Automation of Aluminum Alloy Welding

Abstract

Aluminum and its alloys, due to the characteristic features of aluminum like low density, good strength, ductility, corrosion resistance, and high thermal and electrical conductivities, are being used in a wide number of industrial applications, and the welding processes for aluminum are being automated for high-scale manufacturing. However there are some difficulties associated with the welding of aluminum for different welding processes. This work aims to describe and compare various methods for the automated fusion welding processes of aluminum alloys. It is almost exclusively made by reviewing several research articles and literature and discusses about the fusion welding process for aluminum alloys, its benefits, drawbacks, and usual applications. The most usual defects generated by these processes are also the target of this analysis. Two comparison tables are also presented and commented on, one comparing the features of each type of welding process and the other on comparing quantitative parameters, like the welding speed and weld type, as well as the quality of the weld, from various real world applications. It was found that the automated welding of aluminum is a well-established subject and that solutions for most industrial automated aluminum welding needs can be further explored.

Full text

International Review of Mechanical Engineering (I.RE.M.E.), Vol. 8, N. 1
ISSN 1970 - 8734 January 2014
Manuscript received and revised December 2013, accepted January 2014 Copyright © 2014 Praise Worthy Prize S.r.l. - All rights reserved
145
Automation of Aluminum Alloy Welding
P. Kah
Abstract – Aluminum and its alloys, due to the characteristic features of aluminum like low
density, good strength, ductility, corrosion resistance, and high thermal and electrical
conductivities, are being used in a wide number of industrial applications, and the welding
processes for aluminum are being automated for high-scale manufacturing. However there are
some difficulties associated with the welding of aluminum for different welding processes. This
work aims to describe and compare various methods for the automated fusion welding processes
of aluminum alloys. It is almost exclusively made by reviewing several research articles and
literature and discusses about the fusion welding process for aluminum alloys, its benefits,
drawbacks, and usual applications. The most usual defects generated by these processes are also
the target of this analysis. Two comparison tables are also presented and commented on, one
comparing the features of each type of welding process and the other on comparing quantitative
parameters, like the welding speed and weld type, as well as the quality of the weld, from various
real world applications. It was found that the automated welding of aluminum is a well-
established subject and that solutions for most industrial automated aluminum welding needs can
be further explored. Copyright © 2014 Praise Worthy Prize S.r.l. - All rights reserved.
Keywords: Aluminum, Automation, Sensor, Hybrid Laser Arc Welding
I. Introduction
Aluminum alloys are characterized by their high
resistance to corrosion, high thermal and electrical
conductivities, good strength, ductility, and low density.
These features ensure their high suitability for a
variety of industrial applications especially in the sectors
like aerospace and automobile where the strength-to-
weight ratio plays a critical role for weight reduction and
fuel economy.
Welding is used to join aluminum alloys in a wide
number of industries but several welding problems incur
when joining aluminum alloy sections that have to
possess high mechanical properties. These are due to the
chemical and structural composition of the precipitation
hardened alloys which can cause a considerable drop in
the mechanical properties in the welding zone and also
have the possibility of hot cracking in high-strength
aluminum alloys with a high content of alloying
elements.
Of particular note is its affinity for oxygen: aluminum
typically has a surface layer of aluminum oxide that
restricts the flow of molten metal in the weld pool during
welding and leads to the formation of very poor welds
containing oxide inclusions.
Another difficulty is that aluminum absorbs a high
quantity of hydrogen in the molten state generating
hydrogen porosities when the weld pool solidifies [1],
[2].
Automated welding is widely considered by industries
as a tool to enhance productivity, weld quality, and in
some cases lower the manufacturing costs.
Automatic aluminum welding can be achieved by
using different processes like tungsten inert gas (TIG)
welding, metal inert gas (MIG) welding, and other
common welding technologies. Welding processes
moving at a high speed with a too short dwell time (less
than 0.3 s), which varies with the intensity of the heat
source, must be automated and cannot be controlled
manually as shown in Fig. 1.
Fig. 1. Maximum weld travel velocity, heat source spot size, and
interaction time with respect to the intensity of the heat source [3]

P. Kah
Copyright © 2014 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 8, N. 1
146
The welding processes with a heat source more
intense than in arc welding (i.e. electron beam, laser
beam welding etc.) have shorter dwell times and are
automated [3]. The ability of a robot controller to ramp
up and down currents and voltages plays a critical role in
welding aluminum. In conjunction with robotic
manipulators controlling welding torches, there might be
a need for real time sensing technology of the weld pool
and seam to intelligently adapt the welding parameters
and torch position for an automated welding system.
The automated welding systems referred to in this
work are composed of the welding torch, a power source,
and a robotic manipulator or other type of positioning
system for the torch. The torch may also have vision
sensing hardware for parameters and position control
attached to it. This paper conducts a comparison of the
productivity and quality of these automated processes,
making use of several sources and examples.
II. Automated Technologies for Welding
of Al-Alloys
II.1. TIG
Automation of TIG welding for aluminum alloys is
mostly used with alternate current (AC-TIG) because of
the lack of oxide layer removal with DC electrode
negative (DCEN) [4]. Compared to the conventional
continuous current, pulsed current is beneficial in TIG
welding [5]. The TIG process parameters such as pulse
current, secondary current, pulse frequency, pulse duty
cycle and % of He in Ar plays a vital role in predicting
weld geometry [6]. The automation in TIG is achieved
by adapting manual techniques integrating a
conventional robotic manipulator. Most welding robots
are “teach and playback” robots, which commonly need
sensing technology and control to surpass mismatch in
welding variables, like errors of pre-machining, fitting of
the work piece, and in-process thermal distortions [7].
Power sources with control capabilities for pre-flow,
post-flow, and high-frequency arc starting are normally
used. The benefits of aluminum TIG welding are the high
weld quality, aesthetic appearance, and the possibility to
weld in all positions. The downsides are the difficulty to
use for small material thicknesses of below 2 mm [8],
slow speed, low deposition rate, and the need for highly
skilled operators. TIG is normally suitable for
applications in which weld integrity and aesthetics are
more important than speed. Some typical aluminum TIG
welding applications are piping systems, chemical and
pressure vessels, gas bottles, thin sheet metal work, and
work requiring intricate torch manipulation [9].
The benefits of automation in TIG welding are its
repeatability, precise heat control, the exactness of
penetration to meet the challenging quality standards,
and reduced operator costs and training time [10].
The drawbacks of TIG mechanization are the need to
provide the welding fixture with more accurate and
consistent weld preparations than required in manual
welding [4].
II.2. MIG
Being a continuously fed wire process, MIG welding
is easily mechanized [4]. When welding aluminum,
special control is needed, normally adapting the welding
parameters for the welding steps: striking an arc, weld
formation, arc end, and burn back. Starting the arc can be
the most difficult step in aluminum MIG welding [11].
Typical MIG automatized welding cells use a DC
pulsed power source. However, AC pulsed MIG welding
is suitable for thin aluminum sections because of its high
melting coefficient, reduced heat input, less penetration
[12], and reduced average temperature of molten droplets
transferred [13], which makes it useful to weld thin
aluminum alloy sheets, for example, in the
manufacturing of car bodies [14]. Pfeifer and Rykala [8]
suggested that for welding of thin-walled (0.5 to 2mm)
aluminum components, a low-energy welding process
(ColdArc, CMT, and STT) with a consumable electrode
in inert gas shield gives the best result characterized by
high aesthetics, as well as very good mechanical and
plastic properties. Good applications for AC pulsed MIG
include those that are prone to problems like a lack of
fusion, warpage, burn-through, spatter, a lack of puddle
control, and poor bead appearance [15]. However, there
might be a problem with shallow penetration in AC
pulsed MIG welding at high speeds. DC pulsed MIG arc
welding is frequently used in joining aluminum alloys
and has higher productivity compared to TIG and greater
convenience in use than laser welding. However, DC
pulsed MIG can bring problems such as burn-through,
the formation of holes in the bead, and low gap
tolerances when used to weld thin aluminum at high
welding speeds (higher than 2 m/min) [14].
Automated MIG welding is more flexible than TIG
welding and is able to weld more complex weld shapes.
Sensor technology may also be needed to provide
correction for component tolerances and fit-up variation,
ensuring that the weld path is optimized for every
individual work [15]. The automation of MIG results in
higher welding speeds and currents which means less
heat input, narrower heat affected zones (HAZ), less
distortion, deeper penetration, less need for large welding
preparations with fewer weld passes, and reduced
porosity apart from those listed in later Table I [4].
The drawbacks of automating the MIG welding of
aluminum are the need for more accurate and consistent
weld preparations, more planning for realizing the full
benefit, and the need for capital expenditure to purchase
manipulators and handling equipment [4].
II.3. Tandem MIG
Tandem MIG welding is an automated process
developed to achieve high welding speed at which
conventional MIG welding leads to a poorer joint quality
and weld pool turbulence due to increased current.

P. Kah
Copyright © 2014 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 8, N. 1
147
This dual wire process uses two electrodes insulated
from each other which allows each one to have different
electrical potentials. The lead arc contributes to the
formation of the fusion shape and control of penetration.
The slave arc controls bead appearance and helps to
stabilize the weld pool formed by the lead heat input
[17]. This process is commonly used with DC pulse
control, having the phases of the master and slave pulse
waveforms 180° shifted [17]. The maximum output of
the process is realized if accurate sensing of the weld
joint and its variables is incorporated [18].
The tandem MIG welding of aluminum alloys
increases process efficiency in terms of higher-speed
welding with increased deposition and better molten
metal availability. It exerts smaller welding thermal
effect, thus conferring advantages in terms of the
suppression of welding defects and improvement of the
mechanical properties of welded joints [17]. Fig. 2 shows
a comparison between single wire and tandem wire MIG
welding with the tandem wire having double the weld
speed of the single wire MIG weld.
II.4. Plasma Arc Welding
Similar to TIG welding, plasma arc welding (PAW) is
suited for automation [19] to weld aluminum in a
variable polarity mode, using the DCEN period to deeply
penetrate the work piece and a DCEP period to remove
the oxide film. Variable polarity PAW consists of three
segments: the start-up segment, in which the keyhole is
generated, the main body segment during which the
keyhole is maintained dynamically, and the terminal
segment, during which the keyhole collapses and the
crater is filled.
Very often, it is difficult to get a smooth transition
from the start-up segment to the main body segment, and
the weld easily fails as the keyhole weld pool is not
properly generated [23]. Precise control of the current
and plasma gas flow rate is needed to maintain arc
stability [21], [22], keyhole and penetration stability [23],
and a difficult to achieve smooth transition from the
start-up segment to the main body segment [24].
PAW has the advantages of higher control and
accuracy, as well as being able to weld at lower current
levels than other arc welding processes.
This process is especially advantageous in the welding
of miniature components such as needles, wires, light
bulb filaments, thermocouples, probes, and some surgical
instruments where the TIG arc start would damage the
part [20]. The tensile strength of welds by PAW is lower
than those made by MIG or TIG. Therefore, PAW can be
used for the welds that do not require high strength, but
high aesthetics is the basic requirement [8]. PAW has a
number of advantages compared with conventional TIG
mainly because of the cylindrical and constricted plasma
column, which provides less sensitivity to process
variables than with the TIG process and a stable,
controllable arc at low currents [4].
II.5. Plasma- MIG Welding
A combination of PAW and MIG within a single torch
[25], plasma–MIG welding utilizes a MIG wire fed
through a plasma–arc torch and provides better control of
the metal and heat transfer to the work piece [26]. This
process allows a higher combined welding current to be
used than for the MIG wire alone with a high current
density and a higher deposition rate than MIG can
achieve [4]. It also brings less welding spatter and fumes,
as well as low porosity with clean and good weld bead
appearance [27], [31]. Plasma–MIG is generally used in
automated applications [4]. This process was suitable and
successfully applied in welding aluminum, such as tank
trailers and tubes to flanges [28]–[30]. Fig. 3(a) shows a
schematic diagram of the plasma–MIG weld system and
Fig. 3(b) shows a general layout of the robotic plasma–
MIG weld system.
II.6. Laser Welding
Aluminum is one of the most difficult materials to
melt with laser [33] due to the poor coupling (absorption
of the beam energy by the metal being welded), high
thermal conductivity, high reflectivity, and low boiling
point [4]. As the wavelength of the laser increases, the
coupling becomes poorer [34]. It was reported that a
Nd:YAG (solid state laser) with a characteristic
wavelength of 1.06 µm provides better coupling with
aluminum than a CO2 laser with a characteristic
wavelength of 10.6 µm [35].
Fig. 2. Comparison of single wire and tandem wire MIG welding of an aluminum fuel tank [18]

P. Kah
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148
Figs. 3. (a). Schematic diagram of plasma-MIG weld system [31], [32], (b) General layout of a robotic plasma-MIG welds system [31]
Porosity, the loss of alloying elements, and
solidification cracking (in the case of heat treatable
aluminum alloys) are the most common problems
encountered in the laser welding of the alloys [36].
For monitoring the welding phenomenon and to
stabilize the automated welding process, vision
technology can be used, and technologies using cameras
are actively being developed to monitor and control the
welding process [37].
The low gap tolerance of laser welding constricts its
use and makes it difficult to weld joints where zero gap is
not easily maintained.
CO2 laser welding has low flexibility due to the use of
parabolic reflectors or transmissive systems for very
concentrated beams with energy densities above 40
kJ/mm2 and is used for butt welds most of the times. Gas
laser welding allows very high welding speeds due to
high power, thus decreasing the microstructural scale and
alloying element loss resulting in stronger weld beads
[38]. In Nd:YAG, a solid state laser welding, the
wavelength of light is ten times lower than in gas laser.
Therefore, solid state laser welding permits a better
coupling of the beam with the parent metal. This also
permits the laser light to be transmitted via fiber optics,
rather than by the use of copper mirrors that are used to
manipulate the light in the CO2 laser; this gives greatly
improved flexibility, allowing the use of a robot [4].
II.7. Hybrid Laser Arc Welding
Hybrid laser arc welding techniques are used for the
synergic effect achieved by coupling two technologically
different welding methods together. The objective is that
the main positive aspects of one involved technology
allow overriding the main drawbacks of the other, and
vice versa. This process is commonly automated and uses
the principles of automation in the arc welding process in
conjunction with laser welding.
MIG as the arc component in hybrid welding results in
the best productivity compared to TIG or the plasma arc
process [8].
One of the main problems found with aluminum laser
welding, i.e. porosity formation, was found to be reduced
in laser–MIG welding and influenced by the MIG
welding process current [39].
Also, the problem of low gap tolerance of the laser
beam is improved with AC pulsed MIG. Laser + AC
MIG has been found to be more suited to weld thin
aluminum sheets, widely used in the automotive industry
at high welding speeds, using the laser to solve the bead
irregularity and shallow penetration problem in AC
pulsed MIG welding at high speeds while maintaining
good gap tolerance [14].
One disadvantage of hybrid welding is the high
number of variables, not only the welding parameters
from both processes but also a number of physical
variables from the mutual position of the laser focus head
and MIG welding torch. Hybrid Laser + MIG is applied
to many application areas that need deep penetration and
gap bridging on thick plates or heavy sections for the
shipbuilding industry.
It has recently been introduced in thin aluminum sheet
welding for the automotive industry, in car bodies such
as the AUDI A8 [40] and VW Phaeton [41]. Fig. 4(a)
illustrates a schematic diagram of hybrid laser + MIG
welding, and Fig. 4(b) shows the welding head for the
laser–MIG hybrid system developed by KUKA.
II.8. Process Comparison Based on Prior Research
Table I presents a comparison of aluminum fusion arc
welding using knowledge acquired from the review of
research referenced in this work.
II.9. Quantitative and Qualitative Comparison
in Real cases
Table II is a collection of data from various journal
articles and documentation from welding technology
manufacturers. Its purpose is to enable the comparison of
performance and usability in some real cases of
automated welding with aluminum alloys.

P. Kah
Copyright © 2014 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 8, N. 1
149
Figs. 4 (a) Schema of laser–MIG hybrid system [42] (b) KUKA KS Hybridtec (Laser–MI system) [43]
TABLE I
ALUMINUM WELDING PROCESSES COMPARISON
Process Speed Material thickness
welded with good
quality
Gap
tolerance Ease of
automation Applications Comments Automation benefits
TIG _ _ Limited to small
thicknesses + _
Piping systems, chemical
and pressure vessels, thin
sheet metal work
requiring intricate torch
manipulation [9].
Suited for applications
in which weld integrity
and aesthetics are
more important than
speed [9].
Repeatable, precise
heat control and
exact penetration.
Reduced operator
costs [10].
DC Pulsed
MIG + Wide range of
thicknesses + + + +
General construc
tion, ship
building, railroad, storage
tanks, pressure vessels,
large diameter pipelines,
and armored vehicles [9].
Flexible and versatile
technology
Higher productivity
and speed. More
consistent quality
and aesthetic bead
shape, reduced
defects, less heat
input, no high-
skilled welders [4].
AC Pulsed
MIG + Suitable for small
thicknesses + + +
Thin aluminum sheets,
should be very suitable
for the automotive
industry
Specialized for thin
aluminum sheet
welding
Tandem MIG + + Wide range of
thicknesses + + + Same as MIG with higher
productivity requirements
Achieves higher
productivity than MIG
Used in the
automated mode
only [44]. Reduced
weld stations &
short payback period
[45].
PAW _
Normally used for
small thicknesses,
can also weld thick
material in some
cases
+ + +
Critical products, such as
external fuel tanks for
space shuttles, missile
shells, and nuclear
magnetic resonance
devices
High quality and low
currents
Improved quality,
productivity, and
flexibility [46].
Plasma/
MIG + Wide range of
thicknesses + + + + Tank trailers and tubes to
flanges
Less welding spatter,
low porosity, clean,
and good weld bead
Generally used in
automated
applications [4].
CO2 Laser + + + +
Wide range of
thicknesses, suitable
for the thickest
materials
_ _ + + + Automotive and
aerospace industries Low flexibility, high
power
Used in the
automated mode
only.
Nd:YAG Laser
+ + + Wide range of
thicknesses _ _ + + + Automotive and
aerospace industries Good flexibility
Hybrid Laser-
MIG + + Wide range of
thicknesses + +
Application areas that
need deep penetration and
gap bridging on thick
plates for the heavy and
shipbuilding industry, thin
aluminum sheets for the
automotive industry
High number of
parameters, high
welding speed with
good quality to join
thin aluminum alloys
Hybrid Laser-
(DC)MIG + +
Wide range of
thicknesses, suitable
for thin aluminum
sheets
+ + Thin aluminum sheets Suited for high speed
welding of thin
aluminium sheets
++++: Excellent, +++: Very good, ++: Good, +: Ok, - : Difficult, - - : More difficult