Download MIG welding of aluminium materials made easy

MIG welding of aluminium materials made easy
Norbert Knopp / Heinz Lorenz, Mündersbach and Robert Killing, Solingen, Germany
Introduction
“Higher, further, faster” is today more than just a maxim in top-class sport; there are always new challenges in
modern technology which follow this principle. Whether it’s the height of skyscrapers, the span of bridges or the
speed of trains, materials and welded joints are required to meet increasingly high demands and welding
technology is expected to function without any errors.
For example, to realise the technology required for higher speeds and/or lower power consumption, lightweight
construction techniques have established themselves in vehicle construction (Figure 1). As well as modified
designs, this has required in particular the use of aluminium and aluminium alloys of high-strength steels.
Due to its physical properties, aluminium as a material does many different things during welding and even
experienced steel welders have initial difficulties when required to switch to aluminium. However, thanks to
continued advancements in power sources, welders have a functional tool for welding aluminium materials. This
article covers this topic.
to a spray arc even at relatively low current
intensities. Without MIG pulse welding, out-ofposition welding on thicker aluminium structures
would be therefore not possible at all.
Due to the low solubility of aluminium in hydrogen in
the solid state, complicated cleaning work of the
material surface and some pre-heating is required in
order to keep pore formation within reasonable limits.
This is because aluminium oxide is hygroscopic.
The points mentioned above indicate that for errorfree welds on aluminium, the welder needs to be well
versed in the properties of aluminium, and in terms of
the technology, only the best is good enough for
aluminium welding.
1
Why is welding aluminium different from
welding steel
Table 1 compares the physical properties of iron and
aluminium. It becomes clear that there are significant
differences between the two metals in some
important properties for welding [1].
Specifically, the physical properties of aluminium can
have the following negative effects:
2
The shielding gas is also a critical factor
Previously it was mainly argon that was used as the
shielding gas for MIG welding aluminium. Using this
shielding gas means that a quiet, low-spatter spray
arc can be set and the pulse arc can also be used to
good effect using argon. The low heat retention and
the poor thermal conductivity of argon create a wide
Figure 1
MIG aluminium welding in vehicle construction
Physical property
The density of aluminium is so low that aluminium
oxide is heavier than the metal itself. On the other
hand, the melting point of the aluminium oxide is very
high. Both promote the formation of oxide inclusions
in the weld.
The low melting point of aluminium – it melts even
before it starts to glow significantly – makes it difficult
to weld through the root. The molten pool drops
through more easily.
The high degree of thermal conductivity– more than
three times as high as iron – requires intensive heat
feeding. The undesirable consequences of excessive
heat dissipation may be bonding errors, insufficient
single layer welding and pores. Large material
thicknesses therefore need to be pre-heated to
ensure adequate fusion penetration and sufficient
exhalation of the weld.
Due to the high electrical conductance of aluminium,
the range of the short arc is very small and it changes
© 2002 EWM HIGHTEC WELDING GmbH
Iron
Aluminium
Density of the metal
g/cm3
7.85
2.7
Density of the oxide
3
3.7
3.4
Melting point of the metal
°C
1539
660
Melting point of the oxide
°C
1460
...1580
2050
cm3/100 g
8
0.05
Heat-conductance
W/(cm x K)
0.58
2.2
Electrical conductance
S x m/min2
10
35
Solubility in hydrogen in
neutral state1)
1)
g/cm
immediately on solidification
Table 1
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Unit
Differences in the main physical properties
between iron and aluminium in relation to welding
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seam in the upper part, which, however, leaves the
seam with a narrow, finger-shaped fusion penetration
shape in the lower part.
Helium as a shielding gas does not have these
disadvantages, thus producing wider and deeper
fusion penetration in comparison to argon. Pure
helium is rarely used for MIG welding, but instead
argon/helium mixtures containing between 30 to 70
% of helium (the remainder being argon).
When deeper fusion penetration is not required, e.g.
when welding thinner materials, the greater degree of
heat retention and the improved thermal conductivity
of shielding gases containing helium can be
converted at greater welding speeds. Figure 2
clarifies this point in a schematic diagram. Thanks to
the high-energy arc, the pre-heating can also be
reduced or may not be necessary at all up to certain
thicknesses.
Argon/helium mixtures require different parameter
settings than when welding with pure argon. By
adding helium to the argon, the electrical
conductance of the arc path is reduced. This means
that at the same wire feed rate, the arc becomes
shorter the greater the amount of helium used, if the
Figure 3
2
strength of 300 N/mm . In comparison, Si/Mn-alloyed
wire electrodes for welding steel with a diameter of
2
1.2 mm achieve strengths of over 900 N/mm [2]. The
buckling strength of aluminium wire is also
correspondingly lower. For this reason, only short
tube packages can be used and the internal wire
feed tube should be made from plastic due to the
improved gliding properties. In addition, it is essential
that the feed rollers do not damage the soft surface
of the wire. Greater pressure points between the
rollers must therefore be avoided. Multi-roller drive
units have proven to be the most useful here (Figure
4). Instead of the steel, trapezoidal groove, the drive
rollers should ideally have a rounded groove.
With longer feed paths, intermediate feeds are
required or push/pull torches can be used, where the
wire is not just advanced by the machine, but is also
drawn forwards into the torch (Figure 5). With torches
of this type, even thinner wire electrodes made from
aluminium can also be transported across longer
distances without problems. Small spool torches can
also be used for very thin wire electrodes. An even
wire feed is important because irregularities in the
Shielding gas
Argon
Argon/Helium 50/50
Argon/Helium 50/50
260 A / 27 V
260 A / 32 V
260 A / 32 V
vS 100%
vS 100%
vS 140%
Figure 2 Fusion penetration profile with different shielding
gases (material AlMg3, 1.6mm ∅ wire electrode)
arc voltage is not adjusted. At 50 % He / 50 % Ar, this
equates to a voltage increase of around 4-5 Volts.
The low density of helium also requires greater
shielding gas flow rates to provide sufficient shielding
against the atmosphere.
Shielding gases containing helium are more
expensive than pure argon. However, this is more
than compensated by the higher welding speed
possible and the fact that pre-heating may no longer
be required.
3
Modern machines make welding easier
As mentioned above, only the best equipment is
good enough when welding aluminium because of
the difficult conditions produced by the material. This
applies both to the characteristics of the power
source and in a very specific way to the wire feed
units. Today, digital inverters (Figure 3) are generally
used as power sources for these welding tasks.
There are special demands on all components of the
wire feed unit when welding aluminium because the
wire electrodes used are very soft and therefore bend
very easily when being fed through. A few figures for
comparative purposes: A wire electrode made from
pure aluminium has a level of strength after drawing
2
which is normally only a little over 100 N/mm .
Aluminium alloys can at the very best provide a wire
© 2002 EWM HIGHTEC WELDING GmbH
Inverter power sources for MIG/MAG welding
Figure 4
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View into a wire feed unit with 4-roller drive
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Figure 5
setting process is part of standard modern MIG/MAG
systems.
The ideal working characteristics for frequently used
welding tasks are saved on the machine. All the
operator of the system then needs to do, for
example, is use buttons to set the material being
welded, the required wire diameter and the
connected shielding gas. This calls up the ideal preprogrammed working characteristic. The output can
be infinitely adjusted on a rotary dial and individual
requirements relating to the optimum arc length can
also be set using a correction control. Figure 6 shows
the control panel on a modern welding system
equipped with even more sophisticated settings. In
the centre part, the welding task can be set using jog
buttons. As well as the material, the wire electrode
diameter and the shielding gas, it is still possible to
specify whether solid wire or flux-cored wire is used
for welding, or whether there are special tasks at
hand such as MIG soldering or deposit welding.
The different levels of electrical conductance of pure
aluminium, AlSi alloys and AlMg alloys require
modified welding voltages. For this reason, as can be
seen on the control panel, some characteristics are
pre-programmed for these material groups, whereby
an argon/helium mixture can also be selected in
addition to pure argon.
As the system is a multi-process system, the required
changes to the characteristics are made in the centre
panel, as well those for other processes (TIG, MMA).
In the left-hand part of the display, the output can
then be set on the top rotary dial, the centre rotary
dial can be used to correct the arc length and the
lower dial changes the arc dynamics electronically.
More on this later. The current intensity and voltage
relating to the selected operating point are shown on
the display along with the weldable sheet metal
Push/Pull torch
wire feed process are reproduced as fluctuations in
the welding parameters.
4
Good welding results need the right
welding parameters
In MIG/MAG welding, two adjustments are normally
always required. The wire feed rate and thus the
welding current intensity are set on the wire feed unit,
and the arc length and therefore the welding voltage
are set by choosing a suitable characteristic on the
power source. This manual setting requires
considerable experience from the welder. However,
not every company always has sufficiently well
trained welders available. Modern MIG/MAG systems
provide simplified options for setting the welding
parameters.
5
Synergetic setting of welding parameters
It started as early as the 70s with one-dial operation
where a single rotary potentiometer was used to set
the output by changing the wire feed and the same
adjustment knob in a specific translation ratio was
connected to infinite characteristic adjustment for
modifying the voltage simultaneously. It was also
possible to correct the operating point to a certain
extent using a second knob.
Today a more far-reaching simplification of the
Figure 6
Control panel of the PHOENIX 300 EXPERT PULSE welding machine
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end of the wire after the first contact with the surface
of the workpiece to give the arc space, with the
exception of the first few steps in MIG welding. Also
this is not normally required because the arc burns
freely at the ignition point due to the high current
density at the ignition point itself. With aluminium it’s
rather a different story. Due to the good electrical
conductance, the transition resistance between the
end of the electrode and the surface of the workpiece
is relatively low and the heating there is
correspondingly low. It is therefore advisable to ignite
using a current pulse defined in terms of the height
and the time.
If this occurs at too high a wire feed speed, however,
the retracting wire extinguishes the arc, which is still
very small to begin with. This results in repeated
ignition processes in quick succession before the arc
length is sufficiently ionised and the arc is burning
stably. To avoid this, a lower wire feed speed, known
as wire creep, is used for ignition to achieve a gentle
positioning of the wire onto the workpiece.
In modern digital power sources these ignition
processes are already programmed for the individual
wire and gas combinations.
Once the first arc is burning, the fusion penetration
continues to be very low on the still cold base
material and this can result in fusion penetration
errors at this stage. For this reason, increased
welding energy is used for a short time to start the
welding process. How this type of program works is
shown in Figure 8. First of all, the shielding gas
begins to flow. This starts the wire electrode moving
at the programmed “creep speed” and ignites the arc
when it comes into contact with the workpiece using
the programmed ignition current. For an adjustable
time, the welding process is started at increased
welding power in order to avoid cool points (Pstart).
Only then does the machine switch over to the actual
working program (PA).
thickness. The welding data used can be saved and
retrieved at a later time.
6
It’s much easier with a pulse arc
In MIG pulse arc welding, the current and voltage
pulse between a low base level and a short-term
higher pulse-shaped level. With the values for the
pulse current height and the duration of the pulse set
correctly, the drip transition takes place in the same
rhythm as the pulse frequency (Figure 7). This
ensures virtually spatter-free welding.
The duration of the pulse arc covers the entire range
from the lowest to the highest current intensities. In
this process, the pulse frequency increases with the
output from around 15 Hz in the lower range to
around 300 Hz at high current intensities.
For MIG welding aluminium the pulse arc is
particularly important because as mentioned above,
the duration of the short arc is very small, meaning
that out-of-position welding on slightly thicker
materials is only possible using a pulse arc. The heat
feeding can also be more easily regulated using a
pulse arc.
Unlike normal MIG welding, with MIG pulse welding a
greater number of parameters needs to be set,
namely the level of the base current (voltage), pulse
current (voltage), base time and pulse time, in
addition to the wire feed. Modern digital arc models
use this to calculate other parameters, such as the
pulse frequency. As these values are also specific to
the material, the option of synergetic settings (Figure
6) offers an additional advantage here. These
characteristics are also saved in the machine for
MIG/MAG pulse arc welding.
7
Special programs are the icing on the cake
In the software on modern MIG/MAG systems, not
only sets of parameters but also entire welding
programs can be saved and retrieved later on.
Some of these programs are explained in more detail
below. They normally operate according to a working
characteristic saved on the machine, which is
selected on the basis of the material, wire electrode
diameter and shielding gas.
8
Safe ignition is important
With MIG/MAG welding it is not usual to retract the
Figure 7
Drip detachment in MIG/MAG pulse welding
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Figure 8
the power source is not switched off, however, the
arc continues to burn for a time on the wire (wire
burn-back) before the power source is switched off as
well. This prevents the end of the wire dipping into
the end-crater and freezing solid inside it. The free
burning phase must not be too long, however,
because otherwise the arc melts onto the contact
nozzle.
Another disadvantage of an excessively long freeburn time is that a thick drip forms on the end of the
wire. The manual welder cuts this using a side cutter
before the next ignition process. With mechanical
welding this is not generally possible, however. Here
the thickened end of the wire can result in ignition
faults. Modern systems therefore also provide the
option of throwing off the drip formed on the end of
the wire with a final pulse. The result is a properly
spiked end of the wire that can be easily ignited next
time. If the arc needs to be fully extinguished, a short
post-flow phase is used for the shielding gas. Only
then is the end program complete.
Ignition program
9
The working program (PA)
The appropriate current intensity and voltage for the
actual welding process are programmed in the
working program. If required, these values in the
working program can also be changed, e.g. when the
arc approaches corners or edges, the current and
voltage can be reduced to avoid overheating.
11
Special functions
Modern MIG/MAG systems provide even more
options that can be used to control the welding
process.
12
Setting the arc pressure
As mentioned above, the operating panel of the
welding system shown in Figure 6 provides the option
of adjusting the dynamics. This means that the arc
can be adjusted to be harder or softer. This occurs
via an adjustable current dynamic, similar to a choke
effect. In the short arc range, which is of little
significance for welding aluminium, this can be used
to accelerate the drip separation process by reducing
the choke effect. This makes the spatter formation
slightly greater, but the arc slightly more directionally
stable.
Of more interest is the adjustable dynamic in MIG
pulse welding. Here the reduction in the choke effect
means that the pulse current increases. This also
increases the effective current intensity resulting from
the base current and pulse current. One
consequence of this is that the concentric magnetic
field surrounding the arc is amplified. This constricts
the arc and increases the pinch effect. The harder
arc becomes less easily diverted by magnetic fields
and generates narrower, deeper fusion penetration.
The dynamics must be set according to the relevant
conditions, i.e. when welding thinner metal sheets a
softer arc is preferable, and with thicker sheets, a
harder arc.
10
Avoiding welding seam irregularities using
an end program
It is desirable to keep the craters that form at the end
of the seam as small as possible in order to avoid
end-crater shrinkage cavities and cracks. The manual
welder achieves this in conventional systems by
making circular movements across the end point of
the seam with the arc extended, before the arc
extinguishes. With modern MIG/MAG systems an
end program (PEND) can be entered (Figure 9). The
working current intensity is first run down for a
specific period (slope-down), held at a lower level for
a certain time and then the wire feed is turned off. As
t
PEND
I
t
13
Super pulse function
In addition to pulses for targeted drip separation in
the range between around 15 and 300 Hertz
depending on the set current intensity, modern
MIG/MAG welding systems also provide the option of
pulsing at just a few Hertz (Figure 10). Here the
welding power with adjustable frequency and pulseduty factor is switched backwards and forwards
t
Figure 9
End program
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between a high and a low level. This pulsing at low
frequency has no effect on the drip separation
process. However, this does produce reduced heat
feeding. This can be used when welding root passes,
when welding in the V-up position and when welding
thin sheet metal. Sometimes this pulsing is also used
when welding final passes. This produces rough, but
very even seam ripples, as is familiar from TIG pulse
welding (Figure 11). This type of seam appearance is
sometimes desirable for visual reasons.
Figure 11 Appearance of a MIG seam welded using a low
pulse frequency
14
Online or offline programming
The individual stages in the programs described can
be switched to non-latched or latched mode in
manual welding using torch switches, or in
mechanised welding in a time-controlled way using
the program saved for the process, for example.
The programs can be programmed on the welding
machine. As programming time is unproductive time,
however – no welding can be carried out during
programming – the programming work can also be
carried out using special software on a computer or
laptop.
15
Conclusion
Modern, digital, inverter-based MIG/MAG systems
(Figure 12) not only provide very good welding
properties, but also excellent ease-of-use during the
welding process. The user can access the expertise
of the machine manufacturer who will have saved not
only the appropriate characteristics (JOBs) for
different welding tasks in the system, but who also
allows entire welding programs to be stored in the
power sources matched to the welding task. This is
Figure 12 MIG welding aluminium using push/pull torches
16
Literature
[1] Killing, R.: Was ist beim Schmelzschweißen von
Aluminium anders als beim Schweißen von Stahl
(The differences between the fusion welding of
aluminium and welding steel). Der Praktiker,
H.11/1995, pages 562-567, DVS-Verlag Düsseldorf
[2] Killing, R.: Das Beste ist gerade gut genug dafür
(Only the best is good enough), Metallbau, H.2/1996,
pages 73-75, Caldewey-Verlag, München
First publication magazine:
„Metallbau 9/2002“
Figure 10 Current output wave with superpulses
especially important when welding aluminium that
due to some of its physical properties places higher
demands on the exact settings of the welding
systems. In addition to the use of the JOBs entered
by the machine manufacturer, the user can also
create and archive various program sequences and
call them up again later as required.
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