Analysis of Metal Transfer in Gas Metal Arc Welding

This study shows that the transition of metal transfer mode in gas metal arc welding occurs much more gradually than is generally believed

BY Y-S. KIM AND T. W. EAGAR

ABSTRACT. Droplet sizes produced in transfer. These transfer modes show dif- metal transfer phenomenon. These have GMAW are predicted using both the ferent arc stabilities, weld pool penetra- had limited success. static force balance theory and the pinch tions, spatter production, porosity pop- In this study, the droplet size and instability theory as a function of weld- ulation and level of gas entrapment. droplet transfer frequency are analyzed ing current, and the results are compared Lesnewich (Ref. 1) showed that the mode both theoretically and experimentally. with experimental measurements. The of metal transfer depends on many op- In the first section of this paper, the equi- causes for the deviation of predicted erational variables such as welding cur- librium drop sizes are calculated using droplet size from measured size are dis- rent, electrode extension, electrode di- the static force balance analysis and the cussed with suggestions for modification ameter and polarity. Later, A. A. Smith pinch instability analysis. In the second of the theories in order to more accu- (Ref. 2) reported that an entirely differ- section of this paper, measurements of rately model metal transfer in GMAW. ent type of metal transfer mode is pro- droplet si'ze at different welding currents The mechanism of repelled metal trans- duced when using carbon dioxide gas are compared with the theoretical pre- fer is also discussed. The transition of shielding as compared with argon shield- dictions. The limitations of the static metal transfer mode has been considered ing. force balance theory and the pinch in- as a critical phenomenon which changes With many factors influencing metal stability theory in the prediction of the dramatically over a narrow range of transfer, theoretical models such as the droplet size are discussed. In order to welding current. This transition has been static force balance theory (Refs. 3-5) account for the deviation between these investigated experimentally using high- and the pinch instability theory (Refs. theories and the experimental data, a speed videography which shows that the 6-8) have been proposed to explain the modification of the static force balance transition is much more gradual than is theory is proposed. The modified theory generally believed. The mechanism of is tested using a pulsed current welding the transition is discussed using a modi- experiment. fied static force balance theory. KEY WORDS Previous Studies Introduction Modeling Factors Affecting Metal Transfer Modes In gas metal arc welding (GMAW), GMAW Metal Transfer there are various modes of metal trans- Droplet Size Predict The operational variables affecting fer such as globular, repelled globular, Transfer Frequency the mode of metal transfer are the weld- projected spray, streaming, and rotating Taper Formation ing current, composition of shielding Shielding Gas gas, extension of the electrode beyond Electrode Extension the current contact tube, ambient pres- Y. S. KIM is Assistant Professor, Department Static Force Bal. Theory sure, active element coatings on the elec- of Metallurgy and Materials Science, Hong Pinch Instability Theory trode, polarity, and welding material. Ik University, Seoul, Korea. T. W. EAGAR is Measurement Among these variables, welding current Co-Director, Leaders for Manufacturing Pro- is the most common variable that the gram, Richard P. Simmons Professor of Met- welder adjusts to obtain the desired allurgy, Department of Materials Science and Engineering, Massachusetts Institute of Tech- metal transfer mode. At low welding cur- nology, Cambridge, Mass. rents, globular transfer mode occurs,

WELDING RESEARCH SUPPLEMENT I 269-s while spray transfer mode occurs at rel- metal transfer modes. where Cn is the drag coetficient, Ap is atively higher welding currents. 1) Static Force Balance Theory. The the projected area on the plane perpen- At the lower welding current range static force balance theory postulates dicular to the fluid flow, p, is the den- of the spray transfer mode, projected that the drop detaches from the elec- sity of the fluid, dnd vl is the velocity of transfer occurs in which the droplet di- trode when the static detaching forces the gas. Therefore, the plasma drag force ameter is approximately the same as the on the drop exceed the static retaining acting on the liquid drop can be approx- diameter of the electrode. As the weld- force. Four different forces are usually imated by moditying Equation 4 to allow ing current increases, the metal transfer considered: the gravitational force, elec- for [tie area uccupied by the electrode. mode changes from projected transfer tromagnetic force, and plasma drag The surfcice tension force, which acts mode to streaming transfer mode, then force are detaching forces, while the sur- to retain the liquid drop on the electrode to a rotational transfer mode. This sets a face tension force is a retaining force. is given as follows: practical upper limit on the current be- The gravitational force is due to the mass cause unstable metal transfer begins of the drop and acts as a detaching force where u is the radius of the electrode with rotational transfer. when welding in the flat position: and is the surface tension uf the liquid The transition of metal transfer modes metal. described above are observed in se- Waszink (Ref. 22) investigated the quence only when the welding material relative magnitudes of the detaching is steel and the shielding gas has an where R is the droplet radius, p.1 is the forces and showed good agreement with argon-rich composition. With other ma- density of the drop, and g is the gravita- experimental results within the range or" terials and with other shielding gases, tional constant. globular transfer, however, in the spray not all metal transfer modes are ob- The electromagnetic force on the transfer mode, the theory deviates sig- served. When carbon dioxide, helium, drop results from divergence or conver- nificantly rrom the experiment. and nitrogen are used as shielding gases, gence of current flow within the elec- In addition to the above-mentioned the repelled globular transfer mode is trode. When the current lines diverge in limitations, the static force balance the- usually observed (Refs. 9-1 1) and nei- the drop, the Lorentz force, which acts ory has clifticulties in explaining several ther streaming transfer nor rotational at right angles to these current lines, cre- metal transfer phenomena in GMAW. transfer is observed. When mixtures of ates a detaching force. The electromag- Firstly, the effect or electrode extension argon and carbon dioxide are used, the netic force is given by Lorentz's law: on metal transter is difficult to explain rate of drop transfer was found to in- since the electrode extension will not crease linearly with the composition of affect the force balance. Secondly, the the argon gas (Ref. 12). analyses of metal transf~rusing this the- Considering the effects of shielding where 7is current density and B is mag- ory have been performed mostly using gas on the metal transfer modes de- netic flux. steel electrodes and argon shielding gas. scribed above, there have been several By assuming that the current density Other systems which produce a re- attempts to increase the workable range on the drop is uniform, the total electro- pelling transter mode cannot be ex- of welding current by suppressing the magnetic force on a drop can be ob- plainer! with this theorv rotational transfer mode using argon- tained by integrating Equation 2 over the 2)Pinch Instability Theory. The pinch based shielding (Ref. 13). When helium current conducting surface of the drop instabili~vtheorv wcis (leveloped from and/or carbon dioxide are added to the (Ref. 3). the Ravleigh instability moclei (Ref. 23) argon gas, the range of welding current of a liciuicl cylindrical column. Since for projected spray transfer is greatly in- spheres can have a lower iree energy creased. than the liquid column, a disturbance of In addition, when a shorter electrode the propel ~vavelengtliin [lie liquid ~ol- extension and a larger electrode diame- unin tends to cause the liquid column ter are used, the transition current is in- to break up into drops. Rayleigh derived creased (Ref. 1). Amson (Ref. 14) ob- the conditions for the liciuid column in- served that as the pressure increases, the lability ~i:~urninga simple sinusoidal transition current increases. However, where I is the welding current and p.,, is pett~irtx~~iorifor an ir~viud5v-itern. The Perlman, etal. (Ref. 19, found that when the permeability of free space. The ge- perturbation of the cylinder is solved the pressure reaches 5 atm, spray trans- ometry used in Equation 3 and a graph with an exponential function of the form fer becomes irregular and fluctuation of of fy as a function of the conduction zone the voltage increases. The use of thin angle is given in Fig. 1. When the con- coatings on the electrode consisting of duction zone is small such that the cur- alkali, alkaline earth, rare earth ele- rent lines converge, f2 becomes nega- ments, and certain oxides have been tive, i.e., the electromagnetic force acts shown to increase the stability of metal as a repulsive force. However, when the transfer (Refs. 16-1 8). conduction zone is large enough so that the current lines diverge, f2 becomes Review of Existing Theories positive and the electromagnetic force p =density, A = wavelength of the fluc- of Metal Transfer becomes a detaching force. tuation, lt7,iq) modified Bessel func- The plasma drag force on the liquid = tion of the first kind order m, l',,,(q) = the There are two well-quoted theories drop can be estimated by considering iirst derivative of lm(v. of metal transfer. These are the static the drag force on a sphere immersed in When m = 0 (sausage mode), the force balance theory (Refs. 3-5) and the a fluid. The drag force on a sphere im- maximum frequency is obtained with pinch instability theory (Refs. 6-8). In mersed in a fluid of uniform velocity q = 0.690, iiri(.l the most probable drop addition to these theories, the plasma field is (Ref. 23) force theory (Ref. 19) and the critical ve- size (Rn) is calculated to be hvice the di- locity theory (Ref. 20) have been pro- imeter of the liquid column. posed to explain the transition between The pinch instability theo'ry of metal

270-s I JUNE 1993 - transfer analysis postulates that the pinch force on the liquid column of molten metal due to the self-induced electromagnetic force enhances the Detaching break-up of the liquid column into '1 .I droplets. An approxiniate analytical so------lution of the critical wavelength of this instability has been derived (Ref. 24) : Retaining

As seen in Equation 8, the welding 30 60 90 120 150 current reduces the critical wavelength Fig. 1 - Variation of f-, as a function of the instability of the liquid jet and thus ANGLE THETA IN DEGREES of q value. decreases the droplet size. In this way, the pinch instability theory claims to explain the general trend of decreasing drop size with increasing welding cur- 6 is assumed to be 150 deg, which is the For the surface tension force, it is as- rent. conduction zone angle when the droplet sumed that the interface between the liq- Anno (Ref. 25) derived the frequency size is twice the size of the electrode. uid drop and solid electrode is perpen- of fluctuation for a viscous jet with a sur- As seen in Fig. 1, the value of 6 does not dicular to the electrode axis. Also, the face charge and showed that viscous ef- change significantly when 6 is larger diameter of the drop holding neck was fects and surface charges have stabiliz- than 60 deg. Thus, this assumption will assumed to be the same size as the elec- ing effects. Allurn (Ref. 8) showed that not cause any significant error in the trode diameter. The surface tension data the viscous effects are negligible in liq- electromagnetic force calculation. were assumed to be: 0.9 N/m for alu- uid metal, but the stabilizing effects of For the drag force, the value of Cn in minum (Ref. 27), 1.3 N/m for titanium surface charge are significant in the low Equation 4 depends on the Reynold's (Ref. 28), and 1.8 N/m for steel (Ref. 27). welding current range. number of the shielding gas. Since the The total detaching force under vari- The pinch instability theory suffers velocity of the plasma in GMAW is not ous droplet size at a certain welding cur- the same problems as the static force available, the plasma velocity was as- rent is the summation of the electromag- balance theory. These include difficul- sumed to be 100 m/s, which is the same netic force, the gravitational force, and - ties in explaining the effect of electrode as the plasma velocity in GTAW (Ref. the plasma drag force. At the crossover extension, and the repelling mode of 27). The Reynold's number with this ve- point, where the surface tension hold- metal transfer. locity is calculated to be approximately ing force and the total detaching forces 3) Other Theories. When using steel 9000, which lies in the Newton's law meet, the equilibrium droplet size is de- electrodes with argon shielding, the region. Thus, the values of CD for the termined. Figure 2 shows the total de- transition of metal transfer mode from plasma is 0.44 (Ref. 21). For less-devel- taching force at various welding currents globular to spray transfer has been re- oped plasma jets, 10 m/s was used for as a function of droplet size for steel ported to occur over a very narrow cur- the velocity of the fluid. The value CD electrodes with shielding gas velocities rent range: less than 10 A (Ref. l). In an for 10 m/s gas flow rate is also calcu- of 10 m/s, respectively. The crossover attempt to explain the sharp transition lated to be 0.44. Ap is the projected area point at each welding current defines in metal transfer mode as found by of the sphere exposed to the fluid on a the equilibrium droplet size from the Lesnewich (Ref. 1), Needham, etal. (Ref. plane perpendicular to the direction of static force balance theory. The equilib- 191, using the static force balance the- the motion and is given by Equation 10 rium droplet size of the steel electrode ory, has proposed that the transition oc- with shielding gas velocity of 10 m/s and curs when the welding plasma starts to 100 m/s are summarized in Fig. 3. exert a drag force on the drop.

Calculation of Equilibrium Drop Size

From the static force balance theory, the droplet size can be calculated under the assumption that the drop detaches from the electrode when the sum of the detaching forces equals the holding force :

(holding force) (detaching force) (9) in calculating these forces acting on the liquid drops, a number of assumptions are made. Firstly, in calculating the 0 0.12 0.16 0.20 0.24 0.28 electromagnetic force on the droplet, the electrons are assumed to condense DROP RADIUS (CM) uniformly on the liquid droplet only and Fig. 2 - Variations of the detaching force as the droplet size at the steel electrode tip increases. The assumed argon plasma velocity is 10 mh.

WELDING RESEARCH SUPPLEMENT I 271-s WELDING CURRENT IA1 WELDING CURRENT (A)

Fig. 3 - The equilibrium droplet size of the steel electrode calcu- Fig. 4 - The forces acting on the drop at a steel electrode tip (argon Sated from the static balance theory. The shielding gas is assumed to plasma velocity: 1OOm/s). be argon.

The effect of velocity differences in predicts drop sizes which are much rent electrode positive (DCEP) condi- the shielding gas is not significant and smaller than the equilibrium drop size tions. An alumina tube was inserted into decreases as the current increases. This predicted by the static force balance the contact tube, leaving only 5 min (0.2 is because the influence of the electro- theory. in.) for contact length rather than the magnetic force becomes dominant as normal contact length of 24 rnm (0.94 current increases (Fig. 4). At low weld- Experimental Procedures in.) in commercially available contact ing currents, the welding p1asm.a does tubes. This reduced the variability in not form a strong enough jet to produce Mild steel (AWS ER70S-3), alu- joule heating due to changes in the lo- a 100 m/s velocity jet, thus, it is likely minium alloy (AA1100, AA53561, and cation of the current contact. that the droplet size will follow the pre- titanium alloy (Ti-6AI-4V) were used in Analysis of metal transfer was per- diction of the 10 m/s jet. However, as the experimental portion of this study. formed using high-speed videography the welding current increases, the The shielding gases used were pure with a laser back-lighted shadowgraphic droplet size will follow the prediction argon, pure helium, 25%Ar-75%He, method (Ref. 30). In this method, a spa- of the 100 m/s jet. In addition, this pre- 50%Ar-50%He, 75%Ar-25%He, Ar- tial filter is located at the focal point of diction shows that the droplet size de- 20/002 and carbon dioxide. The weld- the objective lens, where the parallel creases smoothly as the welding current ing equipment included a constant cur- laser light becomes focused. Thus, the increases. rent power supply, and a voltage con- spatial filter transmits most of the laser The droplet size from the pinch in- trolled electrode feed with a "low iner- light and excludes most of the intense stability theory is calculated using Equa- tia" motor. The transistorized welding arc light. The high-speed video camera tion 8. Assuming that the most probable current regulator used in this study was is capable of producing images at ii max- wavelength of the instability is kc/0.696, of the constant current type that can sup- imum 1000 full frame pictures per sec- the droplet size is calculated and plot- ply DC current with less than 1% ripple ond (pps). In most of the analyses of ted as a function of welding current in (Ref. 29). This system is capable of puls- metal transfer, this maximum 10(10 pps Fig. 5. The droplet size decreases con- ing the DC current to a maximum of 5 filming rate was used. The droplet trans- tinuously as welding current increases; kHz for small superimposed signals. The fer rate was measured for ten seconds however, the pinch instability theory welding was performed under direct cur- and an average droplet transfer rate for

Globular++- Spray 0.240 10 m/s

0.180 ** 0.120 a 0 Theory

200 300 400 180 210 240 270 300 WELDING CURRENT IA1 WELDING CURRENTIAI

Fig. 5 - The droplet size of steel electrodes when shielded with Ar- Fig. 6 - Comparison ofpredicted and measured droplet size of the 2%02calculated from the pinch instability theory. steel electrode when shielded with Ar-2%OJ. ! each welding condition was calculated. Table 1 - The Combination of Welding Parameters Used in This Study The droplet size was measured from the still image on the screen once every sec- : ond for ten seconds and averaged. The Shielding Gas Electrode - Extension Arc Length Welding : variation in measured droplet size and Argon Helium CO; (mm) (mm) Current frequency was *5%. The parameters used as variables in Mild steel (a) (a) (a) 16,26,36 14:Ar 180-420 A 6-He this study include welding current, weld- 8"ZOz ing material, shielding gas, electrode ex- Aluminum (a). . 16,26,36 14:Ar 80-220 A tension, and arc length. The combina- (1100,5356) tions used are shown in Table 1. All the Ti-6Al-4V (a) welding was performed as bead-on- plate, and the plates were prepared such (a) Welding was performed that there was no mill scale on the sur- dieted from the pinch instability theory with helium and carbon dioxide shield- face. The electrode diameter used was is much too small in the globular trans- ing. Because of errors in direct measure- 1.6 mm in.) for all materials. fer mode region and does not tend to- ments of droplet size due to distortion, The arc length using argon shielding ward the experimentally determined the average droplet sizes that are plot- gas was constant at 14 mm (0.55 in.), droplet sizes at high currents. ted are not the directly measured data. but with helium and carbon dioxide Figure 7 shows the measured fre- The average sizes were calculated by di- shielding gas, the arc lengths were 6 and quency of drop transfer. Again, no sign viding the melting rate of the electrode 8 mm (0.24 and 0.31 in.),respectively, of a sharp transition in the metal trans- by the frequency of metal transfer, both because of the higher electrical resistiv- fer rate is present. Using a definition of of which were experimentally mea- ities of these welding plasmas. spray transfer mode in which the droplet sured. Figure 10 shows the variation of diameter is the same as the diameter of droplet transfer frequency as a function Measurement of Droplet Size the electrode, one obtains a transition of welding current for both of the shield- and Transfer Frequency at a current of approximately 255 A. The ing gases. transition from projected spray transfer With helium shielding, at low weld- Effect of Welding Current mode to streaming transfer mode is also ing currents, the droplet sizes are much Figure 6 shows the experimental re- a gradual phenomenon. The droplet size bigger than predicted by the static force sults of droplet size measured as a func- decreases gradually, and it is very diffi- balance theory. As the welding current tion of welding current when shielded cult to define the transition current based increases, a change occurs in the droplet with Ar-2% oxygen gas using a 2.6 cm on either droplet size or frequency. size around 240 A. This jump corre- (1.02 in.) electrode extension. The sponds to the transition from the repelled droplet size predicted from the static Effect of Shielding Gas globular transfer mode to the projected ' force balance theory and the pinch in- spray transfer mode. In the spray trans- stability theory is also shown in the fig- Figure 8 shows the repelled transfer fer mode, the droplet size predicted from ure. As seen in Fig. 6, the measured mode just before detachment, when the static force balance theory agrees droplet size decreases gradually as weld- shielded with carbon dioxide gas. The fairly well with the experimentally mea- ing current increases, and there is no drops are blown away from the base sured droplet size. With carbon dioxide sharp transition. In the globular transfer metal by the strong cathode jet, and they shielding, a repelled transfer mode was mode, the predicted drop size from the are distorted significantly. As the weld- observed up to the maximum welding static force balance is reasonably close ing current increases, the droplet size current tested in this study, i.e., 400 A. to the measured values. As the welding and the distortion of the drops become The droplet size continued to decrease current increases, the predicted droplet less pronounced. as the welding current increased, and size starts to deviate from the measured The average droplet size is plotted in there was no dramatic change in the values significantly. The drop size pre- Fig. 9 as a function of welding current droplet size as current was increased.

WELDING CURRENTIAI

Fig. 7 - Frequency of drop transfer of the steel electrode with Ar- Fig. 8 - Repelled metal transfer of steel electrodes shielded with 2%0, shielding. CO, gas.

WELDING RESEARCH SUPPLEMENT I 273-s I-

: CO, : He I -

L-Q: Â I I AA O A I A AAA 150 200 250 300 350 4 WELDING CURRENT (A)

Fig. 9 - Experimentally measured droplet size for steel electrodes Fig. 10 - Frequency of drop transfer of steel electrodes shielded with shielded with helium and carbon dioxide. helium and carbon dioxide.

With these shielding gases, streaming trode extensions: 2.6 and 3.6 cm (1.02 narrow range of welding current in both transfer mode was not observed. and 1.43 in.). A steel electrode shielded alloys 1 100 and 5356. In order to explain the repelled trans- with Ar-2% O2 gas was used. As the The droplet size in the globular range fer mode observed with these shielding electrode extension increases from 2.6 is within the range of the prediction from gases, it is necessary to introduce a re- to 3.6 cm, the current of the globular- the static force balance theory. As the pelling force into the static force bal- projected spray transition deceases from welding current increases, thedroplet size ance theory. A possible candidate for 255 to 240 A. As seen in Fig. 11, the ef- from the static force balance theory devi- the repelling force in helium and car- fect of electrode extension on metal ates significantly from the experimental bon dioxide shielding gases is the cath- transfer is significant. However, this ef- results as in the case of steel electrodes. ode jet force. The cathode jet force in fect is very difficult to understand in The current of the globular-projected GTAW has been estimated as in Equa- terms of either the static force balance spray transfer transition was 120 A. tion 11 (Ref. 31). theory or the pinch instability theory. For the Ti-6AI-4V electrode, the mea- The possible cause for the effect will be sured droplet sizes are shown in Fig. 13. --/v- cathode jet - discussed in a later section of this paper. With this electrode, two different metal 871 transfer modes are shown: repelled Effect of Welding Material transfer due to a strong cathode jet (Fig. In the range of welding current from 14) was observed at low welding cur- 200 - 400 A, the cathode jet force is cal- Metal transfer with aluminum and rents, and projected transfer was ob- culated to vary from 1 X lo-* to 6 X 10-2 with Ti-6AI-4V electrodes in addition to served at high welding currents. As the N, which is comparable to the detaching the steel electrodes was studied. Figure welding current increases, there is a sud- force. 12 shows the variation of the droplet size den reduction in the droplet size. This with welding current for an aluminum jump corresponds to the transition from Effect of Electrode Extension electrode, along with the droplet size the repelled globular transfer mode to predicted by the static force balance the- the projected spray transfer mode, as is Figure 1 1 shows the droplet size vari- ory. The transition from globular trans- also found in steel electrodes shielded ation with welding current at two elec- fer to spray transfer occurs over a very with helium gas.

A: Extension 3.3 CM s a: Extension 2.6 CM 0.160

0 1 * I I I I , - 150 200 250 300 120 150 180 21 0 240 WELDING CURRENT 1AI WELDING CURRENTIAI Fig. 1 1 - Effects of electrode extensions on the droplet size of steel Fig. 12 - Comparison of preclictecl c~ndmeasured droplet size of sn electrodes. The shield gas is Ar-2%0,. iiluminum 1 100 electrode with Ar-2% shielding.

274-s I JUNE 1993 In the projected spray transfer region, the droplet size predicted by the static force balance theory predicts larger droplet sizes compared with the experimental results. It is believed that the deviation is caused by the value of the surface tension used: the surface tension for the Ti-6Al-4V alloy was unavailable and the surface tension for pure titanium was used in the calculation. As an illustration of the strength of Fig. 13 - Comparison the cathode jet force with Ti-6AI-4V of predicted and mea- electrodes, Fig. 15 shows successive pic- sured droplet size ofa tures of a trajectory of the drop detached Ti-6Aj-4V electrode with Ar-2%0-, shield- from the electrode and repelled by the me. cathode jet in the welding plasma. The WELDING CURRENT (A1 welding material used was zirconium, which has similar thermophysical prop- erties as the Ti-6Al-4V electrode, and the shielding gas was argon. Initially, the drop travels toward the weld pool. As it approaches the weld pool, the cathode jet becomes stronger such that the velocity of the drop is reduced and finally the drop is [ejected away from the weld pool by the cathode jet, causing spatter on the base plate. This phenomenon is also frequently observed with the Ti-6Al-4V electrode. As the welding current increases, the anode jet dominates the plasma flow, and the droplet transfer mode changes from repelled transfer to projected spray transfer.

. The Cause of Droplet Size Deviation between the Static Force Balance Theory and the Experimental Results

The discrepancy of the droplet size from the static force balance theory as compared with the experimental data may be explained by examining the va- lidity of the assumptions made in the calculation of the static force balance theory. One of the most important as- Fig. 14 - Repelled metal transfer of Ti-6Al-4Velectrodes caused by the strong cathode jet sumptions is that the electrode should force. The shielding gas is Ar-2%02 shielding. remain cylindrical and maintain its full diameter at the point where the drop is formed. If the diameter of the neck is changed either by surface melting or by Table 2 - Tapering of Electrode a1 Various Welding Currents with a Sleel Electrode Shielded deformation, the holding force will be with Ar-2% 02 affected as can be seen from Equation Welding current (A) 205 237 253 28 1 310 2.c The geometry of the drop holding Taper length (mrn) 0 2.4 2.7 4.1 5.1 neck was investigated under various conditions, and it was found to be significantly changed due to formation of a taper at the electrode tip - Table 2. As seen in Table 2, the tapering of the electrode occurs in the range of welding currents where the discrepancy between the theoretical droplet size and experimental data becomes significant. Figure 16 shows a fully developed taper at high welding current (>280 A). The ,- tapering of the electrode occurs because the anode spot reaches this surface of the electrode and generates condensa- Fig. 15 - Successive pictures of a drop rejected by the strong cathode jet from the base metal. tion heating on the cylindrical surface The electrode material /< zirconium and the shielding gas is pure argon.

WELDING RESEARCH SUPPLEMENT I 275-s 0. Â Â Experimental Data Â n A :Modified Static force Balance Theory @*Â

180 210 240 270 300 WELDING CURRENT (A)

Fig. 16 - A typical shape of the taper formed at the end of steel elec- Fig. 17 - Comparison of droplet size preriic :"ci !IL !)it ~:~:11i:ton e trode when shielded with Ar-2%0,. balance theoryand by the modified ,t~ticr'orcr t~~ii~vi:' (i~eon iritti measured values. of the electrode (Ref. 32). When enough by measuring the size of the drop hold- tional arec>ot the c~~i?ctro;l;~.The ~ori- heat is generated on the surface, the sur- ing neck at different welding currents densation heat is :eneraleri when clec- face will melt and the liquid metal will and substituting the tapered electrode troni enter the- pic-ctroilo ;ion1 the be swept downward by either the gravi- radius for the radius of the drop holding plasma When shielded nith ,irqon, tational force and/or the plasma drag neck in Equation 5. It was assumed that some portion of this heat e'lters onto the force. When this melting and sweeping most of the welding current flows di- cylindri~~ilsurface since eki. irons ion- action occurs overa significant length rectly into the drop. As seen in the fig- dense nut onl~on the burtiice or tlie Iic- of cylinder, a taper will develop at the ure, the droplet size predicted from the uid drop, but iilio on the ,urii'ite 01tlii., end of the electrode. modified theory follows the measured solid electrode. The !DJI~hi\it \i tl'ed~e\i~ The tapering of the electrode tip will droplet size. gener~t~riby the r-IM Ti<,il rrs~<;i,n~(2 ot reduce the effective diameter of the drop The effect of the electrode extension the electrorle iinrl oci urs ~,n~'ornil\in- holding neck, thus reducing the holding may also be explained by considering side the electrode. force in Equation 5. The reduced hold- the taper formation. The total heat input When there is no i on(lr;>.it,on hfiit ing force will produce a smaller droplet into the electrode occurs via electron input on the i vlmcirn.al 5Llll.K e 01 the size thus creating a streaming droplet condensation heat and joule heat as de- elec trode, the tpnjpt3r.itiircor ;tic joliil transfer mode. On the other hand, with fined by Equation 12 electrode will increiiv' J;. tho ,oulv lie,it steel electrodes shielded with helium generation inci-eiises. rlius 11 !tli loii:;i~: and with Ti-6Al-4V electrodes shielded electrode extension, :he nlrliiii'-; the with Ar gas, tapering of the electrode is cylinclricial surtcice IMpcrir!? i ,in

In order lo lei1 the tdpei-inq lhcor) pulsed current welcnnt; i-x~erinient, Fig. 18 - Comparison 0.300 with which taperinv,cCiin hts ( ontrolled between the droplet Current Cycle (%) were designed. The o\perimentiil pro- size of the steel elec- 0.240 cedures tor the pulsed current fie~diris trode form the static may be found elsewhere [Ref i 3). Fi'-;- force balance theory ure 18 show the miriinium droplet iizes and minimum droplet 0.180 Theoretical size from pulsed cur- measured at 300-, 400- and iO0-A pe,ik rent welding. The currents, .alonf; with thi-1 droplet \izo pre- shielding gas is dicted from the static force baLince the- Ar-2'X~0,5hieldiiig. ory. Steel electrode;, shielded Â¥i ith Ar - 2% oxygen were used at biiir currents of 180

WELDING RESEARCH SUPPLEMENT I 277-s & 17. Agusa, K., Nishiama, N., and Tsuboi, demic Press, New York. 28. Hand Book of Chemistry and Physics, J. 1981. MIG welding with pure argon shield- 22. Waszink, J. H., and Graat, L. H. J. 63rd ed. 1982. F-28, CRC Press, Boca Raton, Ã ing-arc stabilization by rare earth additions 1979. Der einflues der gasstroemung auf die Fla. to electrode wires. Metal Construction 9: tropfenabloesung beim plasma-mig scheis- 29. Eickhoff, S. T. 1988. Gas metal arc 570-574. sen. CrosseSchweisstechnische Tagung, DVS welding in pure argon. M.S. thesis, M.I.T., 18. Lucas, W., and Amin, M. 1975. Effect Berichte 57: 198-203. Cambridge, Mass. of wire composition in spray transfer mild 23. Lord Rayleigh, 1897. Proceedings of 30. Allemand, C. D., Schoeder, R., Ries, steel MIG welding. Metal Construction 2: the Mathematical Society, pp. 10, 4. D. E., and Eagar, T. W. 1985. A method of 77-83. 24. Lancaster, J. F. IIW Document No. filming metal transfer in welding arc. Weld- 19. Needham, J.C., Cooksey, C. J., and 21 2-429-78. ingjournal 64(1): 4547. Milner, D. R. 1960. Metal transfer in inert- 25. Anno, J. N. 1977. The Mechanics of 31. Lin, M. L. 1985. Transport processes gas shielded-arc welding. British Welding Liquid Jets. Lexington Books, Lexington, affecting the shape of arc welds. Ph.D thesis, Journal 7(2): 101-1 14. Mass. M.I.T., Cambridge, Mass. 20. Waszink, J. H., and Van Den Heuvel, 26. Chapman, 5. 1980. Glow Discharge 32. Kim, Y. S. 1989. Metal transfer in gas C. J. P. M. 1982. Heat generation and heat Processes. lohn Wilev and Sons, New York, metal arc welding. Ph. D thesis, M.I.T., Cam- flow in the filler metal in CMA welding.Weld- 53. bridge, Mass. ing Journal 61 : 269-5 to 282-5. 27. Mondain-Monval. The physical prop- 33. Kim, Y. S., and Eagar, T. W. Metal 21. Szekley, J. 1979. Fluid Flow Phenom- erties of fluids at elevated temperature. IIW transfer in pulsed current gas metal arc weld- ena in Metals Processing. pp. 256-257. Aca- Document No. 212-264-73. ing. To be published in welding journal.

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WELDING JOURNAL

278-s I JUNE 1993