Weldability of Thin Sheet Metals during Small-Scale Resistance Spot Welding using an Alternating-Current Power Supply

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1 1090 Journl of ELECTRONIC MATERIALS, Vol. 29, No. 9, 2000 Zhou, Gormn, Regulr Tn, nd Issue Pper Ely Weldility of Thin Sheet Metls during Smll-Scle Resistnce Spot Welding using n Alternting-Current Power Supply Y. ZHOU, 1,3 P. GORMAN, 2 W. TAN, 1 nd K.J. ELY 2,4 1. University of Wterloo, Deprtment of Mechnicl Engineering, 200 University Avenue West, Wterloo, Ontrio, N2L 3G1, Cnd. 2. Edison Welding Institute, Microjoining nd Plstics, 1250 Arthur E. Adms Drive, Columus, Ohio, 43221, USA. 3. e-mil: nzhou@uwterloo.c. 4. e-mil: kevin_ely@ewi.org. The resistnce weldility of 0.2-mm-thick sheet luminum, rss, nd copper in smll-scle resistnce spot welding (SSRSW) ws studied. The effects of electrode mterils nd process prmeters on joint strength nd nugget size were investigted. The welding current rnges for SSRSW of the sheet metls were determined sed on the minimum current tht produced required nugget dimeter nd mximum currents tht did not result in electrode-sheet sticking or weld metl expulsion. A qulittive nlysis indicted tht resistnce weldility of the metls is not only determined y their resistivity (or therml conductivity) ut is lso ffected y other physicl properties (such s melting point, ltent het of fusion nd specific het). Key words: Smll-scle resistnce spot welding, weldility, thin sheet metl, joint strength, electrode mterils, process prmeters INTRODUCTION Smll-scle resistnce spot welding (SSRSW) is one of the microjoining processes, in which weld is formed etween two workpieces through the loclized melting nd colescence of smll volume of the mteril(s) due to the resistnce heting cused y the pssge of electric current. The het otined cn e expressed s 1,2 Q = I 2 Rt (1) where, Q is the het genertion, I is the welding current, R is the resistnce of the workpieces, nd t is the durtion of the current (weld time). The resistnce includes contct resistnce t the electrode/ workpiece interfces nd t the fying interfce etween the two workpieces, nd ulk resistnce of the workpieces nd electrodes. These resistnce vlues chnge during the process nd their reltive mgnitudes control the process. Among them, the contct resistnce t the fying interfce, which is influenced y mteril chrcteristics (such s clenliness, surfce roughness, hrdness nd plting mterils), nd electrode force, is elieved to e criticl fctor ffecting the process, especilly t the erly stges in (Received Novemer 11, 1999; ccepted My 19, 2000) the heting cycle. 3 7 The formtion of molten metl nugget depends on the interply of het genertion nd het dissiption in the workpieces. The ltter is governed minly y the mteril s therml conductivity nd the geometry of the workpieces nd electrodes. Since, for most metls, the therml nd electricl conductivities re correlted, it is elieved tht electricl resistivity is one of the most importnt mterils properties ffecting mterils weldility during resistnce spot welding (RSW). 3,7 Extensive reserch nd development work hs een crried out in the re of lrge-scle RSW (LSRSW) of sheet metls for pplictions in the utomotive industry, minly on reltively thick sheet steels (thicker thn mm), nd, to much smller extent, on sheet luminum-sed lloys. 3,7 In study of RSW of 0.8-mm-thick steels, Dickinson et l. 4 proposed tht RSW comprises series of stges, nmely, () surfce rek down, () sperity collpse, (c) heting of the workpieces, (d) molten nugget formtion, nd (e) nugget growth nd mechnicl collpse. 4 Similrly, Gould 5 indicted tht nugget formtion nd development could e chrcterized s function of welding vriles (either weld time or current) y four steps: () incution, () rpid growth, (c) stedily decresing growth rte, nd (d) weld metl expulsion. Weld metl expulsion (WME) occurs when the dim- 1090

2 Weldility of Thin Sheet Metls during Smll-Scle Resistnce Spot Welding using n Alternting-Current Power Supply 1091 eter of the molten metl is lrger thn the contct dimeter nd severe WME cn reduce the joint strength ecuse of the loss of metl volume. 3 7 According to the Americn Welding Society, 7 weldility is the cpcity of mteril to e welded under the imposed friction conditions into specific, suitly designed structure nd to perform stisfctorily in the intended service. There re mny wys to define the weldility of mteril in RSW (electrode tip life, welding current level nd current rnge, etc.); the current rnge nd the electrode life re two most commonly used tests. The current rnge is determined y evluting the minimum nd mximum current levels (under certin process conditions) permissile for required joint properties. The utomotive experience shows tht the strength of resistnce spot welded joints cn e correlted to the dimeter of the weld nuggets; therefore, under certin process conditions, certin level of welding current is generlly required to produce weld with minimum nugget dimeter. 3 5 However, too high welding current my result in WME nd hence reduction in joint strength. The electrode deteriortes during welding ecuse of the interctions etween electrode tip nd workpieces. Electrode tip life my e chrcterized s the numer of welds tht cn e mde efore loss of properties of the welds. The current rnge test is most commonly used since the electrode life test is generlly very time consuming. However, there is n incresing reserch interest on electrode tip life since reduced electrode life ecomes one of the mjor fctors ffecting resistnce weldility of coted steels nd luminum lloys for the utomotive pplictions. 8,9 Electrode-sheet sticking (ESS) occurs when excessive het genertion produces loclly melted res t the electrode-sheet interfce. 8 If the molten metl solidifies efore the electrodes seprte from the workpieces t the end of the weld cycle, the workpieces my stick to the electrodes nd smll force would e needed to seprte them. If the molten metl remined molten when the electrodes seprted from the workpieces, the welding opertor would not experience the electrode-sheet sticking; however, the locl surfce res ffected y melting (e.g., resultnt voids) my e reveled y microscopic exmintion. If the welding current is incresed to eyond the level when ESS occurs, the electrodes might weld to the workpieces. ESS should e minimized ecuse it contriutes to reduced electrode tip life. 8 The ppliction of resistnce welding in the friction of electronic devices nd components (e.g., tteries for implntle pcemker) is generlly termed s micro-, fine, or smll-scle resistnce welding since the metl sheets to e welded re reltively thin or smll in dimeter (<0.5 mm) Little work hs een pulished in the open literture on smllscle resistnce spot welding (SSRSW) despite the ever-incresing pplictions of the technology. Becuse of limited informtion ville, it is common prctice for production engineers to scle down the welding conditions suggested for lrge-scle RSW (e.g., from Reference 3) to suit their welding requirement. However, there re mny differences etween SSRSW nd LSRSW, e.g., SSRSW uses different welding equipment (with much more precise electricl nd mechnicl control), nd much lower electrode force. Furthermore, mterils to e welded in SSRSW re mostly non-ferrous metls. 13 The ojectives of this work re to study the weldility of thin sheet luminum (Al), rss, copper (Cu), nd develop prcticl guidelines for selection of process prmeters nd electrode mterils for SSRSW of thin sheet metls. MATERIALS AND EXPERIMENT PROCEDURE Bse metls used in this study included 0.2-mmthick Al (commercilly pure 1100-H18, full-hrd temper), rss (70 wt.%cu-30 wt.%zn, hlf-hrd cold rolled), Cu (commercilly pure 110, nneled). Lpwelded joints (Fig. 1) were mde using test coupons cut to pproximtely 40-mm long nd 6-mm wide. Joint qulity ws evluted using peel test (Fig. 1) tht ws performed using Chtillon Digitl Force Guge DFIS 2 t speed of 38 mm/min. Nugget Fig. 1. Schemtic of setup for () resistnce spot welding nd () peel test

3 1092 Zhou, Gormn, Tn, nd Ely RESULTS Vrious filure modes were oserved during the peel testing of welded joints, nmely, interfce fil c Fig. 2. Schemtic showing joint filure modes during peel test: () filure long interfce, () filure through nugget, nd (c) filure s utton pullout. dimeter ws estimted y mesuring the dimeter of pullout uttons during the peel test. Peel-tested smples were lso exmined under stereomicroscopy nd scnning electron microscope (SEM) for the existence of expelled metl trpped etween the sheets, which is considered to e the result of WME. The weldility of these mterils ws evluted y their permissile welding current rnges. Although electrode tip life ws not quntified in this work, ESS ws monitored through SEM exmintion of the sheet surfces tht were djcent to electrodes during welding. An lternting-current (t 60 Hz) power supply ws used for SSRSW; the RMS (Root Men Squre) current vlues were mesured using Miychi MM- 336A weld checker. Both Clss 2 (chromium copper lloy) nd Clss 14 (molydenum) electrodes 3 used in this work were commercilly ville t tip-fce dimeter of 3.2 mm nd shnk dimeter of 6.4 mm (Fig. 1). Unlike LSRSW, 3,7 the electrodes were not wter-cooled during SSRSW. The whole welding process ws semi-utomticlly controlled, i.e., n ir-pressure system ws triggered y foot pedl to pply electrode force fter two overlpped specimens were mnully plced etween the opposing electrodes. Welding current ws delivered to the stck fter the force hd reched preselected vlue. Welding current, rise time (fixed t 2 cycles in this study), nd weld time were ll preselected s inputs on the welding controls; however, squeeze time ws not mesured nd, more importntly, cooling time were neither controlled or mesured. Prior to welding, the smple surfces were clened using methnol. Fig. 3. () Peel force nd () nugget dimeter versus welding current using different electrodes (Clss 2 nd Clss 14) nd electrode forces (in kilogrm) for the rss joints. Weld time is 8 cycles. ure, weld filure, nd utton pullout (Fig. 2). Interfce filure ws due to lck of onding or only wek onding etween sheets (Fig. 2). Once weld nugget formed, joints generlly filed through the nugget when the nugget dimeter ws smll or y utton pullout when it ws ove certin size (Fig. 2 or c). Brss Figure 3 shows the plots of peel force or nugget dimeter versus welding current for the rss joints when using Clss 2 or Clss 14 electrodes, nd t weld time of 8 cycles. Both welding current nd electrode force ffected nugget size nd joint strength. Stereoscopic nd SEM oservtions indicted tht weld nuggets generlly ppered very porous, which is elieved due to the very low oiling temperture of zinc (907 C). Zinc will voltilize from the molten metl even with slight superhet. 14 When using Clss 2 electrodes, WME (Fig. 4) strted t welding current of out 2.0 ka (corresponding to nugget dimeter of out 0.8 mm). ESS ws not experienced; however, surfce voids were oserved when the welding current ws 2.6 ka (Fig. 4). If minimum nugget dimeter of 0.4 mm (corresponding to joint strength of out 3 kg) is required, the minimum current needed is out 1.6 ka. The mximum permissile current cn e set t 2.6 kg since

4 Weldility of Thin Sheet Metls during Smll-Scle Resistnce Spot Welding using n Alternting-Current Power Supply 1093 Fig. 4. () An exmple of pulled utton from rss joint with 2.6-kA welding current, Clss-2 electrodes, 4.5-kg electrode force nd 8-cycle weld time. Note the metl tht ws squeezed out during WME (pointed y rrow) nd voids t the utton surfce; () detils of the voids t the utton surfce. Tle I. Welding Current (in kilomps) for 0.4-mm-Dimeter Nugget, WME, nd ESS Sheet Metls Al Brss Cu Electrodes Minimum* Expulsion Sticking Suggested Rnge Clss 2 Clss 14 Clss 2 Clss 14 Clss 2 Clss > >2.1 ~ ~ * The minimum current is determined to produce 0.4-mm-dimeter of weld nuggets. Electrode force is 4.5 kg nd weld time is 8 cycles. Fig. 5. () SEM microgrph showing sheet surfce tht ws djcent to n electrode in rss joint using Clss-14 electrodes, 4.5-kg electrode force, 1.4-kA welding current nd 8-cycle weld time. Note ig hole resulting from molten metl nd melted re (pointed y n rrow); () Detils of the re tht is pointed out y n rrow in (). Note the solidifiction ptterns nd mny other smll melting res. WME did not result in reduction in joint strength nd severe ESS reduces the electrode tip life. Therefore, the current rnge for SSRSW of rss using Clss-2 electrode, 4.5-kg electrode force nd 8-cycle weld time cn e recommended s ka (Tle I). When using Clss 14 electrodes, no WME ws oserved t welding currents up to 1.8 ka. Incresing electrode force from 4.5 kg to 6.8 kg incresed the current threshold to form weld from out 1.0 ka to 1.2 ka (Fig. 3). ESS strted t welding current of ka; incresing electrode force ppered to reduce the tendency of ESS. If gin minimum 0.4-mm

5 1094 Zhou, Gormn, Tn, nd Ely Fig. 6. () Peel force nd () nugget dimeter versus welding current for the Al joints using Clss-14 electrodes, 8-cycle weld time nd different electrode forces (in kilogrm). nugget dimeter is required nd severe ESS is to e voided, the current rnge for SSRSW of rss using Clss-14 electrode, 4.5-kg electrode force nd 8-cycle weld time cn e selected s ka (Tle I). A comprison of the minimum welding current indictes tht lower welding current ws needed to join rss when using Clss 14 electrodes compred with tht when using Clss 2 electrodes (Fig. 3 nd Tle I). However, ESS ws more severe when using Clss 14 electrodes. The onset current for ESS is lower thn tht for WME when using Clss 14 electrodes, ut higher thn tht for WME when using Clss 2 electrodes (Tle I). It is elieved tht ESS ws cused y locl melting t the electrode-sheet interfce (Figs. 4 nd 5). When using Clss 14 electrodes, the ESS ws worse ecuse higher electricl resistivity nd lower therml conductivity of Clss 14 electrodes compred to Clss 2 electrodes would result in higher temperture t the electrode-sheet interfce. Aluminum Figures 6 8 show the plots of peel force or nugget dimeter versus welding current for the Al joints. No effect of electrode force on joint strength nd nugget dimeter ws oserved when welding Al, which is Fig. 7. () Peel force nd () nugget dimeter versus welding current for the Al joints using Clss-14 electrodes, 13-cycle weld time nd different electrode forces (in kilogrm). different from the welding of rss. The reson my e due to the existence of tencious luminum oxide. When using Clss 14 electrodes, incresing welding current incresed the joint strength first, nd then decresed the joint strength, which my e the result of WME nd/or incresed softening of the hetffected zone (HAZ). At 8-cycle weld time (Fig. 6), WME strted t welding current of out 1.0 ka (corresponding to 0.8-mm nugget dimeter pproximtely); lrge voids were oserved on pullout uttons when WME occurred. ESS ws experienced when welding current exceeded ka; incresing electrode force ppers to decrese the tendency of ESS. SEM nlysis showed the ESS ws gin cused y Al surfce melting (Fig. 9). Incresing weld time decresed the current threshold to form weld ut ppered to reduce the joint strength for the higher current vlues (compring Fig. 6 to Fig. 7) nd increse the tendency of ESS nd WME. The current rnge would e recommended s ka for SSRSW of Al using Clss-14 electrodes, 4.5-kg electrode force nd 8-cycle weld time (Tle I). The minimum current is determined t 0.4-mm nugget dimeter (corresponding to joint strength of out 0.8 kg). The mximum current is set t the onset current for WME since the joint strength strted to decrese.

6 Weldility of Thin Sheet Metls during Smll-Scle Resistnce Spot Welding using n Alternting-Current Power Supply 1095 form weld, ut the dt is very scttered nd regression nlysis indicted tht this effect is not sttisticlly significnt (ssuming confidence level of 90%).15 A limited numer of trils lso indicted no decrese in joint strength nd nugget dimeter when the weld time ws reduced from 8 cycles to 4-6 cycles.15 The current rnge would e recommended s ka for SSRSW of Al using Clss-2 electrodes, 4.5-kg electrode force nd 8-cycle weld time (Tle I). The minimum current is gin determined sed on minimum 0.4-mm nugget dimeter. The mximum current is set t 2.1 ka since the joint strength did not decrese when the current is lrger thn the onset current for WME. Similr to the welding of rss, lower welding current ws needed to join Al when using Clss 14 electrodes compred with tht when using Clss 2 electrodes. However, the permissile current ws much smller when using Clss 14 electrodes thn tht when using Clss 2 electrodes (Tle I). Copper Fig. 8. () Peel force nd () nugget dimeter versus welding current for Al joints using Clss-2 electrodes, 8-cycle weld time nd different electrode forces (in kilogrms). When using Clss 2 electrodes, WME strted t welding current of out 1.7 nd 2.0 ka (corresponding to pproximtely 0.7-mm nd 0.8-mm nugget dimeter) when the electrode force ws 2.3 kg nd 4.5 kg, respectively. There is no WME oserved t welding currents up to 2.1 ka when the electrode force ws 6.8 kg. Therefore, incresing electrode force decresed the tendency of WME. No ESS ws experienced when welding currents were up to 2.1 ka. Although Fig. 8 ppered to show tht incresing electrode force decreses the current thresholds to There ws only limited success in SSRSW of Cu. The resons re tht the power required is very high due to very high therml conductivity nd low electricl resistnce of Cu, nd the high het genertion cuses severe ESS or even welding etween the Cu sheets nd electrodes. When using Clss 2 electrodes, the peel force ws out 1 kg when welding current ws out ka (Fig. 10). Joint strength could e further improved y incresing welding current; however, ESS ecme incresingly severe s well, nd the electrodes nd Cu sheets welded together when the welding current ws 3.8 ka (Fig. 11). WME lso occurred when the welding current ws 3.8 ka. The joints produced t this current level chieved joint strength of 3 4 kg nd the dimeter of the pullout utton ws out 1 mm. Limited trils t shorter weld time (4 6 cycles) hs shown tht weld time hs little effect on the nugget size nd joint strength;15 therefore, welding using Clss 2 electrodes cn e done t shorter weld time thn 8 cycles. When using Clss 14 electrodes, the joint strted to form t welding current of out 2.0 ka nd the peel force ws out 1 kg (Fig. 10). ESS ws oserved in ll c Fig. 9. () SEM microgrph showing sheet surfce djcent to n electrode in n Al joint using Clss-14 electrodes, 4.5-kg electrode force, 1.1-kA welding current nd 8-cycle weld time. Detils of () the surfce melting holes nd (c) the melting t grin oundries in ().

7 1096 Zhou, Gormn, Tn, nd Ely Fig. 10. () Peel force nd () nugget dimeter versus welding current for the Cu joints using 4.5-kg electrode force, 8-cycle weld time nd different electrode forces. of these joints (Fig. 12). Joint strength ws improved to out 5 kg when the welding current ws out 2.4 ka. However, it ws ovious from the color chnge t oth the electrode tips nd Cu specimen tht they were over-heted. Joint strengths were decresed to kg t reduced weld times of 2 6 cycles when the welding current ws held constnt t 2.4 ka. This indictes tht weld time hs much lrger effect on the nugget formtion nd joint strength when using Clss 14 electrodes compred to tht when using Clss 2 electrodes. Cu is one of those metls tht re the lest suitle for RSW ecuse of its low electricl resistivity nd high therml conductivity.3,7 Only limited success ws chieved in this study. Oservtions on WME nd ESS when using different electrodes were similr to those for Al nd rss. The effect of weld time on nugget formtion nd joint strength ws clerly shown when using Clss 14 electrodes. Further work is needed to study the SSRSW of Cu ecuse of its wide use in electronic pplictions. DISCUSSION Weldility of Sheet Metls: Process Prmeters Process prmeters (welding current, electrode Fig. 11. () Pullout utton from Cu joint using Clss-2 electrodes, 4.5kg electrode force, 8-cycle weld time nd 3.8-kA welding current. Note the WME nd lso frctured re on the top of the utton pointed y n rrow. () Detils of the re tht is pointed out y the rrow in (). Frctured surfce ws cused y the weld etween the electrode nd sheet Cu. force, weld time, etc.) ll my ffect the joint strength nd nugget size. However, welding current is the most significnt vrile ffecting nugget formtion nd growth ecuse the power generted is proportionl to the squre of welding current s indicted in Eq. 1. The requirement for welding current is lso relted to other process vriles, e.g., lower welding current ws needed when using Clss 14 electrodes compred with tht when using Clss 2 electrodes ecuse of the higher electricl resistnce nd lower therml conductivity of the Clss 14 electrodes. When welding rss, electrode force strongly ffects the current threshold to form weld since electrode force strongly influences the contct resistnce y plsticlly deforming locl contct points nd reking down surfce contminnt lyers.16 Once molten metl zone is formed, contct resistnce is gretly reduced nd its role in nugget development is decresed. Although lower electrode force could reduce the current requirement for forming

8 Weldility of Thin Sheet Metls during Smll-Scle Resistnce Spot Welding using n Alternting-Current Power Supply 1097 Fig. 12. Pullout utton from Cu Joint using Clss-14 electrodes, 4.5- kg electrode force, 8-cycle weld time nd 2.2-kA welding current. Note the surfce voids tht were cused y the ESS. welds y incresing the contct resistnce, it my lso led to unstle/inconsistent resistnce vlues t the contct interfces, 17 which is undesirle in terms of process control. Very high contct resistnce my lso cuse initil splshing/expulsion t the interfce. 18 However, this electrode force effect ws not oserved when welding Al, which my e due to the existence of tencious luminum oxide. It hs een shown tht the sttic contct resistnce t the room temperture etween Al sheets ws not ffected y the electrode force from 0.5 to 10 kg. 15 The mximum nugget dimeter without WME ws out 0.8 mm in this study nd ws much smller thn the electrode tip dimeter of 3.2 mm (lthough the expulsion did not result in gret reduction of the joint strength). This is very different from the LSRSW in which nugget cn generlly grow to size tht is similr to the tip dimeters of the electrodes without WME. 8 The reson for this difference is due to much smller electrode force used during SSRSW. In other words, electrode force determines the mximum nugget dimeter without WME when the electrode geometry is kept constnt; this spect hs een shown y computer simultion. 6,19 It is thought tht this mximum nugget dimeter should lso e relted to mterils chrcteristics, lthough no such reltionship hs een oserved in this work. It hs een reported tht higher electrode forces rodened the process window of welding current, 16 which my e ecuse electrode force cn increse the onset current for WME more thn it cn increse the threshold current to form weld. However, lrge electrode force cn led to excessive surfce indenttion, which is often undesirle during microjoining or precision welding. The effect of weld time ws oserved when using Clss 14 electrodes in which the higher electricl resistivity nd lower therml conductivity of Clss 14 electrodes contriuted to the nugget formtion nd growth. A longer weld time llows more het to e conducted to the sheet metls. However, longer weld time would increse the softening effect t the HAZ nd hence decrese the joint strength when welding cold-worked sheet metls (such s the sheet Al in this study). It hs een oserved tht cold-worked Al (1100-H8) will lose lmost 80% of its originl strength t 200 C. 14 ESS ws mjor prolem when using Clss 14 electrodes compred with Clss 2 electrodes during SSRSW of Al, rss nd Cu lthough using Clss 14 electrodes leds to lower welding current requirement. Higher electricl resistivity nd lower therml conductivity of the Clss 14 electrodes would result in higher temperture t the electrode-sheet interfce, which would promote ESS. Higher electrode force usully reduces the contct resistnce t the electrode-sheet interfce nd, hence, would decrese the het/temperture generted t the interfce nd hence my reduce the tendency of ESS. Incresing rise time or dding current pre-pulse re other wys to reduce the ESS ecuse they cn grdully reduce the contct resistnce etween the electrode nd the workpiece when the current is low, hence reducing the het/temperture generted t the interfce. 15 Weldility of Sheet Metls: Bse Metl Physicl Properties The weldility of Al, rss, nd Cu cn e compred sed on the welding current required to produce given nugget dimeter (e.g., for 0.4-mmnugget, Tle I). Cu is included in Tle I for comprison lthough further work is needed to develop the process prmeters to efficiently weld Cu. It cn e seen tht the weldility of these metls cn e listed in decresing order of Al, rss, nd Cu when using oth Clss 2 nd Clss 14 electrodes, which is not exctly in the sme order of their resistivity or therml conductivity (i.e., rss > A l > Cu, Tle II). 20,21 The current for given nugget dimeter when using Clss 14 electrodes for given metl is lwys lower thn tht when using Clss 2 electrodes, which is resonle ecuse of the higher electricl resistivity nd lower therml conductivity of the Clss 14 elec- Tle II. Physicl Properties Used for Clcultions 20,21 Al Brss Cu Zn Melting point ( C) T (K) Therml conductivity (W/m/K) Electricl resistivity (µωcm) Density (g/cm 3 ) Specific 20 C * (J/kg/K) Ltent het of fusion * (J/g) Totl het (q N ) y Eq * The specific het of rss nd ltent het of fusion of rss re ssumed to e 70% of tht of Cu plus 30% of tht of Zn.

9 1098 Zhou, Gormn, Tn, nd Ely trodes compred to the Clss 2 electrodes. The following is n ttempt to provide qulittive explntion on the oserved weldility order of Al > rss > Cu when using Clss 2 electrodes. Weld nugget formtion depends on the interply of het genertion nd het dissiption in the electrodeworkpiece system. Mthemticlly, Q G = Q N + Q L (2) where, Q G is the het genertion, Q N is the totl het required to form weld nugget, nd Q L is the het loss y conduction into the workpieces nd electrodes, which is determined y the therml conductivities nd geometricl shpes of the workpieces nd electrodes. Assuming Q L = fq N, Eq. 2 ecomes, Q G = (1 + f)q N (3) where f is rtio determined y the reltive mgnitude of Q L nd Q N. Recll, het genertion cn e expressed s, Q G = I 2 Rt (4) where, I is the welding current, R is the electricl resistnce of the workpiece, nd t is the weld time. Therefore, the het genertion is determined y oth process prmeters (i.e., welding current nd weld time), nd the electricl resistivity nd geometricl shpe of the workpieces. The het generted from the electrodes is neglected in this nlysis, which is good pproximtion since the electricl resistivity of the Clss 2 electrodes is very low. The het generted from the contct resistnce t fying interfces is lso neglected to keep the nlysis simple nd workle, which my e resonle when the nugget is firly well developed. The totl het required to form weld nugget (Q N ) includes t lest two prts: the first to het the weld metl to its melting point nd the second to melt the weld metl to form molten nugget (other fctors, such s the het to overhet the molten metl, re neglected in this nlysis). Therefore, Q N = q N V = (ρc p T + ρh) V (5) where, q N is the totl het to form weld nugget per unit volume, ρ is the density of the weld metl, C p is the specific het, T is the temperture rise from the room temperture to the melting point, V is the volume of weld nugget, nd H is the ltent het of fusion per unit volume. Comining Eqs. 3 5 leds to, It f V q 2 N = ( 1+ ) (6) R To compre the welding current required to produce given nugget dimeter t n identicl weld time for different sheet metls, the ove eqution cn e rrnged to e, I Al : I rss : I Cu = ( 1+ ) : ( 1 ) : ( 1 ) + + f q f q f q N N N R R R Al rss Cu (7) Tle III. Comprison of Experimentl nd Clculted Welding Current to Produce 0.4-mm-Dimeter Weld Nuggets Al Brss Cu Experimentl Clculted Note: Both experimentl nd clculted current vlues were normlized using the current for Al using Clss 2 electrodes, respectively. Since the geometry of sheets nd electrodes were identicl for Al, rss, nd Cu, the resistivity of sheet metls cn e used to replce R in Eq. 7. It is lso ssumed tht the rtio of het loss versus the totl het required to from weld nugget (i.e., the f vlue) is identicl in the welding of ll the sheet metls, so the term (1 + f) cn e cncelled out from Eq. 7. With these ssumptions, the current vlues in Eq. 7 re clculted using metl s resistivity nd q N (Tle II) nd then normlized using the clculted current for Al: I Al : I rss : I Cu = 1.0 : 1.3 : 2.9 (8) All these clculted current vlues re listed in Tle III to e compred with the experimentl results. Tle III indictes tht the current order of the experiment is the sme s tht provided y Eq. 8, which implies tht weldility is not only ffected y metls electricl resistivity (or therml conductivity), ut lso ffected y other physicl properties (such s melting point, het of fusion, specific het). However, it is worth pointing out tht the ove nlysis is just qulittive (or semi-quntittive). While the order is correct, the rtios do not gree quite so well ecuse of the very complexity of het genertion nd dissiption in the process. For exmple, no considertion of contct resistnce nd electrode resistnce, nd the ltent het due to Zn vporiztion were included in Eqs. 5 nd 7. The ssumption of n identicl rtio of het loss versus the totl het required to from weld nugget for the sheet metls is questionle since, in relity, the f vlue is neither constnt in the process nor constnt for ll different mterils. However, more detiled nd quntittive nlysis requires numericl modeling. 6,19 CONCLUSION The resistnce weldility of 0.2-mm-thick sheet luminum, rss, nd copper in smll-scle resistnce spot welding (SSRSW) ws studied. The effects of electrode mterils nd process prmeters (welding current, electrode force, nd weld time) on joint strength nd nugget size were investigted. The welding current rnges for SSRSW of the sheet metls were determined sed on the minimum current levels tht produce required nugget dimeter nd the mximum current vlues tht did not result in electrode-sheet sticking (ESS) or weld metl expulsion (WME). Other mjor findings from this study cn e sum-

10 Weldility of Thin Sheet Metls during Smll-Scle Resistnce Spot Welding using n Alternting-Current Power Supply 1099 mrized s follows: Al nd rss re reltively esier to resistnce weld compred with Cu ecuse of their reltively higher electricl resistnce nd lower therml conductivity. It ws found tht resistnce weldility of sheet metls is not only determined y resistivity (or therml conductivity) ut lso ffected y other physicl properties (such s melting point, ltent het of fusion nd specific het). Welding current, electrode force nd weld time ll ffect joint strength nd nugget dimeter, with welding current hving the strongest effect. Incresing electrode force incresed the current threshold to form weld when welding rss; however, no such effect is oserved during welding of Al. Incresing electrode force seems lso to increse the onset current for WME nd ESS. The effect of weld time ws significnt when using Clss 14 electrodes in which the higher electricl resistivity nd lower therml conductivity of Clss 14 electrodes contriuted to the nugget formtion nd growth. The mximum nugget dimeter tht did not result in WME ws much smller thn the electrode tip dimeter, which is quite different from lrge-scle resistnce spot welding in which the mximum nugget dimeter is similr to the electrode tip dimeter. A lower welding current is needed when using Clss 14 electrodes compred with tht when using Clss 2 electrodes. However, the permissile current rnge is much smller ( ka) when using Clss 14 electrodes thn tht when using Clss 2 electrodes (1.0 ka) in the welding of sheet Al nd rss. ACKNOWLEDGEMENT The uthors would like to thnk Dr. G. Kelkr t the Edison Welding Institute nd Dr. B. H. Chng t the University of Wterloo for their useful discussion nd suggestions. We lso cknowledge the dontion of equipment used in this project y Unitek Miychi Corportion. REFERENCES 1. K.I. Johnson, editor, Introduction to Microjoining (Aington, MA: TWI, 1985). 2. C.A. Hrper, Hndook of Mterils nd Processes for Electronics (New York: McGrw-Hill, 1970). 3. Resistnce Welding Mnul, fourth edition (Phildelphi, PA: Resistnce Welder Mnufcturers Assocition, 1989). 4. D.W. Dickinson, J.E. Frnklin, nd A. Stny, Welding Journl, Welding Reserch Supplement 59, 170-s to 176-s (1980). 5. J.E. Gould, Welding Journl, Welding Reserch Supplement 66, 1-s to 10-s (1987). 6. D.J. Browne et l., Welding Journl, Welding Reserch Supplement 74, 417-s to 422-s (1995). 7. AWS, Welding Hndook, Vol. 1, Welding Technology, eighth edition (Mimi, FL: Americn Welding Society, 1987). 8. M.R. Finly, Austrlin Welding Reserch CRC No. 18 (Octoer 1996). 9. E.P. Ptrick, J.R. Auhl, nd T.S. Sun, Understnding the Process Mechnisms Is Key to Relile Resistnce Spot Welding Aluminum Auto Body Components, SAE Technicl Pper #840291(Wrrendle, PA: SAE, 1984). 10. H.R. Lst et l., IEEE Trnsctions on Components, Hyrids, nd Mnufcturing Technology 22, 338 (1999). 11. J.J. Fendrock nd L.M. Hong, IEEE Trnsctions on Components, Hyrids, nd Mnufcturing Technology 13, 376 (1990). 12. C. Knpp, G. Crevensten, nd D. Skinner, ICAWT 98: Joining Applictions in Electronics nd Medicl Devices (Columus, OH: EWI, 1998), pp Y. Zhou, C. Reichert, nd K.J. Ely, ICAWT 98: Joining Applictions in Electronics nd Medicl Devices (Columus, OH: EWI, 1998), pp AWS, Welding Hndook, Vol. 3, Mterils nd Applictions Prt 1, eighth edition (Mimi, FL: Americn Welding Society, 1996). 15. Y. Zhou, P. Gormn, nd K.J. Ely, unpulished work (Edison Welding Institute, 1998). 16. J.G. Kiser, G.L. Dunn, nd T.W. Egr, Welding Journl, Welding Reserch Supplement 61, 167-s to 174-s (1982). 17. W.L. Roerts, Welding Journl, 30, 1004 (1951). 18. E.V. Bumieris nd E.S. Lutsuk, Welding Interntionl 7, 988 (1993). 19. C.L. Tsi, O.A. Jmml, J.C. Ppritn, nd D.W. Dickinson, Welding Journl, Welding Reserch Supplement 71, 47-s to 54-s (1992). 20. R.R. Tumml nd E.J. Rymszewski, editors, Microelectronics Pckging Hndook (New York: Vn Nostrnd Reinhold, 1989). 21. E.A. Brndes, Smithells Metls Reference Book, sixth edition (London: Butterworths, 1983).