ORIGINAL ARTICLE

Qualitative and quantitative analysis of misaligned electrodedegradation when welding galvannealed steel

Bobin Xing1& Shaohua Yan1

& Haiyang Zhou1& Hua Chen2

& Qing H. Qin1

Received: 13 December 2017 /Accepted: 23 March 2018 /Published online: 7 April 2018# Springer-Verlag London Ltd., part of Springer Nature 2018

AbstractElectrode misalignment, induced from the flexibility of welding machine gun of spot welder or the potential poor fit-up, leads tothe high tendency of expulsion and asymmetric nugget shape. Yet very few studies have focused on the influence of misalign-ment on the electrode degradation and microstructure evolution of the electrode, especially when welding galvannealed steel.This paper investigates the degradation of such misaligned electrodes when spot welding galvannealed steel. To reveal itsmechanism, the electrodes welded with galvannealed steel were examined after 10 and 200 welds with the slight misalignment.The electrodes were found to experience more severe degradation compared to the results from previous studies with the alignedelectrodes. The results from energy-dispersive spectrum (EDS) analysis further confirmed that δ-Fe–Zn phase, a barrier fromgalvannealed coating for isolating Cu and Zn formation, was not uniformly distributed on the electrode tip. As a result, the initialelectrode pitting took place after 50 welds. Furthermore, electron backscatter diffraction (EBSD) quantitatively analyzed therecrystallized grains of the worn electrodes, which underwent rotation under the asymmetric pressure distribution under mis-alignment. The calculated Taylor factor via EBSDmapping also indicated the declined portion of <111> grains accounted for thelow deformation resistance of the worn electrode. Finally, electrode displacements were simultaneously collected in the exper-iments, of which the peak values accurately predicted the heat generation for each spot weld and accordingly predicted theelectrode life.

Keywords Resistance spot welding . Electrode wear . Electrode misalignment . Electrode displacement . Electron backscatterdiffraction

1 Introduction

Resistance spot welding (RSW) is extensively utilized forjoining metal sheet in the automobile industry. Due to its com-bined high yield strength and ductility, steel has been used askey components, such as A and B pillars, in the body-in-whitestructure. Galvannealed Fe–Zn alloy (GA) is coated on steelfor higher corrosion resistance. It comprises several layers of

Fe–Zn intermetallic compounds after annealing at 500 °C [1].However, the electrode life when welding zinc-coated steel ismuch shorter than that when welding bare steel, which ismeasured by the number of welds made before any correctiveactions on the electrodes [2]. Frequent electrode tip dressingnot only affects the productivity in the plant environment butalso leads to reduced number of spot welds produced beforereaching the minimum height for the electrode tip.

Electrode mushrooming and pitting, due to electrode soft-ening and the formation of alloy compound, have been sug-gested to be the primary issues during the welding of zinc-coated steel [3]. Parker discussed the electrode wear mecha-nism of welding hot-dip galvanized steel [4]. The formation ofβ-brass and γ-brass on the electrode was elucidated, and thegradual softening of the electrode against the welds producedwas disclosed with the microhardness test. Holliday investi-gated the electrode softening in-depth, and he found the re-crystallizing process was predominant in hardness reduction[5]. Williams found that the electrode life obtained for Fe–Zn

Electronic supplementary material The online version of this article(https://doi.org/10.1007/s00170-018-1958-1) contains supplementarymaterial, which is available to authorized users.

* Qing H. QinQinghua.Qin@anu.edu.au

1 Research School of Engineering, The Australian National University,Acton, ACT 2601, Australia

2 Center for Advanced Microscopy, The Australian NationalUniversity, Acton, ACT 2601, Australia

The International Journal of Advanced Manufacturing Technology (2018) 97:629–640https://doi.org/10.1007/s00170-018-1958-1

alloy steel was longer than that for conventional hot-dip zinc-coated steel [2]. The electrode life was mainly determined bythe composition of the Fe–Zn coatings. Hu compared the mi-crostructures of worn electrodes welded with three differenttypes of GA steel, where the composition of Fe–Zn phasevaried [6]. The layers formed on the surface of the electrodetip were determined by the content of GA coating, as theoutermost layer of the GA coating can be picked up by theelectrode. The Fe-rich phase, such as δ Fe–Zn, functioned as aphysical barrier to prevent the fast alloying of brass phase,allowing longer electrode life than that for galvanized steelwelding. In addition, a number of studies have been devotedto understanding the electrode wear mechanism in GA steels,including GA coating on hot stamping steel [7], alternativeweld schedule between GA steel and bare steel [8], and con-tact region characterization of worn electrodes [9]. In the plantenvironment, electrode misalignment is usually induced bythe flexibility of the welding machine arm for positioningthe electrodes and the mechanical deformation of the sheets(poor fit-up) [10]. Howe investigated the short electrode life ofsteel under electrode deflection [11]. In addition, Tang usedbare steel and galvanized steel to investigate the influence ofwelding machine stiffness [12]. However, the effect of elec-trode misalignment on electrode life and its mechanism ingalvannealed steel is not fully understood.

Electrode misalignment has been found to substantiallyaffect the welding quality in RSW. It can be divided into axialmisalignment and angular misalignment, as shown in Fig. 1,where the rotation and off-centering of the electrodes arefound. Misalignment in electrodes results in a much narrower

weldability lobe, and more heat is required to produceadequate-sized nugget [13, 14]. Moreover, electrode forceneeds to be moderate to avoid producing expulsion or under-sized welds. The electrode misalignment affects the value anddistribution of the force in the workpiece at the expulsion,especially for angular misalignment [15, 16]. According toISO 18278-2:2004, the acceptable alignment can be identifiedbased on the carbon imprint, where the difference in the hor-izontal length L1 and vertical L2 is less than 1 mm [17].Though the magnitudes of rotation and off-centring are small,their impacts on the heat generation are substantial. Nielsenshowed that the asymmetric nugget was formed due to theelectrode misalignment in the three-dimensional numericalsimulation [14]. However, the joint strength is not consider-ably reduced by the electrode misalignment via numericalsimulation and experiments. In addition, Kim suggested thateven a tiny magnitude of misalignment might result in anuneven temperature profile [18]. From the existing studies,the electrode misalignment is considered to influence the in-termetallic compound (IMC) formation rate and IMC compo-sition due to distinct peak temperature at the tip surface, espe-cially at the edge of the tip. Moreover, the temperature-relatedrecrystallization process is likely to be different on themisaligned electrode. As δ Fe–Zn from GA coating is key toprolong the electrode life, electrode life of galvannealed steelmight be considerably affected by the electrode misalignment.

Though alloying formation and recrystallization take placein a short time for every weld, the alloying product will accu-mulate on the electrode tip, and the tip will grow in the axialdirection. The appearance of the electrode will be affected bypitting and mushrooming. The intensities of current on thefaying surface and electrode/workpiece surface are affected;as a result, small nugget or no melting is seen. Imaging pro-cessing and online monitoring techniques have been carriedout to characterize the electrode wear. Image processing meth-od includes carbon imprints of the electrode [19], outlines ofthe electrodes [20, 21], and the indentation marks [22, 23].Electrode displacement (ED), induced by thermal expansionof the sheets, was carried out to monitor electrode wear [24].The signals collected from the worn electrodes were substan-tially distinct those from the brand-new electrodes. The peak

Fig. 1 Schematic illustration of electrode misalignment. a Axialmisalignment. b Angular misalignment

Table 1 Chemical composition (wt%)

Material C Si S P Mn Al Fe

G2S 0.035–0.07 ≦ 0.02 ≦ 0.02 ≦ 0.02 0.20–0.30 0.02–0.07 Bal.

Table 2 Welding parameters used in this study

Welding current Welding time Electrode force Hold time

11 kA 200 ms 2.7 kN 500 ms

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value of both the ED curve and the ED velocity were stronglyrelated to the nugget development [25]. Alternatively, theimage-based electrode displacement showed a better accuracyin revealing the actual thermal expansion from the basemetals, where the contribution of electrode tip was eliminated[26, 27]. In addition, the temperature of the electrode, mea-sured by the thermocouple, was also used as an index formonitoring the electrode wear in austenite steels [28].Although ultrasonic C-scan is widely used to determine thefinal nugget size in the plant environment, the misalignmentof the electrode could make ultrasonic C-scan inaccuratelymeasure the nugget diameter. [29]. Thus, it is excluded fromthe electrode wear characterization in this study, as electrodetip appearance becomes irregular when the electrode ap-proaches the end-of-life.

In this study, effects of electrode misalignment on the elec-trode wear during welding of GA steel are investigated. Anendurance test with galvannealed steel is carried out. Carbonimprint, electrode outline, and electrode displacement signalare collected throughout the tests to evaluate the electrodewear under the misalignment. The recrystallization and IMCformation affected by the misalignment are characterized viaoptical microscopy (OM), scanning electron microscope(SEM), and electron backscatter diffraction (EBSD) analysis.This study further develops the understanding of electrodedegradation of galvannealed steel in the plant environment,where the perfect electrode alignment is challenging.

2 Methodology

2.1 Materials

One-millimeter-thick G2S galvannealed steel was used.Chemical compositions of the galvannealed steel are listedin Table 1. The yield strength and ultimate tensile strengthare 290 and 350 MPa, respectively. The minimum zinc massof the coating is 100 g/m2. Six-millimeter-diameter Cu–Crtruncated electrodes were used, where the raw copper barwas manufactured via cold extrusion.

2.2 Experimental procedure

The endurance test was carried out on a single-phase ACpedestal welder, with a frequency of 50 Hz. The welding pa-rameters listed in Table 2 were designed for producingacceptable-sized welds. The weld rate was 10 welds/min.The welding machine arm for positioning the upper electrodewas manually rotated to 5° to create angular electrode mis-alignment due to the flexibility of the spot welder. A largemisalignment angle could result in expulsion in the work-piece, which is not considered in this study. The level of elec-trode alignment was characterized via the carbon imprints, as

suggested in ISO 18278-2:2004 [17]. The carbon imprint ofthe electrode alignment is presented in Table 3, where a min-imal difference among (L1–L2) meets the criteria of electrodealignment in ISO 18278-2:2004. The tensile-shear specimenswere prepared with the size of 100 × 25 mm. For every 50welds, 5 tensile-shear specimens were tested under anInstron 4505, with the crosshead velocity of 5 mm/min. Ifthe averaged tensile-shear strength dropped to 80% of theinitial value, the electrodes were considered to reach theend-of-life.

The experimental set-up for signal collection is shown inFig. 2. Electrode displacement, welding voltage across elec-trodes, and welding current were simultaneously recorded.The electrode displacement system consisted of a MICRO-EPSILON non-contact laser displacement sensor(optoNCDT 1402-5) and an objective beam. Rogowski coilwas used to record the current waveforms, and the electricalpotentials of electrodes were recorded from the secondarycircuit. The sampling frequency was 10 kHz. The originalED oscillated at the frequency of 100 Hz corresponding tothe changed temperature of the sheets, which is twice of thatof the electric current [30–32]. A low-pass Butterworth filterwas adopted to remove the components of signal oscillation.Carbon imprints of the electrodes were taken for misalignmentcheck on the basis of 50 welds during the test. The binarycarbon imprints were then analyzed under ImageJ to calculatethe tip area and tip diameter [33]. Pictures at side view and topview of the electrodes were captured with a CCD camera atthe end of electrode life. Active contour segregation method(Snake Active Contour package) was applied on the capturedimages to track the electrode outline under Matlab [20, 34].Moreover, the width of the electrode tip, a parameter to char-acterize the level of mushrooming, was calculated usingCullen’s method [21].

2.3 Metallography and microstructure evolution

The cross-sections of spot-welded samples were examined at50-weld intervals. The worn electrodes were prepared by thestandard metallurgical method for OM and energy-dispersiveX-ray spectroscopy (EDS) at 10 and 200 welds. The samplesfor EBSD were grinded/polished with SiC papers and 3 and1 μm diamond suspension, then the 0.02 μm colloidal silicasuspension was used for the final mechanical-chemicalpolishing. The etching reagent (25 ml ammonia + 5 ml hydro-gen peroxide + 25 ml distilled water) was used to reveal thegrain boundary and recrystallization regions of the copper

Table 3 Initial configuration of the electrode alignment

L1 (horizontal) L2 (vertical) Carbon imprints

5.96 mm 5.86 mm

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electrodes. Four percent nital reagent was used to etch GAsteel.

The samples were analyzed under a stereomicroscope forlow-magnification images and a light microscope for high-magnification images. Hitachi 4300 with Oxford X-MaxEDS detector was used to quantitatively characterize the basemetals and the electrode after welding. The compositions ofthe IMCs were analyzed with Oxford Inca software. Theworking configurations for EDS were an accelerating voltageof 15 kV, a working distance of 25 mm and probe current of600 pA. The EBSD analysis was carried out under a ZeissUltraplus FESEM with Nordlys Camera, where a workingdistance of 25 mm, beam current of 4 nA, and an acceleratingvoltage of 15Vwere implemented duringmapping. To reducethe scanning time, the step sizes were selected as 1 and0.25 μm for the base metals and recrystallization regions,respectively. The rolling direction (RD) was perpendicular tothe electrode surface in this study. Lastly, the Vickers micro-hardness of the worn electrodes was measured by a micro-hardness tester (MHT-1, Matsuzawa), with a load of 100 g

and 15 s load time. The positions measured in Vickers micro-hardness test are demonstrated in Fig. 3, in which the distancebetween every two indentation marks was 0.5 mm.

3 Results

3.1 Electrode life of the misaligned electrodes

Electrode life of the misaligned electrodes was determined viathe carbon imprints of the electrodes and the tensile shearstrength. Table 4 presents the carbon imprints of the upperelectrode throughout the endurance test. At 0 welds, the mis-alignment was introduced at the beginning of the electrode lifevia adjusting the machine gun of the spot welder. The upperelectrode quickly developed small voids at the center of theelectrode at weld number of 50, which is faster than the cavityformation at 200 welds in the previous study [35]. The smallvoids at the center further enlarged in axial directions alongthe test, and the final diameter of the cavity was approximatedto 2.2 mm at 200 welds. The side views of the CCD imagesare presented in the supplementary material, where the annu-lus electrode morphology was not discernible from the sideview.

Fig. 2 Experimental set-up

Fig. 3 Schematic diagram of the positions of microhardness tests

Table 4 Carbon imprints of the upper electrode from 0 to 200 welds

Weld Number Carbon Imprints

0

50

100

150

200

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The changes in electrode tip morphology and the tensileshear strengths are summarized in Fig. 4. At 200 welds, theweld strength dropped to 80% of the initial value, which wasconsidered as having reached the end-of-life. The area of theelectrode tip decreased with an increase in the number ofwelds, while the tip diameter slightly increased. Considerablepitting at the center of the tip was observed, due to the pickingup of the brittle intermetallic compounds by the electrodes.Limited mushrooming was found in the carbon imprints dueto the reduced electrode life.

3.2 Electrode displacement curves of worn electrodes

Electrode displacement was directly recorded and processedfrom the laser triangulation sensor. Figure 5 depicts the elec-trode displacement signals of GA steel, from which distinctcurves are observed due to electrode wear. The thermal ex-pansion of the sheet metals directly contributed to the elec-trode displacement. Electrode current intensity increased atthe contact region between the electrode and workpiece, dueto the severe pitting. However, it is noted that the peak valuesof the ED curves gradually declined with increasing number

of welds, as shown in Fig. 5a. In addition, the heat generatedper unit time was found to be proportional to electrode dis-placement velocity in Fig. 5b. The potential reason for de-creasing thermal expansion is presented in the Sect. 4.

4 Discussion

The electrode misalignment led to a much shorter electrodelife than those from previous studies [8, 36–38]. The rapiddecline in the joint strength possibly affected by the electrodemisalignment is firstly discussed in Sect. 4.1. The IMC for-mation and softening influenced under the electrode misalign-ment are also presented in Sects. 4.2 and 4.3, respectively.

4.1 The influence of electrode misalignmenton the weld strength

The electrode misalignment led to a short electrode life whenwelding GA steel. An apparent decline in the joint strengthand load-bearing capacity was found from 0 to 200 welds inSect. 3.1, as shown in Fig. 6. Joint strength refers to the peak

Fig. 5 a Electrode displacement signals. b Electrode displacement velocity

Fig. 4 a Tip diameters and areas from the carbon imprints. b Tensile shear strengths

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load in the tensile shear test, and load-bearing capacity de-notes the area under the curve before reaching the peak loadin the tensile shear test [39]. At 0 welds, an uneven indentationmark was observed due to the electrode misalignment, asmanifested in Fig. 7a. The nugget was found to be larger onthe left side of the cross-section due to the shorter electricalcurrent path [12]. No void was found at the same side of thenugget. At 50 welds, the joint strength suddenly dropped. Avoid shown in Fig. 7b was found on the left side of the nuggetas a consequence of electrode misalignment. In the tensileshear mode, the nugget size determines the predominant fail-ure mode of the joint [40]. The void at the nugget edge con-siderably reduces the effective diameter of the fusion zone,which in turn undermines the load-bearing capacity in pull-

out mode. The tensile strength was found to drop again at 150welds, where voids were found at the edge and center of thenugget. The relatively small void at the center does not sub-stantially affect the joint strength [41, 42]. Wang identified thecritical pitting diameter in electrode wear of steel welding[43]. Electrode wear contributed to solidification voids at thecenter of the faying surface. The little current was transferredat the center of the faying surface when the pitting was greaterthan 3 mm. As a result, a substantially large solidification voidwas formed. In this study, the final cavity diameter at 200welds was 2.2 mm, indicating the electrode wear has a minorinfluence on the nugget strength.

In this study, the electrodes underwent severe pitting andminor mushrooming before reaching the end-of-life. The

Fig. 7 Cross-sections of thenuggets. a 0 welds. b 50 welds. c,d Side and center at 150 welds

Fig. 6 Load-displacement curvesFig. 8 SEM image of the GA coating

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cavity in the electrode tip induced a wider spread of the cur-rent, resulting in low current density and less heat generated.Nevertheless, the ED curves, a measure of thermal expansionof the sheets, showed no increase in the peak value. The centerregion with severe pitting did not have any contact with theworkpiece; little current flow through the region. This regionwas melted by the heat conduction from the surrounding con-tact regions. Based on Wang’s numerical simulation, the vol-ume of fusion zone was inversely proportional to the cavitysize [43]. Thus, the thermal expansion of the base metals isexpected to decline in the worn electrodes, which matches theobserved ED curves.

4.2 The influence of electrode misalignmentof alloying product on the tip surface

To predict the potential IMC formation, the coating layer ofthe base metal was examined via SEM and EDS. The GAcoating thickness was around 13 μm, which was averagedfrom ten measurements in Fig. 8. The EDS results were sum-marized in Table 5. Due to annealing of the zinc layer at500 °C, the GA coating consisted of δ, δ + Γ1, and Γ1, where

the content of Fe gradually grew. δ phase was considered as aFe-rich barrier to prolong the electrode life during welding ofgalvannealed steel [36].

The affected tip surfaces were observed via CCD camera.The top views of the electrode at 10 welds and 200 welds arepresented in Fig. 9a, b. The appearance of the worn electrodeshad moderately changed from the brand-new electrode. Novisible mushrooming or pitting could be found on the elec-trode tip at 10 welds. The gray layers are observed on theelectrode surface, suggesting potential alloying of base metaland electrode. The worn electrodes were sectioned at the cen-ter of the electrode tip as shown in Fig. 9c, d, and two sides ofthe worn electrodes were used for EDS and EBSD, respec-tively. Thus, they were mirror symmetric in the grain struc-tures and IMC layers. The projected temperature lines wereadopted from the simulated temperature fields, suggesting thepotential shape of the recrystallization zone [14].

To reveal the composition of IMC products at differentregions of the worn electrode, an EDSmapping of the selectedregions of the electrode at 10 welds is presented in Fig. 10.Different alloying products are found at two edges of the elec-trode due to an uneven temperature profile. In Fig. 10b, theFe–Zn phase was found on the outermost layer of the GAelectrode, which came from the zinc coating of the basemetalsas reported in Hu’s study [36]. However, the δ Fe–Zn phasewas limited to the tiny outermost layer as shown in Fig. 10a,where inefficient localized heat could not reach the meltingpoint of the Fe–Zn phase. In conventional steel spot welding,the temperature at the tip surface could reach 600–700 °C, andthe melting temperature of δ Fe–Zn is around 660 °C [36].

Fig. 9 Top views of the wornelectrodes. a 10 welds. b 200welds. c Schematic diagram of thecross-section of the wornelectrode for OM and EBSD. dSchematic diagram of the cross-section of the worn electrode forEDS

Table 5 Chemical compositions in the GA coating

Site of point Fe (wt%) Zn (wt%) Possible phase

A 10.6 89.4 δ

B 13.6 86.4 δ + Γ1

C 23.5 76.5 Γ1

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Electrode misalignment made the δ phase partially melt andaccumulate on the electrodes. Minimal Fe content could befound in the middle and innermost layers of the electrode. Thethicknesses of alloy products in Fig. 10a, b were measured tobe ~ 5 and ~ 15 μm.

Severe electrode pitting took place at 200 welds with GAsteel. The cross-section of the electrode and the SEM results ofedges of the electrode tip at 200 welds are manifested inFig. 11. It is found that recrystallization took place and adistinct boundary between the base metal and recrystallizedregion formed, similar to the microstructures previously re-ported [44]. Two regions (marked as α and β in Fig. 11a)located at the edge of the electrode tip were further investigat-ed with the SEM and EDS analysis. It is noted that differentalloy compositions were formed in two regions depending on

the local temperature field and contact condition. The aver-aged thickness of the alloy layers at α and β grew to 10.5 and29.5 μm, respectively, which is considerably wider than thoseat 10 welds. Multiple layers were identified in each region viaEDS analysis. Data in Table 6 were from the analysis of eachlayer based on the BSE images of the specimens. Region βwas found to attain Cu–Zn alloys, and Fe–Zn phase was alsopicked up from the base metals, while region α only com-prised β + γ Cu–Zn at point A and α +β Cu–Zn at point B.Though the δ Fe–Zn phase was identified from the GA coat-ing, region α showed a restricted distribution of Fe, whichindicates the barrier with low Fe cannot effectively protectCu from the pickup of the base metal. Similar to sample no.2 in Hu’s study [36], a much faster wear-out is expected toinitiate in those Fe-free regions.

Fig. 11 Metallographicexamination of the worn electrodeat 200 welds. a stereo microscopeexamination at × 15magnification. b SE image atregion α. c BSE image athighlighted area. d SE image atregion β

Fig. 10 a SEM image of region α of the worn electrode at 10 welds. b SEM image of region β of the worn electrode at 10 welds

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4.3 The influence of misalignment on softeningmechanism

The temperature field in the regions of the electrode tip underelectrode misalignment was non-uniform. To investigate theinhomogeneous softening, Vickers microhardness tests werecarried out on the etched cross-sections of the electrodes.Figure 12a compares the microhardness of the cross-sectionsof the worn electrodes at 10 and 200 welds. It is noted thatsoftening in the electrode was proportional to the number ofwelds, regarding the reduction of hardness and the softeningarea affected. A substantial reduction in hardness after 10welds was observed at measurement line of 0.1 mm awayfrom the electrode tip, and the softening region gradually en-larged with the number of welds.When 200 welds were made,the softening zone reached ~ 1 mm. Furthermore, the influ-ence of misalignment on softening was also examined bymicrohardness test as shown in Fig. 12b, c. The electrodemisalignment substantially influenced the distribution of elec-trode hardness. A moderate reduction in the electrode hard-ness was found at the right side of the electrode when welding10 samples since more heat was generated due to the misalign-ment. A strong oscillation in the hardness profile was noticedalong the electrode tip at 200 welds. The electrode experi-enced severe copper pickup at the center of the tip, and theelectrode surface gradually developed more asperities. Thus,the peak temperature of electrode significantly increased at the

localized asperities and thereby aggregated softening effect onthese sites.

From Fig. 11a, a clear boundary between the base metaland recrystallization region is seen. Recrystallization processaccounted for the softening of the worn electrode. The uneventemperature field introduced by electrode misalignment alsoaffected the grain morphology in the recrystallization region.Previous studies only qualitatively examined the recrystal-lized regions with OM images. To quantitatively understandthe microstructure evolution under the electrode misalign-ment, EBSD mapping was implemented on the cross-sections of the worn electrodes at 10 and 200 welds, as dem-onstrated in Fig. 13. Several recrystallization regions of inter-est are marked in the OM images in Fig. 13a, b. Regions (f–g)were taken at the same distance from the electrode surface,while region (f) was closer to the electrode contact region,where a high temperature was attained at the contact regionof the misaligned electrodes.

EBSD mapping revealed the recrystallization and grain ro-tation due to the electrode misalignment. Since the electrodewas manufactured from the cold extruded copper rod and noannealing was carried out before the welding, the base metalpresented a very strong fiber texture in <100> and <111>direction, as shown in Fig. 13c. A strong fiber texture <111>existed at the recrystallization region at 10 welds in Fig. 13e.Then, the fiber texture <111> kept weakening at 200 weldssince more grains with different orientations could be found inthe recrystallization regions shown in Fig. 13f–g. The aver-aged grain diameters in rolling direction (RD) and transversedirection (TD) were summarized in Table 7. Compared to basemetals, the grain diameter has drastically declined in RD di-rection from ~ 13 to 2.5 μm, and smaller grains could beobserved in the recrystallization regions, even after 10 welds.More importantly, a rotation in the grain orientation was read-ily seen in Fig. 13e, f over the asymmetric pressure under themisaligned contact. 2D finite element simulation on angular-misaligned electrodes showed that higher pressure wasattained in proximity to the shortest electrical current path,and an asymmetric pressure distribution was seen [12]. The

Fig. 12 a Averaged Vickers microhardness of the electrodes. The influence of electrode misalignment on microhardness. b 10 welds. c 200 welds

Table 6 Element compositions at the alloying regions on the electrodeafter 200 welds

Sites of point Fe (wt%) Cu (wt%) Zn (wt%)

A 10.9 32.5 56.4

B 0.8 52.7 46.3

C 0.4 97.9 1.1

D 2.5 37.8 59.7

E 1.1 54.4 44.2

F 51.4 16.8 31.4

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grains were re-orientated in the direction normal to the in-clined pressure.

The weakened texture in the recrystallized region requiredless deformation energy against the electrode force. To evalu-ate the deformation resistance of the grains from the wornelectrode, the Taylor factors along the electrode force direction(RD direction) of the mapped regions were calculated inFig. 14. The Taylor model assumes that the slip of polycrystalrelies on the five independent slip planes. Among all combi-nations of five slip systems, the active combination has theminimum value of accumulated slip. The Taylor model furthersuggests the slip systems are hardened at the same rate. For auniaxial deformed polycrystal, the incremental work per unitvolume due to the external stress σx:

dw ¼ τdy ¼ σxdεx ð1Þσx=τ ¼ dy�

dεx¼ M ð2Þ

where M is the Taylor factor, τ is the critical resolved shearstress (CRSS) for slip on all slip systems, dw is work per

volume with a grain, dεx is the incremental strain in the uni-axial deformation direction, and dy is the incremental slip onindividual slip system.

Distinct distributions of the Taylor factors are observed,and they give an insight on how the recrystallization tunedthe mechanical properties of the electrode tips. The partitionfractions of the mapped regions at different Taylor factorranges are summarized in Table 8. The Taylor factor in thisstudy is calculated based on the slip system <111>/{110} inface-centered cubic (FCC) metal. For a randomly texturedpolycrystalline FCC metals, the Taylor factor under a uniaxial

Fig. 13 Etched cross-section of the worn electrodes. a 10 welds. b 200 welds. EBSDmapping of the base metal region. c 10 welds. d 200 welds. EBSDmapping of the highlighted areas of the recrystallization regions. e at 10 welds. f, g at 200 welds

Fig. 14 Taylor factors of the selected regions in Fig. 13

Table 7 Average grain diameters at the mapped regions in Fig. 13

Mapped regions (c) (d) (e) (f) (g)

d in RD direction (μm) 13.98 12.99 2.89 2.57 2.87

d in TD direction (μm) 6.06 4.94 3.62 2.58 2.55

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tension Mt is 3.06 [45]. The Mt value (3.06) matched one ofthe discernible peaks in Fig. 14. To some extent, it disclosesthe strength of texture in the mapped regions. The base metalin the electrodes attained a combination of fiber textures of<100> and <111> direction. As a result, the relative frequencyatM = 3.06 of the base metal at 10 welds is the lowest. Due tothe high temperature and inclined pressure experienced by theelectrode, the grains underwent dynamic recrystallization andthe <111> and <100> fiber texture weakened. This processwas reflected by the increased relative frequency at M = 3.06in the recrystallization regions (f–g). The <100> fiber texturewas weakened simultaneously, where the low relative fre-quency was found at all recrystallized regions (e–g).

Moreover, the distribution of the Taylor factor indicates thedeformation work required for a polycrystalline metal. Thegrains with high Taylor factor will require more deformationenergy [46–48]. The rotation of grains due to the inclinedpressure led to a substantial decline in the portion of <111>fiber texture. Thus, the fractions of the recrystallization re-gions (f–g) in Fig. 13 at higher Taylor factor (3.39 <M <3.67) are 31.97 and 26.7%, respectively, which is the lowestamong all the mapped regions. Compared to base metal re-gions with high deformation resistance, the recrystallizationregions around the electrode tip were likely to be deformeddue to the decreased portion of high Taylor factor.

5 Conclusion

Electrode misalignment was found to substantially affect theelectrode wear mechanism when welding galvannealed steel.The electrode life was much shorter under the preset misalign-ment than those with proper alignment. Following conclu-sions are drawn from this study:

& The voids formed at the edge of the electrode decreasedthe nugget diameter, causing the reduction in load-bearingcapacity of the nugget. On the other hand, the voids at thecenter of the nuggets due to annulus electrode morphologydid not reach the lower limit.

& The Fe-rich layer from GA steel welding, preventing di-rect alloying of Zn–Cu, was found to non-uniformly form

across the electrode tip surface. This phenomenon led tocopper pickup, electrode pitting, and ultimately annuluselectrode morphology.

& Recrystallization was the main mechanism for electrodesoftening when welding GA steel. The microstructures ofthe recrystallization regions were quantitatively analyzedby EBSD mapping, where the rotation of the grains tookplace via the asymmetric pressure distribution close to theshortest electrical current path.

& Some techniques to monitor the electrode wear whenwelding steel under electrode misalignment were present-ed. Image analysis such as carbon imprints captured elec-trode pitting. Besides, electrode displacement preciselydescribed the electrode wear on the misaligned electrodes,as the heat generated for nugget was sensitive to the elec-trode tip conditions.

A small magnitude of electrode misalignment may not re-sult in a substantial decline in the nugget strength at the be-ginning. However, it could lead to a moderate decrease in theelectrode life and slightly aggressive degradation mechanism.To prolong the lifespan of the electrode, it is important tointroduce an appropriate monitoring tool to distinguish theelectrode misalignment from the relatively perfect alignments.

Acknowledgements The authors would like to acknowledge the techni-cal assistance from the Centre for AdvancedMicroscopy at the AustralianNational University. The authors would like to acknowledge Dr. Yi Xiaofrom the Australian National University for the discussion.

Funding information The financial support from the Australian ResearchCouncil (Grant No. LP130101001) is fully acknowledged.

Publisher’s note Springer Nature remains neutral with regard to jurisdic-tional claims in published maps and institutional affiliations.

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