Fiber Laser Welding of 1700-MPa, Ultrahigh-Strength Steel

Abstract

This paper investigated the appearance, microstructure, and mechanical properties of laser welded joints for 1700-MPa, ultrahigh-strength steel (UHSS) with 6 mm thickness. The results indicate welding speed was the dominant factor that affected appearance quality. A sound weld without underfill can be obtained when the speed was limited to approximately 2-3 m/min. Two softening mechanisms exist in the heat-affected zone (HAZ). In the intercritical heataffected zone (ICHAZ), ferrite appears and the hardness drops sharply. For the subcritical heat-affected zone (SCHAZ), the microstructure was tempered martensite with precipitation of carbides and the high temperature temper results in the minimum hardness. With a martensitic structure in the weld metal and a narrow softening zone in the HAZ, more than a 94% joint efficiency was achieved in tensile testing. Only about a 40-deg maximum bending angle was obtained during a bend test, as plastic deformation mainly concentrates in the softening zone due to nonuniform hardness distribution. The study also found that the existence of porosities in the joints had significant affects on the ductility, toughness, and fracture mechanism, but little affect on the joint strength. For the joints without porosities, the fractures from tensile and bend tests both initiated at the zone with the minimum hardness and propagated along the 45-deg direction. But when porosities existed in the joints, fractures occurred completely in the weld metal, which resulted in a smaller elongation and bending angle as well as impact energy. However, the tensile strength was similar to an average value.

Effect of post-weld heat treatment on microstructure and mechanical properties of DP800 and DP1200 high-strength steel butt-welded joints using diode laser beam welding

“…Advanced high-strength steels are extensively used in automotive sectors due to their extraordinary combined properties of strength, ductility, formability, high-strength to weight ratio and low CO 2 emissions [1][2][3][4][5]. Dual phase (DP) steels are a group of low-carbon micro-alloyed steels, and the term 'dual phase' indicates that the steel has two distinct phases, i.e.…”