Dry deep drawing of aluminum for automotive production

Abstract

Lubricants are commonly used in metal forming processes to reduce the friction between the workpiece and the forming tool to protect semi-finished products and goods against corrosion and to reduce the load on the tool. One aim of environmentally friendly production technologies is to achieve dry forming without the use of lubricants. The goal of this project is to enable the dry forming of aluminum alloys during deep drawing by locally increasing the tool load capacity using customized tool coatings. The suitability of two types of carbon-based coatings, amorphous carbon and CVD diamond coatings will be investigated for dry contact with aluminum. In addition to coating, tribological effective microstructuring should improve the material flow and wear resistance of the used tools. The advantage of the amorphous carbon coatings is the deposition process, which enables a large area deposition as well as a good adhesion strength on steel substrates. Dry strip drawing and deep drawing tests with amorphous carbon coated tools showed higher friction coefficients compared to lubricated tests without any coating and a high adhesive wear. A reduction of the contact ratio from 100% to 87.5% resulted in a decrease of the sliding friction value by 20%. Dry tribological ball-on-plate tests of different coatings against aluminum showed that a polished microcrystalline CVD diamond coating is most promising to enable dry aluminum forming with a long lifetime of the coated tool. By in situ silicon carbide sublimation in a diamond deposition process, a possibility has been demonstrated for atmospheric CVD processes to reduce the electrical resistance of CVD diamond layers without the use of toxic gases. The electrical resistance of the coating could be reduced with a silicon doping concentration in the order of 1020 cm-3 in range between 104 Ω and 106 Ω. Electrical discharge machining of CVD diamond coatings has been made possible by silicon doping, which means that the specific resistance has been reduced to below 100 Ω∙cm.