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Characterization of electrical contact conditions in spot welding assemblies
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 117–124
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Characterization of electrical contact conditions in spot welding assemblies P. Rogeon a,∗ , P. Carre a , J. Costa a , G. Sibilia b,c , G. Saindrenan b ´ Laboratoire d’Etudes Thermiques Energ´etiques et Environnement, Universit´e de Bretagne Sud, Rue de Saint-Maud´e, BP 92116, 56321 Lorient Cedex 3, France b Laboratoire G´enie des Mat´eriaux, Ecole Polytechnique de l’Universit´e de Nantes, La Chantrerie, Rue Christian Pauc, BP 50609, 44306 Nantes Cedex 3, France c PSA Peugeot Citro¨en, Centre Technique de V´elizy, MXP/CEB/ASG, Route de Gisy, 78943 V´elizy Villacoublay Cedex, France a
a r t i c l e
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a b s t r a c t
Article history:
In this study the interfacial conditions encountered in electrodes and sheets assemblies
Received 11 September 2006
used in resistance spot welding process are characterized. Electrical contact resistances are
Received in revised form
measured on a specific device, allowing to rise high pressure and elevated temperature.
14 December 2006
Measurements concern electrode–sheet (E/S) and sheet–sheet (S/S) interfaces, considering
Accepted 19 April 2007
coated and non-coated steel sheets. The evolutions of the electrode profiles according to the number of weld are presented. The quick wear of the electrodes surfaces is also measured, depending on the type of sheets (coated or none coated). A numerical approach is chosen to
Keywords:
simulate the squeezing stage, to show the consequences of modifying the electrode profiles
Spot welding
on the initial contact surfaces at the beginning of the heating stage.
Electrical contact resistance
© 2007 Elsevier B.V. All rights reserved.
Interface Coated steel sheet
1.
Introduction
Spot welding resistance is a process largely used in the automotive industry because of its capacity to assemble thin sheets. From an industrial point of view, the introduction of new families of steel (in particular high-tensile steels) requires adjustments to the traditional techniques of welding. From a theoretical point of view, such a process is a complex problem, due to the combination of coupled phenomena. Previous studies agree with the complexity of the physical couplings (Sibilia, 2003; Sibilia et al., 2003; Feulvarch et al., 2006; Murakawa and Zhang, 1998; Chang and Zhou, 2001; Li and Kimchi, 2000), and focus on the essential role of the contact or interfacial
∗
conditions occuring between electrode and sheet (E/S) and sheet and sheet (S/S), respectively (James et al., 1997; Babu et al., 2001; Vogler and Sheppard, 1993; Thornton et al., 1996). The role of the contact conditions must be considered at two scales. During the heating stage the area of the apparent contact surfaces controls directly the macro-constriction of the current in the assembly; increasing the current density in the different resistances (contact resistances at the E/S and S/S interface and bulk resistances arising from bulk resistivity of the materials) increases the thermal energy in the assembly. Surface defects (roughness, oxides, etc.) force the current to flow through a few microscopically small areas. These microconstriction effects locally increase the electrical resistance
Corresponding author. E-mail addresses: [email protected] (P. Rogeon), [email protected] (P. Carre), [email protected], [email protected] (G. Sibilia), [email protected] (G. Saindrenan). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.04.127
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independently of the bulk resistance of the material. These phenomenon associated with the existence of the interface is the contact resistance. Note that the same analysis can be made for thermal conduction. However in this case, heat can also crosses the interstitial medium by conductive or radiative transfer. Although the contact resistance can be easily defined in abstracto, it is not a real physical object since: • it cannot be separated from the bulk of the piece, • its depth is not defined. For these reasons, measurement of contact resistances is difficult so much more its values are heavily disturbed by the measurement itself and because it is very sensitive to external parameters (temperature, pressure, pollution, etc.). Thus, to characterize the interfacial conditions, an experimental device has been primarily developed to measure electrical contact resistance according to temperature and pressure. The effect of electrode wear on the apparent contact surfaces sizes at the end of the squeezing stage was also studied through experimental and numerical approaches. The measurement of contact resistances, through in situ techniques during welding as considered by some authors (James ´ et al., 1997), (Thieblemont, 1992) does not permit the control of the most important parameters. Indeed probable evolutions of contact surfaces sizes and current density in the assembly could occur. Furthermore interface temperatures are also unknown. However, dynamic resistance evolutions measured in situ during the process, could indicate above all the time during which the contact resistances are important. As Babu (Babu et al., 2001), Vogler (Vogler and Sheppard, 1993) and Thorton (Thornton et al., 1996) do, the contact resistances are measured here on a specific ex situ device; in this way, contact surfaces are controlled, furthermore the thermal and pressure conditions encountered during the heating stage (fast heating rates, high temperature gradients and elevated pressure) are simplified. The profiles of new electrodes initially curved are quickly flattened while the number of weldings increases (Dupuy, 1998); that modify apparent contact surfaces, with important consequences on the formation of the weld nugget (Khan et al., 2000). For coated and non-coated steels, the change in the electrode profile according to the number of welds is measured. The sizes of the contact surfaces at the end of the squeezing stage are also calculated through a numerical simulation, when considering two types of electrode profiles (curved and flattened). These two combined effects explain the essential role of the contact conditions occuring at the different interfaces in the spot welding process. Futhermore this good knowledge of the contact resistances with temperature and pressure before each weld constitutes a prerequisite condition to any relevant analysis or modeling of the spot welding process.
under strong pressure. The measurement technique ex situ is consistent with those used by Babu (Babu et al., 2001), and Vogler (Vogler and Sheppard, 1993). The samples are placed in a power press (MTS Cynergie 1000 5 kN), between two stamps connected through electrical wires to a current supply power (Tektronix CPS250 or Hewlett Packard 6034A). The current is measured precisely with an amperemeter (Keithley 2000). Contrary to the above-mentioned authors, not one resistance was measured between two sheets but rather N resistances between N + 1 sheets. To accomplish this a sample stack technique is used whose aim is to amplify the measured signal. The two samples at the extremity of the stack are connected ` with copper wires to a nanovoltmeter (Voltmetre Keithley 2002) allowing us to measure with accuracy the potential difference at the boundaries. For studying the two types of contact (S/S) and (E/S), stacks are constituted with samples in steel and with samples in steel and copper, respectively. In the case of S/S contact, measuring the stack resistance with N + 1 samples results in the addition of an initial resistance R0 (because of the two instrumented samples in contact with the first sample), N contact resistances RCE and N bulk resistances RO steel . Each new sample introduces into the stack an additional bulk resistance and an additional contact resistance. Thus, the total resistance RN of the stack corresponding to the N measurement according to N, R0 , RCE and RO steel is: RN = p × N + R0
2. Characterization of electrical contact resistances 2.1.
Method
Measurement of electrical contact resistances (RCE) were achieved on a specific device (Fig. 1) according to temperature,
Fig. 1 – Schematic representation of the experimental device for electrical contact resistance measurements.
(1)
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where p = RCE(S/S) + RO
(2)
steel
Concerning the E/S contact, at each step a couple of samples is added. The total resistance of the stack corresponding to the N measurement can then be written according to Eq. (1): with p = 2 × RCE(E/S) + (RO
steel
+ RO
copper )
(3)
In both cases, because of a linear evolution of the resistance stack RN with N it is possible to calculate the slope p and so extract RCE.
2.2. Materials, samples preparation and experimental conditions 2.2.1.
Materials and properties
Three types of steel sheets are considered in this study (Fig. 2): two coated with zinc and one none coated.
2.2.1.1. XSG and XES steel sheet. The first steels considered in this study are non-alloyed steels, rolled at room temperature, ¨ for embossing process (Norme PSA Peugeot Citroen, 2000). The designation introduced by PSA Peugeot-Citroen for these steels is XSG or XES, respectively, according to the presence of zinc coating or not. They have low carbon contents (interstitial free (IF) type). Vickers hardness measurements lead to the following mean values—XSG: 98 HV0.1 and XES: 110 HV0.1 .
2.2.1.2. DP600 steel sheet. DP600 is a dual phase steel, rolled at room temperature (Llewellyn and Hillis, 1996). The composition of 80% ferrite and 20% martensite leads to a good resistance corresponding to new industrial requirements (HV0.5 = 202). The main mechanical properties at room temperature for XSG, XES and DP600 are given in Fig. 2. The DP600 and XSG steel sheets are galvanized (10 m thick).
2.2.1.3. Copper alloyed samples. The welding electrodes are made of copper alloyed with chromium (0.4–1%) zirconium (0.02 and 0.15%). To measure the E/S contact resistance, we have used specific samples in the same copper alloyed. At
Fig. 3 – Curves RCE(S/S) = f(P) at room temperature: (a) case of uncoated sheets XES and confrontation to the literature and (b) cases of coated sheets XSG and DP600.
room temperature the hardness value is around 170 HV0.1 , the yield stress value between 470 and 570 MPa and the Young modulus 120 GPa.
2.2.2.
2.2.3.
Fig. 2 – Mechanical properties of the different steels at room temperature and ε˙ = 1.67 × 10−4 s−1 .
Preparation technique of samples
Preparation of the samples is an important and critical phase of the resistance measurements. Indeed, even small defects remaining on the samples surfaces, above all oxides or barbs on the edges can change radically the contact resistances. The steel samples (dimensions 10 mm × 10 mm and 7 mm × 7 mm) are cut in thin strips (thickness 1.5 mm) by using a circular saw and sprayed simultaneously with lubricant. The samples are immediately cleaned with clear water and dried. All the edges are burnished to take off the barbs under water spray and finally dried another time. The copper samples are obtained following the same protocol, from a cylindrical bar, of the same alloy as the electrode. The copper samples surfaces are systematically polished to retrieve the same surface as on a new electrode. The shape of the samples assigned to be connected with copper wires present little change: the dimensions (10 mm × 12 mm × 1.5 mm and 7 mm × 10 mm × 1.5 mm) keep free a little strip allowing to fix the copper wires outside the contact zone.
Experimental conditions
Preliminary experiments at room temperature confirm that contact resistance varies mainly until pressure reach 20–30 MPa, as already shown by Babu (Babu et al., 2001) and Vogler (Vogler and Sheppard, 1993) (Fig. 3a and b). However
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when the pressure increases from 40 to 80 MPa, the contact resistance values are divided by a factor two! So we decided to work at two pressures level 40 and 80 MPa for the experiments whatever the temperature; that is relatively closed to the mean pressure value encountered at the contact surfaces in the welding assembly (around 100 MPa). In some case the maximum stress values at the level of the (E/S) and (S/S) interfaces could exceed 200 MPa at the end of the squeezing phase (cf. Section 3.3, Figs. 12 and 13) depending on the apparent contact surface sizes! Two heating collars (power 600 W each) are fixed on the stamps in stainless steel (Fig. 1). The heat flux crosses and heats the stamps and the stack of samples by conduction. Four thermocouples (type K, diameter 0.1 mm) are introduced in each stamp. The two thermocouples at the extremity of each stamp allowed us to calculate the mean temperature of the samples stack. Under welding conditions the interfaces are subjected to extreme thermal rates (several thousands of degrees Celsius per second) and high temperature gradients (several hundreds of degrees Celsius at the E/S interface only, for symetrical assembly). The thermal conditions are different and simplified in the ex situ device. The temperature at the level of the heating collar is programmed as a step until 550 ◦ C and remained constant: the stack is heated by conduction and the rise until 500 ◦ C takes less than 15 min. During the rise of temperature the voltage at the extremities of the stack and the temperatures in the stamps are registered. The scanning frequency, for these two measurements, was 0.1 Hz.
2.3.
Experimental results
2.3.1. Electrical contact resistance at S/S interface 2.3.1.1. Non-coated steel—contact XES/XES. From the evolutions of stack resistances with temperature, the evolution of RCE(S/S) (Fig. 4) is deduced by calculation from equation (3) for the various temperatures. In particular a fast collapse of RCE with the temperature can be noticed: since at 250 ◦ C, the value already fell of by a factor of 4. This strong reduction is confirmed by measurements of Vogler and Sheppard for temperatures lower than 200 ◦ C (Vogler and Sheppard, 1993), but ´ has not been found by Thieblemont (Thieblemont, 1992). For the contacts between non-coated sheets, at room temperature and even under strong pressure, it is established that the
Fig. 4 – Curves RCE(S/S) = f(T)—case of uncoated sheets XES and confrontation to the literature.
Fig. 5 – Curves RCE(S/S) = f(T)—cases of coated sheets XSG and DP600 and confrontation with the literature.
contact ratio does not exceed a few percentage of apparent surface (Salgon et al., 1998). When the temperature increases, the mechanical properties of the material (yield stress, Young modulus, etc.) drop quickly while the electrical resistance increases. These antagonistic effects act in a contradictory way on RCE. For the contacts between non-coated sheets, the main factor seems to be, from our results, an increase in the contact rate ratio which leads to a radical reduction in RCE with temperature.
2.3.1.2. Coated steel—contact XSG/XSG et DP600/DP600. A first series of measurements carried out with a current of 200 mA, reveals that the voltage at the extremity of the stack are disturbed by thermoelectric effects (Seebeck effect in particular). In fact, stacks with coated sheets form a succession of alternated zinc and steel layers which, in the presence of an even weak temperature gradient, generate a few micro Volts of the same order of magnitude as the potential differences measured at the boundaries of the stack, above all when the temperature is low. In order to prevent the measurements of this thermoelectric noise, two modifications to the protocol were brought: first by increasing the current from 200 mA to 1.5 A, in order to increase the signal-to-noise ratio; secondly by programming the current in as a square signal (1.5 A during 10 s), one is able to observe the thermoelectric noise during the period (10 s) when the current is zero, allowing a correction by removing this parasitic value when the current flows. The RCE increases at first with temperature, then it reaches a maximum, around 350 ◦ C, just before the melting point of the coating (Fig. 5), and finally falls with the approach of 420 ◦ C. This behavior very specific to the contact between coated sheets is not found in the literature. It can nevertheless be explained by the great malleability of zinc (low elastic limit and Young modulus values). These characteristics of the coating must make it possible to reach, under 40 MPa and already at ambient temperature, a very high contact ratio. While the temperature rises, the major effect is the increase of the zinc resistivity, rather than the contact surface ratio, resulting in an increase in RCE with the temperature. Let us specify that our measurements show that the contact resistance is multiplied by 2.75 between 30 and 350 ◦ C, an interval in which the
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temperature until 200 ◦ C and more slowly after. In the case of the dissymmetric contact E/S with copper and coated sheets, the governing phenomena is the increase of the contact ratio with temperature, while it’s the electrical conductivity of the coating in the case of RCE(S/S) (Section 2.3.1, Fig. 5) until the melting of the zinc layers. In the case of E/S contact, like in the case of S/S contact (Fig. 5), the evolutions of RCE with temperature, for the two pressure levels, are similar whatever the steel DP600 or XSG (Fig. 7a). Furthermore the RCE(E/S) evolution found at 80 MPa appears relatively close to that given by ´ Thieblemont (Thieblemont, 1992), in spite of the differences in the shape of the evolutions (Fig. 7b). Fig. 6 – Curves RCE(E/S) = f(T)—case of uncoated sheets XES and confrontation with the literature.
electrical conductivity of zinc is divided by 2.6 (Llewellyn and Hillis, 1996). Then, when the temperature reaches the melting point of coating, its mechanical properties break down which cause a very important crushing of the asperities and a rise in the contact ratio. This phenomenon leads to a rapid reduction of the electrical contact resistance until 420 ◦ C. The comparison between electrical contact resistance values obtained at 80 and 40 MPa leads to the following remarks: the same type of evolution can be observed in the two cases and the values are much lower at 80 MPa except when the temperature reach 420 ◦ C, where they become very low corresponding to a good contact in the two case. The general shape of the curves RCE(S/S) consolidates the particular role which we allot to zinc in the behavior interfaces: whatever the material substrate, only the properties of the coating determines the evolution of the contact resistance with temperature. This behavior results from an antagonistic contribution of the electric properties with the mechanical properties as predicted by the model of Babu but for uncoated sheets (Babu et al., 2001)!
2.4.
Accuracy and sources of errors
Different sources of errors can affect the values of the contact resistance RCE, and they can be classified in two groups: deterministic and random type, respectively. Oxides, barbs, and dirt which could appear on surfaces during the samples preparation provide random errors; furthermore with industrial steel sheets, the roughness is certainly non-uniform on all the surface samples. This could lead to scattered measurements witch remain although representative of the reality. Inaccuracies about, the samples dimensions when cutting, there position in the stack, the current and the voltage measurements, and the resistivity values of steel, lead to deterministic errors. In the case of the uncoated sheet XES the scattering is relatively important (around 0.003 mm2 ) at low temperature
2.3.2. Electrical resistance at E/S contact 2.3.2.1. Non-coated steel—contact copper/XES. Fig. 6 presents the variation of RCE(E/S) with temperature compared with the results in the literature. As for the S/S contact types (Fig. 4), it could be observed a continuous decrease, initially fast till 250 ◦ C, then less marked. As in the case of RCE(S/S), our results are closed to those of Vogler (Vogler and Sheppard, 1993) rather ´ than from those of Thieblemont (Thieblemont, 1992). In this case, like for RCE(S/S) (Section 2.3.1), the increase of the contact ratio is the major effect.
2.3.2.2. Coated steel—contact copper/XSG and copper/DP600. Quite important differences between the evolutions of RCE(E/S) with temperature can be observed at the two pressure levels (Fig. 7a). At 80 MPa initial values of RCE at room temperature are already very low and correspond to high contact ratio: consequently values of RCE vary slowly with temperature at this pressure. At 40 Mpa initial values at room temperature are relatively higher and correspond to lower contact ratio. At this pressure the values of RCE(E/S) decrease first quickly with
Fig. 7 – Curves RCE(E/S) = f(T) cases of coated sheets: (a) comparison between XSG and DP600 and (b) XSG and confrontation with the literature.
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Fig. 8 – Curves RCE(S/S) = f(T) with scattering effect—case of uncoated sheets XES.
the nugget, such as annular or crescent shapes. However, with the repetition of the weldings, various phenomena contribute to the degradation of their ends (pollution and erosion of the profile, (Dupuy, 1998; Dong et al., 1998; Dong and Lu, 1999)). To mitigate the effects of this continuous evolution, the reference frame of spot welding at PSA recommends an incrementing of the welding current by steps, and the grinding of the electrodes after a certain number of welded points. One presents in this paragraph the experimental characterization of the modification of the profile of the electrodes during their use. Dong (Dong and Lu, 1999) proposed an analytic model to calculate the contact area size at the E/S interface taking into account the wear of the electrode face; however, although two types of electrodes are considered (truncated and radiused), a flat profile is supposed in each case. Inversely Kan (Dupuy, 1998) have shown that electrode profiles initially plate are quickly blunted with a high curvature radius (R 125 mm), essential condition to achieve satisfying weld nugget. Yeung (Yeung and Thornton, 1999) has proposed a numerical parametric model using conjugate heat analysis to predict the temperature rise at the E/S interface, which contributes to the electrode surface damage. A high maximum temperature was found at the electrode face, and no significant E/S temperature changes were found when changing the cooling cap parameters.
3.1. Fig. 9 – Curves RCE(S/S) = f(T) with scattering effect—case of uncoated sheets XES.
and decreases significantly as soon as temperature reaches 200 ◦ C (Figs. 8 and 9): a similar behaviour has been observed for coated sheets. The scattering effect becomes negligible after 300 ◦ C. This excellent repeatability indicates not only the quality and the accuracy of our measurements but also that all the errors are certainly negligible as regard the very low values of RCE at high temperature. This also confirm that measurement scattering at low temperature is certainly due to industrial steel sheet surfaces presenting roughness singularities witch disappear when the temperature increases and becomes sufficient to allow their crushing.
3.
Profilometric study
The variation of contact area sizes during the heating stage influences directly the formation of the welded point. The literature shows moreover the dominating role played by the profile and the state of the electrode contact surface in this ´ evolution (Thornton et al., 1996; Thieblemont, 1992; Dupuy, 1998; Dong et al., 1998; Dong and Lu, 1999). The two types of electrodes used within the framework of this study are machined ends whose contact face is a segment of a sphere (diameter Ø6 mm and initial curvature radius R 40 mm). This specific geometry makes it possible, in particular, to avoid alignment problems of the electrodes during the squeezing phase, and to force the current to the center of the assembly; as a consequences, this helps prevent against defects of
Experimental device and trial run
The various factors which can influence the modification of the electrodes contact surface are in our case, the materials, the type of electrode Ø6 and the welding parameters (force and current). The welding parameters as well as the current increments are defined according to a standard (E34.03.180, 2001). The principle of this series of measurements consists in the realization of a given number N of welded points, and in the analysis of the electrodes set used for these N weldings. The frequency of measurements is defined according to N = 1, 3, 5, 10, 25, 50, 75, 100, 150 and 300. The technical solution adopted to measure the evolution of the electrodes surface profiles is based on the use of a mechanical feeler, interdependent of an inductive sensor: the tests were carried out at the Laboratory Metrology of PSA-Sochaux (Fig. 10).
3.2.
Experimental results
Concerning the welding of non-coated sheets, one observes an evolution of the initial profile, radiused, towards a partial flattening of the electrode contact surface. After 50 welds at most (N = 50), the active face becomes partially flat. The profile characteristic of a worn electrode after 300 welds (N = 300) is compared to the initial curved profile (Fig. 11). One also reports on Fig. 11 the profile corresponding to an electrode having welded coated sheets (N = 300): one notes in this last case the essential role of the coating on the consumption of the electrode, leading to a total flattening of the profile. One can notice that the worn profile is not completely flat but presents, in the modified part, a curved profile with a high radius of curvature very different from the initial one. Consequently the wear of the initial profile of the electrode contact surface did not prevent successful weld nuggets.
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Fig. 10 – Experimental device for the electrode profile measurement.
3.3. Initial contact areas sizes at the beginning of the heating stage To study the incidence of the electrode contact surface profile on the contact areas at the two interfaces (E/S) and (S/S), the squeezing phase was numerically simulated by considering profiles of new and worn electrodes. At the end of the squeezing stage, the contact area is at the initial value for the heating stage, although it can change during this period. It will determine, through the macroconstriction effect, the initial current density in the sheets assembly. The finite element code Sysweld (ESI group France) was used to calculate the mechanical deformations and stresses during the squeezing stage. Only the case of non-coated steel sheets is presented here. The geometry was supposed axisymetric, and elastoplastic deformations and stresses are calculated in a radial plane in order to simplify the analysis. The effort applied corresponds to a standard at PSA and the mechanical characteristics of materials at room temperature (Fig. 2) are deduced from (Sibilia, 2003). The contact radii values are given by the evolutions of the normal stresses along each interface. The
Fig. 11 – Comparison between different electrode profiles: new and worn.
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results obtained with a new electrode profile (curved profile R 40 mm), are compared to those corresponded to a worn electrode; the eroded profile is obtained after N = 150 weldings on non-coated steel sheets, which corresponds to the fifth of the usual maximum number of welds for the electrodes. Fig. 12 shows the normal stress evolutions along interface E/S for the new and worn electrodes. It can be noticed that the erosion of the profile involves a notable increase of the contact radius from 1.5 to 2.5 mm. In the same way, Fig. 13 shows the normal stresses along the S/S interface for the new and worn electrodes. The wear of the electrode involves an appreciable increase of the contact radius from 1.8 to 2.7 mm. Because of the quick wear of the electrodes profile, incremental modification of the welding current must be done to ensure the achievement of welded points. During the welding stage, contact radii will continue to change due to complex mechanisms (Robin et al., 2002; Feng et al., 1998). Concerning the welding of coated sheets, the wear of the electrodes profiles is more important, and leads quickly to the flattening of the initial profile (Fig. 11). For this reason, the consequences on the initial contact areas are the same as previously observed for non-coated sheets. Moreover in this case, the initial contact radii at the beginning of the welding phase are closer to the radius of the electrode surface (3 mm). During the heating stage, the fusion of the coating layers during the first welding cycle leads to the very fast formation of zinc rings at each interface.
Fig. 12 – Theoretical distribution of stress at the E/S interface—case of uncoated sheets XES.
Fig. 13 – Theoretical distribution of stress at the S/S interface—case of uncoated sheets XES.
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4.
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Conclusion
The aim of this work was to determine the contact conditions at the interfaces electrode–sheet and sheet–sheet which constitute essential variables in the control of spot welding process. The formation of the weld nugget is indeed strongly dependent on the phenomena at the interfaces. The contact areas influence directly the macro-constriction of the welding current in the assembly: they vary throughout complex way, along the different phases of the process, are affected by the configuration of the assembly and by the profile of electrodes contact surface. The surface defects lead to micro-constriction effects taken into account by the electrical and the thermal contact resistances definition. Primarily electrical contact resistances measurements depending to the temperature and under pressure have been carried out on a specific device. A complete data file of electrical contact resistances, according to temperature, for non-coated and coated sheets, and for E/S and S/S interfaces was determined. In the case of non-coated sheets, the contact resistance decreases with the temperature, the increase of the contact ratio is the major effect. Concerning coated sheets the contact resistances values are lower, and at the S/S interface the contact resistance first increases until 350 ◦ C and than decreases till 450 ◦ C. This behaviour very specific to the contact between coated sheets has not been found yet in the literature. In this case the decrease of the coating electrical conductivity drive first the evolution of RCE till the melting point of coating where its mechanical properties break down which cause a very important crushing of the asperities and a rise in the contact ratio. Secondly the experimental analysis showed that with the repetition of welding, the electrode surface profile, initially curved, quickly presented a ‘flat part’ whose size increases with the number of welds. Moreover the flatness of the profile is much more important with coated sheets. A finite element analysis of the macroscopic deformations during the squeezing phase was made, by considering in the model the new and worn electrode profiles. Axi-symetrical deformation was assumed in order to simplify the analysis. The increase of the contact radii, due to electrode wear, leads to the incremental modification of the welding current during the process to ensure the achievement of welded points. With coated sheets the contact radii at the beginning of the welding stage is closer to the electrode radius, especially for worn electrodes. Consequently, with lower contact resistance values and larger contact areas, coated sheets involve higher welding current.
references
Babu, S.S., Santella, M.L., Feng, Z., Riemer, B.W., Cohron, J.W., 2001. Empirical model of effects of pressure and temperature on electrical contact Resistance of metals. Sci. Technol. Weld. Join. 6 (3), 126–132. Chang, B.H., Li, M.V., Zhou, Y., 2001. Comparative study of small scale and ‘large scale’ resistance spot welding, in: H. Cerjak (Ed.), Mathematical Modelling of Weld Phenomena VI, Graz, Octobre 1–3, pp. 923–937.
Dong, P., Lu, F., 1999. Model for estimating electrode face diameter during resistance spot welding. Sci. Technol. Weld. Join. 4 (5), 285–289. Dong, P., Li, M.V., Kimchi, M., 1998. Finite Element analysis of electrode wear mechanisms: face extrusion and pitting effects. Sci. Technol. Weld. Join. 3 (2), 59–64. ´ ´ Dupuy, Th., 1998. La degradation des electrodes lors du soudage ˆ ´ ` ENSMP. par points de toles zinguees, These ¨ “Soudage electrique ´ E34.03.180.G, Norme PSA Peugeot Citroen, ´ ´ par resistance: Techniques de soudage—Specifications ´ erales”, ´ gen Mars 2001. Feng, Z., Babu, S.S., Santella, M.L., Riemer, B.W., Gould, J.E., 1998. An incrementally coupled electrical-thermal-mechanical model for resistance spot welding. In: Proceedings of the Fifth International Conference on Trends in Welding Research, Pine Mountain, Juin 1–5. Feulvarch, E., Rogeon, P., Carre, P., Robin, V., Sibilia, G., Bergheau, J.M., 2006. Resistance spot welding process: experimental and numerical modelling of the weld growth mechanisms with care to contact conditions. Numer. Heat Trans. Part A. Appl. 49, 345–367. James, P.S., Chandler, H.W., Evans, J.T., Wen, J., Browne, D.J., Newton, C.J., 1997. The effect of mechanical loading on the contact resistance of coated aluminium. Mater. Sci. Eng. A 230, 194–201. Khan, J.A., Xu, L., Chao, Y.-J., Broach, K., 2000. Numerical simulation of resistance spot welding process. Numer. Heat Trans. Part A 37 (5), 425–446. Li, V., Kimchi, M., 2000. Modeling of resistance spot welding high-strength steel. In: Proceedings of the International Body Engineering Conference’00, Body Assembly and Manufacturing, vol. 34, Detroit, Octobre 3–5, 2000. Llewellyn, D.T., Hillis, D.J., 1996. Dual phase steels. Ironmaking Steelmaking 23 (6), 471–478. Murakawa, H., Zhang, J., 1998. FEM simulation of spot welding process (report 1)—effect of an initial gap on nugget formation. Trans. JWRI 27 (1), 75–82. ¨ Produits plats en acier B53 3106, Norme PSA Peugeot Citroen, ´ a` froid, soudable extra-doux pour emboutissage lamines Septembre 2000. Robin, V., Sanchez, A., Dupuy, T., Soigneux, J., Bergheau, J.M., 2002. Numerical simulation of spot welding with special attention to contact conditions, in: H. Cerjak (Ed.), Mathematical Modelling of Weld Phenomena 6, Graz-Seggau, Austria, October 2001, The Institute of Materials, London. Salgon, J.-J., Quemener, O., Belghali, M., Bransier, J., 1998. ´ Resistance thermique de contact statique. Evaluation ´ ` a` deux resistances ´ experimentale d’un modele issu d’une ´ description probabiliste des deformations de l’interface. ´ erale ´ Revue gen de Thermique 37, 284–294. ´ Sibilia, G., 2003. Modelisation du soudage par point—influence ´ e. ´ These ` de Doctorat, des conditions interfaciales sur le proced Ecole Polytechnique de Nantes. Sibilia, G., Rogeon, P., Saindrenan, G., 2003. Experimental validation of an electrical-thermal-metallurgic predictive model in resistance spot welding. In: Proceedings of the Second International Conference on Thermal Process Modelling and Computer Simulation, Nancy, 31 Mars–2 Avril. ´ ´ ´ Thieblemont, E., 1992. Modelisation du soudage par resistance ` INPL. par points, These Thornton, P.H., Krause, A.R., Davies, R.G., 1996. Contact resistances in Spot Welding. Weld. Res. (Suppl.), 402s–412s. Vogler, M., Sheppard, S., 1993. Electrical contact resistance under high loads and elevated temperatures. Weld. J. 72 (6), 231–298. Yeung, K.S., Thornton, P.H., 1999. Transient thermal analysis of spot welding electrodes. Weld. Res. (Suppl.), 1s–6s.
journal homepage: www.elsevier.com/locate/jmatprotec
Characterization of electrical contact conditions in spot welding assemblies P. Rogeon a,∗ , P. Carre a , J. Costa a , G. Sibilia b,c , G. Saindrenan b ´ Laboratoire d’Etudes Thermiques Energ´etiques et Environnement, Universit´e de Bretagne Sud, Rue de Saint-Maud´e, BP 92116, 56321 Lorient Cedex 3, France b Laboratoire G´enie des Mat´eriaux, Ecole Polytechnique de l’Universit´e de Nantes, La Chantrerie, Rue Christian Pauc, BP 50609, 44306 Nantes Cedex 3, France c PSA Peugeot Citro¨en, Centre Technique de V´elizy, MXP/CEB/ASG, Route de Gisy, 78943 V´elizy Villacoublay Cedex, France a
a r t i c l e
i n f o
a b s t r a c t
Article history:
In this study the interfacial conditions encountered in electrodes and sheets assemblies
Received 11 September 2006
used in resistance spot welding process are characterized. Electrical contact resistances are
Received in revised form
measured on a specific device, allowing to rise high pressure and elevated temperature.
14 December 2006
Measurements concern electrode–sheet (E/S) and sheet–sheet (S/S) interfaces, considering
Accepted 19 April 2007
coated and non-coated steel sheets. The evolutions of the electrode profiles according to the number of weld are presented. The quick wear of the electrodes surfaces is also measured, depending on the type of sheets (coated or none coated). A numerical approach is chosen to
Keywords:
simulate the squeezing stage, to show the consequences of modifying the electrode profiles
Spot welding
on the initial contact surfaces at the beginning of the heating stage.
Electrical contact resistance
© 2007 Elsevier B.V. All rights reserved.
Interface Coated steel sheet
1.
Introduction
Spot welding resistance is a process largely used in the automotive industry because of its capacity to assemble thin sheets. From an industrial point of view, the introduction of new families of steel (in particular high-tensile steels) requires adjustments to the traditional techniques of welding. From a theoretical point of view, such a process is a complex problem, due to the combination of coupled phenomena. Previous studies agree with the complexity of the physical couplings (Sibilia, 2003; Sibilia et al., 2003; Feulvarch et al., 2006; Murakawa and Zhang, 1998; Chang and Zhou, 2001; Li and Kimchi, 2000), and focus on the essential role of the contact or interfacial
∗
conditions occuring between electrode and sheet (E/S) and sheet and sheet (S/S), respectively (James et al., 1997; Babu et al., 2001; Vogler and Sheppard, 1993; Thornton et al., 1996). The role of the contact conditions must be considered at two scales. During the heating stage the area of the apparent contact surfaces controls directly the macro-constriction of the current in the assembly; increasing the current density in the different resistances (contact resistances at the E/S and S/S interface and bulk resistances arising from bulk resistivity of the materials) increases the thermal energy in the assembly. Surface defects (roughness, oxides, etc.) force the current to flow through a few microscopically small areas. These microconstriction effects locally increase the electrical resistance
Corresponding author. E-mail addresses: [email protected] (P. Rogeon), [email protected] (P. Carre), [email protected], [email protected] (G. Sibilia), [email protected] (G. Saindrenan). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.04.127
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independently of the bulk resistance of the material. These phenomenon associated with the existence of the interface is the contact resistance. Note that the same analysis can be made for thermal conduction. However in this case, heat can also crosses the interstitial medium by conductive or radiative transfer. Although the contact resistance can be easily defined in abstracto, it is not a real physical object since: • it cannot be separated from the bulk of the piece, • its depth is not defined. For these reasons, measurement of contact resistances is difficult so much more its values are heavily disturbed by the measurement itself and because it is very sensitive to external parameters (temperature, pressure, pollution, etc.). Thus, to characterize the interfacial conditions, an experimental device has been primarily developed to measure electrical contact resistance according to temperature and pressure. The effect of electrode wear on the apparent contact surfaces sizes at the end of the squeezing stage was also studied through experimental and numerical approaches. The measurement of contact resistances, through in situ techniques during welding as considered by some authors (James ´ et al., 1997), (Thieblemont, 1992) does not permit the control of the most important parameters. Indeed probable evolutions of contact surfaces sizes and current density in the assembly could occur. Furthermore interface temperatures are also unknown. However, dynamic resistance evolutions measured in situ during the process, could indicate above all the time during which the contact resistances are important. As Babu (Babu et al., 2001), Vogler (Vogler and Sheppard, 1993) and Thorton (Thornton et al., 1996) do, the contact resistances are measured here on a specific ex situ device; in this way, contact surfaces are controlled, furthermore the thermal and pressure conditions encountered during the heating stage (fast heating rates, high temperature gradients and elevated pressure) are simplified. The profiles of new electrodes initially curved are quickly flattened while the number of weldings increases (Dupuy, 1998); that modify apparent contact surfaces, with important consequences on the formation of the weld nugget (Khan et al., 2000). For coated and non-coated steels, the change in the electrode profile according to the number of welds is measured. The sizes of the contact surfaces at the end of the squeezing stage are also calculated through a numerical simulation, when considering two types of electrode profiles (curved and flattened). These two combined effects explain the essential role of the contact conditions occuring at the different interfaces in the spot welding process. Futhermore this good knowledge of the contact resistances with temperature and pressure before each weld constitutes a prerequisite condition to any relevant analysis or modeling of the spot welding process.
under strong pressure. The measurement technique ex situ is consistent with those used by Babu (Babu et al., 2001), and Vogler (Vogler and Sheppard, 1993). The samples are placed in a power press (MTS Cynergie 1000 5 kN), between two stamps connected through electrical wires to a current supply power (Tektronix CPS250 or Hewlett Packard 6034A). The current is measured precisely with an amperemeter (Keithley 2000). Contrary to the above-mentioned authors, not one resistance was measured between two sheets but rather N resistances between N + 1 sheets. To accomplish this a sample stack technique is used whose aim is to amplify the measured signal. The two samples at the extremity of the stack are connected ` with copper wires to a nanovoltmeter (Voltmetre Keithley 2002) allowing us to measure with accuracy the potential difference at the boundaries. For studying the two types of contact (S/S) and (E/S), stacks are constituted with samples in steel and with samples in steel and copper, respectively. In the case of S/S contact, measuring the stack resistance with N + 1 samples results in the addition of an initial resistance R0 (because of the two instrumented samples in contact with the first sample), N contact resistances RCE and N bulk resistances RO steel . Each new sample introduces into the stack an additional bulk resistance and an additional contact resistance. Thus, the total resistance RN of the stack corresponding to the N measurement according to N, R0 , RCE and RO steel is: RN = p × N + R0
2. Characterization of electrical contact resistances 2.1.
Method
Measurement of electrical contact resistances (RCE) were achieved on a specific device (Fig. 1) according to temperature,
Fig. 1 – Schematic representation of the experimental device for electrical contact resistance measurements.
(1)
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where p = RCE(S/S) + RO
(2)
steel
Concerning the E/S contact, at each step a couple of samples is added. The total resistance of the stack corresponding to the N measurement can then be written according to Eq. (1): with p = 2 × RCE(E/S) + (RO
steel
+ RO
copper )
(3)
In both cases, because of a linear evolution of the resistance stack RN with N it is possible to calculate the slope p and so extract RCE.
2.2. Materials, samples preparation and experimental conditions 2.2.1.
Materials and properties
Three types of steel sheets are considered in this study (Fig. 2): two coated with zinc and one none coated.
2.2.1.1. XSG and XES steel sheet. The first steels considered in this study are non-alloyed steels, rolled at room temperature, ¨ for embossing process (Norme PSA Peugeot Citroen, 2000). The designation introduced by PSA Peugeot-Citroen for these steels is XSG or XES, respectively, according to the presence of zinc coating or not. They have low carbon contents (interstitial free (IF) type). Vickers hardness measurements lead to the following mean values—XSG: 98 HV0.1 and XES: 110 HV0.1 .
2.2.1.2. DP600 steel sheet. DP600 is a dual phase steel, rolled at room temperature (Llewellyn and Hillis, 1996). The composition of 80% ferrite and 20% martensite leads to a good resistance corresponding to new industrial requirements (HV0.5 = 202). The main mechanical properties at room temperature for XSG, XES and DP600 are given in Fig. 2. The DP600 and XSG steel sheets are galvanized (10 m thick).
2.2.1.3. Copper alloyed samples. The welding electrodes are made of copper alloyed with chromium (0.4–1%) zirconium (0.02 and 0.15%). To measure the E/S contact resistance, we have used specific samples in the same copper alloyed. At
Fig. 3 – Curves RCE(S/S) = f(P) at room temperature: (a) case of uncoated sheets XES and confrontation to the literature and (b) cases of coated sheets XSG and DP600.
room temperature the hardness value is around 170 HV0.1 , the yield stress value between 470 and 570 MPa and the Young modulus 120 GPa.
2.2.2.
2.2.3.
Fig. 2 – Mechanical properties of the different steels at room temperature and ε˙ = 1.67 × 10−4 s−1 .
Preparation technique of samples
Preparation of the samples is an important and critical phase of the resistance measurements. Indeed, even small defects remaining on the samples surfaces, above all oxides or barbs on the edges can change radically the contact resistances. The steel samples (dimensions 10 mm × 10 mm and 7 mm × 7 mm) are cut in thin strips (thickness 1.5 mm) by using a circular saw and sprayed simultaneously with lubricant. The samples are immediately cleaned with clear water and dried. All the edges are burnished to take off the barbs under water spray and finally dried another time. The copper samples are obtained following the same protocol, from a cylindrical bar, of the same alloy as the electrode. The copper samples surfaces are systematically polished to retrieve the same surface as on a new electrode. The shape of the samples assigned to be connected with copper wires present little change: the dimensions (10 mm × 12 mm × 1.5 mm and 7 mm × 10 mm × 1.5 mm) keep free a little strip allowing to fix the copper wires outside the contact zone.
Experimental conditions
Preliminary experiments at room temperature confirm that contact resistance varies mainly until pressure reach 20–30 MPa, as already shown by Babu (Babu et al., 2001) and Vogler (Vogler and Sheppard, 1993) (Fig. 3a and b). However
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when the pressure increases from 40 to 80 MPa, the contact resistance values are divided by a factor two! So we decided to work at two pressures level 40 and 80 MPa for the experiments whatever the temperature; that is relatively closed to the mean pressure value encountered at the contact surfaces in the welding assembly (around 100 MPa). In some case the maximum stress values at the level of the (E/S) and (S/S) interfaces could exceed 200 MPa at the end of the squeezing phase (cf. Section 3.3, Figs. 12 and 13) depending on the apparent contact surface sizes! Two heating collars (power 600 W each) are fixed on the stamps in stainless steel (Fig. 1). The heat flux crosses and heats the stamps and the stack of samples by conduction. Four thermocouples (type K, diameter 0.1 mm) are introduced in each stamp. The two thermocouples at the extremity of each stamp allowed us to calculate the mean temperature of the samples stack. Under welding conditions the interfaces are subjected to extreme thermal rates (several thousands of degrees Celsius per second) and high temperature gradients (several hundreds of degrees Celsius at the E/S interface only, for symetrical assembly). The thermal conditions are different and simplified in the ex situ device. The temperature at the level of the heating collar is programmed as a step until 550 ◦ C and remained constant: the stack is heated by conduction and the rise until 500 ◦ C takes less than 15 min. During the rise of temperature the voltage at the extremities of the stack and the temperatures in the stamps are registered. The scanning frequency, for these two measurements, was 0.1 Hz.
2.3.
Experimental results
2.3.1. Electrical contact resistance at S/S interface 2.3.1.1. Non-coated steel—contact XES/XES. From the evolutions of stack resistances with temperature, the evolution of RCE(S/S) (Fig. 4) is deduced by calculation from equation (3) for the various temperatures. In particular a fast collapse of RCE with the temperature can be noticed: since at 250 ◦ C, the value already fell of by a factor of 4. This strong reduction is confirmed by measurements of Vogler and Sheppard for temperatures lower than 200 ◦ C (Vogler and Sheppard, 1993), but ´ has not been found by Thieblemont (Thieblemont, 1992). For the contacts between non-coated sheets, at room temperature and even under strong pressure, it is established that the
Fig. 4 – Curves RCE(S/S) = f(T)—case of uncoated sheets XES and confrontation to the literature.
Fig. 5 – Curves RCE(S/S) = f(T)—cases of coated sheets XSG and DP600 and confrontation with the literature.
contact ratio does not exceed a few percentage of apparent surface (Salgon et al., 1998). When the temperature increases, the mechanical properties of the material (yield stress, Young modulus, etc.) drop quickly while the electrical resistance increases. These antagonistic effects act in a contradictory way on RCE. For the contacts between non-coated sheets, the main factor seems to be, from our results, an increase in the contact rate ratio which leads to a radical reduction in RCE with temperature.
2.3.1.2. Coated steel—contact XSG/XSG et DP600/DP600. A first series of measurements carried out with a current of 200 mA, reveals that the voltage at the extremity of the stack are disturbed by thermoelectric effects (Seebeck effect in particular). In fact, stacks with coated sheets form a succession of alternated zinc and steel layers which, in the presence of an even weak temperature gradient, generate a few micro Volts of the same order of magnitude as the potential differences measured at the boundaries of the stack, above all when the temperature is low. In order to prevent the measurements of this thermoelectric noise, two modifications to the protocol were brought: first by increasing the current from 200 mA to 1.5 A, in order to increase the signal-to-noise ratio; secondly by programming the current in as a square signal (1.5 A during 10 s), one is able to observe the thermoelectric noise during the period (10 s) when the current is zero, allowing a correction by removing this parasitic value when the current flows. The RCE increases at first with temperature, then it reaches a maximum, around 350 ◦ C, just before the melting point of the coating (Fig. 5), and finally falls with the approach of 420 ◦ C. This behavior very specific to the contact between coated sheets is not found in the literature. It can nevertheless be explained by the great malleability of zinc (low elastic limit and Young modulus values). These characteristics of the coating must make it possible to reach, under 40 MPa and already at ambient temperature, a very high contact ratio. While the temperature rises, the major effect is the increase of the zinc resistivity, rather than the contact surface ratio, resulting in an increase in RCE with the temperature. Let us specify that our measurements show that the contact resistance is multiplied by 2.75 between 30 and 350 ◦ C, an interval in which the
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temperature until 200 ◦ C and more slowly after. In the case of the dissymmetric contact E/S with copper and coated sheets, the governing phenomena is the increase of the contact ratio with temperature, while it’s the electrical conductivity of the coating in the case of RCE(S/S) (Section 2.3.1, Fig. 5) until the melting of the zinc layers. In the case of E/S contact, like in the case of S/S contact (Fig. 5), the evolutions of RCE with temperature, for the two pressure levels, are similar whatever the steel DP600 or XSG (Fig. 7a). Furthermore the RCE(E/S) evolution found at 80 MPa appears relatively close to that given by ´ Thieblemont (Thieblemont, 1992), in spite of the differences in the shape of the evolutions (Fig. 7b). Fig. 6 – Curves RCE(E/S) = f(T)—case of uncoated sheets XES and confrontation with the literature.
electrical conductivity of zinc is divided by 2.6 (Llewellyn and Hillis, 1996). Then, when the temperature reaches the melting point of coating, its mechanical properties break down which cause a very important crushing of the asperities and a rise in the contact ratio. This phenomenon leads to a rapid reduction of the electrical contact resistance until 420 ◦ C. The comparison between electrical contact resistance values obtained at 80 and 40 MPa leads to the following remarks: the same type of evolution can be observed in the two cases and the values are much lower at 80 MPa except when the temperature reach 420 ◦ C, where they become very low corresponding to a good contact in the two case. The general shape of the curves RCE(S/S) consolidates the particular role which we allot to zinc in the behavior interfaces: whatever the material substrate, only the properties of the coating determines the evolution of the contact resistance with temperature. This behavior results from an antagonistic contribution of the electric properties with the mechanical properties as predicted by the model of Babu but for uncoated sheets (Babu et al., 2001)!
2.4.
Accuracy and sources of errors
Different sources of errors can affect the values of the contact resistance RCE, and they can be classified in two groups: deterministic and random type, respectively. Oxides, barbs, and dirt which could appear on surfaces during the samples preparation provide random errors; furthermore with industrial steel sheets, the roughness is certainly non-uniform on all the surface samples. This could lead to scattered measurements witch remain although representative of the reality. Inaccuracies about, the samples dimensions when cutting, there position in the stack, the current and the voltage measurements, and the resistivity values of steel, lead to deterministic errors. In the case of the uncoated sheet XES the scattering is relatively important (around 0.003 mm2 ) at low temperature
2.3.2. Electrical resistance at E/S contact 2.3.2.1. Non-coated steel—contact copper/XES. Fig. 6 presents the variation of RCE(E/S) with temperature compared with the results in the literature. As for the S/S contact types (Fig. 4), it could be observed a continuous decrease, initially fast till 250 ◦ C, then less marked. As in the case of RCE(S/S), our results are closed to those of Vogler (Vogler and Sheppard, 1993) rather ´ than from those of Thieblemont (Thieblemont, 1992). In this case, like for RCE(S/S) (Section 2.3.1), the increase of the contact ratio is the major effect.
2.3.2.2. Coated steel—contact copper/XSG and copper/DP600. Quite important differences between the evolutions of RCE(E/S) with temperature can be observed at the two pressure levels (Fig. 7a). At 80 MPa initial values of RCE at room temperature are already very low and correspond to high contact ratio: consequently values of RCE vary slowly with temperature at this pressure. At 40 Mpa initial values at room temperature are relatively higher and correspond to lower contact ratio. At this pressure the values of RCE(E/S) decrease first quickly with
Fig. 7 – Curves RCE(E/S) = f(T) cases of coated sheets: (a) comparison between XSG and DP600 and (b) XSG and confrontation with the literature.
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Fig. 8 – Curves RCE(S/S) = f(T) with scattering effect—case of uncoated sheets XES.
the nugget, such as annular or crescent shapes. However, with the repetition of the weldings, various phenomena contribute to the degradation of their ends (pollution and erosion of the profile, (Dupuy, 1998; Dong et al., 1998; Dong and Lu, 1999)). To mitigate the effects of this continuous evolution, the reference frame of spot welding at PSA recommends an incrementing of the welding current by steps, and the grinding of the electrodes after a certain number of welded points. One presents in this paragraph the experimental characterization of the modification of the profile of the electrodes during their use. Dong (Dong and Lu, 1999) proposed an analytic model to calculate the contact area size at the E/S interface taking into account the wear of the electrode face; however, although two types of electrodes are considered (truncated and radiused), a flat profile is supposed in each case. Inversely Kan (Dupuy, 1998) have shown that electrode profiles initially plate are quickly blunted with a high curvature radius (R 125 mm), essential condition to achieve satisfying weld nugget. Yeung (Yeung and Thornton, 1999) has proposed a numerical parametric model using conjugate heat analysis to predict the temperature rise at the E/S interface, which contributes to the electrode surface damage. A high maximum temperature was found at the electrode face, and no significant E/S temperature changes were found when changing the cooling cap parameters.
3.1. Fig. 9 – Curves RCE(S/S) = f(T) with scattering effect—case of uncoated sheets XES.
and decreases significantly as soon as temperature reaches 200 ◦ C (Figs. 8 and 9): a similar behaviour has been observed for coated sheets. The scattering effect becomes negligible after 300 ◦ C. This excellent repeatability indicates not only the quality and the accuracy of our measurements but also that all the errors are certainly negligible as regard the very low values of RCE at high temperature. This also confirm that measurement scattering at low temperature is certainly due to industrial steel sheet surfaces presenting roughness singularities witch disappear when the temperature increases and becomes sufficient to allow their crushing.
3.
Profilometric study
The variation of contact area sizes during the heating stage influences directly the formation of the welded point. The literature shows moreover the dominating role played by the profile and the state of the electrode contact surface in this ´ evolution (Thornton et al., 1996; Thieblemont, 1992; Dupuy, 1998; Dong et al., 1998; Dong and Lu, 1999). The two types of electrodes used within the framework of this study are machined ends whose contact face is a segment of a sphere (diameter Ø6 mm and initial curvature radius R 40 mm). This specific geometry makes it possible, in particular, to avoid alignment problems of the electrodes during the squeezing phase, and to force the current to the center of the assembly; as a consequences, this helps prevent against defects of
Experimental device and trial run
The various factors which can influence the modification of the electrodes contact surface are in our case, the materials, the type of electrode Ø6 and the welding parameters (force and current). The welding parameters as well as the current increments are defined according to a standard (E34.03.180, 2001). The principle of this series of measurements consists in the realization of a given number N of welded points, and in the analysis of the electrodes set used for these N weldings. The frequency of measurements is defined according to N = 1, 3, 5, 10, 25, 50, 75, 100, 150 and 300. The technical solution adopted to measure the evolution of the electrodes surface profiles is based on the use of a mechanical feeler, interdependent of an inductive sensor: the tests were carried out at the Laboratory Metrology of PSA-Sochaux (Fig. 10).
3.2.
Experimental results
Concerning the welding of non-coated sheets, one observes an evolution of the initial profile, radiused, towards a partial flattening of the electrode contact surface. After 50 welds at most (N = 50), the active face becomes partially flat. The profile characteristic of a worn electrode after 300 welds (N = 300) is compared to the initial curved profile (Fig. 11). One also reports on Fig. 11 the profile corresponding to an electrode having welded coated sheets (N = 300): one notes in this last case the essential role of the coating on the consumption of the electrode, leading to a total flattening of the profile. One can notice that the worn profile is not completely flat but presents, in the modified part, a curved profile with a high radius of curvature very different from the initial one. Consequently the wear of the initial profile of the electrode contact surface did not prevent successful weld nuggets.
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Fig. 10 – Experimental device for the electrode profile measurement.
3.3. Initial contact areas sizes at the beginning of the heating stage To study the incidence of the electrode contact surface profile on the contact areas at the two interfaces (E/S) and (S/S), the squeezing phase was numerically simulated by considering profiles of new and worn electrodes. At the end of the squeezing stage, the contact area is at the initial value for the heating stage, although it can change during this period. It will determine, through the macroconstriction effect, the initial current density in the sheets assembly. The finite element code Sysweld (ESI group France) was used to calculate the mechanical deformations and stresses during the squeezing stage. Only the case of non-coated steel sheets is presented here. The geometry was supposed axisymetric, and elastoplastic deformations and stresses are calculated in a radial plane in order to simplify the analysis. The effort applied corresponds to a standard at PSA and the mechanical characteristics of materials at room temperature (Fig. 2) are deduced from (Sibilia, 2003). The contact radii values are given by the evolutions of the normal stresses along each interface. The
Fig. 11 – Comparison between different electrode profiles: new and worn.
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results obtained with a new electrode profile (curved profile R 40 mm), are compared to those corresponded to a worn electrode; the eroded profile is obtained after N = 150 weldings on non-coated steel sheets, which corresponds to the fifth of the usual maximum number of welds for the electrodes. Fig. 12 shows the normal stress evolutions along interface E/S for the new and worn electrodes. It can be noticed that the erosion of the profile involves a notable increase of the contact radius from 1.5 to 2.5 mm. In the same way, Fig. 13 shows the normal stresses along the S/S interface for the new and worn electrodes. The wear of the electrode involves an appreciable increase of the contact radius from 1.8 to 2.7 mm. Because of the quick wear of the electrodes profile, incremental modification of the welding current must be done to ensure the achievement of welded points. During the welding stage, contact radii will continue to change due to complex mechanisms (Robin et al., 2002; Feng et al., 1998). Concerning the welding of coated sheets, the wear of the electrodes profiles is more important, and leads quickly to the flattening of the initial profile (Fig. 11). For this reason, the consequences on the initial contact areas are the same as previously observed for non-coated sheets. Moreover in this case, the initial contact radii at the beginning of the welding phase are closer to the radius of the electrode surface (3 mm). During the heating stage, the fusion of the coating layers during the first welding cycle leads to the very fast formation of zinc rings at each interface.
Fig. 12 – Theoretical distribution of stress at the E/S interface—case of uncoated sheets XES.
Fig. 13 – Theoretical distribution of stress at the S/S interface—case of uncoated sheets XES.
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4.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 117–124
Conclusion
The aim of this work was to determine the contact conditions at the interfaces electrode–sheet and sheet–sheet which constitute essential variables in the control of spot welding process. The formation of the weld nugget is indeed strongly dependent on the phenomena at the interfaces. The contact areas influence directly the macro-constriction of the welding current in the assembly: they vary throughout complex way, along the different phases of the process, are affected by the configuration of the assembly and by the profile of electrodes contact surface. The surface defects lead to micro-constriction effects taken into account by the electrical and the thermal contact resistances definition. Primarily electrical contact resistances measurements depending to the temperature and under pressure have been carried out on a specific device. A complete data file of electrical contact resistances, according to temperature, for non-coated and coated sheets, and for E/S and S/S interfaces was determined. In the case of non-coated sheets, the contact resistance decreases with the temperature, the increase of the contact ratio is the major effect. Concerning coated sheets the contact resistances values are lower, and at the S/S interface the contact resistance first increases until 350 ◦ C and than decreases till 450 ◦ C. This behaviour very specific to the contact between coated sheets has not been found yet in the literature. In this case the decrease of the coating electrical conductivity drive first the evolution of RCE till the melting point of coating where its mechanical properties break down which cause a very important crushing of the asperities and a rise in the contact ratio. Secondly the experimental analysis showed that with the repetition of welding, the electrode surface profile, initially curved, quickly presented a ‘flat part’ whose size increases with the number of welds. Moreover the flatness of the profile is much more important with coated sheets. A finite element analysis of the macroscopic deformations during the squeezing phase was made, by considering in the model the new and worn electrode profiles. Axi-symetrical deformation was assumed in order to simplify the analysis. The increase of the contact radii, due to electrode wear, leads to the incremental modification of the welding current during the process to ensure the achievement of welded points. With coated sheets the contact radii at the beginning of the welding stage is closer to the electrode radius, especially for worn electrodes. Consequently, with lower contact resistance values and larger contact areas, coated sheets involve higher welding current.
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