Relationship of Coating Failure to Deformation in the Deep Drawn Cup

Abstract Deformation of the metal substrate in the deep drawing process was characterized by macroscopic appearance and strain distribution. The results of the analysis were correlated with the modes of coating failure. The characteristic ears and valleys at the top of the cup and buckling of the metal in the valley regions resulted from instabilities raised during the cupping process. Earing was a plastic instability that originated from the combined factors of the crystallographic unit cell and the anisotropy of the rolled metal substrate. Buckling was an elastic instability that occurred when the valleys lost constraint before the ears. Stopping the forming process at intermediate positions before the cup was completely formed revealed four stages in the cupping process. Stage I was the initial indentation with formation of six ears. Stage II was conversion from 6 ears to 4 ears separated by alternating deep and shallow valleys. Stage III, loss of constraint in the deep valleys, led to buckling. Stage IV was total loss of constraint. Relationships between the deep drawing process as described by the 4 stages and coating failure were explored with two coating formulations: one that did not fail during deep drawing and one that failed. Two failure modes were observed on the deep drawn cup. Mode I, coating delamination at the top of the cup, was related to the bucking instability. Mode II, loss of adhesion along the cup wall, was caused by elastic retraction of the unconstrained cup in stage IV.