Application of Finite Element Simulation in Metal Forming Tribology

The impact of numerical methods, particularly, the Finite Element Method (FEM), in providing tribological information relevant for lubricant formulations and general study of tribological variables is discussed. FEA was used to study surface evolution at the tool-workpiece interface for tube hydroforming and forging processes. The study has demonstrated that material properties, geometric complexity, and loading paths have significant influence on surface evolution, which ultimately predetermines lubrication mechanisms. A tribo-module that can output relevant tribovariables from finite element simulations has been developed. Using this module, surface evolution history of a deforming part can be traced. The tribo-module can be used by a tribologist to study and gather tribological information for a specific metal forming process. The surface evolution data also help in determining plausible lubrication mechanisms and possible lubricant candidates, and can play a significant role in the development of new lubricants. INTRODUCTION The main goals of numerical simulation in manufacturing process are to reduce manufacturing costs and increase quality and productivity. For instance, process simulation can be used to develop forming dies and establish process parameters by a) predicting metal flow and final dimensions of the part, b) preventing flow induced defects such as laps (forging) and excessive thinning and wrinkling (sheet forming), c) predicting temperatures (warm forming operations) so that part properties, and die life can be controlled, and d) predicting and improving grain flow and microstructure. Tribology is another area where the application of numerical modeling through Finite Element Analysis (FEA) and Molecular Dynamic (MD) Simulation are steadily gaining wide acceptance. Substantial tribological studies involving numerical simulations on metal forming have mainly focused on the development of friction models which can accurately characterize friction at the tool-workpiece interface [Wilson 1979]. Coulomb’s and constant shear friction models commonly used in process modeling are unrealistic, thus various attempts to develop nonlinear friction models using internal variables have been developed [Wilson 1979]. Theoretical models that involve simultaneous solutions of the equations governing the flow of lubricant and plastic deformation of the workpiece have been developed, and implemented in FE codes [Meng 1993]. The models are, however, limited to simple processes. In order to improve FE modeling of hot forging processes, Schmid, [2007] developed friction and heat transfer modules which take into account lubricant film thickness and real area of contact. Groche [2007] developed an FE scheme to study the influence of grain size and crystallographic texture on surface evolution. To accurately describe boundary lubrication, researchers have attempted to use molecular dynamic (MD) simulations coupled with FEA [Ham 1997]. Practical predictive ability of MD for application in areas such as nano and micro manufacturing tribology is, however, far away due to highly idealized nature of MD simulations, and also the method is computationally intensive. Despite the advancements in numerical methods and computer technology, lubricant developers have not benefited much from these tools. This may be due to the fact that the science pertaining to tribochemistry and tribomechanics have been studied separately. Presently lubricant developers are facing great challenges; there has been increasing demand for the development of environmentally friendly lubricants, self lubrication surface systems through die coatings, and effective lubricants for net shape forming of complex parts and emerging processes such as micro and nano manufacturing. This paper addresses some potential applications of numerical modeling in establishing tribodata via finite element analysis for enhancement of tribological performance and development of metal forming lubricants. NUMERICAL MODELING IN METAL FORMING TRIBOLOGY One of the major factors that can benefit tribochemists in lubricant development is to be able to describe quantitatively the evolution of boundary surfaces of the deforming part and their tribological derivatives. Though this is extremely a difficult task, the advancement in numerical computational capabilities, has paved a way toward better and efficient examination of the boundary surfaces. A typical metal forming discretization problem used in finite element formulations is represented by the functional, ds u F dV ε kδ ε k dV ε δ σ I I I i