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Microstructural and Mechanical Characterization of a Fibre Laser Welded T-joint in High Strength Steel
P-L. Zhang, Z-S. Yu, H. Yan and Q-H. Lu

The experiment was conducted on the 907A high strength steel T-joint weldment with double pass laser based on the fiber laser welding system. The welds were fully characterized by means of metallography, microhardness and tensile test. It can be concluded that when the welding speed and defocus amount remain unchanged, the width and penetration of two weld beads increase with the increase of laser power. Uniform thin lathy martensite was found in the weld seam, which may have something to do with the preheating by the first scan and the effect of heat treatment of the second pass laser. Residual austenite and a small amount of carbides were found in the heat affected zone (HAZ). The microhardness of weld seam and HAZ are all higher than the base metal. Dimple and cleavage steps feature were easy to be found from the fractograph of the tensile samples, which can be concluded that ductile-brittle compound fracture was reached.

Keywords: Fibre laser, 907A high strength steel, T-joint weldment, laser welding, microstructure, phase transformation, microhardness, tensile strength

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