Download Low Energy Arc Joining Process for Materials Sensitive to Heat

Low Energy Arc Joining Process for
Materials Sensitive to Heat
S. F. Goecke, EWM Mündersbach, Germany
Modern, ultra-lightweight places demands on welding technology that simply cannot be met with traditional
shielding gas welding processes. Variants of the robust arc welding process need to be developed which feed very
little heat into the material but which still guarantee strong connections. The coldArc is a variant of the MIG/MAG
process that meets these demands. In this process, all interventions in the process flow are carried out directly in
the power source without mechanical intervention in the wire feed, which means that standard welding torches can
be used and the process can also be used to produce excellent manual welding results.
1
After a specific arc burning duration, a drop forms on
the tip of the electrode which, as the arc is relatively
short, quickly comes into contact with the molten pool,
and the arc goes out. The surface tension of the pool
draws the drop away from the electrode tip – in the
final phase of the separation process if the bridge has
already been constricted, the pinch effect also
contributes to this via the Lorentz Force as well as the
Joule heating effect from the rapidly increasing current
density – and after the liquid bridge between the
electrode and the workpiece breaks, the arc re-ignites.
What happens in terms of the electrical forces is also
shown in Figure 1. At the start of the short circuit, the
voltage falls because the electrical resistance of the
liquid bridge is now lower than the previous resistance
level in the arc. At the same time, the current starts to
increase to the value of the short circuit current. As
soon as the bridge between the electrode and the
workpiece breaks, the voltage then increases very
quickly as there is an increase in voltage required to
ignite the arc. The voltage fall starting at that point is
very slow, however, due to the inductivities in the
welding current circuit, the re-ignition process takes
place under a relatively high electrical output. In this
process, part of the liquid bridge can evaporate in an
explosive way if not counteracted by sufficient choke
effect in the current circuit in advance of the increase
in current. The consequence is either significant
spatter formation or a very low process dynamic
through to instability.
For welding tasks requiring low heat effects, e.g. when
welding very thin metal sheets with poor fit up, it is
much more damaging because the weld metal drops
through at the re-ignition point, creating a hole. When
welding metal sheets with surface finishes, e.g. zincplated sheets, there is also a risk of the surface
coating evaporating and burning away next to the joint
and on the reverse side. With higher strength steels,
softening can occur if the heat feeding is too great.
The normal short arc, otherwise an excellent tool for
welding thin sheets, is therefore not suitable for these
types of welding tasks which are extremely sensitive
to heat.
Introduction
In addition to the concepts of "higher, further, faster"
which have represented the challenges to the modern
world of technology for many years, recently a
demand for "easier" has also come to the fore. This
applies primarily in vehicle construction where fuel
can be saved during acceleration, driving and when
braking by reducing weight, which in turn preserves
resources, reduces costs and protects the
environment.
This more recent demand has produced increasingly
lightweight models which are only made possible by
the use of thinner high-strength steel metal sheets,
normally plated, and lighter materials such as
aluminium and magnesium. However, this type of
lightweight design places demands on welding
technology which simply cannot be met using
standard welding machines. This meant it was
necessary to develop new processes that expose the
joint to an extremely low level of heat during the
welding process.
The coldArc is just such a process.
2
The short arc; the conventional method for
low-energy welding
The short arc is used in MIG/MAG welding in the
lower power range, i.e. at lower currents and lower
voltages. In this process a form of material transfer is
used which features cyclical, repeating arc phases
and short circuit phases, Figure 1.
Phase 1
Arc burning
Phase 2
Short-circuit
Phase 3
Short-circuit resolution
and renewed burning phase
Us
t
Is
t
Figure 1 Material transfer (schematic), current and
voltage outlines in short arc welding.
© 2005 EWM HIGHTEC WELDING GmbH
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3
Phase 1
Arc burning
Approaches to improving the short arc
process
There have been no shortage of attempts to improve
the behaviour of the short arc, especially on re-ignition
after the short circuit, and to use a short arc with
reduced heat feeding. As early as the 1980s, attempts
were made to reduce the current immediately before
the short circuit bridge breaks and then to provide a
high voltage pulse to ease the re-ignition process.
This did reduce the spatter formation, but the heat
feeding was only slightly reduced, [1] and [2]. Further
steps down this path were the modified short arc
ChopArc, [3] and [4], which as a process-safe MAG
welding process achieved considerable progress,
especially in the minimum thickness sheet range 0.8 0.2 mm. In addition, an adaptive control system was
developed here which optimised the process quality in
real-time, [5].
More recent developments have worked with a
discontinuous wire feed, i.e. the duration of the short
circuit is reduced so that the wire electrode is
retracted slightly during the short circuit so that the
short circuit bridge breaks more easily. This means
that a lower energy welding process with low spatter
formation can be achieved, [6]. Because a push-pull
drive with two wire feed motors with high dynamics is
required, this process is more suitable for automated
welding and is only used in combination with welding
robots.
4
Phase 3
Short-circuit resolution
and renewed burning phase
Us
t
Is
t
Figure 2 Material transfer (schematic), current and
voltage outline in the coldArc process.
It is used as a guideline value when controlling the
current. However, the continuous measurement of the
voltage with the corresponding reaction to all changes
in voltage is required to achieve this (highly dynamic
instantaneous value regulation). A digital signal
processor (DSP) can then be used to extract the
power from the arc immediately before re-ignition in a
period of less than 1 µs, Figure 2, so that the reignition takes place very gently.
So that a sufficient quantity of molten material can be
formed immediately on the electrode tip however,
there is an increase in the amount of energy required.
Immediately after the arc re-ignites, the current is
therefore raised back up again for a defined short
period to what is known as the melt pulse. Only then
is the current lowered to an extremely low basic level
to minimise further melting, and the next cycle begins.
This melt pulse after each short circuit generates a
melting cone of a constant size on the electrode which
means that process continues very smoothly and
evenly. This is the only way it has been possible to
work at extremely low currents in the phases between
the short circuits, without the wire melting further or
the arc going out. All this goes to make up the very
low-energy coldArc process.
Figure 3 shows a sequence of images from a highspeed film, which highlight the very even material
transfer and the gentle ignition of the arc.
coldArc – successful joining in tasks
demanding low heat
Development work with the aim of achieving a lowenergy process without mechanical intervention in the
wire feed process, resulted in a process variant in
which all necessary interventions in the process take
place in the power source alone. This variant of the
MIG/MAG process, known as coldArc, is also a short
arc process, and is called such due to the cyclical
change between the arc and short circuit phases. As
the electrical output during the re-ignition process is a
critical criterion for successfully welding thin sheets,
active intervention is carried out in the outline of the
power intake for the overall process, however, in other
words during the arc phase, in the short circuit phase
and especially when re-igniting the arc, Figure 2, the
voltage outline remains the same as in the normal
short arc process.
© 2005 EWM HIGHTEC WELDING GmbH
Phase 2
Short-circuit
5.
What the coldArc process can do
The outline of the arc output on arc re-ignition is
shown in Figure 4. The advantages of the coldArc
process in comparison to the standard short arc at the
moment of re-ignition and immediately afterwards
become very clear. Here the output at the moment of
arc re-ignition is considerably lower not just as an
absolute value.
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Figure 3
Sequence of the material transfer in the coldArc process taken from high-speed pictures, 8,000 B/s.
The copper-based wires have a melting point of
around 1000 °C. In comparison to the same type of
MAG welding, the heat loading of the surface layers is
already therefore considerably reduced. These are
even more protected if MIG brazing is carried out
using zinc-based solder with melting intervals of
around just 450 °C. The use of these wires is only
possible however, if the short circuit current is strictly
limited and the general heating feeding is also
reduced considerably. The vaporisation temperature
of the zinc/aluminium alloys used for arc brazing is
just under 900 °C, in other words still below the
melting temperature of copper alloys.
In fact, immediately after the arc ignites, the output is
reduced in an exceptionally dynamic and controlled
way, and then, after the arc has been stabilised,
increased to the defined melting of the electrode tip in
a pulsed way.
A process of this type can be used for many welding
tasks, especially in vehicle construction where the
normal short arc is no longer suitable.
Even just a few years ago, it was assumed that the
MIG/MAG process should be used for steel over a
panel thickness of 0.7 mm and for aluminium over
3 mm [7]. The panel thicknesses in vehicle
construction today are becoming increasingly thin,
however. They already go down to as low as 0.3 mm,
and 0.2 mm is already being tested for composite
construction work. The difficulties in achieving an
even groove are even greater if there are larger air
gaps to be bridged. This is a typical task for the
coldArc process.
Standard short arc
PS
coldArc arc
Figure 5 Manual coldArc brazed joint of 0.8 mm
electrolytically plated steel plate with 4.0 mm
air gap, 1.0 mm CuSi3 wire.
Output on
re-ignition
Figure 4 Minimised arc output of the coldArc process
on re-ignition.
For some time now, different welding techniques have
been used on surface-coated metal sheets, in other
words, using copper-based filler material for arc
brazing. This helps to preserve the zinc layer, but
difficulties can arise if there is a larger air gap. With
the coldArc process, on the other hand, even larger
air gaps can be bridged with the filler material.
Figure 5 shows 0.8 mm thick zinc-plated steel metal
sheets which have been brazed manually with air
gaps of as large as 4 mm in the vertical down position
using 1.0 mm CuSi3 wire with a moderate current of
50 A and a voltage of 13.5 V coldArc.
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It is not possible to fusion-weld these two materials
directly because inter-metallic Al/Fe phases form
which are exceptionally brittle, Figure 8.
Figure 6 Electrolytically galvanised steel metal
sheets, fillet joint on the lap joint with zinc
wire brazed using the coldArc process.
On re-ignition, the short circuit bridge would therefore
vaporise immediately in an explosive way and blow
away the lightweight welded material if the short
circuit current is not reduced considerably.
With the coldArc process, MIG brazing with zincbased wires is possible for the first time without
restrictions.
Figure 6 shows the surface and the reverse side of a
lap joint on 0.75 mm thick galvanised metal sheets
which have been joined using this low-melting wire.
The zinc layer is completely preserved, both
immediately next to the groove and on the reverse
side. In the brazing process it would have become
completely liquid, but it would not have vaporised.
In vehicle construction work, mixed joints between
steel and aluminium are also increasingly being used.
Figure 8 Phase diagram for iron/aluminium.
The phase diagram shows that iron or steel and
aluminium offer virtually no solubility with one another.
In each mixed ratio, Fe/Al phases occur which feature
brittle characteristics. Experience therefore shows that
a proportion of Al/Fe phases in the molten material of
over 10% must be avoided in all cases.
When using zinc as the filler material, a joint can then
be created between these two materials, where the
aluminium is partially melted, whereas the steel, to
avoid brittleness in the molten material, may only be
moistened. This means that a welded joint is created
on one side, and a brazed joint on the other. Figure 7
shows an overview picture and a detailed picture of
this type of joint, brazed using the coldArc process
with a zinc-based wire, and an application from
vehicle body construction. The strength values
achieved with zinc wires in the fillet weld on the lap
joint are in the range of the strengths of aluminium
wrought alloys and of MIG brazed joints using copperbased wire. With butt joints, slightly lower strength
values are achieved.
Even here the use of push/pull torches is not required;
completely normal MIG/MAG welding torches can be
used for coldArc welding and coldArc brazing.
Other typical applications for coldArc brazing and
coldArc welding are shown in Figure 9 to Figure 13.
Figure 7 Mixed aluminium/steel joints with zinc-based
wires;
Top: Overview picture
Bottom left: Detailed picture
Bottom right: Car door
© 2005 EWM HIGHTEC WELDING GmbH
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7
Figure 9
Hot dipped steel metal sheets, 0.7 mm, fillet
on the lap joint, with 1.0 mm Zn wire coldArc
brazed with 0.35 m/min, U=13.5 V, I=40 A.
Figure 10
Al-St mixed joint, 0.7 mm hot dip steel and
1.0 mm AlMg, fillet weld on the lap joint, with
1.0 mm Zn wire coldArc brazed at
0.35 m/min, U=13.5 V, I=40 A.
Figure 11
Al-St mixed joint, 1.0 mm AlMg and 0.7 mm
hot-dip steel, fillet weld on the lap joint, with
1.0 mm AlSi5 wire coldArc brazed at
1.1 m/min, U=14.5 V, I=60 A.
Figure 12
Steel sheet, 1.0 mm, butt joint, 1 mm gap,
1.0 mm G4Si1 wire, coldArc welded at 2.0
m/min, U=19 V, I=136 A.
Figure 13
CrNi sheet, 0.5 mm, fillet weld on lap joint,
0.8 mm wire, coldArc welded at 2.0 m/min,
U=16.5 V, I=90 A.
© 2005 EWM HIGHTEC WELDING GmbH
Literature
[1] Kabushiki Kaisha Kobe Seiko Sho: Output control
of SC welding power source, PatNr.: US
4546234, Kobe Steel, Japan, 1984
[2] The Lincoln Electric company: STT – Surface
Tension Transfer, Pat.Nr.: EP 0324960 B1, USA,
1989, and EP 1232825 A3, USA, 2002
[3] Goecke, S. F. and L. Dorn: Research on the
influence of process control and shielding gas
composition on spatter formation and groove
geometry in MAG short arc welding of thin steel
sheets less than 0.5 mm thick, Final Report DFG
(Deutsche Forschungsgemeinschaft; German
Research Association) Do 202/26-3, (2000)
[4] Goecke, S. F., L. Dorn and M. Hübner: MAG
ChopArc welding for minimum-thickness sheets ≥
0.2 mm. Individual conference report in the
conference volume for the Great Welding
Conference – GST 2000, Nürnberg, 27-29 Sep.
2000, German Welding Association Reports,
Volume 209, (2000), pages 163-168
[5] S. F. Goecke, E Metzke, A Spille-Kohoff, M
Langula: ChopArc – MSG arc welding for ultralightweight
construction,
bmb+f-gefördertes
Verbundprojekt, final report, Fraunhofer IRB
Verlag, 2005, ISBN 3-8167-6766-4
[6] G Huismann: Direct control of the material
transfer, the Controlled Short Circuiting (CSC)MIG process, ICAWT 2000: Gas Metal Arc
Welding for the 21st Century, Dec. 6-8, 2000,
Orlando, Florida
[7] R Killing: Handbook of Welding Techniques, Part
1 – Arc welding, specialist book series on welding
technology, volume 76/I, DVS-Verlag Düsseldorf
1999
[8] S. Kröger and R. Killing: Software for creating
and managing parameters for MIG/MAG welding,
German Welding Association Yearbook 2004,
pages 150-161, DVS-Verlag, Düsseldorf 2003
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