Measurement of contact resistance values

Depending on the manufacturing method, surfaces of sheet metal semi-finished products generally have an uneven, rough surface structure. An oxide skin forms the outer surface layer on uncoated metallic materials under ideal conditions.

This can be supplemented with specific surface modifications, e.g. anti-corrosion surface coatings, in the case of technical materials such as sheet metal semi-finished products.

The type and design of the sheet metal surface (including coating material(s), one-sided/two-sided coating, texturing, cleaning process, pretreatment, application of lubricants) can significantly affect the contact resistance values of the sheet metal parts and are therefore highly significant for the size of the welding zone, the electrode life and thus the welding result of resistance welding.

An automated test method for determining the contact resistance values on sheet metal semi-finished products made of copper has been developed in collaboration with Zwick GmbH & Co. KG. The method described in the ISO 18594 standard was transferred to applications involving copper materials in the field of electrical engineering. In order to test the measurement setup in the sense of factory calibration, comparison measurements are performed on a reference material (copper sheet gold plated on both sides). The measurements are performed for three load levels (100 N, 200 N and 300 N).

The objective of the test is to determine the contact resistance. The purpose of the test is to compare semi-finished sheet metal products in the sense of production monitoring or receiving inspections or to detect changes in the contact resistance values.

In realizing the device, a conventional resistance measurement device was implemented in a universal testing machine (load range: ± 2.5 kN) from Zwick GmbH & Co. With this system, the individual settings for controlling the resistance measurements (e.g. measuring range, start and duration of the measurement, data transfer) can be made directly through the user interface of the testXpert program system. Test evaluation and logging are also performed using the standard software routines of  testXpert. This permits simple expansion/retrofitting of existing Zwick testing systems.

The work was performed as part of the project titled "Untersuchung zum Einfluss der Oberflächengüte und Fügeteilgeometrie auf die Qualität von Fügeverbindungen an Kupferwerkstoffen im Automotivebereich" (study on the influence of surface quality and the geometry of joined parts on the quality of joints on copper materials in the automotive sector) at the Esslingen University of Applied Sciences.

ISO 18594: Resistance spot-, projection-, and seam-welding – Method for determining the transition resistance on aluminium and steel material

The studies are being funded by Deutsches Kupferinstitut Berufsverband e.V. (DKI), Düsseldorf, as part of the immediate-action program "Beschäftigung für Absolventen im Wissenschaftsbereich" (MINT) of the Baden-Württemberg Ministry of Education and Research to promote the employment of science graduates.

Prof. Dr.-Ing. Martin J. Greitmann and Dipl.-Ing. Graf Nicolas Strachwitz
Esslingen University of Applied Sciences