Effect of medium-speed forming on formability of tailor welded blanks

Despite the widespread use of tailor welded blanks (TWB) in manufacturing auto-body panels and considering that different forming processes are performed at various speeds, it is necessary to examine the forming speed consequences. So, this study aims to investigate the forming speed effect on the formability of TWB and its constituent sheets. TWB consisting of St14 steel sheets of different thicknesses were welded using a fiber laser. Uniaxial tensile tests were conducted at various strain rates, and formability experiments were carried out at the quasi-static and medium-speed forming to construct forming limit curves (FLC). Tensile test results showed that some strain-hardening indicators decreased by strain rate increasing. The flow stress of base metals at different strain rates was successfully predicted by the constitutive model considering variable parameters. At mediumspeed forming of base metals, the peak of major strain and fracture moved to the dome center. But, the strain concentration of most TWB samples at two forming speeds occurred in the thin sheet near the dome center. Also, the limiting dome height (LDH) at medium-speed forming decreased. Medium-speed FLC of base metals shifted downward and to the more positive minor strain compared to quasi-static FLC. Medium-speed FLC of TWB was lower than quasi-static FLC, especially on the tension-compression side. The reduction of medium-speed formability was attributed to the strain-hardening lowering, friction reduction, and inertial effects due to increased forming speed.