Evaluation of the mechanical properties of welded metal in tailored steel sheet welded by CO2 laser

Automotive manufacturers have taken more interest in tailored sheet metals for improving rigidity, weight reduction, crash durability, and cost savings so that their application to auto-bodies has increased. However, since tailored sheet metals do not behave like an un-welded sheet metals in press forming operations, stamping engineers no longer rely only on conventional forming techniques. Furthermore, there is no clear understanding of the characteristics of welded metal which influence the overall press formability of tailored sheet metals. Recently, computer simulations have been prevailing for the evaluation of formability. Unfortunately, the mechanical properties of tailored sheet metal have to be defined quantitatively in simulation. In this study, analytical equations are formulated in order to find the mechanical properties of the welded metal in tailored sheet metal welded by CO2 laser. Based on a force-distribution assumption, the constitutive behavior of the welded metal is investigated using uniaxial tensile test results for the base metals and the tailored sheet metal. Then, the strength coefficient, work-hardening exponent, and plastic strain ratio of the laser-welded metal are calculated from those of the base metals and the tailored sheet metal. In addition, the existence of weld defects in the welded metal is indirectly detected by examining the slope of the strength coefficient of the welded metal.