188 PHILIPS TECHNICAL REVIEW VOLUME 20

INFLUENCE OF THE PELTIER EFFECT IN RESISTANCE WELDING

During tests on a spot-welding machine for thefabrication of grids for radio valves, the directionof current flowwas found to have a marked influenceon the strength of the weld. It will be shown belowthat this at first sight surprising phenomenon ean heexplained by assuming it .to be due to the Peltiereffect.

The situation is illustrasedsschematically infig. 1.Two molybdenum wires are simultaneously weldedcrosswise to a thicker nickel wire, over and under

....~~--------~~~~------4-~cl~~~-----------+----------~--~Fig. 1. Sketch of arrangement for spot-welding two molyb-denum wires to a nickel wire. If the current i flows from topto bottom in the diagram, an amount of heat Qp (Peltier heat)is developed at the upper weld and approximately the sameamount of heat Qp' is absorbed at the lowerweld. As a result theupper weld has a higher temperature than the lower.El and E2are the (copper) electrodes.

the latter. The welding current is supplied by thedischarge of a capacitor through the primary coilof the welding transformer; as a function of time ithas roughly the form shown infig. 2. The current iseffectively unidirectional; in fig. 1, for example, itflows from top to bottom 1). Thus, in the upperweld it flows from molybdenum to nickel and in thelower weld from nickel to molybdenum. Now, thePeltier effect is the thermo-eleetric phenomenon thatwhen current flowsthrough a junction of two dissim-

1) The current pulse of opposite polarity, which followsthefirst sharp peak, transports just as much electric charge,but it is longer drawn out and reaches nothing like theamplitude of the first peak. For this reason it has littleimportance in the welding process. .

537.322.15: 621.791.76

îlar conductors, heat is developed or absorbed atthat junction, depending on the direction of thecurrent. Whether Peltier heat will be developed orabsorbed can be deduced from the behaviour ofthe conductors in question when made to form athermocouple. From fig. 3 and its caption we seethat in the situation of fig. 1, Peltier heat will be gen-erated at the upper weld and absorbed at the lower.The Peltier heat is added to or siIbtracted from theohmic heating, as the case may be. According to thisreasoning, then, the upper weld develops the highertemperature. It is in fact found that when the currentpulse is just high enough to weld the upper wire

i

tlmsec

95614

Fig, 2. The welding current-pulse as a function of time .

firmly, the lower weld is not yet properly consoli-dated. If the current pulse is raised sufficiently toweld the lower wire firmly, the upper wire usuallymelts through. If the direction of current flow isreversed the same phenomena occur, but now withthe roles of the upper and lower wires interchanged.

Although critical adjustment of the current pulsemakes it possible to obtain satisfactory welds atboth the upper and the lower stations, it takes onlya few welds before surface contamination of theelectrodes attenuates the pulse enough to prevent agood weld at the lower wire. When the electrodeshave been cleaned, a number of good double weldscan again be made.

Ni 95615

Fig. 3. In a thermocouple formed by Ni and Mo, the currenti at the hot junction (temperature T + LIT) flows from Nito Mo.The Peltier effect opposes this current flow, i.e, it heatsthe cold junction (temperature T; heat Qp is liberated here)and cools the hot junction (where heat Qp' is absorbed). Thuswhen a current is driven through an Mo-Ni junction, heatingoccurs when the current direction is from Mo to Ni.

1958/59, No. 7 PELTIER EFFECT IN RESISTANCE WELDING 189

In principle, the Peltier effect occ~rs whenevertwo dissimilar materials are jointed by resistancewelding. The fact that the effect is so unusuallypronounced in the case described is partly attribut-able to its relatively high value at an Ni-Mo junc-tion, but is mainly due to the fine gauge (0.1 mm)of the molybdenum wire used. With such- fine wirea relatively slight rise in the heat developed at thewelding zone tips the scales between non-adhesionand a melted-through wire.

40mV

vt

Ft,CC:

-15

cient, for which one of the Thomson relations isrelevant 2): .

dVP=T-.

dT(2)

T is the temperature of the junction in "K. V is thethermo-electromotive force III a thermocoupleformed by the two metals concerned, one junctionof which is held at a constant tempcrature. whilethe other has the temperature T. In fig. 4a the

tFe

pt12000C' \

O~~~~~~~~~~~~400 800

400 600 800 1000 1200 1400 'K-T

-5

956'6

bFig. 4. a) Thermo-e.m.f, V of a number of metals, with respect to platinum, as a functionof the temperature T of one of the junctions. The other junction is at 273 "K (0 "C), Thepositive sign means that at the cold junction the thermo-e.m.f, is directed from the metalto the platinum 3).b) Values of dV/dT (the so-cálled thermo-electric power) as a function of:r... derived fromthe curves in (a). As a result of the Peltier effect a junction is heated when a current flowsfrom the metal with the higher value of dV/dT to the metal with the lower.

We have already touched on the connection be-tween Peltier heat and the behaviour of a thermo-couple made from the contacting metals in question.We shall examine this connection more closely, inorder to be in a position to predict whether in agiven case the Peltier effect may be expected toinfluence a weld significantly or not.

Let Qp be the Peltier heat developed per second,i.e. the Peltier power. This is proportional to thecurrent i through the junction:

Qp :__ Pi.

Thè proportionality factor P is the Pelfier coeffi-

thermo-e.m.f.'s of several metals with respect toplatinum are plotted as a function oftemperature 3).The thermo-e.m.f, for any two metals is found fromthis graph by measuring the vertical distance be-tween the curves for the metals in question From

(1)

2) See e.g.S.G.Starling, Electricity andmagnetism, Longmans,London 1939, 6th edition, pages 210-212. A treatment ofThomson relations, taking account of irreversible processes,will be found in R. C. Tolman and P. C. Fine, On theirreversible production of entropy, Rev. mod. Phys. 20,51-77, 1948, in particular pages 70-72.

3) The curves are plotted from data given by W. F. Roeserand H. T. Wensel in:Temperature, Report of a symposiumheld at New York in 1939, Reinhold, New York 1941,pages 1309-1310. .

190 PHILIPS TECHNICAL REVIEW VOLUME 20

fig. 4b, which sets out the derivatives with respectto T of the curves in fig. 4a, the values of dV/dT(the so-called thermo-electric power) can be foundin the same way for an arbitrary combination ofmetals. It can be seen from fig. 4b that in the caseof Fe-Mo, for example, the values of dV/dT areappreciably smaller than for Ni-Mo. This impliesthat if an iron wire were substituted for tbe nickelwire in fig. 1, it should be easier to obtain good si-multaneous welds of the upper and lower wires.

3m

cas,e of Cu-Mg junctions, give rise to additionalheating of the positive electrode and to cooling ofthe negative electrode. To '~heckthis, measurementswere, made of the the;rmo-e.m.f. in one of thc alloysused in the investigation (fig. 5). The resultsshowed that the positive electrode does indeedattain the higher temperature (fig. 6).The observations and considerations discussed

above prompted a more comprehensive experimen-tal and theoretical investigation of the influence

l'JpvjoC

la

dV 8d1t 6

4

2Cu 0 Cu

400 6OO"C 0 200 400 eo«:-T -T

s II 95617

2V

t

Fig. 5. a) Thermo-e.m.f, Vof copper with respect to an alloy ofMg with 1.5% Mn plottedagainst the temperature T; b) the derivative of this curve with respect to temperature,again as a function of T.

Such is indeed found to be the case. Evcn withFe-Mo the Peltier effect still makes itself felt, for ata certain setting of the current pulse the upper wireis completely welded and the lower one is still loose,However, a slight increase of current is sufficientto weld both wires firmly. The weld cycle can thenbe repeated thousands of times without it becomingnecessary to clean the electrodes and without therebeing any tendency for the wires to melt throughat one side or break away at the other.The Peltier effect also probably explains why, in

spot welding, one electrode tip often deterioratesmore rapidly than the other (burning and pick-up;the latter being the transfer of electrode copperto the workpiece, or of material from the workpieceto the electrode.) Hess et al.4) reported in 1947a detailed investigation of this phenomenon asobserved in the spot welding of magnesium alloysheet - a case of considerable importance in aircraftconstruction. They found that when welding withcurrent pulses which were mainly unidirectional thepositive electrode suffered more than the negative.If the Peltier effect were the cause it should, in the

4) W. F. Hess, T. B. Cameron and R. A. Wyant, Observationsof electrode tip pickup and tip life in the spot welding ofmagnesium alloy sheet, Welding Journal 26, Supplement,pp. 433s-442s and 484s, 1947. '

of the Peltier effect in resistance welding. Thisinvestigation was undertaken in the Philips labora-tory at Aachen 5). The photograph in fig. 7, which

Fig. 6. The Peltier effect may largely explain why in spot-welding one electrode often wears faster than the other. Inthe welding of Mg sheet, the positive electrode gets hotter,according to fig. 5b, and hence deteriorates earlier thanthe other 4). .

5) A report on this investigation, by S. Scholz, will shortlybe published elsewhere.

1958j59, No. 7 PELTIER EFFECT IN RESISTANCE WELDING 191

Fig. 7. Cross-section of a weld of two Ni wires on a Ni-Cr wire.Current flows from the top to the bottom wire. In accord withfig. 4b, the lower weld has fused farther than the upper. Wiregauge 0.5 mm. This photograph was kindly put at our disposalby S. Scholz of the Philips Laboratory at Aachen.

originates from this source, shows clearly the asym-metry that results from making a double weld suchas in fig. 1 when the metal combination shows alarge Peltier effect.

Energy involved in the Peltier effect

We shall now make a theoretical estimate of theenergy released, or absorbed, in the form of Peltierhcat during the welding process. We can then com-pare this energy with the total energy used formaking a weld.

The Peltier heat is given by

f dVi X T~ dt.

dT

The integration is carried out over the durationof the welding process; fig. 2 gives i as a functionof time t. In fig. 8, dVld T is plotted againstT for the combination Ni-Mo (the curve is derivedfrom fig. 4b). The same figure shows T(dV/dT) asa function of T. We assume further that the tem-perature T of the weld zone varies as a function oftime t in the manner shown in fig. 9, it being esti-

mated that the maximurn temperature remainsabout 100 oK below the melting point of Ni (approx.1700 OK). With the aid of fig. 8 we can now plotthe Peltier coefficient P = T(dV/dT) as a functionof t, and also the Peltier power Qp = Px i. This isdone in fig. 9. The area bounded by the latter curvegives the total Peltier heat, which is found to be4.2 X 10-2 joule. The capacitor, which contains the

80P

Î60

7/

P=T~ /'

"-V/ ~

r--_-

V........~Ni-MOáV

v- I--- ~ V/'

VV j...---'

V 200 4{X) 600 800 lOOG 1200"C

40

IOGmV

20 20

o o4{X)

95619800 /000600

Fig. 8. The thermo-electric power dVjdT for the combinationNi-Mo as a function of T (derived from fig. 4,b), and T(dVjdT)as a function of T.

welding energy, has a capacitance C of 6.7 X 10-6farad and is charged to a voltage Vc of 980 V. Theenergy released upon discharge is tCVc2 = 3.2joule. Estimating that about half of this generatesuseful heat in the immediate vicinity of both junc-tions, then 0.8 joule is available per weld. Thecalculated Peltier heat comes to about 5% of thisvalue. The quantities of heat available for the oneweld and for the other thus differ by 10%. Thismakes it a plausible assumption that the Peltiereffect causes a significant discrepancy between thetwo welds.

/oow806040

95620

Fig. 9. To calculate the Peltier heat at a junction between Niand Mo, it is assumed that the temperature T of the weldingzone varies as a function of time t in the manner shown. Alsoplotted as a function of t are the welding current i, the Peltiercoefficient P = T(dVjdT) and the Peltier power Qp = Pxi.

192 PHILIPS TECHNICAL REVIEW VOLUME 20

The Thomson effectFinally, a word about the Thomson effect. This

is the phenomenon that when an electric current iflows through a wire in which a temperature gra-dient exists, an amount of heat, in addition to thejoule heat, will be developed or absorbed, dependingon the direction of the current. The Thomson heatdeveloped, or absorbed, per second in an elementdx of such a wire is given by

dTdQTh = (J i- dx.

t· .&Ix

The proportionality factor (J, which is the Thomsoncoefficient, is dependent on the materialof thewire and on the temperature.

In our case considerable temperature gradientswill arise in the molybdenum wires between theweld zones and the welding electredes. It is there-fore reasonable to ask whether perhaps the Thom-son effect too will have any significant influenceon the weld. The Thomson coefficients of a numberof metals (including Mo) have been measured as afunction of temperature by Lander 6); see jig. 10.To be able to say anything about the influence ofthe Thomson effect we assume that at the momentwhen the weld has reached its maximum tempera-ture (1600 OK) the temperature of the electrodetip is 600 "K, We assume furthermore that thetemperature gradient in an Mo wire is linear from

G) J. J. Lander, Measurements of Thomson coefficients formetals at high temperatnres and of Peltier coefficientsforsolid-liquid interfaces of metals, Phys. Rev. 74, 479-488,1948.

. weld to electrode. Near the junction, where thetemperature is higher than e--> 1000 oe, the Thomsoncoefficient in the Mo wires is negative (see fig. 10).A simple calculation shows that in this part of anMo wire the Thomson power QTh at the momentunder consideration is about 5 W. From fig. 9we see that the Peltier power is then about 80 W.

(3)

20pVfJ<,

10at a-la

Agk::::s ::::::,.R-, Au"""

~-<,---«.~ <, /<,

<, ~ tx-:.......<,

-...t:._d r-,<,<,

<,<,

<,

-20

-JO

-4a

-50

-60300 7500 7000 2100 sor«

-T 95621

Fig. 10.Thomson coefficientsa for various metals as a functionof temperature, after Lander G). The sign convention adoptedis: a positive value of a means that heat is developed when thetemperature gradient and the electric current are in oppositedirections.

900 1200600

The Thomson heating is thus much smaller than thePeltier heating and may be assumed to have nosignificant influence on the weld. It will have lessinfluence still if thicker wires are welded, for in thatcase the Thomson heat will not be so concentratedat the junction.

T. C. BALDER.