanoscale Al-X intermetallic-based and Fe-based solid solution and exhibited better thermal stability;Lastly,the interface layer and its both adjacent sides achieved lattice matching with the lattice distortion of Mg alloy/steel double-solid solution interfacial structure,which was conducive to improve the performance of the joint further. |
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