Predicting galling behaviour in deep drawing processes

Deep drawing is a sheet metal forming process which is widely used in, for example, the automotive industry. With this process it is possible to form complex shaped parts of sheet metal and it is suitable for products that have to be produced in large numbers. The tools for this process are required to meet high demands, because these tools have to create high quality products while at the same time withstanding large forces. As a result, tooling is expensive. Damage might cause tools to fail during production and one such failure mechanism is galling. Galling is a mechanism whereby material transfer occurs from the sheet to the tool, where it forms lumps on the surface, and these lumps subsequently cause scratching into the sheet. Currently the occurrence of galling in real sheet metal forming applications is rather unpredictable. In this thesis a model is presented from which the galling tendency of a sheet material in forming operations can be predicted. To investigate the phenomenon of material transfer from a sheet to a tool surface, experiments are performed on a single asperity scale. Observations from these experiments are used to formulate a single asperity lump growth model. Beside adhesion between the transferred material and the tool surface, the mechanical stability of the formed lump is taken into account. In order to approach the galling situation in real contact between a sheet and a tool, a multi asperity lump growth model is developed. The multi asperity lump growth model is based on a developed contact model combined with the single asperity lump growth model. The multi asperity model is used to investigate the influence of a number of parameters on galling in deep drawing. According to the model, important parameters are the sliding distance, the contact pressure and the relative strength of the lump compared to the sheet material. A galling performance indicator is formulated on the basis of results from the multi asperity lump growth model. The results are split into two components: a galling impact factor, which is determined on contact scale, and operational conditions, contact pressure and sliding length, that are obtained from a finite element simulation of the deep drawing process. The indicator is formulated in such a way that it can be implemented as a post processor for finite element simulations. The galling model is validated by galling tests and successfully applied to deep drawing of a cup.