Influence of anode composition on metal transfer in GMAW
Stéphane PELLERIN 1, Flavien VALENSI 1,2, Nadia PELLERIN 3, Emmanuel VERON 3, Francis BRIAND 4
1
GREMI, University of Orléans / CNRS, Rue G.Berger, BP 4043, F-18028 Bourges cedex (France)
LAPLACE, Université de Toulouse, UPS, INPT, 118 route de Narbonne, F-31062 Toulouse Cedex 9 (France)
3
CEMHTI, CNRS/Université d’Orléans, UPR3079, Av. de la Recherche Scientifique, F-45071 Orléans cedex 2
4
CTAS-Air Liquide Welding, Saint Ouen l’Aumône, F-95315 Cergy-Pontoise cedex (France)
2
Abstract: Various transfer modes can occur in gas metal arc welding (GMAW)
process, according to the choice of the welding current and shielding gas
composition. The spray transfer mode, that users prefer, is obtained for a high
enough current under pure argon; for a lower current or when CO2 is added in the
gas in sufficient concentration, the transfer becomes globular, with a lower
quality. The aim of the present work is to define the conditions allowing to get
the spray transfer mode for the highest possible CO2 concentration in the
shielding gas and/or the lowest possible arc current value. Particular attention
was paid to the microstructural and chemical evolution of anode wire in various
welding conditions. The formation of an oxide gangue around the molten metal
has been evidenced and correlated to the globular mode establishment: this
insulating and highly viscous layer hinders the current transfer and prevents the
detachment of small droplets required in spray arc. Complementary studies show
the benefit of using cored wires with specific elements (especially alkaline), and
the influence of these minority elements contained in the anode on the behavior
of the arc welding, according to changing of the properties and microstructure of
the electrode, and thus the metal vapor source term in the models.
Keywords: GMAW, spray, globular, anode, microstructure
1. Introduction
Metal vapors have a significant, and in some cases
dominant, influence in many applications of
atmospheric-pressure plasmas. Particularly, in arc
welding processes, metal vapor is formed by the
evaporation of molten metal in the weld pool, and in
the case of gas–metal arc welding, in the wire
electrode and droplets. The presence of these metal
vapors can have a major influence on the properties
of the arc and the size and shape of the weld pool.
While the influence of metal vapor has long been
recognized, it is only recently that diagnostic and
computational tools have been sufficiently welldeveloped to allow this influence to be more
thoroughly examined and understood [1].
In the MIG-MAG ("Metal Inert Gas"- "Metal Active
Gas") welding process, the plasma arc burns
between the extremity of a fusible electrode (usually
anode) and metal plate (usually cathode). The
transfer mode of the melted metal in the arc depends
mainly on nature of the used gas, electrode
dimensions and composition, and the density of the
welding current; the regime of transfer by shortcircuits ("Short-arc"), the "globular transfer" and the
"spray-arc" are the most frequent ones. The nature of
the applied shielding gas has a strong influence on
quality of the welding process. In particular, increase
of the percentage of carbon dioxide in argon induces
increase of the transition current value from the
globular to spray metal transfer mode.
We have investigated in particular the globular and
spray transfer modes, for solid and cored wire. The
arc shape and electrode configuration during
welding have been studied using high-speed video
recording, in order to get criteria to characterize the
transfer mode. This study allows also to get an
estimation of the current flow from the anode and
the current line geometry. The microstructure of the
consumable anode tip, collected after the welding
sequence, has been studied by scanning electron
microscopy (SEM) and electron dispersion
spectroscopy (EDS) analysis. We studied the
influence of electrode initial composition on the
transition condition in regard to arc current and CO2
concentration.
2. Experiments
The experiments were organized around a welding
set SAFMIG 480 TRS PLUS generator with a
SAFMIG 480 TR 16 kit [2]. The welding was
performed under reverse-polarity (wire-anode,
workpiece-cathode) in the constant current mode.
The tests have been performed using 1.2 mm
diameter wires of various compositions. A first
group of experiments has been made with solid
wires and a second one using cored wires. These
wires allow studying a wide range of composition,
but the final wire composition can differ slightly
form the initial wanted composition. The results
interpretation must then be made by taking into
account the actual wire composition.
The gas used was pure argon and argon-CO2
mixtures. The composition was set with two mass
flowmeters regulating the argon and CO2 supply
from two different gas bottles.
As the dynamic of the process is fast, a high-speed
video camera (up to 3000 frames per second) was
used, fitted out with a blue interferential filter
centered at 469.2 nm with a FWHM of 3 nm. In this
wavelength window, the argon continuum can be
considered as only dependant of the square electron
density Ne2. No argon line is observed but metallic
elements ones, such as iron or manganese [2], which
have a greatest electric conductivity. Then the
brightest part of the recorded pictures should
correspond to the highest conductivity zone, which
will help to define the shape of the current lines in
the plasma.
The procedure to study the tip of the consumable
electrode microstructure was to collect it after the
end of the welding sequence, assuming that the
cooling in the shielding gas was fast enough to cause
a quench. This has been validated using another
method, allowing collection in a water or oil bin of
metal droplets with a breakthrough cathode. The
droplet constituting the tip of the electrode was then
cut and polished to get a cross section in the axial
direction. The samples were studied using scanning
electron microscopy (SEM) to identify the various
phases and X-ray spectroscopy (EDS) to get a
qualitative analysis of the present elements.
3. Gas influence on the arc shape in
MIG-MAG welding plasma
The observations carried out with a reference wire
(70 S) show that the type of gas has a significant
influence on the plasma column shape.
In the case of pure argon at an average current of
330 A the arc column has a form similar to that
observed in the case of the spray-arc mode of metal
transfer. The arc shape is almost not modified in a
presence of a small admixture of CO2 in the mixture
of shielding gas. Then the arc is very stable, what
ensures great effectiveness and good quality of
welding. The arc consists of a central core
containing vaporized electrode material (Fe, Mn, Cr,
Cu spectral lines were observed), which is well
limited and highly luminous and surrounding plasma
(argon spectral lines were observed) much lower
luminous. These observations are consistent with
one described by Lancaster [3]. The end of the wire
was tapered and completely surrounded by the arc,
in accordance with description of this mode of
molten metal transfer presented in other works [3, 4,
5, 6]. According to the results observed by Rhee [7]
the break-up stream length becomes shorter and the
size of the droplets increases with CO2
concentrations.
When the amount of CO2 in the shielding gas
exceeds 9% with an average current of 330 A, one
can clearly observe detachment of droplets. The arc
loses stability, produces sputter and intensive fume
formation. The arc shape is significantly modified
(longer and more diffusive) when the percentage of
CO2 exceeds 12%. The molten metal transfer
completely changes: large droplets are formed at the
end of the electrode as a result of the transition to the
globular transfer mode.
To establish benchmarks, measurements were
conducted to determine the rate of CO2 causing the
transition at various current intensities with the wireanode 70 S for various compositions of shielding
gas. The transfer mode is studied by varying the
current intensity between 200 and 410 A (See Figure
1). A polynomial approximation of order 2 allows,
by extrapolation, to determine a theoretical current
of 2825 A for the transition to the regime of axial
spray under pure CO2.
explains the bell arc shape associated to globular
transfer or short arc. The thickness of this gangue is
decreasing when current value increases or CO2
content decreases. So, it has not been observed in
spray transfer mode for pure argon as shielding gas.
In those conditions, the plasma column has a welldefined conical shape and the curvature on the
current lines is clearly modified.
The interpretation of the transition from spray mode
to globular regime by the formation an oxide gangue
that prevents the detachment of fine droplets, is an
innovative alternative compared to the commonly
accepted explanation based on the effect of
electromagnetic forces [8]. However, these two
approaches stay compatible, seeing that this bad
conductor gangue can explain the change in the
geometry of the current lines and hence the change
in sign of electromagnetic forces.
5. Effect of the metal
composition at high CO2
Figure 1. Transition between the globular and spray arc transfer
modes expressed from CO2 concentration in gas mixture and arc
current intensity [Experimental conditions: wire 70 S, Ø ≈ 1,2
mm ; Dg = 20l/mn]
core
wire
To confirm all these hypotheses, experiments are
made with various types of modified anode wires.
The performance of a wire is defined in relation to
the position of the transition: the best is when it is
observed for the lowest current and highest CO2
content.
4. Influence of the microstructural
evolution of the anode tip
Microstructural observations of the quenched drops
from MIGMAG welding experiments have shown
the presence of precipitates and of an oxide gangue,
comparable to slag in iron metallurgy that is typical
of globular transfer and short-arc. Their composition
depends of the gas nature, with strong oxygen
enrichment when CO2 is added. The analysis of
elements present in the gangue and in the
precipitates show a high concentration of minority
elements in steel, such as silicon, aluminum or
manganese, as oxide phases.
Figure 2 Transition current intensity as a function of CO2
concentration in shielding gas for various wires
CO2 in shielding gas favors the gangue formation by
chemical oxidation-reduction reactions. This bad
conductor gangue hinders the current transfer and
the arc needs a larger attach zone on anode. That
On Figure 2, data are plotted in a graph similar to
Figure 1 giving the current transition value as a
function of CO2 concentration. The fitting curve of
theses points can be read in both directions: for a
given current, one can see the maximum CO2
concentration allowing getting spray transfer, or for
a given CO2 concentration the current needed to
reach the transition. The area at the left of the curve
corresponds to parameters providing spray transfer,
whereas the area at the right to the globular transfer
mode: the more the position of the curve is shifted to
the right and the more the wire is efficient. Two
criteria must then be studied: firstly the overall
position of the curve and secondly the behavior for
low currents.
In order to validate the extension of results obtained
with solid wires (SW) to the core wires (CW),
experiments have been made with a cored wire of
similar composition to one of the tested solid wire
[9]. Results showed that the three main transfer
modes existed almost the same way as with solid
wires, with comparable transition characteristics.
The silicon dioxide SiO2 can be responsible of the
globular transfer according to the two studied ways:
it has a high viscosity and a low electric
conductivity. The Figure 2 shows the results for two
wires with decreasing concentration in silicon, one
being solid wire (AS26) and the second cored wire
(G002): the studied silicon decrease allows in
average obtaining spray transfer for CO2
concentration higher than for the reference wire,
with a maximum concentration above 35% while the
70S reference wire limit remains below 20% for a
410 A arc current. However, if this result confirms
the hypothesis of the oxide layer influence on arc
stability, it cannot lead to industrial application, as
silicon is needed to guarantee the mechanical
properties of the weld.
The behavior of zirconium adjunction in the wire
G005 is slightly better than similar wire (AS26) for
which silicon concentration is similar. The aim was
to create a conductive oxide gangue that would not
prevent arc attachment and modify current line
geometry.
Finally, the influence of three alkali elements
(lithium, sodium and potassium) on the transition
parameters allowed a large improvement with a
spray transfer occurring above 50% of CO2. The
lowering of the melting temperature of the gangue,
due to the presence of alkali, can explain theses
results: the gangue becomes more fluid at the same
given temperature and then loses its mechanical
properties hindering small droplets detachment.
Conclusion
The interpretation of the transition from spray mode
to globular regime by the formation an oxide gangue
that prevents the detachment of fine droplets, is an
innovative alternative compared to the commonly
accepted explanation based on the effect of
electromagnetic forces.
The influence on transition of various suitable
adjunctions in the wire has been studied to
counteract the negative effects of this gangue:
adjunctions of zirconium in the wire allow creating
a conductive oxide gangue that would not prevent
arc attachment and modify current line geometry.
but the most important effect is due to adjunctions
of alkali elements (Li, Na and K): the lowering of
the melting temperature of the gangue, due to the
presence of alkali, makes the gangue more fluid at
the same given temperature and then it loses its
mechanical properties hindering small droplets
detachment.
These very promising results allow hoping a control
of the metal transfer mode, taking into account the
chemical reactions of oxidation-reduction occurring
between melted metal drops and the shielding gas.
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