23rd International Colloquium Tribology - January 2022 415 Contact and Lubrication Aspects on Predicting the Contact Area in Lubricated Hot Rolling André Rudnytskyj AC2T research GmbH, Wiener Neustadt, Austria TU Wien, Vienna, Austria Corresponding author: andre.rudnytskyj@ac2t.at Martin Jech AC2T research GmbH, Wiener Neustadt, Austria Josef Leimhofer AMAG rolling GmbH, Ranshofen, Austria Stefan Krenn AC2T research GmbH, Wiener Neustadt, Austria Georg Vorlaufer AC2T research GmbH, Wiener Neustadt, Austria Markus Varga AC2T research GmbH, Wiener Neustadt, Austria Carsten Gachot TU Wien, Vienna, Austria 1. Introduction In the context of metal forming, bulk forming processes such as lubricated hot rolling involve a series of physical and chemical events in the contact zone between roll and workpiece [1]. In order to optimize such processes in terms of energy consumption, it is desirable to quantify and predict the friction forces due to the tribological aspects of the contact. For a sound tribological understanding of the friction conditions, it is required to estimate the real contact area [2,3]. In turn, the quantification of real contact area requires an accurate description of the material deformation behaviour, characterization of the surface topography, and defining the role of the lubricant. 2. Materials and Methods Timeand temperature-dependent plastic deformation is an important aspect of hot rolling of aluminium alloys of the 6xxx material grade. Assisted by high temperature tests on aluminium alloys 6061 and 6016, the material properties were modelled through advanced thermo-viscoplastic constitutive equations such as Eq. (1), calculating the material parameters (Q,A,a,nʹ) using Python programming language and its numerical libraries. Despite having similar properties at room temperature, the two alloys significantly differ with varying temperature and strain rate. The material laws of flow stress (σ f ) as a function of strain, strain rate, and temperature (ε,ἑ,T) were implemented in the numerical analysis of a contact applying the Finite Element Method (FEM), by use of the commercial software COMSOL Multiphysics ® . In order to investigate whether equation (1), which was derived from compression experiments, is appropriate to be used in a contact analysis, hardness tests at room and elevated temperatures were performed and compared to a FE indentation model. A contact patches approach in conjunction with a FE database [4] allows customized treatment of the surface and user-defined material models to be included in a contact analysis (Figure 1). 416 23rd International Colloquium Tribology - January 2022 Contact and Lubrication Aspects on Predicting the Contact Area in Lubricated Hot Rolling Figure 1: Surface topography is evaluated at chosen separation and contact patches are identified. Each individual contact patch (or asperity) is analysed in a FE simulation with the implemented material model. The presence of the lubricant and the characteristics of the topography may lead to a limitation in the real contact area if the lubricant is entrapped between asperities. Along with the development of an inlet film thickness, entrapped lubricant in so-called lubricant pockets could provide normal support, thus decreasing the real contact area. 3. Results The contact patches approach along with the user-defined material model allows to calculate the real contact area and contact load under tribological conditions of interest. These can be a set temperature or an approaching velocity, which can be related to actual rolling process parameters. The importance of correctly setting the appropriate temperature of the contact is evidenced by the FE database results, which is a consequence of the temperature-dependent material properties. Figure 2: Contact load of different sized asperities at different temperatures [5]. Additionally, different sizes of asperities resulted in different mean contact pressures (Figure 2). Such results indicate that not only the right operating conditions such as temperature and speeds must be well defined, but also the representation of the surface topography plays an important role. 4. Acknowledgments Part of this work was supported by the Austrian COM- ET-Program (K2 Project InTribology, no. 872176) and carried out at the “Excellence Centre of Tribology” (AC2T research GmbH). The government of Lower Austria supported the endowed professorship tribology at the TU Vienna (grant no. WST3-F-5031370/ 001-2017). References [1] Schey, J.A. “Tribology in metalworking: friction, lubrication, and wear”. American Society for Metals, 1983. [2] Nielsen, C. V., and N. Bay. “Review of friction modeling in metal forming processes.” Journal of Materials Processing Technology 255: 234-241; 2018. [3] Wilson, W., et al. “Real area of contact and boundary friction in metal forming.” Int. J. Mech. Sci. 30: 475-489; 1988. [4] Shisode, M.P., et al. “ Semi-analytical contact model to determine the flattening behavior of coated sheets under normal load.” Tribology International 146: 106-182; 2020 [5] Rudnytskyj, A., et al. “Influence of the 6061 aluminium alloy thermo-viscoplastic behaviour on the load-area relation of a contact”. Materials 14, 1352, 2021.