Current Welding Practice Leseprobe

Fronius International GmbH
Current Welding Practice:
CMT Technology

_____________________________________________________ PRACTICAL SOLUTIONS
March 2013
Fronius International GmbH
Current Welding Practice:
CMT Technology
Cold Metal Transfer - a new gas metal arc welding process

Bibliographic information published by the Deutsche Nationalbibliothek
The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliographie;
detailed bibliographic data are available in the Internet at
http://dnb.ddb.de.
English Edition
Volume 11
ISBN 978-3-945023-36-5
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© DVS Media GmbH, Düsseldorf · 1 st Edition 2014
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List of authors
List of authors
Authors:
Jürgen Bruckner
Stephan Egerland
Karin Himmelbauer
Andreas Millinger
Manfred Schörghuber
Dominik Söllinger
Andreas Waldhör
Contributors:
Josef Artelsmair
Tamara Dekum
Ekkehard Kepplinger
Angelika Kogler
Michael Mittermaier
Thomas Rauch
Horst Scheiböck
Herbert Staufer
Page 3

Summary
Summary
This technical reference book concerns a new gas metal arc welding (GMAW) process
with a reversing wire movement, which represents an extension of short-arc welding
technology. Due to the reduced heat input, this process is called "Cold Metal
Transfer" (CMT).
In addition to the process description, the different process variants and possible
settings are presented. The new areas of application that this opens up and
limitations of the process are discussed in relation to the welding of joints, brazing
and surface welding on different materials and compared with conventional welding
processes.
Page 5

Foreword
Foreword
In the history of gas metal arc welding (GMAW), there have been few major process
developments. They include, for example, the controlled pulsed arc technique or the
programmable characteristic curve and dynamics. The introduction of the CMT
process in 2004 was one such milestone, because this technology opened up
applications previously considered impossible. CMT was a success from the start with
both automated and manual welding systems due to the self-regulating process
characteristics.
Behind the name "Cold Metal Transfer" is a group of people who had the vision and
faith to make the impossible happen. Foremost among them was Klaus Fronius,
without whose determination to co-write the textbooks of the welding industry
this project would not have been possible. Besides inventive thinking, developing
the technologies of the future also calls for enormous perseverance. The dedicated
CMT development team had these characteristics in abundance, so that they were
eventually able to bring this product development to a successful conclusion.
Today, the CMT process is used in a wide range of industries with virtually every
material one can think of.
Fronius International GmbH takes this opportunity to thank all those who made the
development of this great technology possible, as well as our customers who have
placed their confidence in us over many years and provided us with practical
examples for the textbook. Our thanks are also due to the authors, who pooled their
knowledge of welding practice and product development in order to write this
textbook.
As well as considering the theoretical principles, this book also clearly describes the
various possibilities offered by the CMT process.
We hope you enjoy reading this book and that it provides you with new insights.
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Cold Metal Transfer - Classification of the welding process | Chapter 1
1 Cold Metal Transfer – Classification of the
welding process
CMT is short for "Cold Metal Transfer" and describes a welding process with reduced
heat input. The CMT process involves a completely new method of droplet
detachment, combined with a reversing filler wire movement.
With the conventional short arc process, the wire is fed continuously to the
workpiece. When a short-circuit occurs, the current is increased in order to break the
short-circuit and reignite the arc. With the CMT process, on the other hand, droplet
detachment and reignition are achieved in a controlled fashion by a backward
movement of the filler wire.
Vd[m/min]
Short-circuit phase
CMT cycle time
Arc phase
Fig. 1: CMT process flow with wire feed, current and voltage
Figure 1 shows the process states and progression of wire feed rate, welding current
and voltage in the CMT process. The wire is fed to the workpiece until a short-circuit
occurs and the arc is extinguished. The direction of wire movement is then reversed,
i.e. the wire is withdrawn from the workpiece. This breaks the short-circuit and the
arc is reignited. The wire movement is then reversed again and the described process
starts again from the beginning. Depending on the characteristic curve for the filler
metal, shielding gas and electrode diameter, this reversing movement takes place in
a frequency range from 50 to 130 Hz.
CMT: Reignition with wire
backward movement
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Chapter 1 | Cold Metal Transfer - Classification of the welding process
Short-circuit phase
Arc phase
Fig. 2: Short arc process flow with wire feed, current and voltage
Short arc: Reignition
with welding current
Figure 2 shows the short-circuit and arc phase for the short arc with constant wire
feed. Droplet detachment occurs due to the surface tension of the liquid weld pool.
As soon as a short-circuit occurs, the current increases owing to the physical
properties. In the simplest case, this is limited or regulated according to a power
curve characteristic. Local overheating due to the high short-circuit current
promotes the constriction of the weld pool bridge. When this liquid metal bridge
melts through, the arc ignites at relatively high short-circuit current and high arc
force. This can lead to spatter and instability.
The current rise and decay functions can be optimised with digitally controlled
machines. The principle of reignition at a relatively high power level is however
necessary for physical reasons and at the same time represents a significant
difference compared with the CMT process, which mechanically forces the
interruption of the short-circuit bridge and reignition through the backward
movement.
1.1 Limitations compared with conventional welding processes
Three essential features distinguish the CMT process from conventional gas metal arc
welding (GMAW) processes:
Reversing
filler wire
Spatter-free
reignition
The movement of the filler wire is directly integrated into the welding process
control, with movement not only towards the workpiece, but alternately back and
forth. With conventional GMAW processes, the wire feed rate is either constant or it is
subject to a controlled, predetermined time scheme.
Droplet transfer to the weld pool and reignition are virtually currentless, because
droplet detachment takes place during the backward movement from
the short-circuit of the filler wire. With a conventional short arc, the droplet can only
be detached from the wire end with higher currents in the short-circuit, which
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Cold Metal Transfer - Classification of the welding process | Chapter 1
– particularly on reignition – requires a significantly higher welding current and
results in more spatter.
Due to the recurring short-circuit and the defined wire backward movement, the arc
length is mechanically kept constant, thereby maintaining accurate and stable arc
length control. With conventional processes, on the other hand, the arc length is
determined via a voltage measurement and is subject to fluctuations caused by the
welding speed, workpiece surface and resistance changes.
Mechanical arc
length adjustment
1.2 Power range of the CMT arc
At higher welding currents, a short-circuit of the filler wire no longer occurs. At the
same time this represents the upper limit of the pure CMT process. The lower limit is
below the conventional short arc process, i.e. the heat input is significantly lower here.
Figure 3 provides an overview of the different welding processes and their power ranges.
Further developments based on a combination with the pulsed arc in the case of
CMT Pulse (see Chap. 3.2.) or conversely with a polarity change in the case of CMT
Advanced (see Chap. 3.3.) result in a power range from minimal heat input to well
above the transition arc.
Fig. 3: Comparison of different GMAW processes in a current-voltage chart
The influence of the welding process on wetting and on heat input when welding at
the same filler wire feed rate can be represented very clearly.
CMT with lower
heat input
For the following comparison, in each case a weld bead was carried out without
process corrections and without support/backing strip with the same parameters
(welding speed, shielding gas, length of free wire end and torch inclination) onto two
different sheets of 1.5 mm and 2 mm thickness.
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Chapter 1 | Cold Metal Transfer - Classification of the welding process
Base metal:
S235; sheet upper row = 1.5 mm, sheet lower row = 2.0 mm
Filler metal:
G3Si 1; ø = 1.0 mm
Shielding gas:
M21 = 18% CO 2 , remainder argon
Torch position:
α = 0°; ß = 0°; Contact Tip to Workpiece Distance (CTWD) = 12 mm
Welding speed:
v s = 100 cm/min
Wire feed rate:
v d = 6.5 m/min
CMT MAG-k CMT Pulse MAG-p
E = 1.4 kJ/cm
I s = 150 A, U s = 15.5 V
E = 2.1 kJ/cm
I s = 170 A, U s = 20.5 V
E = 1.8 kJ/cm
I s = 130 A, U s = 23.0 V
E = 2.0 kJ/cm
I s = 130 A, U s = 25.5 V
Fig. 4: Comparison of heat input of different welding processes at the same wire feed rate on two different sheet thicknesses:
Sheet = 1.5 mm (upper row), sheet = 2 mm (lower row)
Figure 4 shows that a difference in sheet thickness of only 0.5 mm has a huge
influence on weld sagging. With the chosen deposition rate, the difference in heat
input is thus particularly impressive on the 1.5 mm thick sheet.
The different form of the penetration and wetting can be seen from the second series
of images on the 2 mm thick sheet. With the same wire feed rate, in this power range
the short arc (MAG-k) and the pulsed arc (MAG-p) have roughly the same energy per
unit length of weld (E) and the pulsed arc achieves full penetration just before the
weld begins to fall through. Combining the CMT process with the pulsed arc (CMT
Pulse) makes it possible to increase the energy per unit length of weld and allows
a penetration depth close to that obtained with the pulsed arc.
1.3 CMT process corrections
Generating and processing CMT characteristics are complex tasks requiring special
measuring apparatus (oscilloscope, high-speed camera, etc.) and extensive
background knowledge. However, correction parameters are available that allow the
user to optimise the process.
Arc length correction
This correction adjusts the spatial extent of the arc plasma column. A short arc has
favourable effects on welding speed and avoidance of undercutting, while a long arc
has positive characteristics in terms of broad wetting and edge formation.
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Cold Metal Transfer - Classification of the welding process | Chapter 1
With conventional processes, the arc length is corrected by increasing or reducing
the welding voltage or the wire feed rate. With the CMT welding process, on the
other hand, the relationships are more complex.
With CMT, the arc length is reduced by means of a shorter wire return travel time;
process frequency increases. Conversely, with positive arc length correction, the
return travel time is increased, the arc becomes longer and process frequency
decreases.
Varying the
arc time
Arc length correction
Shortening the arc phase
Increases droplet frequency
Lengthening the arc phase
Reduces droplet frequency
Dynamic correction
This correction adjusts the duration and properties of the short-circuit break. A high
dynamic correction is characterised by soft reignition, while conversely a low
dynamic correction results in hard reignition with a high arc force.
With conventional welding processes, dynamic correction is used to adjust the rate
of current increase. With the CMT welding process, this behaviour is reproduced with
the short-circuit current before the short-circuit break.
Reducing the dynamic correction produces a higher reignition current and hence
a higher arc force. Increasing the dynamic correction reduces the short-circuit current
on reignition.
Varying short-circuit
current on reignition
High arc force on reignition
Low arc force on reignition
1.4 Components of CMT welding
A CMT system differs from a conventional welding system in that it involves
a special push-pull welding torch, a wire buffer and a modified process control
system. The block diagram in Figure 5 illustrates the welding wire control system
and signal profiles.
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Chapter 1 | Cold Metal Transfer - Classification of the welding process
Dynamic correction
Main feed Wire buffer Push-pull feed
Sensor
Memory
high-speed bus
Digital process
control
Power
supply unit
Control panel
Robotic
control
Welding
circuit
Power source
Fig. 5: Block diagram of a CMT system
Fast data
communication
Highly dynamic
push-pull drive
Wire buffer
The core of this arrangement is the power source (1) of the digitally controlled
welding system. This allows the dynamic wire movement to be integrated into the
welding process control and different movement profiles to be specified, depending
on the filler metal and shielding gas combination. Selection of the characteristic
curve and interventions in the welding process are carried out by the user via a
control panel (6) or a robotic control system.
The CMT-specific high-frequency wire movement is made possible by a special
push-pull welding torch (4) with integrated AC servo motor. The motor is gearless
and wear-free and is located immediately before the tube bend (5) close to the
welding process.
The wire buffer (3) decouples the two drives, thus ensuring a trouble-free wire feed.
The highly dynamic push-pull motor (4) can draw the filler wire from the wire buffer
store or replenish it by a backward movement, depending on the process phase.
The main feed motor (2) delivers wire continuously from the wire reel or wire drum
and fills the wire buffer.
Robotics-capable CMT welding system
The equipment for automated CMT welding consists of a mechanised torch and
a robotics-capable hose package. A viewing window in the wire buffer lets you see
how much wire is left and the external wire feed hose provides a quick-change
connection. The control panel with a long data cable allows process intervention
even at some distance from the power source.
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Cold Metal Transfer - Classification of the welding process | Chapter 1
Manual CMT system
With the manual CMT system, the AC servo motor is built into a push-pull welding
torch with pistol grip. In terms of size and weight, the pistol is similar to a
conventional arc welding gun with DC motor. The wire buffer is integrated into the
hose package and protected with a leather sleeve.
Fig. 6: Automated CMT system (left), manual CMT system (right)
CMT for tandem welding
For the CMT Twin welding process, two complete CMT welding systems are needed.
In a common high-performance welding torch, the two filler wires are brought
together in one gas nozzle, but the two circuits remain isolated. The power sources
are connected with a high-speed bus and processes such as welding start-up take
place together and in a synchronised fashion.
Fig. 7: CMT Twin system set-up
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Advantages of the CMT welding process | Chapter 2
2 Advantages of the CMT welding process
2.1 Start and end of welding
Start of welding
Reproducible, spatter-free ignition of the arc at the start of the welding process is not
always easy to achieve. Figure 8 shows a conventional ignition process. Ideally the
lower end of the wire should melt immediately after contact, whereupon an arc is
ignited. This touch start requires high rates of current increase and welding currents
which generally result in spatter in view of the high arc forces.
Fig. 8: Conventional ignition at the contact point
If, when the filler wire makes contact, the contact resistance is small and the rate of
current increase is too low, ignition cannot take place as shown in Figure 8.
The ignition sequence shown in Figure 9 occurs. The welding wire is heated most
strongly by the ignition approximately midway between the contact tube and the
workpiece. The further feed motion causes the wire to bend and subsequently melt
through at the midway point. The lower part of the wire is thrown aside by the
pressure of the igniting arc.
Fig. 9: Poor ignition throwing out a large amount of spatter at the start of welding
If the wire feed movement can be synchronised with the arc ignition, when the filler
wire touches the workpiece (= short-circuit), the feed can be stopped and then
moved backwards.
At a reduced current, an arc is ignited during the reverse movement, which preheats
the workpiece and melts the filler wire. After a defined arcing time, the feed is again
reversed and the CMT welding process starts. Since a high short-circuit current is no
longer needed for ignition, welding starts virtually free of spatter. This type of
ignition is referred to as "Spatter Free Ignition" (SFI).
SFI: Swing ignition
through backward
movement of the
filler wire
Fig. 10: Spatter-Free Ignition (SFI) with CMT cycles
The key advantage of CMT ignition (i.e. a combination of SFI with CMT cycles) is that
welding can be started much more quickly.
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Chapter 2 | Advantages of the CMT welding process
Hammering slag
from the wire end
Thanks to the dynamic drive technology of the CMT welding process,
non-conductive oxides on the tip of the filler wire or slag islands in multi-pass welds
can be removed by an implemented algorithm. It is very important to get rid of these
oxides, since ignition cannot take place otherwise.
The implemented algorithm controls the filler wire so that it oscillates as it
approaches the workpiece. Upon electrical contact, SFI ignition takes place
immediately. With insulating contact, a mechanical load is applied to the slag surface
due to further oscillation or "hammering", until it is released from the filler wire tip.
This function is referred to as "slag hammer".
Fig. 11: Slag hammer with detaching slag
End of welding
No dome formation due to
currentless wire withdrawal
To guarantee a reproducible welding start, the shape and surface of the end of the
filler wire after welding has ended are crucially important. Low-melting slag may
collect at the filler wire tip which, as non-conductive deposits, can hinder the next
welding start.
The amount of slag is related to the molten volume of the wire tip after the end of
the welding process. In a conventional system (without wire reversal), the length
of the free wire end is adjusted with the semifusion of a last drop. This means that,
at the end of the process, an arc attaches to the wire end and a ball (dome) forms.
In the case of CMT, at the end of the welding process the filler wire is drawn
currentlessly out of the weld pool. Therefore semifusion cannot occur and dome
formation is avoided. To prevent the wire from getting welded into the solidifying
weld pool, this process must be carried out very quickly - this again is only possible
with dynamic wire feed systems.
2.2 Process stability
Continuity of droplet formation and droplet detachment is the most important
prerequisite for a stable welding process with constant arc length. This can be
influenced by the following negative factors:
Fluctuations in the actual voltage due to oxidation and/or impurities
In a conventional metal inert gas (MIG) process, the arc length is controlled
exclusively via the arc voltage. Any oxidation and/or impurities of the parts to be
joined affect the measurements and thus have a negative impact on voltage
regulation.
CMT without
voltage-dependent
arc length control
The CMT process on the other hand eliminates these negative factors altogether.
For the first time the short-circuit is used as the most appropriate time for length
adjustment for industrial applications. The arc length is measured and controlled
"mechanically", i.e. independently of the arc voltage. For this purpose the wire is
withdrawn after the short-circuit for a defined period and at a defined speed.
This gives the path length, which defines the arc length. External influences therefore
no longer interfere with the control process.
Page 18