Optimization of hot strip drawing test methodology for hot forming of aluminium alloys

Abstract: The development of lightweight design for automotive applications has lasted for many years and is still increasing. Vehicles with light structures use less fuel, emit less CO2, and are better for the environment. The next generation of lightweight vehicle structures will be realized using materials with a high strength-to-weight ratio, corrosion resistance, and high bending stiffness such as high-strength aluminium alloys. Increased formability, lower spring-back, and the ability to incorporate age-hardening heat treatments into the process are all advantages of the hot forming of aluminium alloys. However, because aluminium is prone to adhesion even at low temperatures, it can be difficult to avoid its transfer onto the tool. When forming aluminium, lubrication is always required to lower the interfacial shear strength and avoid direct contact between the interacting surfaces, to promote low friction and wear. However, lubricants can fail and significant issues with material transfer arise. Because of this, interruptions are needed for tool refurbishing, this issue affects process efficiency. Developing new or improved lubrication technologies to prevent adhesion and high friction, surface engineering solutions, as well as effective testing platforms related to aluminium forming, are vital. A significant challenge when studying the interaction between lubricant, aluminium, and tools often result in poor reproducibility of tests and uncertainties regarding the effect of lubricant thickness/weight on the friction behavior. Problems associated with reproducibility also affect modelling and simulations of the forming operation, as there are many uncertainties in the experimental campaigns, thus affecting the validation stages. The current study aim is to optimize the tribology test methodology used in the hot strip drawing method, with emphasis on the lubricant application methodology, to improve the reproducibility of tribological tests. Lubricant application methodology was developed by using air-brush equipment, and a consistent amount of lubricant. Different lubricant weights were characterized using two different types of lubricants (graphite-based and polymer-based lubricants), and then the tribological response was evaluated after the tribological tests. Lubricants were applied on cast iron and tool steel (CrN coated) to take into consideration the effect of the surface material and topography on the reproducibility, and consistency of the lubricant application methodology. The hot strip drawing method was used to perform tests at 350°C temperature, 10MPa pressure and 100mm sliding distance. To simulate the forming conditions encountered in hot stamping of aluminium, solubilization of a 6XXX aluminium alloy at 540°C was done before testing. An optical microscope, 3D optical profilometry, and SEM were used for the characterization of specimens after the tribological tests. With the lubricant application method, good reproducibility was obtained and it was determined that, in the case of cast iron, as the amount of both lubricants (polymer-based, graphite-based) increases the effective sliding also increases, but tool steel (CrN coated) shows slightly good results only when lubricant amounts are highest.