Download Overview of modern arc processes and their metal transfer methods

The 6th International Conference – Innovative technologies for joining advanced materials
Overview of modern arc processes and their metal transfer
methods in the case of GMA welding
H. Cramer, D. Böhme, L. Baum, M. Dudziak
German Welding Institute, SLV München, NL der GSI mbH, Germany
E-mail: [email protected]
Abstract
The utilisation of modern, electronically clock-pulse-controlled
welding devices with innovative digital process control and
regulation techniques permits a heat input adapted to the joining
task and optimised metal transfer methods in the GMA arc.
CMT, coldArc, Cold Weld, STT, SpeedPulse, PCS, forceArc,
Rapid Weld, SpeedArc, DeepARC etc. can frequently be read in
the specialist literature as process names from various device
manufacturers and designate modern arc processes and their
metal transfer methods in the case of GMA welding in different
power ranges. Due to the large number and diversity of the
process names, even the well-versed users have difficulty in
finding an overview and, above all, a classification and
assignment of the new arc technologies for the previously wellknown arc types and metal transfer methods since the modified
metal transfer methods have not yet been taken into
consideration in standards and technical bulletins at the
moment.
In conjunction with the lecture, the present manuscript is
intended to give an overview and possible classifications in
an independent and neutral form. It also provides assistance
with regard to the assignment of the new process names to
the previously well-known arc types such as the short,
pulsed and spray arcs.
Rapid Weld, SpeedArc, DeepARC etc. and promise the users
significantly better welding results and savings with regard
to the fabrication by means of welding technology.
Prozess variants and metal transfer methods on
the basis of the short arc
The arc process in which the metal is always transferred in the
short circuit is designated as the GMA short arc, i.e. the arc is
extinguished during the droplet transfer into the weld pool. In
the case of the classical welding power sources (transformer,
rectifier and choke), the dynamics of the welding process are
determined by the adjustment of the electric circuit and by the
design-related structure of the welding device. Due to
changing boundary conditions (e.g. alteration in the arc length
caused by wire feed irregularities), this may lead to
disturbances with spatter. In particular, mains voltage
fluctuations give rise to process irregularities. Nevertheless,
good welding process sequences can be achieved by exerting
an active influence on the choke properties.
Introduction
The general trend towards lightweight construction with
complex thin-walled structures in conjunction with highstrength and ultrahigh-strength materials and heat-sensitive
mixed joints, the pressure resulting from the global
competition to organise the fabrication by means of welding
technology in a more efficient and thus more economically
viable way and the constantly rising quality demands on
welded joints are setting ever more stringent requirements on
the GMA welding process.
Modern electronic welding devices with innovative, digital
control and regulation concepts permit optimised and
modified metal transfer methods in the GMA arc and thus
offer new solution approaches for these challenges.
Numerous manufacturers of GMA welding devices advertise
these modified metal transfer methods on the basis of the
short, pulsed and spray arcs with new process names such as
CMT, coldArc, Cold Weld, STT, SpeedPulse, PCS, forceArc,
tima12
Figure 1. Current/voltage/power course and metal transfer of
a conventional short arc
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The 6th International Conference – Innovative technologies for joining advanced materials
Because of their high switching (clock) frequency (up to
over 100 kHz) and the utilisation of very quick digital signal
processors, modern clock-pulse-controlled welding devices
with computer-controlled power sections permit optimised,
event-oriented current/voltage regulation and thus the
outstanding adaptation of the welding process to the joining
task. Therefore, they can compensate for certain
disturbances. Building upon this, a number of device
manufacturers have redefined arc processes and have given
them catchy process names such as CMT, coldArc,
ColdMIG, Control Weld, STT etc. All these processes have
one thing in common. They make use of the principle of
metal transfer with short circuiting while exploiting
individually adapted, mostly patented regulation strategies
and thus avoid the spattering during the arc reignition to a
great extent.
According to Fig. 2, this new form of the "electronic" GMA
short arc can be divided into two control principles in a
simplified form:
¾ Short arc with a constant wire feed direction and the
utilisation of trigger points in the current/voltage course.
¾ Short arc with wire electrode retraction during the
reignition phase of the short arc with the aid of a quickly
changing wire feed drive on the welding torch for the
additional support of the droplet detachment.
reignition of the arc is detected via another trigger threshold
and the electrical power during the arc reignition is limited
by a very quick drop in the welding current. Fig. 3 shows
schematised current courses of triggered short arc variants
with a constant wire feed direction using a wire electrode
with continuous positive polarity (representative of Variant
B on Fig. 2).
Figure 2. Possible classification of the modern process
variants and metal transfer methods on the basis of the short
arc
The use of additional trigger points exerts a defined
influence on the current/voltage course. For example, the
beginning of the short circuit is detected via a minimal
voltage threshold. The current rise can be adjusted in this
way. If the short circuit bridge is ripped open because of the
increasing current, the quickly rising voltage after the
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Figure 3. Schematised current courses of modern short arc
variants with the utilisation of trigger points in the process
sequence (representative of Variant B on Fig. 2)
The particular advantage of the GMA short arc with trigger
points primarily relates to the fact that very good adaptation
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The 6th International Conference – Innovative technologies for joining advanced materials
of the process to the joining task is achieved in this way. The
energy input and the pressure on the weld pool can be
controlled in a better manner and, if necessary, can also be
reduced in this way. The welding spatter can thus be avoided
in most cases (Fig. 4).
a) Reduced power
cycle (Variants C and E on Fig. 2). During the negative
polarity, the arc literally envelops the end of the wire
electrode, thus causing the formation of larger droplets at the
end of the wire (Fig. 6). The heat input and the penetration
effect on the component are consequently lower in the
negative phase.
b) High power peak
Figure 3. Schematised current courses of modern short arc
variants with the utilisation of trigger points in the process
sequence (representative of Variant B on Fig. 2)
Figure 6. Influence of the wire electrode polarity during
GMA welding with alternating current Tables and Figures
Fig. 7 shows a current/voltage course of a short arc variant
using a wire electrode with intermittent negative polarity.
Due to the infinitely variable negative polarity proportions,
the heat input and the deposition rate can be adjusted to the
joining task in a targeted way.
Stromstärke [A]
Spannung [V]
If additional trigger points are used in order to control a quickly
changing wire feed, this provides direct mechanical support for
the droplet detachment in the short circuit phase (Variant D on
Fig. 2). The electrical energy required for the dissolution of the
short circuit bridge can be reduced further in this way. The
CMT process is the practical implementation of this sequence.
Immediately after the occurrence of the electrical short circuit,
the wire electrode is retracted as far as a stipulated value using a
special wire feed on the welding torch and thus supports the
droplet detachment. After the arc reignition, the wire feed
direction is reversed once again and the wire electrode is fed
towards the weld pool for another short arc cycle and droplet
transfer operation (Fig. 5). In addition, the changing movement
of the wire electrode leads to improved electrical contacting.
The exact and reproducible forward and retraction movements
of the wire electrode guarantee a high process stability and a
constant arc length.
Figure 5. Wire electrode movement and schematised
current/voltage course of the CMT process (figure from
Fronius)
For the GMA welding of thin sheets and/or for the bridging
of wider gaps during the welding, a few manufacturers offer
short arc variants with alternating current proportions (wire
electrode with intermittent negative polarity) in the short arc
tima12
Figure 7. Schematised current/voltage course of a short arc
variant using a wire electrode with intermittent negative
polarity (figure from Cloos)
Finally, Fig. 8 provides an overview and assignment of
modern short arc designations of Variant B on Fig. 2 on the
arc working range diagram.
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The 6th International Conference – Innovative technologies for joining advanced materials
consequently with a higher pressure on the weld pool.
However, the continuously acting arc leads to a somewhat
higher heat input than with the AC short arc variants.
Figure 8: Overview and assignment of modern short arcs of
Variant B (Fig. 2) on the arc working range diagram
Process variants and metal transfer methods on
the basis of the pulsed arc
The pulsed arc of modern electronic welding devices can also
be controlled and regulated very much more precisely today
because of the high switching (clock) frequencies. The
essential pulsed arc parameters such as the level and duration
of the background and pulsed currents, the pulsed current rise
and drop rates as well as the pulse frequency can be adjusted
infinitely. The very quick digital signal processors permit
different regulation strategies and modulation types (I/I and
U/I modulations or their combination) for a high process
stability in the background and pulsed current phases and
ensure low-spatter droplet transfer even in the case of pulsed
arcs with very short settings. In order to raise the deposition
efficiency, it is possible to program and implement current
courses with (for example) pulses which do not detach any
droplets and with subsequent metal transfer using short
circuits or also hold times or delays in the pulsed current drop
rate for reliable droplet detachment in the case of particular
materials and/or for multidroplet metal transfer methods. The
manufacturers advertise these special current programs using a
wire electrode with continuous positive polarity and constant
deposition efficiencies with names such as SpeedPulse, Speed
Weld, Turbo-Pulse, WiseFusion etc. and, with their
directionally stable arcs with very short settings in most cases,
these programs extend the previously classical working range
of the pulsed arc not only towards the short arc but also
towards the spray arc (Variant B on Fig. 9).
Similar to the case of a few short arc variants, alternating
current proportions in the pulsed arc cycle are also utilised for
the pulsed arc welding in the lower power range, thus making
use of the already mentioned advantages of a wire electrode
with intermittent negative polarity for the GMA welding of
thin sheets and for a better gap bridging capacity (AC pulsed
arc welding, Variant C on Fig. 9). Depending on the duration
of the negative polarity, the deposition rate, the penetration
effect and the heat input into the component can be controlled
very well. Compared with the AC short arc variants, the
droplets in the case of the AC pulsed arc are transferred with
an additional accelerating force because of the pinch effect in
the pulsed phase (a minimum current density must be
exceeded for droplet transfer without any short circuits) and
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Figure 9. Possible classification of the modern process
variants and metal transfer methods on the basis of the
pulsed arc
A certain special position is occupied by pulsed arc variants
with periodically changing deposition efficiencies (Fig. 10).
While periodically changing pulsed arc powers, i.e.
alternating pulses with higher and lower energies ("high and
low current phases"), have been successfully utilised for the
MIG welding of aluminium materials with characteristic
rippling on the weld surface for many years already (Variant
D on Fig. 9), it has, in most cases until now, only been
possible to implement periodically changing metal transfer
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The 6th International Conference – Innovative technologies for joining advanced materials
types such as short arc / pulsed arc by means of constant job
changeovers with the aid of a robot controller.
Figure 10. Overview and assignment of modern pulsed arcs
with a periodically changing deposition efficiency on the arc
working range diagram
Modern electronic power sections with digital signal
processors now permit nearly any combination possibilities
and delay-free switching operations between various metal
transfer types. The process names listed in Variant E on Fig. 9
are mostly periodically changing short and pulsed arcs in
already prepared welding programs (characteristics). The
energy-richer pulsed arc ("high current phase") is responsible
for the sufficient heat input and the penetration and the
subsequent short arc ("low current phase") for the intermediate
cooling of the weld pool. These alternating combinations
serve to improve the control of the weld pool especially for
GMA welding out of position (PF, PE and PC). The duration
of the short and pulsed arc cycles and, in the case of a few
manufacturers, even the exact sequence (number) of the
droplet transfer operations with and without short circuits can
be altered variably (Fig. 11). The combined welding process
can thus be optimally adapted to the joining task.
Process variants and metal transfer methods on
the basis of the spray arc
Because of the high attainable deposition efficiencies, the
spray arc is often utilised for the GMA welding of filler and
cover passes in favourable welding positions on higher sheet
thicknesses. The conventional spray arc is characterised by
metal transfer with fine droplets and practically no short
circuits. With regard to the parameter setting of the spray
arc, the specialist literature and the practical welder training
already draw attention to a spray arc with a short setting and
a typically crackling arc noise in order to avoid any
undercuts, any excessive magnetic arc blow especially at the
end of the component and any excessive alloy burn-out. This
arc noise is characteristic of droplet transfer methods with
some short circuits. As a consequence of the high current
density, the droplet necks (short circuit bridges) are already
constricted extremely by the acting pinch forces so that the
duration of the short circuits and the welding current rises
associated with these are relatively short and hardly any
firmly adherent welding spatter arises during the arc
reignition. With the correctly set spray arcs of conventional
rectifiers (step-switched or thyristorised), it has already been
possible to obtain very good welding results in relation to the
deposition efficiency, the penetration and the spatter
avoidance.
Because of the high regulation speed of computer-controlled
power sections, the arc voltage of modern spray arc variants
can be lowered a little further without any significant
increase in the spattering. This results in very short,
directionally stable spray arcs with a high plasma pressure
on the molten pool. The spray arc literally burns in a molten
pool crater so that any welding spatter which may arise is
also collected in the weld pool in most cases (Fig. 12).
a) Shortly before the short
circuit bridging
b) Shortly after the short
circuit dissolution
Figure 12. Typical molten pool crater and metal transfer
methods of spray arcs with very short settings
Figure 11. Schematised current/voltage course of periodically
changing short and pulsed arcs, in each case with ten droplet
transfer operations without a short circuit and four to five
droplet transfer operations with a short circuit
tima12
In the case of longerlasting, more massive short circuiting
operations between the droplet or the droplet chain and the
weld pool, the digital regulators intervene in an eventoriented way after the so-called trigger thresholds have been
reached and limit the electrical energy during the arc
reignition due to a very quick welding current drop (welding
current smoothing). Those manufacturer-related regulation
strategies for gentle, low-spatter short circuit dissolution
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The 6th International Conference – Innovative technologies for joining advanced materials
described in Point 2 are often utilised in the case of the short
spray arc variants as well.
A few manufacturers superimpose a more or less higherfrequency pulsed current on the actual welding current or use
a direct current with a certain harmonic content and thus
promise an additional constriction of the spray arc and the
formation of small-volume droplet necks and droplet chains
for easier short circuit dissolution.
Fig. 13 shows a selection of modern spray arc variants
available on the market at the moment and their assignment
to the well-known arc working ranges.
The modern process variants and arc technologies are useful
tools in order to raise the quality and productivity during
GMA welding. However, they cannot alter the physical
fundamentals and laws of the arc or of the droplet
detachment or replace any external disturbing influences
(e.g. deficient surfaces of the components and of the wire
electrodes, excessive tolerances of the workpieces, of the
weld preparation and during the torch manipulation,
draughts etc.) or the corresponding training and qualification
of the welders and installation operators.
The users continue to be located in the "field of tension"
between the lowest possible heat input (e.g. for material
and/or distortion reasons) and a welded joint nevertheless
without any lack of fusion.
We thank all the device manufacturers for the provided
information.
References
[1]
[2]
[3]
Figure 13. Overview and assignment of selected modern
spray arc variants on the arc working range diagram
If their spray arc variants are utilised, the manufacturers
promise the users a significantly higher productivity,
amongst other reasons due to the reduction in the weld
volume and to the number of required welding passes and
beads as a consequence of very acute weld preparation and
bevel angles
[4]
[5]
[6]
[7]
Summary and prospects
3. Lichtbogenkolloquium des FA3 und des V2 des DVS,
27.03.2007 in der SLV München
Erfahrungsaustausch MSG-Schweißen "Die 'kalten' Verfahren",
25.04.2007 in der SLV München
Baum, L.: Der Schutzgasschweißer, Teil II: MIG-/MAGSchweißen, 4. vollständig überarbeitete Auflage, DVS-Verlag
GmbH, Düsseldorf 2007
Schmidt, Klaus-Peter: Lichtbogenschweißen und -Löten:
Modulationsarten bestimmen den Werkstoffübergang, den
Lichtbogen, die Wärmeeinbringung und das Nahtaussehen, DVSJahrbuch 2006
Platz, J., Wiegand, C.: MIG-MAG Schweißen und -Löten mit
modernster Gerätetechnik, DVS-Berichte Band 267, DVS Media
GmbH, Düsseldorf 2010
Jaeschke, B.: Der wirtschaftliche MSG-Lichtbogenschweißprozess
durch moderne Gerätetechnologien, DVS-Berichte Band 267, DVS
Media GmbH, Düsseldorf 2010
Budig, B.: EWM-forceArc - ein kraftvolles Werkzeug zum MIG/MAG-Schweißen, Erstveröffentlichung DVS-Jahrbuch 2005
From the wide range of design possibilities offered by the
GMA process because of the electronic control and
regulation concepts of modern welding power sources and
devices, only a selection of the modern process variants
available on the market at the moment are introduced here.
However, they already show very clearly with what high
precision an arc with a consumable electrode can also be
implemented. The device manufacturers offer these, in part,
very complex process variants with a large number of
variable parameters for the users in already prepared
programs (characteristics) with relatively simple operability.
The new generation of devices with power electronics
contributes to the general improvement in the process
stability from the arc ignition to the end crater filling.
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