Improvement of formability for hot stamping of aluminum alloy sheets by press motion control

In this work, the formability of Al-Mg-Si aluminum alloy sheets during hot stamping has been improved by controlling the press motion. In the hot stamping, a heated blank sheet is cooled partially by contacting with the dies; therefore, its deformation proceeds non-isothermally. The resulting temperature distribution leads to cracking at low forming depths during deep drawing despite the excellent tensile properties of the alloy at high temperatures. Local deformation, i.e. necking, tends to occur in a high-temperature area that is not in contact with the dies until the press reaches the bottom dead center. To suppress this deformation, the potential cracking area near the walls was cooled and hardened by holding the heated blank sheet between the blank holder and the upper die before the forming step. The proposed technique was applied to the square cup forming of a 1.0 mm thick 6000 series aluminum alloy sheet. As a result, the forming limit depth was increased to twice depth with 1.0-s die-holding duration before the forming step as compared to that obtained for the standard hot stamping procedure without the press motion control. (paper)