Effect of Lubrication on Friction in Bending under Tension Test-Experimental and Numerical Approach

Tomasz Trzepiecinski,Hirpa G Lemu

doi:10.3390/met10040544

Tomasz Trzepiecinski, Hirpa G Lemu

Open Access

https://doi.org/10.3390/met10040544

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Abstract

This paper is aimed to determine the value of coefficient of friction (COF) at the rounded edge of the die in the sheet metal forming operations using the bending under tension (BUT) test. The experimental part of the investigations is devoted to the study of the frictional resistances of low alloy steel sheet under different strains of the specimen, surface roughnesses of the tool and for different lubrication conditions. Three oils are destined for different conditions of duties in the stamping process. Numerical modeling of the material flow in the BUT test has been conducted in the MSC.Marc program. One of the objectives of the numerical computations is to know the type of the contact pressure acting on the cylindrical surface countersample in the BUT test by assuming the anisotropic properties of the metallic sheet. It has been found that the COF in the rounded edge of the die does not vary with increasing sheet elongation. Taking into account that normal pressure increases with increasing specimen elongation and workpiece material is subjected to strain hardening phenomenon, the COF value is very stable during the friction test. The effectiveness of the lubrication depends on the balance between two mechanisms accompanied by friction process: roughening of workpiece asperities and adhesion of the contacting surfaces. In the case of high surface roughness of tool due to a dominant share of ploughing, all of the lubricants used were not able to decrease the COF in a sufficient extent. The used lubricants were able to reduce the value of friction coefficient approximately by 3–52% in relation to the surface roughness of rolls.

Highlights

The presence of friction in sheet metal forming (SMF) processes is generally an unfavorable phenomenon and causes changes in the force and energy parameters: the total work and power needed for the process increases, which causes the forces and unit pressures on the tool surfaces to increase
Numerical analyses were carried out on the numerical model corresponding to the friction of samples against the surface of rolls carried out on the numerical model corresponding to the friction of samples against the surface of with a roughness Ra = 0.32 μm (Figure 6a); for these conditions the largest difference in coefficient of friction (COF) values rolls with a roughness Ra = 0.32 μm (Figure 6a); for these conditions the largest difference in COF
In the continued to increase with the elongation of the sample and it reached its stable range at a elongation range between 3.5% and 15%, a constant value of COF was obtained and this result was deformation of 3.5%

Summary

Introduction

The presence of friction in sheet metal forming (SMF) processes is generally an unfavorable phenomenon and causes changes in the force and energy parameters: the total work and power needed for the process increases, which causes the forces and unit pressures on the tool surfaces to increase. Thermal dissipation of friction work at high values of unit pressure and relative speeds leads to a significant increase in temperature in areas adjacent to the contact surface. Friction has a number of negative effects in SMF processes [5,6,7].

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