Research On Mechanism Of Mechanical Properties For Automotive Friction Spot Welded Joints Made Of Aluminum With Thin Walls

The development demand of electric vehicles will raise the lightweight to a higher priority,and expanding the application of high-performance Al-Mg alloys is the key solution to reduce autobody frame weight.At current stage,aluminum thin-wall welding assembly mainly adopts fusion welding,which has a series of problems such as large crack tendency,large deformation and higher manufacturing cost under high energy consumption.Resistance spot welding?RSW?joints usually become the weak link of vehicle structure.Friction stir spot welding?FSSW?has advantages of solid-phase low-temperature bonding and insensitivity to surface of alloy wall,which is a suitable method for aluminum sheet joining.At present,most of relevant reports focus on the direction of materials science,but lack of engineering application considerations for automotive manufacturing and service scenarios.And the influence mechanism of the process on mechanical properties of the joint is still not clear,which leads to the lack of failure prediction and crack resistance under working conditions of vehicle use.This paper focuses on the influence of processing factors in various manufacturing links on forming properties,from aspects of base metal characteristics,parameters design,forming process,post-weld microstructure and service joining performance,the mechanism of mechanical properties for automotive friction spot welded joints made of aluminum with thin walls is systematically studied,aiming to provide experimental basis and method support for strength design and failure prediction in terms of automotive joint structure.Firstly,mechanical properties of typical automotive aluminum substrates of 5052H32and 6061T6 are studied in this paper.The constitutive response law under coupling effects of stress state and strain rate is revealed.It is found that the flow stress of alloys is mainly controlled by strain strengthening,the failure strain depends on the level of strain rate,and the impact deformation energy increases significantly.Effective mechanical characterization models were developed for the elastic-plastic deformation,damage evolution and ductile fracture respectively,which laid the material foundation for the subsequent welding capability simulation.A design approach of weld parameters matching the optimized properties with the working load of joints is proposed.Based on the shear-peel strength factor relationship model,combined with the load transfer distribution law of body welds,the coordinated optimization of multi-axial bearing capacities in accordance with the specific vector force field characteristics of automotive joints is realized.A robust parameter design scheme of vehicle for purposely improving post-weld performance is formed.In addition,the evolution laws of welding temperature field and thermal stress field are simulated,and a performance prediction model of heterogeneous joint covering the influence of thermo-mechanical process is developed.By using a moving heat source model combined with Gauss surface and double ellipsoid and a series of thermal simulation conditions close to the actual production,the forming state variables are related to mechanical behavior.The results show that the cut-off temperature to achieve alloy modification is about 375?,and the residual compressive stress around the Hook tip is conducive to increase the threshold of crack initiation.The temperature regulation mechanism of solid-phase welding is mainly to achieve the negative feedback balance of its own system by controlling the heat production rate through metal softing behavior.The validity of thermal analysis and property evaluation is verified by the measured field temperature,shape of tissue partition and bearing capacity.Furthermore,the influence of quantitative characteristics of post-weld microstructure on anti-cracking property of joints is studied from the perspective of fracture mechanics.Applying a fracture parameter of J integral as local crack resistance criterion,a numerical evaluation model for progressive cracking is established by using virtual crack closure method based on the weak cohesion bonding characteristics of defect.It is found that the crack tends to be unstable with the increase of the growth rate?da/dS?,the crack resistance increases with the increase of the hook bend angle at cracking initiation,and at the later stage of expansion,the anti slip property is positively correlated to the number of second-phase precipitated particles of Mg₂Si in the nugget.Besides,the microstructural causes of strength difference under different base metal combinations are analyzed,thus providing a micro dependent mechanism for joint property manipulation.Finally,the service joining performance between friction welding joint and resistance welding joint is compared.The rate dependent failure transition rule and dynamic vibration performance are also explored.Unified failure envelope criteria under multiaxial stress are given,FSSW has higher bearing capacity in the working stress space.The impact strength of joints show an obvious high-speed strengthening,and the fracture energy absorption of FSSW is about 32.8%higher than that of RSW.The dynamic connection stiffness curves are obtained by using frequency response transfer function tests,indicating that the FSSW response amplitude is about 30.2-56.7%of the RSW value under the same excitation.According to the multi-performance evaluation standard of autobody spot connection,it is considered that the friction welding joint has superior comprehensive service performance as a whole.To sum up,in the design and manufacturing of autobody structure,the prediction accuracy of CAE model can be improved by applying friction spot weld to the integration of aluminum thin-walled parts while considering its property law.A comprehensive study on the mechanism of mechanical properties is of practical significance to promote the upgrading of body spot welding process,which is beneficial to improve overall structural performance of the body to meet the requirements in respects of safety and comfort.