Thick and thin film lubrication models for bulk forming simulations

Abstract

Most of finite element softwares for numerical simulation of bulk metal forming solve the mechanical behaviour at the tool‐workpiece interface with Amonton’s law, the constant friction law or any combination of both. These classical friction laws are suited for boundary and mixed lubrication regimes but can’t model thick or thin film lubrication conditions. In this work, two new constitutive laws are discussed and developed for their implementation in a proprietary finite element software for bulk forming modeling. The first formulation considers the flow of a low viscosity fluid between the roughnesses of the workpiece and die surfaces. The actual asperity geometries are modeled with triangular periodic functions. The pressure distribution as well as the corresponding friction stress inside the tool‐workpiece interface are analyzed for lubricant film thickness lower than the roughness magnitude. The second formulation deals with the case of thick film lubricants of temperature dependent viscosity. The Couette and the Poiseuille flows are associated for the evaluation of the friction stress for a linear distribution of the temperature in the thickness of the tool‐workpiece interface. These new interface constitutive laws are introduced in interface elements with the fluid mass balance and the film thickness changes. They are coupled with the plastic wave theory for mixed lubrication regime. The upsetting of a cylinder between two flat rough dies and the well known ring test are presented as first applications.