Characterization of Spot Welding Behavior by Dynamic Electrical Parameter Monitoring

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1 Characterizatin f Spt Welding Behavir by Dynamic Electrical Parameter Mnitring The use f tw parametersdynamic resistance and critical expulsin energyis prpsed fr cntrlling resistance spt welding BY D. W. DICKINSN, J. E. FRANKLIN AND A. STANYA ABSTRACT. A prgram was undertaken t develp techniques fr studying the resistance spt welding prcess. A dynamic electrical parameter mnitring device was designed t simultaneusly recrd the instantaneus values f vltage, current, pwer, and resistance during spt welding. The data btained using this technique have been analyzed in terms f the relatinships f these parameters t the phenmena ccurring during the frmatin f a spt weld, t the effects f changes in welding variables n electrical parameters, and t the effects f variatins in steel cmpsitin and prperties. The phenmena ccurring during spt weld frmatin (surface breakdwn, asperity cllapse, heating f the wrk pieces, mlten nugget frmatin, nugget grwth, and mechanical cllapse) can be understd thrugh analysis f dynamic resistance curves. If expulsin ccurs during a spt weld, it als is readily detected frm this parameter. A generalized resistance curve cnsists f an initial very rapid drp in resistance t a minimum value within the first few cycles f current flw. This is fllwed by a rise t a maximum and finally a gradual decrease. Dynamic resistance measurements can be used in btaining a better understanding f lbe curves. The shape f the dynamic resistance curve fr welds near the lbe bundary is crrelated t the bserved characteristics f these welds. The effects f changes in welding current and electrde frce can als be related t the dynamic resistance curves. Spt weldability varies as a functin f steel chemistry, and an imprved understanding f these material effects is btained by cmparing dynamic resistance curves fr varius high strength steels. Expulsin is analyzed in terms f pwer curves integrated t btain ttal weld energy input. It was fund that expulsin ccurs when the ttal useful energy int the weld exceeds a critical value. It is prpsed that these tw parameters, dynamic resistance and critical expulsin energy, can be used t cntrl the spt welding prcess. Intrductin It has lng been knwn that the prcess f spt welding ccurs thrugh the lcalized melting and calescence f a small vlume f material due t the heating caused by the passage f electric current. This heating is equivalent t the prduct f the current squared times the ttal resistance f the material t be spt welded. During the prcess f spt welding, hwever, bth the current and the resistance cntinually change as the material is heated and melts. Until recently, spt weld characteristics have been determined by measurement f the initial r static resistance and the initial current surge at the nset f welding, r an average RMS current value btained during welding. Recent papers,'" 6 hwever, have indicated that in rder t fully characterize the spt welding prcess, Based n a paper presented at the AWS 60th Annual Meeting held in Detrit, Michigan, during April 26, D. W. DICKINSN, I. E. FRANKLIN and A. STANYA are with the Research Center, Republic Steel Crpratin, Independence, hi a cntinuus r dynamic mnitring system is needed. This system wuld mnitr and recrd instantaneus changes in the electrical functins during welding. Several authrs 156 reprt cntinuus variatins in the electrical parameters during welding f mild steel sheet t be typified by the sketches presented in Fig. 1. After apprximately the first cycle, an increase in vltage acrss the welding electrdes and a decrease in current flwing thrugh the weld zne ccurs until a "peak regin" is reached. Thrughut the remaining prtin f the weld cycle, the vltage decreases t a cnstant value while the current increases t a cnstant value. These changes in vltage and current can als be represented as instantaneus r dynamic resistance. Several authrs 1,56 have calculated the dynamic resistance changes thrughut the welding cycle by dividing the instantaneus vltage by the instantaneus current (R = E/l). This dynamic resistance trace is als pltted in Fig. 1. After an initial drp, it t rises t a peak in the first prtin f the weld cycle, drpping ff later in the cycle. The appearance f this dynamic resistance trace and hence the basic welding mechanisms can be altered by changes in welding variables such as weld time, electrde frce, and verall weld current. Bhattacharya and Andrews 1 have made the fllwing bservatins: 1. With lw current values, the resistance trace des nt shw a prminent peak; with increased welding current, hwever, a welldefined peak appears. 2. With higher current values, the maximum r peak resistance ccurs 170sl JUNE 1980

2 earlier during a weld. 3. Fr welds prduced with high currents, the resistance trace indicates a lwer value f resistance twards the cmpletin f weld time. 4. A splash weld (expulsin), caused by excessive welding current, is characterized by a sudden step in the resistance trace. Savage 6 has attempted t explain the shape f the resistance curve n theretical grunds. When analyzing the resistance after each half cycle f current, he nted rectificatin which is believed t indicate the presence f xide films fr times as lng as 6 t 8 cycles n sme welds. He has als attributed the rise in resistance after the initial few cycles t be the result f the material heating. The latter decrease in resistance appears t cincide with the grwth f the fused zne. The sudden drp in resistance upn expulsin was attributed t the increase effective cntact area prvided by the expelled metal trapped between the sheets. These bservatins lay a pathway fr effective utilizatin f dynamic electrical measurements in mnitring and cntrl f spt welding. Hwever, befre such cntrl can be utilized, a characterizatin f these electrical measurements thrughut the entire range f acceptable spt weld parameters must be made. ne methd fr determining the range ver which acceptable spt welds are btained n plain carbn sheet is thrugh the use f spt weld lbe curves. A typical spt weld lbe curve is presented in Fig. 2. A lbe curve is a graphical representatin f ranges f welding variables ver which acceptable spt welds are frmed n a specific material welded with a preselected electrde frce. The lbe curve is determined by making spt welds using different weld time/weld current cmbinatins. Welds made with currents and/r times exceeding the upper curve experience expulsin n welding and are, therefre, cnsidered unacceptable. Welds made with currents r times belw the lwer curve have insufficient size nuggets r ex N EXPULSIN WELD, CYCLES Fig. 1 Schematic representatin f dynamic electrical parameters during spt welding f mild steel' _l > 2 _i.. " : ' ' : : ' ' ' A^AJ NUG SMALL R \ BRITTLE NUGGETS'fe V: III a EXPULSIN ;:\ LEVEL. ^^yk RMS SECNDARY CURRENT, AMPS Fig. 2Typical spt weld lbe curve hibit brittleness during tearing and are likewise cnsidered unacceptable. nly welds made with weld currents and times lying within the lbe area are acceptable. nce the lbe curve is knwn, characterizatin f the dynamic electrical measurement can be made at critical areas n the curve. Such critical areas are a) arund the lwer limit line, b) at expulsin, and c) within the acceptable regin. Investigatin Prcedure bjective The bjectives f this investigatin were t: 1. Build a spt weld mnitring system capable f measuring dynamic values f vltage, current, resistance, and pwer. 2. Relate the phenmena ccurring during spt weld frmatin t the changes in these basic electrical measurements thus cnfirming sme earlier speculatins and develping new insight. 3. Characterize the dynamic electrical measurements ver the range f weld parameters giving acceptable spt welds (i.e., ver the lbe curve area) as a first step t using these electrical measurements fr cntrl f the spt welding prcess. Materials The spt weldability f several heats f plain carbn and high strength ht rlled and cld rlled steels was investigated. Specific chemical cmpsitins f the materials fr this investigatin are listed in Table 1. These include plain carbn, rimmed, renitrgenized, Cbadded, Cb + Vadded and Cb 4 Mnadded steels. Als, sme welds were made n a Type 304 stainless steel. Except where specifically nted, mst f the data discussed are related t the welding f a in. (0.8 mm) thick plain carbn aluminumkilled material (material A in Table 1). Equipment All welding was perfrmed n tw TaylrWinfield airperated electric resistance spt welding machines. These are equipped with Weldtrnic cntrllers fr electrnic phase cntrl (heat adjustment) and Duffers current analyzers fr simultaneus measurements f weld time and secndary RMS weld currents. Electrde frces were accurately set at the recmmended frce 7 fr the sheet thickness being welded and checked several times thrughut the welding prgram using a universal frce gauge. Electrnic Circuits fr Mnitring the Dynamic Electrical Parameters A schematic f the circuits used t develp the welding parameters is shwn in Fig. 3. The instantaneus vltage acrss the electrdes is sensed by prbes attached t the electrde hlders. Table 1 Chemical Analyses, % Materia Type Nminal gage, in. C Mn Si S P Cu Ni Cr Cb V Al N A B C D E F G H"» Plain C, AK Renitrgenized High Cb, lw Mn Lw Cb, high Mn High Cb, high Mn Plain C CbV 304 stainless < <.08 <.08 <.008 < < , C WELDING RESEARCH SUPPLEMENT I 171s

3 + T RECRDER I00 WELD CURRENT, AMPERES Fig. 4Lbe curve fr plain carbn AK steel ( in., i.e., 0.81 mm) Fig. 3Schematic f circuits used t develp spt welding dynamic electrical parameters Prbes in this lcatin sense the entire vltage drp attributed t the electrdes and the wrk pieces. ther wrkers 6 have attached prbes at different lcatins, such as the sheets being welded; hwever, prbes permanently munted t the electrdes r electrde hlder are mre durable and perfrm ver lng weld campaigns. The sensed vltage is amplified fr prper scaling. The amplified vltage signal is then simultaneusly sent t a switch (SW1) and an analg cmputing mdule which cnverts the instantaneus vltage int a signal that is prprtinal t the rtmeansquare r effective vltage. The utput f the vltage RMS mdule is then sent t SW1, an analg divider mdule input, and a recrder channel. The instantaneus current is sensed by a HallEffect device, amplified fr prper scaling and sent t SW1 and the input f an analg RMS mdule which cnverts the instantaneus signal int a signal that is prprtinal t the effective r heating value f the welding current. The RMS current signal is then sent t SW1, the input f an analg divider mdule and a recrder channel. When SW1 is in psitin 1, the instantaneus vltage and current signals are cnnected t an analg multiplier mdule whse utput is prprtinal t the prduct f the instantaneus vltage and current which is the instantaneus pwer dissipated acrss the welding electrde. When SW1 is in psitin 2, the utputs f the vltage and current mdules are cnnected t the inputs f the multiplier and the utput signal is the prduct f the effective vltage times the effective current. The prduct f the instantaneus vltage and current is equal t the prduct f the effective vltage and current when vltage and current are in time synchrnizatin ("in phase"). The utput f the multiplier is sent t a channel f the recrder. The signals frm the current and vltage RMS mdules are als cnnected t the inputs f an analg divider mdule. This mdule divides the vltage signal by the current signal and develps a new signal which is prprtinal t the changing resistance (dynamic resistance) between the electrdes during the welding cycle. This signal is then sent t a recrder channel. The high speed strip chart recrder can then simultaneusly recrd the effective welding current, effective vltage acrss the welding electrde, pwer dissipated during the welding cycle, and the changing resistance between the electrdes during the welding cycle. Determinatin f Lbe Curve fr Plain Carbn Steel The in. (0.81 mm) plain carbn aluminumkilled material was selected as the base material. The spt weldability lbe curve was determined fr 2 CYCLES 4 CYCLES this material using cnventinal prcedures. 7 (Lbe curves were determined fr all materials; nly the plain carbn material is presented.) The weld times, in half cycles, and the secndary RMS weld current were measured with a Duffers current analyzer. The lbe curve determined is presented in Fig. 4. As expected, this plain carbn material exhibited a wide range f acceptable welding currents and is cnsidered t be weldable. N "brittle" centerline nugget failures were nted fr this material; therefre, the lwer bundary f the lbe curve represented the limit f acceptable size nuggets as required in specificatins. 7 Dynamic Electrical Measurement Dynamic Resistance Measurements arund Lwer Lbe Bundary As a means f characterizing the dynamic electrical parameters in relatin t the spt weldability lbe curve, several welds were made with a weld current held cnstant (at apprximately 8000 A) and varying weld time frm 2 t 12 cycles. A full cmplement f dynamic electrical measurements was made. This allwed the characterizatin f electrical parameters arund the lwer lbe bundary (insufficient nugget size line) f the lbe curve presented in Fig CYCLES I 2 CYCLES Fig. 5Dynamic resistance curves arund lwer lbe bundary 172s I JUNE 1980

4 TEMPERATURE INCREASE ASPERITY SFTENING NUGGET GRWTH MECHANICAL CLLAPSE Fig. 6Theretical dynamic resistance curve ,000 WELD CURRENT, AMPS Fig. 7Effect f secndary RMS current level n dynamic resistance The electrical parameter prving the mst useful was the dynamic resistance. Figure 5 shws the dynamic resistance curves determined alng with metallgraphic crsssectins f the respective welds. The curves presented thrughut are thse btained by cnnecting the peak pints f the dynamic resistance fr each half cycle f weld time. The weld made fr tw cycles shwed evidence f heating, but n melting was seen n the weld crss sectin. The dynamic resistance curve drpped frm its initial high value t a minimum and just started t increase again at the terminatin f the tw cycle weld time. n the fur cycle weld sme melting was nted. This was accmpanied by further develpment f the dynamic resistance curve. The resistance shwed a substantial increase after passing thrugh the minimum pint. The six cycle weld develped a nugget diameter apprximately equal t the minimum acceptable diameter and, therefre, fell clse t the lwer limit f the lbe curve. The resistance curve fr this weld was fully develped with weld terminatin near the peak in the curve. The weld made at twelve cycles fell well within the acceptable regin f the lbe curve. The weld nugget was fully develped and the dynamic resistance curve exhibited the full characteristic shape as bserved by thers. 1 "" Interpretatin f Dynamic Resistance Curves Based n the abve analysis and upn similar analyses f ther authrs, 1 " 6 the fllwing interpretatin fr the typical shape f the dynamic resistance curves is given. With reference t Fig. 6, the stages f spt weld frmatin can be described as fllws: Stage /. The wrk pieces are brught int cntact under the pressure prvided by the electrde frce. This creates areas f electrical cntact at the pints where asperities n the surfaces meet. Vltage is applied between the electrdes causing current t flw at the micrcntact pints. The resistance between electrdes at this pint is equal t the sum f the bulk resistance f the tw wrk pieces, the tw electrdetwrk cntact resistances, and the wrktwrk cntact resistance. Under nrmal cnditins, surface films, xide layers, r ther cntaminants will be present n the wrk pieces. Since these are essentially insulatrs, the initial cntact resistance will be very high. Therefre, the initial generatin f heat will be cncentrated at the surfaces, especially at the wrktwrk cntacts. This heat will cause the surface cntaminants t break dwn, resulting in a very sharp drp in resistance. Stage II. Immediately after the breakdwn f surface cntaminants, metaltmetal cntact exists. Hwever, the surface resistance may still remain relatively high due t the limited area fr current flw prvided by the asperity cntacts. Heating then is cncentrated at the wrktwrk surface, and temperature in this regin and in the bulk material will increase. As heating prgresses, the asperities sften and the cntact area increases thus causing resistance t decrease. At the same time increasing temperature results in increasing resistivity, thus prviding an ppsite effect. The cmpetitin between these tw mechanisms determines whether resistance is increasing r decreasing and thus determines the psitin f the a minimum. Eventually, the increase in cntact area will be vercme by the increasing temperature effect, and the ttal resistance will begin t rise. Stage ///. During this perid, the increase in resistivity resulting frm increasing temperature dminates the resistance curve. The end f Stage III shuld crrespnd t lcal melting beginning t ccur at the asperity cntacts. The transitin t Stage IV will prbably ccur near the inflectin pint in the curve (d L R/dt 2 = 0). Stage IV. Three mechanisms influence Stage IV. The bulk f the wrk pieces cntinue t increase in temperature, thus causing resistivity and resistance t increase. But, the heat being generated als causes additinal melting t ccur at the surfaces, increasing the size f the mlten regin and the crsssectinal area available fr current flw. This mechanism causes a resistance decrease. Als, increased sftening will result in sme mechanical cllapse, shrtening the path fr current flw and decreasing resistance. The P peak is a cnsequence f the temperature beginning t stabilize, while nugget grwth and mechanical cllapse begin t dminate, and therefre resistance starts t decrease. Stage V. Beynd the fi peak, the grwth f the mlten nugget and mechanical cllapse cntinue t cause resistance t decrease. If the nugget grws t a size such that it can n lnger be cntained by the surrunding slid metal under the cmpressive electrde frce, expulsin will ccur. This series f events, Stages I thrugh V, ffers a cnsistent interpretatin f the shape f the dynamic resistance curves bserved fr spt welds made in the plain carbn AK material. Variables expected t cause significant variatin in the shape include current level, electrde frce, and material being welded. These factrs are cnsidered belw. Effect f Current Level n Dynamic Resistance Curve In rder t examine me effect f WELDING RESEARCH SUPPLEMENT 1173s

5 Fig. 8Effect f electrde frce n dynamic resistance curve varying current level upn the shape f the dynamic resistance curve, a number f welds were made in the (0.81 mm) plain carbn AK steel at 12 cycle weld time with varying current levels. The current levels investigated were: 1. Thse that fell just belw the acceptable lbe regin, where mlten nuggets were frmed but were f insufficient size. 2. Thse that fell within the acceptable regin f the lbe curve. 3. Thse experiencing expulsin. Thus, a full range f current levels were studied. The results are presented in Fig. 7. At lw currents, when an undersize r brittle nugget frms, the curves shw a distinct initial resistance spike which decays within the first tw cycles. At currents near the middle f the acceptable welding range, the initial spike is still present but the decay is mre rapid. At expulsin level currents the decay is s rapid that the initial spike is barely detected. Since the resistance spike is believed t be a result f the breakdwn f surface films and the cllapse f asperities during the initial stages f the welding pulse, the time required fr it t decay will depend n the rate at which heat is supplied t the weld, i.e., the pwer (E X I). Therefre, at lw currents a measurable amunt f time is required fr sufficient heat t be supplied; hence, a distinct decaying spike is detected. At high currents, surface film breakdwn and asperity cllapse are s rapid that the instrument is nt capable f recrding the initial spike. In this case, Regin I f the curve in Fig. 6 is just barely bserved and the a minimum is reached almst immediately after current begins t flw. With regard t the /3 maximum, Battacharya and Andrews 1 indicate that this is the pint at which melting f sheettsheet cntacts first ccurs. They als prpse that the decreasing slpe f the curve near /? is a result f partial cllapse f the sheettsheet interface brught abut by sftening. The present wrk indicates that melting ccurs prir t the /3 peak and that the peak itself merely signifies the < QC UI z IS t UI QC n. III SE IB cc % t ce < a. Z C I ' M _l_l_ A u. M. I K* ' i i i u.* \ E u^ MATERIAL A MATERIAL B MATERIAL C PLAIN RENITRGENIZED LW Mn/ CARBN AK HIGH Cb LJ " \ la. i m i 0 12 MATERIAL D HIGH Mn/ LWCb L u l^\ A ^ 0 12 MATERIAL E HIGHMn/ HIGH Cb WELD, CYCLE Fig. 9Cmparisn f dynamic resistance curves fr five different materials (A = acceptable nugget, U = undersize nugget, B = brittle nugget, E = expulsin) pint at which resistance drp due t mlten nugget grwth and mechanical cllapse vercmes the resistance rise resulting frm increasing temperature. Figure 5 supprts the hypthesis that melting ccurs prir t the /3 peak. The /? peak in Fig. 5 ccurs at abut 6 cycles, but this Fig. 5 shws that a substantial amunt f melting has ccurred after nly 4 cycles. N melting ccurred up t 2 cycles. Therefre, because the /J peak is a balance pint between a resistance increase and a resistance drp, the psitin f the ft peak must be dependent upn heating rate (i.e., current level). At very high currents (expulsin level), the ft peak is reached early in the welding pulse (after 45 cycles in Fig. 7). In the middle current range /? is reached after 56 cycles. At lw currents, 7 r mre cycles pass befre the maximum. This behavir indicates that at lwer currents heating rate is slwer; therefre, mre time is required fr nugget grwth and mechanical cllapse t ccur. Since heating is prprtinal t pwer input (P = l*r), faster rates f heating and shrter times t reach j3 shuld be assciated with higher current levels r higher resistance. Effect f Electrde Frce n Dynamic Resistance Curve In rder t examine the effect f electrde frce n the shape f the dynamic resistance curve, spt welds were made in the in. (0.81 mm) plain carbn AK material with electrde frces varying frm 350 t 1500 lb (159 t 680 kg). The dynamic resistance traces btained frm three f these welds are presented in Fig. 8. As the frce was increased, the curve flattened such that the maximum resistance decreased and /? was shifted t later times in the welding pulse. Als, as the frce increased, there was a general decrease in the resistance level. Since all the welds were made at identical tap and heat cntrl settings, which resulted in similar current levels ( A Duffers current), decreasing resistance results in a decrease in the rate at which energy is supplied t the weld (P = IR). Thus, the fi peak is shifted t lnger times, and nugget frmatin is delayed as electrde frce is increased. Lwer electrde frces, therefre, shuld favr nugget frmatin. This hypthesis is cnfirmed by the results f nugget pullut tests n the welds represented in Fig. 8. Ductile nugget pullut ccurred nly at the lwest electrde frces. At higher frces, nuggets were brittle, and at the highest electrde frces n nugget was frmed. This cnfirms earlier wrk 8 which indicated that higher electrde frces tended t mve the lbe curve t the right. Effect f Material n Dynamic Resistance Curve The material variable may include nt nly the cmpsitin f the steels, but als the surface cnditin. Surface effects have been studied 6 8 and have been fund t have nticeable effects n spt weldability. Likewise, chemistry variatins have als been fund t have nticeable effects n spt weldability. 38 T demnstrate the effect f material n the shape f the dynamic resistance curve, full lbe curves (nt presented herein) were determined fr materials A thrugh E (Table 1). Using the lbe curves as a baseline, dynamic resistance traces were determined fr 174s I JUNE 1980

6 a. ii > QC z CD >,.*" '" ACCEPTABLE NUGGET..." '"..' '<y ACCEPTABLE, 1 NUGGET LW TAP SETTING HIGH TAP SETTING rn ri r r ass* 'Gy EXPULSIN FULL SINE WAVE, % (HEAT CNTRL),.,.,.,M, 100 Fig. 10 Input energy as a functin f heat cntrl (% full since wave) and tap setting. The RMS vltage is als included fr reference H 4 > cc Z z 3 _ x I** 1 v~""? THERMAL CNDUCTIVITY AT I600 F (CAL cm/sec cm 2<l C) \J \ V^ \ S A: > RLLED SS ELECTRICAL RESISTIVITY AT I600"F (MICRHM cm) Fig. 11Expulsin energy as a functin f electrical resistivity, and thermal cnductivity a range f nugget cnditins frm undersize nugget t expulsin at 12 cycle weld current, exactly as indicated fr material A in Fig. 7. The dynamic resistance curves btained at three different secndary RMS current levels fr all five materials are cmpared in Fig. 9. Fr each weld, the appearance f the nugget is als indicated. Cmparisns between the five steels represented in Fig. 9 can be made by cnsidering: 1. The shape f the dynamic resistance curves assciated with acceptable welds. 2. Shape differences assciated with acceptable welds in different materials. The general shape f the dynamic resistance curve as prpsed earlier, cnsisting f an initial spike which decays rapidly fllwed by a secndary j3 peak, is bserved fr all five materials. It is clear that the curves assciated with acceptable welds in all materials exhibit a prnunced /? peak. Undersized welds resulted when the weld cycle was terminated befre r just slightly after reaching the /? peak. Expulsin resulted in a sharp drp in resistance. It shuld be nted that the lw current (unacceptable) weld in the renitrgenized steel resulted in a "brittle" nugget whse failure ccurred alng the weld cunterline during testing. The resistance curve, hwever, exhibited a distinct /? peak. As wuld have been predicted, it is clear frm metallgraphic sectins that large weld nuggets were btained n this weld. The prblem, therefre, in achieving gd welds in the renitrgenized steel is nt related t the inability t frm a nugget, but rather t the brittleness f the nugget. Since, at this pint, we are able t interpret the dynamic resistance curves nly in terms f nugget frmatin and nt in terms f weld metal prperties, it is difficult t relate resistance behavir t the weldability f the renitrgenized steel. Further wrk including the investigatin f cmpsitinal effects, as mentined abve, will be required t determine if additinal infrmatin can be derived frm the dynamic resistance curves. The variatin in the dynamic resistance curves assciated with different materials is als clearly evident in Fig. 8. The higher manganesecntaining steels and the renitrgenized steel shw higher verall resistance levels than des the plain carbn material. This is prbably the result f the high bulk resistance f these materials. Even mre distinct, is the rapid decay f the initial spike in material D. This may be the result f an early surface breakdwn fr this material. In all cases, hwever, the higher allyed materials develp the ft peak f earlier weld times. Thus, the evaluatin f the effect f variatin in material chemistry and surface cnditin is pssible by means f analyzing the dynamic resistance curves. Analysis f Dynamic Electrical Parameters at Expulsin As indicated earlier in Fig. 1, the expulsin pint n the dynamic resistance curve is nted by a sharp drp in electrical resistance. This drp in resistance is prbably the result f mechanical cllapse arund the nugget reducing the thickness f material thrugh which additinal current must flw tgether with the increased effective cntact area prvided by the expelled metal trapped between the sheets. In rder t btain additinal insight int the criteria fr expulsin, the dynamic electrical parameters were measured. Values f instantaneus pwer were recrded fr a series f spt welds and the energy input was determined by measuring the area under the pwer vs. time curve. During the curse f this investigatin, it became apparent that the best data were btained n the welds with the lnger weld times. Fr this reasn, thicker plain carbn sheet (0.053 in., r 1.3 mm), material F, was selected s the 15 cycle weld times culd be used. Welds were made with tw different transfrmer tap settings and heat settings (percent f full sine wave) ranging frm 60 t 100%. The results btained frm the pwer and energy determinatins are presented in Fig. 10. Here, the ttal weld energy input is pltted as a functin f the heat cntrl setting (percent full sine wave) up t the expulsin pint. Thereafter, fr higher heat cntrl settings, bth the ttal energy int the spt weld as well as the energy int the weld until expulsin ccurred is pltted. It is clear frm these data that acceptable weld nuggets are btained, irrespective f transfrmer tp setting r heat cntrl setting, up t a critical energy. It is speculated that the nset f expulsin in steels can be related t the critical energy input. At that pint, a balance between energy input by l 2 R heating and energy ut by cnductin, cnvectin r radiatin is n lnger maintained. The mlten nugget grwth ccurs uncntrlled thus resulting in expulsin. This being the case, the critical expulsin energy shuld be a functin f material parameters such as electrical resistivity and thermal cnductivity. Materials with high electrical resistivity and lwer thermal cnductivity will tend t develp the unstable cnditin at lwer energy input value. As a means f cnfirming this, critical expulsin energy values were determined fr a CbV material (material C) and a Type 304 stainless steel (material H). The results presented in Fig. 11 WELDING RESEARCH SUPPLEMENT 1175s

7 clearly shw the effect f the higher resistivitylwer cnductivity materials. Use f Dynamic Electrical Parameters fr Cntrl f Spt Welding Thrughut the wrk described herein, crrelatins have been made between the dynamic electrical parameter bservatins fr spt welds and the lcatin f thse spt welds relative t the acceptable weld lbe regin. Fr welds exhibiting undersized nuggets (i.e., fr welds made with weld parameters belw the acceptable lbe range), the dynamic resistance curve des nt shw an bvius ys peak. Acceptable welds are thse which have welding parameters set such that an bvius /3 peak in the dynamic resistance curve is btained, but als such that the critical expulsin energy input is nt reached. Thus, the lwer and upper bundary n acceptable spt welds is knwn. Equipment which can mnitr the dynamic parameters, cmpare the values with preset values, and then cntrl the spt weld heat cntrl and/r weld time t maintain the parameters within the bundary limits will cntrl the spt welding prcess. Cnclusins 1. A dynamic electrical mnitr system was built which allwed the cntinuus mnitring f vltage, current, resistance, and pwer during the spt welding prcess. 2. The shape f the dynamic resistance curve was related t the phenmena ccurring during spt welding. The ccurrence f the predminant ft peak signified the prductin f an acceptable size nugget. 3. Variatins in the shape f the dynamic resistance curve were related t variatins in material, secndary RMS current, and electrde frce. 4. A critical energy level fr expulsin was bserved. 5. Using infrmatin established fr dynamic resistance and critical expulsin energy, a spt weld cntrl mechanism was prpsed. References 1. Bhattacharya, S., and Andrews, D. R., "ResistanceWeld Quality Mnitring," Sheet Metal Industries (July 1972), pp Andrews, D. R., and Bhattacharya, S., "Quality CntrlResistanceWeld Mnitring fr Prductin," Metal Cnstructin and British Welding lurnal (May 1973), pp Jhnsn, K. I., "Quality CntrlResistance Welding QualityCntrl Techniques," Metal Cnstructin and British Welding lurnal (May 1973), pp Hayward, B. P., British Welding lurnal, 1967, 14 (11), pp Rberts, W. L., "Resistance Variatins During Spt Welding," Welding lurnal, 30(11), Nv. 1951, pp Savage, W. F., Nippes, E. F., and Wassell, F. A., "Dynamic Cntact Resistance f Series Spt Welds," Welding lurnal, 57(2), Feb. 1978, Research SuppL, pp. 43s t 50s. 7. Frd Mtr Cmpany Spt Welding Specificatin BA Dickinsn, D. W Haser, J. M., Ries, G. D., "Spt Weldability Cmparisn f Selected HSLA Steels," Republic Steel Research Reprt (January 1975). WRC Bulletin 254 Nvember 1979 (1) A Critical Evaluatin f Plastic Behavir Data and a Unified Definitin f Plastic Lads fr Pressure Cmpnents by J. C. Gerdeen (2) Interpretive Reprt n Limit Analysis and Plastic Behavir f Piping Prducts by E. C. Rdabaugh (3) Interpretive Reprt n Limit Analysis f Flat Circular Plates by W. J. 0'Dnnell These three reprts summarize a furyear effrt by the PVRC Task Grup n "Characterizatin f the Plastic Behavir f Structures" t meet the need fr unified and standardized methds fr limit analysis n plastic cllapse determinatins. Publicatin f this reprt was spnsred by the Pressure Vessel Research Cmmittee f the Welding Research Cuncil. The price f WRC Bulletin 254 is $13.50 per cpy. Please include $3.00 fr pstage and handling. rders shuld be sent with payment t the Welding Research Cuncil, 345 East 47th St., Rm 801, New Yrk,NY sl JUNE 1980

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