LASER WELDING ON BARE & COATED STED STEELS FOR TAILOR WELDED BLANK APPLICATIONS

Abstract

Prior to forming process, the blanks which are assembled and welded to make a joint are called tailor welded blanks. These welded blanks may have different thicknesses, materials and coatings (can also be uncoated). TWBs when welded using laser technique are called LWB. LWBs had created their own position in automotive industries by meeting the high economic, environmental and governmental standards and demands. The superior advantages in cost reduction, weight reduction while maintaining structural integrity and crash performance made LWBs, the most widely used blanks in automotive industry. Employing LWBs causes inhomogeneity (disadvantage) which depends on the material properties used to weld the blanks. Such disadvantage affects the formability of LWBs. AHSS steels like Dual Phase (DP) poses high strength and good forming abilities but poor deep drawing characteristics thus, the interest of incorporating deep drawing steels like IF steel which has excellent formability traits but low strength. Hence, the combination of dual phase steel (high strength) with IF steel (excellent forming abilities) is justifiable. However, welding of DP steel causes softening in SC-HAZ resulting in premature failure and reduces formability. The aim of this thesis is to study the effect of weld design on formability where different grades of DP steel (GI coated/ uncoated) and IF steel (GI coated) are assembled to fabricate LWBs. In this work, certain areas were focused which may play detrimental role on formability. Irrespective of combination of steel grades, all the LWBs are fabricated in butt joint configuration with optimized parameters along the perpendicular direction of rolling. The thesis primarily deals with the optimization of parameters like speed, power and heat input for best weld bead geometry. The depth of penetration has been examined with respect to heat input. The HAZ widths of different parent metals aided to calculate the ratio of division of heat input experienced by respective parent metals. Later, this research discusses about the evaporation behaviour of zinc after laser welding. This study mainly focuses on formability of welded blanks and how they are affected with respect to thickness and strength ratio. With the help of 3D-DIC image acquisition forming of all the samples were video captured and FLD of parent metals and welded samples had been generated by using second order point of inflexion method and the vegter FLC line. The curve that separates the safe and unsafe extremes of forming is called FLC. The necking region and position of fracture is studied in detail with the help of screen printing and 3D-DIC. EBSD Characterization of the welded sample was carried out to find the two steels behaviour after welding.