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Spot Weld Propert ies When WeldingW i t h ExpulsionA Comparative Study

Reduced strength does not necessarilyresult when resistancespot welds are made under expulsion conditions

BY M. KIMCHI

ABSTRACT. A high proportion of resistance spot welds are made under expuls ion condit ions and have been assumedto be of lower strength and quality thanwelds made without expulsion. Thispaper provides comparative data onweld propert ies made wi th and wi thoutexpulsion under various welding condit ions.Although increased indentation occurswhen expulsion condit ions are used, thewelds are not necessari ly of reducedst rength. At welding currents just overthe expulsion criterion normally used forreference, there is generally some loss ofstrength on the order of 5%. W ith further

current increases, considerable growth inthe nugget size and strength areob ta ined . The strength increase andindentation de pen d to a great extent onthe electrode geometry and other w e l d ing conditions. Elastic stress analysis tosupport the experimental results is presented.Welding under expulsion condit ions inthe range where imp roved w eld s trengthis obtained causes rapid ele ctrode deteriorat ion. The extent and consequences ofthe electrode deterioration are l ikewisedependent on electrode geometry. Thisstudy involved only bare high strength

low alloy (HSLA) and low carbon steels;the results should not be extrapolated toany of the coated steels.In t roduc t ion

Excessive heating in resistance weldingresults in metal expulsion during theweld ing operat ion. Good weld ing prac-Paper presented at the 64th Annual AWSConvention heidinPhiladelphia, Pennsylvania,during April 24-29, 1983.M.KIMCHI isanAssociate WeldingEngineerWelding Research, Armco Inc. Research &Technology Middletown, Ohio.

t ice is considered t o be o ne that operatesjust below init ia l expulsion. While published resistance spot welding schedulesassume that welds are made withoutexpulsion, this seldom occurs in production where a high percentage of spotwelds are actually made with expulsion.Investigators and text books state thatexpulsion welds are undesirable andweak because furt her increase in current does not increase the size of thespot, but seriously injures its metallurgicalstructure and markedly increases theinde ntatio n (Ref. 1). Ho we ver , no evidence or data to support the abovestatements were found in the l i terature.The investigation described in thispaper was undertaken to prov ide comparative test data on weld propertiesmade with and without expulsion undervarious welding conditions. Resistanceweld ing e lec trode l ife when w eld ing w i thexpulsion is also a part of this study. Onlybare HSLA and low carbon steels wereconsidered. An elastic stress analysis providing support to the experimentalstrength results is presented in theAppendix .

P ro c e d u reMaterials used in this investigation andtheir chemical composit ions are presented in Table 1. W eld ing schedules usedare l isted in Table 2. Welds were madejust below the expulsion point for eachparticular schedule, and the welding current was subsequently increased in increments to well above the expulsion point.Spot weld sample geom etry is show n inFig. 1.Weld indenta t ion was measured, andthe samples were tested in tensile-shearto determine the ult imate load. A numberof tension-tension fatigue tests were conducted with constant load amplitude to amaximum value of 450 lb (204 N) and

Fig.1 Spot weldspecimenR = 0.2 (R = minimum stress/max stress).The cyclic loading rate was 20 Hz.Welds were examined meta l lograph icaliy to verify nugget size determined bythe peel test; welds were also inspectedfor defects and examined for fai luremodes. To analyze the weld nuggetgrowth dur ing expu ls ion weld ing, addi t ional instrumented tests were made todetermine the welding current cyclewhere expulsion f irst occurred.

Electrode l i fe tests were performed inorder to evaluate the effect of weldingwith expulsion on electrode deteriorationand to determine the consequences ofextensive use of expulsion welding inproduct ion.Results and Discussion

Figure 2 shows the effect of weldingabove the expulsion point on spot weldstrength using flat faced (FF) truncatedelectrodes with a 30 deg side angle. At acurrent sett ing just above the expulsionpoint, a reduction in strength on theorder of 5% is apparent. At higher current sett ings, s ignif icant improvement intension-shear strength was observed.The relative effect of electrode force isalso presented in Fig. 2. As would beexpec ted , increasing the electrode forceincreases the current level required toreach expulsion and also increases thestrength of the weld made just below theexpulsion point.

The effect of weld t ime on tension-shear strength is shown in Fig. 3. Strongerwelds were made wi th the longer weldtime schedule (16 cycles) just below and

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Table 1Materials a n d C h e m i c a l C o m p o s i t i o n ,Mate rial and thickness C S

0.038 in . (0 .97 mm) low carbo n0.036 in . (0 .91 mm) HSLA(a)0.098 in. (2.49 mm)HSLA

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T3roo

1350

1 3 0 0 -

1250

1200 -

1150 ;: E x p u l s i o n

I 9.5 1 1 i12C u r r e n t (kA)

Fig. 4 Detailed analysis of the initial expulsion portion of the tension-shear curve. M aterial 0.038in. thick low carbon steel; electrode force 450 lb (2 kN); electrodes 0.25 in. diameter 30 degFF2300i

2200 -t

= 2100o

| 2000

1900-

1800 -

13.7% Indentation i>

- 12.3*4Ind.

A 11*4 Ind.1.4*4 I n d .

Ex p u l s i o n \ :

- I 1 1 1 1 1.170 .190 .210 .230 .250 .270 . 290N u g g e t D i a m e t e r (in. )

Fig. 6Tension-shear strength as a function of weld nugge t diameter when welding withexpulsion; note the percent indentation. Material0.036 in. thick HSLA steel; weld time- 12cycles; electrode force 450 lb (2 kN); electrodes 0.25 diameter 30 deg FF

(0

oCOo1

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c0)TJc

4540353025201510

5 \

P- 45FF 3.5

E x p u l s i o n 3 -

30FF

o5 , 2 .5

scj 1-83 Rad.

r7 -|9 1 1 13 i 1 5 i1 7

450 lbs. 800 lbs. Force

Expu ls ion

11 1

1 3 15 17C u r r e n t ( k A )

Fig. 8 Electrode indentation at various current levels and the effect ofelectrode geometry. Material0.036 in. thick HSLA steel; weld time12 cycles; electrode force-800 lb (3.6 kN)

Current (kA)

e x p u l s i o n p o i n t ) u s in g 0 .2 5 i n . ( 6 . 3 5 m m )d i a m e t e r 4 5 d e g t r u n c a t e d e l e c t r o d e ,r e s u l ts in r a p i d d e t e r i o r a t i o n o f t h e e l e c t r o d e ; n u g g e t d i a m e t e r d r o p p e d f r o m0 .2 7 i n . ( 6 . 8 6 m m ) t o 0 .2 0 i n . ( 5 . 0 8 m m )a f t e r 5 0 0 w e l d s a n d t h e n s ta b i l i z e d . S ur f a c e e x p u l s i o n w a s o b s e r v e d , a n d t h ee x p u l s i o n c u r r e n t c h a n g e d f r o m 1 0 , 5 0 0to 7 ,5 0 0 a m p e r e s ( A ) a f t e r 1 , 5 0 0 w e l d s .N o s i g n i fi c a n t c h a n g e i n n u g g e t s i z e w a so b s e r v e d u s i n g 3 i n . ( 7 6 m m ) r a d i u s f a c e de l e c t r o d e s i n t h e l i f e t e s t . A f t e r 2 0 0 0w e l d s t h e e x p u l s i o n c u r r e n t i n c r e a s e df r o m 9 , 7 0 0 t o 1 1 , 5 0 0 A .T h e a p p a r e n t b e h a v i o r o f t h e t w oe l e c t r o d e g e o m e t r i e s c a n b e e x p l a i n e db y t h e d i f f e r e n t d e t e r i o r a t i o n p h e n o m e n a s h o w n i n F i g . 1 3 . In the case o f a f l a t

Fig. 9 - Tension-tension fatigue life of expulsion welds at maximu m loadof 450 lb (2.0 kN) and the ratio of minimum stress to maximu mstress = 0.2effect of electrode force. Material0.038 in. thick lowcarbon; weld time 12 cycles; electrodes - 0.25 in. diameter 30 degFF.X6 (reduced 52 on reproduction)f a c e d e l e c t r o d e , d e t e r i o r a t i o n b e g i n s a tp o i n t E ( e d g e ) a n d r e s u l ts in d e c r e a s e de l e c t r o d e d i a m e t e r a n d i n c r e a s e d c u r r e n td e n s i t y F i g . 14 . In th e case o f rad iusf a c e d e l e c t r o d e s , d e t e r i o r a t i o n b e g i n s a tp o i n t C ( c e n te r ) a n d r e s u l t s i n i n c r e a s e de l e c t r o d e d i a m e t e r a n d d e c r e a s e d c u r r e n t d e n s i t y .

I n a d d i t i o n t o s h o r t e r e l e c t r o d e t i p l if ew h e n w e l d i n g w i t h e x p u l s i o n i n p r o d u c t i o n , t h e a m o u n t o f i n d e n t a t i o n m a yp r e s e n t a p r o b l e m if a p p e a r a n c e is a ni m p o r ta n t f a c to r . F o r s u c h a p p l i c a t i o n s ,i n d e n ta t i o n g r e a te r t h a n 5 % is u s u a l lyc o n s i d e r e d u n d e s i r a b l e . T h e e x c e s s i v es p a r k i n g c h a r a c te r i s t i c o f e x p u l s i o n s p o tw e l d i n g s h o u l d b e r e c o g n i z e d a s ap o te n t i a l s a fe t y h a z a r d .

Conclusions1. E l e c t r o d e i n d e n ta t i o n i s n o t o f

p r i m e i m p o r t a n c e in d e t e r m i n i n g s p o tw e l d s t r e n g t h .

2. A n i n c r e a s e i n w e l d i n g c u r r e n t w e l lab ov e the in i t i a l exp u ls io n po in t resu l ts i na n i n c r e a s e i n n u g g e t d i a m e te r w i t ha t t e n d a n t i n c r e a s e in s h e a r s t r e n g t h .

3 . S p o t w e l d s m a d e w i t h c u r r e n t l e v e ls a b o v e t h e i n i ti a l e x p u l s i o n p o i n ts h o w e d s i g n i f i c a n t i m p r o v e m e n t ( u p t o68%) i n fa t i gue l i fe due to an i nc rease i nn u g g e t d i a m e t e r .4 . F o r c u r r e n t s e t t i n g s b e l o w a n d j u s ta b o v e t h e e x p u l s i o n p o i n t , b e t t e r w e l ds t r e n g th w a s o b ta i n e d u s i n g 8 0 0 lb ( 3 6 3N ) w e l d i n g f o r c e t h a n t h e 4 5 0 l b ( 2 0 4 N )

121110

9876 -543H21

E X P U L S I O N

N O E X P U L S I O N

r-10 11 12 13 14 15 16 17Current (kA)

Fig. 10 Time of expulsion current flow at various current settings.Material - 0.038in . thick lo w carbon steel; electrode force - 450 lb (2 kN);electrodes 0.25 in. diameter 30 deg FF

7100 AmpsNo Expulsion

9 30 0AmpsExpulsion

11800 AmpsExpulsion

13600AmpsExpulsion

Fig. 11Sp ot welds made at various current levels below and aboveexpulsion point. Note the nugget diame ter and indentation. Material0.036 in. thick HS LA steel; electrode force-450 lb (2 kN); weldtime 12 cycles; electrodes 0.25 in. diameter 30 deg FF. Nital etch,X12 (reduced 43 on reproduction)

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Low Expulsion Current8000Amps

D :

Hich Expulsion Current13600 Amps

D,

a a- , ElectTOde D i a m e t e r i Fig. 12 Welds made with different expulsion current settings. Note the failurelocation in relation to the electrode indentation and nugget diameter. Material-0. 036 in. thick HSLA steel; electrode force - 45 0 lb (2 kN); we ld time - 12cycles; electrodes 0.25 in. diameter 30 deg FF. Nital etch, X10 (reduced 50on reproduction)

TI- D a -

Da = E l e c t r o d e d i a m e t e r b e f o r e w e l d i n g .D = E l e c t r o d e d i a m e t e r a f te r w e l d i n g .

Fig. 13 Differences in deterioration of flat-faced and rad ius-faced electrodes

Fig. 14-Electrode deterioration-0.25 in.(6.35 mm ) diameter flat face, 45 deg truncatedelectrode after 1500 welds

r e c o m m e n d e d f o r th is g a u g e o f s te e l .5 . S t r o n g e r w e l d s w e r e m a d e w i t h

l o n g e r w e l d t i m e b e l o w a n d a b o v e t h ei ni ti al e x p u l s i o n p o i n t . H o w e v e r , l o w e rs t r e n g th a t i n i t i a l e x p u l s i o n w a s o b ta i n e d .N o l os s i n s t r e n g th w a s o b s e r v e d u s i n gth e s h o r t w e l d t i m e s c h e d u l e .

6 . E l e c t r o d e i n d e n ta t i o n w a s g r e a te s tw i t h t h e 4 5 d e g t r u n c a t e d e l e c t r o d e s ,l e a s t w i t h t h e 3 i n . ( 7 6 m m ) r a d i u s f a c e de l e c t r o d e , w h i l e t h e i n d e n t a t i o n w i t h 3 0d e g t r u n c a t e d e l e c t r o d e s w a s i n t e r m e d i a te .

7 . l u s t a b o v e t h e e x p u l s i o n p o i n t ,e x p u l s i o n o c c u r s n e a r t h e e n d o f t h ew e l d i n g t i m e . A t h i g h e r c u r r e n t l e v e l s ,e x p u l s i o n t e n d s t o o c c u r e a r l i e r i n t h ew e l d i n g p e r i o d , a n d t h e w e l d c o n t i n u e st o g r o w d u r i n g t h e r e m a i n i n g w e l d i n gt i m e p e r i o d .

tfwt22 Total Load = tf Wt,lit dw

4V- dw UTTTTt

(JWt,Fig. 15 Tension-shear specimen

tfwt22

8 . W h e n w e l d i n g w i t h e x p u ls i o n , t w oa c c e l e r a t e d e l e c t r o d e d e t e r i o r a t i o n

m e c h a n i s m s w e r e o b s e r v e d . In f l a t - f a c e de l e c t r o d e s d e t e r i o r a t i o n s ta r t s a t t h ee d g e s , w h i l e in r a d i u s - f a c e d e l e c t r o d e sd e t e r i o r a t i o n b e g i n s a t t h e c e n te r .

9 . A l t h o u g h g o o d re s u lt s w e r e r e p o r t e d i n t h e r a d i u s f a c e d e l e c t r o d e l if e t e s t ,m o r e w o r k i s n e e d e d t o d e t e r m i n e t h e

fe a s i b i l i t y o f u s i n g e x p u l s i o n w e l d i n g i np r o d u c t i o n . W e l d i n g w i t h e x p u l s i o n u s i n gf l a t - f a c e d e l e c t r o d e s is t h e l e a s t d e s i r a b l eo f t h e tw o a l t e r n a t i v e s .

References1. Resistance W elder M anufac turers ' Association. Resistance Welding Manual, 3 rd ed i t i on , vol. 1: 121-124. Philadelphia, Pennsylvania.2. American We ld ing Society. WeldingHandbook, 7th edi t ion, vo l . 3: 2-11 . Mia mi ,Florida.3. lohn son , K. E., Hann ah, M. D.,Roswel l ,S.L, and Dinsdale, W. D. 1973 (Nov.) . Qual i tycon t ro l . Metal C onstruction and British W elding journal: 401-406.4. Armco Internal Report .

Ackno wledgmentT h e a u t h o r w i s h e s t o t h a n k A r m c o

R e s e a r ch s ta f f a n d m a n a g e m e n t f o r s u p p o r t i n g t h is w o r k . T e c h n i c a l c o n t r i b u t i o n sb y T . P . J o s e p h , D . H . O r t s , G . C . Schmida n d F . A . W a s s e l l a r e g r e a t l y a p p r e c i a t e d .

AppendixThe fo l l ow ing e las t i c s t ress ana lys i s i s

i n t e n d e d t o e x p l a i n t h e t e n s i o n - s h e a re x p e r i m e n t a l r es u lt s d e m o n s t r a t i n g t h er e l a t i v e i m p o r t a n c e o f w e l d s a m p l e v a r i a b l e s w h i c h i n f l u e n c e t e n s i o n - s h e a r

i r t T =

Fig. 16- Specimen bottom half

t fWta _ 4d W t,n d 2 n d 2

X - Average Shear Stressin the Weld 6Wti I

Fig. 17Analysis of failure at point B

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strength. Depicted in Fig. 15 is a tension-shear specimen for which:w = spec imen wid th ,d = nugget diameter,t i = sheet thickness,t2= Vi nugget thickness,a = average applied stress,