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Analysis of Metal Transfer in Gas Metal Arc Welding

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Analysis of Metal Transfer in Gas Metal Arc Welding 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.
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