Laser Weldability of High Strength Steel Sheets in Fabrication of Tailor Welded Blanks

A tailor welded blank (TWB) consists of materials different in thickness, tensile strength, and other properties, and is now widely used for fabricating automotive bodies. TWBs are used not only to save materials and reduce cost in stamping and assembling, but also to improve fuel efficiency by reducing the weight of automobile bodies and to improve the stiffness. This report presents the conditions required at welds of TWBs composed of 590MPa class high strength steel sheets with high thickness ratios. Further, from the viewpoint of material properties, basic characteristics such as mechanical and fatigue properties and press formability of TWBs having a tensile strength of 590 MPa or more were clarified. The main conclusions obtained are as follows: (1) The weld thickness ratio TW/T1 must be kept at 0.8 or higher regardless of material properties and sheet thickness ratios, where TW is the thickness of the weld, and T1 is the thickness of the thinner base metal. (2)The elongation of weld joints is estimated systematically by the sum of weld metal hardness and base metal hardness. (3)The fatigue strength of welded joints is lower than that of base metal regardless of equal thickness or differential thickness welded joint. Also, as the thickness ratio increases for the differential thickness joints, the fatigue strength becomes low. (4)The formability of welded joints is inferior to that of the base metal because the weld is hard. When the weld heat input is high and thus the heat-affected zone is softened in case of 980MPa class high strength steel sheets, the formability decreases further.