Improvement of the square shell drawability of aluminum sheet by means of differential lubrication in flange.

For the improvement of square shell drawability, the so called ""Strain Relief Effect"" is utilized, that is, the metal flow from the corner to the straight side region in a flange is positively promoted by friction decrease at the corner or/and friction increase at the edge of the straight side. The following points were clarified : (1) Increasing the blank size over the square shell limiting drawing ratio, the fracture punch load Pcr increases approximately linearly. However, it becomes constant when the blank size reaches large enough. (2) In this case, the crack initiates and grows along the periphery of the punch shoulder. The crack initiation and growth are restricted locally within the corner region of the punch shoulder. (3) The strain relief effect makes the distribution of meridian tension along the punch shoulder uniform and prevents the local crack. Thus, it improves markedly the square shell drawability.