Numerical and experimental investigation of dry deep drawing of aluminum alloys with conventional and coated tool surfaces

Abstract The demand for light weight concepts in the automotive industry motivates a permanently increasing amount of aluminum alloys in the body in white. Aluminum alloys cause challenging tribological conditions in sheet metal forming due to their adhesion tendency towards tool steel. Therefore, in conventional deep drawing lubricants are applied to separate tool and workpiece to reduce friction and avoid wear. Global trends like increasing sustainability and a demand for an efficient usage of resources encourage the abandonment of environmental harmful lubricants. However, the realization of lubricant free – so called dry – deep drawing is accompanied by an intensive interaction caused by the direct contact between sheet and tool. Especially for aluminum alloys, this leads to increased friction and galling. Therefore, surface modifications are necessary for realizing dry forming. Due to the large batch sizes in the automotive industry surface modifications of the workpiece do not seem appropriate. In former studies, the tool-sided application of carbon based coatings revealed promising results in laboratory tribological tests without lubrication. However, these tests do not mirror the conditions of the entire deep drawing process. In this study, dry deep drawing was analyzed numerically and experimentally in comparison to lubricated tests. With conventional tools the direct contact led to adhesive wear and failure of the rectangular cup. Thus, deep drawing was not possible without lubrication. Applying a ta-C coating led to decreasing friction and thus lower forming forces under dry conditions. A tool- and workpiece-sided surface characterization revealed that ta-C coatings prevent adhesion and improved workpiece quality. The experimental results like punch force, part geometry and sheet thickness were used for validation of the finite element (FE) model. Comparing experimental and numerical results revealed that modelling of dry forming is possible with conventional FE methods as long as the friction coefficients are determined under the same tribological conditions without distinctive wear occurrence.