Microstructure and tensile properties of the different thickness ASSAPH440 high strength steel/DC52D+ZF45 galvanized cold rolled sheet dissimilar metal welding joint by CO₂ laser welding for automobile manufacturing

Abstract

In order to adapt to the development trend of a lightweight automobile and meet the safety requirements of key automotive parts, it is necessary to study the relationship between microstructure and mechanical properties of the dissimilar and different thickness materials welded joint. In this work, a dissimilar welded joint of high strength vehicle steel ASSAPH440 with 2.0 mm thickness/ galvanized cold rolled sheet DC52D+ZF45 with 2.6 mm thickness formed by CO 2 laser welding was studied. The microstructure of the welded joint was analyzed by metallographic structure and mechanical properties were discussed in terms of their tensile strength and microhardness. The results indicated that a high-quality welded joint with an average tensile strength of 643 MPa and an elongation of 22.7% was obtained at a welding speed of 30 mm s −1 and the laser power of 2200 w in CO 2 laser welding, the tensile strength of laser welded joint was more than that of the both based metals. The center of the welding seam was composed of slender lath martensite with a few small white pieces of ferrite. The microstructure of the Heat Affected Zone (HAZ) on the DC52D+ZF45 side was smaller than that of the HAZ on the ASSAPH440 side, and its mechanical properties were superior. The highest microhardness was obtained at the center of the cross section of the welding seam, and the microhardness of the welding seam on the DC52D+ZF45 side was higher than that of the welding seam on the ASSAPH440 side. The tensile fracture appeared at the base metal of DC52D+ZF45 steel as a result of the strength of ASSAPH440 was more than that of DC52D+ZF45. The performance of the laser welded joint met the technical requirements of automobile manufacturing engineering. manufacturing of the entire body of an automobile in Europe, the United States and Japan [5][6][7]. Currently, structural body parts made from ordinary steel plates are being replaced by high-strength steel plates. The highstrength steel sheets for vehicles generally have a tensile strength above 350 MPa. It not only has high tensile strength but also high yield strength, high weight loss potential, high impact absorption energy, high formability and low planar anisotropy.However, laser welding of high-strength steel is difficult because of the complicated metal state in the process of laser welding. Under the condition of the extremely fast heating and cooling speed during the laser welding, a large amount of non-equilibrium structure will appear in HAZ of welded joint, which hurts softening behavior of the welded joints [8][9][10]. Xia [11] studied the laser welded joints of high-strength steel with a tensile strength ranging from 450∼980 Mpa. They found that all joints existed the softening zones, and the degree of softening is related to the volume of martensite. As the heat input increases, the content of martensite in the softening zone decreases and the degree of softening increases. Consequently, the tensile strength of the welded joint was reduced. Sreenivasan [12] found that the forma...

A Novel Method of Defect Detection Based on Experimental Investigation of Galvanized Steel Welded Plates Using Wobble Laser Process

“…Based on these results, microstructures and the determined mechanical properties indicate good weldability of this material, and hardening is occurred in the weld seam [19]. Luo et al [20] have performed experimental mechanical and metrological characterization of different thicknesses ASSAPH440 high strength steels welded by CO 2 laser welding and reported that the highest microhardness comes about at the center of the welding cross-section. In the same context, Chen et al [21] developed a vent hole in CO 2 laser welding to improve the welding quality and to minimize the porosities generation.…”