| With the aggravation of energy crisis and environmental pollution,the automotive manufacturing is facing more and more severe challenges.Vehicle lightweight is a favorable choice to improve fuel efficiency and reduce CO2 emissions.Aluminum alloy and high strength steel are important lightweight materials,the use of aluminum/steel composite structure in automobile design and manufacturing is an effective measure to realize automobile lightweight.Subsequently,the welding reliability of aluminum/steel composite structure has become an urgent problem to be solved.Owing to the huge differences in physical and chemical properties between aluminum alloy and high strength steel,the weldability of aluminum/steel dissimilar metals is seriously deteriorated,and the welded joint performance can not meet the requirements of automobile performance,which has become one of the main technical bottlenecks restricting automobile lightweight.Resistance spot welding has the advantages of low cost,high production efficiency and easy automation,it is widely used in automobile manufacturing.Therefore,the research on weldability and joint fatigue behavior of aluminum alloy/high strength steel dissimilar metals resistance spot welding has important theoretical significance and practical value.5083 aluminum alloy/DP980 high strength steel dissimilar metals connection was completed by process tape assisted resistance spot welding in this dissertation.Firstly,the microstructures and mechanical behavior of aluminum/steel resistance spot welded joint were studied.The results showed that the aluminum alloy/high strength steel resistance spot welded joint had the characteristics of fusion-brazing joint,consisting of aluminum alloy nugget zone,high strength steel solid phase transformation zone and aluminum/steel interface zone.The nugget of aluminum alloy was characterized by epitaxial crystallization,the morphology ofα-Al grain was cellular crystal,dendritic crystal and equiaxed dendrite crystal from fusion line to nugget center.The grains in the transformation zone of high strength steel were refined,and it was mainly composed of ferrite and pearlite.During welding,the inter-diffusion of Fe and Al atoms occurred in the aluminum/steel interface zone,and the intermetallic compound（IMC）layer with double-layer structure was formed by the interfacial metallurgical reaction:needle-like Fe4Al13 phase adjacent to aluminum alloy side and tongue-like Fe2Al5 phase adjacent to high strength steel side.The formation of IMC layer by interfacial metallurgical reaction was a necessary condition for accomplishing resistance spot welding of aluminum/steel.According to the thermodynamic and kinetic analyses of IMC layer formation,the formation free energy of Fe4Al13 was the lowest in Fe-Al intermetallic compounds,followed by Fe2Al5;the growth kinetic coefficient of IMC layer Fe2Al5>Fe4Al13.The growth model of IMC layer was proposed on this basis.The hardness distribution of aluminum/steel resistance spot welded joints was non-uniform.The average microhardness of aluminum and steel base metals was 78HV and225HV,respectively.Since the interface zone was mainly composed of Fe4Al13 and Fe2Al5intermetallic compounds,its microhardness was as high as 431HV.There were two fracture modes of interface fracture and button fracture in aluminum/steel spot welded joints under tensile-shear loading.Compared with the interface fracture joint,the button fractured joint had stronger bearing capacity.The nugget diameter had obvious effects on the tensile-shear properties and fracture mode of aluminum/steel spot welded joints.The tensile-shear force of the joint increased with increasing of nugget diameter.When the nugget diameter was greater than 7mm,the joint fracture mode changed from interface fracture to button fracture,which was mainly due to the increase of bearing area and bearing capacity of interface area.The research results showed that welding parameters（welding current,welding time and electrode force）had obvious effects on microstructures and properties of aluminum/steel spot welded joints,and the main reason was to change the resistance heat and temperature field in the welding area.With the increase of welding current（10k A-15k A）or welding time（100ms-300ms）,the nugget diameter,IMC layer thickness and indentation rate of spot welded joints increased.It was mainly due to the increase of resistance heat and temperature in the welding area,the increase of the volume of molten aluminum alloy and the promotion of interfacial metallurgical reaction;the increase of the joint tensile-shear force was mainly due to the increase of nugget diameter,the increase of the joint bearing area and the formation of continuous interface IMC layer.The welding current of 16k A（welding time of 350ms）led to the reduction of tensile-shear force of the joint,which was mainly related to the defects such as cracks and pores in the nugget.With the increase of electrode force（3.5k N-6.0k N）,the nugget diameter and IMC layer thickness of the joint decreased,which was mainly due to the decrease of resistance heat and temperature in the welding area;the decrease of tensile-shear force of joint was mainly related to the decrease of nugget diameter,the decrease of bearing area of joint and the formation of discontinuous IMC layer.Based on results above,the orthogonal test method was used to optimize the welding parameters.The results showed that the significant order of the influence of welding parameters on the tensile-shear properties of spot welded joints was welding current>welding time>electrode force;the optimized welding parameters were welding current 15k A,welding time 300ms and electrode force3.5k N.The aluminum/steel resistance spot welding connection was achieved by using the optimized welding parameters.The nugget diameter,IMC layer thickness and indentation rate of the joint were 10.02mm,2.82μm and 6.5%,respectively,meeting the quality requirements of the relevant standards for spot welded joint.The average tensile-shear force of the joint reached 5.15k N,which was the button fracture mode.Therefore,it was an effective method to optimize welding parameters by orthogonal test regulating the resistance heat and temperature field in the welding area for improving the microstructures and properties of aluminum/steel spot welded joints.The optimized welding parameters（welding current 15k N,welding time 300ms and electrode force 3.5k N）were used to complete 5083 aluminum alloy/DP980 dissimilar metals resistance spot welding connection.By changing the fatigue load,the effects of fatigue load on the fatigue life,fracture mode and fracture mechanism of spot welded joints were studied.The results showed that the fatigue life of spot welded joints increased with the decrease of fatigue load.Under this test conditions,the fatigue limit value of spot welded joint was 1.08k N,which was 21%of the tensile-shear force（5.15k N）of the joint.The spot welded joints were button fracture mode under the fatigue load>3.5k N,the interface fracture mode under the fatigue load range of 1.5k N-3.5k N and the aluminum base metal fracture mode under the fatigue load<1.5k N.The fatigue failure process of spot welded joints included three stages:crack initiation,crack propagation and instantaneous fracture.The notch at the root（aluminum nugget edge）of the joint surface between aluminum alloy and high strength steel base metal was the main factor affecting crack initiation.The fatigue life of the spot welded joint mainly depended on the crack propagation stage.With the continuous propagation of the crack,the effective bearing area of the joint decreased,the stress increased,the crack propagation accelerated,and finally led to the instantaneous fracture of the joint.Increasing the nugget diameter and improving the interfacial bonding properties were beneficial to improve the fatigue resistance of aluminum alloy/high strength steel spot welded joints.Abaqus software was used to establish a numerical analysis model for resistance spot welding of aluminum alloy/high strength steel,and the distribution characteristics and variation rules of temperature field and stress field in welding zone were studied.The results showed that the temperature field distribution of aluminum/steel spot welding was non-uniform and asymmetric.The temperature（～1300℃）of high strength steel center was lower than its melting point and the aluminum alloy nugget temperature（～870℃）was higher than its melting point,demonstrating that the aluminum melted while the steel remained solid during spot welding.The aluminum/steel interface temperature（～1220°C）satisfied the conditions for the formation of the intermetallic compound layer.According to the simulation results of the stress field distribution in the welding zone,there were an annular high stress region on the surface of the aluminum alloy nugget and a stress concentration at the root（aluminum melting edge）of the bonding surface of aluminum and steel base metals.It was consistent with the tensile-shear test and fatigue test results of aluminum/steel spot welded joints.The simulation results of the nugget diameter of aluminum/steel spot welded joints were basically consistent with the experimental results. |
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