Expulsion and Its Influence on Weld Quality

Transcription

1 Expulsin and Its Influence n Weld Quality Welding schedules can be scientifically selected t effectively prevent expulsin while still btaining sizable welds BY H. ZHANG ABSTRACT. Experiments have been cnducted t understand the influence f expulsin n weld quality. The cnclusin was that expulsin may prduce undesirable features t spt welds in bth appearance and strength. Existing mdels f expulsin predictin have been reviewed n their merits and limitatins. Differences between welding steel and welding aluminum allys have been highlighted using the statistical mdels. Expulsin prbability is illustrated graphically as a functin f welding current, time and electrde frce. Tw- and threedimensinal expulsin bundaries have shwn the distinguishing characteristics f steel and aluminum welding, and act as a basis fr better understanding the influence f welding parameters and the welding prcess n expulsin. Intrductin A cmmn phenmenn in resistance spt welding is expulsin, i.e., the ejectin f mlten metal during welding. Expulsin happens at either the faying surface r the electrde/wrkpiece interfaces, as shwn in Figs. 1 and 2, respectively. The latter may severely affect surface quality and electrde life, but nt the strength f the weld if it is limited t the surface. On the ther hand, expulsin at the faying surface is undesirable in terms f weld quality because it invlves lss f liquid metal frm the nugget during welding. The risk f expulsin is especially high in spt welding f aluminum allys because f the very dynamic and unstable characters f the prcess, which is related t the applicatin f a high current in a shrt welding time, and the in- H. ZHANG is with the Department f Mechanica I Engineering and Applied Mechanics, University f Michigan, Ann Arbr, Mich. trinsic electrical, thermal, metallurgical and mechanical prperties f aluminum allys. The causes f expulsin are bth technical- and human-related. In the latter case, expulsin is ften used as a visual indicatr f a "crrect" welding prcess in steel welding. T achieve a weld size as large as pssible t meet certain requirements, a prevalent practice is t use a large welding current, ften clse t r beynd expulsin limits. Expulsin limits are als ften deliberately exceeded in prductin t reduce variatin in weld quality caused by randm factrs. Because f the lss f metal during expulsin, hwever, discntinuities such as vids and prsity that may reduce weld strength are usually intrduced t the nugget. Expulsin als has a negative influence n adhesives (weld bnding). The need t eliminate nncnfrming welds in sheet metal assembly makes it necessary t reduce expulsin in resistance spt welding. Predictin and cntrl f expulsin are then f imprtant practical interest. In this article, expulsin as a frequently bserved phenmenn is discussed, and its pssible impact n weld quality and perfrmance is presented. Theries and mdels f expulsin are then reviewed, and their implicatins fr KEY WORDS Aluminum Drawing Steel Expulsin Resistance Spt Welding Tensile-Shear Test Weld Schedule Weld Nugget the fundamental understanding f the expulsin phenmenn are discussed. This all may lead t a guideline fr effectively cntrlling expulsin while still btaining large-sized welds. Influence f Expulsin n Spt Weld Quality Expulsin ften prduces undesirable features in a spt weld. It is frequently linked t excessive surface indentatin, electrde wear, sheet distrtin r separatin and vlume defect frmatin. Figure 3 shws a large cavity frmed as a result f a deficit in vlume due t a heavy expulsin (Ref. 1). There are tw ppsing pinins regarding the influence f expulsin n the perfrmance f a spt weld. One is that expulsin des nt decrease weld perfrmance and it is acceptable in limited ranges, a view represented by the wrks f Kimchi (Ref. 2) and Karagulis (Ref. 3). Anther pinin is that expulsin has a detrimental effect n weld perfrmance and appearance and shuld be suppressed, as suggested by Newtn, et al (Ref. 4), and Ha, et al. (Ref. 5). This difference in pinin basically stems frm the difference in materials tested. The first pinin was drawn frm experiments n steels, while the latter was n aluminum allys. But, generally, it is agreed that expulsin may induce sme unfavrable features t the weldment. T quantify the influence f expulsin n weld quality r weld perfrmance, a series f experiments was cnducted n drawing steels (DS). In the first experiment, a galvanized DS f 0.78 gauge was used. A fixed welding time (200 ms r 12 cycles) and electrde frce (2.8 kn) were chsen, and the welding current was altered in a small range, as fllws: Case A: 7.7 ka, with very lw expul- WELDING RESEARCH SUPPLEMENT I 373-s

2 Fig Ejected and slidified metal frm the laying interface (shwn by the arrws) n a peeled bare steel weld buttn. Fig Trace f expulsin frm the electrde-sheet interface in welding an aluminum ally. that welding parameters used t create these welds were very clse t each ther. A clser lk at the micrstructures f the nuggets and the HAZ reveals Fig Vids in an aluminum weld nugget due t heavy expulsin. very little difference between welds with and withut expulsin prbability. Case B: 7.8 ka, with mderate expulsin prbability. Case C: 7.9 ka, with high expulsin prbability. Case D: 8.0 ka, with very high expulsin prbability. These cases were selected based n a statistical study f expulsin by Zhang, et sin in untested specimens. Examinatin f the micrstructures f tested specimens als shws n significant difference. Clumnar structures can be clearly seen inside the nugget, and precipitates are visible arund grain bundaries in the HAZ. Basically, n significant difference has been bserved in the appearance f the welds. The nly visible difa/. (Ref. 6). They represent welding ference between the expelled and schedules in a range between welding with very lw and very high pssibilities f expulsin. The small range f welding current was used t prduce welds f similar size (and ther physical attributes), with sme having expulsin, and thers nt. Weld quality usually refers t bth weld appearance r physical attributes and weld perfrmance characteristics. Figure 4 shws measurements f weld attributes and their ranges f variatin. Cmpared t the nes withut expulsin, welds with expulsin have slightly smaller nugget diameters and less penetratin, similar heat-affected zne (HAZ) size and ttal weldment thickness, and slightly larger electrde indentatin marks. The small difference in the physical appearance might be due t the fact nrmal welds is that specimens with expulsin usually have larger distrtin and sheet separatin. The specimens were als tensileshear tested. The peak lad shws very little difference between welds with and withut expulsin -- Fig. 5. Generally, hwever, welds with expulsin have lwer maximum displacement, especially lwer energy. Definitins f these quantities can be fund in the wrk by Zhu, et al. (Ref. 7). The measurements f specimens with expulsin als shw larger ranges f variatin. The static energy is imprtant as it can be directly related t the impact strength f a weld, as shwn by Zhang, et al. (Ref. 8). These differences wuld be neglected if nly peak lad were measured, as in many wrks n expulsin where peak shear-tensin lad was taken as the sle measure f weld strength. Large variatin, lwer maximum displacement and energy can be clearly attributed t expulsin. Indeed, displacement and energy exhibit an imprtant aspect f weld strength, i.e., ductility f the weldment. A gd weld shuld have sufficient lad-carrying capacity (peak lad), as well as certain ductility. Using peak lad as the nly measure f weld strength is nt nly an incmplete descriptin f weld quality, it may als be misleading, such as in the case f brittle welds. An examinatin f tested specimens revealed that specimens with expulsin generally fracture in the HAZ near the nugget (Fig. 6A), while the nes withut expulsin failed farther away frm the nugget. Therefre, the difference in gemetry (difference in distrtin) may cntribute the mst t the difference in strength. It may als indicate that the small difference in the strength f the HAZ may influence the verall perfrmance. In anther experiment, the effect f expulsin was investigated using welding schedules f larger ranges. The material used was a 0.8-mm galvanized drawing steel. T create varius sizes f nuggets and heat-affected znes, tw electrde face diameters (6 and 10 mm) and several welding schedules were used. The electrde frce was between 600 and 1667 Ib (2.67 and 7.4 kn), weld current was in the range f ,232 A, and weld time was between 6 and 24 cycles (100 ms and 400 ms). Figure 7 shws the dependence f peak lad and displacement n average weld buttn size. Bth peak lad and maximum displacement increase with buttn size. The trend is interrupted when expulsin happens. Cmpared 374-S [ NOVEMBER 1999

3 with welds f similar size withut expulsin, the nes with expulsin shw drps in peak lad (abut 10%) and maximum displacement (abut 30%). A similar phenmenn is bserved in energy -- Fig. 8. In this set f experiments, under the same welding cnditins, welds with expulsin shw slightly larger nugget and buttn sizes, but lwer strength than their cunterparts withut expulsin. Frm the analysis f the experimental results, the fllwing cnclusins can be drawn: 1) Welds with expulsin may have lwer strength than the nes f similar sizes withut expulsin. 2) Buttn diameters crrelate well with tensile-shear strength when there is n expulsin. 11 6ll.,-~ 4.0 ~ 3.0 ~ 2.1)?.0., i, 1, 1, 1, i, l, l r l, l, l, l, i, i, i I ' I ' I ' I ' I I ' I ' i ' F ' I 0.0,, I,,, E3,II~I,, DiNa~eetr HAZ Size ~~ Of t' nctratln Fig Physical attributes measured n sectined specimens. Fr each measurement, welds with and withut expulsin are cmpared, and the ranges f variatin are shwn fr minimum, mean and maximum values. Expulsin Prcess and Detectin T understand the expulsin phenmenn and t predict expulsin limits, several theries r hyptheses n the mechanisms f expulsin have been prpsed. Davies (Ref. 9) stated that the electrde frce causes expulsin. The mlten metal f a weld nugget is squeezed ut by the electrdes, and expulsin happens. This statement is cntrary t bservatins f many ther researchers. Expulsin takes place, as stated by Dickinsn, et al. (Ref. 10), when ttal useful energy applied t the weld exceeds a certain value defined as "critical expulsin energy," which is a functin f physical prperties/characteristics f the given material. This critical expulsin energy, as well as the ttal useful energy, is difficult t calculate in practice. Expulsin was linked by Wu (Ref. 11 ) t excessive current densities, either by "grss" peak welding current r by highly lcalized/micrscpic cntact areas f faying surfaces with increased resistance by xidatin r cntaminatin, at the early stage f spt welding. Althugh these studies prvided certain insights t the understanding f the expulsin mechanism, they are nt practical in a prductin envirnment. The latest thery f expulsin is that it happens when the frce frm the nugget (due t the internal pressure in a liquid nugget caused by melting, liquid expansin and ther factrs) exceeds the frce frm the electrdes (Ref. 1). As shwn abve, expulsin may decrease the strength f a spt weld; therefre, it shuld be avided. In general, it invlves interactins f electrical, thermal, mechanical and metallurgical prcesses. Fr instance, the electric current determines the heat input rate, which in turn influences the frmatin f I ~ 6.0 "0 ~ 4.0 ~ 2.0 N 0.0 [ 1 N Expulsin Expulsin, I I, Maximum Peak Lad (kn) Displacement (ram) Energy (J) Fig Strengths tested under tensile-shear cnditins. the nugget, the temperature distributin in the weldment and, ultimately, expulsin. Other factrs, such as surface cnditins, material strength (especially yield strength), lading and thermal cnditins als influence expulsin. Because f its cmplex nature, it is difficult t predict r cntrl expulsin. A few attempts have been made t detect expulsin by varius techniques. Accrding t published wrks, expulsin can be detected by measuring dynamic resistance (Ref. 10), acustic emissin (Ref. 12), electrde Fig Tensile-sheared specimens: A -- Withut expulsin; B -- with expulsin. WELDING RESEARCH SUPPLEMENT [ 375-s

4 376-s I NOVEMBER Z 5'5,~ 5.0 ~"~ 4.5 ~,~ ~00 am 8 J O I ' ' ' ' 3.0,,,l.... l,,,,l,,j,l,,,,l,,, Average Buttn Size (mm) I Lad- n expulsin L L n Displacement- n expulsin Lad - expulsin Displacement - expulsin O 10.0 ~ 8.0 ~ 6.0.~. 4.0 ~ 2.0,--, tm m ' l ' ' ' ' I ' ' ' ' I ' ' ' ' l ' ' ' ' I ' O OO %, I,,, L I i i L L ].,,, I,,, ~ I i Average Buttn Size (mm) 0 Fig. 7-- Peak Iad and maximum displacement vs. average weld buttn size. Fig Energy measured vs. average buttn size. displacement (Ref. 13, 14) and electric signals (Ref. 14). Mst f these measurements are difficult t make and ften t expensive fr use in prductin. The measurement prpsed by Ha, et al. (Ref. 14), seems prmising. It describes a rbust methd f mnitring expulsin in aluminum spt welding in high-vlume prductin, based n electric signals fr bth AC and medium-frequency DC welding machines. Attempts were als made in detecting expulsin by researchers early n, such as using acustic emissin by Havens (Ref. 15) in Mst f these effrts were generally fr expulsin detectin nly. Thus, crrectins f welding parameters, if pssible, can nly be made n subsequent welds t avid expulsin after making the previus weld. Therefre, the effectiveness f these methds in eliminating expulsin is limited. A cmmn way t study expulsin is cnstructing "lbe" diagrams, assuming nugget frmatin and expulsin as functins f welding current and time. Althugh they are widely used in practice t determine welding schedules, such diagrams nly explre the expulsin phenmenn and suggest reasns fr expulsin in terms f welding time and welding current. N fundamental understanding can be drawn frm them. T avid expulsin, it is imprtant nt nly t detect but als t predict expulsin, either by n-line r ff-line predicting. Expulsin Predictin and Preventin There are a number f effrts in mdeling expulsin prcess; sme f them were reviewed by Senkara, eta/. (Ref. 1 ). Of them, three mdels appear mst prmising: 1) Gemetry cmparisn mdel. 2) Frce balance mdel. 3) Statistical mdel. In the fllwing sectins, these mdels are summarized, and their advantages and disadvantages are discussed. Gemetry Cmparisn Mdel In a series f papers by researchers at Alcan Aluminum C. (Refs. 4, 16, 17), a mdel was prpsed and verified based n the cmparisn f weld nugget size with the cmpressive zne size. In the mdel, the radius f a mlten weld nugget is calculated (using a finite element mdel) as a functin f welding parameters. Expulsin takes place when the radius f the grwing nugget exceeds that f the cmpressive frce frm the electrdes, as illustrated in Fig. 9. This mdel catches the characteristics f expulsin phenmenn, Le., the liquid metal is ejected when there is insufficient cntainment (by cmpressive frce) thrugh part f the slid periphery f a nugget. Fairly gd agreement was btained by the authrs when the mdel predictin was cmpared with ex- perimental results (Ref. 17). Hwever, because f limitatins f numerical simulatin techniques and the number f cases that can be simulated, this mdel is difficult t use in accurately predicting expulsin in practice. The welding prcess and cnditins were idealized because f limitatins f numerical mdeling, and the criterin depends slely n the cmparisn f gemetric dimensins. Expulsin is usually predicted at a later stage f welding using this mdel, as a nugget needs time t grw t a certain size. Hwever, there are bservatins that expulsin ften happens at early stages f welding when the size f the mlten metal is cnsiderably smaller than that f the cmpressive zne supplied by the electrdes. Expulsin ccurs nt nly frm imprper welding time, current r insufficient electrde frce, but is als related t pr electrde cnditins, electrde alignment and wrkpiece cnditins. In practice, there are rarely cases in which electrde alignment and fitup are perfect. It has als been recgnized that sme materials tend t expel mre than thers. It is difficult t apply the gemetry cmparisn mdel t such cases mainly because f the limitatins f finite element mdeling. Frce Balance Mdel Based n the understanding f the physical prcesses during resistance

5 WFI DING RFqFARCH ql Ippl FhAFNIT I 277_c spt welding, an expulsin mdel was prpsed by several researchers (Ref. 1 ). Althugh there are many cmplicated reasns fr expulsin, its basic prcess was described by the interactin between the frces frm the liquid nugget and its surrunding slid cntainment. An expulsin criterin was prpsed by cmparing the electrde frce with that frm the liquid nugget, and expulsin was predicted when the latter exceeded the frmer. An effective electrde frce, instead f an applied/nminal electrde frce, was used in the mdel. A methdlgy was prpsed fr determining the effective electrde frce by analyzing lading cnditins and lcatins f the nugget and electrdes. By thermdynamics analysis, the internal pressure in the liquid nugget caused by melting, liquid expansin and ther factrs can be evaluated, and the frce frm the liquid nugget can be calculated by knwing the internal pressure and the dimensins f a nugget. The tw majr frces and a schematic illustratin f the mdel are shwn in Fig. 10. In practice, the applied electrde frce is rarely aligned with the ttal frce frm the liquid nugget because f cmplicatins in electrde gemetry such as electrde wearing, electrde alignment and part fitup. Therefre, the applied electrde frce, in many cases, is nt the same as the ne used t cntain the liquid nugget frm expulsin. The "effective" electrde frce was intrduced t accurately represent the frce used t suppress the frce frm the liquid nugget. A simple way f estimating effective electrde frces was depicted by Senkara, et al. (Ref. 1). There are fur majr pressure cmpnents in the liquid nugget in resistance spt welding: 1) Slid-t-liquid phase transfrmatin (melting). 2) Expansin in the liquid state. 3) Vaprs frm the liquid metal. 4) Decmpsitin f surface agents (such as lubricants). The ttal pressure in the liquid nugget is the summatin f all these cmpnents: P = Pmelt + Pexp. + Pvapr + Plubr. (I) The frce frm the liquid nugget nt its slid surrundings can be estimated nce the pressure and the prjected area in the directin f cncern are knwn. This mdel prvides a criterin fr expulsin predictin and a systematic methdlgy t analyze frces invlved in resistance spt welding. The mdel was verified by experiments with gd Fig Schematic f the gemetry cmparisn mdel. Expulsin happens when rm> r F. Fig. I0 -- Schematic f the expulsin criterin used in the frce balance mdel (Ref.. I). agreement. A quantitative knwledge f the evlutin f nuggets in shape and lcatin is the basis fr accurate predictin f expulsin using this mdel, n which mre research is needed. Slatistical Mdel The gemetry cmparisn mdel and frce balance mdel generally predict expulsin with the knwledge f nugget size and ther gemetric factrs. It is nt always pssible t btain these quantities. Besides, randm factrs, such as electrde alignment and wrkpiece fitup, are nt easy t include in these mdels. Welding current and time are nt directly reflected in the mdels, which makes the use f these mdels incnvenient. Chsing crrect welding schedules (usually electrde frce, welding current and welding time) is still the preferable way t cntrl expulsin. Based n these cnsideratins, a mdel was prpsed n expulsin predictin based n a statistical analysis by Zhang, et al. (Ref. 6). In their wrk, a statistical methd was prpsed t utline the expulsin limits by statistically planning experiments and analyzing results. Unlike previus wrks n expulsin limits, expulsin was nt regarded as an event happening at a single welding schedule; rather, its ccurrence was treated as a prbability that spans frm n expulsin t 100% f welds having expulsin, cnsidering randm factrs that culd influence expulsin. Predicting expulsin prbability is useful in design and prductin where a certain percentage, rather than a definite number f cnfrming welds, is ften mre meaningful. Because the mdel was built based n the belief that expulsin is a phenmenn

6 378-s I NOVEMBER 1999 A 'L" \ I \.,v / 3.0 \\,L...J -''~ \ I B j...~ ] t.z/t i\ T 12~ /I ~ ~, ~ = -' \ 6: " i "-\ \-/ ~ " / ~'5 ~ 5.~5 \ t.f.-/ ~ ~ ' / 35 j A -- / with prbability f ccurrence, experiments need t be planned and cnducted in accrdance. Welding schedules shuld be chsen arund ptential expulsin bundaries, and adjustment n welding schedules needs t be cnstantly made during experiments accrding t previus bservatins t effectively cpe with the change f expulsin limits. In the experiments in Ref. 6, several electrde frces (instead f ne single frce) were used, and welding current and time were varied fr fixed electrde frces. Once experimental data are cllected, a statistical analysis can be perfrmed t establish the analytical relatinship between expulsin prbabilities and welding schedules thrugh mdel selectin, identifying influential effects and estimating magnitudes f effects. A statistical mdel determined thrugh the abve prcedure has the fllwing characteristics: 1) Cntrary t traditinal lbe diagrams, expulsin is treated as a dependent f bth deterministic effects and randm factrs. Therefre, its bundary is determined as a prbabilistic range, rather than a line. 2) All-imprtant welding parameters -- i.e., welding current, time and electrde frce -- are included in the mdeling, unlike traditinal lbe diagrams usually cnstructed using a fixed electrde frce. 3) Influential welding parameters n expulsin can be determined using such mdels, and it prvides guidelines fr suppressing expulsin in practice. Summary f Expulsin Mdels Amng existing mdels n expulsin, the gemetry cmparisn mdel, the frce balance mdel and the statistical mdel have been verified and shwn t have the greatest prmise in effec- tively cntrlling expulsin. As with ther mdels, they have prs and cns, described belw: 1) The gemetric cmparisn mdel deals with expulsin withut the knwledge f nugget frmatin and influence f welding parameters. It captures the imprtant gemetric aspect f expulsin. But, it relies n finite element simulatin t prvide gemetric dimensins, which seriusly limits its use in the prductin envirnment. 2) The frce balance mdel prvides a systematic prcedure t calculate and analyze frces invlved in resistance spt welding, and it prvides a criterin fr expulsin. It reveals the physical prcesses invlved in resistance welding that lead t an understanding f expulsin phenmenn. It requires input frm ther surces fr dimensins. It can be used t develp welding schedule guidelines t suppress expulsin. 3) The statistical mdel treats expulsin as a phenmenn with prbability f ccurrence, unlike previus wrks n this subject. Such a methdlgy is adequate in dealing with physical prcesses with uncertainty. Quantitative predictins r mdels f expulsin limits can be btained by applying the generic statistical methdlgy n particular material/ welding systems. The cncept f expulsin prbability makes it pssible t btain the largest pssible spt welds withut expulsin. As a rutinely bserved phenmenn in resistance spt welding, expulsin is undesirable because f its adverse effects n weld strength, electrde wear, weld appearance and ther factrs. It is true that welds with expulsin, especially steel welds, can ften meet r exceed industrial requirements. But this des nt justify the practice f welding deliberately abve the expulsin limits. Expulsin shuld be suppressed t achieve high weld quality and ptimal electrde life, which are especially imprtant in aluminum welding. Dependence f Expulsin n Welding Parameters Using the statistical mdel, Zhang, et al. (Ref. 6), attempted t explain the dependence f expulsin prbability n electrde frce, welding current and welding time. Expulsin limits were fund dependent n welding time fr the steel, while the dependence was very weak fr welding aluminum allys. The influence f electrde frce is small fr steel welding, but it is large fr AI welding. In general,

7 i i i the transitin frm n expulsin t all expulsin is narrw fr steel and wide fr aluminum, implying that aluminum welding may have a wider uncertainty in terms f expulsin. Deeper understanding f expulsin phenmenn can be btained by analyzing the statistical mdels develped by Zhang, et al. (Ref. 6). The mdels are as fllws: Drawing Steel (DS) L g[px/(1-px)] = ( x 102) + ( x 102)/+ ( x 10-~)z + ( x 10')F + ( x 10~)F + (-1.49 x 10-s)x 2 + ( x 10')F + ( x 10-')P + ( )P + ( x 10-')Ix + ( )1F + ( x 10-2)'~F+ ( x 10"3)/2x +( )PF + (-5.12 x 105)x~F + ( )1F 2 = f(i, ~, F) (2) AA5754 Lg[px/(l-px)] - ( x 10') + (2.172 x 10~)/+ (3.9 x 10-3)'C + (6.56 x 10-')F + (-2.79 x 10-2)12 + (-1.4 x 10-3)/'t + ( x 10-')IF + (-8.0 x 10"gxF + (2.7 x 10-3)12F + (3.0 x 10-91xF =- f(i, "C, F) (3) AA6111 Lg[px/(1-px)] = ( x 10') + (1.15 x 10')/+ (9.81 x 10-2)'c +(-3.15 x 10-qF + (-1.69 x 10-2)P + (6.17 x 10-2)F 2 + (-6.3 x 10-3)lx + (-1.37 x 10-~)IF + (1.0 xl 0-4)Fx + (2.8 x 10-3)12F - f(i, z, F) (4) where / is current (ka), x is time (ms) and F is electrde frce (kn). Because these mdels describe expulsin prbabilities as a dependent f all three welding parameters, it is pssible t depict expulsin bundaries graphically in a 3-D space. In Fig. 11, surfaces f expulsin prbabilities f 0.05 are pltted using Equatins 2, 3 and 4. The figure shws distinguishing characteristics f welding steel and aluminum allys. Fr steel welding, the expulsin prbability surface cvers the crner f (/ = min., F = max., x = min.), as shwn in Fig. 11A. Therefre, expulsin is cntained when the welding schedule falls in the area surrunded by the surface. Accrding t this mdel, when current and time are t large, r electrde frce is t small, expulsin is likely t happen in the A 6.0, ' ; i ranges studied. I Unlike the expulsin 5.5 A 5% expulsin 95% expulsin 1 surface f the drawing steel, AA5754 shws very ~" 4.5 little dependence n welding time and slight 4.0 : dependence n electrde ~ ~. frce -- Fig. 11B. The in- ~ 3.5 ; : fluence f electric current 3. vershadws thse f time and electrde frce. 2.5 Zne I Zne II Zne III ' The weak influence f 2.0 welding time stems frm s I ] the fact that expulsin usually happens at an early stage during weld- Current (ka) ing, and, therefre, ttal u welding time des nt matter much. The shape f 10.0 the expulsin surface als implies a pssible critical 8.0 expulsin current fr this aluminum ally. The ex- Z pulsin surface fr weld- 6.0 ing AA6111 is different frm that fr welding 4.0 AA5754. Electrde frce plays an imprtant rle, and expulsin depends 2.0 strngly n welding time 0.0 when welding current is Cur/~t (ka 3) lw and electrde frce is small- Fig. 11C. In general, welding current is the dminant factr in aluminum expulsin. C s. The differences in the * [ ' ' ' 1 ' ' ' 1 ' ' ' 1 ' ' ' 1 ' ' 1 ' ' ~ 1 ' ' ' 1 shape f expulsin sur- 7. faces reflect the imprtant 6.0 differences between welding steel and welding 5. aluminum allys. These u~ 4.0 differences stem frm the ~ 3.0 significantly different electrical, thermal, metal- 2. lurgical and mechanical prperties between these 1.0 tw materials. Althugh 0.0 these mdels d nt prvide exact infrmatin n physical prcesses, they suggest pssibilities n hw physical parameters influence nugget frmatin and expulsin. As the surfaces represent lw expulsin prbability (0.05), welding schedules chsen n this surface can prduce large nuggets withut the risk f expulsin. Generally, welding time has far less significance in influencing expulsin than the ther parameters. A clse lk, ~ 95% expulsin Zne I 'Zne li" Zne lii I,,, I k,, I, ~, l ~,, I,,, I,,, 4 ~,, I Current (ka) Fig Electrde frce vs. welding current fr expulsin limits. A -- Drawing steel, time = 200 ms; B -- AA5754, time = 110ms; C-- AA6111, time = 50 ms. f hw expulsin bundaries depend n welding current and electrde frce reveals sme insights n the expulsin phenmenn. Figure 12 shws expulsin prbabilities f 0.05 and 0.95 as functins f current and electrde frce. Fr WELDING RESEARCH SUPPLEMENT I 379-s

8 a fixed welding time, electrde frce needed t cntain expulsin increases with current. But this increase is nt cnstant. As shwn in Figure 12, electrde frces increase rapidly when current is lw, then they reach a state f cnstant increase. Cntrlling expulsin by increasing electrde frce is difficult with further increase in current. Accrding t the expulsin characteristics, the influence f electrde frce can be categrized int three parts, with respect t the current znes as marked in the figure. In Zne I, when current is lw, the influence f randm factrs such as asperity and wrkpiece fitup is significant. Therefre, a small variatin f current may induce significant changes in expulsin prbability. Hwever, the influence f randm factrs r variatins can be suppressed by increasing electrde frce. In Zne II, the dependence f expulsin n frce and current is mre predictable. A windw exists in this zne fr welding with a high degree f certainty f expulsin ccurrence. In this regin, the expulsin mdel prpsed by Senkara, et al. (Ref. 1), shuld dminate. When welding current further increases, the influence f electrde frce n expulsin diminishes, as in Zne III. This is primarily due t the interactin between electrdes and the nugget during welding. Large current (fr a fixed welding time) prduces a large weld nugget, and the electrde frce will nt be able t cntain the liquid nugget, n matter hw big the frce is, nce the edge f the nugget grws beynd the cntainment f the electrde. This may happen in tw situatins. One is when part f the nugget is beynd the cmpressive regin created by electrde frce, and the ther is when the nugget size is cmparable t the cmpressive zne by the electrde, which can be apprximated by the electrde face size. Therefre, the fundamental cncept f the gemetry cmparisn mdel is cnsistent with the expulsin behavir in Zne III. In general, welding aluminum allys has larger uncertainty f expulsin than welding steel, as shwn in Fig. 12. Summary Althugh expulsin is cmmn in resistance welding, its mechanisms and influence n weld quality have nt been well understd. In this article, attempts have been made t advance knwledge n expulsin prcess and cntrl. In summary, the fllwing have been achieved: 1) Experiments have been cnducted n the influence f expulsin. It has been cncluded that expulsin has adverse effect n weld appearance and weld perfrmance. 2) Several mdels have been analyzed and cmpared n their merits, applicable areas and limitatins. 3) Unlike ther expulsin mdels, the statistical mdel directly links expulsin t welding schedules. Insights f the expulsin phenmenn can be btained using statistical mdels. 4) Welding steel has different characteristics frm welding aluminum allys, as illustrated by the statistical mdels. Adjusting electrde frce is generally an effective way t cntrl expulsin, which is cnsistent with the basis f the frce balance mdel. 5) When the nugget grws t a size clse t that f the electrde face, increasing electrde frce is n lnger effective, and the cncept f the gemetry cmparisn mdel is mre applicable. It shuld be emphasized that welds with expulsin can ften satisfy specificatins r requirements based n strength. Hwever, suppressing expulsin prvides an pprtunity t imprve weld quality and tl life. Based n the mdels reviewed in this article, it is pssible t develp an n-line expulsin predictin and cntrl algrithm fr use in welding practice. References 1. Senkara, J., Zhang, H., and Hu, S. J. Expulsin predictin in resistance spt welding. Submitted t Welding Jurnal. 2. Kimchi, M Spt weld prperties when welding with expulsin -- a cmparative study. Welding Jurnal 63(2): 58-s t 63-s. 3. Karagulis, M. J Cntrl f materials prcessing variables in prductin resistance spt welding. Prc. AWS Sheet Metal Welding Cnference V, Paper N. B5, American Welding Sciety, Miami, Fla. 4. Newtn, C., Brwne, D. J., Thrntn, M. C., Bmer, D. R., and Keay, B. E The fundamentals f resistance spt welding aluminum. Prc. AWS Sheet Metal Welding Cnference VI, Paper N. E2, American Welding Sciety, Miami, Fla. 5. Ha, M., Osman, K. A., Bmer, D. R., Newtn, C. J., and Sheasby, P. G Online nugget expulsin detectin fr aluminum spt welding and weld bnding. SAE Paper N Zhang, H., Hu, J. S., Senkara, J., and Cheng, S. A statistical analysis f expulsin limits in resistance spt welding. T appear in ASME Jurnal f Manufacturing Science and Engineering, February Zhu, M., Hu, S. J., and Zhang, H Critical specimen sizes fr tensile-shear testing f steel sheets. Welding Jurnal 78(9): 305- s t 313-s. 8. Zhang, H., Zhu, M., Hu, S. J., Bhr, J., and Trjanwski, W. H Impact strength measurement f spt welds. Sheet Metal Welding Cnference VIII, Oct., Try, Mich., Paper Davies, A. C The Science and Practice f Welding. Vl. 2, the practice f welding. Cambridge University Press, Cambridge, U.K., 10th ed., p Dickinsn, D. W., Franklin, J. E., and Stanya, A Characterizatin f spt welding behavir by dynamic electrical parameter mnitring. Welding Jurnal 59(6): 170- s t 176-s. 11. Wu, K. C The mechanism f expulsin in weld bnding f andized aluminum. Welding Jurnal 56(8): 238-s t 244-s. 12. Vahavils, S. J., Carls, M. F., and Slykhuse, S. J Adaptive spt-weld feedback cntrl lp via acustic emissin. Materials Evaluatin 39(10): Kilian, M., and Hutchenrenther, A Mnitring and cntrl f electrde indentatin. Prc. AWS Sheet Metal Welding Cnference VI, Detrit, Mich., Paper N. C4, American Welding Sciety, Miami, Fla. 14. Ha, M., Osman, K. A., Bmer, D. R., and Newtn, C. J Develpments in characterizatin f resistance spt welding f aluminum. Welding Jurnal 75(1 ): 1 -s t 6-s. 15. Havens, J. R Cntrlling spt weld quality and expulsin. SME Technical Paper Series, AD Brwne, D. J., Chandler, H. W., Evans, J. T., and Wen, J Cmputer simulatin f resistance spt welding in aluminum -- part I. Welding Jurnal 74(10): 339-s t 344-s. 17. Brwne, D. J., Chandler, H. W., Evans, J. T., James, P. S., Wen, J., and Newtn, C. J Cmputer simulatin f resistance spt welding in aluminum -- part II. Welding Jurnal 74(12): 417-s t 422-s. Welding Jurnal Call Denis Mulligan at (800) s I NOVEMBER 1999