Concept development of a method for identifying friction coefficients for the numerical simulation of clinching processes

Abstract

In order to reduce fuel consumption and thus pollutant emissions, the automotive industry is increasingly developing lightweight construction concepts that are accompanied by an increasing usage of aluminum materials. Due to poor weldability of aluminum in combination with other materials, mechanical joining methods such as clinching were developed and established in series production. In order to predict the relevant characteristics of clinched joints and to ensure the reliability of the process, it is simulated numerically during product development processes. In this regard, the predictive accuracy of the simulated process highly depends on the implemented friction model. In particular, the frictional behavior between the sheet metals as well as between the sheet metal and clinching tools has a significant impact on the geometrical formation of the clinched joint. No testing methods exist that can sufficiently investigate the frictional behavior in sheet materials, especially under high interface pressures, different relative velocities, and long friction paths, while allowing a decoupled consideration of the test parameters. This paper describes the development of further testing concepts based on a proven tribo-torsion test method for determining friction coefficients between sheet metal materials for the simulation of clinching processes. For this purpose, the correlation of interface pressure and the relative velocity between aluminum and steel sheet material in clinching processes is investigated using numerical simulation. Based on these findings, the developed concepts focus on determining friction coefficients at interface pressures of the above materials, yield stress, as well as the reproduction of the occurring friction conditions between sheet metal materials and tool surfaces in clinching processes using tool substitutes. Furthermore, wear investigations between sheet metal material and tool surface were carried out in the friction tests with subsequent EDX analyses of the frictioned tool surfaces. The developed method also allows an optical deformation measurement of the sheet metal material specimen by means of digital image correlation (DIC). Based on a methodological approach, the test setups and the test systems used are explained, and the functionality of the concepts is proven by experimental tests using different sheet metal materials.

Increasing the accuracy of clinching process simulations by modeling the friction as a function of local joining process parameters

“…13 Additionally, another conceptual extension for the mapping of a joining tool–joining part contact was presented. 13…”

“…Eq. 13 assumption and is known as the friction factor model, where the frictional shear stress is represented by the friction factor m multiplied by shear yield strength k of the softer contact partner. While the friction factor model is well suited for high interface pressures, it leads to an overestimation of the frictional shear stress at low interface pressures.…”

“…Further investigations showed that this friction test is able to accommodate interface pressures above the initial yield stress by using a two-part encapsulation. 13 Figure 2 points out that the two-part encapsulation does not generate any additional friction surface due to a defined gap setting.…”

“…Due to the fact that the screw test bed can only absorb very low normal forces through a lack of stiffness, a two-column frame is used to apply the normal force to the specimens in conjunction with a hydraulic cylinder. 13…”

“…Two-column frame (left) and technical data of the screw test bed (right) for conducting friction tests. 13 …”