| With the rapid development of the automotive industry,the safety of vehicles,as well as energy consumption and environmental pollution,have become increasingly prominent.How to ensure vehicle safety while achieving lightweight is a pressing issue.The most effective way is to apply new high-strength,lightweight materials to replace traditional autobody materials,and among them,22 Mn B5 hot-stamping boron steel,which has the advantages of high strength,low cost and excellent price-performance ratio,is gradually being favored.In recent years,in order to enhance crash safety performance objectives,it has been demanded to design tailored parts with zoned strength and toughness properties in some boron steel structural components.The hot stamping patchwork blanks technology is a new method of producing tailored parts,with advantages such as good applicability and low cost,and is increasingly being used in the manufacture of tailored parts.However,during the patchwork blanks hot stamping process,there are variations in thickness in different areas,resulting in unequal material flow and decreased forming properties,susceptible to problems of wrinkling and cracking.The arrangement of numerous welded joints is also a problem that needs to be solved.If the arrangement of welding joints is unreasonable,such as the number of weak areas is too few,welding joints are not strong enough,it is extremely vulnerable to cracking and falling off during the hot stamping process,and even directly lead to the parts scrap.To investigate and solve the forming quality problems during the patchwork blanks hot stamping process.Study of welding parameters（welding current,welding time,electrode pressure）on the impact of welding quality.Conducting high temperature tensile shear tests and high temperature cross tension tests on 22 Mn B5 boron steel spot welded joints according to the patchwork blanks hot stamping process.In order to study the welded joints’ room temperature mechanical properties after hot stamping,microhardness tests and microstructure observations were performed.Established a welded joint high temperature damage model according to the results of high temperature experiments,and based on this,the hot stamping thermal-mechanical coupling finite element model of a certain vehicle A-pillar patchwork blanks was built.The influence of the weld joint arrangement on the patchwork blanks forming properties was also investigated and optimization of the weld joint location parameters was applied to parts analysis.In addition,tests on hot stamping of A-pillar patchwork blanks were conducted to verify the validity.Finally,combining finite element simulation,central composite design,response surface method and multi-objective optimization,the process of A-pillar patchwork blanks hot stamping is optimized and explored the influence law of forming properties.The key study findings are as follows:（1）Ideal welding window for welding current I= 7.0-8.0kA,welding time t = 16-24 cyc,electrode pressure F = 3.0-4.5k N.After heat treatment the welded joint without heat affected zone compared to the original welded joint.The microstructure of the base material and the fusion zone is fine and homogeneous martensite.The hardness distribution of the fusion zone and the base material after heat treatment is approximately horizontally distributed and about 500 HV.The peak load of welded joints in high-temperature tensile shear and cross tension experiments decreases with increasing temperature,the decline is smaller and the displacement and energy absorption work under peak load is greater at 700-750°C,so patchwork blanks are more suitable for forming at around 700-750°C.（2）According to the experimental results of welded joints at high temperatures,by adopting a Spotweld model,the failure criterion established by the stress state and plastic strain law could effectively predict the high temperature mechanical properties of the welding spot.The results of the A-pillar patchwork blanks hot stamping thermalforce coupled finite element model indicated that the strength of the welding spots satisfied the forming requirements,and the forming defects were mainly caused by the excessive thinning of the base material around the local welding spots.The cracked position of the patchwork blanks is mainly distributed in the base material around the weld joint of the thinner main blank,and mainly caused by the thinning rate increased dramatically when the welding spot reached the die fillet.By optimizing the welding spot arrangement,the thinning rate of the base material around the welding spot was decreased from 33% to less than 12%.The thickness distribution of different sections was compared with the simulated and the experimental results and the error was less than 10%.The experimental and simulation results show that the established finite element model for hot stamping of patchwork blanks is valid and can be used for hot stamping formability prediction.（3）By using the central composite experimental design method for the optimization variables,the response surface model built with the simulated calculation results can realistically describe the functional relationship between the patchwork blank objective function and the optimization variables process parameters.By analyzing the interaction between the optimizing variables and the response variables,the influence laws of each optimizing variable on the response variables were obtained.The multi-objective optimization algorithm NSGA-II was used to obtain a uniformly distributed set of Pareto optimal solutions using the optimized target response surface as the fitness function.Then,the data was used as the hot stamping process parameters for experimental verification of the hot stamping process,and the optimized A-pillar patchwork blank parts were without wrinkling and cracking defects.The optimization results show that the combination of central composite design,response surface methodology and multi-objective optimization methods can be effectively applied to optimizing the forming performance of patchwork blanks. |
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