Aluminium welding FACTS ABOUT 1 2008

Aluminium welding
09 2008
FACTS ABOUT
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CONTENTS
Introduction ...................................................................................... 3
Physical properties ........................................................................... 3
Heat treatment. ................................................................................ 4
Strenght of weldment ....................................................................... 5
Filler metal ........................................................................................ 6
Surface preparation .......................................................................... 7
Reducing distortions ........................................................................ 8
Flame straightening.......................................................................... 8
Corrosion .......................................................................................... 8
MIG welding ..................................................................................... 9
Equipment ....................................................................................... 9
Pulsed MIG welding ........................................................................ 9
Welding procedure ........................................................................... 9
Starting the welding procedure ...................................................... 9
Procedure ........................................................................................ 9
Ending procedure ............................................................................ 9
TIG welding..................................................................................... 10
Equipment ..................................................................................... 10
Welding procedure ........................................................................ 10
Plasma arc welding......................................................................... 11
Variable-polarity plasma arc welding ............................................ 11
Laser welding .................................................................................. 12
Shielding gases ............................................................................... 12
Selecting shielding gas ................................................................... 13
Work environment .......................................................................... 13
Weld defects.................................................................................... 15
Solidification cracking ................................................................... 15
Incomplete fusion ......................................................................... 15
Porosity .......................................................................................... 15
Inclusion ........................................................................................ 15
Detection ....................................................................................... 15
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INTRODUCTION / PHYSICAL PROPERTIES
Introduction
Physical properties
Aluminium is a rapidly growing material and has found
many new applications as an engineering material.
Growth is taking place mainly in the transport sector i.e.
cars, buses, trains and marine vessels. Joining is a key
technology in many cases and especially welding of
aluminium, which is shown by the growing interest in
aluminium welding. Many fabricating facilities are used
to welding steel and the lack of knowledge about
aluminium welding has often hindered the use of
aluminium in many cases.
The physical properties of a material have a significant
influence on the welding properties. A comparison with
steel is shown in Figure 1.
Aluminium suffers from large deformations when subjected
to heat and this is due to the large thermal conductivity of the
material.
Aluminium has properties that differ substantially from
those of steels. The most interesting aspect of aluminium
usage is the weight saving that becomes possible. Weight
savings of 40-60% are often mentioned in many cases,
reducing fuel consumption in transport vehicles. The
possibility of using profiles also offers new possibilities in
design. The excellent corrosion properties also motivate
the use of aluminium.
Property
Aluminium
Steel
Melting temperature
>570°C
>1500°C
Density
1/3
1
Modulus of elasticity
1/3
1
Thermal conductivity
5
1
Linear expansion
2
1
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HEAT TREATMENT
Heat treatment
Pure aluminium has poor mechanical properties and is
therefore not used in load bearing constructions. The
metal is therefore usually alloyed and heat treated or
hardened to obtain the required properties.
The main groups of aluminium alloys are: Al-Cu,
Al-Mn, Al-Si, Al-Mg, Al-Si-Mg, Al-Zn. In Europe, the most
common type of classification for base materials is the
AA classification. The different types can be seen in
Figure 2.
The lXXX, 3XXX, 4XXX and 5XXX alloys are strain
hardened while the 2XXX, 6XXX and 7XXX alloys are
heat treated. A so-called temper designation is put after
the alloy to show how the hardening has been performed.
The most common temper designations for wrought
alloys are:
“H” -strain hardened. Applies to products which are
strain hardened through cold-working. The “H” is always
followed by two or more digits. The first digit indicates
basic operations and the second digit indicates degree of
strain hardening.
“T” -thermally treated to produce stable tempers. Applies
to products which have been heat-treated, sometimes to
produce a stable temper. The “T” is generally followed by
one digit indicating the specific sequence of treatments.
Common designations are the T4 (solution heat treated
and naturally aged) and the T6 (solution treated and
artificially aged).
AA
Terminology
Alloy type
Typical applications
1xxx
Non-alloyed
Packaging, decorative applications
2xxx
Copper
Aircraft sheet construction
3xxx
Manganese
Generalpurpose applications, strip
4xxx
Silicon
Filler wire
5xxx
Magnesium
Marine components, pressure vessels,
railroad cars
6xxx
Silicon + Magnesium
Automotive frames
7xxx
Zinc
High strength aircraft applications
8xxx
Other alloying
elements
Figure 2. The AA classification for aluminium alloys.
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STRENGTH OF WELDMENT
Strength of weldment
The heat from welding has great influence on the internal
structure of the aluminium material. The strength in the
HAZ (heat affected zone) is reduced, sometimes by as
much as 50%, due to the thermal treatment the material
receives during welding. The width of the HAZ depends
on the degree of metallurgical change which in turn
depends on thickness and geometry of the joint, the
welding process, the welding procedure, preheat and
interpass temperature and the thermal effects of backing
and fixtures. The HAZ in MIG or TIG welding seldom
extends more than 13 mm from the weld centreline, but
for design purposes it is often assumed to be the double.
The type of alloy plays a role as to how the strength
decreases and to what degree. Figure 3 shows the hardness
distribution for two types of alloys in the 6XXX and 5XXX
series. Observe how the hardening method affects the
distribution.
Figure 3. Hardness distribution
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FILLER METAL
Filler metal
MIG welding is always conducted with filler metal and TIG
welding is conducted with or without. Filler metal is chosen
in accordance with Figure 4.
The main factors which influence the filler alloy selection
include the following:
• Freedom from hot cracking
• Weld metal strength
• Weld metal ductility
• Corrosion resistance
The filler metal is not in itself hardenable, which implies
that no hardening procedure can harden the weld after
welding. When a good colour match is needed between the
weld bead and the surface, AlSi5 should be avoided.
Not much work has been done on developing filler metals
for aluminium for the last 20 years. An increased interest
has been shown during the recent years due to the increased
amount of welding that is being performed on aluminium.
New filler metals will be commercially available that offer
increased strength and reduced mis-match, which in turn
reduces material consumption and allows new possibilities
in design.
• Weld performance at elevated temperatures
• Weld metal fluidity
• MIG electrode wire feedability
• Weld metal colour match with base material after
anodising
Base
material
AIZn5Mg1
AISi1Mg
AIMg4
AIMg5
AIMg2.5
AIMg1
AIMn1
AI99.5
Properties
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
AI99.5
AIMn1
AISi5
AISi5
AISi5
AIMg5
AIMg5
AISi5
AISi5
AIMg5
AIMg5
AIMG5
AIMG5
AIMG5
AIMG3.5
AIMG3.5
AIMG3.5
AIMG3.5
AI99.5
AI99.5
AI99.5
AI99.5
AI99.5
AI99.5
AI99.5
AI99.5
AI99.5
AI99.5
AI99.5
AI99.5
AISi5
AISi5
AISi5
AIMG5
AIMg5
AISi5
AISi5
AIMg5
AIMg5
AIMg5
AIMg5
AIMG5
AIMg3.5
AIMg3.5
AIMg3.5
AIMg3.5
AI99.5
AI99.5
AIMn1
AIMn1
AIMn1
AIMn1
AIMn1
AIMn1
AIMg1
AIMg5
AIMg5
AISi5
AIMg5
AIMg5
AIMg5
AISi5
AIMg5
AIMg5
AIMg5
AIMg5
AIMg5
AIMg3.5
AIMg3.5
AIMg3.5
AIMg3.5
AIMg3.5
AIMg3.5
AIMg3.5
AIMg3.5
AIMg2.5
AIMg5
AIMg5
AISi5
AIMg5
AIMg5
AIMg5
AIMg5
AIMg5
AIMg5
AIMg5
AIMg5
AIMg5
AIMg3.5
AIMg3.5
AIMg3.5
AIMg3.5
AIMg4
AIMg5
AIMg5
AIMg5
AISi5
AIMg5
AISi5
AIMg5
AIMg5
AIMg5
AIMg5
AISiMg
AISi5
AIMg5
AIMg5
AISi5
-
AIZn5Mg1
AISi5
AIMg5
AIMg5
AISi5
Properties:
1. Highest strenght
2. Best weldability
3. Best corrosion resistance
4. Best colour match after anodising
Figure 4. Filler metal selection for welding aluminium alloys
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SURFACE PREPARATION
Type
SS-EN AW
Rm base material
(MPa)
Welding method
1100
>70
>70
-
>70
>70
AISi5
4043
>150
>150
AISi12
4047
-
-
AIMg3
5554
>200
>200
AIM4.5Mn
5183
>280
>280
AIMg5
5356
>280
>280
AI99.5
AI99.5Ti
Figure 5. Common filler metals
Surface preparation
The cleaning of the surface is necessary in order to achieve
the best welding results. Dirt, oil residues, moisture and
oxides must be removed, either with mechanical or chemical
methods. Hydrogen bearing mixtures represent the largest
problem because they are broken down into atomic hydrogen
in the arc, causing gas porosity in the weld. Normal shop
practise is to mechanically remove the oxide layer by brushing
with a rotating stainless steel brush, scraping or peening.
Light pressure should be used when brushing. Excessive
pressure might lead to locally overheating and distortion
of the metal surface. Chemical treatment includes cleaning
with alcohol or acetone. The chemical treatments may
demand access to costly equipment which often impedes
this treatment. The amount of cleaning necessary largely
depends on how much care has been taken to keep the
metal clean and dry in storage and in subsequent handling
during fabrication.
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REDUCING DISTORTIONS / CORROSION
Reducing distortions
The following practical hints are good to keep in mind, in
order to reduce distortions due to shrinkage.
• Use highly productive welding methods with the
lowest heat input as possible.
• Use maximum welding speed.
• Aluminium does not change colour when heated,
which makes it difficult to know when the right
temperature is reached. Therefore the temperature
must be measured. Brazing flux, small pieces of wood
or measuring instruments can be used for this pur
pose. When flame straightening non-hardenable
aluminium alloys, which are less sensitive to cracks,
thermal crayons or templesticks can be used.
• Allow the sheets to move freely.
• If longitudinal and transverse joints meet, weld the
transverse first.
• The gap between flame straightening temperature and
melting temperature is small. This means that the
heating must be done very carefully, or there is a risk of
melting.
• If butt and fillet joints meet, weld the butt joints first.
• Begin welding in the centre of the structure and
proceed symmetrically outwards.
• Aluminium has very high heat conductivity, which
means that heat is rapidly led away from the heated
area. This must be compensated by using a large
nozzle.
• Use fixtures that provide even cooling.
• Weld as little as possible.
These rules are the basis of the welding plan, i.e. the order
in which the weld should be performed. The key words are
low heat input and symmetrical welding.
Flame straightening
Even if the above mentioned measures are taken, it
could be difficult to get welded parts completely free
from distortions. One efficient and long-established
method of correcting distorted parts is flame straightening.
In flame straightening an oxy-fuel flame is used to quickly
heat a limited area of a component or assembly until the
material in this area becomes plastic. The temperature when
this happens is 350 -400°C for aluminium. The material
within the heated area expands, but the expansion is
limited by the surrounding cold material. Upon cooling,
the material in the heated area contracts more than it
expanded when heated and the component or assembly
is straightened out. By using external restraining devices
the straightening effect can be reinforced.
Other things to bear in mind when flame straightening
aluminium are to clean the area to be heated and to use
some kind of restraining device to prevent cold parts of the
workpiece to move when the heated metal expands.
Corrosion
The heat input the welding causes sometimes reduces the
normally so superb corrosion properties of aluminium.
It is the area next to the weld and the weld bead that loses
corrosion resistance due to the creation of a coarse-grained
structure. Solidification cracks that have not been repaired
represent a big problem because the corrosive media easily
opens the cracks and results in a corrosive attack. The 6XXX
and the 7XXX alloys are most sensitive to corrosion after
welding. Pure aluminium and the non-hardenable alloys
are more resistant or are not affected at all. Prolonged
service at elevated temperatures (>65°C) causes 5XXX
series alloy with more than 3% magnesium to be sensitive
to stress corrosion.
Aluminium is suitable for flame straightening. As it has
a high thermal expansitivity the straigthening effect is
good. The surface is not so sensitive to the flame and
aluminium can be cooled quickly. But the following
factors complicates flame straightening of aluminium:
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MIG WELDING / WELDING PROCEDURE
MIG welding
MIG welding is an arc welding process which utilises
a wire as combined melting electrode and filler metal
in a direct current (DC), electrode positive arc and inert
shielding gas. Neither alternating current nor direct
current, electrode negative arc has any practical application. The welding current, arc length and electrode wire
speed are controlled by the welding machine and set by
the operator:
Equipment
The following equipment and consumables are used for
the process:
• A DC power source designed for MIG welding.
• An electrode feeder and gun combination. The feeder
is often of push-pull type, which means that the wire
is pushed through a feed pipe and at the same time is
drawn by the gun.
• A shielding gas supply with pressure regulator and
flowmeter.
• A supply of cooling water when required.
• Aluminium and aluminium alloy electrode wire.
Pulsed MIG welding
Pulsed MIG welding is a variation of MIG welding.
It maintains an arc at low current and superimposes
short periodic pulses of high current in order to detach
and transfer drops of molten metal from the electrode
to the weld pool. The pulses are usually set to give one
drop per pulse. The result is that a thicker electrode can
be used in thin material together with less spatter, less
deformation and less posttreatment. The average current
becomes low, but the metal transfer occurs at high current,
which is necessary for spray transfer and for a stable arc.
Other advantages that can be seen when welding
aluminium is that pulsed welding enables slower
welding speed, which can be an advantage on complex joints where extra time may be needed for torch
manipulation. It also enables better control of the
bead shape. Pulsed MIG welding requires a power
source which can supply the twodifferent current
levels. Modern power sources for pulsed welding
have a “built in” set of parameters that follow the wire
feed speed setting (synergetic power sources). This
implies that it is much easier to use this technique in
practise compared to older types of power sources where
all pulse parameters had to be adjusted by the operator.
Welding procedure
Starting the welding procedure
When start and stop plates are not used, the arc should be
ignited approximately 25 mm in front of the starting point
and is then returned to the starting point where the welding
commences. This pre-warms the material and results in
better penetration and minimises the risk of coldflow and
porosity in the beginning of the weld. Another method is
to ignite the arc beside the starting point and then move it
to the starting point. The extra weld bead that this creates
can thereafter be machined.
Figur 6. Schematics of MIG welding
Procedure
Developing a qualified welding procedure requires
establishing an optimum setting for each parameter; and
the maximum setting for each parameter. The sequence
of steps in the development process usually approximates
the following:
• The average welding current should principally be
related to the metal thickness, although the joint type
may also have a support backing. The current
determines the heat and hence the penetration.
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TIG WELDING
• The arc length has an effect on penetration. Long arcs
are somewhat less penetrating than short, but give
wider weld beads.
• The welding speed for semiautomatic MIG welding is
to some degree reliant on the welder, but speeds in the
range of 5-13 mm/s are most common. Automatic MIG
welding has been reported at speeds up to 42 mm/s.
• The gun angle is influenced mainly by the welding
speed, i.e. the higher the speed, the greater the angle
from the vertical, all to ensure adequate gas shielding
of the arc and weld pool. Angles from 5 to 15° are
normal for semiautomatic welding, but up to 30° can
be used with automatic welding.
Ending procedure
To minimise the risk of cracks at the end of the bead when
start/stop plates are not used, the welding speed should
be increased so that the weld pool is reduced followed by a
backing of the torch before the arc is extinguished.
TIG welding
The TIG welding process uses a non-consumable tungsten electrode with either alternating current (AC),
direct current with positive electrode or direct current
with negative electrode. Alternating current is mainly
used for aluminium. The TIG process was developed
earlier than the MIG process and was earlier applied
to all metal thicknesses and joint types. Today, TIG is
limited to thin plates of aluminium up to 7 mm, although
the DC mode is suitable for thicknesses up to 26 mm.
Equipment
• An AC power source designed for TIG welding.
• Shielding gas supply.
Figure 7. Schematics of TIC welding.
Welding procedure
• The welding current used is related to the thickness of
the material, because the arc must be hot enough to
give the required penetration. Normally the pool is
ready for filler addition after 3-4 seconds. The average
welder is most comfortable at welding speed of 4 to 5
mm/s when using AC-TIG welding.
• The electrode size must be chosen to suit the current
level. An electrode which is too small will overheat
and the molten tip will become unstable and can lead
to droplets of tungsten being transferred into the
weld pool. It is crucial that the electrode is not covered
with thorium. Electrodes covered with zirconium are
preferable.
• The electrode size and the current level affect the arc
and the size of the weld pool. These factors will in
turn influence the selection of the gas cup diameter. A
too small gas cup will not give the necessary gas
shielding. A too large gas cup is wasteful of gas and
interferes with the welder's view of the weld pool. The
inner diameter of the gas cup should be approximately
4 times the electrode diameter.
• Welding torch with all necessary cables and hose for
power, gas and coolant and with a gas cup.
• A tungsten electrode of suitable type and size.
• Filler metal is usually in straight lengths but
occasionally as spooled wire.
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PLASMA ARC WELDING
Plasma arc welding
Plasma arc welding (PAW) was a development of the DCTIG process, when it was discovered that reducing the
gas orifice size not only increased the gas velocity but also
increased the arc temperature and concentrated the arc
energy. Severe constriction of the arc plasma produced
a cutting arc which resulted in the plasma arc cutting
process (PAC). By moderating the constriction, it was
discovered that the arc was suitable for welding, even with
some advantages over the TIG process for aluminium. One
difference between the processes can be observed visually:
the TIG arc is conical and the plasma arc is cylindrical.
The plasma welding arc produces a very hot and well
defined column of ionised gas. This is normally used
to melt completely through the base material, to form
a “keyhole”. The arc moves slowly forward to melt the
leading edge of the keyhole and molten metal flows
around the perimeter of the hole and solidifies behind
the arc to form the weld. Filler metal can be added at the
leading edge ofthe keyhole. The vertical upward welding
position is preferred for this process. It is possible
to weld up to 6 mm thickness in all positions, but in
vertical upwards position up to 20 mm thickness can be
welded. Plasma welding is practically always mechanised
or automised.
Two gases are supplied to the arc: a plasma gas and
a shielding gas. Argon is often used in both cases for
aluminium.
Variable-polarity plasma arc welding
The variable-polarity plasma process combines the
advantages of plasma arc welding with the additional
benefits of arc cleaning, provided by periodic bursts of
positive electrode energy. Variablepolarity plasma welding
has relatively low arc-travel speeds when compared to
other arc welding methods and especially compared to
MIG welding, but the fact that a single pass will replace
multiple passes needed by other methods sometimes
motivates its use.
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LASER WELDING / SHIELDING GASES
Laser welding
Shielding gases
Laser welding has been used on steel for some time, but
laser welding of aluminium is a rather new application.
There are essentially two types of lasers used for sheet
metal welding: CO2 and Nd:YAG lasers. The Nd:YAG
laser emits light in the infrared range with a wavelength
of 1.06μm with a maximum, continuous power of
lkW. The low wavelength enables the light to travel in
fiberoptic cables, which in turn makes the Nd:YAG flexible,
making it suitable for robotics applications.
If the power is pulsed, power up to 10kW can be reached.
The Nd:YAG is usually used for precision work on thinner
sheet. The CO2 laser is a gas laser using CO2 together with
N2 and He as laser medium. The wavelength is longer
than that of Nd:YAG, 10.6μm which implies that the light
can not pass through fiberoptics.
Besides protecting the molten metal and the electrode
from the oxygen in the air, the role of the shielding gases
is also to provide a stable arc and help avoid defects being
introduced into the weld. For aluminium welding, inert
gases are used i.e. argon and helium and mixtures of these
two. The advantages that can be reached with helium are
a result of the higher arc power combined with the better
heat conduction that helium provides. This influences the
penetration (see Figure 8). The penetration when welding
with helium is deeper and broader than with argon. This
can be utilised either by using the good penetration to
weld thicker material or by increasing the welding speed
in thinner material. The most common solution is to use
argon for MIG and AC TIG welding because the process
is usually easier to control with this gas. Pure helium
is seldom used except in DC TIG welding where the
increased heat is necessary to break up the oxide film.
Helium is more expensive than argon and the flow rate
must be increased because helium is lighter than argon (a
factor 10) due to lower density.
The process is characterised by high welding speeds,
deep penetration effect and low heat input. This makes
the laser suitable for welding overlap joints. The high
welding speed is preferred for long, one dimensional
welds. Another advantage is that the process only requires
one-sided access.
Frequently used shielding gas mixtures when welding
thicker aluminium plates are: (Ar/He) 70/30, 30170 or
50/50.
Very small additions of oxidising components such as CO2
and NO can be used without affecting the quality adversely.
They can actually improve arc stability. CO2 is suitable for
welding AlMg alloys with MIG, but cannot be used for TIG
welding since this rapidly destroys the tungsten electrode.
The addition of 0.03 % NO can also be used for TIG and
MIG welding in order to reduce ozone levels.
Figure 8. Penetration profiles in aluminium welding. TIG
welding at 225 A. Travel speed: 500 mm/min.
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SELECTING SHIELDING GAS / WORK ENVIRONMENT
Selecting shielding gas
Work environment
As mentioned, inert gases such as argon and helium or
mixtures of these are used as shielding gases in MIG
and TIG welding of aluminium. It is very important that
the purity of the gas is preserved all the way to the arc. If
there is any leakage in the welding equipment, the gas
will be contaminated. Holes in the hose package or water
leakage from water cooling systems create large problems. The different shielding gas alternatives are
shown in Figure 9 together with their European Norm
designations.
The intensity of the arc when welding aluminium is
much greater than when welding many other materials.
The emission of particulate fume and gases depends
on welding method, filler metal and type of alloy. TIG
welding produces less fume than MIG, due to the lower
energy of the arc and the fact that the filler metal is
not placed in the extremely hot centre of the arc. When
MIG welding, the highest amount of fume is produced
by AlMg5 filler metal.
The arc welding processes also induce an environmental
load on the welder and people working close to the
welding workplace. Dust is created in the form of fumes
and particles. The particles are often large in size and
fall down close to the workplace, but fume particles are
smaller and can travel far from the workplace. Much
effort is directed towards minimising air pollution. One
of the largest problems encountered when welding is the
large amounts of ozone that are created. The ultraviolet
light created when welding strikes molecules in the air
(a single oxygen molecule, O2 consists of two oxygen
atoms, O) splits the oxygen molecule to form two separate
oxygen atoms. (O2 becomes O+O). When the single oxygen
atom encounters a new oxygen molecule, it combines to
form ozone, O3). MIG welding of aluminium produces
more ozone than TIG welding. The amount is also
dependant on welding current, arc length, welding time
and type of alloy. Silicon filler metal produces the largest
amount of ozone, followed by pure aluminium (15-20%
lower) and magnesium alloyed wire (3-4 times lower).
MISON, should be used together with other measures in
order to reduce the ozone level, such as ventilation etc.
Shielding gas
ISO 14175 group
AGA Designation
Argon
I1
Argon
Helium
I2
Helium
Argon + 300ppm NO
Z
MISON Ar
Ar + 30%He + 300ppm NO
Z
MISON He30
Ar + 50% He
I3
VARIGON He50
Ar + 70% He
I3
VARIGON He70
Figure 9. Shielding gases for aluminium welding.
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There are five ways of reducing the ozone concentrations
in the welding working environment:
1. Lower the ozone producing UV intensity
(130-240mm)
2. Increase the ozone reducing UV intensity
(230-280mm)
3. Increase, or add the amount of catalytic dust
(i.e. increase the amount of welding fumes)
Of these alternatives only 1 and 5 are practically possible.
The first alternative is to lower the ozone producing
intensity through using helium. The wavelength in which
ozone is produced is then somewhat changed. This means
that less ozone is created in the remote zone. The ozone
close to the welder is not removed. The second alternative,
using a shielding gas that reacts with ozone, is a method
AGA has chosen. It includes adding nitric oxide, NO,
(275ppm± 25ppm which equals max 300ppm or 0.03%).
The NO dissociates the ozone molecules into oxygen
nitrogen dioxide, NO2.
4. Increase the thermal decomposition
5. Add a shielding gas that reacts with ozone
Figure 10. The relative amount of ozone formed during
MIG welding of different alloys.
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WELD DEFECT
Weld defects
Defective welds are those that contain discontinuities
serious enough to affect the weld strength or corrosion
resistance. The defects are the results of incorrect metal
preparation, welding procedures or techniques. Common
types include cracks (longitudinal, transversal -not so
common - or crater cracks), excessive porosity, incomplete
fusion, undercuts and inadequate penetration. Incorrect
weld size and shape are also considered as weld defects.
Solidification cracking
Solidification cracking is the result of high thermal
expansion combined with a brittle alloy structure at,
and just above, the solidification temperature. The
metallurgical weakness may result from the wrong filler
alloy, too little filler metal in the weld, too small weld
for the base material thickness or too low welding
speed. If the correct filler metal is selected which
increases the plasticity in the critical temperature range,
the cracking can be avoided. Another way to reduce
solidification cracking is to reduce transverse stress or
increase the amount of edge preparation, or sometimes
both. The 6XXX alloys are particularly sensitive to
solidification cracking.
Incomplete fusion
Incomplete fusion is perhaps the most serious of the
different defects, since it is difficult to detect, and
weakens the joint considerably. It is the result of weld
metal failing to coalesce with the base metal or with
other weld metal. Incomplete fusion may result from
insufficient current, insufficient edge preparation, too
long arc or attempting to weld over oxidised surfaces.
The latter is avoided by cleaning off the oxide properly
before welding.
Method
Radiography
Incomplete
Pores Cracks
fusion
X
X
Penetrant
Ultrsonic
Eddy current
Porosity
Porosity causes much concern despite the fact that, unless
it is severe or aligned, it usually has less effect on weld
strength than other defects. It is rather easily detected
through standard radiography and thus has become a
highly regulated defect. Porosity is caused by hydrogen
gas trapped in the metal as itcools. The sources of
hydrogen are many, such as moisture and dirt (oil and
grease). To control porosity, it is essential to eliminate
these contaminants by correct metal preparation and
control of the welding procedure. Welding procedure is
important; the longer the weld remains fluid, the greater
is the opportunity for the hydrogen to escape. For this
reason, TIG welds usually have less porosity than MIG
welds. The shielding gas, regardless of composition,
should therefore have a purity of at least 99.95% with the
lowest possible moisture and hydrogen content.
Inclusions
Inclusions in aluminium are usually metallic. The most
common is tungsten, transferred through the arc when
TIG welding. Nitrogen can also be a problem because it
readily forms nitrides with aluminium which reduce the
mechanical properties.
Detection
There are several methods of detecting weld defects in
aluminium. Radiographic, penetrant, ultrasonic or eddy
current are all nondestructive (NDT) detection methods
that are readily used on aluminium. Figure 11 shows
what methods are suitable for detecting different types of
defects in aluminium.
Ultrasonic testing is the most effective and used
testing method, but it is important to realise that visual
inspecting is by far the easiest and most inexpensive
method. Frequent visual inspection during welding
can often detect faults early enough to allow corrective
action before a weld is welded over, and thus minimise
repair welding at a later stage.
Incomplete
penetration Inclusion
X
X
X
X
X
X
X
X
Figure 11. NDT detection methods and what they can detect.
15
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Getting ahead through innovation
With its innovative concepts, AGA is playing a pioneering role in the global market. As a technology leader, our task is to
constantly raise the bar. Traditionally driven by entrepreneurship, we are working steadily on new high-quality products
and innovative processes.
AGA offers more. We create added value, clearly discernible competitive advantages and greater profitability. Each concept
is tailored specifically to meet our customers’ requirements – offering standardized as well as customised solutions. This
applies to all industries and all companies regardless of their size.
Sweden | AGA Gas AB | 08-706 96 50 | www.aga.se
Finland | Oy AGA Ab | 010-2421 | www.aga.fi
Norway | AGA AS | 23 17 72 00 | www.aga.no
Denmark | AGA A/S | 32 83 66 00 | www.aga.dk
Iceland | ISAGA ehf. | 577 3000 | www.aga.is
Estonia | AS Eesti AGA | 6504 500 | www.aga.ee
Latvia | AGA SIA | 70 23 900 | www.aga.lv
Lithuania | AGA UAB | 27 87 788 | www.aga.lt
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AGA – ideas become solutions
01/09/08 13:32:06