Cold piercing of magnesium alloy billet with high aspect ratio

Cylindrical billet of a magnesium alloy AZ31B (Mg–3%Al–1%Zn) is pierced at room temperature by applying counter pressure to make a hole of an aspect ratio (initial billet height/piercing punch diameter) greater than 1.0. During piercing, the billet is kept in a container and the counter pressure is applied to the billet by a counter punch from the opposite side of the piercing punch. The effects of die clearance and counter pressure on the quality of the created hole surface are examined. In the case of piercing without counter pressure, the hole surface is very rough due to fracture irrespective of the clearance between the punch and the die. When a high counter pressure is applied, a hole with good shear surface quality is obtained. By applying the counter pressure, the height of the pierced billet becomes slightly higher and the height of the slug ejected from the hole becomes lower because of the change of the deformation mode from pure shear deformation to combined forward–backward extrusion.