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8/7/2019 BOC 216312 GasShielding Arc Welding AUS LR FA
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1
Gas-shielded Arc
Welding of Aluminium
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02
Contents
03 Gas-shieldedarcweldingofaluminummaterials
04 Theinfluenceofconductivity
05 Theinfluenceofhydrogensolubilityandsolidificationrange
06 Preventionofcracks
10 SeamPreparationinbuttwelding
10 Shieldinggases
11 Summary
12 TechnicalSpecifications
Pulsed Arc
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Table 1: Physical properties of various aluminium alloys.
Alloy Electricalconductivity@20C SolidificationrangeC
Symbol EN Number International Sm/mm W/cm K
Al 99.5 3.0255 1050A 33.5 35.5 2.26 2.29 659 658
Al Mg 5 3.3355 5056A 14 19 1.20 1.34 625 590
Al Mg 4.5 Mn 3.3547 5083 15 19 1.20 1.30 640 575
Al Mg Si 0,5 3.3206 6060 26 35 2.0 2.4 650 615
Al Mg 1 Si Cu 3.3211 6061 23 26 1.63 640 595
Al Zn 4.5 Mg 1 3.4335 7020 21 25 1.54 1.67 655 610
G Al Si 1.2 3.2581 6082 17 26 1.3 1.9 580 570
G Al Si 10 Mg 3.2381 6005 17 26 1.3 1.9 600 550
Gas-shielded arc welding of aluminium materials
The handling of aluminium differs from that ofsteel in many respects as each material requiresspecic treatment. To prevent corrosion, thetwo materials should be stored and processedseparately right from the beginning. Likewise,only appropriate tools should be used foreach material. Tools previously used for othermaterials, e.g. carbon steel, must be cleanedbefore use with aluminium. Aluminium
requires tooling with a larger rake angle,the right grinding wheels and wire brusheswith stainless steel bristles.
Thephysicalpropertiesofaluminiumanditsalloys
Anyone welding aluminium for the rst time, either on its own or
in conjunction with steel, should rst of all consider the physical
properties of aluminium alloys. Their excellent electrical and
thermal conductivity together with the solidication range are
ideal properties for the welding of this non-ferrous material.
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04
1 2 3
Electrical conductivity, Sm/mm 15 19 24 32 34 36
Current / voltage 250A / 26V 300A / 28V 340A / 29V
Depth of penetration 3mm 7mm 9mm
Wire diameter and feed rate 1.6mm; 8m/min
Travel speed rate 50cm/min
The inuence of electrical conductivity
The electrical conductivity of a wire electrodeis basically inuenced by the nature andcomposition of the alloy elements. The lowerthe alloy proportion, the better the electricalconductivity at the stick-out of the wire and thehigher the welding current and voltage (withconstant wire feed). This means the arc outputrises, improving the weld geometry in the process(Figure 1). Changing the type of wire electrode
because of the base material, therefore requiresadapted set values for the wire feed (= weldingcurrent) and welding voltage.
Theinfluenceofthermalconductivity
The thermal conductivity of aluminium (Table 1) which is
signicantly better than that of structural steel (app. 0.5W/cm.K),
makes rapid welding more difcult. It also reduces the depth
of penetration and leaves the melting pool with less time for
solidication, thereby affecting the degassing, which can lead
to lack of fusion and pores. To prevent these problems, the
following should be done:
Preheating
Increasing the arc output
Under certain circumstances, additional preheating during
the welding of thick cross-sections
Figure 1: MIG welding: Inuence of electrical conductivity on weld geometry.
Filler alloy:
1. SG-AlMg 5
SG-AIMg 4.5 Mn
2. SG-AlSi 5
3. SG-Al 99.5 Ti
Shielding gas Argon
Base metal AlMg3
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The inuence of hydrogen solubilityand the solidication range
Hydrogen is the main cause of pores duringthe welding of aluminium. In Figure 2, thehydrogen solubility of iron is compared to thatof aluminium. The signicant surge in solubilitywhen solidication takes place (at about 660C)and the low rate of solubility in the solid stateshow that hydrogen in aluminium (if the rateof cooling is too rapid) can easily be frozen in,thereby creating pores.
The solidication range can be considered as a yardstick for
determining the formation of pores. If there is no solidication
range or only a slight one, the aluminium welding deposit
solidies so rapidly that degassing cannot fully take place,
thereby causing porosity. Table 1 shows that the solidication
range increases as the alloy content rises with a corresponding
reduction in the risk of pore formation. There are ve main
reasons for the occurrence of pores and lack of fusion in the
welding of aluminium materials:
High thermal conductivity
Good hydrogen solubility in the molten state
A noticeable surge in solubility when solidication occurs
at about 660C
Low solubility in the solid state
No or virtually no solidication range
Figure 2: The hydrogen solubility of aluminium and iron.
AI
Fe
Temperature [C]
Hydrogensolubility
[cm/100g]
100
50
10
5.0
1.0
0.5
0.1
0.05
0.01
659 1536
500 1000 1500 2000 2500
0.036
0.7
8
28
50
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Prevention of cracks
Heat cracks in the weld seam occur wheneverthere is a high silicon and /or magnesiumcontent (Figure 3). The size of the alloyproportion required in the wire electrodedepends on the base material and the degreeof agitation anticipated.
Shouldaluminiumoxide(AI2O
3)
onthesurfaceberemoved?
To make welding easier and prevent defects in the weld,
aluminium oxide close to the weld is usually removed. The
reasons for this are:
Al2O
3has a considerably higher melting point (app. 2050C)
than aluminium (about 500C 600C, depending on the alloy)
Al2O
3is heavier than aluminium and can lead to inclusions
Al2O
3is hygroscopic and binds to moisture, which leads
to the formation of pores
Tendencytoheat-cracking
0 1 2 3 4 5 %alloy
Mg
Si
Preventionofcratercracks
The considerable shrinkage which aluminium is prone to
frequently leads to crater cracks. This can be prevented if
the following action is taken:
Filling in the nal crater just before it solidies either
manually or through the application of a crater lling
program (Figure 4)
Place the craters on a drain plate, if possible
Return the arc at the end of the seam to the seam alreadywelded and then turn off
Crater cracking Crater lled
Figure 4: MIG welding of aluminium: avoidance of crater cracks.
Figure 3: Tendency to heat-cracking of aluminium.
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As the oxide skin reforms in a very short time, it has to be
removed not only before but also during welding while
the layers are building up. This can be done:
Mechanically by means of brushing and grinding
With the assistance of the cleaning effect, through the electrical
current, with the electrode having positive polarity (Figure 5)
With adequate heat concentration, using about 75 to 100%
helium in TIG welding, with DC and a tungsten electrode with
negative polarity
Wire brushes are used to remove the oxide skin and the smoke
precipitate. Brushes with worn-out ends can no longer cut and
brush the surface effectively. Far from removing the Al2O
3, it
simply presses it in, causing smudging (Figure 6).
However, a lack of aluminium oxide during TIG welding causes
the arc to burn erratically, resulting in:
A rather wide weld with an irregular delineation
A dark precipitate, and
Almost no cleaning zone (see Figure 7, page 8)
In Figure 7, the left hand side shows that because of the oxide
present, the arc has burnt steadily. A relatively wide cleaning
zone and a uniform weld delineation can be clearly seen.
The cleaning effect from the electrical current can be explainedas follows:
If the electrode has a positive polarity (whether it is a wire or
tungsten electrode), the positively charged gas ions coming from
the positive pole hit the oxide skin with great kinetic energy and
shatter it (Figure 5, below).
+
Ions
+
Electrons
AIO
1.77eV
3.95eV AIAI
Figure 5: Cleaning effect (Al2O
3elimination) through electrical current.
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AIO
rounded bristle ends (worn out)
no cleaning action
sharp-edged bristles
shaving effect
Figure 6: Inuence of bristle end form on removal of Al2O
3.
Figure 7: AC TIG welding: weld region with and withoutaluminium oxide.
withoutA120
3withA1
20
3
180A 180ACurrent
Base metal : AI Mg3, 3mm thick, square butt joint
Wire : SG AI Mg5, 1.6mm dia.
Shielding gas : Argon
The earlier theory about the cleaning effect relates to the
electrons escaping from the aluminium. The following fact
argues against this the electron emission is also subject to
physical laws. The electrons emerge from the aluminium oxide
at 1.77 electron volts, whereas they would have needed 3.95
electron volts from aluminium (Figure 5, right-hand side).
Corresponding to Figure 7, Figure 8 shows the current and
voltage patterns during TIG welding for the welds shown.
This conrms the view previously expressed concerning the
behaviour of the arc in terms of the presence or absence of
aluminium oxide.
The effect of brushing (particularly rotating brushes) which
should effectively remove all the oxide is questionable in many
cases, especially during TIG welding.
Figure 8: AC TIG welding: current and voltage patterns for Figure 7.
withoutA120
3withA1
20
3
Base metal : AI Mg3, 3mm thick, square butt joint
Wire : SG AI Mg5, 1.6mm dia.
Shielding gas : Argon
08
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Alushields aresuperior in most
cases with regardsto economics
and quality.
Figure 9: AC TIG welding: current-voltage cycle with scrapedand brushed weld region.
WeldregionbrushedWeldregionshaved
Base metal : AI Mg3, 3mm thick, square butt joint
Wire : SG AI Mg5, 1.6mm dia.
Shielding gas : Argon
The right-hand part of Figure 9 (the brushed weld area) shows
a relatively uniform current and voltage pattern. Although it had
been believed that the Al2O
3had been removed using a rotating
brush, the welding reaction and the appearance of the weld
showed that hardly any oxide had been removed, leaving the
arc to burn at a steady rate.
However, if no aluminium oxide or hardly any is detected in the
weld, this is basically due to the cleaning effect of the electrical
current. This incidentally occurs in MIG as well as TIG welding.
Whether the aluminium oxide skin should be removed in
the weld region and if so, to what extent, depends on thecircumstances of each case. A lack of oxide can make the arc
behave erratically, creating atmospheric humidity and eddying,
leading to the formation of pores. Compromise, therefore, is
often needed. Pores can also occur if the weld area still shows
signs of moisture. Simply drying the area, using a neutral
oxy-acetylene ame, is often the solution. At the same time,
the weld is preheated.
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Seam preparation inbutt welding
Figure 10 Butt welding: chamfering the root side plate edges.
Wrong
Plate edges
not chamfered
Oxides not properly removed
Oxide notch
In butt welding, the plate edges of the underside should be
chamfered (Figure 10). This avoids an oxide chamfer on the
root side, making it easier to scour out the oxides from the
front faces.
Shielding gasesAluminium requires gases of an inert nature, with argon being
frequently used. Although more expensive from a cubic metre
price point of view, in many cases, Argon-Helium mixtures
(Alushield) are superior to Argon in terms of economics and
quality. Two examples illustrate the point.
Example:MIGwelding
Both girth welds and the straight bead on the cylindrical
housings for some high-voltage switches (Figure 11) should as
much as possible be pore-free. With argon as the shielding gas,
this could only be achieved through costly re-nishing work
using the TIG process.
The use of the shielding gas mixture 50% argon and 50% helium
(Alushield Universal) led to a drastic reduction in the amount
of re-nishing work the welds were virtually free from pores,
together with a reduction in the consumption of wire electrodes.
In spite of the higher gas price for the mixture, there was a
production cost saving of approximately 60%.
Figure 11 MIG Welding of cylindrical housings for high voltage switches.
Correct
Plate edges chamfered
Oxides completely removed
Perfect root formation
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Example:TIGwelding
In a tender submitted for the manufacture of telescopic braces
masts (Figure 12), the calculations were on TIG welding with
argon. When the structure had been installed, a further
calculation was done, which showed a reduction of 10% in the
total manufacturing costs with regard to the tender price. This
gure was achieved because the replacement of argon with a
mixture of 50% argon and 50% helium (Alushield Universal)
increased the welding speed, leading to a reduction in the
welding time and labour costs. The overall result was a 10%
reduction in production costs.
SummaryAluminium and its alloys need to be treated quite differently
from steel. A knowledge of its physical properties such as its
electrical and thermal conductivity, as well as its solidication
range can be of great help in welding of aluminium materials.
The amount of aluminium oxide to be removed depends in
the rst place on what is required of the weld in terms of
quality and secondly on the welding process. Wire brushesare not particularly suitable for removing oxide. Argon-helium
(Alushield) mixtures are often better than argon with regard
to quality and cost where practical. Whenever practical, push
pull guns should always be used with the leads kept as short
as possible.
Figure 12 TIG welding of telescopic braced masts.
11
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CylinderSizes
Content
(m3@STP) GaugePressure(kPa@15C)
Outlet
Connection
J 13.8 30,000* AS 2473 Type 10
G 8.6 16,900E2 4.1 20,000
D2 2.0 20,000
D 1.6 15,800
Pack Sizes
KAY15 227.2 30,000* AS 2473 Type 10
JAY15 207.7 30,000*
KAY9 136.3 30,000*
KAY4 60.6 30,000*
Cylinder colour (to AS 4484): Peacock blue body, shoulder and neck.Not all cylinders and packs are available at all BOC outlets. Please check with your local BOC branch on 131 262. Other sized
cylinders and packs may also be available on request.* Indicates tted with a pressure regulating valve outlet connection, 6,0009,000 kPa.
Gas Purity
Argon >99.995%
Note: Higher grades and purities ofthis product are available from BOC.Specications are included in the ScienticGases and Equipment Manual. Contact1800 658 278 to obtain a copy.
ArgonWeldingGrade,Compressed(Ar)GasCode061
12
Technical Specications
Applications
All the major applications
of argon are related to the
production, processing
and fabrication of metals.
The role of argon is
nearly always to excludeatmospheric air from
contact with metal alloys.
An example of this is as a
purging gas to protect weld
areas, such as the inside of
pipes during welding
The predominant gas used
in GMA and TIG welding
applications. Argon is used
alone or mixed with other
gases such as helium, carbon
dioxide, oxygen, nitrogen orhydrogen
Used on a wide range of
ferrous and non-ferrous
materials for welding
A major component in most
shielding gas mixtures for
arc welding due to low
ionising potential and low
density
Each mixture is often specic
to welding a particular
material or to a particular
process. For example, argon
is used as a shielding gas
for TIG welding aluminium,
titanium and copper
Features
Colourless, tasteless and
odourless
Inert to all materials at all
temperatures and pressures
Heavier than air, argon will
collect in low-lying areas,
ducts and drains
An asphyxiant
Benefits
Argon produced by BOC has
a minimum purity level of
99.995% ideal for welding
applications
The low ionising potential
allows easy forming of awelding arc without reacting
with the metal components
being welded
When welding thicker
materials, other gases are
added to the argon base
to produce a more uid
weld pool
12
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Applications
Boat and ship building
Tankers
Truck body work
Water heaters and heat
exchangers
Piping and balustrades
Light busbars
Features
Excellent arc stability
High heat input efciency
Low distortion
Flatter weld bead with low
reinforcement
Faster welding speed
Good fusion characteristics
Little or no spatter
Used in dip, spray and
pulsed transfer modes
Benefits
Lower spatter reduces
clean-up time
Improved weld metal
properties
Easy to use
Good appearance and nish
with low reinforcement
levels
Increased productivity due
to fast weld speeds
Can be used on robotic
machines
AlushieldLightGasCode079
Gas Composition
Helium 27%
Argon Balance
CylinderSize
Content
(m3@STP)
GaugePressure
(kPa@15C) OutletConnection
G 7.8 16,900 AS 2473 Type 10
Pack Size
KAY4 55.6 30,000* AS 2473 Type 10
Cylinder colour (to AS 4484): Peacock blue body, brown shoulder and neck.Not all cylinders and packs are available at all BOC outlets. Please check with your local BOC branch on 131 262.* Indicates tted with a pressure regulating valve outlet connection, 6,0009,000 kPa.
Technical Specications
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AlushieldUniversalGasCode507
Applications
Shop tting and frames
Heat exchangers
Tanks and vessels
Repairing vehicle body work
Rail carriages
Copper work
MIG Brazing: dissimilar
joints, automotive joints
Features
Ideal for automatic
and applications
Can be used for copper and
stainless steel TIG welding
Stable welding arc
Excellent appearance
and nish
Excellent fusion
characteristics
Suitable for applications
where penetration is critical
Benefits
Faster welding speed
Low defect levels
Strong weld
No post weld cleaning
Improved productivity
Reduced porosity
Gas Composition
Helium 50%
Argon 50%
CylinderSize
Content
(m3@STP)
GaugePressure
(kPa@15C) OutletConnection
G 7.3 16,900 AS 2473 Type 10
Technical Specications
14
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Applications
Aluminium castings
Ship building and
armoured vehicles
Heavy aluminium fabrication
Road and rail transport
Chemical and
petrochemical plants
Copper and aluminium
busbars
Features
Excellent arc stability
High heat input efciency
Low distortion and
oxidisation potential
Wide bead shape with
low reinforcement
Faster welding speeds
Good fusion characteristics
Reduced spatter
Benefits
Lower spatter reduces
clean-up time
Improved weld metal
properties
Easy to use
Reduced reject rates
Lower risk of defect levels
Good appearance and high
quality nish with low
reinforcement levels
Increased productivity
due to fast weld speeds
Can be used on robotic
machines
AlushieldHeavyGasCode069
Gas Composition
Argon 25%
Helium Balance
CylinderSize
Content
(m3@STP)
GaugePressure
(kPa@15C) OutletConnection
G 7.3 16,900 AS 2473 Type 10
Currently not available in packs.
Cylinder colour (to AS 4484): Brown body, peacock blue shoulder and neck.
Not available at all BOC outlets. Please check with your local BOC branch on 131 262.
Technical Specications
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For more information contact theBOCCustomerServiceCentreon:
Australia
www.boc.com.au
BOC is a trading name of BOC Limited, a member of The Linde Group. BOC Limited 2010. Reproduction without permission is strictly prohibited.
Details given in this document are believed to be correct at the time of printing Whilst proper care has been taken in the preparation no liability
BOCLimitedABN 95 000 029 729
Riverside Corporate Park10 Julius AvenueNorth Ryde, NSW 2113Australia
9-0402AEQAUS05105K