Arc brazing - Innovative, safe and economical
Norbert Knopp, Mündersbach and Robert Killing, Solingen, Germany
Introduction
After welding and together with adhesion and mechanical joining, arc brazing is an important process for
connecting metallic components. In comparison to welding, less heat is introduced into the joining area in arc
brazing, meaning less damage to surface layers such as zinc plating and less distortion of components. Alongside
conventional soft soldering using the wave soldering process, a soldering iron or flame, and brazing using a flame
or an electric brazing furnace, arc brazing has become firmly established in manufacturing over the past few years
with the increasing use of thin sheet metal with metallic plating.
The term arc brazing covers a wide field, as it includes a whole range of soldering procedures. Some of these
procedures, such as MIG/MAG brazing, use consumable electrodes, others, such as TIG and plasma soldering,
use non-consumable tungsten electrodes. So far, however, application of the procedure is concentrated mainly on
brazing thin metal sheets or fine metal sheets, especially sheet metals with metallic plating.
The intention of this brochure is to demonstrate the factors involved.
1
plated materials are found most often in vehicle
construction,
household
appliances
and
ventilation and air conditioning technology.
Zinc plating can be applied in various ways. Hot
galvanising can be carried out in a continuous
operation, known as continuous galvanising
(Sendzimir galvanising), or by dipping in zinc
baths. The latter is known as hot dip
galvanising.
In
addition
there
is
electrogalvanising and metal spraying of zinc
through thermal spraying. Thin coatings of
aluminium,
zinc/aluminium,
zinc/nickel,
aluminium/zinc,
aluminium/silicon
and
aluminium/nickel can be applied to steel
surfaces by hot dipping.
The plating thickness produced by the various
procedures varies greatly [1]. The thickest
coatings occur with hot dip galvanising,
measuring around 80 micrometers. The next
thickest in order are continuous galvanising
(around 20 micrometers) and galvanised zinc
plating at a few micrometers in thickness. In hot
dip galvanisation, various iron/zinc coatings are
deposited on the surface of the iron first,
followed by a coating of pure zinc. Figure 1
shows galvanic zinc plating on an iron surface
with a thickness of 8 micrometers; Figure 2
shows a zinc plating of around 80 micrometers
produced using loose piece plating.
The thicker the zinc plating, the greater are the
Definition of welding/brazing
The differences between fusion welding and
brazing are listed in Table 1. Whereas in
welding the weld deposit and the parent metal
are melted around the joining area and joined by
the hardening of the fluid area of the materials,
in brazing only the solder is melted and the
connection to the parent metal is provided via
adhesive forces and diffusion.
The different connecting mechanisms also result
in the lower energy requirement in soldering and
consequential advantages such as reduced
damage to the material and lower levels of
distortion. The reduced heat feeding involved
has other advantages which will be covered in
later sections.
2
It all comes down to the plating
To protect against corrosion, steel panels and
profiles are often plated with metal. These
Fusion welding as defined in
DIN 1910
Brazing as defined by
DIN 8505
Brazing is a process for
joining metallic materials
using a molten filler
material (solder), where
appropriate using flux.
The
welding
temperature
of
the
solder is below that of
the parent metals being
joined. These are wetted
Fusion welding is welding using without being melted.
molten metal limited to the
joining area, with or without filler
metal.
Welding is the joining of
materials in the weld zone using
heat and/or force with or without
filler metal. This can be
facilitated or simplified using
welding
consumables,
e.g.
shielding gas, welding powder or
paste. The energy required for
welding is supplied by an
external source.
Table 1
Differences between fusion welding and brazing
Figure 1
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Galvanic zinc plating – 8µm thick
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weld metal in form of bubbles. The mechanism
behind mechanical pore formation is illustrated
in Figure 3. This results in - usually tube-shaped
- chains of pores in the weld metal.
The problem of mechanical pore formation
diminishes with a reduction in thickness of the
items being joined. This is because in thin
sheets very dense gaps are quite unlikely, and
also because solidification is slower, improving
degassing. Even so, groove shapes without
gaps are preferable when welding materials with
plated surfaces. Pores can also be formed,
however, if zinc vapour becomes trapped by
overflowing weld deposit.
3.3
Deterioration of welding properties
difficulties arising in welding and brazing.
Zinc vapour penetrating the arc area due to the
significant affinity of zinc and oxygen also
reduces the welding efficiency of the arc
process. Free oxygen binding increases the
surface tension on the forming droplet and
produces a coarse material transfer combined
with additional spatter formation. This applies to
manual arc welding using stick electrodes, but
especially to MAG welding. In this case the
greater spatter effect also means that the
shielding gas nozzle must be cleaned more
often. As the viscosity of the weld deposit
increases, the welding speed will often have to
be reduced.
3
3.4
Figure 2
Loose piece zinc plating - 80µm thick
Problems when welding materials with
zinc-plated surfaces
When welding zinc-plated surfaces, various
problems can occur which become more
complex with increasing thickness of the zinc
coating [2].
3.1
Cracking
Zinc mixing into the weld deposit may create
zinc deposits on the grain boundary, causing
liquid metal embrittlement cracks in the weld
seam through internal stress. However, the
stress level required to form cracks must only be
anticipated with higher strength steels and
greater sheet thicknesses, which means that
cracking in thin metal sheets has no great
significance for welding.
3.2
3.5
Combustion of the zinc plating
Next to the seam, the protective zinc plating is
partially destroyed during welding. The same
applies to the underside of the joint for thinner
metal sheets. If these areas are very narrow –
less than 2-3 mm – they do not necessarily have
to be reworked, because the surrounding baseer
zinc protects the iron cathodically. If wider zones
are damaged, however, these must be re-plated
just as the weld seam itself, e.g. by applying
coating materials, applying solder coating or by
Pore formation
Due to the low boiling point of zinc (907° C), the
substance may vaporise under the effect of the
heat from the welding process. If this occurs at
the surface only, the zinc vapour produced may
escape as smoke. If the zinc is in a narrow gap
such as in a fillet or lap weld, however, the zinc
vapour cannot escape, leading to a pressure
build-up in the welding gap, which, when it
reaches a sufficient level, is released into the
© 2003 EWM HIGHTEC WELDING GmbH
Heavy smoke formation
Due to evaporating and combusting zinc, smoke
formation increases during welding, meaning
that the permissible MWC (Maximum Working
area Concentration) values for zinc oxide and
for dust are generally exceeded. In this case,
extraction of the welding smoke is required at
the point of origin.
Figure 3
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Principle of mechanical pore formation
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Figure 5
Figure 4
Both inert gases (MIG brazing) and inert gases
with low proportions of active gases (MAG
brazing) are used as shielding gas.
Direct current power sources with constant
voltage characteristics are used for brazing. For
working in the short arc range, conventional
power sources can be used. For pulse arc
brazing, electronic power sources (Figure 6) are
preferable. Due to the lower electric output
generally used for brazing, it is important that
the power sources can be precisely adjusted,
especially in the lower range, and that they have
a fast control response.
Schematic diagram of MSG brazing as per [3]
thermal spraying.
The difficulties described here for welding zincplated structures are much less marked in arc
brazing due to the lower melting point of the
solder and the lower heat feeding. The
advantages of brazing will be covered below,
but first the procedure for arc brazing and the
solders used are described in more detail.
4
Procedures of arc brazing
The principles, processes and systems for arc
brazing are described in detail in German
Welding Association (DVS) Information Sheet
0938, Part 1 [3]; a second sheet containing
application notes is in preparation. The
procedures can be sub-divided into metal
shielding gas (MSG) and tungsten shielding gas
(TSG) brazing processes.
The principle behind the processes is essentially
similar to the corresponding welding processes,
except that a filler material (solder) with a lower
melting range than the parent metal is used. In
addition, the technique of brazing is slightly
different and the process requires different
parameter settings.
5
Four-roller drive
6
Principle of and equipment for TIG brazing
As with TIG welding, in this process the arc
burns between a consumable tungsten
electrode and the workpiece. Inert gases are
used as shielding gases. TIG brazing can be
carried out by hand with manually fed brazing
rods or using partial mechanisation where the
brazing wire is fed by a separate drive unit
(Figure 7). The power sources have decreasing
characteristics and should be precisely
adjustable especially in the lower range. Uniform
direct current is used for the welding process.
Principle and equipment for MIG/MAG
brazing
Figure 4 illustrates the principle behind
MIG/MAG brazing. The solder is drawn from the
spool on the feed roller unit by the feed rollers
and fed to the torch via a tube package. The
tube package should not be longer than 3 m if
the wire is only pushed. Push/pull drives, where
the wire electrode is both pushed and pulled,
are ideal. Optimum feed properties are also
provided by a 4-roller drive (Figure 5). The wire
feed hoses should be made of plastic due to
their low sliding resistance. In continuous
operation, the use of water cooled torches is
recommended.
© 2003 EWM HIGHTEC WELDING GmbH
Figure 6
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Inverter power sources for MIG/MAG brazing
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8
Other processes in arc brazing
In addition to the processes mentioned above,
plasma MIG brazing, which combines a TIG and
a MIG arc, can also be used. Separate settings
of the two arcs in this hybrid process can be
used to achieve specific effects.
9
Figure 7
7
Brazing additives
Copper-based alloys (bronzes) are generally
used in arc brazing.
The main alloys are listed in Table 2. For zincplated metal sheets, silicon bronze SG-CuSi3 is
mostly used. For aluminium-plated metal sheets,
aluminium bronze is more appropriate. The table
also includes the melting ranges of alloys. They
are not much above 1000 °C and are therefore
significantly lower than the melting range of the
parent metal.
Recently several special alloys for arc brazing
have emerged, e.g. tin bronzes with a higher
Sn-content (10 %) or silicon bronzes with an
increased Mn content. In aluminium bronzes,
nickel additives are also found.
The brazing additives are available in diameters
of 0.8 to 1.2 mm. The most frequently used
brazing wire diameter is 1.0 mm.
Schematic diagram of TIG brazing as per [3]
Principle of and equipment for plasma
brazing
Plasma brazing can be carried out with uniform
or pulse-shaped current. The power sources
have decreasing characteristics; the principle
behind the process is given in Figure 8. The
constriction of the arc by a water cooled copper
nozzle increases its energy density. Two
shielding gas flows are required; one for the
plasma gas and one for the shielding gas. The
brazing additive in-feed is mechanised. As in
TIG brazing, the power supply is not dependent
on the quantity of brazing material used. To
increase fusion performance and welding speed,
the filler material can also be pre-heated by
resistance heating through connection to a
separate power source (plasma hot-wire
brazing).
As the tungsten electrode is positioned far back
in the torch, the system is equipped with a high
voltage pulse ignition aid. This first ignites a
non-transferring auxiliary arc burning between
the electrode and the restricting nozzle. This
pre-ionises the subsequent arc path so that the
transferring arc can then ignite touchlessly
without a high voltage ignition pulse.
10
Shielding gases
Whereas with the tungsten brazing process only
inert gases (normally argon) are used as
shielding gas due to the service life of the
tungsten electrode, the use of argon with low
proportions of active components in MSG
brazing has certain advantages over pure argon.
In our own tests with SG-CuSi3 brazing wire,
adding 2.5 % CO2 or 1 % O2 resulted in
improved stability of the arc, lower porosity and
better flow characteristics of the solder. The
heat feeding is increased slightly. The best
visual appearance of the brazed seam and
particularly good arc characteristics were
achieved using argon + 2 % N2. However, pore
formation was slightly greater.
How the various shielding gases affect other
brazing materials is shown in Table 3.
Type
DIN code
Alloy base
Melting
range °C
Silicon
bronze
SG-CuSi3
Cu + 3%
silicon
910-1025
Tin bronze
SG-CuSn6
Cu + 6% tin
910-1040
Aluminium
bronze
SG-CuAl8
Cu + 8%
aluminium
1030-1040
Table 2
Figure 8
The main brazing materials used in arc brazing
Schematic diagram of plasma brazing as per [3]
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Flow
Seam
Heat feeding
Porosity
Arc
characteristics
Designation
Draft
Limiting
span length
Butt weld on
buttjoint
0 < S < 2tmin
Fillet weld on tee
joint
0 < S < 2tmin
Wire
Shielding
gas
SG-CuSi3
Argon
+
o
++
+
o
99%Ar, 1%O2
++
+
o
++
++
Fillet weld on lap
joint
0 < S < 2tmin
Fillet weld on lap
joint
0 < S < 2tmin
Fillet weld on lap
joint (3 plate
joining)
0 < S < 2tmin
Egde joint weld
0 < S < 2tmin
Flange weld
0 < S < 2tmin
+
SG-CuAl8
97.5%Ar,
2.5%CO2
++
+
o
+
++
98%Ar, 2%N2
++
--
o
++
++
98%Ar, 2%H2
+
--
o
+
o
70%Ar,
30%He
+
+
+
+
o
Argon
+
+
++
+
+
+
99%Ar, 1%O2
+
+
Figure 9
o
+
+
o
+
+
the coating in front of the torch and the rising
vapour does not enter the arc area. Arc stability
is not reduced, and the shielding nozzle
becomes less dirty. The negative affects of the
zinc coating described above for welding do not
therefore occur, or are at least greatly reduced.
The arc also cleans the bond level. Therefore,
no flux is usually required for arc brazing.
Occasionally, brazing on a slip of forward
brazing material is recommended, so that the
arc does not burn directly on the parent metal
[1]. This means that less zinc combusts, but
fusion penetration and the mixing of iron into the
solder are reduced. Iron can only be dissolved in
small quantities in copper materials. The
proportion of iron over and above the absorption
limit is present in the braze seam in the form of
rounded foreign body inclusions, and the
mechanical properties of the joint are impaired.
The fear that bonding errors may occur due to
the forward solder is unfounded with proper
application, as the bond in brazing is not created
by partial melting. The melting temperature of
the solder is sufficient to melt the zinc layer still
present on the surface (419 °C), enabling it to
form an alloy with the solder. A type of special
brass is produced as an intermediate layer,
creating a material-inclusive bond. There is also
a diffusion of copper into the parent material,
which reinforces the bond.
With arc brazing little heat should be fed in. On
the one hand, this protects the metallic coating
against excessive damage, and reduces the
warping of the normally thin items being joined,
on the other hand significant heat feeding is not
required anyway because of the low melting
+
SGCuSn10
Table 3
11
97.5%Ar,
2.5%CO2
+
98%Ar, 2%N2
--
+
-
-
o
98%Ar, 2%H2
--
-
-
o
o
70%Ar,
30%He
++
+
++
++
++
Argon
++
o
++
+
+
99%Ar, 1%O2
++
+
o
++
++
97.5%Ar,
2.5%CO2
+
+
o
++
++
98%Ar, 2%N2
++
--
o
--
++
98%Ar, 2%H2
--
--
--
--
--
70%Ar,
30%He
+
o
+
+
+
+
+
+
Effect of shielding gases in MSG brazing
Workpiece preparation
Due to the reduced sheet metal thickness
concerned, mechanical working of the edges of
the parts to be joined is not normally necessary.
Butt joints, fillet welds on a T- and lap joints,
seams on unprepared faces and flanged seams
occur (Figure 9).
To ensure good wetting between the solder and
the parent material, the joining area should be
bare metal and free of impurities.
12
12.1
Carrying out arc brazing
MSG brazing
In MSG brazing the torch position is slightly
inclined (10-20 °). This means that the forward
heat and the arc result in partial vaporisation of
© 2003 EWM HIGHTEC WELDING GmbH
Seam types and gap widths in MSG brazing
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Sheet thickness [mm]
1
1.5
2
3
Arc voltage [V]
14.0
14.3
14.5
17
72
90
118
Welding current intensity 55
[A]
Wire feed [m/min]
2.3
3.4
4.5
6.0
Shielding gas: Argon – 12 l/min; brazing wire: SG-CuSi3 –
1.0 mm thick; parent metal: Mat. no. 1.0312; brazing
position: flat, horizontal vertical; torch angle: 10-20°
incline; brazing speed: 50-70 cm/min; seam type: fillet
weld on lap joint.
Table 4:
Guidelines for MIG brazing in the short-arc range;
values as per [3]
range of the solder. MSG brazing can therefore
be carried out in the short arc range or in the
lower output range of the pulse arc. Depending
on sheet thickness, the current intensities are
around between 50 and 120 amperes, with
pulse brazing slightly lower. Table 6 and Table 4
contain guideline values for setting the
equipment.
In
modern
MSG
systems,
characteristics
pre-programmed
by
the
manufacturer can be called up for the brazing
process.
Figure 10 Torch inclination and wire feed angle as per [4]
12.2 WSG brazing
TSG brazing is also carried out with less energy,
but unlike MSG brazing the torch is used as in
backhand welding. The arc should be directed
more towards the molten pool. That way the zinc
plating is not combusted directly, but the forward
heat still results in a partial melting loss of the
plating. With this technique, however, the risk of
the parent metal melting is lower. In mechanised
brazing it is even possible to position the brazing
wire slightly underneath the arc by means of the
relevant wire feed setting, so that the arc is
partly burning on the melting end of the wire.
This further reduces the risk of iron inclusion.
Opinion as to the best way of feeding the
brazing wire still varies. Whereas in the
diagrams of the principle in the German Welding
Sheet thickness [mm]
Sheet thickness [mm]
1
1.5
2
3
Arc voltage [V]
24
26
30
32
Pulse current intensity:
[A]
80
100
110
120
Basic
[A]
38
50
55
60
current
intensity
Pulse frequency [Hz]
2.5 – 3.0
Wire feed [m/min]
4.0
6.5
8.5
11
Shielding gas: Argon – 12 l/min; plasma gas: Argon – 0.9
l/min; brazing wire: SG-CuSi3 – 1.0 mm thick; parent
metal: Mat. no. 1.0312; brazing position: flat, horizontal
vertical; torch position: neutral; brazing wire feed: inclined
in the direction of work; brazing speed: 50-70 cm/min;
seam type: fillet weld on lap joint
Table 6:
Guideline values for plasma brazing with a
pulsed arc; values interpolated as per [3]
Association (DVS) Information Sheet 0938-1 in
Figure 7 and Figure 8 the wire is inclined in the
direction of work, in [4] positive results have
been reported from a backhand brazing
direction (Figure 10).
Guideline values for setting the plasma
equipment are given in Table 6
1
1.5
2
3
16.0
17.0
18.0
19.0
13
Welding current intensity 25
[A] *)
45
65
90
Pulse frequency [Hz]
20
35
60
80
Pulse duration [ms]
1.5
1.6
1.7
1.7
Figure 11 shows the upper- and underside of a
brazed joint on galvanised vehicle bodywork
with a thickness of 1 mm; in comparison, Figure
12 shows the undersides of MAG welds on the
same panels.
)
Arc voltage [V] *
Wire feed [m/min]
2.0
3.0
4.5
6.0
*) Effective values
Shielding gas: Argon – 12 l/min; brazing wire: SG-CuSi3 –
1.0 mm thick; parent metal: Mat. no. 1.0312; brazing
position: flat, horizontal vertical; torch angle: 10-20° incline;
brazing speed: 50-70 cm/min; seam type: fillet weld on lap
joint
Table 5:
Figure 11 Upper side and underside of an MSG brazed joint
on bodywork panels
Guideline values for MIG brazing using a pulse
arc; values as per [3]
© 2003 EWM HIGHTEC WELDING GmbH
Results of brazing tests
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MAG plug welding with G3Si1
Figure 12 Underside of an MSG weld on vehicle bodywork
It is clear that due to the lower molten pool
temperature in brazing, the zinc plating on the
underside is only slightly oxidised, but not burnt.
On the upper side only a very narrow zone
where damage may have occurred can be seen.
Due to the cathodic protective effect of zinc, this
does not need to be repaired if it is narrower
than around 2 mm. Part of the liquefied zinc on
the surface also runs back behind the arc and
creates an alloy with the bronze on the edge of
the brazed seam. The seam itself is corrosionresistant due to the alloy it consists of. This
MIG plug brazing with SGCuSi3
Figure 14 Comparison of hole welding/hole brazing on
vehicle bodywork panels
14
14.1
Errors in arc brazing
Pores
As the melting ranges of the solders used in arc
brazing are only slightly above the boiling point
of zinc, zinc vaporisation and thus pore
formation is very low in brazing. A certain
amount of internal porosity of the brazed seams
can be tolerated if the mechanical properties of
the joint are not unacceptably impaired.
14.2
Cracks
Due to the differing heat expansion coefficients
of solder and the parent metal, cross-wise
cracks can occur in the brazed joints. This
cracking is increased if brazing is carried out on
cold-worked structures under tensile stress. The
brazed seams should therefore not be much
longer than 50 mm. Longer seams may be
produced using a series of consecutive short
seams.
15
Durability of the joints
It can be expected that the durability of brazed
joints is at least that of the worked parent
metals. Our own tests were conducted on fillet
welds at the lap joint on car body panels (1mm
thick). All shear tension samples cracked in the
parent metal, Figure 15, while on the sheet
cross-section, average strength values of 330
N/mm2 were measured. These values are not
surprising because the partial penetration of the
solder into the brazing gap enlarges the actual
load-bearing surface. This behaviour cannot be
expected with higher strength steels (yield point
>260 N/mm2) and with thicker metal sheets (>
around 1.5 mm). In this case preliminary tests
should be carried out to determine whether the
Figure 13 Cross-sections of MSG brazed joints on vehicle
bodywork panels (s=0.8mm) with different gap
sizes
means that re-plating as in welding is not
required.
Figure 13 shows cross-sections of MSG-brazed
fillet weld bonds on a lap joint. Although the
capillary effect of the solder in arc brazing is not
as great as in autogenic brazing, the solder has
penetrated sufficiently into the brazing gap. The
gap bridging capacity is good. A small amount of
mixing with the parent metal is detectible only in
the lower part-illustration.
Figure 14 as a comparison shows a hole weld
and hole brazing on vehicle bodywork panels.
Better durability can be expected of the brazed
than the welded seam due to the greater
bonding area.
Figure 15 Strength of brazed joints
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17
Literature
[1] Killing, R.: MIG Brazing on Thin Metal
Sheets, Metallbau Heft 9/1999, p. 52 – 54
Callwey-Verlag, Munich
[2] Killing, R.: Living with Coatings, Metallbau
Heft 5/1997, p. 32 – 35 Callwey-Verlag, Munich
[3] German Welding Association (DVS)
Information Sheet 0938-1 Arc Brazing –
Principles and Requirements for System
Technology
–
DVS-Verlag
Düsseldorf
September 2001
[4] B. Bouaifi, B. Quaissa, J. Tuchtfeld, AitMekideche and C. Radscheid: Plasma Brazing
Galvanised Metal Sheets – Status and Trends in
Vehicle Bodywork, DVS-Berichte Band 204,
DVS-Verlag Düsseldorf 1999
Figure 16 Brazed seams on the rear panel of a car
durability is sufficient for the intended purpose.
This is not necessary if one of the metal sheets
has a lower yield strength or is thinner.
Tests with hole welds and hole brazings, as
shown in Figure 14, indicated the advantage of
brazed joints, as anticipated from the bond
surfaces. In this case all samples cracked in the
parent metal, while the welded points became
detached from the parent metal at a lower load.
For higher strength steels and thicker metal
sheets, the same applies as described above.
16
Example applications
Arc brazing has attained significant importance
particularly in vehicle bodywork construction. As
early as the 1970s, there were short brazed
seams on the VW Beetle, which at that time
were however produced using an oxy fuel gas
flame. Even before the introduction of arc
brazing
into
the
automobile
industry,
manufacturers were “welding” garage doors
made from galvanised materials using bronze
wire and the MIG procedure. MIG brazing was
not a concept at that time.
Today, all car manufacturers deploy arc brazing,
with both MSG brazing and plasma brazing
being used. This applies mostly to short seams
on the bodywork, which are brazed either
manually or using industrial robots. Figure 16
shows these MSG brazed seams on the rear
panel of a car.
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