Whitening mechanism in the drawing of pre-coated aluminum sheets

Abstract

Pre-coated aluminum sheets are superior in cost performance and productivity compared to aluminum sheets coated after press forming. Functional pre-coated aluminum sheets, with superior surface characteristics such as lubricity and corrosion resistance as well as aluminum's inherent lightness and excellent heat conductivity, have been increasingly used in electrical and other industrial fields. The serious problems that result from the forming of pre-coated aluminum sheets, namely peeling and whitening, can be attributed to the properties of the coating film. These problems have to be defined in terms of the properties of the coating film and then solved to create top quality products that are not characterized by peeling and whitening. Using aluminum sheets coated with polyester films at various glass transition temperatures, whitening in drawing of pre-coated aluminum sheet was investigated in this study. The effect of the application of lubricants on whitening behavior based on the stretching test was also investigated. Results showed that the degree of whitening was directly affected by the kind of lubricants used. Whitening was only caused in areas where lubricants were applied, and micro-cracks were only observed in the areas where whitening occurred. In addition, whitening was observed only in films with high glass transition temperatures. Based on these results, it was clear that whitening is attributable to the occurring of micro-cracks. As films with high glass transition temperatures are hard, excessive internal stress cause of cracks tend to concentrate on local area. It is considered that the adsorbed lubricant on the polyester films acts to decrease the mechanical strength of the films. As a result of the lubricant adsorption, the film can not withstand internal stress in stress-concentrated regions, which in turn causes micro-cracks to develop.