Influence of Welding Machine Mechanical Characteristics on the Resistance Spot Welding Process and Weld Quality

Transcription

1 nfluence of Weldng Machne Mechancal Characterstcs on the Resstance Spot Weldng Process and Weld Qualty How machne stffness, frcton, and movng mass can affect weldng results BY H. TANG, W. HU, S. J. HU, H.Y. ZHANG, Z. FENG, AND M. KMCH ABSTRACT. Mechancal characterstcs of resstance spot weldng machnes, such as stffness, frcton, and movng mass, have complex nfluences on the resstance weldng process and weld qualty. n ths paper, these nfluences are systematcally nvestgated through experments. The mechansms of the nfluences are explored by analyzng process sgnatures, such as weldng force and electrode dsplacement, and other process characterstcs, such as electrode algnment. A better understandng of the nfluences s acheved through ths analyss. Ths study shows machne stffness and frcton affect weldng processes and weld qualty. t also confrms the movng mass does not sgnfcantly affect the process and qualty of resstance spot weldng. ntroducton A resstance spot weldng (RSW) machne conssts of two dstnct subsystems: electrcal and mechancal. The characterstcs of the mechancal subsystem, such as machne stffness, frcton, and mass, play mportant roles n the functonalty and performance of a weldng machne, and subsequently nfluence weldng process and weld qualty. Accordng to publshed lterature, research on the nfluence of weldng machnes started n the 197s. Early work focused on the dfferences n machne types. For example, Ganowsk and Wllams nvestgated the nfluence of machne type on electrode lfe for weldng znc-coated steels (Ref. 1). Kolder and Bosman stud- H. TANG s wth Advance Manufacturng Engneerng, DamlerChrysler Corp., Detrot, Mch. W. HU and S. J. HU are wth the Dept. of Mechancal Engneerng, Unversty of Mchgan, Ann Arbor, Mch. H. ZHANG s wth the Dept. of Mechancal, ndustral and Manufacturng Engneerng, Unversty of Toledo, Toeldo, ho. Z. FENG s wth Engneerng Mechancs Corp. of Columbus, Columbus, ho. M. KMCH s wth Edson Weldng nsttute, Columbus, ho. led the nfluence of equpment on the weld lobe dagrams of an HSLA steel at fve stes (Ref. 2). Satoh and hs coworkers concluded machne type was an mportant factor n weld performance based on ther experments on four types of weldng machnes (Refs. 3, 4). More recently, researchers have addressed the effects of ndvdual machne characterstcs on varous aspects of the weldng process. Several studes were conducted on the nfluence of machne stffness and electrode dsplacement. Hahn et al. (Ref. 5) found large dsplacement of electrodes resulted n defects of electrode contact and decrease of weld qualty. Wllams (Ref. 6) dscovered an ncrease n throat depth and electrode stroke decreased electrode lfe. Smlarly, Howe (Ref. 7) found electrode deflecton sgnfcantly nfluenced electrode lfe. Dorn and Xu (Ref. 8) showed the stffness of a lower arm had an effect on the oscllaton and the mean value of electrode force when electrodes were n contact wth the workpeces. The nfluence of machne frcton, or the frcton between the movng parts and the statonary parts n contact wthn a weldng machne, has also been studed. Satoh et al. (Ref. 3) found that frcton had effects on nugget dameter and sheet separaton. They also notced that weld expanson occurred manly n a drecton perpendcular to the electrode axs f the frcton effects were sgnfcant. Dorn and Xu (Refs. 8, 9) concluded an ncrease n KEY WRDS Alumnum Alloys Resstance Weldng Weldng Machne Desgn frcton reduces oscllaton of electrode force durng touchng. They further found that the ncrease n frcton reduced the tenson-shear force and torson moment of welds. The movng mass of RSW machnes was found less mportant to weld qualty than stffness and frcton. Satoh et al. (Refs. 3, 4) dd not fnd much nfluence from movng mass on weld nugget formaton. They stated that the optmal weght of the movng part exsted for electrode lfe n relaton to the natural frequency of a RSW machne. Dorn and Xu (Ref. 9) observed that the movng mass affected vbraton at low frcton wth a rgd lower arm. However, they dd not detect any clear nfluence of the mass on weld qualty. Theoretcal attempts were made by Gould and hs coworkers on the dynamc behavor of movng parts of a weldng machne (Refs. 1, 11). n a recent study, Tang et al. (Ref. 12) performed a systematc nvestgaton of the dependence of electrode force on machne mechancal characterstcs, weldng process parameters, and materals. n summary, these researchers have made valuable contrbutons to the understandng of the effects of machne characterstcs on weldng. However, several ssues reman to be resolved n ths research area. Frst, the results of the prevous studes were manly descrptve and lacked convncng explanatons. Due to the complexty of RSW, t s dffcult to obtan explct expressons of the nfluences only from comparsons. Second, weld qualty was not emphaszed n ther studes, although mprovng weld qualty has been one of the man concerns n both academc research and ndustral practce. n addton, mportant mechancal characterstcs of a resstance weldng machne have not been systematcally studed n prevous works. t s desrable yet challengng to obtan a scentfc and systematc relatonshp between machne characterstcs and weld qualty. Ths research attempts to address ll.1~.1 MAY 23

2 - nfluences - Modfcaton - nfluences WELDNG RESEARCH Process sgnature analyss n... n..... Upper Structure (Movng) / / / / / / J / Machne characterstcs Weld qualty evaluaton \ \ \ \ \ \ Lower Structure (Statonary) Fg. 1 --An approach to studyng machne nfluence. Fg. 2 - of machne frcton ( Ref. 12). ~" 6. "116. E _ hgh stffness ~ " ±... E o ~"-... T'-'---"... g ~.~ 4.2 ~ ,.. 4, ~ g - 3., Smm bare steel.8ram galvanzed steel 1.7ram bare steel 6.8kA,2.67kN,12cycle 7.5kA, 2.67kN,12cycle 6.SkA, 2.67kN,12cycle r-:. ~" ~" o) T ~ ~ ~15Jo6 "~ EE 8. on ~ / l '-: o ~: ' g g 6oo oo ~1[ u3 H/ 2 lp -~ 4.~~---1 orgnal stffness----~ ~ 11.oo 4, ~' #~ ~T - hgh stffness ~-'-~.L :~ 3 L - - m ~~" lnnn ~,o 1ram A 2ram A.8mm bare steel 1.7mrn bare steel... 35kA, 3.56kN 35kA, 5.34kN 9.5kA, 2.67kN 9.5kA, 2.67kN Fg. 3 - of stffness on weld strength (weld gun). Fg. 4 - chne). of stffness on weld strength (2-kVA pedestal ma- some aspects of the relatonshp and also to understand how weld qualty s affected. Both expermental and process sgnature analyss technques were utlzed n ths study. Changes n machne mechancal characterstcs are usually drectly reflected by varous process sgnatures recorded durng weldng. Therefore, the nfluence of machne characterstcs on weld qualty can be explaned by analyzng the process sgnatures. The electrode force was chosen as the prmary process sgnature n ths study. The nfluence of machne characterstcs on weld qualty may be largely explaned through the analyss of the force characterstcs because other process parameters, such as weldng current and tme, are not drectly related to machne mechancal characterstcs. Electrode dsplacement durng weldng s also mportant because t reflects the expanson, meltng, growth, and soldfcaton processes. Thus, both force and dsplacement were studed n ths nvestgaton of the relatonshp between RSW machnes and weld qualty. n the context of ths work, weld qualty refers to the geometrc characters of a weld such as ndentaton, the appearance of expulson, and the tensle-shear strength (peak load) of a weld. The new approach attempted n ths study combnes expermental nvestgaton (as performed by prevous nvestgators) and process sgnature analyss -- Fg. 1. Expermental nvestgatons were sys- tematcally desgned and conducted. Weld qualty was emphaszed n the study. Based on the expermental results and on analytcal and numercal studes, the relatonshp between the ndvdual characterstcs and weld qualty was explored through process sgnature analyss. Expermental nvestgaton and Process Analyss Ths expermental nvestgaton was carred out on weldng machnes wth modfed mechancal characterstcs. Such characterstcs were then lnked to weld qualty, and the relatonshp was establshed through process sgnature analyss. The focus was placed on the sgnals durng WELDNG JURNAL >~'t

3 6. 5. ~ 4. z a) 3. u_ 2. Hgl~ s ~ Low stff~ss."..!.~.:.; Expulson Tme (sec.)." Weld stage,11 r Tme (sec.) Fg Force comparson under dfferent machne stffzesses (Ref 12). Fg Dsplacement comparson under dfferent machne stffnesses. Table 1 -- nfluence of Stffness on Expulson Lmts Materal Current (ka) - Current (ka) - ncrease n Low Stffness Hgh Stffness Expulson Current (ka).8-mm Bare mm Galvanzed mm Bare Fg Deflecton of machne structure. weldng,.e., durng the applcaton of electrc current. n addton, the hold stage was analyzed because t nfluences the soldfcaton of lqud nuggets. The majorty of experments were carred out on a 75-kVA pedestal weldng machne. Some experments were performed usng a 42-kVA C-type gun and a 2-kVA pedestal weldng machne. Trun- cated-cone electrodes were used n all experments. Weldng force, electrc current, and thckness of metal sheets were chosen as experment varables. To determne the weldng current used n the experments, weld lobe tests under varous condtons were conducted frst. The current profle when weldng expulson happened was recorded. The weldng currents used for all followng desgned experments were selected near the expulson lmts, or the mnmum currents at whch expulson s observed. n order to ensure the relablty of experments, fve welds were made under each condton. n the experments, the process sgnals were recorded for subsequent analyses. The force was measured by a stran gaugebased load cell, whle a lnear varable df- ferental transducer was used to measure the dsplacement. The current was also recorded n order to capture the exact weldng stage duraton. Several measures were selected as crtera for evaluatng weld qualty. The tensleshear strength was used as the prmary ndex of qualty. Weld ndentaton was also measured. Each of fve specmens was measured three tmes for the ndentaton. Furthermore, metallurgcal examnatons were conducted on selected specmens. Materals used n the experments are steel and alumnum sheets commonly used n automotve applcatons, and ther specfcatons are lsted n the ndvdual sectons. The mechancal characterstcs of the weldng machnes,.e., machne stffness, frcton, and movng mass, were modfed. n order to ensure the consstency of testng condtons, only one characterstc was modfed at a tme for specfc experments. Two levels of these ndvdual characterstcs were used n comparatve experments. The modfcatons of weldng machnes are brefly descrbed n the followng sectons. Refer to an earler paper (Ref. 12) for detaled descrptons. Stffness The stffness of weldng machnes reflects ther ablty to resst the deflecton under loadng. Snce the upper structures of the weldng machnes used n ths study are much stffer than the lower structures, only the stffness of the lower structures was consdered and altered. The stffness was modfed n two ways. Reducng stffness was realzed by addng sprngs between the lower electrode and ts support structure for the pedestal weldng ma- l:l~'~ MAY 23

4 chnes, whle ncreasng stffness was acheved by addng a supportng leg from the floor to the lower arm for the C-type gun. Therefore, two levels of stffness were acheved on each machne: 4.3 kn/mm and 13. kn/mm for the gun; 8.8 kn/mm and 52.5 kn/mm for the 75-kVA pedestal weldng machne. Frcton Frcton exts between two contact surfaces only when there s a relatve movement or a movng tendency. For both the pedestal weldng machnes and the C-type gun used n ths study, only the upper structure s movable durng weldng. A specal devce offerng adjustable addtonal frcton (Fg. 2) was desgned and mounted on the pedestal weldng machnes n order to vary the frcton force. n the experments, two dfferent frcton stuatons were consdered: the orgnal setup and one wth an addtonal.36-kn (8-1b) frcton. Movng Mass Because some parts of the upper structures of the weldng machnes move durng weldng, a 2-kg weght was added on the movable parts to change ther mass. Two levels of movng mass of the upper structures were consdered for the pedestal weldng machnes: the orgnal weght (about 4 kg) and the orgnal weght plus addtonal weght. nfluence of Machne Stffness ; < 2r Fg Geometrc model for axal msalgnment and contact area. A ~ ~-- 7_ S-~-- ~-_~=~-~ -~-5~--L" ~+~+ Fg FEA model of weld gun body. B The nvestgaton was conducted on both gun-type and pedestal-type weldng machnes. Three types of DS (drawng steel) steel specmens were used:.8-mm bare, 1.7-mm bare, and.8-mm galvanzed. The weldng tme was 12 cycles for most experments, and the weldng current and force are lsted n Fgs. 3 and 4. Error bars are used n these and other fgures to show the ranges of expermentally observed values, and sold dots and numercal values ndcate the averages of the data. Expulson, whch has been proved undesrable (Ref. 14), and tensleshear peak load (Ref. 15) are dscussed n the followng. Expulson Tests were conducted frst for the nfluence of machne stffness on the expulson lmts and then for the selecton of approprate currents for subsequent experments. n the tests, weldng current was ncreased gradually whle the force and tme were unchanged. The experment results show the expulson lmt rses wth stffness, as shown n Table 1. Because hgher expulson lmts allow hgher weldng cur- Fg FEA models for pressure analyss. A -- Perfect algnment; B -- msalgnment.!+: : L.:: n,+.! ~~'. :+;~:~ : + E... Pc"c ;llglllwllt (} / lh11 xllcct5 NSallgntlWllt, 11 ] tl}lll ~h'('tx ' )crlcct ;thl'hw, ; T/ ~1,~'~t~ M~,ahgn[;cnt ] ~ lt}lll ~+C[~ Fg Comparson of pressure WELDNG JURNAL ll#l~"l

5 WELDNG RESEARCH Z 8.oo orgnal frcton-a~ ~ =16. 7" t] addt nal fdct n~- " d' l ~ _ - ~ ~ 1 5- Z~ o.9 5. ~ F $ ~ 3. t--- ~ ~ t kN 4.kN 4.kN 2.67kN.8 mm.8 mm 1.7 mm 1.7 mm Weldng condton 9.r 8. ~ :rd~:.~oalnfr:~: lc~"~ t 8.57 ~ ~. 7.] 6.oF ~ 4 ~.! "*a.4~ 3 ~6 a_, 3. ~-... ~L....--_.._-._~ ' 3.56 kn 5.34 kn 3.56 kn 5.34 kn 1 mm A 1 mm A 2mm A 2mm A Weldng condton Fg. 12- nfluence of frcton on wem strength (steel). Fg nfluence of frcton on weld strength (alumnum) 5..,:,.~.,,... :..:~, -:.~ ~-:-;,~'~.:.,~::'..~"2 '-) -.-'~-, L: :.:,.~, J,'~', :~-t,'-"j,'~ 5""~: ~ -'.'.; Fg WeM cross sectons wth dfferent frcton (.8-mm steel). A -- Wthout addtonal frcton; B -- wth addtonal frcton. rents, potentally larger welds can be made wthout expulson. Peak Load A comparatve experment was conducted under dentcal condtons wth varous machne stffnesses. As shown n Fg. 3, weld qualty s mproved n terms of tensleshear strength (peak load) wth machne stffness. However, the mprovement s not sgnfcant because only about a 3% dfference exsts and the data ranges overlap. Smlar experments were performed usng the 2-kVA pedestal weldng machne. n these experments, both steel and alumnum specmens were used:.8- mm and 1.7-mm bare steel, and 1-mm and 2-mm alumnum alloy (AA5754). The experment results show the stffness s slghtly benefcal to weld qualty n terms of tensle-shear strength -- Fg. 4. However, the experment data dd not show the nfluence of machne stffness on weld ndentaton. The experment results of both gun-type and pedestal-type machnes are consstent. n summary, machne stffness slghtly mproves weld qualty n terms of weld strength under the same condtons and sg- nfcantly rases weldng expulson lmts. Further analyss was made on the nfluence of the machne stffness on two aspects of the weldng process: the characterstcs of the weldng force and electrode dsplacement, and electrode algnment. Electrode Force n weldng steels usng the pedestal weldng machne, electrode force ncreases mmedately after electrcal current s appled -- Fg. 5 (Ref. 12). The maxmum ncrease of electrode force s about 5-1% of ts preset value. The ncrease s due to thermal expanson of the weldment and the constrant mposed by machne stffness. When a machne s stff, the constrant s strong and results n a large force. The ncreased electrode force mposes a forgng force on the nugget, whch s benefcal to preventng weldng expulson. Electrode Dsplacement Electrode dsplacement may be the best ndcator of nugget ntaton and growth, and other characterstcs (such as expulson, etc.) durng weldng. Although the amount of electrode movement durng weldng vares accordng to the stffness of the weldng machne, smlar dsplacement characterstcs have been observed when weldng wth dfferent machne stffness, as shown n Fg. 6 where relatve dsplacement between the upper and lower electrodes was plotted. t takes a smlar amount of expanson (.e., d = d2) for nuggets to grow, but wth the stffer machne t takes longer for expulson to happen. Ths s because a stffer machne provdes a larger constrant or force, whch delays expulson accordng to Ref. 13. n the case of Fg. 6, weldng tme was set at 16 cycles and expulson occurred at the ffth cycle (.83 s) n the weld stage when the machne stffness was low. However, expulson happened almost at the end of weldng when stffness was hgh. f weldng tme had been set at 1 cycles, the expulson would have not occurred under the hgher stffness, but stll could have occurred under the lower stffness. The amount of relatve dsplacement of the electrodes of the stffer weldng machne s larger when expulson happens, as shown n Fg. 6, as the machne asserts more squeezng force on the softened weld. NF,',Tol~..1 MAY 23

6 ---~t. Fg Weld cross sectons wth dfferent frcton (1.7-mm steel). A -- Wthout addtonal frcton; B -- wth addtonal frcton. Fg Weld cross sectons wth dfferent frcton (2-mm alumnum). A -- Wthout addtonal fncton; B -- wth addtonal frcton. Table 2 -- Pressure on Fayng Surface under Dfferent Electrode Algnments Perfect Algnment (.8-mm sheets) Msalgnment (.8-ram sheets) Perfect Algnment (1.7-mm sheets) Msalgnment (1.7-mm sheets) Electrode Algnment deally, electrodes should be algned durng the RSW process because a msalgnment nduces unfavorable features to the process and weld qualty. Msalgnments, ether axal or angular, may cause rregularly shaped welds and reduce weld sze because msalgnments result n asymmetrcal dstrbuton of force and current. A msalgnment can result from machne deflecton under an appled force, as shown by a fnte element model (Fg. 7), even f the electrodes appear algned under no loadng or very low loadng. Such msalgnment s often gnored n practce. n ths secton, analytcal and numercal (fnte element modelng) analyses are conducted to provde a quanttatve understandng of the reducton n contact area that affects the weldng current densty, and the effect on the pressure dstrbuton due to deflectonnduced msalgnment. Maxmum At Center 131 MPa 73.7 MPa 299 MPa 38.5 MPa 16 MPa 78.4 MPa 195 MPa 45.3 MPa Axal msalgnment reduces the (overlapped) contact area between the sheets, as shown n Fg. 8. The actual contact area (Cr), n percentage of electrode face area, can be approxmated by Equaton 1. t can be seen from the equaton that the reducton n contact area s strongly correlated wth the axal msalgnment. C r = 2rZ arccos(2~r )- 28~ r2~--(~)2 (1) 'ff, F 2 where r s the radus of electrode face; 5 s the axal msalgnment. A msalgnment due to structural deflecton can be quantfed when the geometry and mechancal propertes of a weldng machne are known through theoretcal or numercal analyses. Fnte element analyss (FEA) s a convenent tool for evaluatng the deflecton and correspondng msalgnment. A 3-D FEA model was created for a gun body usng ANSYS Fg. 9. n the model, the gun body was modeled by tetrahedral sold elements; the nterface between sheets and electrodes was modeled by nonlnear contact elements. Under an electrode force of 2.67 kn (6 lb), the electrodes msalgn.75 mm axally and.28 deg n angle. The electrode face dameter was assumed 6.4 mm, and the contact area was reduced to 85% of the electrode face area accordng to Equaton 1. Therefore, a weldng schedule desgned on perfect algnment condtons may not be adequate, and weld strength may be reduced due to msalgnment. Furthermore, a contact pressure analyss was conducted based on the above msalgnment nformaton,.e.,.75-mm axal and.28-deg angular msalgnment usng a 2-D FEA model for the electrodes and workpeces -- Fg. 1. The pressure dstrbuton on the fayng surface under 2.67 kn (6 lb) of electrode force s shown n Fg. 11 and some data are lsted n Table 2. The average pressure wth perfect algnment on the fayng face s 83. MPa. Under deal algnment, hgh pressure always occurs around the electrode edge, whch plays a role n contanng the molten nugget and preventng possble weldng expulson. When the electrodes msalgn, the pressure dstrbutes asymmetrcally. bvously, ths asymmetrcal pressure dstrbuton s unfavorable n terms of expulson preventon WELDNG JURNAL ll,,.,ll~.'~

7 - nfluence - nfluence Wthout a d d ~.1. ~ -~- : v U "o "o v 2,o / '' n ' / N N "~ ]N T Tme (sec) ~ \ ~Wth addtonal frcton] Wthout addtonal frcton ~ t, Tme (sec) Fg Comparson of electrode force wth dfferent frcton. Fg Comparson of electrode dsplacement wth dfferent frcton. 9. E 8. orgnal mass.-----~ ~ addtonal mass ~ " , 9. orgnal mass -.--~ ~ 8. addtonal mass ~ " ~ 1 19.(3 ~z~ ) a, ~ =~ ~ 5,L ka 1. ka 6.8 ka 1. ka 2.67 kn.8 mm 4. kn.8 mm 2.67 kn 1.7 mm 4. kn 1.7 mm Weldng condton Z o o LL t- u) _$ t ~ ~ 16. ~ 5. _ :~ ~ 4. ~ ka 1. ka 6.8 ka 1. ka 2.67 kn.8 mm 4. kn.8 mm 2.67 kn 1.7 mm 4. kn 1.7 mm Weldng condton Fg of mass on wem strength (steel). Fg. 2 - of mass on weld strength (alumnum). (Ref. 13) and electrode lfe. Therefore, expulson happens easly when electrode msalgnment exsts. n addton, the analyss results show sheet thckness s another factor on pressure dstrbuton. Thcker sheets can reduce the asymmetrcal pressure on the fayng surface, whch partally explans why machne stffness has less nfluence on weldng expulson for weldng thcker sheets. nfluence of Machne Frcton on Weld Strength and Mcrostructure n the experments, weldng specmens were.8- and 1.7-mm bare steel and 1- and 2-mm alumnum sheets. The weldng tme was 12 cycles. The current was 7.1 ka for steel and 35 ka for alumnum. The force was set at two levels: 2.67 kn (6 lb) and 4. kn (9 lb) for steel; 3.56 kn (8 lb) and 5.34 kn (12 b) for alumnum. Fgures 12 and 13 show the comparsons of the jont strength, wth data range bars, under dfferent condtons. Based on the comparsons, t can be concluded that frcton s unfavorable for both steel and alumnum weldng. However, for some combnatons of parameters, strength reducton s not statstcally sgnfcant snce the data ranges overlap. n general, the nfluence of frcton vares wth weldng condtons. n order to gan further understandng, the welded specmens were sectoned and examned through standard metallographc technques. Typcal cross sectons are shown n Fgs n the cross sectons, t s easy to recognze there s ncomplete fuson on the fayng surface under hgher frcton condton. For.8-mm steel (Fg. 14), ncomplete fuson on the fayng surface was observed when the machne had addtonal frcton. Ths could be the reason for the reducton of weld strength. For 1.7-ram steel (Fg. 15), t s obvous the weld under greater frcton has shrnkage porosty. A smlar stuaton was observed n the weld of 2- mm alumnum-- Fg. 16. However, the nternal porosty may not affect the tensleshear strength of the welds n some cases, as reported n Ref. 16 for alumnum welds. lp, P.15.1 MAY 23

8 - Axal.2 % E.1 t- Q),< o ' A ^ ~,~., t'~.~ Weld staqe " ~ J Tme (sec.) Z v (1) o LL " 2 _.m LU Wth a d d ~ ~ \ o r Tme (sec.) ' Fg Acceleraton of nugget expanson n electrode axal drecton. Fg Force comparson wth dfferent mass. Therefore, machne frcton adversely nfluences weld qualty. Although the nternal dscontnutes may not always be harmful, premature falures may happen f ncomplete fuson exsts. Effect on Electrode Force The frcton n RSW machnes contrbutes sgnfcantly to total electrode force. Under the same preset value, the actual force wth larger frcton s smaller than that wth smaller frcton -- Fg. 17. When large frcton exsts, electrode movement s sluggsh and cannot promptly follow the contracton. Thus, t s most lkely nternal dscontnutes, such as porosty, appear n the nugget. The desred stuaton should be that the electrodes move freely n the weldng and holdng stages. Effect on Electrode Dsplacement The dsplacement sgnal provdes explct nformaton on weld ndentaton. An example of 2-mm alumnum weldng under 3.56-kN force s shown n Fg. 18. The electrodes extrude less nto the workpeces wth larger frcton. The dfference n dsplacement was about.1 mm as electrodes were retracted. The ntal mpact of electrodes onto workpeces s also reduced by frcton, whch may be benefcal to electrode lfe. n summary, machne frcton nfluences weldng process and weld qualty. The frcton always opposes electrode movement and makes t dffcult for electrodes to follow nugget expanson durng weldng and contracton durng coolng. The latter effect may help the creaton of nternal dscontnutes n welds. Wth large machne frcton, electrodes ndent less nto workpeces for alumnum weldng. nfluence of Movng Mass on Weld Qualty o~ t~ 1 9o < 8 - o 7 Varous experments were conducted by alterng the mass of the upper structure of the pedestal weldng machnes. The selected m 6 5 weldng schedules were the same as used n the early part of the experments of ths work. Some results are shown n Fgs. 19 and 2. The results, n Fg terms of the tensle-shear strength of welds and weldng expulson lmts, do not show any sgnfcant nfluence of machne movng mass for both steel and alumnum weldng. Therefore, the effect of movng mass of a weldng machne can be neglected n desgn. The nsgnfcance of the effect of the movng mass was expected because of the small amount of moton of electrodes durng weldng. The effect of mass, n the form of dynamc force, can be sgnfcant only when weld volume thermally expands wth a large acceleraton. The dynamc force can be obtaned f movng mass and the acceleraton of electrode movement are known. The force (F) can be calculated by F=ma where m s the movng,,, J Axal Msalgnment (mm) msalgnments vs. contact areas. mass and a s acceleraton, whch can be approxmated by the followng dfferental equaton: x(t+l)-2x(t)+x(t-1) (2) a= (At) 2 where x(t) s the measurement of electrode dsplacement durng weldng at the nstant t; and ~ t s the samplng nterval. For 1.7-mm steel, weldng wth 6.8-kA current and 2.67-kN (6-1b) force, as an example, the calculated acceleraton of weld expanson s shown n Fg. 21. From the calculated results, the largest acceler- WELDNG JURNAL lp.kll~.~

9 aton occurred durng the frst cycle n the weld stage, but was only.23 m/s'-. Afterward, the acceleraton was nearly zero. f the movng mass s 4 kg, then maxmum dynamc force s 9 N (2 lb). ts effect s very small compared wth the appled electrode force. n addton to ts small magntude, the dynamc force happens only at the very begnnng of the weld stage; therefore, t has lttle effect on weld formaton. The effect of movng mass on the force s neglgble (Fg. 22), and the mass nfluence on weld qualty s expected to be nsgnfcant. However, the nfluence may be sgnfcant durng the touchng of electrodes; therefore, movng mass may nfluence electrode lfe. Gudelnes on Machne Desgn As shown n prevous sectons, mechancal characterstcs of weldng machnes have complex nfluence on weld qualty. Based on the understandng obtaned n ths study, gudelnes can be developed for desgnng weldng machnes to mprove weld qualty. n general, hgh machne stffness mproves weld qualty because t ensures good electrode algnment, provdes large forgng force, and rases expulson lmts. Therefore, hgh stffness s recommended for the structure desgn of RSW machnes. However, the approprate level of stffness should be determned because extremely hgh stffness s nether achevable nor necessary. Based on the applcatondependent relatonshp between machne stffness and the electrode algnment due to structure deflecton, a crteron can be establshed n terms of electrode msalgnments rather than the abstract stffness. n order to ensure weld qualty, a certan amount of contact area (overlap of electrode faces) should be mantaned. f a certan percentage of contact s deemed necessary for achevng reasonable weld strength, then the axal msalgnment should be lmted accordngly. Wth dfferent szes of electrode faces, an acceptable msalgnment can be obtaned through Equaton 1, or estmated from Fg. 23 where r s the radus of electrode face. For a specfc weldng machne, ts structure can be desgned to ensure a certan percentage of contact area. For nstance, when a 9% contact s assumed, the allowable msalgnment can be specfed as.31,.47,.63, and.79 mm for electrode radus (r) of 2, 3, 4, and 5 mm, respectvely. Smlarly, for some types of weldng guns, such as long-arm scssors guns, angular msalgnment should also be a concern n order to avod strong pressure and current concentratons. Accordng to the FEA analyss of ths study, a.28-deg an- gular msalgnment wth.75-mm axal msalgnment result n sgnfcant asymmetrcal pressure dstrbuton. Such a concluson should be consdered as a reference only because ths calculaton dd not consder thermal-mechancal nteracton occurrng durng weldng. However, t ndcates possble nfluence of machne stffness on electrode algnment and possbly on weld qualty. n practce, a certan amount of angular msalgnment can be compensated by the fact that electrodes are usually slghtly worn after a few welds, and by usng dome-shaped electrodes. n general, establshng a tolerance of axal and angular msalgnments for RSW machne desgn needs further FEA and expermental study. As dscussed n prevous sectons, machne frcton s generally unfavorable to weld qualty. Frcton should be kept as small as possble for ths reason. There are several practcal ways to mnmze frcton. For nstance, the movng parts of RWS machnes should be supported by a roller gude, such as ball screws, rather than by a sldng mechansm. A specfc mechansm may be desgned so that the frcton s appled only durng the hold stage to ncrease forgng force. Accordng to ths study, movng mass of RSW machnes has no sgnfcant effect on weld qualty. Therefore, the mass or weght should be mnmzed to reduce the mpact at touchng for mproved electrode lfe and to mprove gun portablty for energy and ergonomc consderatons. Further research s needed to obtan optmal machne and process desgn. Summary n ths paper, the nfluence of RSW machne mechancal characterstcs on weldng process and weld qualty was systematcally nvestgated. Both expermental and analytcal studes were performed to understand such nfluences. n general, machne stffness has a postve nfluence on expulson preventon and weld qualty. Specfcally, hgh stffness can reduce electrode msalgnment, ncrease expulson lmts, and provde forgng effect. Thus, hgh stffness s recommended. Machne frcton should be reduced whenever possble because of ts negatve effects on weld qualty, especally on nternal dscontnutes. Machne movng mass shows no nfluence on weld qualty. n addton to expermental nvestgatons, a systematc approach s developed based on analyzng process nformaton. From such analyses, new understandng of the nfluences of the machne mechancal characterstcs can be obtaned, and gudelnes for the desgn of RSW machnes can be developed. Acknowledgment The authors are grateful for the support of the Advanced Technology Program (NST/ATP) through the ntellgent Resstance Weldng (RW) Consortum. References 1. Ganowsk, E J., and Wllams, N. T Advances n resstance spot and seam weldng of znc-coated steel strp. Sheet Metal ndustres 49(11 ): Kolder, M.W., and Bosman, A.W.M nfluence of the weldng equpment on the weldablty lobe of an HSLA-steel. W Doc. No Research and Development, ljmuden, The Netherlands. 3. Satoh, T., Katayama, J., and Nakano, T Effect of mechancal propertes of spot weldng machne on spot weld qualty. W Doc. no , pp Satoh, T., Nakamura, T., and Matsuyama, K Current status of research and development on resstance spot weldng machnes n Japan. 5. Hahn,., Budde, L., and Hantzsch, D nvestgatons on the nfluence of the mechancal propertes of spot weldng tongs on the weldng process. Weldngand Cuttng, No. 1, pp Wllams, N.T., Chlvers, K., and Wood, K The relatonshp between machne dynamcs of pedestal spot weldng machnes and electrode lfe. lw Doc. No p Howe, E The effect of spot weldng machne characterstcs on electrode lfe behavor on two welders. Proceedngs ofaws Sheet Metal Weldng Conference V, Detrot, Mch. 8. Dorn, L., and Xu, P nfluence of the mechancal propertes of resstance weldng machnes on the qualty of spot weldng. Weldngand Cuttng, No. 1, pp Dorn, L., and Xu, P Relatonshp between statc and dynamc machne propertes n resstance sport weldng. Weldngand Cuttng, no. 1, pp Gould, J. E., and Dale, W. N Theoretcal analyss of weld head moton. Sheet Metal Weldng Conference V, Detrot, Mch. 11. Gould, J. E., Feng, Z., Chou, J., and Kmch, M Analytcal models for the mechancal response of a resstance spot weldng machne. CRP Report SR992. Edson Weldng nsttute. 12. Tang, H., Hou, W., Hu, J. S., and Zhang, H. 2. Force characterstcs of resstance spot weldng of steels. Weldng Journal 79(7): 175-s to 183-s. 13. Senkara, J., Zhang, H., and Hu, J.S. 22. Expulson predcton n resstance spot weldng. To be publshed by Weldng Journal. 14. Zhang, H Expulson and ts nfluence on weld qualty. Weldng Journal 78(11): 373-s to 38-s. 15. M. Zhou, M., Hu, S. J., and Zhang, H Crtcal specmen szes for tensle-shear testng of steel sheets. Weldng Journal 78(9): 35-s to 313-s. 16. Thornton, P. H., Krause, A. R., and Daves, R. G The alumnum spot weld. Weldng Journal 75(3): 11-s to 18-s. l!,'~!$."t MAY 23