Metal Tranfer

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    WELDING RESEARCH

    -s229WELDING JOURNAL

    ABSTRACT. Dual-bypass gas metal arc welding (DB-GMAW) is a modifiedGMAW process. In this novel process, the

    base metal current is decreased from themelting current by adding two tungstenelectrodes to a conventional GMAW sys-tem. The resultant bypass arcs change theforces affecting on the droplet, and the re-sultant metal transfer becomes more de-sirable. To understand the desirablechanges in the metal transfer, this paperapplies established theories to analyze thechanges in the forces acting on the dropletand the effects of these changes on themetal transfer behaviors. Analysis showsthat the bypass arcs and currents lower thecritical current needed to achieve the de-

    sired spray transfer. Experimental resultsobtained by a high-speed camera show thatthe analysis agrees with experimental data.

    Introduction

    Gas metal arc welding (GMAW) is anarc welding process that establishes an arcbetween a continuously fed consumablemetal electrode and the workpiece. Dueto its high productivity, it has become oneof the predominant methods to join met-als. In recent years, more and more alu-minum alloy welded structures have beenwidely applied. The use of aluminum as analternative material in more applicationshas brought a higher requirement andchallenge to aluminum welding (Ref. 1).As one of the widely used aluminum weld-ing methods, the GMAW process needsimprovements in order to achieve higher

    weld quality and higher productivity.Since the characteristic of metal transferin GMAW significantly affects the weld

    quality especially with respect to its mi-crostructure, porosity formation, strength,and fatigue properties, etc., researchershave made great efforts to study the metaltransfer in GMAW (Refs. 27). This paperstudies the metal transfer in welding alu-minum using a novel process that can re-duce the heat input.

    Low base metal heat input and arcpressure are often critical in meeting spec-ified requirements in aluminum welding(Ref. 8). In a traditional GMAW process,it operates in the globular metal transfermode at relatively low continuous wave-

    form currents. However, this transfermode is characterized by periodic forma-tion of large droplets that detach from theelectrode primarily by the gravitationalforce and are typically associated with arcinstability (Ref. 9). At higher currents, thetransfer mode changes to the desirablespray mode that offers high depositionrate and desirable arc stability, but at theexpense of high heat inputs that may betoo much for many aluminum welding ap-plications. Moreover, solidification or hotcracking due to the overheating of basemetal is also a critical weld quality con-cern. In order to solve this problem,pulsed gas metal arc welding (GMAW-P)has been developed. In GMAW-P, thepulse parameters can be adjusted to con-

    trol the droplet transfer mode, heat input,droplet size, or droplet velocities for dif-ferent applications. However, to achieve

    the spray transfer, the peak current has tobe greater than the transition current,which is relatively high. This relativelyhigh peak current produces a large arcpressure that can easily generate meltthrough in complete-joint-penetration ap-plications especially during aluminumwelding. Further, the parameters for thepulse waveform need to be determined ac-cording to material, shield gas, and wirediameter.

    Recently, a modified GMAW processhas been developed at the University ofKentucky (Ref. 10) to decouple the melt-

    ing current (which melts the wire and de-termines the deposition rate) into the basemetal current (which determines basemetal heat input and arc pressure) and abypass current. This process is formed byadding a bypass tungsten electrode to by-pass part of the melting current that wouldotherwise, in conventional GMAW, alsoflow into the base metal. This modifiedprocess is referred to as double-electrodeGMAW or DE-GMAW. It can achieve thedesirable spray transfer at a wide range oflow base metal (continuous-waveform)currents. To further reduce the currentneeded for the desirable spray transfer toaddress the need of aluminum welding forlow heat input and low arc pressure, theauthors have further modified the processto result in dual-bypass GMAW (DB-GMAW) in which two bypass tungstenelectrodes are used. Experimental resultshave indicated that the bypass arcs in DB-GMAW do affect the arc forces and theirdistribution and consequently affect themetal transfer process. This paper is thusdevoted to the analysis of the influences ofbypass arcs on the forces acting on thedroplet and their influences on metaltransfer during DB-GMAW of aluminum.

    KEYWORDS

    Dual-Bypass Gas Metal ArcWelding (DB-GMAW)

    CurrentBypass ArcsDropletBase Metal Heat InputSpray Transfer

    Analysis of Metal Transfer and CorrelatedInfluences in Dual-Bypass

    GMAW of Aluminum

    Various factors, including the welding gun setting, joint penetration,and base metal heat input during this process were examined

    BY Y. SHI, X. LIU, Y. ZHANG, AND M. JOHNSON

    Y. SHI is with the State Key Laboratory of GansuAdvanced Nonferrous Metal Materials, LanzhouUniversity of Technology, Lanzhou, China. Along with X. LIU and Y. ZHANG(ymzhang@engr.uky.edu) , SHI is also with theCenter for Manufacturing, University of Kentucky,Lexington, Ky. M. JOHNSON is with Los Alamos

    National Lab, Los Alamos, N.Mex.

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    WELDING RESEARCH

    SEPTEMBER 2008, VOL. 87-s230

    Principles of DB-GMAW

    A dual-bypass GMAW process shownin Fig. 1 was at the University of Kentucky.As illustrated, the system includes a con-stant voltage (CV) power supply to pro- vide the base metal currentIcv, and twoconstant current (CC) power supplies to

    provide the left and right bypass currents:Ileft andIright. The positive terminals of allthree power supplies are connected to-gether to the GMAW gun (which providethe bypass tungsten electrodes), respec-tively. In this way, the total melting currentthat melts the wire is the sum of currents,i.e.,I=Ibasemetal +Ileft +Iright. Thus, thebase metal current that controls the basemetal heat input and arc pressure imposedon the base metal can be much less thanthe total melting current. It has been ver-ified by experiments that the total meltingcurrent is determined by the preset wire

    feed speed (WFS) and the welding voltage

    for the CV powersupply. Hence, thebase metal currentcan be decreasedby increasing thebypass currents be-cause their sum is aconstant. Becausethe bypass currentsare provided bytwo CC power sup-plies and can be ad- justed freely, theDB-GMAW can

    provide a largerange of base metalcurrents for eachset of wire feedspeeds and welding voltages to meet

    the needs from different applications fordifferent heat inputs and arc pressures.While the effects of DB-GMAW on thebase metal heat inputs and arc pressuresare straightforward, an analysis is neededin order to understand its effects on themetal transfer process.

    Force Analysis

    Conventional GMAW

    Previous research indicates that thetype of transfer obtained is determined bythe welding current magnitude and polar-ity, the wire composition, and activation.It also is affected by the wire diameter andwire extension (Ref. 11). The reason thatthese factors have an impact on metaltransfer is because all these parameterscan change how the forces act on thedroplet.

    In conventional GMAW, the major

    forces acting on the droplet include thegravity, electromagnetic force (Lorentzforce), aerodynamic drag force, surfacetension, and vapor jet force (Ref. 12). Ac-cording to the static-force balance theory(SFBT) (Ref. 3), the balance of theseforces determines the metal transferprocess, i.e., droplet formation, size, andfrequency. These forces are shown in Fig.2.

    The force due to gravity can be ex-pressed as

    where rd is the droplet radius, is thedroplet density, andg is the acceleration ofthe gravity.

    The surface tension is given as (Ref.12)

    whereR is the electrode radius, while isthe surface tension coefficient.

    The aerodynamic drag force can be ex-

    pressed as (Ref. 12)

    where Cd is the aerodynamic drag coeffi-cient, fandvfare the density and fluid ve-locity of the plasma. This force is higherwith higher droplet radius and plasmavelocity.

    The vapor jet force is given by (Ref. 12)

    Fm

    dIJv

    f

    =0 4( )

    F v r C a f f d d = 0 5 32 2

    . ( )

    F R = 2 2( )

    F mg r gg d

    = =4

    31

    3 ( )

    Fig. 1 Illustration of DB-GMAW. Fig. 2 Major forces acting on the droplet in GMAW.

    Fig. 3 Schematic of forces affecting droplet in DB-GMAW.

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    WELDING RESEARCH

    -s231WELDING JOURNAL

    wherem0 is the total mass vaporized persecond per ampere, I is the welding cur-rent, andJis the vapor density.

    The electromagnetic force, Fem, isgiven by (Ref. 13)

    where 0 is the magnetic permittivity,Iisthe welding current,ri is the exit radius ofthe current path, andru is the entry radiusof the current path. At the time thedroplet is initially formed, the radius ofthe droplet is smaller than the arc radius.At this particular time,rirw (rw is the ra-dius of the welding wire),rura (ra is theradius of anode area). After the appear-ance of droplet neck, ri rn (rn is thedroplet neck radius) andrura.

    The balance of the forces on a dropletis given by

    For spray transfer, Ref. 13 calculated Fg,Fem,F, andFawhen the welding currentis 300 A and the droplet mass is 30 mg.Calculation indicated that the influencefrom Fg and Fa to droplet is relativelysmaller; Fv obviously influences thedroplet only under large welding currents(Ref. 12). Therefore, the electromagneticforce is the do