EXPERIMENTAL INVESTIGATION OF MECHANICAL PROPERTIES OF LASER WELDED DUAL PHASE STEEL JOINTS

mechanical engineering

Dual phase (DP) steels have been widely used in the automotive industry to reduce vehicle weight and improve car safety. In such applications welding and joining have to be involved, which would lead to a localized change of the microstructure and property, and create potential safety and reliable issues under dynamic loading. The aim of the present study is to find out optimised parameter for laser welding and examine the rate-dependent mechanical properties, deformation and fracture behaviour of DP600 & DP980 steel and its welded joint (WJ) produced by Nd:YAG laser welding over a wide range of strain rates. The lasers welding were performs using three variable parameter laser power, welding speed & pulse duration. Taguchi based optimization of laser welding parameter was done with the help of L9 orthogonal array to achieve the maximum ultimate tensile strength. Taguchi base optimization of laser welding parameter shows that for the given condition, 400 W of laser power, 0.6 mm/sec welding speed & 6 ms pulse duration have as the optimal parameter. From ANOVA, laser power has the most significant contribution with 91.2%, followed by pulse duration 5.8% & welding speed 1.7 %. Laser welding results in not only significant micro hardness increase in the fusion zone (FZ) and inner heat-affected zone (HAZ), but also the formation of a softened zone in the outer HAZ. The microstructure of laser welded joint shows the formation of mostly marten’s tic structure in the fusion zone as that area experiences very high cooling rate during solidification corresponding to the optical micrograph representing the FZ microstructure of laser welded joint. DP600 welded joints exhibited an obvious yield point phenomenon at all strain rate, while the smooth & plastic flow remained in the DP980 welded joints.

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