Performance of resistance spot weld caps coated with Ni and Fe aluminide alloys by electro spark deposition on hot dip galvanized steel

Ultrasonic Test Results of Resistance Spot Welds by Uncoated Electrode Cap

The UT signals of the welded sample obtained by the uncoated cap are shown in Fig. 3b. It has been determined that the signals are logarithmic, noiseless and a homogeneous core is formed in the test graph, where the signal intensity is observed to be usual. In the experiments with samples having a lower threshold tolerance of 1.05 mm thickness, the resistance spot weld thickness was measured to be 1.23 mm in the welded sample made with the uncoated electrode cap. This is an average value for most of the welded samples which are accepted as a “pass” by the manufacturer.

Chisel Test Results of Resistance Spot Welds of Uncoated Electrode Caps

The post-weld chisel test of the welded sample made with the uncoated cap is given in Fig. 3c. The nugget core is nicely formed and the chisel damage at the edge of both facing sides of the metal sheets reveals that there was sufficient joining along the weld nugget. The face of electrode cap is given in Fig. 3d and a layer with an average thickness of 5.69 µm is visible, which is possibly an indication of brassing due to Zn evaporation during the spot welding. It is brittle and reduces the life of electrode caps by chipping of edges and forming large cracks extending to the unaffected part of the electrode caps (Harlin et al., 2003Harlin, N., Jones, T.B., Parker, J.D. (2003). Weld growth mechanism of resistance spot welds in zinc coated steel. J. Mater. Process. Technol. 143-144, 448-453. https://doi.org/10.1016/S0924-0136(03)00447-3.; Chen and Zhou, 2006Chen, Z., Zhou, Y. (2006). Surface modification of resistance welding electrode by electro-spark deposited composite coatings: Part I coating characterization. Surf. Coat. Technol. 201 (3-4), 1503-1510. https://doi.org/10.1016/j.surfcoat.2006.02.015.).

Microhardness Results

In the hardness tests performed on the 10^th, 20^th, 30^th, 40^th and 50^th samples cut from the plates. The most noticeable variation in hardness occurred in the caps coated with the Fe₃Al intermetallic alloy. As in Table 4, it has been determined that the welded sample joined with Fe₃Al coated caps with the coating voltages of 66 V and 112 V are harder than the others. The plate that shows the largest variability among their hardness is the one welded with the cap coated with Ni₃Al at 158 V. A sharp hardness difference of 13.1 HV_0,1 was observed between the 10^th weld and the 30^th weld. Plates with the lowest hardness deviation were determined as those welded with the Ni₃Al and NiAl coated caps at 112 V. Although a metallurgical alloying is present between the coating and the substrate (Korkmaz, 2015Korkmaz, K. (2015). Investigation and characterization of electrospark deposited chromium carbide-based coating on the steel. Surf. Coat. Technol. 272, 1-7. https://doi.org/10.1016/j.surfcoat.2015.04.033.; Onan et al., 2022Onan, M., Şahin, O., Yıldırım, E., Talaş, Ş. (2022). Effect of WC based coatings on the wear of CK45 sheet metal forming dies. Int. J. Surf. Sci. Eng. 15 (4), 265-280. https://doi.org/10.1504/IJSURFSE.2021.120959.), having a distinct elasticity modulus between them is a very challenging situation. During the course of applying pressure and being exposed to heat from the lap joint accelerate the wear process of coating and Cu caps with a secondary effect from deformation of copper caps. After certain number of pressing during RSW, in addition to the zinc accumulating on the surface, nugget hardness is expected to show variation as it is seen in Table 4. The variation in hardness in the series is not profoundly distinctive but a gradual change in some series is still observable. Apart from small changes, they are all within the similarity range of 200 HV. These results are lower than the bulk Fe₃Al, FeAl, Ni₃Al and NiAl aluminide intermetallic alloys’ hardness values, which is possibly due to the layer effect i.e. alloying with the substrate and low thickness of the aluminide coatings. Within the voltage series, changes are again not profound. In terms of type of electrodes, the aluminide intermetallics having ordered phases of bcc B2 superstructure (NiAl and FeAl) are expected to have a higher hardness due to their increased brittleness as a result of a lower number of slip planes that are activated during the plastic deformation compared to A₃B type of intermetallics, i.e. Fe₃Al (D0₃ superstructure, bcc) and Ni₃Al (L1₂ superstructure, fcc). It is also possible that the increased hardness of B2 superstructures is due to the thermal vacancies occurring after the high temperature treatment and antisite vacancy formation and fast cooling rate (Morris and Munoz-Morris, 2005Morris, D.G., Munoz-Morris, M. (2005). The stress anomaly in FeAl-Fe3Al alloys. Intermetallics 13 (12), 1269-1274. https://doi.org/10.1016/j.intermet.2004.08.012.; Krein et al., 2007Krein, R., Schneider, A., Sauthoff, G., Frommeyer, G. (2007). Microstructure and mechanical properties of Fe₃Al - based alloys with strengthening boride precipitates. Intermetallics 15 (9), 1172-1182. https://doi.org/10.1016/j.intermet.2007.02.005.). However, such defects may result in an easy glide of dislocation planes at high temperature but not at low temperatures. Assuming that such defects were retained until room temperature due to having a sufficiently high cooling rate, the hardness is believed to be well affected by such defects (Schaefer et al., 1999Schaefer, H-E., Frenner, K., Würschum, R. (1999). High temperature atomic defect properties and diffusion processes in intermetallic compounds. Intermetallics 7 (3-4), 277-287. https://doi.org/10.1016/S0966-9795(98)00121-6.; Terada et al., 2002Terada, Y., Ohkubo, K., Mohri, T., Suzuki, T. (2002). Thermal conductivity of intermetallic compounds with metallic bonding. Mater. Trans. 43 (12), 3167-3176.).

Ultrasonic Test Results and Post Weld Coating Microstructures

The interpretation of the ultrasonic test (UT) graphics obtained from the RSW specimens is given in Fig. 5. All contact surface echoes that exceed 80% were measured for all weld nuggets. Ni₃Al/66 V, Ni₃Al/112 V, NiAl/66 V and Fe₃Al/66 V spot welds were found to be the most homogenized weld nuggets, showing logarithmic and noiseless reduction, respectively. The worst performance was from FeAl/158 V RSWs. The deformations, in terms of RSW thickness detected because of the UT tests, are plotted in Fig. 6. If the threshold limit is set below 30% as a risk percentage and set it as 1.05 mm for the combined total thickness of 1.5 mm, all welded samples made with electrode coated with 66 V are suitable for pass mark, while only those coated with Ni-based aluminide intermetallics at 112 V can be considered suitable for the ESD coating condition; however, the performance of the coatings may differ in practice, hence, it should be tested in situ. The most striking finding is that the coating voltages have a direct effect on the RSW deformation, with respect to the type of coating electrode material, and the type of electrode has a profound effect on the nugget cross section thickness of RSW. Figure 6 shows that the lowest cross section thickness is obtained with FeAl and Fe₃Al with the exception of Ni₃Al at the coating voltage of 158 V. This behaviour may be related to the thermal conductivity of the coating. The thermal conductivity of NiAl is approximately 80 W/mK while it is less than 25 W/mK for the Ni₃Al alloys. The thermal conductivity of the Fe₃Al is lower than that of the FeAl, which is lower than that of the NiAl and the Ni₃Al (Athi et al., 2009Athi, N., Cullen, J.D., Al-Jader, M., Wylie, S.R., Al-Shamma’a, AI., Shaw, A., Hyde, M. (2009). Experimental and theoretical investigations to the effects of zinc coatings and splash on electrode cap wear. Measurement 42, 944-953. https://doi.org/10.1016/j.measurement.2009.02.001.). Hence, the coatings with Fe aluminides have lower thermal conductivity than the Ni aluminides. This would affect the heat transfer from the weld zone and create a barrier for heat flow towards the cooled electrode caps (Bozkurt et al., 2016Bozkurt, A., Çakmakkaya, M., Çetkin, A., Talaş, Ş. (2016). Heat transfer and electrical characteristics in spot welding with composite coated caps. Proc. International Conference on Welding Technologies and Technology, (ICWET’16), Ankara.). Figure 6 show that the low thermal conductivity of the Fe aluminides produces a lower cross-sectional thickness for 112 V and 158 V coatings except for the NiAl and the Ni₃Al (for 158 V is lower). For the Ni₃Al-158 V case it is obvious that the thickness of the coating was greater than the rest of the coatings. This results in a low heat conductivity from the heat generating interface, i.e. lap joint, towards the RSW caps and, hence, causes the accumulation of heat at the nugget zone, increasing the heat in the weld region and making it more plastically easy to deform. Similar mechanisms are valid for the low thermal conductivity alloys, i.e. Fe aluminides, as it is shown in Fig. 6. The best performance is observed with NiAl, which has the highest thermal conductivity amongst the aluminide type intermetallic alloys studied in this work.

Macrostructural and microstructural analysis of resistance spot welds by coated caps

The cross-sectional images of the 30^th RSWs are shown in Fig. 7. A clear nugget structure was observed in welded samples made with the Fe₃Al/66 V, the Ni₃Al/66 V, the Ni₃Al/112 V and moderately with the NiAl/112 V and the NiAl/66 V caps. Among them, the most homogeneous one was observed in the RSW with the Ni₃Al/112 V. Microstructural examinations of the Fe₃Al/112 V, the Fe₃Al/158 V, the FeAl/112 V, the FeAl/158 V, the Ni₃Al/158 V and the NiAl/158 V showed that excessive nugget deformations were observed within the melting zone extending to the surface.

The microstructures of the coated caps taken at 100x magnification from the cross-section are shown in Fig. 8. The average values of the coating thicknesses obtained from the optical images were analyzed and given in Fig. 9. In addition, the coating topologies were examined and the differences between the coatings were compared. All coated electrodes show a good metallurgical bonding with the copper base metal and mostly homogeneous and continuous line of coating is present along the coating layer. The phase transformation points of these aluminide intermetallics alloys are important in their behaviour. Intermetallics have order-disorder transformation points of 1310 ºC for the FeAl, 552 ºC for the Fe₃Al, 1300 ºC for the Ni₃Al and 1630 ºC for the NiAl alloy (Schaefer et al., 1999Schaefer, H-E., Frenner, K., Würschum, R. (1999). High temperature atomic defect properties and diffusion processes in intermetallic compounds. Intermetallics 7 (3-4), 277-287. https://doi.org/10.1016/S0966-9795(98)00121-6.; Krein et al., 2007Krein, R., Schneider, A., Sauthoff, G., Frommeyer, G. (2007). Microstructure and mechanical properties of Fe₃Al - based alloys with strengthening boride precipitates. Intermetallics 15 (9), 1172-1182. https://doi.org/10.1016/j.intermet.2007.02.005.). Most intermetallics are profoundly stable in their properties below their order-disorder transition temperature and, hence, high hardness and high elasticity module are common for such alloys at low temperatures, in this respect, the temperatures around 200 ºC at the electrodes are not greatly high to defer their properties, however, at the contact point the temperatures are very high and the properties of the intermetallic layers are not preserved on cooling (Mikno and Bartnik, 2016Mikno, Z, Bartnik, Z. (2016). Heating of electrodes during spot resistance welding in FEM calculations. Arch. Civ. Mech. Eng. 16 (1), 86-100. https://doi.org/10.1016/j.acme.2015.09.005.). However, long heat treatment regimes are needed to obtain fully intermetallics structures, fast cooling of coating layer just after RSW may not be sufficient to conserve such properties. This may not be valid in the NiAl system because of its high order-disorder temperature but the rest of the intermetallic coatings are not fully functional in ordered state unless the cooling medium is sufficiently effective in reducing the interface temperature. Figures 8 and 9 show that because the post weld thickness of the Ni₃Al intermetallic layer on copper cap is relatively high, this may have caused an increase in interface resistance and temperature. Such an increase in the interface temperature would increase the plasticity of the metal sheets being joined, reducing the thickness of lap joint by the deformation. The examination of the coatings on copper caps shows that the coating thicknesses obtained with high voltages are usually higher than the other coating voltages. Among the caps coated with 66 V, the most different topology was observed in the Ni₃Al, while the highest value of the coating layer was measured in the FeAl/66 V. Among the caps coated with 112 V, the most different topology was also seen in the Ni₃Al, and scattering in thickness and microcracks were observed in the coating layer. The highest coverage was also measured in the Ni₃Al. Material loss and microcracks were observed in the FeAl and the Ni₃Al between the caps coated with 158 V. Except for the NiAl, coating thicknesses were measured at similar levels for other coatings. The most stable coating increase was observed in the Fe₃Al and the NiAl coatings, and no irregularities were observed in these two coatings in terms of coating behaviour and topology. As it is seen in Fig. 10, Zn appears to have diffused across the coating into the matrix in most electrodes, even though the coating layer is formed by the accumulation of micro droplets unlike of such as powder metallurgy products (Biliz et al., 2020Biliz, İ., Bakkaloglu, A., Kilic, M. (2020). The effect of process parameters on microstructure and porosity of layered NiAl(Co/Cr) alloy produced by SHS method. J. Polytech. 23 (1), 161-169. https://doi.org/10.2339/politeknik.557592.). The Fe aluminide coatings are less effective with preventing Zn diffusion into the coating first and then the Cu matrix. However, the Ni based coatings are more effective in such a way that Zn diffusion as much as Ni has been observed in the Ni₃Al series. It can be noted that the Ni₃Al is more resistant to Zn diffusion from the surface through the coating layer because the Zn line scan does not penetrate to the matrix as did in the NiAl and the Fe-Al series. This would in practice increase the life span of the Cu electrode which is prone to cracking and erosion by Zn diffusion from the Zn coated steels (Bhattacharya, 2018Bhattacharya, D. (2018). Liquid metal embrittlement during resistant spot welding of Zn-coated high-strength steels. Mater. Sci. Technol. 34 (15), 1809-1829. https://doi.org/10.1080/02670836.2018.1461595.). In the NiAl series, Zn diffusion is similar to FeAl and Fe₃Al series, suggesting that Al content may be related to the diffusion capacity of Zn in the Cu matrix. The material loss from the surface is more pronounced in the FeAl/112 V, the FeAl/158 V series and the Fe₃Al/158 V specimen. There are more cavities in the FeAl/112 V and FeAl/158 V which may be a result of the high temperature corrosion and the Zn-rich phase erosion due to compaction pressure active upon the coating layer during the RSW operation. It is noted that the Fe based intermetallic aluminide alloys are in general more resistant to sulfidation than oxidation at high temperatures compared to the Ni based intermetallic aluminide alloys (Talaş, 2018Talaş, Ş. (2018). Nickel Aluminides. in, Intermetallic matrix composites. R. Mitra (Ed.), Woodhead Publishing, pp. 37-69. https://doi.org/10.1016/B978-0-85709-346-2.00003-0.). Different thermal properties such as the thermal expansion capacities of matrix and the coating layer also play an important role in the formation of cracks and cavities. The ESD coating layer is formed differently such that the ESD deposition is layered, and each micro arc is practically formed in a manner of brick lying. This would pose a problem when the dynamic forces are in question since the layers do not behave as a bulk metal, acting as either soft or very hard region with a weak interface compared to bulk metals. Such weakness can be seen as a source of degradation leading to cavities and fall-outs as shown in Fig. 10, that is, the NiAl 158 V series. As it is seen in the NiAl and the Ni₃Al series (Fig. 10), there is also a tension crack between the matrix and the coating, which may be due to the differences in the thermal properties of the coating interface components. The coefficient of thermal expansion values for the FeAl and the Fe₃Al are 19.5x10^-6 K^-1 and 25x10^-6 K^-1, while the coefficient of thermal expansion values are 23.5x10^-6 K^-1 for Cu, 18x10^-6 K^-1 for Ni, 15x10^-6 K^-1 for NiAl and 19x10^-6 K^-1 for Ni₃Al (Wang and Reeber, 1996Wang, K., Reeber, R.R. (1996). Thermal expansion of copper. High Temp. Mater. Sci. 35, 181-186.; Wang et al., 2004Wang, Y., Liu, Z-K., Chen, L-Q. (2004). Thermodynamic properties of Al, Ni, NiAl, and Ni3Al from first-principles calculations. Acta Mater. 52 (9), 2665-2671. https://doi.org/10.1016/j.actamat.2004.02.014.; Masset et al., 2009Masset, P., Texier, D., Schütze, M. (2009). Coefficients of thermal expansion of (Ni0.5Al0.5) (1-x)Hfx alloys (x= 0...0.2). Mater. Sci. Technol. 25 (7), 874-879. https://doi.org/10.1179/174328408X372083.; Švec et al., 2013Švec, M., Hanus, P., Vodičková, V. (2013). The coefficient of thermal expansion of Fe₃Al and feal - type iron aluminides. Manuf. Technol. 13 (3), 399-404. https://doi.org/10.21062/ujep/x.2013/a/1213-2489/MT/13/3/399.). It is likely that Cu-NiAl combinations are more prone to contraction stresses during the RSW process with fast heating and cooling conditions.

Chisel Test Results of Resistance Spot Welds of Coated Electrode Caps

Figure 11 shows the specimens after the destructive tests. The RSW nuggets with high rupture resistance were observed for the Ni₃Al/66 V, the Ni₃Al/112 V, the NiAl/66 V, and the NiAl/112 V, respectively. The Ni₃Al/158 V and the NiAl/158 V RSWs broke off more easily in a shear mode compared to the 66 V and the 112 V. The ruptures in the welded sample occurred with the formation of tearing at the edges of the RSW nugget, showing some ductility. It was observed that the RSW nuggets joined with the FeAl coated caps were also smaller in size, supporting the UT test. In the Fe₃Al tests, the RSW made with the 112 V and the 158 V coated caps resulted in premature rupture with an appearance that the borders of the nuggets were not clearly defined. The thickness of the coating in the 158 V series implies that, as the thickness of the coating increases, the joule effect due to the heating by electric current is less effective in producing sufficient heat at the interface of two mating sheet metals, leading to the insufficient formation of the weld nugget core and, hence, a premature failure.