Study On Microstructures And Mechanical Properties Of Dissimilar Thickness Dual-phase Steels DP1180/DP590 Laser Welded Joints

The important means to improve the safety in car manufacturing are multi-materials design and high strength steel,with high strength steel or thick plates in high need collision locations and low strength steel or sheet in low need locations.During the manufacturing the parts,firstly joining dissimilar thickness and different high strength steels by laser welding,then plastic forming of the joining plates.Based on the production technology and structure characteristics,the paper investigated the microstructure and mechanical properties of dissimilar thickness dual-phase steels DP1180/DP590 laser welded joints.Due to the different thickness,microstructure,chemical composition,physical properties and mechanical properties,the dissimilar thickness dual-phase steel laser welding are complex.The laser welding parameters must accord with the requirements of DP1180 and DP590.The FZ of DP1180 consists of martensite and ferrite,and the microstructure of HAZ was uneven and had softening zone.The fracture occurred in HAZ while tensile test and cupping test,the strength and elongation were 1251 MPa and 4.9%,and the cupping value was 6.87 mm.Compared with DP1180 laser welded joints,the mechanical properties of DP590 joints were quite different,The fracture occurred at BM while tensile test,the elongation was 19.5%,the fracture as the type of parabola occurred in weld area while cupping test,the cupping value was 10.51 mm.Compared with laser welded of DP1180 and DP590,the DP1180/DP590 laser welding process was hard to control.There was great difference between the FZ and the BM in DP1180/DP590 laser welded joints,which reduced the strength of the joints.Investigated the microstructures and mechanical properties of dissimilar thickness dual-phase steels DP1180/DP590 laser welded joints.The results showed that the FZ was inverted trapezoid,which composed of martensite and ferrite,and the alloying elements of FZ were evenly.The size of HAZ of DP1180 was larger than the DP590.The microhardness of the FZ was higher than the HAZ and BM,the HAZ on the DP1180 side was softened.The fracture occured on the BM of DP590 side while TW tensile test and the elongation was 14.7%.The fracture occured on the parallel section while LW tensile test,and the equivalent strength was 1170 MPa.The crack first occured on the DP590 side when the punch located in the weld seam,the cupping value was 9.46 mm.According to study on microstructure and properties of laser welded joints with DP1180,DP590 and the morphological characteristics of DP1180/DP590,the effect of parameters on the microstructure and mechanical properties of DP1180/DP590 laser welded joints were studied.Increased the laser power or reduced the speed can increase the weld width of the joint,and the grain sizes of FZ and HAZ became bigger,the elongation and cupping value reduced.When the defocusing value from-2.0mm to 0,the weld width of front side increased and weld width of back side first increased and then decreased,the grain sizes of FZ and HAZ became bigger.The experimental results showed that the main factors affecting the quality index of DP1180/DP590 laser welded joint were laser power,welding speed and defocusing value and the optimum laser welding parameters were 1.4k W,2.1m/min and-1.0mm.