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Aluminium alloys welding processes: Challenges, joint types and process
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DOI: 10.1177/0954405413484015
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Proceedings of the Institution of Mechanical
Engineers, Part B: Journal of Engineering
Manufacture
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Aluminium alloys welding processes: Challenges, joint types and process selection
Muyiwa Olabode, Paul Kah and Jukka Martikainen
Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 2013 227: 1129
originally published online 23 May 2013
DOI: 10.1177/0954405413484015
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Review Article
Aluminium alloys welding processes:
Challenges, joint types and process
selection
Proc IMechE Part B:
J Engineering Manufacture
227(8) 1129–1137
Ó IMechE 2013
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DOI: 10.1177/0954405413484015
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Muyiwa Olabode, Paul Kah and Jukka Martikainen
Abstract
Aluminium and its alloys have gained increasing importance in structural engineering due to advantageous properties
such as light weight, ease of machining and corrosion resistance. This article presents surface-related challenges facing
aluminium welding, specifically weld process limitations and joint limitations. The methodological approach is a critical
review of published literature and results based on eight industrial welding processes for aluminium and six joint types. It
is shown that challenges such as heat input control, hot cracking, porosity and weldable thickness vary with the process
used and that there is no optimal general weld process for all aluminium alloys and thicknesses. A selection table is presented to assist in selection of the optimal process for specific applications. This study illustrates that knowledge of weld
limitations is valuable in selection of appropriate weld processes.
Keywords
Aluminium alloys, aluminium oxide, shielding gases, anodising, aluminium welding process selection
Date received: 17 September 2012; accepted: 4 March 2013
Introduction
Aluminium and its alloys are widely used in welding
industries due to economic advantages such as light
weight, good corrosion resistance, high toughness,
extreme temperature capabilities and easy recyclability.1 Aluminium alloys are used for construction of airplanes, cars, rail coaches and marine transports.
Aluminium alloys are used in manufacture of tanks
and pressure vessels because of their high specific
strength, good heat conductivity and beneficial properties at low temperatures.2 Aluminium is the second
most used metal after iron and steel in the industry; for
example, aluminium is the second most used material
taking about 15% of total body weight of average cars
and about 34% in Audi A2.3 There are comprehensive
reviews on the uses and applications of aluminium and
its alloys.4,5 Welding is a means of joining metals by
creating coalescence due to heat. The work piece is
melted at the joint point (weld pool) that solidifies on
cooling. Welding of aluminium alloys is important for
fabricating structural constructions and mechanical
fabrications like aircrafts. However, welding has problems and can be challenging. Welding defects common
to aluminium include porosity, hot cracking, incomplete fusion and so on.2,6
Researches7,8 have shown that welding aluminium
demands greater caution compared with steel, particularly as regards the amount of heat input and pre-weld
cleaning, and that acceptable weld processes for aluminium joints are limited because the weldable thickness
varies considerably with the different welding processes.
It is therefore of interest to study the limitations facing
aluminium welding, particularly joint- and processspecific limitations.
The aim of this article is to present a comprehensive
guide to understanding aluminium-welding challenges.
In the field of aluminium welding, there are eight
industrially common welding processes and six basic
joint types that have been analysed. For comparison
purposes, a table is designed that shows the influence of
joint and process limitations on optimum welding process selection. The remainder of this article is divided
into two main parts, which are surface-related welding
challenges and joint types and process limitations.
Lappeenranta University of Technology, Lappeenranta, Finland
Corresponding author:
Muyiwa Olabode, Lappeenranta University of Technology, Skinnarilankatu
34, 53850 Lappeenranta, Finland.
Email: muyiwa.olabode@lut.fi
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1130
Proc IMechE Part B: J Engineering Manufacture 227(8)
Evaluation of the findings shows that there is no singular optimum process for welding aluminium.
However, understanding of the limitations of individual welding processes helps in selection of the optimal
process for specific aluminium weld applications.
Surface-related welding considerations
A clean, smooth and protected surface is important in
pre-weld aluminium structures to ensure good aluminium weldments except in high energy density welding
processes like hybrid laser beam welding (LBW) (using
pulsed metal inert gas (MIG)).9 It is therefore important to understand different surface-related phenomena
and their effect on the weldability of the work piece. In
addition, knowledge of preventative measures ensuring
the attainment of acceptable welds, despite any adverse
surface effects, is also important.
Presence of aluminium oxide surface
Oxide formation in aluminium occurs due to the strong
chemical affinity of aluminium for oxygen on exposure
to air. The aluminium oxide thickness increases as a
result of thermal treatment, moist storage conditions
and electrochemical treatment (anodising).10–14 It is
also important to note that Al2O3 melts at about
2050 °C, while aluminium alloys melts at about 660 °C9
(as illustrated in Figure 1). Therefore, the layer is
removed by pickling or dry machining just before weld.
However, the difference in melting point is not a problem during the processing by means of high energy density welding processes; it can also be an advantage, for
example, the presence of oxide layer during laser welding increases the absorptivity of aluminium and its
alloys to laser radiation.15,16 It should be noted, that a
main challenge in applying most joining technologies to
aluminium is its tendency to form a thick, coherent
oxide layer. This oxide layer has a melting temperature
much higher than that of aluminium itself; moreover, it
has a significant mechanical strength. Therefore, this
oxide layer can remain as a solid film (or fractured in
small particles) due to the flow of the molten material,16
even when the surrounding metal is molten. This can
result in severe incomplete fusion defects. Therefore,
the removal of the oxide layer just before welding is
important.
The aluminium oxide layer is, furthermore, an electrical insulator, and the layer may sometimes be thick
enough to prevent arc initiation. In MIG processes, a
thick oxide layer can produce erratic electrical commutation in the gun’s contact tube, resulting in poor
welds.
It is thus evident that aluminium oxide has to be
removed before welding because it compromises the
quality of the weld. Generally, the oxide removal can
be done by mechanical processes like brushing with a
stainless steel brush, cutting with a saw or grinding with
semi-flexible aluminium oxide grinding discs.9 Some
welding processes enhance additional oxide removal
processes, for example, in ultrasound metal welding
processes (UW), oxides and contaminates are removed
by high-frequency motion, thus providing metal–metal
contact and allowing for the work pieces to bond properly.17 In hybrid laser MIG-welding of aluminium
alloys, the MIG-welding process has a cleaning effect
that removes the aluminium oxide layer. However, it is
recommended that pre-weld cleaning of the weld surface should be carried out preferably by pickling or dry
machining.18 In gas-shielded arc welding, aluminium
oxide removal from the weld pool can be done by cathode etching (which is controlled chemical surface corrosion done to reveal the details of the microstructure).19
A direct current passes through the electrode connected
to the positive pole of the power source. There is thus a
flow of electrons from the work piece to the electrode
and the ions flow in the opposite direction, bombarding
the work piece surface. The aluminium oxide film is
broken and dispersed by the ion bombardment, thereby
allowing the flowing weld metal to fuse with the parent
metal. It is advantageous to remove the aluminium
oxide layer before welding because2,9
1.
2.
3.
It significantly reduces the amount of hydrogen
porosity in the weld.
It helps to improve the stability of the weld process
especially in tungsten inert gas welding (TIG).
It allows for complete fusion of the weld. Cathode
cleaning is important in TIG process as the oxide
starts to form immediately after wire brushing.
Figure 1. Schematic of aluminium showing its oxide layer and the anodised surface.
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Olabode et al.
1131
Table 1. Chemical treatments for cleaning and oxide removal.9
Solution
Concentration
Temp (°C)
Procedure
Container material
Purpose
Nitric acid
50% water
50% HNO3
(technical
grade)
5% NaOH in
water
Concentrated
HNO3
18–24
Stainless steel
Removal of thin oxide
film for fusion welding
Mild steel
Stainless steel
Removal of thick oxide
film for all welding and
brazing operations
Sulphuric chromic
acid
5 L H2SO4
1.4 kg CrO3
40 L water
70–80
Antimonial
lead lined steel
tank
Removal of films and
stains from heat treating
and oxide coatings
Phosphoric chromic
acid
1.98 L of 75%
H3PO3
0.65 kg of CrO3
45 L of water
95
Immerse 15 min
Rinse in cold water
Rinse in hot water
Dry
Immerse for 10–60 s
Rinse in cold water
Immerse 30 s
Rinse in cold water
Rinse in hot water
Dry
Dip for 2–3 min
Rinse in cold water
Rinse in hot water
Dry
Dip for 5–10 min
Rinse in cold water
Rinse in hot water
Dry
Stainless steel
Removal of anodic
coatings
Sodium hydroxide
followed
by nitric acid
70
18–24
Aluminium oxide can also be removed by chemical
etching or pickling. Table 1 presents chemical treatments for oxide layer removal.9
One of the causes of the oxide layer is from anodisation, which is an electrochemical process by which a
metal surface is converted into a decorative, durable,
corrosion resistant anodic oxide finish.20,21 Anodisation
utilises the unique ability of amorphous alumina to build
up an even porous morphology22 formed in alkaline and
acidic electrolytes. During anodising, aluminium oxide is
not applied like paint or plating. Rather, it is integrated
fully with the underlying aluminium substrate.
Therefore, it cannot peel or chip off. The anodic oxide
structure is highly ordered and porous, thereby allowing
for further processing like sealing and colouring.20
The reasons for the utilisation of anodisation are to
increase corrosion resistance and ensure the metal surface is fade proof for up to 50 years,23 to improve decorative appearance, to increase abrasion resistance and
paint adhesion, to improve adhesive bonding and lubricity, to provide unique decorative colours or electrical
insulation, to permit subsequent plating, to detect surface flaws, to increase emissivity and to permit application of photographic and lithographic emulsions.14,20,24
Anodising of aluminium alloys is generally advantageous. However, it poses challenges for aluminium
welding because the arc cleaning effect of the AC current cannot remove the double layer (the anodised layer
and oxide layer as in Figure 1). Before welding, the
anodised surface needs to be removed.20
Shielding gas selection
Shielding gas protects the molten weld pool from the
atmosphere, which is important because aluminium has
a tendency to react with atmospheric air to form oxide
and nitrides. The shielding gases commonly used in
welding aluminium and its alloys are inert gases such
as argon and helium.
Argon is used as a shielding gas for manual and
automatic welding. Argon is cheaper than helium, and
the use of argon produces a more stable arc and
smoother welds. However, argon gives lower heat input
and lower attainable welding speed, and therefore there
is the possibility of a lack of fusion and porosity in
thick sections. In addition, use of argon can result in a
black sooty deposit on weld surfaces, although this can
be wire brushed away. It has been observed that with
helium shielding gas, the arc voltage is increased by
20%, resulting in a higher, hotter arc, deeper penetration and wider weld beads. This implies that the criticality of arc positioning (aids avoidance of missed edge
and insufficient penetration defects) is lower with
helium. There is a reduction in the level of porosity
when helium shielding gas is used because the weld
pool is hotter and there is slower cooling, which allows
hydrogen to diffuse from the weld pool. Due to the
higher heat produced, the use of helium allows that
welding speeds up to three times higher than with
argon. The high cost of helium and the inherent arc
instability mean, however, that helium is used mainly
in mechanised and automatic welding processes.9
It is common practice to use a mixture of helium and
argon as it provides a compromise on the advantages of
each gas. Common combinations are 50% or 75% of
helium in argon, which allow for better productivity by
increasing the welding speed and provide a wider tolerance for acceptable welds. The purity of the shielding
gas is of importance. At the torch, not at the cylinder
regulator, a minimum purity requirement of 99.998%
and low moisture levels of less than 250 °C (less than
39 parts per million (ppm) H2O) are expected.9
Generally, the shielding gas should be selected with the
following considerations.2,9,25,26
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1132
Proc IMechE Part B: J Engineering Manufacture 227(8)
Table 2. MIG shielding gases for aluminium.26
Metal transfer mode
Shielding gas
Characteristics
Spray transfer
100% Argon
35% Argon–65% Helium
Best metal transfer and arc stability, least spatter, and good cleaning action.
Higher heat input than 100% argon; improved fusion characteristics on thicker
material; minimises porosity.
Highest heat input; minimises porosity; least cleaning action
Argon satisfactory on sheet metal; argon–helium preferred for thicker base
material.
Short circuiting
25% Argon–75% Helium
Argon or Argon + Helium
Table 3. Effect of shielding gas on aluminium welding.9,29–34
Shielding gas
Relative effect (100% argon as the reference)
100% Ar
Ar + He
100% He
Gas flow
Arc voltage (MIG)
Arc (MIG)
Weld seam width and depth
Nominal
Nominal
Nominal stability
Nominal width and depth
Weld seam appearance
Penetration
Welding speed
Lack of fusion
Porosity
Pre-heating
Heat production
Cost of shielding gas
Nominal smoothness
Nominal depth and roundness
Nominal welding speed
Nominal
Nominal
Nominal
Nominal warmth
Nominal price
Higher
Higher
More unstable
Higher width
Shorter depth
Smoother
Deeper and more round
Higher attainability
Lower
Lower
Less needed
Warmer work piece
More expensive
Highest
Highest
Most unstable
Highest width
Shortest depth
Smoothest
Deepest and most round
Highest attainability
Lowest
Lowest
Least needed
Warmest work piece
Most expensive
MIG: metal inert gas welding.
1.
2.
3.
4.
5.
6.
7.
8.
The gas must be able to generate plasma and a stable arc mechanism and characteristics.
It should provide smooth detachment of molten
metal from the wire and fulfil the desired mode of
metal transfer.
It should protect the welding head (in the arc’s
immediate vicinity), molten pool and wire tip from
oxidation.
It should help to attain good penetration and good
weld bead profile.
It should not affect the welding speed of the
process.
It should prevent undercutting tendencies.
It should limit the need for post-weld cleaning.
It should not be detrimental to the weld metal
mechanical properties.
The recommended shielding gas for welding aluminium using pulsed MIG is argon (99.998%)25,27 at a
flow rate of about 20 L/min.27 A mixture of argon and
helium can also be used and even helium alone. Helium
increases the weld penetration and offers higher arc
energy and thus increased deposition rate,27,28 and it
should be used when the section is greater than 50 mm.9
More details can be seen in Table 2, which presents
MIG shielding gases for aluminium, and Table 3 presents the effects of shielding gases on aluminium welding. Studies have shown that welding of aluminium can
be improved (arc stability) by oxygen doping of inert
shielding gas.29 In addition, the alternating shielding
gases reduces weld porosity.30–32
Joint types and process limitations
This article considers eight industrially accepted welding
processes and six joint types. Joint design is important
because it costs money to buy weld metal. The fillet throat,
weld accessibility and the functionality of the welded work
piece are taken into consideration in this design. The six
joints considered are butt, T-joint, corner, cruciform, edge
and lap joint (see Table 4), which are derived from the
three basic welding joints (fillet, lap and butt joints). Joint
designs are based on the strength requirements, the alloys
to be joined, the thickness of the material, the joint type
and location, weld accessibility and the welding process.
Before choosing the joint design, it is important to note
that welding in the flat or downward position is preferable
in all arc-welding processes, as there is the easier possibility
of depositing high-quality weld metal at a high deposition
rate in a flat position. Additionally, the weld pool is larger,
allowing for a slower cooling and solidification rate, which
enhances the escape of trapped gases in the weld pool. The
flat position reduces weld porosity, reduces weld cost, and
gives the best weld metal quality compared with other
positions. The static tensile strength of the weld is determined by the throat thickness, which must be designed to
ensure that it can carry the workload for which the weld is
designed. Conventional TIG and MIG processes produce
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Limited to thin gauges of
up to 6 mm thickness.
Limited (shallower)
penetration into parent
metal compared to MIG.
With argon, weldable
thickness is limited to
25 mm, and with helium,
it is limited to 75 mm.
Limited operator
acceptability of the
process because of the
relatively high levels of
radiated heat and arc
intensity.
Limited outdoor
application because air
drafts can disperse the
shielding gas.
Limited by the lower
deposition rate, low
tolerance on filler and
base metal, and cost for
thick sections compared
to MIG.
Difficult to penetrate
into corners and into the
roots of fillet welds.
With argon shielding gas,
the economical weld
thickness limit is 10–
18 mm with helium
(DCEN).
Limitation
Limitation
Limited torch distance of
10–19 mm to ensure
properly shielded weld
metal limits flexibility.
TIG
MIG
Limited operator
acceptability of the
process due to the
complex torch
architecture that
requires more
maintenance and
accurate set-back of the
electrode tip with
respect to the nozzle
orifice, which is
challenging.
Limited tolerance of the
process to joint gaps and
misalignment.
Environmentally friendly
welding process because
fumes and spatters are
not generated.
Large down forces
required with heavy duty
clamping necessary to
hold the plates together
during welding.
Exit hole left when tool
is withdrawn.
Insufficient design
guidelines and limited
education for
implementation.
Limited to lower
productivity cases
compared to LBW.
Tool design, process
parameters, and
mechanical properties
database is limited and
only available for limited
alloys and thicknesses
(up to 70 mm).
Plasma TIG weld
thicknesses range can be
less than 2.5–16 mm in a
single pass.
Limited by the high
capital equipment and
material cost compared
to TIG.
Weldable thickness
ranges from 1–50 mm
(single pass).
Limitation
FSW
Plasma MIG weld
thicknesses limited to 6–
60 mm range.
Limitation
PAW
Limited operator
acceptability of the
process due to the large
capital investment
needed, therefore
requiring high volume
production or critical
applications to justify the
expenditure.
Limited fit up tolerance.
Precise fit up (15% of
material thickness)
needed for butt and lap
joints.
Limited conversion
efficiency of electrical
power to focused
infrared laser beam also
called wall plug efficiency
(about 10%–30% and up
to 40% in fibre lasers).
Limitation
LBW
In addition, it requires
access to both sides of
the joint.
Limited operator
acceptability of the
process because, in
thick-sectioned upset
welds; there is lack of
good non-destructive
weld quality testing high
electrode wear rate and
deterioration.
Limited joint designs or
configuration. Seam
welds can generate
unzipping effect.
Lower tensile and fatigue
strength compared to
other fusion welding
processes.
Limited weld thickness
range (0.9–3.2 mm)
Limitation
RW
Can weld up to
450 mm thick plates.
Rapid solidification
rates can cause
cracking in some
materials.
X-rays produced
during welding can be a
health risk.
High weld preparation
costs.
Time delay when
welding in a vacuum.
Work chamber size
constraints.
High cost of
equipment.
Limitation
EBW
Vibration control
strategies are needed
to ensure weld quality
across a wide range of
component geometries
and the thickness of
the weld piece is
limited.
Alternative welding
configurations are
needed to weld a wide
variety of component
geometries and joint
configurations.
Expensive high
powered transducers
are needed to enable
welding of thick gauges,
castings, extrusions,
and hydro-formed
components.
Limitation
UW
DCEN: direct current electrode negative; EBW: electron beam welding; FSW: friction stir welding; LBW: laser beam welding; MIG: metal inert gas welding; PAW: plasma arc welding; RW: resistance welding; TIG, tungsten inert
gas welding; UW: ultrasonic welding.
(f)
(e)
(d)
(c)
(b)
(a)
Butt joint (a)
Lap joint (b)
T-joint (c)
Edge joint (d)
Corner joint (e)
Cruciform (f)
Joints
Processes
Table 4. Joint types and process limitations of aluminium alloys.2,8,9,17,18,35–52
Olabode et al.
1133
1134
Proc IMechE Part B: J Engineering Manufacture 227(8)
Table 5. Weld process selection (the highest factor summation is the best of the processes considered).
Selection factors
Process A (TIG)
Process B (FSW)
Process C (PAW)
Process D (MIG)
Quality of the welded joint
Strength
Elongation
Chemical stability
Weld defects
Penetration
Distortion
Imp.
3
2
2
2
1
1
Ad.
2
2
2
3
3
1
I. Fac.
6
4
4
6
3
1
Imp.
3
2
2
2
1
1
Ad.
2
3
3
1
3
2
I. Fac.
6
6
6
2
3
2
Imp.
–
2
2
2
1
1
Ad.
–
2
3
1
3
2
I. Fac.
–
4
6
2
3
2
Imp.
3
2
2
2
1
1
Ad.
2
3
3
1
3
2
I. Fac.
6
6
6
2
3
2
Suitability for use
Welding thin sheet (\1 mm)
Sheet welding (.3 mm)
Welding Al-Mg alloys
Overhead welding
Variable material thickness
Variable welding speed
Welding of castings
Joining cast to wrought alloys
Repair welds on castings
2
1
1
1
2
1
2
1
2
2
1
2
1
1
1
2
3
3
4
1
2
1
2
1
4
3
6
2
1
1
1
2
1
2
1
2
3
2
2
3
1
2
3
3
3
6
2
2
3
2
2
6
3
6
2
1
1
–
2
1
2
1
2
3
2
2
–
2
3
2
1
1
6
2
2
–
4
3
4
1
2
2
1
1
–
2
1
2
1
2
2
3
2
–
2
2
2
2
2
4
3
2
–
4
2
4
2
4
Suitability for automation
With filler
Without filler
Butt welding \3 mm
.3 mm
1
2
2
1
1
3
1
2
1
6
2
2
1
2
2
1
3
1
2
1
3
2
4
1
1
2
2
1
2
1
2
3
2
2
4
3
1
2
2
1
3
1
2
1
3
2
4
1
Suitability for joint type
Butt joint
Lap joint
1
1
2
3
2
3
1
1
1
3
1
3
1
1
1
1
1
1
–
1
–
1
–
1
3
3
1
1
2
2
3
1
6
6
3
1
3
3
1
1
3
2
2
1
9
6
2
1
3
3
1
1
2
3
3
1
6
9
3
1
3
3
1
1
1
2
3
3
3
6
3
3
Economic aspects
Equipment costs
Maintenance costs
Labour costs
Welder’s training time
P
Process rating ( )
80
89
73
76
Imp.: importance level; Ad.: advantage level; I. Fac.: impact factor; EBW: electron beam welding; FSW: friction stir welding; LBW: laser beam welding;
MIG: metal inert gas welding; PAW: plasma arc welding; RW: resistance welding; TIG, tungsten inert gas welding; UW: ultrasonic welding.
weld metal on the surface of a plate during bead-on-plate
welds to a depth of 3 mm for TIG and 6 mm for MIG.
Therefore, to attain complete penetration for welds over
3 mm (MIG) and 6 mm (TIG), there is the need for bevelling on butt joints, for example. The bevel can be single or
double sided.9
As presented in Table 4, eight considered welding
processes are correlated with their applicability on six
different welding joints. Butt and lap joints are applicable to all the selected weld processes. Cruciform joints
have the least applicability across the processes, which
is due to limited fixturing possibility during welding.
Table 4 provides additional information on the viability of six joint types on the eighth selected welding processes by presenting the process-specific limitations.
An application of this review article is to use the prestated information to influence the selection casespecific optimum welding process. It can be challenging
to determine an appropriate welding process to be used
for aluminium. However, the challenge can be simplified by considering various comparison selection factors
as presented in Table 5. The solution to the challenge is
case specific. An understanding of the selection factors
considered provides better process selection and thus a
better evaluation.
It is important to point out that the scaling is subject
to the designer’s discretion and not completely objective.
The welding designer determines the importance level of
the selected aluminium-welding project by answering a
question like ‘how important is’ strength, elongation,
chemical stability, etc., to the finished product. The
designer defines the importance level on a scale of 1–3
(1= least, 2 = moderate and 3 = high). In a similar
fashion, the advantageous level is determined by answering a question like ‘how advantageous is’ the selected
welding process to the selected consideration. The
importance level is multiplied by the advantage level
and the result is called an impact factor. The impact factor is summed up for each selected welding process, and
the welding process with the highest impact factor summation is selected as the optimal welding process.
Case study
A welding process for high-strength aluminium for aerospace is to be selected. The available welding processes
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Olabode et al.
1135
are as presented in Table 5. A blank table is constructed
and the considered welding processes are selected and
filled into the table.
The selection factors under consideration are as presented in Table 5, which are categorised under quality
of the weld joint, suitability for use, suitability of fillers,
joint suitability and economics. Therefore, at this stage
in the design, the processes row and the selection factor
column are filled in the table.
As the designer, the importance level is determined
and designed on a scale of 1–3, and using a scale of five
is also applicable, but the calculation becomes more
complex. Choosing a scale of 1–3 (1 = low, 2 = moderate and 3 = high), a number is assigned to the considered selection factor. Therefore, at this stage, the
importance level of the selection factor under consideration is filled into the ‘Imp.’ column (Table 5). It is
important to note that the number is the same across
row (all processes) because the importance of a selection factor is independent of the process.
The advantage level is determined and designed by
the designer on the same scaling used for importance
level. If the scaling used in importance level is five, the
scaling of five should be used. In this case, a scaling of
1–3 is used where 1 = low, 2 = moderate and 3 = high.
At this stage, the entire advantageous level column on
Table 5 is filled for all the considered selection factors
into the ‘Ad.’ column.
The calculation for the impact factor and the process
rating is carried out. The impact factor for each considered selection factor is derived by multiplying the
importance level column of each process by advantageous level column of each process. The derived value
is filled into the ‘I. Fac.’ (impact factor) column of
Table 5. The process rating (welding process) is derived
by the summation of all the impact values column of
each process. Therefore, the process rating row is filled
in Table 5.
The optimum weld process is the process with the
highest process rating, which in this case study is process C friction stir welding (FSW).
detrimental when welding anodised aluminium as the
anodised layer has to be cleaned before welding. The
melting point of aluminium alloys is generally around
660 °C and the melting point of aluminium oxide is
2050 °C. It is therefore recommended that the aluminium
oxide layer or the anodised layer be removed, mechanically or chemically, just before welding.
Aluminium alloys have high chemical affinity; therefore only inert gases can be used as shielding gases during welding. Argon and helium gases are used in
aluminium welding to protect the weld pool. The presence of helium increases the arc heat input and therefore allows for deeper penetration compared with
argon gas, but on the other hand, helium is more
expensive than argon. A mixture of helium and argon
is sometimes used to improve weldability of some aluminium alloys. A wider range of shielding gases would
increase the manipulation possibility for aluminium
alloy welding, but currently argon and helium are the
only gases used.
The industrial welding processes considered in this
work include MIG, TIG, plasma arc welding (PAW),
FSW, LBW, resistance welding (RW), electron beam
welding (EBW) and UW. The weldable thickness is a
limitation in all the processes; the highest weldable
thickness of up to 70 mm is achieved with EBW. FSW
produces the best weld because the mechanical property
deterioration is minimal, and the process is friendly as
no fumes or spatters are produced during welding.
The joint configurations considered include the butt
joint, lap joint, T-joint, edge joint, corner joints and
cruciform joint. The butt joint and lap joint are applicable to all the considered welding processes. The possibility of using different joint orientations with the
considered welding processes depends on the manipulation of the work piece (fixturing).
Although FSW produces the best weld for aluminium alloys, the optimal welding process is case specific. The designed table for weld process selection
provides information on how to select the optimal
process based on case-specific considerations for aluminium alloys.
Conclusion
This article examined the surface-related challenges,
joint types and limitations of aluminium alloys with the
focus on providing a guide on how to select an optimal
welding process. Aluminium and its alloys have welding
challenges, which include the presence of aluminium
oxide on surfaces, welding of anodised aluminium and
limited shielding gas options. The aluminium oxide surface is formed when aluminium is exposed to an atmosphere containing oxygen, and the aluminium oxide has
to be cleaned away from the surface before welding
because its causes weld defects like porosity.
The chemical affinity of aluminium for oxygen is utilised for anodising aluminium alloys and then painting
to improve corrosion resistance. However, it can be
Declaration of conflicting interests
The authors declare that there are no conflicts of
interest.
Funding
This research received no specific grant from any funding agency in the public, commercial or not-for-profit
sectors.
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Appendix 1
Notation
EBW
FSW
LBW
MIG
PAW
ppm
RW
TIG
UW
electron beam welding
friction stir welding
laser beam welding
metal inert gas welding
plasma arc welding
parts per million
resistance welding
tungsten inert gas welding
ultrasonic welding
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