Role of counterpunch for square-cup drawing of tailored blank composed of thick/thin sheets

The deep drawability of tailored blanks (TBs) composed of thick and thin sheets is considerably lower than that of their component sheets, since the plane-strain stretching mode is more likely to occur in such TBs as a result of the movement of the weld line at the bottom of the cup during forming. To improve the deep drawability of TBs, a new forming technology using a counterpunch is proposed, where the movement of the weld line during forming is strongly constrained by the counterpunch pressure. From the results of square-cup drawing experiments on several types of TB of mild steel/high-strength steel sheets, it was found that the limiting cup height increases markedly with increasing counterpunch pressure. One good feature of this forming technology is that the action of counterpunch pressure is necessary only on the thicker (or stronger) sheet part, not on the entire cup bottom.