Study on wrinkling limit of commercially pure aluminium sheet metals of different grades when drawn through conical and tractrix dies

This paper deals with the wrinkling limit diagrams of annealed three different grades of commercially pure sheets, namely ISS 19000, ISS 19600 and ISS 19660 having thickness of 2.00 mm when deep drawing into cylindrical cups through conical and tractrix dies using a flat bottomed punch. It is well established that, the use of either a conical die or a tractrix die can enhance the limiting draw ratio compared with that obtainable in a conventional drawing operation. When a conical or a tractrix die is employed the need for a hold-down or clamping ring is eliminated. However, this enhances the propensity of the blank to fail by wrinkling or buckling, particularly in the early stages of a drawing process in which thin sheet blanks are used. It is proved by these researchers and others that the onset of wrinkling takes place when the ratio of strain increments (dɛr/dɛθ) reaches a critical value during the drawing process. This value which could be determined experimentally over which the wrinkling takes place has been shown in the form of wrinkling limit diagrams for the above three different grades. An attempt is also made to rank the aluminium grade that appear to suppress the wrinkling based on results obtained in the form of wrinkling limit diagrams, established for the above three different grades, in terms of the strain increments ratio and the stress ratio parameters. Further, it is observed that the aluminium grade ISS 19660 having high strain hardening index value, low ratio of tensile to yield (σu/σy) and fairly good value of high normalized hardening rate shows better resistance against wrinkling. Drawing through the tractrix die shows better resistance against wrinkling comparing with the conical die.