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Surface & Coatings Technology 317 (2017) 83–94
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Comparative failure analysis of electrodes coated with TiB2-ZrB2 and TiB2-ZrB2/Ni layers Ping Luo, Can Xiong, Chong Wang, Fucheng Qin, Yao Xiao, Zhi Li, Kang Liu, Xu Guan, Shijie Dong ⁎ a b
Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, China School of Materials and Chemical Engineering, Hubei University of Technology, Wuhan 430068, China
a r t i c l e
i n f o
Article history: Received 28 December 2016 Revised 19 March 2017 Accepted in revised form 20 March 2017 Available online 21 March 2017 Keywords: Titanium diboride-zirconium diboride Failure analysis Copper alloy electrode Coating
a b s t r a c t Electrodes with a monolithic TiB2-ZrB2 coating as well as those with a multilayered TiB2-ZrB2/Ni coating were used to resistance spot welding (RSW) Zn-coated steel, in order to investigate the failure behavior of the coatings during welding. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction analysis (XRD), and microhardness measurements were performed to characterize the microstructures of the coatings, the reactions of the electrodes with the Zn coating of the sheets, and the formation of the alloy layer. It was found that the protective coatings significantly improved the surface hardness of the RSW electrodes by inhibiting plastic deformation at their tips, thus slowing the erosion process. In addition, the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes exhibited different failure mechanisms. A combination of the alloying effect and plastic deformation caused the final failure of the ZrB2-TiB2-coated electrode, while the ZrB2-TiB2/Ni coated electrode failed primarily owing to plastic deformation. © 2017 Published by Elsevier B.V.
1. Introduction Zn-coated (galvanized) steel is a superior material for manufacturing automobile bodies (especially those of sedans) because of its good corrosion resistance. During the resistance spot welding (RSW) of galvanized steel sheets, coating materials can readily adhere to the surface of the RSW electrode owing to the low melting point of the galvanized Zn layer (692 K). As a result, Zn atoms diffuse into the bulk of the Cu electrode, leading to a decrease in its electrical and thermal conductivities [1]. Consequently, the deformation of the electrode tip caused by rapid overheating during the continuous RSW process gradually decreases the strength of the welding nugget, which, in turn, lowers the quality of the weld and ultimately leads to the complete failure of the RSW electrode [2,3]. This phenomenon of premature electrode failure not only reduces the efficiency of the welding process, given the resultant increase in the frequency with which the RSW electrode has to be repaired or replaced, but also significantly increases the production cost. Therefore, extending the lifetime of RSW electrodes is an important issue in the automobile industry and needs to be resolved urgently. Previous studies performed with the aim of resolving this issue have produced significant results. One possible solution to the above-described problem is to coat a metal-ceramic layer on the surface of the RSW electrode via electrospark deposition (ESD). For example, Luo ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (S. Dong).
http://dx.doi.org/10.1016/j.surfcoat.2017.03.045 0257-8972/© 2017 Published by Elsevier B.V.
synthesized a TiB2 coating on the surface of an RSW electrode via ESD and analyzed the mass loss of the Cu alloy electrode during the formation of the TiB2 coating by ESD [4]. To improve the poor wettability between the coating and the electrode, an intermediate Ni layer was used [5]. A similar method was used to prepare Ni/(TiCP/Ni)/Ni composite coatings [6,7]. However, it should be noted that all metal ceramics are single-phase materials. It has been shown that the properties of multiphase metal ceramic composites excelled constituent element. Thus, multiphase metal ceramic coatings have been synthesized via ESD. For example, Luo et al. synthesized TiB2-TiC [8] and Al2O3–TiB2/Ni [9] composite-phase coatings via ESD and studied the effects of the coating parameters on coating quality. ZrB2 exhibits higher electrical and thermal conductivities than TiC and Al2O3 [10]. Further, Mroz [11] demonstrated that (Ti, Zr)B2 solid solutions exhibit better mechanical properties than the end-member compositions. This suggests that ZrB2-TiB2 composites may be suitable materials for coating RSW electrodes. Luo et al. [12] successfully synthesized a ZrB2-TiB2 composite coating on Cu-Cr-Zr alloy electrodes via ESD. However, these studies were mainly focused on coating preparation, as well as ways of improving the coating quality and mass transfer during the ESD process, and there have been few studies on the failure mechanism of the coated electrodes. Thus, there is a lack of information regarding the failure mechanisms of coated electrodes during the RSW of galvanized steel sheets. With this goal in mind, in this work, using various characterization techniques, we analyzed the failure mechanisms of RSW electrodes
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Fig. 1. Schematic of equipment used for electrospark deposition (ESD) [8].
Table 1 Welding parameters for testing of electrodes. Electrode
Welding current (A)
Electrode force (kN)
Welding time (cycles)
Hold time (cycles)
Welding speed (welds/min)
Water flow (L/min)
Uncoated ZrB2-TiB2 coating ZrB2-TiB2/Ni coating
9300 8600 8400
2 2 2
7 7 7
10 10 10
30 30 30
2 2 2
Fig. 2. Change in weld nugget size as function of weld current for constant electrode force and different weld times (t1 b t2 b t3 b t4): (a) uncoated electrode, (b) ZrB2-TiB2-coated electrode, and (c) ZrB2-TiB2/Ni-coated electrode.
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Fig. 3. Schematic diagram of measurement setup and coupon with transferred imprints of electrodes.
coated with ZrB2-TiB2 and ZrB2-TiB2/Ni layers during the welding of Zncoated steel sheets. 2. Experimental The RSW electrodes were coated with the ZrB2-TiB2 and ZrB2-TiB2/ Ni layers using a laboratory-made ESD apparatus, which is shown in Fig. 1. The electrodes used in this study were precipitation-strengthened, cold-formed domed flat electrodes consisting of a Cu-Cr-Zr alloy (Cu-1Cr-0.05Zr (mass%)). They were 16 mm in diameter and 23 mm in length and had a flat top, with the diameter of the top surface being 5 mm. The ZrB2-TiB2 coating was formed by directly using a specially sintered ZrB2-TiB2 rod as the anode during the ESD process. These ZrB2-TiB2 composite rods (6 mm in diameter and 30 mm in length) were fabricated by vacuum sintering at 1400 °C for 1 h (ZrB2/TiB2 molar ratio of approximately 1:1). The multilayered ZrB2-TiB2/Ni films were coated by first forming a layer of Ni on the RSW electrode surface by ESD; a commercially available rod of pure nickel was used for the
Fig. 4. Dependence of nugget diameter on number of welds.
purpose. This was followed by the deposition of a ZrB2-TiB2 composite layer onto the Ni-coated electrode via ESD. The ESD process was performed at a voltage of 12 V using a capacitance of 2000 μF; the coating duration was 2 min and the thickness of the coatings was approximately 10–20 μm. The lifetimes of the coated RSW electrodes were measured using a YR-350SA2HGE single-phase AC resistance spot welder. Sheets of DX56D + Z double-sided hot-galvanized Zn-coated steel (thicknesses of 0.7 mm) manufactured by the Shanghai Baosteel Group Corporation were used for the lifetime measurements. The spots were produced sequentially, and the dimensions of the test specimens and the spot distributions were in keeping with the ISO-14270 standard [13]. The same conditions were used for all three electrodes. Welding tests were performed in order to investigate and compare the degradation processes of the electrodes with and without a coating. The same initial weld nugget size was used in all the cases. The parameters used for the tests are listed in Table 1. The weld current was determined by constant-electrode-force weldability lobe measurements performed on the three types of electrodes; these measurements were performed as per the ISO-14327 standard [14]. Fig. 2 shows the increase in the weld nugget size as a function of the weld current for a constant electrode force (2 kN) and different weld times (t1 b t2 b t3 b t4). In this study, the current was used at weld time = 7 cycles, and the initial nugget diameter was 6 ∗ sqrt(t), where t is the thickness of the steel plates (= 0.7 mm). The growth rate of the electrode tip during the welding process was monitored and used as a measure of electrode performance. The rate was determined by the carbon imprint technique, in which an imprint coupon was made by fixing strips of carbon typing paper to a steel endurance coupon covered with plain white paper on both surfaces (top and bottom). A weld force was then applied on the imprint coupon using the electrodes in the absence of a weld current, in order to transfer the carbon ink to the paper. The imprints were measured with a digital Vernier caliper. Fig. 3 shows a schematic diagram of the measurement setup as well as a tip imprint coupon with electrode impressions. In order to investigate the failure behaviors of the coatings during welding, cross-sections of the electrodes were examined after certain numbers of welds using a Nova NanoSEM 450 scanning electron microscopy (SEM) system equipped with an energy-dispersive X-ray spectroscopy (EDX) attachment. The phases were analyzed through X-ray
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Tip diameter of electrode(mm)
Uncoating electrode (Up) Uncoating electrode (Down)
10
(a)
9
8
7
6
5
0
100
200
300
400
500
600
Number of welds 9.0
9.0
ZrB2-TiB2 coating electrode (Up)
ZrB2-TiB2/Ni coating electrode (Up)
ZrB2-TiB2 coating electrode (Down) Tip diameter of electrode(mm)
Tip diameter of electrode(mm)
8.5 8.0 7.5
(b)
7.0 6.5 6.0 5.5 5.0
500
1000
1500
2000
ZrB2-TiB2/Ni coating electrode (Down)
8.5 8.0
(c)
7.5 7.0 6.5 6.0 5.5 -500
2500
0
500
1000
1500
2000
2500
3000
3500
4000
Number of welds
Number of welds
Fig. 5. Diameter of electrode tip as function of number of welds: (a) uncoated, (b) ZrB2-TiB2-coated, and (c) ZrB2-TiB2/Ni-coated electrodes.
Microhardness at elctrode tip (HV at 50g)
190 180
Unoating electrode (Up) Uncoating electrode (Down)
(a)
170 160 150 140 130 120 110 0
100
200
300
400
500
600
Number of welds 900
ZrB2-TiB2 coating electrode (Up)
800
ZrB2-TiB2 coating electrode (Down)
700 600
(b)
500 400 300 200
Matrix microhardness 180 HV
100 0
0
500
1000
1500
Number of welds
2000
2500
3000
Microhardness at elctrode tip (HV at 50g)
Microhardness at elctrode tip (HV at 50g)
900
ZrB2-TiB2/Ni coating electrode (Up) ZrB2-TiB2/Ni coating electrode (Down)
800 700 600
(c)
500 400 300
Matrix microhardness 180 HV
200 100 0
0
500
1000
1500
2000
2500
3000
3500
4000
Number of welds
Fig. 6. Average microhardness of electrode tip as function of number of welds: (a) uncoated, (b) ZrB2-TiB2-coated, and (c) ZrB2-TiB2/Ni-coated electrodes.
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Fig. 7. Cross-sectional morphologies of (a) uncoated, (b) ZrB2-TiB2-coated, and (c) ZrB2-TiB2/Ni-coated electrodes after 500 welds.
diffraction (XRD) measurements. The hardness values of the coatings were measured with a HVX-1000 microhardness tester using a normal load of 50 g for 20 s.
3. Results and discussion 3.1. Plastic deformation As can be seen from the obtained test results (Fig. 4), the ZrB2-TiB2/ Ni-coated electrodes exhibited the longest lifetime, which corresponded to approximately 3700 welds, while the ZrB2-TiB2-coated electrodes exhibited an average lifetime of approximately 2700 welds, which is still much longer than that of the uncoated electrodes (less than 600 welds).
Fig. 8. Average rates of change of electrode tip diameter as functions of number of welds.
Fig. 5 shows the tip diameters of the uncoated, ZrB2-TiB2-coated, and ZrB2-TiB2/Ni-coated electrodes as functions of the number of welds. On comparing the data depicted in Fig. 5a with the results shown in Fig. 5b and c, it can be seen that the increase in the tip diameter of the uncoated electrode was larger than that observed for the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes, indicating that the deposited coatings inhibited the deformation of the electrode tip during RSW; this deformation takes place because of the combined action of the welding pressure and heat. Further, the hardness of the electrode tip is a critical factor affecting the final degree of plastic deformation in the case of electrodes fabricated from the same matrix material. Fig. 6 shows the changes in the average microhardness values of the upper and lower
Fig. 9. XRD spectra of (a) uncoated, (b) ZrB2-TiB2-coated, and (c) ZrB2-TiB2/Ni-coated electrodes after 500 welds.
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Fig. 10. Cu\ \Zn alloy phase diagram [16].
parts of the RSW electrode tips during spot welding. As can be seen from Fig. 6a, the average microhardness of the tip of the uncoated electrode tip is always lower than that of the matrix part and is significantly smaller than those of the tips of the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes (Fig. 6b and c, respectively). Another phenomenon observed during the softening of the electrode tip is “tip mushrooming.” Fig. 7a, b, and c shows the cross-sectional morphologies of the uncoated, ZrB2-TiB2-coated, and ZrB2-TiB2/Nicoated RSW electrodes, respectively, after 500 welds. The region represented by the dashed line in Fig. 7a is a typical example of “mushrooming.” In general, “mushrooming” occurs when the compression stress during spot welding exceeds the yield strength of the electrode at the temperature in question. Because the contact area between the electrode tip and the workpiece exhibits the highest temperature, the resulting plastic deformation (called “mushrooming”) mostly likely occurs near the tip region. This “mushrooming” effect leads to an additional increase in the electrode tip diameter by a magnitude of ΔR (Fig. 7a). As a result, the current passing through the electrode tip decreases, which, in turn, reduces the amount of heat required for the nucleation of the welding nuggets. This causes a deterioration in the quality of the produced weld. In contrast, the application of a coating layer protects the electrode from failure and increases its resistance to deformation, thus preventing “mushrooming” from occurring. Moreover, given the relatively low thermal conductivity of the electrode coating material, it reduces the amount of heat transferred from the welding nuggets to the electrode, thereby suppressing the increase in the temperature of the RSW electrode and its subsequent softening. Fig. 8 shows the average rates of increase in the tip diameters of the upper and lower uncoated, ZrB2-TiB2-coated, and ZrB2-TiB2/Ni-coated RSW electrodes during spot welding. For less than 600 welds (corresponding to the lifetime of the uncoated electrode), the average rate of
increase in the tip diameter of the uncoated RSW electrode was the highest (up to 19%); in contrast, those for the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes were 1% and 4%, respectively. A larger rate of increase of the electrode tip diameter leads to a greater increase in the contact area between the RSW electrode and the workpiece. As has been mentioned above, this, in turn, results in a decrease in the current density, which leads to a decrease in the final diameter of the welding nugget to a level lower than the critical value and thus to electrode failure. As per the obtained results, plastic deformation is one of the primary failure mechanisms of the uncoated electrode. On the other hand, the ZrB2-TiB2 and ZrB2-TiB2/Ni coatings protect against tip softening to a certain degree by suppressing the “mushrooming” effect and decreasing the rate of increase of the electrode tip diameter. 3.2. Alloying reaction Fig. 9 shows the XRD spectra recorded at the surfaces of the uncoated, ZrB2-TiB2-coated, and ZrB2-TiB2/Ni-coated RSW electrodes after 500 welds. The uncoated electrode tip exhibits a CuZn diffraction peak (Fig. 9a), in addition to the Zn reflection; this confirmed that an intense alloying reaction had occurred between the matrix of the RSW electrode and the Zn coating of the galvanized steel sheet during the spot welding process. This Zn diffraction peak was also observed in the cases of the tips of the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes; however, no peaks related to a CuZn alloy phase were detected in the spectra of these electrodes after 500 welds (Fig. 9b and c). The observed formation of Cu\\Zn alloys on the surface of the uncoated electrode during spot welding can be partially attributed to the presence of diverse Cu\\Zn alloy phases at relatively low temperatures (see Fig. 10) and is facilitated by the high temperature of the contact
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Fig. 11. Surface morphology of ZrB2-TiB2-coated electrode (a) and results of elemental mapping analysis for (b) Cu, (c) Zn, (d) Zr, and (e) Ti species after 1000 welds.
zone, which exceeds the melting point of Zn. Babu et al. [15] found that liquid Zn species can diffuse into Cu bulk approximately 500 times faster than solid Zn. Therefore, the melting of Zn noticeably accelerates the alloying reaction between the uncoated RSW electrode surface and the galvanized Zn layer of the steel sheets, which, in turn, decreases the electrical and thermal conductivities of the electrode. Consequently, the electrode tip undergoes rapid overheating and deformation during the continuous spot welding process, which results in a gradual decrease in the strength of the welding nugget, ultimately leading to incomplete welding. Both the ZrB2-TiB2 coating and the ZrB2-TiB2/Ni coating can improve the surface hardness of the electrode and reduce its degree of plastic deformation, thus ensuring that a sufficiently high current passes through the electrode tip during the spot welding process. In addition, the coatings prevent direct contact between the Zn layer of the steel sheets and the matrix of the RSW electrodes, thereby inhibiting the alloying process on the surfaces of the electrodes and increasing their lifetime. 3.3. Failure of coated electrodes Fig. 11 shows the cross-sectional morphology of the ZrB2-TiB2-coated electrode after 1000 welds and the results of an elemental mapping analysis for Cu, Zn, Zr, and Ti species on the surface of the electrode (Fig. 12a–e show the corresponding data for the ZrB2-TiB2/Ni-coated electrode). A comparison of Figs. 11a and 12a shows that the coating layer of the ZrB2-TiB2-coated electrode remained intact but exhibited
significant internal cracking and interfacial delamination after 1000 welds. In contrast, the coating layer of the ZrB2-TiB2/Ni-coated electrode contained only a few internal cracks, with there being no noticeable damage to the interface between the coating and the electrode matrix. Owing to the natural properties of the coating layer, the formation of cracks on its surface is inevitable under the repeated actions of the mechanical force and electrical-resistance-related heating experienced during the spot welding process. The obtained results indicate that, for the same number of welds, the ZrB2-TiB2/Ni-coated electrode was more resistant to damage than the ZrB2-TiB2-coated electrode, owing to the buffering effect of the Ni layer. Fig. 13 shows the cross-sectional morphology of the ZrB2-TiB2-coated electrode after 2000 welds and the results of an elemental mapping analysis for Cu, Zn, Zr, and Ti species on the surface of the electrode. It can be seen that the integrity of the coating layer on the surface of the ZrB2-TiB2-coated electrode was adversely affected by prolonged welding, with a large number of cracks forming across the entire coating surface; the coating even exhibited peeling in a few areas. The corresponding XRD spectrum contains diffraction peaks related to Cu\\Zn compounds (Fig. 14a) in addition to the Cu, Zn, ZrB2, and TiB2 reflections, indicating that the surface of the ZrB2-TiB2-coated electrode was significantly affected by the alloying process. The main reason for the observed phenomenon is that the severe damage experienced by the coating layer (such as cracking and peeling) leads to the creation of channels for the diffusion of Zn species into the electrode matrix; this promotes the alloying reaction on the electrode surface.
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Fig. 12. (a) Surface morphology of ZrB2-TiB2/Ni-coated electrode and results of elemental scanning analysis for (b) Cu, (c) Zn, (d) Ti, (e) Zr (e), and (f) Ni species after 1000 welds.
The cross-sectional morphology of the ZrB2-TiB2/Ni coated electrode and the results of an elemental mapping analysis for Cu, Zn, Ti, Zr, and Ni species on its surface after 2000 welds are shown in Fig. 15a–f. Despite the large number of cracks, the coating layer was not damaged and still completely covered the surface of the electrode matrix, thus inhibiting the electrode alloying process. The related XRD spectrum also confirmed the absence of products from the alloying reaction (Fig. 14b). Fig. 16 shows the cross-sectional morphology of the ZrB2-TiB2-coated electrode and the results of an elemental mapping analysis for Cu and Zn species on its surface after electrode failure (corresponding to 2700 welds). It can be seen from the figure that the coating layer has almost completely peeled off from the electrode surface, owing to the plastic deformation and alloying processes. Fig. 17 shows the cross-sectional morphology of the ZrB2-TiB2/Nicoated electrode and the results of an elemental mapping analysis for Cu, Zn, Ti, Zr, and Ni species on its surface after electrode failure (corresponding to 3700 welds). As can be seen from the figure, a portion of the coating layer remains on the surface of the failed electrode; this could be attributed to the strong adhesion between the coating and the electrode matrix. 3.4. Failure analysis of coated electrodes The results described above indicate that the failure mechanisms of the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes were different (the related processes that occur at their tips are illustrated in Figs. 18
and 19, respectively). The failure of the ZrB2-TiB2-coated electrode (Fig. 18a) is caused by the generation of cracks within the coating layer under the action of the electrode force and heat during spot welding (Fig. 18b), which ultimately leads to its delamination. After the electrode is pulled back, Zn atoms from the galvanized steel sheet rapidly fill the surface area from which the coating layer is peeled off owing to the tensile stress generated by the adhesion of the coating to the electrode matrix (Fig. 18c); this aids the Cu\\Zn alloying reaction. As a result, the electrode contact area protected by the coating layer is reduced significantly during prolonged welding (Fig. 18d). The unprotected RSW electrode undergoes destructive plastic deformation and Cu\\Zn alloying, which further accelerate its failure. On the other hand, the existence of the Ni buffer layer on the surface of the ZrB2-TiB2/Ni-coated electrode significantly improves its plasticity (Fig. 19a), despite the presence of cracks generated during the spot welding process. In contrast to the case for the single-coating layer, the diffusion of Ni atoms results in the relative content of the plastic phase in the ZrB2-TiB2 layer being higher, thus enhancing its deformation capacity. The phenomenon of “edge curling” typically occurs near the electrode cracks during the spot welding process (see Fig. 19b and the corresponding insets). However, despite its enhanced plasticity, the ZrB2-TiB2/Ni coating layer is still not as elastic as the electrode matrix. As a result, the size of the crimped edge is smaller than that for the uncoated electrode. The stress is usually concentrated in the joint area of the crimped edge. Thus, the latter readily undergoes cracking
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Fig. 13. (a) Surface morphology of ZrB2-TiB2-coated electrode and results of elemental mapping analysis for (b) Cu, (c) Zn, (d) Zr, and (e) Ti species after 2000 welds.
Fig. 14. XRD spectra of (a) ZrB2-TiB2-coated and (b) ZrB2-TiB2/Ni-coated electrodes after 2000 welds.
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Fig. 15. (a) Surface morphology of ZrB2-TiB2/Ni-coated electrode and results of elemental mapping analysis for (b) Cu, (c) Zn, (d) Zr, (e) Ti, and (f) Ni species after 2000 welds.
Fig. 16. (a) Surface morphology of ZrB2-TiB2-coated electrode and results of elemental mapping analysis for (b) Cu and (c) Zn species after 2700 welds.
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Fig. 17. (a) Surface morphology of ZrB2-TiB2/Ni-coated electrode and results of elemental mapping analysis for (b) Cu, (c) Zn, (d) Ti, (e) Zr, and (f) Ni species after 3700 welds.
and is ultimately peeled off from the coating layer during welding, thus preventing further increases in the diameter of the electrode tip (Fig. 19c). In addition, the fact that the coated layer does not undergo delamination minimizes its probability of being peeled off. Hence, Zn-filled regions are ultimately formed at the tip of the ZrB2-TiB2/Ni-coated electrode after prolonged welding. However, the mechanisms of formation of these regions in the single-layered ZrB2-TiB2 coating and the multilayered ZrB2-TiB2/Ni coating are different. In the former system,
Zn-filled regions are mainly generated by peeling, while in the latter one they are formed owing to crack propagation. Although the ZrB2TiB2/Ni coating is ultimately destroyed, the part of it remaining on the electrode surface contributes to dispersion strengthening (Fig. 19d), as was confirmed previously by the results of the microhardness tests performed on the electrode tips after electrode failure (see Fig. 6c). The enhanced ability of the ZrB2-TiB2/Ni coating to prevent deformation and inhibit element diffusion through the coating-matrix interface results
Fig. 18. Failure mechanism of ZrB2-TiB2-coated electrode.
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Fig. 19. Failure mechanism of ZrB2-TiB2/Ni-coated electrode.
in the lifetime of the ZrB2-TiB2/Ni-coated electrode being higher than that of the ZrB2-TiB2-coated electrode. Thus, the results obtained in this study revealed that the failure of the ZrB2-TiB2/Ni-coated electrodes is mainly associated with the increase in the tip diameter caused by plastic deformation. Nevertheless, this deformation is relatively smaller than that observed in the uncoated electrodes, which explains the effect of the ZrB2-TiB2/Ni coating on the lifetime of RSW electrodes. Single-layered ZrB2-TiB2 coatings can also improve the service life of RSW electrodes; however, their effectiveness is limited by the inherent defects in the coating-matrix interface.
relatively higher coating-matrix bonding strength; thus, the coating film was not completely delaminated during spot welding, and its protective effect endured until electrode failure occurred eventually, owing to plastic deformation. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51375150), the International Science & Technology Cooperation Program of China (Grant No. 2016YFE0124300) and Hubei Provincial Key Laboratory of Green Materials for Light Industry open fund (Grant No. 201611A07).
4. Conclusions References 1. Protective coatings significantly improve the surface hardness of RSW electrodes by inhibiting the plastic deformation of their tips. In addition, the coatings can effectively slow the erosion process; in the case of the ZrB2-TiB2-coated electrodes, erosion occurred only after 2000 welds, while for the ZrB2-TiB2/Ni-coated electrodes, it occurred even later. 2. The failure mechanisms of the ZrB2-TiB2-coated and ZrB2-TiB2/Nicoated electrodes were different. In the former case, in the early stage of the RSW process (for fewer than 2000 welds), owing to the cyclic mechanical forces and thermal effects, plastic deformation was inevitable. At the same time, poor bonding between the coating layer and the electrode matrix, as well as the relatively low content of the plastic phase, led to the gradual delamination, damage, and even peeling of the coating film, in which Zn-rich regions were formed and local alloying occurred between the steel-plate coating and the electrode body during the subsequent RSW process. Thus, the alloying effect and plastic deformation in combination lead to the final failure of the ZrB2-TiB2-coated electrode. In contrast, the ZrB2TiB2/Ni coating exhibited a certain degree of plasticity and a
[1] J.D. Parker, N.T. Williams, R.J. Holliday, Sci. Technol. Weld. Join. 3 (1998) 65–74. [2] N.T. Williams, J.D. Parker, Int. Mater. Rev. 49 (2004) 77–108. [3] B.P. Wilson, J.R. Searle, K. Yliniemi, T.B. Jones, D.A. Worsley, H.N. McMurray, ECS Trans. 50 (2013) 53–64. [4] C. Luo, S. Dong, X. Xiong, N. Zhou, Surf. Coat. Technol. 203 (2009) 3333–3337. [5] C. Luo, X. Xiong, S.-j. Dong, Trans. Nonferrous Metals Soc. China 21 (2011) 317–321. [6] J. Zou, Q. Zhao, Z. Chen, J. Mater. Process. Technol. 209 (2009) 4141–4146. [7] Z. Chen, Y. Zhou, Surf. Coat. Technol. 201 (2006) 1503–1510. [8] P. Luo, S. Dong, A. Yangli, W. Yang, Surf. Coat. Technol. 253 (2014) 132–138. [9] P. Luo, S. Dong, A. Yangli, S. Sun, Z. Zheng, H. Wang, J. Asian Ceramic Soc. 3 (2015) 103–107. [10] J.F. Shackelford, W. Alexander, Materials Science and Engineering Handbook, third ed. CRC Press, Boca Raton, 2001. [11] C. Mroz, Am. Ceram. Soc. Bull. 73 (1994) 78–81. [12] P. Luo, S. Dong, A. Yangli, S. Sun, Z. Zheng, Int. J. Surf. Sci. Eng. 10 (2016) 41–45. [13] Specimen dimensions and procedure for mechanized peel testing resistance spot, seam and embossed projection welds, International Standardization Organization, ISO 14270. [14] Resistance welding — procedures for determining the weldability lobe for resistance spot, projection and seam welding, International Standardization Organization, ISO 14327. [15] S. Babu, M. Santella, W. Peterson, AWS Welding Shows, Illinois, Chicago, 2004. [16] A. Handbook, ASM, Metals Park, OH, 1992.
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Comparative failure analysis of electrodes coated with TiB2-ZrB2 and TiB2-ZrB2/Ni layers Ping Luo, Can Xiong, Chong Wang, Fucheng Qin, Yao Xiao, Zhi Li, Kang Liu, Xu Guan, Shijie Dong ⁎ a b
Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, China School of Materials and Chemical Engineering, Hubei University of Technology, Wuhan 430068, China
a r t i c l e
i n f o
Article history: Received 28 December 2016 Revised 19 March 2017 Accepted in revised form 20 March 2017 Available online 21 March 2017 Keywords: Titanium diboride-zirconium diboride Failure analysis Copper alloy electrode Coating
a b s t r a c t Electrodes with a monolithic TiB2-ZrB2 coating as well as those with a multilayered TiB2-ZrB2/Ni coating were used to resistance spot welding (RSW) Zn-coated steel, in order to investigate the failure behavior of the coatings during welding. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction analysis (XRD), and microhardness measurements were performed to characterize the microstructures of the coatings, the reactions of the electrodes with the Zn coating of the sheets, and the formation of the alloy layer. It was found that the protective coatings significantly improved the surface hardness of the RSW electrodes by inhibiting plastic deformation at their tips, thus slowing the erosion process. In addition, the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes exhibited different failure mechanisms. A combination of the alloying effect and plastic deformation caused the final failure of the ZrB2-TiB2-coated electrode, while the ZrB2-TiB2/Ni coated electrode failed primarily owing to plastic deformation. © 2017 Published by Elsevier B.V.
1. Introduction Zn-coated (galvanized) steel is a superior material for manufacturing automobile bodies (especially those of sedans) because of its good corrosion resistance. During the resistance spot welding (RSW) of galvanized steel sheets, coating materials can readily adhere to the surface of the RSW electrode owing to the low melting point of the galvanized Zn layer (692 K). As a result, Zn atoms diffuse into the bulk of the Cu electrode, leading to a decrease in its electrical and thermal conductivities [1]. Consequently, the deformation of the electrode tip caused by rapid overheating during the continuous RSW process gradually decreases the strength of the welding nugget, which, in turn, lowers the quality of the weld and ultimately leads to the complete failure of the RSW electrode [2,3]. This phenomenon of premature electrode failure not only reduces the efficiency of the welding process, given the resultant increase in the frequency with which the RSW electrode has to be repaired or replaced, but also significantly increases the production cost. Therefore, extending the lifetime of RSW electrodes is an important issue in the automobile industry and needs to be resolved urgently. Previous studies performed with the aim of resolving this issue have produced significant results. One possible solution to the above-described problem is to coat a metal-ceramic layer on the surface of the RSW electrode via electrospark deposition (ESD). For example, Luo ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (S. Dong).
http://dx.doi.org/10.1016/j.surfcoat.2017.03.045 0257-8972/© 2017 Published by Elsevier B.V.
synthesized a TiB2 coating on the surface of an RSW electrode via ESD and analyzed the mass loss of the Cu alloy electrode during the formation of the TiB2 coating by ESD [4]. To improve the poor wettability between the coating and the electrode, an intermediate Ni layer was used [5]. A similar method was used to prepare Ni/(TiCP/Ni)/Ni composite coatings [6,7]. However, it should be noted that all metal ceramics are single-phase materials. It has been shown that the properties of multiphase metal ceramic composites excelled constituent element. Thus, multiphase metal ceramic coatings have been synthesized via ESD. For example, Luo et al. synthesized TiB2-TiC [8] and Al2O3–TiB2/Ni [9] composite-phase coatings via ESD and studied the effects of the coating parameters on coating quality. ZrB2 exhibits higher electrical and thermal conductivities than TiC and Al2O3 [10]. Further, Mroz [11] demonstrated that (Ti, Zr)B2 solid solutions exhibit better mechanical properties than the end-member compositions. This suggests that ZrB2-TiB2 composites may be suitable materials for coating RSW electrodes. Luo et al. [12] successfully synthesized a ZrB2-TiB2 composite coating on Cu-Cr-Zr alloy electrodes via ESD. However, these studies were mainly focused on coating preparation, as well as ways of improving the coating quality and mass transfer during the ESD process, and there have been few studies on the failure mechanism of the coated electrodes. Thus, there is a lack of information regarding the failure mechanisms of coated electrodes during the RSW of galvanized steel sheets. With this goal in mind, in this work, using various characterization techniques, we analyzed the failure mechanisms of RSW electrodes
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Fig. 1. Schematic of equipment used for electrospark deposition (ESD) [8].
Table 1 Welding parameters for testing of electrodes. Electrode
Welding current (A)
Electrode force (kN)
Welding time (cycles)
Hold time (cycles)
Welding speed (welds/min)
Water flow (L/min)
Uncoated ZrB2-TiB2 coating ZrB2-TiB2/Ni coating
9300 8600 8400
2 2 2
7 7 7
10 10 10
30 30 30
2 2 2
Fig. 2. Change in weld nugget size as function of weld current for constant electrode force and different weld times (t1 b t2 b t3 b t4): (a) uncoated electrode, (b) ZrB2-TiB2-coated electrode, and (c) ZrB2-TiB2/Ni-coated electrode.
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Fig. 3. Schematic diagram of measurement setup and coupon with transferred imprints of electrodes.
coated with ZrB2-TiB2 and ZrB2-TiB2/Ni layers during the welding of Zncoated steel sheets. 2. Experimental The RSW electrodes were coated with the ZrB2-TiB2 and ZrB2-TiB2/ Ni layers using a laboratory-made ESD apparatus, which is shown in Fig. 1. The electrodes used in this study were precipitation-strengthened, cold-formed domed flat electrodes consisting of a Cu-Cr-Zr alloy (Cu-1Cr-0.05Zr (mass%)). They were 16 mm in diameter and 23 mm in length and had a flat top, with the diameter of the top surface being 5 mm. The ZrB2-TiB2 coating was formed by directly using a specially sintered ZrB2-TiB2 rod as the anode during the ESD process. These ZrB2-TiB2 composite rods (6 mm in diameter and 30 mm in length) were fabricated by vacuum sintering at 1400 °C for 1 h (ZrB2/TiB2 molar ratio of approximately 1:1). The multilayered ZrB2-TiB2/Ni films were coated by first forming a layer of Ni on the RSW electrode surface by ESD; a commercially available rod of pure nickel was used for the
Fig. 4. Dependence of nugget diameter on number of welds.
purpose. This was followed by the deposition of a ZrB2-TiB2 composite layer onto the Ni-coated electrode via ESD. The ESD process was performed at a voltage of 12 V using a capacitance of 2000 μF; the coating duration was 2 min and the thickness of the coatings was approximately 10–20 μm. The lifetimes of the coated RSW electrodes were measured using a YR-350SA2HGE single-phase AC resistance spot welder. Sheets of DX56D + Z double-sided hot-galvanized Zn-coated steel (thicknesses of 0.7 mm) manufactured by the Shanghai Baosteel Group Corporation were used for the lifetime measurements. The spots were produced sequentially, and the dimensions of the test specimens and the spot distributions were in keeping with the ISO-14270 standard [13]. The same conditions were used for all three electrodes. Welding tests were performed in order to investigate and compare the degradation processes of the electrodes with and without a coating. The same initial weld nugget size was used in all the cases. The parameters used for the tests are listed in Table 1. The weld current was determined by constant-electrode-force weldability lobe measurements performed on the three types of electrodes; these measurements were performed as per the ISO-14327 standard [14]. Fig. 2 shows the increase in the weld nugget size as a function of the weld current for a constant electrode force (2 kN) and different weld times (t1 b t2 b t3 b t4). In this study, the current was used at weld time = 7 cycles, and the initial nugget diameter was 6 ∗ sqrt(t), where t is the thickness of the steel plates (= 0.7 mm). The growth rate of the electrode tip during the welding process was monitored and used as a measure of electrode performance. The rate was determined by the carbon imprint technique, in which an imprint coupon was made by fixing strips of carbon typing paper to a steel endurance coupon covered with plain white paper on both surfaces (top and bottom). A weld force was then applied on the imprint coupon using the electrodes in the absence of a weld current, in order to transfer the carbon ink to the paper. The imprints were measured with a digital Vernier caliper. Fig. 3 shows a schematic diagram of the measurement setup as well as a tip imprint coupon with electrode impressions. In order to investigate the failure behaviors of the coatings during welding, cross-sections of the electrodes were examined after certain numbers of welds using a Nova NanoSEM 450 scanning electron microscopy (SEM) system equipped with an energy-dispersive X-ray spectroscopy (EDX) attachment. The phases were analyzed through X-ray
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Tip diameter of electrode(mm)
Uncoating electrode (Up) Uncoating electrode (Down)
10
(a)
9
8
7
6
5
0
100
200
300
400
500
600
Number of welds 9.0
9.0
ZrB2-TiB2 coating electrode (Up)
ZrB2-TiB2/Ni coating electrode (Up)
ZrB2-TiB2 coating electrode (Down) Tip diameter of electrode(mm)
Tip diameter of electrode(mm)
8.5 8.0 7.5
(b)
7.0 6.5 6.0 5.5 5.0
500
1000
1500
2000
ZrB2-TiB2/Ni coating electrode (Down)
8.5 8.0
(c)
7.5 7.0 6.5 6.0 5.5 -500
2500
0
500
1000
1500
2000
2500
3000
3500
4000
Number of welds
Number of welds
Fig. 5. Diameter of electrode tip as function of number of welds: (a) uncoated, (b) ZrB2-TiB2-coated, and (c) ZrB2-TiB2/Ni-coated electrodes.
Microhardness at elctrode tip (HV at 50g)
190 180
Unoating electrode (Up) Uncoating electrode (Down)
(a)
170 160 150 140 130 120 110 0
100
200
300
400
500
600
Number of welds 900
ZrB2-TiB2 coating electrode (Up)
800
ZrB2-TiB2 coating electrode (Down)
700 600
(b)
500 400 300 200
Matrix microhardness 180 HV
100 0
0
500
1000
1500
Number of welds
2000
2500
3000
Microhardness at elctrode tip (HV at 50g)
Microhardness at elctrode tip (HV at 50g)
900
ZrB2-TiB2/Ni coating electrode (Up) ZrB2-TiB2/Ni coating electrode (Down)
800 700 600
(c)
500 400 300
Matrix microhardness 180 HV
200 100 0
0
500
1000
1500
2000
2500
3000
3500
4000
Number of welds
Fig. 6. Average microhardness of electrode tip as function of number of welds: (a) uncoated, (b) ZrB2-TiB2-coated, and (c) ZrB2-TiB2/Ni-coated electrodes.
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Fig. 7. Cross-sectional morphologies of (a) uncoated, (b) ZrB2-TiB2-coated, and (c) ZrB2-TiB2/Ni-coated electrodes after 500 welds.
diffraction (XRD) measurements. The hardness values of the coatings were measured with a HVX-1000 microhardness tester using a normal load of 50 g for 20 s.
3. Results and discussion 3.1. Plastic deformation As can be seen from the obtained test results (Fig. 4), the ZrB2-TiB2/ Ni-coated electrodes exhibited the longest lifetime, which corresponded to approximately 3700 welds, while the ZrB2-TiB2-coated electrodes exhibited an average lifetime of approximately 2700 welds, which is still much longer than that of the uncoated electrodes (less than 600 welds).
Fig. 8. Average rates of change of electrode tip diameter as functions of number of welds.
Fig. 5 shows the tip diameters of the uncoated, ZrB2-TiB2-coated, and ZrB2-TiB2/Ni-coated electrodes as functions of the number of welds. On comparing the data depicted in Fig. 5a with the results shown in Fig. 5b and c, it can be seen that the increase in the tip diameter of the uncoated electrode was larger than that observed for the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes, indicating that the deposited coatings inhibited the deformation of the electrode tip during RSW; this deformation takes place because of the combined action of the welding pressure and heat. Further, the hardness of the electrode tip is a critical factor affecting the final degree of plastic deformation in the case of electrodes fabricated from the same matrix material. Fig. 6 shows the changes in the average microhardness values of the upper and lower
Fig. 9. XRD spectra of (a) uncoated, (b) ZrB2-TiB2-coated, and (c) ZrB2-TiB2/Ni-coated electrodes after 500 welds.
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Fig. 10. Cu\ \Zn alloy phase diagram [16].
parts of the RSW electrode tips during spot welding. As can be seen from Fig. 6a, the average microhardness of the tip of the uncoated electrode tip is always lower than that of the matrix part and is significantly smaller than those of the tips of the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes (Fig. 6b and c, respectively). Another phenomenon observed during the softening of the electrode tip is “tip mushrooming.” Fig. 7a, b, and c shows the cross-sectional morphologies of the uncoated, ZrB2-TiB2-coated, and ZrB2-TiB2/Nicoated RSW electrodes, respectively, after 500 welds. The region represented by the dashed line in Fig. 7a is a typical example of “mushrooming.” In general, “mushrooming” occurs when the compression stress during spot welding exceeds the yield strength of the electrode at the temperature in question. Because the contact area between the electrode tip and the workpiece exhibits the highest temperature, the resulting plastic deformation (called “mushrooming”) mostly likely occurs near the tip region. This “mushrooming” effect leads to an additional increase in the electrode tip diameter by a magnitude of ΔR (Fig. 7a). As a result, the current passing through the electrode tip decreases, which, in turn, reduces the amount of heat required for the nucleation of the welding nuggets. This causes a deterioration in the quality of the produced weld. In contrast, the application of a coating layer protects the electrode from failure and increases its resistance to deformation, thus preventing “mushrooming” from occurring. Moreover, given the relatively low thermal conductivity of the electrode coating material, it reduces the amount of heat transferred from the welding nuggets to the electrode, thereby suppressing the increase in the temperature of the RSW electrode and its subsequent softening. Fig. 8 shows the average rates of increase in the tip diameters of the upper and lower uncoated, ZrB2-TiB2-coated, and ZrB2-TiB2/Ni-coated RSW electrodes during spot welding. For less than 600 welds (corresponding to the lifetime of the uncoated electrode), the average rate of
increase in the tip diameter of the uncoated RSW electrode was the highest (up to 19%); in contrast, those for the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes were 1% and 4%, respectively. A larger rate of increase of the electrode tip diameter leads to a greater increase in the contact area between the RSW electrode and the workpiece. As has been mentioned above, this, in turn, results in a decrease in the current density, which leads to a decrease in the final diameter of the welding nugget to a level lower than the critical value and thus to electrode failure. As per the obtained results, plastic deformation is one of the primary failure mechanisms of the uncoated electrode. On the other hand, the ZrB2-TiB2 and ZrB2-TiB2/Ni coatings protect against tip softening to a certain degree by suppressing the “mushrooming” effect and decreasing the rate of increase of the electrode tip diameter. 3.2. Alloying reaction Fig. 9 shows the XRD spectra recorded at the surfaces of the uncoated, ZrB2-TiB2-coated, and ZrB2-TiB2/Ni-coated RSW electrodes after 500 welds. The uncoated electrode tip exhibits a CuZn diffraction peak (Fig. 9a), in addition to the Zn reflection; this confirmed that an intense alloying reaction had occurred between the matrix of the RSW electrode and the Zn coating of the galvanized steel sheet during the spot welding process. This Zn diffraction peak was also observed in the cases of the tips of the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes; however, no peaks related to a CuZn alloy phase were detected in the spectra of these electrodes after 500 welds (Fig. 9b and c). The observed formation of Cu\\Zn alloys on the surface of the uncoated electrode during spot welding can be partially attributed to the presence of diverse Cu\\Zn alloy phases at relatively low temperatures (see Fig. 10) and is facilitated by the high temperature of the contact
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Fig. 11. Surface morphology of ZrB2-TiB2-coated electrode (a) and results of elemental mapping analysis for (b) Cu, (c) Zn, (d) Zr, and (e) Ti species after 1000 welds.
zone, which exceeds the melting point of Zn. Babu et al. [15] found that liquid Zn species can diffuse into Cu bulk approximately 500 times faster than solid Zn. Therefore, the melting of Zn noticeably accelerates the alloying reaction between the uncoated RSW electrode surface and the galvanized Zn layer of the steel sheets, which, in turn, decreases the electrical and thermal conductivities of the electrode. Consequently, the electrode tip undergoes rapid overheating and deformation during the continuous spot welding process, which results in a gradual decrease in the strength of the welding nugget, ultimately leading to incomplete welding. Both the ZrB2-TiB2 coating and the ZrB2-TiB2/Ni coating can improve the surface hardness of the electrode and reduce its degree of plastic deformation, thus ensuring that a sufficiently high current passes through the electrode tip during the spot welding process. In addition, the coatings prevent direct contact between the Zn layer of the steel sheets and the matrix of the RSW electrodes, thereby inhibiting the alloying process on the surfaces of the electrodes and increasing their lifetime. 3.3. Failure of coated electrodes Fig. 11 shows the cross-sectional morphology of the ZrB2-TiB2-coated electrode after 1000 welds and the results of an elemental mapping analysis for Cu, Zn, Zr, and Ti species on the surface of the electrode (Fig. 12a–e show the corresponding data for the ZrB2-TiB2/Ni-coated electrode). A comparison of Figs. 11a and 12a shows that the coating layer of the ZrB2-TiB2-coated electrode remained intact but exhibited
significant internal cracking and interfacial delamination after 1000 welds. In contrast, the coating layer of the ZrB2-TiB2/Ni-coated electrode contained only a few internal cracks, with there being no noticeable damage to the interface between the coating and the electrode matrix. Owing to the natural properties of the coating layer, the formation of cracks on its surface is inevitable under the repeated actions of the mechanical force and electrical-resistance-related heating experienced during the spot welding process. The obtained results indicate that, for the same number of welds, the ZrB2-TiB2/Ni-coated electrode was more resistant to damage than the ZrB2-TiB2-coated electrode, owing to the buffering effect of the Ni layer. Fig. 13 shows the cross-sectional morphology of the ZrB2-TiB2-coated electrode after 2000 welds and the results of an elemental mapping analysis for Cu, Zn, Zr, and Ti species on the surface of the electrode. It can be seen that the integrity of the coating layer on the surface of the ZrB2-TiB2-coated electrode was adversely affected by prolonged welding, with a large number of cracks forming across the entire coating surface; the coating even exhibited peeling in a few areas. The corresponding XRD spectrum contains diffraction peaks related to Cu\\Zn compounds (Fig. 14a) in addition to the Cu, Zn, ZrB2, and TiB2 reflections, indicating that the surface of the ZrB2-TiB2-coated electrode was significantly affected by the alloying process. The main reason for the observed phenomenon is that the severe damage experienced by the coating layer (such as cracking and peeling) leads to the creation of channels for the diffusion of Zn species into the electrode matrix; this promotes the alloying reaction on the electrode surface.
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Fig. 12. (a) Surface morphology of ZrB2-TiB2/Ni-coated electrode and results of elemental scanning analysis for (b) Cu, (c) Zn, (d) Ti, (e) Zr (e), and (f) Ni species after 1000 welds.
The cross-sectional morphology of the ZrB2-TiB2/Ni coated electrode and the results of an elemental mapping analysis for Cu, Zn, Ti, Zr, and Ni species on its surface after 2000 welds are shown in Fig. 15a–f. Despite the large number of cracks, the coating layer was not damaged and still completely covered the surface of the electrode matrix, thus inhibiting the electrode alloying process. The related XRD spectrum also confirmed the absence of products from the alloying reaction (Fig. 14b). Fig. 16 shows the cross-sectional morphology of the ZrB2-TiB2-coated electrode and the results of an elemental mapping analysis for Cu and Zn species on its surface after electrode failure (corresponding to 2700 welds). It can be seen from the figure that the coating layer has almost completely peeled off from the electrode surface, owing to the plastic deformation and alloying processes. Fig. 17 shows the cross-sectional morphology of the ZrB2-TiB2/Nicoated electrode and the results of an elemental mapping analysis for Cu, Zn, Ti, Zr, and Ni species on its surface after electrode failure (corresponding to 3700 welds). As can be seen from the figure, a portion of the coating layer remains on the surface of the failed electrode; this could be attributed to the strong adhesion between the coating and the electrode matrix. 3.4. Failure analysis of coated electrodes The results described above indicate that the failure mechanisms of the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes were different (the related processes that occur at their tips are illustrated in Figs. 18
and 19, respectively). The failure of the ZrB2-TiB2-coated electrode (Fig. 18a) is caused by the generation of cracks within the coating layer under the action of the electrode force and heat during spot welding (Fig. 18b), which ultimately leads to its delamination. After the electrode is pulled back, Zn atoms from the galvanized steel sheet rapidly fill the surface area from which the coating layer is peeled off owing to the tensile stress generated by the adhesion of the coating to the electrode matrix (Fig. 18c); this aids the Cu\\Zn alloying reaction. As a result, the electrode contact area protected by the coating layer is reduced significantly during prolonged welding (Fig. 18d). The unprotected RSW electrode undergoes destructive plastic deformation and Cu\\Zn alloying, which further accelerate its failure. On the other hand, the existence of the Ni buffer layer on the surface of the ZrB2-TiB2/Ni-coated electrode significantly improves its plasticity (Fig. 19a), despite the presence of cracks generated during the spot welding process. In contrast to the case for the single-coating layer, the diffusion of Ni atoms results in the relative content of the plastic phase in the ZrB2-TiB2 layer being higher, thus enhancing its deformation capacity. The phenomenon of “edge curling” typically occurs near the electrode cracks during the spot welding process (see Fig. 19b and the corresponding insets). However, despite its enhanced plasticity, the ZrB2-TiB2/Ni coating layer is still not as elastic as the electrode matrix. As a result, the size of the crimped edge is smaller than that for the uncoated electrode. The stress is usually concentrated in the joint area of the crimped edge. Thus, the latter readily undergoes cracking
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Fig. 13. (a) Surface morphology of ZrB2-TiB2-coated electrode and results of elemental mapping analysis for (b) Cu, (c) Zn, (d) Zr, and (e) Ti species after 2000 welds.
Fig. 14. XRD spectra of (a) ZrB2-TiB2-coated and (b) ZrB2-TiB2/Ni-coated electrodes after 2000 welds.
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Fig. 15. (a) Surface morphology of ZrB2-TiB2/Ni-coated electrode and results of elemental mapping analysis for (b) Cu, (c) Zn, (d) Zr, (e) Ti, and (f) Ni species after 2000 welds.
Fig. 16. (a) Surface morphology of ZrB2-TiB2-coated electrode and results of elemental mapping analysis for (b) Cu and (c) Zn species after 2700 welds.
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Fig. 17. (a) Surface morphology of ZrB2-TiB2/Ni-coated electrode and results of elemental mapping analysis for (b) Cu, (c) Zn, (d) Ti, (e) Zr, and (f) Ni species after 3700 welds.
and is ultimately peeled off from the coating layer during welding, thus preventing further increases in the diameter of the electrode tip (Fig. 19c). In addition, the fact that the coated layer does not undergo delamination minimizes its probability of being peeled off. Hence, Zn-filled regions are ultimately formed at the tip of the ZrB2-TiB2/Ni-coated electrode after prolonged welding. However, the mechanisms of formation of these regions in the single-layered ZrB2-TiB2 coating and the multilayered ZrB2-TiB2/Ni coating are different. In the former system,
Zn-filled regions are mainly generated by peeling, while in the latter one they are formed owing to crack propagation. Although the ZrB2TiB2/Ni coating is ultimately destroyed, the part of it remaining on the electrode surface contributes to dispersion strengthening (Fig. 19d), as was confirmed previously by the results of the microhardness tests performed on the electrode tips after electrode failure (see Fig. 6c). The enhanced ability of the ZrB2-TiB2/Ni coating to prevent deformation and inhibit element diffusion through the coating-matrix interface results
Fig. 18. Failure mechanism of ZrB2-TiB2-coated electrode.
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Fig. 19. Failure mechanism of ZrB2-TiB2/Ni-coated electrode.
in the lifetime of the ZrB2-TiB2/Ni-coated electrode being higher than that of the ZrB2-TiB2-coated electrode. Thus, the results obtained in this study revealed that the failure of the ZrB2-TiB2/Ni-coated electrodes is mainly associated with the increase in the tip diameter caused by plastic deformation. Nevertheless, this deformation is relatively smaller than that observed in the uncoated electrodes, which explains the effect of the ZrB2-TiB2/Ni coating on the lifetime of RSW electrodes. Single-layered ZrB2-TiB2 coatings can also improve the service life of RSW electrodes; however, their effectiveness is limited by the inherent defects in the coating-matrix interface.
relatively higher coating-matrix bonding strength; thus, the coating film was not completely delaminated during spot welding, and its protective effect endured until electrode failure occurred eventually, owing to plastic deformation. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51375150), the International Science & Technology Cooperation Program of China (Grant No. 2016YFE0124300) and Hubei Provincial Key Laboratory of Green Materials for Light Industry open fund (Grant No. 201611A07).
4. Conclusions References 1. Protective coatings significantly improve the surface hardness of RSW electrodes by inhibiting the plastic deformation of their tips. In addition, the coatings can effectively slow the erosion process; in the case of the ZrB2-TiB2-coated electrodes, erosion occurred only after 2000 welds, while for the ZrB2-TiB2/Ni-coated electrodes, it occurred even later. 2. The failure mechanisms of the ZrB2-TiB2-coated and ZrB2-TiB2/Nicoated electrodes were different. In the former case, in the early stage of the RSW process (for fewer than 2000 welds), owing to the cyclic mechanical forces and thermal effects, plastic deformation was inevitable. At the same time, poor bonding between the coating layer and the electrode matrix, as well as the relatively low content of the plastic phase, led to the gradual delamination, damage, and even peeling of the coating film, in which Zn-rich regions were formed and local alloying occurred between the steel-plate coating and the electrode body during the subsequent RSW process. Thus, the alloying effect and plastic deformation in combination lead to the final failure of the ZrB2-TiB2-coated electrode. In contrast, the ZrB2TiB2/Ni coating exhibited a certain degree of plasticity and a
[1] J.D. Parker, N.T. Williams, R.J. Holliday, Sci. Technol. Weld. Join. 3 (1998) 65–74. [2] N.T. Williams, J.D. Parker, Int. Mater. Rev. 49 (2004) 77–108. [3] B.P. Wilson, J.R. Searle, K. Yliniemi, T.B. Jones, D.A. Worsley, H.N. McMurray, ECS Trans. 50 (2013) 53–64. [4] C. Luo, S. Dong, X. Xiong, N. Zhou, Surf. Coat. Technol. 203 (2009) 3333–3337. [5] C. Luo, X. Xiong, S.-j. Dong, Trans. Nonferrous Metals Soc. China 21 (2011) 317–321. [6] J. Zou, Q. Zhao, Z. Chen, J. Mater. Process. Technol. 209 (2009) 4141–4146. [7] Z. Chen, Y. Zhou, Surf. Coat. Technol. 201 (2006) 1503–1510. [8] P. Luo, S. Dong, A. Yangli, W. Yang, Surf. Coat. Technol. 253 (2014) 132–138. [9] P. Luo, S. Dong, A. Yangli, S. Sun, Z. Zheng, H. Wang, J. Asian Ceramic Soc. 3 (2015) 103–107. [10] J.F. Shackelford, W. Alexander, Materials Science and Engineering Handbook, third ed. CRC Press, Boca Raton, 2001. [11] C. Mroz, Am. Ceram. Soc. Bull. 73 (1994) 78–81. [12] P. Luo, S. Dong, A. Yangli, S. Sun, Z. Zheng, Int. J. Surf. Sci. Eng. 10 (2016) 41–45. [13] Specimen dimensions and procedure for mechanized peel testing resistance spot, seam and embossed projection welds, International Standardization Organization, ISO 14270. [14] Resistance welding — procedures for determining the weldability lobe for resistance spot, projection and seam welding, International Standardization Organization, ISO 14327. [15] S. Babu, M. Santella, W. Peterson, AWS Welding Shows, Illinois, Chicago, 2004. [16] A. Handbook, ASM, Metals Park, OH, 1992.