Reibmodell

Design of a friction model for the description of tribological conditions in cold forging of semi-finished products with structured surfaces

 

Problem

In the cold forging industry, large process simulations of forming operations are gaining significance in the development of efficient tools and processes. The Increasing pressure to reduce costs requires fast and meaningful process simulations. These simulations aspire to minimize the number of experiments and simplify the procedures of critical moments in production processes.

Furthermore, there is a need to research alternative lubricant systems to enhance existing tribological systems. Bonder techniques are currently a standard method. While they provide good lubrication, they are characterized by poor environmental compatibility and the cleaning procedures require substantial efforts.

Modern FE-systems provide sophisticated modelling- and calculating algorithms. However, complex tribological systems can only be accommodated by simple friction models at the interface between work piece and tool system. [Figure 1] shows the friction models mainly implemented. On the left side, the friction model formulated by Coulomb. On the right side, the friction coefficient law.

The effect of strong surface modifications locally occurring cannot be illustrated with these models. Local meshing of the work piece surface with element structures that are sufficiently fine could represent the effects of structured surfaces. The computing operations necessary exceed by far current computing systems.

Figure 1: Friction models
Figure 1: Friction models

Purpose

In line with a DFG-Project, a semi-empirical friction model is set up, which satisfies current requirements of the cold forging industry.

New approaches for the reduction of process forces and wear will be integrated. This friction model may be implemented in FE-development-environments. During the development, the friction model will be tested in msc-superform.

The new friction model is based on physical laws and includes all relevant parameters that influence the tribological system. It will be applicable to other tribological systems without requiring special parameters.

Solution

Based on the design of friction models for unstructured work pieces, surfaces for structured work piece surfaces will be developed. In order to identify the friction coefficients of different process states, the slide-bulge-machine of the PtU will be modified and enhanced. In the new process, the radially symmetric sample will be placed on the shell, bulged by an optimized tool, and drawn over the sliding plate [Figure 2].

Figure 2: Slide-bulge-process
Figure 2: Slide-bulge-process

The adjustability of some parameters guarantees a certain reproducibility in industrial processes. Such parameters include: surface enlargement, contact stress, pressing-velocity, sliding distance, relative velocity and temperature. The verification of the new model is tested in a three-step process with full forward-flow-pressing function.