Study On Microstructures And Mechanical Properties Of Dissimilar Thickness Advanced High Strength Steel DP590/TRIP800 Laser Welded Joints

In the field of automobile manufacturing,the important means to achieve lightweight and improve safety adopt multi-material structure and advanced high strength steels.Laser welding is used to connect advanced high strength steels of different thicknesses and different strengths,and then through precision plastic forming and other processing methods,the performance advantages of various materials can be maximized and the manufacturing cost can be significantly reduced.Based on the production technology and structure characteristics,the paper investigated the microstructures and mechanical properties of dissimilar thickness advanced high strength steel DP590/TRIP800 laser welded joints.The process parameters were explored,(Laser welding speed,laser welding heat input per unit length)on the microstructure and mechanical properties of laser welded joints.reveals the relationship between the microstructures and mechanical properties of dissimilar thickness advanced high strength steel laser welded joints.The qualitative relationship between the formability of Tailored Welding Blank(TWB)and the dimensionless aspect ratio of welded joints in advanced high strength steels with dissimilar thickness was established.The test results show,both DP590 and TRIP800 advanced high strength steels have readily weldable,and two advanced high strength steels can perform laser welding well to meet industrial production requirements.The DP590/TRIP800 laser welding joint consists of three parts: fusion zone(FZ),heat affected zone(HAZ)and base metal(BM).The FZ microstructures is composed of martensite,and the distribution of HAZ is uneven.The grain size of the HAZ near the FZ is larger.As for the mechanical properties of joints,it was observed that the average microhardnesses of BM,HAZ and FZ of the DP590 side were198.2 HV,274.5 HV and 401.8 HV.The average microhardnesses of BM,HAZ and FZ of TRIP800 side were 200.9 HV,274.5 HV and 398.9 HV.The laser welded HAZ of DP590 occurred softening,and softening rate was 0.91.No HAZ softening occurred in TRIP steel.The tensile strength of the TWB was 483 MPa and elongation was 24.1%.The fracture occurs at BM of DP590,the deformation of FZ and HAZ is small,and the fracture surface revealed a large number of dimples,which was ductile fracture.During the Limit Dome Height(LDH)test,the value of DP590,TRIP800 and the TWB were 11.2 mm,9.7 mm and8.0 mm,respectively.The crack was started in the FZ,and then extended along the direction which was vertical to the FZ and extends to the BM.Combining the microstructures and properties of DP590/TRIP800 laser welded joints of dissimilar thickness advanced high strength steel,the process parameters(laser welding speed,heat input per unit length)were explored for dissimilar thickness advanced high strength steel DP590/TRIP800 laser welded joints.Influence and optimize process parameters.After summarizing and analyzing,it is found that as the laser welding speed increases,the joint width decreases continuously,and the FZ and HAZ grains are reduced to different degrees when the laser welding speed is lower.the laser welding heat input per unit length increases.The width of the FZ and the HAZ of the laser welded joint is gradually widened,and the LDH value of the TWB was first increased and then decreased,and the dimensionless aspect ratio of welded joint is first decreased and then increased,and the two are inversely proportional.The effect of heat input per unit length change on elongation is not obvious.when the laser welding heat input per unit length is too high(108 J/mm),both the tensile test and the LDH test break at the FZ,which is brittle fracture,Excessive heat input per unit length causes the material to heat up rapidly during the laser welding process,and rapidly cools down to produce high stress concentrations.Finally,it was determined that under this test condition,when the laser welding heat input per unit length was in the range of 64-80 J/mm,the TWBs with excellent performance could be obtained by welding advanced high strength steel DP590 and TRIP800 of different thickness.