An experimental study to determine electrical contact resistance

Christophe PRADILLE [corresponding author] Christophe.pradille@mines-paristech.fr

CEMEF Mines-ParisTech- CNRS UMR 7635 Rue Claude Daunesse - BP 207 06904 Sophia Antipolis cedex

France

Katia MOCELLIN CEMEF Mines-ParisTech- CNRS UMR 7635

Rue Claude Daunesse - BP 207 06904 Sophia Antipolis cedex

France

Francois BAY CEMEF Mines-ParisTech- CNRS UMR 7635

Rue Claude Daunesse - BP 207 06904 Sophia Antipolis cedex

France

Abstract: Electrical contact resistance is of critical importance in resistance welding and depends on pressure, temperature, surface condition and steel grade. In this paper we present the experimental investigations on the contact electrical resistance with two distinct techniques for different pressures and temperatures and on different types of contacts (electrode-metallic sheet or metallic sheet - metallic sheet).

Keywords: Contact resistance, temperature, pressure, welding.

I. INTRODUCTION

Resistance welding is a process used to join metallic parts with heating deriving from electric current and a forging force. There are several forms of resistance welding, including spot welding, seam welding, projection welding, and butt welding. During these processes, the heating is generated by Joule effect, which is proportional to the total resistance of assembling. The numerical simulation of these processes is an important tool for development, optimization and mastering of high quality joints. However it needs a good knowledge of the electrical resistivity, and more precisely the electrical contact resistance. In fact the electrical contact resistance is of critical importance in resistance welding and

depends on pressure, temperature, surface condition and steel grade. When two metal surfaces with a certain roughness are brought into contact, the real contact area is much smaller than the apparent contact area [1]. This contact area depends also of the temperature and the pressure. The applied current crosses the interface through this real contact area. This constriction of electric current produces a local increase of current density, which produce a local heating by Joule effect. The objective of the present work is to establish a model of contact resistance that can be applied in simulation of resistance welding. In this paper we present the experimental investigations on the contact electrical resistance with two distinct techniques for different pressures and temperatures and on different types of contacts (electrode-metallic sheet or metallic sheet - metallic sheet). For a better precision of the electrical contact resistance measurements a four-point method is used. The use of the numerical simulation allows evaluating the uncertainty associated with each method and also allows explaining some difficulties and results. The results are also compared with measurements given during the real process.

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II. EXPERIMENTAL METHOD

In order to investigate the influence of temperature and pressure on electrical resistance, a static method is employed for the experiment.

A. The test equipment. Gleeble® systems [2,3] are used by some researchers, but in our case a mechanical press equipped by a furnace has been preferred. The mechanical press used is a servo hydraulic testing machine custom built by Dartec, Stourbridge, UK, of 300 kN capacity. The samples can be mechanically loaded following a prescribed program, while the parameters are measured and recorded for later analysis. Concerning the heating of the sample, the machine is equipped with two interchangeable split (clam shell) furnaces, one with kanthal heating elements used for testing at temperature up to 1000 °C and the other with silicon carbide heating elements used for testing at temperatures up to 1250 °C. During the heating, the oxidation of the sample can be minimized using an inert gas like argon. Moreover to improve gas shielding, a crucible has been added on the setup. This crucible is placed around the sample and is saturated with the inert gas. During the heating and the loading the global resistance is measured using a four point method.

B. Four point method. In applications where the wire lead resistance compared to the sample resistance is not negligible, a four point method is generally preferred. However, when measuring very low values of ohms, in the milli or micro Ohm range the two point method is not satisfactory because test lead resistance induces a significant error. The key difference between a four point and a two point sensing is that the separation of current and voltage electrodes in a four point method allows the ohmmeter/impedance analyzer to eliminate the impedance contribution of the wiring and contact resistances. Instruments based on four point measurement work on the following principle are presented on Figure 1. Two current leads C1 and C2, comprise a two wire current source that circulates current through the

sample under test. Two potential leads, P1 and P2 provide a two wire voltage measurement. The resistance value is obtained using the Ohm law. The needed parameters are issued from the measured values of voltage and current. Current and voltage are measured using platinum wire welded on the samples surface.

Figure 1: schematic drawing of four point method

C. Specimens Two different alloys are used to determine the electrical resistance of contact. One of these is a copper alloy which is used for the electrode in the industrial process. The other one will be named alloy X. This alloy has a meting point over 1500°C. The test specimens are circular cylinders (like “pills”) with dimensions ∅ 10x1 mm. The surface roughness of samples is Ra= 0.5±0.1 μm. The specimens are produced and stored in conditions similar to the industrial process. This particular geometry of samples was designed to minimize the effect of material electrical resistivity compared to the electrical contact resistance. In fact, in this geometry, the apparent electrical contact area is large, and the volume of sample is quite small, consequently measure of electrical resistance and electrical contact resistance are quite equivalent.

D. Physical setup The global experimental setup is illustrated in Figure 2. Specimens are compressed between two planes. One of the anvils is fixed on the movable jaws. During the test, the specimen is heating using the furnace. Figure 3 presents a precise view of the sample. On this figure, we note that a pileup of small cylinders is used. This technique was already used and presented

in [4]. The pileup is composed by 21 small pills (11 of alloy X and 10 of copper alloy or 21 of alloy X).

Figure 2: experimental setup

They are placed in a ceramic housing, and on each boundary a copper electrode (for the current alimentation) and a ceramic protection (for electrical insulation) are added. Two wires are welded by percussion on the first and on the last pill. They are used to measure the electrical potential in the pileup.

Figure 3: schematic view and potography of the sample

The objective of this pileup is first to increase the value of the electrical contact resistance to be able to measure it, and then to average this value for the 20 interfaces. During the test the electrical resistance of contact is continuously measured using a micro ohmmeter and a four point method.

E. Measurements A first experiment is necessary to determine the time necessary to have good temperature homogeneity in the sample. Therefore, a stainless steel sample (with

thermal properties very close to the Alloy X properties) was instrumented with four thermocouples K (chrome-Alumel). Three are welded by percussion on the sample surface, one on each boundary, and one on the central part. The last is introduced in the middle of pileup.

Figure 4: position of thermocouples for the temperature

homogeneity test.

As shown in Figure 5, the temperature becomes homogenous very quickly in the sample. Thanks to this point, a thermocouple at the surface of the sample allows a regulation of temperature in the sample. During each test, the sample temperature is kept constant. For the different temperatures, the load can be changed in two ways i.e. continuously or stepwise. In practice, a change by stepwise is preferred. The initial heating time, to prescribe the temperature, is given by the test presented just before, and is controlled by a thermocouple welded on the surface of the sample.

Figure 5 temperature measurement at the different

thermocouple welded on the sample

III. RESULT AND DISCUSSIONS

Figures 6, 7, 8 and 9 list the results from stepwise loading on AlloyX - AlloyX contact and on AlloyX – Copper contact.

Figure 6: electrical contact resistance of Alloy X contact

vs. pressure and temperature

It can be seen that the pressure has a large influence on results of electrical contact resistance. As a general rule, the electrical contact resistance decreases with an increase of pressure. Moreover it seems that the rate of decrease is lower at high pressure than at low pressure. Moreover, the temperature also plays an important role. But it’s more difficult to define a rule. Electrical contact resistance does not vary continuously with the temperature. The same observations are made by Song [5]. The proposed hypothesis is that some variations on the mechanical of the resistance film could be observed. Moreover the rate of decrease of the resistance of the film is larger at high temperature than at low temperature. Another hypothesis can be exposed to explain the observed phenomena, alloy X was very sensitive to the oxidation, and higher is the temperature, lager the oxidation is. This oxidation increases drastically the electrical contact resistance. For example a Cu2O film (1nm of thickness) has an electrical resistivity of

²105 12 mΩ× − at 298K, and for a 10nm film, this electrical resistance increases to ²10 8 mΩ− [6]. Figure 7 illustrates the variation of electrical contact resistance between Alloy X and Copper. Pressure and temperature reveal similar influence as on alloy

X contacts. But, we note that the electrical contact resistance is higher than that of the alloy X contact. Due to the lower resistivity of Copper, the last point is non intuitive, but it perhaps can be explain by a higher flow stress of copper.

Figure 7: electrical contact resistance of Alloy X-copper

contact vs. pressure and temperature

Nevertheless, this experiment design seems not to be the best one and this for different reasons:

• First, as shown in Figure 8 with this sample design, the results are repeatable with difficulty.

Figure 8: reproducibility test on 5 different samples.

Electrical contact resistance vs. pressure at room temperature

• Second, if a different loading cycle is considered during which the temperature changes by step way and the pressure is

maintained constant, the results are significantly different. The increase of electrical contact resistance with temperature is not observed (see Figure 9).

• Then, despite a good and precise manufacturing a large pileup seems quite unstable and consequently the contact is not necessary the same between each “pills”. This point seems to be proved by post test observations. First some signs of frictions have been observed on the ceramic housing, and then some pills in the pileup are welded at the end of the test.

Figure 9: electrical contact resistance of Alloy X contact

vs. temperature and compression force (10kN is equivalent to 133MPa)

In all cases influence of pressure seems similar. The electrical contact resistance decreases with an increase of pressure. In fact, pressure has an effect in at least two different aspects:

• Enlarging the real contact area • Facilitated the rupture of the surface film

At high pressure, both effects become less influential, because the real surface is closer of the apparent surface. The temperature affects the contact resistance in different aspects. The mechanical properties change with the temperature, consequently it is easier to change the contact surface. Indeed the yield stress decrease with temperature. Under the same load, the

real contact area is larger at high temperature. But in the same time, there is an increase of resistance of constriction area. This change in constriction resistance is caused by an increased of material resistivity. And finally, with higher temperature, the oxidation layer may grow at higher rate.

IV. CONCLUSION

This study on the influence of pressure and electrical resistance shows contrasted results. The influence of pressure is quite the same in all experiment, but the influence of temperature seems more contracted and difficult to analyze. In the same time, the observed values of electrical resistance of contact are very close of measurements realized directly on the industrial process. A new investigation of electrical contact resistance seems necessary to validate results. But a new geometry of sample is planed to restrict the unknowns due the actual geometry. The future sample must have a larger area of contact. Moreover, for the conference electrical simulations of the samples will be presented. First simple thermal and mechanical simulations could be explained mechanicals problem. And with a coupled electrical, thermal and mechanical model, we are probably able to explain some observed problems.

REFERENCES

[1] L. Féchant, Le contact éléctrique, Collection SEE, Ed Hermes, Paris, 1996 [2] Using a Gleeble® 1500. second edition, Dynamic Systems, Inc, PO Box

1234, Poestenkill, NY, 12140 [3] Rasmussen, Soren, Information About Gleeble 1500, Departement of

manufacturing engineering and management, technical university of Dannemark

[4] A. Monnier, B. Froidurot, C. Jarrige, R. Meyer, Ph. Testé, "A mechanical, electrical, thermal coupled-field simulation of a sphere-plane electrical contact" 51ième IEEE Holm Conference, Chicago, USA, pp.224-231, Sept. 2005

[5] Q. Song, W. Zhang, N. Bay, An experimental study to determines the electrical contact resistance in resistance welding, Welding Journal, 73-S, May 2005

[6] P. Pascal, Nouveau traité de chimie minérale, Tome III, Masson, 1957