shunting effect_spot weld

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Introduction

In industrial applications of resistancespot welding, there are usually severalwelds arranged next to each other in thesame region of a part. This is usually by de-sign for structural and handling purposes,and may also result from weld repair work.Therefore, a premade neighboringweld(s) often exists when making a newweld. The existing welds may divert theelectric current intended for the new weld,a phenomenon called shunting. As the ap-plied electric current is shared by the weldto be made (the shunted weld) and the ex-isting weld (the shunt weld), the heat gen-erated in the shunted weld may not be suf-ficient for it to grow to the designated size.Although the shunting effect is well rec-ognized by the resistance welding commu-nity, it has been discussed only in a fewpublications such as the work by Howe(Ref. 1), the book by Zhang and Senkara(Ref. 2), and ASM Handbook on Welding(Ref. 3).

The shunting process can be visualizedthrough a simple schematic as shown inFig. 1. The proportions of the electric cur-rent through the shunting and welding

paths are determined by the relative val-ues of electric resistances along thesepaths. These resistances can be catego-rized as follows:

Shunting path: contact resistances atthe electrode-sheet interfaces at both thetop and bottom surfaces, and the bulk re-sistance along the shunting path throughthe shunt weld.

Welding path: contact resistances atthe electrode-sheet interfaces at both thetop and bottom surfaces, bulk resistancesof the top and bottom sheets, and the con-tact resistance at the faying interfacewhere the shunted weld is to be made.

The contact resistances at the elec-trode contact can be assumed identical forthese two paths, and they can be lumpedas Rc for simplicity. The electric circuit ofthe welding setup can be simplified asshown in Fig. 2. In this diagram the totalapplied electric current, I, is split into awelding current and a shunting current, IWand IS, respectively. These two currents,and therefore the amounts of heat, are de-

termined by the relative values of the re-sistances along the shunting and weldingpaths. An estimate of these resistances ishelpful for assessing the amount of elec-tric current and, therefore, heat, divertedthrough the shunting path. However, theresistances along these two paths do notremain constant during welding. The ris-ing temperature, as well as metallurgicalchanges, has a great effect on the resist-ance levels. In addition, the contact area atthe faying interface along the welding pathexperiences a large change from an inti-mate contact of two sheets to a single pieceof metal due to melting, at which point thecontact resistance Rcw disappears. As thecontact resistance dominates the total re-sistance in the welding path, the resistanceratio of the two paths is expected tochange drastically during welding. How-ever, considering the detailed actualprocesses in welding would make it im-possible to analyze the shunting phenom-enon. As a matter of fact, the relative re-sistances at the beginning of welding canbe used to simplify the analysis, and itshould produce a reasonable estimate ofthe shunting effect as the initial state ofthe electrical resistance is crucial in deter-mining the electrical current distributionand the heat generation throughout thewelding process. There are many factorsinfluencing shunting or the distribution ofelectric current during resistance spotwelding. They can be classified into thefollowing three groups:

Electrical Factors

The electrical factors are as follows:Bulk resistance. The bulk resistance is

a function of temperature for a certainmaterial with fixed chemical composition.Such a temperature dependence is signif-icant, although it is not accurately knownfor most of the metals beyond their melt-ing points. As the total resistance alongthe shunting path is proportional to thebulk resistance, a large weld spacingwould result in a long shunting path andthus a small amount of shunting.

Contact resistance. Shunting is also af-fected by the contact resistance at the fay-

Shunting Effect in Resistance Spot WeldingSteels — Part 1: Experimental Study

Shunting in resistance spot welding is affected by the process variables involvedand caution must be taken when welding with narrow weld spacing

BY B. WANG, M. LOU, Q. SHEN, Y. B. LI, AND H. ZHANG

ABSTRACT

Shunting in resistance spot welding is difficult to avoid, as multiple welds arecommonly designed in a specific area for strength and other considerations. Thisexperimental study investigates the effects of several variables on shunting, in-cluding material, surface condition, welding schedule, and other process parame-ters. The experimental results show that weld spacing and surface condition havethe largest effects on shunting. The electrode force in general helps avoid shunt-ing, with the exception that for a narrow weld spacing it actually promotes shunt-ing by reducing the electrical resistance in the shunting path. It was found thatshunted welds may have larger/darker indentation marks than the shunt welds, al-though they are smaller in weld size. In general, it was found that the variables aretightly coupled and their interaction is complex, making it difficult to single out theinfluence of individual factors in shunting.

KEYWORDS

Resistance Spot WeldingShuntingMild SteelDual-Phase SteelWeld Spacing

B. WANG is with Zhejiang Normal University,Jinhua, China. M. LOU, Q. SHEN, and Y. B. LIare with Shanghai Jiao Tong University, Shang-hai, China. H. ZHANG is with University ofToledo, Toledo, Ohio.

Wang Supplement June 2013_Layout 1 5/15/13 1:22 PM 82

ing interface. A resistive surface may formdue to surface contaminants such as ox-ides and greases. The contact resistance atthe sheet faying interface plays a signifi-cant role in determining the proportionsof the shunting and welding currents, es-pecially at the beginning of welding. Its di-rect impact diminishes once melting startsand the contact area disappears. However,the influence of contact resistance existsbeyond melting as it affects the heat gen-eration in the sheet stack-up at the earlystage of welding, which in turn dictates theresistivity distribution and, therefore, fur-ther heat generation as welding proceeds,even when the original contact area iscompletely replaced by the molten metal.It is necessary to consider this effect onlyin the welding path, not the shunting pathas there is no contact area along this path.There are many variables affecting thecontact resistance. For instance, zinc coat-ing for corrosion protection significantlyreduces, while other contaminants pro-mote, contact resistance.

Welding time. Although it is not strictlyelectrical, welding time directly interactswith other electrical factors in welding.For instance, joule heating is proportionalto the welding time. Extending the weld-ing time may lessen the shunting effect bycreating sufficient adhesion at the fayinginterface. It is also beneficial to homoge-nizing the variation in the distribution ofelectrical resistivity in the sheet stack-up.

Welding current. The electrical cur-rent used in welding is directly related tojoule heating. Its effect in shunting is sim-ilar to that of the welding time. In general,a large welding current is preferred to re-duce the shunting effect.

Metallurgical Factors

The metallurgical influence in shunt-ing is reflected in the dependence of elec-trical resistivity on the chemical composi-tions and phase changes of the metals.

Material composition. As the ratio of

electrical current is inversely proportionalto the ratio of the resistances along thewelding and shunting paths, increasing thebulk resistivity of the workpieces de-creases the resistance ratio (Rcw +Rbw)/Rbs and, therefore, increases the pro-portion of welding current. That is, weld-ing a metal with large bulk resistivity mayrequire a small weld spacing to avoid theshunting effect. Therefore, a smaller weldspacing may be needed when weldingsteels than when welding aluminum alloys.Different metals exhibit different rates ofincrease in bulk resistivity with tempera-ture. As seen from Fig. 2.2 of Zhang andSenkara (Ref. 2), aluminum and magne-sium have significantly lower bulk resistiv-ity than iron in their respective tempera-ture ranges up to the melting point. Themetallurgical properties of the coatingsare also of importance, as they directly af-fect the contact resistivity. For instance,hot-dipped galvanized (HDG) steels pos-sess lower contact resistance than galvan-nealed sheets and, therefore, may needsmaller weld spacing because of a largerproportion of current passing through the

welding path due to a lower (Rcw +Rbw)/Rbs ratio.

Phase change. A change in bulk resis-tivity is always associated with the phasetransformation in the solid state, andthere is always a jump in resistivity inmetals when they melt, as seen in Fig. 2.2of Zhang and Senkara (Ref. 2). However,the total resistance monitored duringwelding usually does not reveal such asudden change in resistivity occurring atphase transformation, largely due to thecompeting changes in resistivity duringheating (which increases the total resist-ance) and melting (which decreases thetotal resistance as it eliminates the con-tact resistance).

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Table 1 — Materials Used in the Experiment

Material Sheet Thickness (mm) Coating Yield Strength (MPa)

Mild Steel 1.0 Bare 205Mild Steel 1.5 Bare 205Mild Steel 2.0 Bare 205

DP590 1.2 HDG 590DP780 1.25 HDG 780

Table 2 — Nominal Chemical Composition (wt-%) of the Test Materials

Material Si C Mn S P Cr Mo Al V Fe

Mild Steel 0.01 0.07 0.26 0.012 0.014 — — — — Bal.DP590(a) 0.44 0.12 1.8 0.006 0.021 0.26 — — — Bal.DP780(Ref. 5) 0.23 0.1 2.33 — — 0.03 0.02 0.04 0.06 Bal.

(a) The nominal chemical composition of DP590 is that for DP600 in Ref. 4.

Fig. 1 — Schematic of shunting in welding (adapted from Fig. 2.6 of Zhang and Senkara, Ref. 2).

Fig. 2 — An electric circuit of the welding andshunting paths during resistance spot welding.

Wang Supplement June 2013_Layout 1 5/15/13 1:22 PM Page 183

Mechanical Factors

In addition to the electrical and metal-lurgical effects in shunting, an often over-looked effect comes from the mechanicalaspect of welding.

Electrode force. It directly affects thecontact resistance at the faying interfacealong the welding path. A large electrodeforce creates a contact area with smallelectrical resistance and is, therefore, ben-eficial to reducing the shunting effect.

Yield strength. The effect of electrodeforce is directly influenced by the resist-ance of the sheet metals to deformation.As plastic deformation of the metals isnecessary for creating the electric contactat the sheet faying interface, the yieldstrength of the sheets should be consid-ered a factor in shunting study.

Workpiece geometry. Other influentialmechanical factors include sheet fit-up,electrode geometry (dimensions andshape), and electrode alignment (axialand angular misalignments). Sheet dimen-sions, such as thickness, also directly affectthe mechanical resistance to plastic defor-mation under an electrode force. In gen-eral, any geometric conditions that arebeneficial to creating a close contact at the

faying interface reduce the required weldspacing. Therefore, thick sheets requirelarge weld spacing as they are difficult todeform in order to create intimate contactwhen an electrode force is exerted. The ef-fect of sheet thickness in shunting is alsoreflected in the relative lengths of theshunting and welding paths for electriccurrent.

Because of the large number of vari-ables, their intensive interactions, andtheir (often unknown) temperature-dependent properties, it is not viable toquantify their effects on shunting. Experi-mental studies are common in such inves-tigations. In the work by Howe (Ref. 1),several types of steels of various gaugesand surface (coating) conditions werestudied in order to understand the shunt-ing effect in these materials. As the weldspacing used in the tests was fairly large,larger than those specified by the recom-mendations of the International Instituteof Welding (Ref. 4), the conclusion wasthat weld spacing was not a significant fac-tor. Caution must be taken in practicalwelding as the experiments in Ref. 1 werenot designed to determine the minimumweld spacing.

This investigation explored, in steelwelding, the influence on shunting by a

number of variables, including material(composition and strength), gauge, sur-face condition (coating, use of resistive in-sert), as well as welding parameters, mostof them were not included in the previousstudies. An experimental matrix was de-signed in which several values were se-lected for each of the variables. Their ef-fects were then discussed based on theexperimental observations, and a statisti-cal analysis was conducted to test the sig-nificance of the combinations of thevariables.

Experimental Procedure

Based on the material availability,three types of mild steels (MS) and twodual-phase (DP) steels were selected inthis study. They are as listed in Table 1, andthe nominal chemical compositions ofthese commercial sheet metals are listedin Table 2.

Several surface conditions were cre-ated to understand the effect of contactresistance. Bare steel surface, zinc-coated [hot-dipped galvanized (HDG)]surface, and a plastic film insertion incombination with either bare or coatedsurfaces were created to represent thenormal surface conditions and extremelylow and high resistive surface conditions.The plastic insertion used was a 0.05-mm-thick PVC (polyvinyl chloride) film.As it has a melting temperature range of160° ~ 180°C, a large portion of the plas-tic film at the faying interface disappearsthrough melting and vaporization at thebeginning of welding. As the purposewas to explore the effect of contact re-sistance on shunting, plastic films wereonly used when making the shuntedwelds, not the shunt welds.

The sheet materials were cut into 30- ×155-mm coupons, and welds were madeaccording to Fig. 3, with preselected weldspacing. Two toggle clamps, one on eachside of the electrodes, were used to hold asheet stack-up during welding. Two, three,or four (shunted or test) welds, dependingon the weld spacing, were made using thesame schedule as the first (shunt) weld oneach specimen with fixed spacing. Thewelding schedules were selected to makeideal-sized shunt welds for the respectivesheets, based on experience and trials.

The shunting effect was assessed bycomparing the sizes of the shunt weld andthe shunted ones on the same coupon. Peel-ing a weld coupon to measure the weld sizewas considered first for convenience. How-ever, it was proven impractical as the weldspacing selected in this study was generallymuch smaller than that in a normal weldingsetting in order to obtain a significant shunt-ing effect. A much more tedious approachhad to be adopted. The welds were evalu-ated by longitudinal sectioning of the weld

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Table 3 — Welding Schedules and Weld Spacings

Material Current (kA) Time (ms) Force (kN) Weld Spacing (mm)

1.0-mm Mild Steel 6.0 200 1.8, 2.8 8.0, 15.0, 25.01.5-mm Mild Steel 6.0 400, 500 1.8, 2.8 8.0, 15.0, 25.02.0-mm Mild Steel 6.0 500 1.8, 2.8 8.0

1.2-mm DP590 8.0 500 3.5, 5.0 8.0, 15.0, 25.01.25-mm DP780 8.0 500 3.0, 3.5, 4.3, 5.0 8.0, 15.0, 25.0

Fig. 3 — Welding sequence.

Fig. 4 — Cross-sectional views of the shunt and shunted welds made on a 2-mm bare mild steel, with8-mm weld spacing. The parameters were welding current = 6 kA, welding time = 500 ms, and elec-trode force = 2.8 kN. A — #9 in Table 4 and 1.8 kN; B — #11 in Table 4.

A

B


coupon, and metallurgical examination wasconducted to obtain the dimensions andother characteristics of welds. Fixed weldspacing was used in welding for conven-ience. The welding schedules and weldspacing for welding the sheets shown inTable 1 are listed in Table 3.

Truncated flat-face Cu-Cr-Zr elec-

trodes with a 5-mm tip diameter were usedfor welding the steels. The electrodes wereconditioned by making 50 welds beforebeing used in the experiments.

Results and Discussion

The welded coupons were sectioned

and specimens prepared following stan-dard metallographic examination proce-dures. The weld width was measured onthe sectioned specimens using an opticalmicroscope. The measurements are listedin Table 4, and the effects of various vari-ables are discussed in the followingsections.


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Table 4 — Welding Process Parameters, Measured Widths of the Shunt and Shunted Welds, Difference between the First Shunted and the ShuntWelds (%), and Statistical T-Test Results

# Material Thickness Current Time Force Spacing Surface Shunt Shunted Shunted Shunted Shunted Change Standard t_Value p_Value(mm) (kA) (ms) (kN) (mm) Condition Weld Weld_1 Weld_2 Weld_3 Weld_4 (%) Deviation

(mm) (mm) (mm) (mm) (mm) (mm)

1 MS 1.5 6 400 2.8 8 bare 4.4 4.0 4.1 4.1 4.2 9.1 0.0812 -7.348 0.002662 MS 1.5 6 400 2.8 8 plastic 4.2 3.6 3.7 4.0 3.9 14.3 0.1826 -4.382 0.011233 MS 1.5 6 400 1.8 8 plastic 4.0 3.6 3.7 3.7 3.8 10.0 0.0816 -7.348 0.002664 MS 1.5 6 400 1.8 8 bare 4.1 3.8 3.5 3.7 3.7 7.3 0.1258 -6.755 0.003585 MS 1.0 6 200 2.8 8 bare 4.4 4.0 4.1 4.2 4.2 9.1 0.0957 -5.745 0.005306 MS 1.0 6 200 2.8 8 plastic 4.3 3.7 3.6 3.8 3.9 14.0 0.1291 -8.521 0.002117 MS 1.0 6 200 1.8 8 bare 4.5 4.0 4.2 4.1 4.3 11.1 0.1291 -5.422 0.006328 MS 1.0 6 200 1.8 8 plastic 4.2 2.8 2.7 2.7 2.6 33.3 0.0816 -36.742 09 MS 2.0 6 500 2.8 8 bare 5.0 4.5 4.6 4.6 4.8 10.0 0.1258 -5.960 0.0048110 MS 2.0 6 500 2.8 8 plastic 4.6 4.3 4.2 4.4 4.4 6.5 0.0957 -5.745 0.0053011 MS 2.0 6 500 1.8 8 bare 4.9 4.6 4.7 4.9 4.8 6.1 0.1291 -2.324 0.0209612 MS 2.0 6 500 1.8 8 plastic 4.8 4.5 4.7 4.7 4.8 6.3 0.1258 -1.987 0.0756113 DP780 1.25 8 500 4.3 8 HDG 6.5 6.2 6.3 6.4 6.4 4.6 0.0957 -3.656 0.0179014 DP780 1.25 8 500 3 8 HDG 6.7 6.5 6.5 6.7 6.7 3.0 0.1155 -1.732 0.0934215 DP780 1.25 8 500 3.5 8 HDG 6.6 6.4 6.5 6.5 6.6 3.0 0.0816 -2.449 0.0470916 DP780 1.25 8 500 3.5 8 HDG + plastic 6.5 6.2 6.3 6.2 6.4 4.6 0.0957 -4.700 0.0092117 DP780 1.25 8 500 5 8 HDG 6.3 5.8 6.1 6.2 6.1 7.9 0.1732 -2.887 0.0339018 DP780 1.25 8 500 5 8 HDG + plastic 6.1 6.0 6.0 6.1 6.1 1.6 0.0577 -1.732 0.0934219 DP590 1.2 8 500 5 8 HDG 5.8 5.7 5.7 5.7 5.7 1.7 0 -3.077 0.0281720 DP590 1.2 8 500 5 8 HDG + plastic 5.9 5.8 5.8 5.9 5.9 1.7 0.0577 -1.732 0.0934221 DP590 1.2 8 500 3.5 8 HDG 6.6 6.4 6.4 6.5 6.5 3.0 0.0577 -5.196 0.0070322 DP590 1.2 8 500 3.5 8 HDG + plastic 6.3 6.1 6.1 6.2 6.3 3.2 0.0957 -2.611 0.0422123 DP590 1.2 8 500 5 15 HDG 5.6 4.7 5.1 5.2 16.1 0.2646 -3.928 0.0317824 DP590 1.2 8 500 5 15 HDG + plastic 5.8 5.4 5.5 5.5 6.9 0.0577 -10.0 0.0049525 DP590 1.2 8 500 3.5 15 HDG 6.0 5.3 5.5 5.4 11.7 0.1 -10.392 0.0047226 DP590 1.2 8 500 3.5 15 HDG + plastic 6.2 5.6 5.7 5.9 9.7 0.1528 -5.291 0.0172127 DP780 1.25 8 500 3.5 15 HDG 6.3 5.7 5.8 5.8 9.5 0.0577 -16.0 0.0022828 DP780 1.25 8 500 3.5 15 HDG + plastic 5.6 5.4 5.4 6.5 3.6 0.6351 0.455 0.6504729 DP780 1.25 8 500 5 15 HDG 5.6 5.2 5.4 5.5 7.1 0.1528 -2.646 0.0635830 DP780 1.25 8 500 5 15 HDG + plastic 5.8 5.4 5.5 5.5 6.9 0.0577 -10.0 0.0049531 MS 1.0 6 200 2.8 15 bare 4.2 3.7 3.6 3.8 11.9 0.1 -8.660 0.0067132 MS 1.0 6 200 2.8 15 plastic 4.3 3.4 3.8 3.7 20.9 0.2082 -5.547 0.0156033 MS 1.0 6 200 1.8 15 bare 4.7 4.5 4.5 4.5 4.3 0 -2.887 0.0520334 MS 1.0 6 200 1.8 15 plastic 4.5 4.0 4.1 4.0 11.1 0.0577 -14.0 0.0025535 MS 1.5 6 400 1.8 15 bare 5.5 5.0 5.0 5.1 9.1 0.0577 -14.0 0.0025536 MS 1.5 6 400 1.8 15 plastic 5.3 4.7 4.6 4.8 11.3 0.1 -10.392 0.0047237 MS 1.5 6 400 2.8 15 bare 5.2 4.7 4.8 4.9 9.6 0.1 -6.928 0.0101438 MS 1.5 6 400 2.8 15 plastic 5.1 4.7 4.8 4.7 7.8 0.0577 -11.0 0.0043539 MS 1.5 6 500 2.8 15 bare 4.9 4.7 4.6 4.8 4.1 0.1 -3.464 0.0401640 MS 1.5 6 500 2.8 15 plastic 5.0 4.4 4.6 4.7 12.0 0.1528 -4.914 0.0195941 MS 1.5 6 500 1.8 15 bare 5.2 4.9 4.9 5.0 5.8 0.0577 -8.0 0.0076642 MS 1.5 6 500 1.8 15 plastic 5.3 5.0 5.1 5.1 5.7 0.0577 -7.0 0.0099243 DP590 1.2 8 500 5 25 HDG 6.1 5.7 6.0 6.6 0.2121 -1.667 0.1752444 DP590 1.2 8 500 5 25 HDG + plastic 5.5 5.4 5.4 1.8 0 -1.010 0.3436345 DP590 1.2 8 500 3.5 25 HDG 6.5 6.3 6.4 3.1 0.0707 -3.000 0.0485546 DP590 1.2 8 500 3.5 25 HDG + plastic 6.3 6.1 6.3 3.2 0.1414 -1.000 0.3420647 DP780 1.25 8 500 3.5 25 HDG 6.5 6.2 6.4 4.6 0.1414 -2.000 0.1483448 DP780 1.25 8 500 3.5 25 HDG + plastic 6.0 6.0 6.0 0.0 0 1.0000049 DP780 1.25 8 500 5 25 HDG 6.5 6.1 6.2 6.2 0.0707 -7.000 0.0473250 DP780 1.25 8 500 5 25 HDG + plastic 6.5 6.0 6.3 7.7 0.2121 -2.333 0.1333951 MS 1.0 6 200 2.8 25 bare 5.2 4.8 5.1 7.7 0.2121 -1.667 0.1752452 MS 1.0 6 200 2.8 25 plastic 4.7 4.4 4.6 6.4 0.1414 -2.000 0.1483453 MS 1.0 6 200 1.8 25 bare 5.0 4.6 4.9 8.0 0.2121 -1.667 0.1752454 MS 1.0 6 200 1.8 25 plastic 4.5 4.1 4.3 8.9 0.1414 -3.000 0.0485555 MS 1.5 6 400 1.8 25 bare 6.0 5.5 5.9 8.3 0.2828 -1.500 0.1894456 MS 1.5 6 400 1.8 25 plastic 5.4 5.0 5.3 7.4 0.2121 -1.667 0.1752457 MS 1.5 6 400 2.8 25 bare 5.6 5.2 5.4 7.1 0.1414 -3.000 0.0485558 MS 1.5 6 400 2.8 25 plastic 5.1 4.7 5.0 7.8 0.2121 -1.667 0.1752459 MS 1.5 6 500 2.8 25 bare 5.3 4.7 5.2 11.3 0.3536 -1.400 0.1979660 MS 1.5 6 500 2.8 25 plastic 5.5 4.5 5.2 18.2 0.4950 -1.857 0.1028961 MS 1.5 6 500 1.8 25 bare 5.3 5.0 4.7 5.7 0.2121 -3.000 0.0485562 MS 1.5 6 500 1.8 25 plastic 5.5 4.3 3.0 21.8 0.9192 -2.846 0.11039


Effect of Shunting on Weld Geometry,Microstructure, and HAZ

In addition to the weld size, the metal-lographic cross-sectional views also re-vealed other useful information. Figure4A shows a shunt weld with four subse-quently made shunted welds. The shunt-ing effect is apparent, as there is a cleardifference in heat input, reflected by theweld size, between the shunt and shuntedwelds; and the coarser grains in the shuntweld than in the shunted ones are the re-sult of more concentrated heating whenmaking the shunt weld. In addition to thesmaller size of the shunted welds than ofthe shunt weld, the differences in weldheight, weld shape, and shape of the heat-affected zone (HAZ) are also visible fromthe figure. The first shunted weld (the sec-ond weld in the sequence) is smaller thanthe shunt weld, and its shape is more ovalthan that of the shunt weld, which is morerectangular. The four shunted welds havesimilar shapes, and the weld size slightlyincreases along the welding sequenceaway from the shunt weld. Another differ-ence is the shape of the HAZ. Those of theshunted welds have corners extending tothe edges of the contact areas between theelectrode and sheets, indicating concen-trated heating at these locations. There-fore, the shunted welds may experience adifferent heating from the shunt weld, re-sulting in asymmetrical HAZ due toshunting.

Effect of Electrode Force in Shunting

The electrode force is responsible for

creating sufficent electric contact at thefaying interface. Part of it is consumed todeform the separated sheets and close thegap, and the rest is used to create an areaof electric contact. Sheet separation ordistortion may result from fabrication orfrom welding the shunt weld. The firstshunt weld generates the largest sheet sep-aration because of the undiverted heating,and subsequent shunted welds creategradually reduced separation as the weld-ing current is diverted due to shunting.The effect of electrode force is evident inFig. 4. The coupons in Fig. 4A, B were cre-ated under identical conditions except theelectrode force, which was 2.8 and 1.8 kNfor the specimens in Fig. 4A, B, respec-tively. The first (shunt) weld in Fig. 4B hasa different size and shape from its coun-terpart in Fig. 4A, possibly due to thesmaller contact area at the faying interfacecreated by the smaller electrode forceused when making this weld. The shuntedwelds are drastically different from theshunt weld and from each other in the se-quence. The first shunted one is signifi-cantly smaller than the shunt weld, appar-ently due to the fact that a large portion ofthe applied electric current was divertedfrom the welding path to the shuntingpath, and resulted from the large contactresistance at the faying interface createdunder a small electrode force. The subse-quent shunted welds in the sequence getlarger, away from the shunt weld, due tothe smaller amount of the diverted electriccurrent from their respective shunt welds,the immediate welds prior to the weldsbeing made. It is interesting to see that the4th shunted weld (the 5th in the sequence)

is actually slightly smaller than the 3rdshunted weld (the 4th in the sequence),possibly due to a sufficiently large shunt-ing effect from its shunt weld (4th in thesequence), which grew to a certain sizelarger than its shunt welds. The level ofelectrode force, 1.8 kN, appears insuffi-cient in deforming the separation of 2-mmsteel sheets created by the shunt weld.Under the same electrode force, theshunting effect may become smaller whenwelding more compliant sheets. For in-stance, there is a 5.8% drop in the size ofthe first shunted weld when welding 1.5-mm bare steels (Specimen #41 in Table 4),while the drop is 6.1% when welding 2.0-mm bare steels (Specimen #11 in Table 4).

Effect of Shunting on Weld Size

The shunting effect on weld size alongthe welding sequence is summarized inFig. 5. The shunt and shunted welds on thesame weld coupons are plotted in threegroups according to the weld spacing, re-gardless of the material and welding pa-rameters. In each of the three groups, withweld spacing of 8, 15, and 25 mm, the dual-phase steels take the upper portion in thefigure because more heat was used whenmaking these welds, resulting in largerwelds than the mild steels. There is a di-rect correspondence between the lines inFig. 5 and the specimens in Table 4. Allthree groups of specimens show a sizedrop in the first shunted welds comparedwith their respective shunt welds. Afterthe initial drop, subsequent shunted weldstend to recover from the shunting effectwith an increase in size. The concavedownward trend is observed in most casesin these groups. The second shunted weldsare generally larger than the first shuntedones, and the upward trend is of differentmagnitudes depending mainly on the weldspacing, in addition to other variables. Oneach of the weld coupons with 8-mm spac-ing, the shunted welds continue to grow insize in the welding sequence. The growthrate is slower when the spacing is in-creased to 15 mm, and similar trend is ob-served on the coupons with 25-mm spac-ing. The increase in the shunted weld sizeis the result of an increase in the resistancein the shunting path from the precedingshunt welds and an increase in the contactarea for the welds being made.

There are a few anomalies in Fig. 5. Forinstance, a linear drop in weld size is ob-served in one of the specimens with 25 mmweld spacing (Specimen #62 in Table 4),as a result of significant shunting effectwhen welding 1.5-mm sheets with a smallelectrode force of 1.8 kN and plastic filminsert at the interface.

In Table 4, the welds with an 8-mmspacing are smaller on the MS couponsthan those on the DP steels, as less heat in

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Fig. 5 — Weld widths on the coupons in the order of welding sequence. In the group on the left are thespecimens with four shunted welds and 8-mm spacing; the center group has three shunted welds with 15-mm spacing; and the one on the right has two shunted welds with 25-mm spacing.

Fig. 6 — Typical welded specimens of 1.5-mm bare m ild steel sheets with 8-mm weld spacing. A — Weldsmade with the original bare surfaces (weld specimen #1 in Table 4); B — those made using a plastic filmas insert (weld specimen #2 in Table 4). The welding sequence is from left to right.

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welding was put into the mild steels thaninto the DP steels, which require higherwelding current and longer welding timeto compensate for the zinc coating. In gen-eral, welding HDG DP steels requireshigher electric current, longer weldingtime, and larger electrode force as shownin Table 3. Table 4 also shows that a largerdrop in weld size from the shunt weld tothe shunted ones occurs when a layer ofplastic film was inserted into the faying in-terface on which the shunted welds weremade, compared with welding the samematerials under identical conditions with-out a plastic insertion.

Effect of Shunting on Weld Appearance

Table 4 shows a significant reduction insize in the shunted welds, compared to theshunt weld, in almost all the specimensmade using all the three weld spacings.The shunt and shunted welds on the sameweld coupon can also be distinguished bytheir appearance. Figure 6 shows a seriesof welds made on 1.5-mm bare mild steelsheets with a very tight spacing: 8 mm fromcenter to center. As a result, the impres-sions of the neighboring welds overlap. Byobserving only the indentation marks ofthe welds on the coupons in the figure, onemay conclude that the first weld (the shuntweld) has the smallest weld nugget as itsimpression mark is smaller than those ofthe shunted ones. This was proven untrueas the metallographic examination ofthese specimens reveals the opposite. Theweld widths for the specimens in Fig. 6, aslisted in Table 4, have a reduction of morethan 10% from the shunt weld to the firstshunted one, in both cases with the origi-nal bare surface and the plastic insert.Such an observation, i.e., the welds havinglarge impression marks but smaller sizeshas been obtained in other specimens as

well. Therefore, it is important to recog-nize that in the case of multiple welding, itmay be misleading to judge the weld qual-ity by visual inspection of the weld inden-tation alone. The welds in Fig. 6 haveslightly skewed indentation marks, result-ing from angular misalignment of the elec-trode tips.

In addition to the sizes of the impres-sion marks, another difference in appear-ance can be observed in the distinct burn-ing marks on the surfaces between theshunt and shunted welds, as seen in Fig. 6,with and without the plastic insert. Theshunted welds have darker and larger burnmarks than the shunt weld; even their sizes(widths) are smaller. As a matter of fact,this was observed on all the weldedcoupons, although some of them are notas obvious as others. Several representa-tive welded coupons made on the mildsteels with various spacings are shown inFig. 7. With 8-mm spacing, the shuntedwelds have significantly wider and darkerburn marks. The differences become lessobvious as the weld spacing get larger to15 mm and then 25 mm with original(bare) faying interfaces. The use of plasticinserts at the faying interfaces when mak-ing shunted welds appears to amplify thedifference in burn marks. The appearanceclearly distinguishes the shunt weld fromthe shunted ones even for those of 15- and25-mm spacings when the shunted weldswere made using plastic inserts. Whenwelding zinc-coated steels, as shown inFig. 8 for 1.2-mm DP590 steels, the weldsappear to be very similar judging from theweld marks, even for the 8-mm spacedweld coupons.

The difference in weld marks betweenthe shunt and shunted welds, when weld-ing the uncoated mild steels, can be at-tributed to the role played by the contactresistance at the electrode-sheet inter-

faces. A rise in the contact resistance atthe faying interface, Rcw, increases theshunting current IS and reduces the weld-ing current IW, according to Fig. 2. It is rea-sonable to assume that the contact resist-ance at the electrode-sheet interface isidentical when making the shunt weld andthe shunted weld, as shown in Fig. 2 as Rc.Assume that the total resistance in the sec-ondary loop, excluding Rc, is RT whenmaking a shunted weld, and RT´ whenmaking the shunt weld. The existence ofthe shunting path lowers the resistance inthe weld stack-up, i.e., RT´ > RT. There-fore, the proportion of heating, in terms ofheating rate, resulted from joule heatingat the electrode-sheet interface to the en-tire secondary loop, is not the same whenmaking the shunt weld and the shuntedweld, i.e.,

This means the heating at the elec-trode-sheet interface is faster, relative tothe rest of the secondary loop, when mak-ing a shunted weld than when making ashunt weld. Therefore, the temperature atthis interface rises faster relative to that atthe faying interface than in making theshunt weld. As a result, the resistance atthe electrode-sheet interface increases ata faster pace that in turn produces moreheat at this interface than at the faying in-terface, compared with the heating whenmaking the shunt weld. The larger amountof heat generated at the electrode-sheetinterface in a shunted weld than in a shuntweld is responsible for the larger and/ordarker burn marks on the shunted welds.There is little difference in the indentation

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Fig. 7 — Welded coupons of 1.0-mm bare mild steel sheets, without (left)and with (right) the plastic insert of 8-, 15-, and 25-mm weld spacing.

Fig. 8 — Welded coupons of 1.2-mm HDG steel sheets, without (left) andwith (right) the plastic insert of 8-, 15-, and 25-mm weld spacing.

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marks between the shunt and shuntedwelds when welding coated DP steels, asseen in Fig. 8. This could be attributed tothe fact that the molten zinc from the coat-ing forms a conducting ring at the fayinginterface from the beginning of welding,which smooths out the difference betweenRT´ and RT. The molten zinc at the elec-trode-sheet interface also results in lowcontact resistance, and less heat is gener-ated at this location.

Effect of Weld Spacing on Shunting

Shunting is affected by many factors,and they interact with each in shunting.Therefore, the effect of one factor shouldbe discussed with the consideration ofother factors. Some of the influential fac-tors are analyzed together in the follow-ing, considering the interactions amongthe factors. As shunting is almost alwayssignificant when welding with 8- or 15-mmspacing, the roles of process variables de-pend largely on the weld spacing. There-fore, the influence of various variables onshunting is discussed in the categories ofweld spacing.

1. Mild steels with 8-mm weld spacing.This weld spacing represents an extremeof small weld spacing that may make theprocess parameters affect the welding andshunting processes in a different way fromwelding with larger weld spacing.

For the bare mild steels, the averagechanges in weld size depend on the sheetthickness. They are 10.1, 8.2, and 8.1% forsteels of 1.0, 1.5, and 2.0 mm, respectively,in thickness. The effect of sheet thicknesson shunting comes from two parts: thelengths of the shunting and welding paths,and the resistance to bending under anelectrode force that affects the contact re-sistance at the faying interface. These fac-tors interact with others such as weld spac-ing in affecting shunting.

With the plastic insertion when makingthe shunted welds, the drop in weld size ismore dramatic. An average of 23.7, 12.15,and 6.4% for the 1.0-, 1.5-, and 2.0-mmsheets, respectively, was calculated usingthe data in Table 4. The electrode forcealso appears to be influential. A sharp in-crease in the shunting effect, from a 14%drop to a 33% drop in weld size is ob-served in the 1.0-mm sheets, which is a farlarger change than welding without theplastic insert. Similar to welding bare MSsteels, there is a decrease in shunting ef-fect when increasing the electrode forceon the 1.5- and 2.0-mm sheets with theplastic insert. Therefore, the shunting ef-fect is, in general, inversely proportionalto the sheet thickness, and the plastic in-sertion at the faying interface amplifiessuch an effect.

The increase in shunting with a risingelectrode force on the 1.0-mm sheets, and

a decrease when welding 1.5- and 2.0-mmsheets clearly indicate an interaction be-tween the electrode force and sheet thick-ness. Increasing the electrode force wouldnormally reduce the contact resistance atthe faying interface along the welding pathand therefore, reduces shunting. Whenwelding 1.0-mm sheets, however, thesheets can be easily deformed and the gapat the faying interface between the weldsclosed, as thin sheets have less resistanceto deformation, resulting in increasedshunting.

The 8.0-mm weld spacing used in weld-ing clearly dictates the role the electrodeforce plays in shunting. The 8.0-mm spacebetween the shunt and shunted welds cre-ates a distance less than 4.0 mm betweenthe edges of the welds. The gap along sucha small distance at the faying interface canbe easily closed on the 1-mm stack-upsunder an electrode force. A large elec-trode force is more effective than a smallone in closing this gap in thin sheets, re-sulting in a low electrical resistance alongthe shunting path and a large shunting ef-fect. Closing of such a gap is more difficultto achieve when the stiffness of the sheets,proportional to the sheet thickness, islarge and, therefore, increasing the elec-trode force produces decreased shuntingin 1.5- and 2.0-mm sheets.

2. 1.25-mm HDG DP780 steel sheetswith 8.0-mm weld spacing. Without theplastic insert, the HDG surfaces appear tomake the shunting effect more sensitive tothe electrode force. When the electrodeforce is increased from 3.0 to 3.5 kN, 4.3kN and then 5.0 kN, the decrease in weldsize due to shunting is 3.0, 3.0, 4.6, and7.9%, respectively. That is, a large elec-trode force produces more shunting. Thisis similar to what had happened in the MSas discussed previously, i.e., a large elec-trode force actually helps close the gap be-tween the shunt and shunted welds and el-evate the shunting effect when the weldspacing is small. The molten zinc at thefaying interface easily fills the gap that re-duces the electrical resistance along theshunting path.

3. 1.2-mm HDG DP590 steel sheetswith 8-mm weld spacing. Increasing theelectrode force slightly reduces shuntingwhen welding DP590 sheets with bothzinc-coated only and zinc-coated plusplastic insert surfaces. This is the result ofan interaction among weld spacing, coat-ing, and electrode force. The small weldspacing and the zinc coating together ele-vate the shunting effect, but the loweryield strength (compared to the DP780steels) that makes deforming the sheetseasier under an electrode force, producesless shunting under a large electrodeforce. The opposite effects of electrodeforce on shunting when welding withDP780 and DP590 steels could be attrib-

uted to the difference in yield strengthand, therefore, the different easiness ofcollapsing the contact area and loweringthe contact resistance at the fayinginterface.

4. Welding with 15-mm weld spacing.The effect of electrode force is not consis-tent as observed when welding the MS andDP steels, with or without the plastic in-sert. This could be because a 15-mm spaceapproaches critical weld spacing for someof the sheets at which the shunting is notsignificant and, therefore, the influence ofthe process variables appears to berandom.

5. When welding 1.0-mm MS sheetswith a weld spacing of 25 mm, decreasedshunting is observed with an increasedelectrode force for both bare and plasticfilm-inserted faying interfaces, as the elec-trode force is responsible for reducing thecontact resistance along the welding path.With 25-mm spacing, the gap between thewelds is not easily closed, which is differ-ent from what was observed in weldingwith 8- and 15-mm weld spacings. A simi-lar observation is made on 1.5-mm MSsheets. When welding 1.2-mm DP590 and1.25-mm DP780 sheets with a 25-mm weldspacing, increasing the electrode forceproduces an inconsistent effect in HDGand HDG + plastic insert surfaces. Moreexperiments are needed to draw conclu-sions in such cases.

Statistical Analysis of Various InfluentialFactors

An analysis was performed to deter-mine if, under various combinations ofmaterial and process parameters, theshunting effect is truly significant. A stan-dard statistical procedure was employedto test the statistical significance of the dif-ferences between the shunt and shuntedwelds. An appropriate testing statistic inthis case is a t-value (Ref. 5), as the stan-dard deviations of the weld size popula-tions are unknown. It can be calculatedusing the following formula

where d0 is the width of the shunt weld, dis the mean value of the n shunted weldwidths, and s is the standard deviation ofthese welds. The t-values were comparedwith the critical values of the t-distribu-tion, and corresponding p-values werealso calculated. They are as listed in Table4. For the specimens made using 8-mmweld spacing, 18 out of 22 of them have ap-value below, and most of them farbelow, 0.05; the remaining 4 of them havep-values between 0.07 and 0.09. There-fore, it is reasonable to conclude that 8-

td d

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mm weld spacing results in significantshunting for the materials tested. Whenthe weld spacing is increased to 15 mm, thep-values for 18 out of 20 specimens are farbelow 0.05, and for the rest, one is 0.06 andanother is apparently an outlier with a p-value of 0.65. Therefore, the shunting ef-fect is significant when the weld spacing is15 mm. When the weld spacing is furtherincreased to 25 mm, only 5 out of 20 spec-imens have p-values just below 0.05. Thep-values of the others are apparently high,meaning that the shunted welds are statis-tically not different from their shuntwelds. This analysis indicates that the crit-ical weld spacing for these materials liesbetween 15 and 25 mm.

The t-testing results in Table 4 demon-strate that the influence of the shunt weldson the subsequent shunted welds dependson the size of weld spacing. Using identi-cal welding schedules, the shunted weldsmade on a weld coupon are significantlysmaller than their shunt weld when weld-ing with narrow weld spacing. Such a dropin weld size from the shunt weld to the (es-pecially the first) shunted ones diminisheswhen the weld spacing is sufficiently large.Considering the percent decrease in weldsize from the shunt weld to the firstshunted weld, shown in column 14 of Table4, the effects of various process parame-ters can be estimated. Among the vari-ables involved, the surface condition ap-pears to play a major role in shunting. Asa matter of fact, it overshadowed the in-fluence of all other factors. When anANOVA (analysis of variance, Ref. 5) wasconducted, all factors, except the surfacecondition, appeared to be insignificant.This clearly is not true, and it is the resultof the overwhelming significance of thesurface condition, as the plastic insertused in the experiments and the pure zinc-coated surfaces represent two extremesof excessively high and low contact resist-

ances at the faying interface. ANOVA ap-pears to be inadequate in this situation toanalyze the influence of other factors.

The results obtained in this experi-mental investigation prove that shuntingin resistance welding is a complex process,with multiple influencing factors andstrong interactions among these factors.

Conclusions

In this experimental study, the effectsof various factors such as material, surfacecondition, welding schedule, and otherprocess variables were investigated on sev-eral typical MS and DP steel sheets. Thefindings on shunting in resistance spotwelding can be summarized as follows:

1. Weld spacing is the most influentialfactor in shunting, and increasing weldspacing is the most efficient means ofavoiding shunting;

2. As it determines the proportion ofshunting current, the contact resistancehas a significant effect on shunting. In gen-eral, large contact resistance, created byhighly resistive surface conditions or lowelectrode forces promotes shunting;

3. The chemistry, thickness, and me-chanical strength of the workpiece mate-rial play an important role in shunting.There is a strong interaction betweenthese material parameters and otherprocess variables;

4. Shunting is clearly affected by weldingparameters. The electric current and weld-ing time were selected mainly for makingsizable welds in this study, not for under-standing their effects. Therefore, it is inap-propriate to draw conclusions on these twoparameters, although the experimental re-sults have shown their influence on shunt-ing. On the other hand, shunting is clearly afunction of electrode force, often under theinfluence of other process parameters;

5. Although generally smaller in size,shunted welds may have larger/darkerelectrode impressions than their respec-tive shunt welds. This should be recog-nized in visual inspection of weld quality.

This study shows that shunting is af-fected by almost all of the process param-eters involved in resistance spot welding.Because of the difficulties in systemati-cally/simultaneously studying many fac-tors, shunting experiments are often de-signed to understand the influence of onlya few parameters, with others fixed. Cau-tion must be taken when drawing conclu-sions from such experiments, the experi-mental conditions must be clearly stated,and the limitation of applications wellunderstood.

Acknowledgments

One of the authors, B. Wang, is grate-ful for the financial support provided bythe Zhejiang Provincial Natural ScienceFoundation of P.R. China (Project No.LQ12E05006).

References

1. Howe, P. 1994. Spot weld spacing effecton weld button size. Sheet Metal Welding Con-ference VI, Paper C03. AWS Detroit Section.

2. Zhang, H., and Senkara, J. 2012. Resist-ance Welding: Fundamentals and Applications.CRC Press/Taylor & Francis Group, 2nd edi-tion, Boca Raton, London, New York.

3. Tumuluru, M. D., Zhang, H., and Matte-son, R. 2011. Procedure development and prac-tice considerations for resistance welding. ASMHandbook (Volume 6) on Welding. MaterialsPark, Ohio: ASM International.

4. International Institute of Welding. Proce-dure for spot welding of uncoated and coatedlow carbon and high strength steels, draft. Doc-ument No. III-1005-93, Section 6.

5. Walpole, R. E., Myers, R. H., Myers, S. L.,and Ye, K. 2011. Probability & Statistics for En-gineers & Scientists, 9th edition, Prentice Hall.


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