Correlation Between Electrodes Surface State and Dynamic Resistance During Resistance Spot Welding of 5182 Aluminum Alloy

doi:10.21203/rs.3.rs-4881642/v1

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Correlation Between Electrodes Surface State and Dynamic Resistance During Resistance Spot Welding of 5182 Aluminum Alloy

https://doi.org/10.21203/rs.3.rs-4881642/v1

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This study determines the relationship between the state of the electrodes surface and dynamic resistance during the resistance spot welding of aluminum alloy 5182. For this purpose, dynamic resistance values for each welded spot were analyzed and a correlation between these values and the changing electrode surface state throughout the welding process was examined. The resistance is influenced by the roughness of the electrode surface, which changes during the welding process due to the gradual accumulation of Cu/Al phases. Based on the findings of this study, resistance measurements can be effectively utilized to correlate with electrode state.

Resistance spot welding

Aluminum alloy

Dynamic resistance

Electrode surface roughness

Resistance spot welding (RSW) is a widely used and cost-effective technique for joining steel components in various industries. However, the application of RSW for aluminum alloys is significantly limited due to rapid electrode wear. During the RSW process, aluminum particles from the plate surfaces accumulate on the electrodes, forming brittle Cu/Al intermetallic compounds (IMCs). These IMCs subsequently break down, resulting in erosion and pitting of the electrode [1–3]. The electrode wear mechanism during RSW of the 5182-aluminum alloy has been extensively investigated and described by numerous researchers [4–6]. Ultimately, the electrode wear leads to decrease in welding quality. In addition to the use in the automotive industry, aluminum alloys are used for the manufacture of components in power engineering, such as solar thermal collectors, due to their outstanding properties such as high thermal conductivity, good corrosion resistance and excellent formability and weldability [7]. Resistance spot welding can also be used in the energy sector to weld aluminum panels as a low-cost alternative to laser welding and brazing.

To ensure high-quality joints during RSW of aluminum alloys, it is essential to perform redressing operations after a specific number of weld spots to remove brittle intermetallic phases from the electrode surface [1]. The frequency of redressing is crucial for the process. Shorter redressing intervals can lead to reduced process efficiency, as the electrodes need to be moved to electrode milling station, processed, and returned to the welding area, during which time no welding can be done. On the other hand, longer intervals between redressing operations may decrease downtime, but can compromise the weld spot surface quality. Moreover, to return the electrode surface to its original condition after significant erosion, more material must be removed, which shortens its lifetime [8, 9].

However, to the best of the author's knowledge, there are no studies that directly investigate the correlation between electrode wear and internal signals during RSW of aluminum alloys. Existing research in this area has primarily focused on correlating electrode displacement with weld spot size [10] and detecting weld spatter by analyzing dynamic resistance curves [11, 12]. Both factors – decreased weld spot size and spattering – tend to occur when the electrode is already significantly eroded, making them ineffective indicators for pinpointing the appropriate time for redressing.

In the authors' previous work, a correlation between the occurrence of pores in the welding lens and changes in dynamic resistance was discovered [13]. The objective of the present study is to establish the correlation between electrical resistance and electrode surface state during RSW of aluminum alloys.

The aluminum alloy AW 5182 sheets with Ti-Zr coating were used for the investigations. The chemical composition of the alloy can be found in Table 1. The metal sheets were cleaned of dirt and dust before welding, and no additional coatings were applied.

The Düring Alu X-100 RSW machine with a 1000 Hz medium-frequency inverter DC transformer controlled by the Genius MFI inverter control system from Harms&Wende was used for welding. The dynamic resistance was calculated using current and voltage values. The current was measured with a Rogowski coil, and the voltage was measured with clamps attached directly to the electrodes. A SPATZ Multi04 measuring device from Matuschek was used for data acquisition and processing. The measurement data were recorded for each welding operation at a frequency of 20,000 Hz.

The plates were welded using CuCr1Zr balling electrodes of type A0-16-R40 (ISO 5821). The electrodes were cooled with water, with a total flow rate of 9 l/min.

The optimal welding parameters were chosen so that at the beginning of the process, a high-quality weld spots without welding defects (porosity and cracks) and with the required spot diameter is ensured. The welding parameters were determined in accordance with DIN 14327 [14]. The current was set to 27 kA, the welding time to 100 ms, and the electrode force to 5 kN. To verify the effectiveness of these parameters, shear strength and weld spot size examinations were conducted.

Table 1

Chemical composition of 5182 alloy in wt.% [15]
Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti	Al
0.20	0.35	0.15	0.2–0.5	4–5	0.10	0.25	0.10	Balance

The geometry of the aluminum strips and the arrangement of the weld spots are shown in Fig. 1. The electrodes were cleaned by milling before each welding series.

To evaluate electrode degradation, photographs of the electrode surface (anode) were taken for every 10th weld spot, area roughness and topography of the contact electrode surface (anode) were measured for every 20th weld spot using Keyence VHX-7000 light microscope (ISO 8503). In addition, EDX measurements were carried out for every 20th weld spot using a Thermo Fischer Phenom XL G2 SEM to accurately determine the electrode contact area contaminated with Al and Mg. To reduce testing time and cost, only the anode surface was investigated due to its stronger alloying, which is a result of the Peltier effect [4, 16–18]. Additional experiments were also conducted to determine the time at which the spot formation begins. To assess the influence of electrode wear on spot quality, the Chisel test was conducted on every 5th spot, analyzing fracture type and spot size. Spot diameter was measured along two perpendicular axes and averaged.

Figure 2 shows the electrode (anode) surface and its topography depending on the weld spot number. After 20 weld spots, a silvery trace (patina) in the form of a black spot forms on the surface (due to reflection under a microscope, the silver color of the patina looks completely black). The surface topography shows that the erosion of the electrode is just beginning and occurs at the edge of the electrode - sheet contact zone, where the color of the electrode is gray. After 40 weld spots, ring-shaped depressions at the edge of the electrode-sheet contact zone are observed to become larger and deeper. Thus, it can be assumed that the gray zones are places where brittle intermetallic compounds Cu/Al form, which subsequently fall out, causing depressions to form on the surface of the electrode. Our observations are consistent with Hicken's previous description of this wear behavior [19]. After the 60th weld spot, the entire electrode - sheet contact zone is covered with aluminum compounds, and the peak of the electrode in the center begins to disappear and by the 100th point it completely disappears.

To increase the measurement accuracy and understand when and how electrode erosion occurs, the 2D average electrode profile was also determined (see Fig. 3). After the 20th weld spot, an adhering layer of aluminum is visible, which after the 40th weld spot begins to erode. After the 60th point it is clear, that the electrode profile has become smaller than the original one. In addition, the shape of the electrode changes greatly and an uneven crater begins to form. It is also worth noting that there is not much difference between the electrode profile after the 80th and 100th weld spot. At this point, a rough crater in the center of the contact zone has already formed.

To accurately determine the composition of the electrode surface contaminants, EDX measurements were taken of the contact area of the electrode with a diameter of 5 mm, as can be seen in Fig.. EDX mapping of the electrode after 40 weld spots makes it clear that the black areas on the surface of the electrode contact zone are mainly aluminum coated with a thin oxide film. While there are almost no oxides at the edges of the contact zone, both aluminum and copper are present. The center of the zone is mostly copper. Fig. summarizes the surface areas covered by the Al and Mg. The anode area contaminated with aluminum increases with the number of weld spots, while the amount of magnesium after the 20th spot is stable and does not exceed 4%. After the 60th weld spot, the rate of contamination of the electrode surface with aluminum decreases, which may be due to the erosion of brittle Cu/Al particles.

To better understand the difference between the gray and black zones on the contact surface of the electrode, EDX point measurements of the chemical composition of the anode surface were carried out after 20 weld points (see Fig. 6). As a result of the measurements, the assumption was confirmed that the gray zone is the region of formation of brittle and poorly conductive Cu/Al intermetallic compounds, while the white zone consists mainly of aluminum. By comparing the ratio of elements with known phase diagrams and literature [19], it was possible to determine the composition of Cu/Al particles.

It is noteworthy that contamination and degradation of the electrode surface do not appear to have any significant impact on weld spot size. Figure 6 presents the weld spot size measured using the Chisel test as a function of spot number, showing that the size remains constant and above the minimum requirement \(\:{dp}_{min}=5\sqrt{t}\) [20], where t is the plate thickness.

To quantitatively characterize the state of the anode surface after welding and compare it with the change in dynamic resistance, studies were carried out on the roughness of the contact spot. Figure 8 demonstrates the results of contact area roughness (S_z) measured using a light microscope.

The condition of the electrode surface has been established as a significant factor influencing electrical resistance, predominantly at the initial stages of the welding cycle, prior to the occurrence of lens formation [22]. Given this, it can be inferred that resistance measurements taken in our case within the initial 10 ms of the welding cycle [13] may provide a useful metric for assessing the state of the electrode surface. Figure 9 shows the average electrical resistance values during the first 10 ms, R_{0 − 10ms}, as a function of the spot number. It is noteworthy that the values of S_z (maximum height of the surface) are positively correlated with the average values of electrical resistance for the period 0–10 ms. For comparison, these values can be conventionally divided into three zones: A, B and C.

Initially, the R_{0 − 10ms} values indicate an upward trend until about the 15th point (zone A). Up to this point, Cu/Al plaque is formed on the electrode surface (see Fig. 2). The Cu/Al intermetallic compounds have lower electrical conduction [19] and therefore their presence on the electrode surface can lead to an increase in overall resistance. The roughness S_z begins to increase as the first Cu/Al particles remaining on the electrode surface are distributed unevenly, this can be seen in the EDX results (see Fig. 6).

Following the initial increase, the electrical resistance starts to slowly decrease (zone B). The explanation for this is that the number of intermetallic compounds with poor electrical resistance decreases due to erosion of the electrode contact surface, which reduces the dynamic resistance [4, 19]. It is worth noting a strong decrease in roughness S_z after the 20th point, which does not correlate with a slow decrease in dynamic resistance. The reason for this may be the fact that the dynamic resistance was measured for the entire system, including the cathode, and the roughness S_z was measured only on the surface of the anode. The strong decrease in the roughness S_z is associated with the leveling of the contact area relief: from about the 20th point, hard and brittle Cu/Al intermetallic compounds begin to fall out, and the soft surface of the electrode is smoothed out. Importantly, the point (30th spot) at which the resistance begins to drop significantly is correlated with the onset of porosity formation and surface erosion of the weld joint [13]. It can be attributed to the distorted current flow caused by the uneven distribution of Cu/Al particles [4]. Despite the presence of pores, the Chisel (see Fig. 6) and tensile strength tests [13] indicated that the weld quality is acceptable, which is consistent with previous research showing that pores up to 40% do not significantly impact weld spot quality for aluminum alloys [21]. Figure 3 shows that between 20th and 40th spots, erosion of the resulting aluminum layer occurs, while between 40th to 60th spots, erosion of the copper electrode begins, a crater is formed and the shape of the electrode changes significantly.

After the 60th spot, a flat line can be observed with a slight slope (zone C). At this point, the anode contact area is completely contaminated with aluminum oxide, and the shape of the anode contact area changes from a flat top to a crater. Consequently, there are no significant changes in roughness and resistance.

This study demonstrates that monitoring changes in resistance values can effectively track not only the occurrence of pores in the welding lens, as was presented in the authors’ previous work [13], but also the surface state of electrodes during the resistance spot welding of aluminum alloys. The data show a strong positive correlation between dynamic resistance for the period 0–10 ms welding time and contact area roughness S_z. The slight discrepancy in the correlation can be explained by the fact that the study paid special attention to the state of the anode surface, while the dynamic resistance was measured for the entire system, including the cathode, which is also coated with intermetallic compounds, but much later. The increase in dynamic resistance is associated with the accumulation of oxides and intermetallic compounds on the surface of the electrodes. The decreasing trend of the resistance measurements correlates with the onset of electrode erosion. The resulting crater, covered with aluminum oxides and carbides, prevents the formation of new intermetallic compounds, and therefore the dynamic resistance remains low. In addition, after the dynamic resistance decreases below the values for the first weld spots (without contamination of the electrode), erosion of the electrode surface and crater formation begin. It is worth noting that neither electrode erosion nor the formation of pores inside the lens associated with it reduces the size of the weld spot and the tensile strength of the welded samples. However, their effect on pull-off strength requires further study.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding

The project on which this publication is based was funded by the Federal Ministry of Education and Research under the funding code 02P21Z000. The responsibility for the content of this publication lies with the authors.

Data availability

No data was used for the research described in the article.

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Correlation Between Electrodes Surface State and Dynamic Resistance During Resistance Spot Welding of 5182 Aluminum Alloy

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1. Introduction

2. Experimental procedures

3. Results

4. Discussion

5. Conclusions

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