| Today,for major auto manufacturers,the realization of lightweight production of automobiles has gradually drawn attention with the serious environmental and energy problems.Body weight accounts for 30% of the total car weight,which is mainly made by steel.However,how to accelerate the application of lightweight magnesium(Mg)alloys with achieving reliable connection of Mg and steel has become one of the key technical bottlenecks for automobile filed.Due to enormous differences in the physical properties of Mg and steel and poor interactions,it is difficult to realize the effective connection between Mg and steel by conventional welding methods.This paper proceeds experiments of Mg and steel with lap model by Friction stir keyholeless welding(FSKSW)and studies the bonding mechanism of Mg-steel joint,which provides theoretical guidance for its application.Firstly,it is found that the existing welding equipment and keyholeless method are not suitable for the connection of Mg-steel.In order to obtain the joint with higher bonding strength,a new keyholeless method and a special stirring friction spot welding machine were self-designed for research.After carrying out many long-term experiments with using the self-designed machine and method,the maximum shear load of joint,with appearing smooth and neat of the joint,reaches 10.1KN.Based on the above results,the welding process window of the lap mode with shear load above 9.0KN can be obtained between 1mm thickness DP600 galvanized steel and 2mm thickness AZ31 B Mg alloy by using orthogonal experimental design.And the process parameters are also obtained.In order to study the mechanism of joint bonding,contrast experiments of lap joints among dissimilar AZ31 B Mg alloy,Mg99.50,galvanized DP600,and no zinc coated DP600 sheet were made by FSKSW and vacuum diffusion welding,respectively and the shear load of those joints were proceeding comparative analysis.Experimental results show that metallurgical bonding is not a decisive combination for Mg-steel and there should be an unique connection way for(FSKSW),which is different from traditional fusion welding and solid welding and determines the connect to the lap joints.Further,for those characteristic of unique welding method,this paper launched a series of targeted researches using macro and micro morphology analysis,scanning electron microscopy,X ray spectroscopy,X ray diffraction and mechanical properties test results.And the three-dimensional morphology of the joint interface was reconstructed by means of actual measurement.The results show that: FSKSW bonding way is a kind of compound mode which contains mechanical connection and element diffusion fusion connection and mechanical connection is the main decisive function on joints of Mg-steel with bondinginterface of Mg-steel interlocking each other effecting the connection strength of joint.The elements diffusion in Mg-steel is defined in the reaction between small amounts elements of the base metal and zinc coated metals and a certain amount of the intermetallic compounds and composite oxide can perform auxiliary reinforcement on the mechanical connection strength to some extent.Finally,focusing on the above issues,this paper studies microstructure and formation process of the joint.During welding process,the effect of the process of intense rotation and extruding by the tool and the shoulder makes steel plate localized thermoplastic flow state with mechanical tear and fragmentation and thermoplastic Mg with coercing steel pieces into the region,where the steel occurs flow and deformation,forming close interlocking structure of joint. |
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