Research On The Mechanism Of Laser Composite Bonding Of Magnesium/Steel Dissimilar Metal

Energy conservation,environmental protection and safety has become hot-spot issues at present worldwide.To realize lightweight technology is one of the effective measures to reduce energy consumption and emissions in modern transportation fields such as automobile and aviation.Magnesium alloys,known as"green engineering materials in the 21st century",are used in the field of modern transportation and their connection must be involved with steel.However,Mg and steel are not only hardly solutionized,but also unable to generate IMCs between them.With great differences on physical and chemical performances between Magnesium and steel,metallurgical bonding cannot be achieved,resulting in difficult fusion-welded connection between them.Aiming at the above-mentioned problems in the connection of dissimilar metals between magnesium and steel,in this paper,DP590 dual-phase steel and AZ31magnesium alloy for automobiles were taken as the research objects,proposing the idea of adding interlayer between magnesium and steel and external magnetic field,prefabricating a groove on steel surface etc.to combine with fiber laser-welding heat source,with a research on lap welding on magnesium/steel,to explore the new mechanism of laser composite joining of magnesium/steel dissimilar metals.Using high-speed camera,numerical simulation,theoretical calculation,process experiment,performance test and microscopic analysis and other methods and means,a systematic study was carried out on the weld morphology,interface structure,element distribution,mechanical properties,molten pool temperature field and flow field,intrinsic mechanical properties and thermodynamic properties of interface intermetallic compounds in magnesium/steel joints.The main work is as follows:（1）The laser lap welding test with the interlayer added between the magnesium/steel layers was carried out,and the effect of the Al and Sn interlayer on the microstructure and mechanical properties of the joint interface was studied.Based on the first principles combined with the Toop theoretical model,the thermodynamic properties and intrinsic mechanical properties of the reaction products of different interlayer elements with Mg or Al was calculated to select and design the interlayer elements,to expound the difficulty of the reaction of the Mg-X-Fe ternary system and the diffusion behavior of atoms during the welding process,to clarify the selection and design basis of the added elements,and to explore the change law in the microstructure and properties of the magnesium/steel joints under laser welding conditions before and after adding the interlayer.It was found that the metallurgical connection between magnesium/steel was realized by adding Sn and Al interlayer,and the maximum load of the joint was 730N and 590N,respectively,which were 2.04 times and 1.46 times higher than those without the interlayer.With the addition of Al interlayer,brittle Mg₁₇Al₁₂ compounds were formed on the magnesium side of the joint interface,which limited the performance improvement of the magnesium/steel joint.While the added Sn interlayer made Mg₂Sn compounds with relatively low brittleness and the ductile Fe₃Sn compounds were formed on the steel side interface.The results from above showed that the addition of Sn between magnesium/steel layers was determined as the best interlayer element during magnesium/steel welding.（2）a new structure of magnesium/steel connecting groove interface was explored that a concave groove was prefabricated on the steel surface.A riveted joint construction was presented when magnesium connected with steel.Based on the idea of mechanical and metallurgical composite connection,the added Sn powder in the prefabricated groove compositing with laser revealed the influence rule of the width of the concave groove on the microstructure of the molten pool interface and the joint performance of the magnesium/steel joint and elucidated the diffusion behavior of elements at the interface of magnesium/steel and the formation mechanism of the metallurgical regulation of the new phase at the interface of the joint by the width of the concave groove.It was found that in the system of Mg-Al-Sn-Fe,Al atoms diffused from the matrix to the Fe side,easily forming Fe Al phase and at the steel interface,Fe and Sn atoms were attracted by Al atoms,which promoted the reaction between Fe and Sn atoms.with the increase of groove width,the magnesium/steel joint changed from mechanical bonding mode to mechanical plus metallurgical composite bonding mode;Since the concave groove structure provided a rivet-like mechanical engagement for the magnesium/steel joint and assisted the metallurgical regulation of Sn powder,the performance of the magnesium/steel composite joint had been significantly improved which was 1 times higher than that without prefabricated grooves.This study explored a new connection method for magnesium/steel while optimizing the dimensions of the prefabricated groove structure on the steel surface.（3）On the basis of optimizing the prefabricated groove structure on the steel surface,the coupling relationship between the addition of Sn powder and the laser heat input was further explored.A finite element model of magnesium/steel laser welding with concave groove and Sn powder was established.Through the simulation calculation of magnesium/steel welding temperature field and flow field,the influence rules of the temperature of the welding pool and the change of the flow velocity caused by the laser stirring on the microstructure of the magnesium/steel interface under different heat input conditions was studied.Through the element diffusion at the interface of magnesium/steel joint and the calculation of the intrinsic mechanical properties of the reaction products,the qualitative relationship between the mechanical properties of the joints and the ductility/brittleness of the reaction products under the laser-Sn powder coupling effect was elucidated.It was found that by fixing other welding parameters and increasing the laser power,the flow of the molten pool in the direct laser irradiation area of the magnesium/steel joint was accelerated,and the temperature of the molten pool went up,which promoted the metallurgical reaction of Sn at the steel interface and formed a ductile Fe-Sn phase,which was beneficial to interfacial metallurgical bonding and further improved the performance of magnesium/steel joint.However,excessive laser power gave rise to overreaction at magnesium/steel interface which led to the formation of excessively intermetallic compounds（IMC）and the generation of hot crack defects,resulting in the degradation of joint performance.（4）Based on the prefabricated concave groove and the addition of Sn powder to realize the effective connection of magnesium/steel,the performance of the magnesium/steel joint was further improved by applying an external longitudinal magnetic field,and a new mechanism of the composite connection between the external magnetic field and the concave groove,Sn powder and laser was explored.The results showed that the change of joint performance under different magnetic field intensities was closely related to the thermal electromagnetic force generated by the interaction of metal vapor plasma and thermal current inside the molten pool.The external magnetic field weakened the shielding effect of the metal vapor plasma on the laser energy and improved the absorption rate of the laser energy by the molten pool which promoted interfacial metallurgical bonding.The thermal electromagnetic force generated by the external magnetic field promoted the flow of the molten pool and strengthened the stirring effect on the molten pool.The interface dendrites deviated from the vertical growth direction and are fragmented.the nuclei increased and the grains were refined,which improved the interface grain size and distribution.