Physical modeling and finite element analysis of friction encountered in large deformation processes

The main objective of this research was to investigate the role of frictional forces in large deformation metal forming processes, such as forging and extrusion. The research included analysis of deformation in the above processes using both the Physical Modeling Technique (PMT) and the Finite Element Method (FEM) with the goal of identifying friction effects.

In this research, all experiments were conducted by utilizing PMT to simulate actual metal forming processes by selecting a model material and die geometry that resemble the conditions of an actual metal forming process. All physical modeling experiments were conducted using plasticine, as model material, and plexiglass as the die material.

A new method was developed to obtain constant strain-rate {a — e) curves utilizing the data obtained from a testing machine in which the croshead moves with a constant speed. The flow characteristics and the strain-rate sensitivities of two types of plasticine were determined from several compression tests. Compression test data were analyzed by using a statistical method, 2 factorial design, in order to identify the effects of deformation speed, lubrication and material type on friction and the mechanical behaviors of the materials. A series of ring compression tests were conducted to determine the coefficient of friction, p, and the corresponding calibration curve for different types of lubricants. A new technique, utilizing open-die backward extrusion test, was developed as an alternative method for evaluating the coefficient of friction by relating the percentage of deformation to the extruded height.

The experimental results showed that two types of plasticine have different material properties while being strain-rate dependent at room temperature. It was also shown that the extruded height changes according to the friction conditions at the interface.

In the second phase of this study, ABAQUS, a general purpose finite element code, was employed for the FE analysis of forming processes. Ring compression tests were simulated in order to investigate the effects of material type, deformation speed, barreling, and strain-rate sensitivity on the calibration curves. Compression tests were modeled for different aspect ratios. The effect of aspect ratios on the normal pressure and friction stress was determined. Open-die backward extrusion for different die sizes were also modeled to obtain the normal pressures, frictional stresses, and the calibration curves. Contrary to the results available in the literature, the finite element analysis results showed that every material possesses a distinctive calibration curve which is different than that of a different material.

The experimental and numerical results indicate that material properties play an important role in the behavior of calibration curves obtained from the ring compression test. The numerical results also show that open-die backward extrusion can be used to generate calibration curves for evaluating ì.