Autogenous laser welding of the AA6082-T651 aluminium alloy was investigated with 3 lasers, namely a pulsed laser of 300W, a disk laser of 4kW and a laser marker of 70W. First, the hot cracking susceptibility was studied with two conventional laser welding equipment, without using filler material or heat treatment. Additionally, an attempt was made to weld with a laser marking equipment. Most of the welds made were laser seam welds but, some continuous and laser spot welds were also tested. As each laser was operated with different parameters, more than 400 welds were obtained with a wide range of parameters. Selected welds were studied with visual inspection, dye penetrant inspection (DPI), optical microscopy and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). Results indicated that welds with pulse laser beam tend to develop hot cracking, due to the segregation of silicon-rich low melting point eutectics to the grain boundaries and the development of contraction stresses during solidification. On the other hand, positive results were found with continuous welding since this process results in much longer solidification times and lower stresses. Finally, promising results were obtained with the laser marking machine, which produced high aspect ratio laser seam welds without hot cracking and a penetration of 1 mm using 99.9% of overlap factor. These welds were crack free due to their small weld pool, avoiding segregation effects, and to the heat build-up of successive spots, what had similar effect to solidification in continuous welding.

1. 1
Laser Welding of Low Weldability Materials
Autogenous pulsed laser welding of aluminium alloy AA6082-T651
R. Rodrigues1
*, J. Silva2
, M. F. Vaz1
, L. Quintino1
1
IDMEC, Instituto Superior Técnico, Avenida Rovisco Pais, 1,1049-001 Lisboa, Portugal
2
Carrs Welding, Henson Park, Telford Way Industrial Estate, Kettering, Northamptonshire, NN16 8PX, UK
* Corresponding author: rui.ravail.rodrigues@ist.utl.pt
Abstract
Autogenous laser welding of the AA6082-T651 aluminium alloy was investigated with 3 lasers, namely a
pulsed laser of 300W, a disk laser of 4kW and a laser marker of 70W. First, the hot cracking susceptibility was
studied with two conventional laser welding equipment, without using filler material or heat treatment. Additionally,
an attempt was made to weld with a laser marking equipment. Most of the welds made were laser seam welds
but, some continuous and laser spot welds were also tested. As each laser was operated with different
parameters, more than 400 welds were obtained with a wide range of parameters. Selected welds were studied
with visual inspection, dye penetrant inspection (DPI), optical microscopy and scanning electron microscopy
(SEM) with energy dispersive X-ray spectroscopy (EDS). Results indicated that welds with pulse laser beam tend
to develop hot cracking, due to the segregation of silicon-rich low melting point eutectics to the grain boundaries
and the development of contraction stresses during solidification. On the other hand, positive results were found
with continuous welding since this process results in much longer solidification times and lower stresses. Finally,
promising results were obtained with the laser marking machine, which produced high aspect ratio laser seam
welds without hot cracking and a penetration of 1 mm using 99.9% of overlap factor. These welds were crack free
due to their small weld pool, avoiding segregation effects, and to the heat build-up of successive spots, what had
similar effect to solidification in continuous welding.
Keywords: aluminium alloy AA6082-T651, autogenous laser welding, hot cracking, nanosecond pulse
welding and pulsed laser welding.
Introduction
Laser welding is becoming the main welding choice for many industries. Mass production and cutting
edge industries, like automotive and airspace evidence this trend. This technology offers both mechanical and
economical advantages [1]–[3]. However, process parameters to assure adequate laser welding results have yet
large room for improvement unlike in more conventional welding processes. The aluminium industry is today the
largest non-ferrous metal industry in the world economy [4]. The large scope of aluminium alloys offer a wide
range of physical and mechanical properties, including low density, high specific strength, good corrosion
resistance, good workability, high thermal and electrical conductivity, attractive appearance, and intrinsic
recyclability [5]. This versatility and attractiveness makes aluminium alloys particularly interesting for the above
mentioned industries (as well as many others in metal engineering).
The AA6082-T651 is a relatively recent aluminium alloy that has been replacing the AA6061 because of
its higher strength [6] that is obtained by the higher silicon content in its composition. Although alloys with higher
silicon content are capable, with the proper tempers (T6), to reach higher strength than others of 6xxx series, they
also tend to have cracking problems, notably hot cracking, due to segregation of silicon rich low melting point
eutectics to the grain boundaries during solidification. So, many welding investigations have been carried out
addressing the hot cracking susceptibility of these alloys and respective investigation techniques. Hot cracking,
also referred as solidification cracking is a weld-cracking failure mechanism usually occurring in the weld metal at
elevated temperatures during cooling. It occurs predominantly at the weld centreline or between columnar grains
for the reason that the fracture path of a hot crack is intergranular [7]. There are several theories of solidification
cracking which include the “strain theory”, the “brittleness temperature range theory”, Borland’s “generalized

2. 2
theory” and the “critical speed theory” [8]. All these theories accept that hot cracking is caused by the formation of
a coherent interlocking solid network that is separated by almost continuous thin liquid films. This solid network
ruptures because of the tensile stresses inherent to the solidification of the metal thus forming deep centreline
cracks characteristic of this welding defect [8]–[10].
Laser welding of crack susceptible alloys of the 2xxx, 5xxx and 6xxx series is a widespread practice.
Even though autogenous welding is more economical for industrial applications [11], hot crack susceptible alloys
do not lead to good quality welds and so it is usually necessary the use a filler material with proper dilution ratio to
avoid the crack sensitive compositions. Secondly, regarding autogenous laser welding, the most influential choice
to avoid hot cracks is the laser type. Continuous lasers are capable of providing longer solidification times to avoid
hot cracks (due to reduction of contraction stresses) and are therefore a better choice. Pulsed lasers, on the other
hand, typically make pulses with a few milliseconds and as a result the solidification times are smaller and the
welds are more susceptible to hot cracking [12]. Furthermore, to avoid hot cracks with pulsed lasers, conduction
welding mode is preferred and in some cases pulse shaping is also recommended. Thus, if more penetration is
required and the welding mode enters within the keyhole regime, a solution is yet to be found to avoid hot
cracking [13]. Thirdly, in any case a good joint design of the welds is an important consideration to reduce
undesired stresses [11].
Similar to 6xxx series, the AA6082 alloy has a high hot cracking susceptibility. With that in mind, this
work is meant to complement the existing knowledge on this subject by performing autogenous pulsed laser
welding of AA6082-T651 aluminium alloy producing welds without hot cracks, with 1 mm of weld penetration and
more than 60% of overlap factor.
Experimental Procedure
As mentioned, the main difficulty of this work is the reduction of the hot cracking susceptibility of AA6082
alloy with the increased difficulty of using pulsed lasers instead of continuous lasers. Two Nd:YAG lasers were
employed, namely the AL 300 of ALPHA LASER GmbH and the TruDisk 4002 of TRUMPF GmbH + Co. KG both
installed at Carrs Welding Technologies Ltd (located in Kettering, England) and a laser marker, the G4 Series Z
Type of SPI Lasers UK Ltd installed at the Welding Engineering and Laser Processing Centre (located in
Cranfield, England) (Table 1). All tests were carried out in rectangular blocs with an approximate size of
100x50x10mm of aluminium alloy AA6082-T651 with the standard chemical composition (Table 2). Before
welding block samples were manually grinded with 230, 320-grit sandpapers.
Table 1 Technical data of the 3 lasers [14]–[20]
Name AL 300 TruDisk 4002 G4 Series Z Type
Laser type PW CW (PW option) PW (CW option)
Average power 300 W 4000 W 70 W
Maximum output power 9 kW 4 kW 13 kW
Pulse duration 0.5 – 20 ms Min 1 ms 10 – 520 ns
Maximum pulse energy 90 J - 1 mJ
Table 2 Chemical composition of aluminium alloy 6082 [4], [6]
Composition, wt. %
Si Fe Cu Mn Mg Cr Zn Ti
Unspecified other elements
Al min.
Each Total
0.7-1.3 0-0.5 0-0.1 0.4-1 0.6-1.2 0-0.25 0-0.2 0-0.1 0.05 0.15 Balance
Although the vast majority of the tests were pulsed laser seam welds, some continuous and individual
spot welds were also carried out. Considering that in this work 412 different welds were produced, a large number
of parameters and their respective combination were tested. The parameters tested include pulse duration and
shape, type of gas and flow rate, pulse frequency, welding speed, spot sizes and peak powers. In order to widen
the field of investigation of this research, every laser tested a different range of parameters and so the results
were analysed independently. A general summary of those parameters is presented below (Table 3).

3. 3
Table 3 Summary of the parameters tested
Name AL 300 TruDisk 4002 G4 Series Z Type
Type of welds Seam continuous; seam; spot seam
Pulse duration
and shape
4 ms; no shape and 3
tailored shapes
1, 2 and 20 ms; no shape
and 1 tailored shape
240, 350 and 520 ns; 3 pre-
programed pulse shapes
Gas and flow rate
Argon, 15 L/min and
Heliweld2, 25 L/min
Argon, 15 L/min -
Pulse frequency 8 to 18 Hz 16.4 and 17 Hz 70 kHz
Welding speed 1 to 4 mm/s 4.5 and 25 mm/s 3.6 to 105 mm/s
Spot sizes 0.2 to 0.5 mm 0.2 0.38 and 0.67 mm 0.051, 0.1 and 0.15 mm
Peak powers 1.67 to 6.06 kW 2.8 to 4 kW 5 to 13 kW
The welds produced were visually inspected and some selected welds were further examined with dye
penetrant inspection (DPI), optical microscopes or scanning electron microscope (SEM) with energy dispersive X-
ray spectroscopy (EDS). The initial visual and DPI inspections dictate the choice for further and more thorough
tests with optical microscope, SEM with EDS. Due to the great incidence of cracks in most of the welds
performed, only the most interesting results are presented, namely the SEM results of the welds made with the AL
300 and the TruDisk 4002 lasers. The optical microscope images of the pulsed laser seam welds made with the
G4 Series Z Type laser are also presented.
Results and Analysis
EDS of the face of the welds
The chemical analyses were carried out in a field emission gun scanning electron microscope (FEG -
SEM) (JEOL model 7001F) with an X-ray energy-dispersive system, using an accelerating voltage of 15 kV.
Laser welding is known to cause the loss of low vaporization temperature alloying elements in aluminium
alloys and particularly of magnesium. To verify if these welds suffered similar problem, the face of the welds
produced with the AL 300 laser were chemically analysed with EDS and compared with a prepared sample
surface without welds. The welds analysed were obtained with a spot size of 0.2 mm, peak power of 4.32 kW,
frequency of 10 Hz and welding speeds of 1, 2.5 and 4 mm/s. For each weld, 6 locations were analysed hence
the comparison was made using the mean values of their composition (Figure 1).
Figure 1 Mean chemical composition of the face of the welds vs. prepared sample surface
The comparison indicated that almost no vaporization took place since the amount of magnesium was
nearly the same with a slight decrease of about 4.5%. The amount of oxygen was also similar with a decrease
close to 12.2% which confirms that the gas protection was adequate. Lastly, the silicon content variation was
more noticeable as it decreased significantly of about 40.9%. Since silicon has a higher vaporization temperature
it cannot be lost by vaporization and so, it was assumed that the silicon segregated to the fusion zone of the
welds.
EDS of the fusion zone
Firstly, the welds previously mentioned showed signs of hot cracking with typical centreline cracks in the
fusion zone as seen in Figure 2. Hence, the fusion zone of those welds was also chemically analysed with EDS.
The sample preparation of those welds involved cutting, mounting and polishing with 230, 320, 600, 800, 1000,
5,27
6,00
1,05 1,101,04
1,76
0,00
2,00
4,00
6,00
8,00
Face of the welds Prepared sample surface
Meanwt.%
Locations
O
Mg
Si

4. 4
2400, 4000-grit sandpapers followed by 3 µm then 1 µm diamond particles. Finally the welds were chemical
etched with Keller’s reagent for 30 seconds. The fusion zones of those welds were examined to study the precise
chemical composition of the surfaces of the cracks and of the surrounding material without cracks.
LongitudinalCross-section
1 mm/s (AL 300 laser) 2.5 mm/s (AL 300 laser) 4 mm/s (AL 300 laser)
Figure 2 SEM images of the longitudinal section (face) and cross-section of the fusion zone of the welds
For each weld, 4 locations were analysed, namely 2 at the crack surface and 2 without cracks as shown
in Figure 4. The difference in composition between both locations, with and without cracks, was made once more
using the mean values of their composition (Figure 3). Additionally, the evolution of mean wt. % of oxygen,
magnesium and silicon in the locations with cracks for each welding speed was also determined (Figure 5).
Figure 3 Mean chemical compositions of the locations with vs. without cracks
6,32 01,38 0,49
88,35 99,51
3,95 0
0,00
50,00
100,00
With crack Without crack
Meanwt.%
Locations
O
Mg
Al
Si

5. 5
1mm/sofweldingspeed2.5mm/sofweldingspeed4mm/sofweldingspeed
Figure 4 EDS locations of the fusion zone of the welds
The mean chemical compositions of both locations, with and without cracks, indicate that elements are
present in substantial amounts in the cracks: oxygen (6.32 wt. %); magnesium (1.38 wt. %) and silicon (3.95 wt.
%). On the other hand, the locations without cracks are free of those elements, except of magnesium but in
smaller amounts with 0.49 wt. %. These results suggest that predominantly oxygen and silicon are responsible
for, or are in correlation to the hot cracking of the welds.

6. 6
Figure 5 Mean chemical compositions of locations with cracks for each welding speed
Besides welding speed, all other parameters were kept constant. The influence of welding speed was
analysed separately for each element. First, it was noted that using higher welding speeds increased the amounts
of oxygen and silicon at the locations with cracks. It is known that, unlike magnesium and silicon, which can only
come from the weld itself, oxygen may come from the various surface oxides [21], [22] or directly from the
surrounding atmosphere when gas protection is ineffective. However, the analysis of the face of the welds
indicated that oxygen came from the surface oxides. On the other hand, faster welding speeds lead to lower heat
inputs and faster solidification times. Also, with faster welding speeds, elements that are present at the surface
and that can vaporize, like oxygen, are more susceptible to be trapped inside the fusion zone. Hence, the
presence of oxygen in the welds can be related to the solidification times in the following manner: a lower welding
speed increases the solidification time which allows more oxygen to escape from the metal weld pool and less
oxygen to be left inside and segregate to the locations with cracks.
Silicon and magnesium come from the weld itself, mainly from the compound magnesium silicide
(Mg2Si), which forms during the precipitation hardening treatment and contributes to the mechanical properties of
the alloy [23]. The chemical composition of the locations without cracks indicated 99.51 wt. % of aluminium. This
suggests that the silicon and some of the magnesium detected at the locations with cracks came from the
surrounding weld metal causing it to be almost depleted. Interestingly, the amount of silicon increased
significantly with increasing welding speeds but the amounts of magnesium did not follow the same pattern. The
increase of silicon content with welding speed can be related, yet again to the solidification time. Lower welding
speeds lead to higher heat inputs and longer solidification times. These longer solidification times allow more of
the silicon to precipitate into the bulk material. Hence, forming these precipitates decreased the amount of silicon
found in solid solution at the surface of the cracks with these analyses.
Overall, the results of the chemical compositions of the fusion zone suggest that hot cracking is caused
by depletion of hardening elements from the neighbouring metal to the crack sensitive locations through
segregation during solidification. The amount of silicon found at the crack locations was irregular, with results
going from min. 1.45 wt. % to max. 10.07 wt. % and was overall high with an average of 3.95 wt. %. In contrast, at
the location without cracks there was no silicon detected. This complete depletion of silicon weakened the
strength of the material in these locations. Furthermore, this silicon segregated to the grain boundaries where it
accumulated and reached high amounts resulting in the material becoming brittle and cracking with the
solidification stresses. On the other hand, the magnesium at the locations with cracks was roughly the same for
all welding speeds with amounts going from min. 1.02 wt. % to max. 2.27 wt. % and had an average of 1.38 wt.
%. Additionally magnesium was also found at the location without cracks with amounts up to 1.5 wt. %. Thus,
unlike silicon, magnesium amounts cannot be interpreted to cause hot cracking and further analysis is necessary
to clarify this possibility.
Summarizing, the EDS analysis showed that the hot cracking was in part caused by the presence and
accumulation of oxygen and silicon at specific locations, the grain boundaries. It was also shown that oxygen
must come from the oxides on the surface of the material whereas silicon as one of the main alloying elements of
this aluminium alloy comes from the bulk. Consequently, only oxygen can be reduced or even completely
removed with an appropriate surface preparation.
Crack free continuous welds
Two continuous welds were made with the TruDisk 4002 laser and observed with a SEM. These welds
had a spot size of 0.38 mm, welding speeds of 25 mm/s and power of 2.8 and 3.4 kW. Both SEM images of the
face of the welds (Figure 6) and their fusion zone (Figure 7) evidenced that there was no hot cracking. Such
1,38
6,07
9,65
1,24 1,10 1,761,91
3,25
6,76
0,00
5,00
10,00
1 mm/s 2,5 mm/s 4 mm/s
Meanwt.%
Welding speed
O
Mg
Si

7. 7
interesting results confirmed that an adequate cooling rate of the weld pool is essential to accommodate the
stresses of the solidification and thus avoid hot cracking [24].
2.8 kW 3.4 kW
Figure 6 SEM images of the face of the continuous welds
2.8 kW 3.4 kW
Figure 7 SEM images of the fusion zone of the continuous welds
Crack free pulsed seam welds
Finally it was attempted to make pulse seam welds with the G4 series Z Type laser using very short
pulse durations. As this laser is designed for laser marking and micro-machining applications this unconventional
welding attempt was an innovative research and showed very promising results.
In terms of parameters these welds had a spot size of 0.051 mm, frequency of 70 kHz, welding speed of
3.6 and 35.7 mm/s and pulse duration of 240, 350 and 520 ns. Additionally, two welds were also made with 10
passes, spot size of 0.051 mm, frequency of 70 kHz, welding speed of 35.7 mm/s, pulse duration of 240 and 520
ns. Figure 8 show optical images of the welds made with the G4 series Z Type laser.

8. 8
10 passes Single pass
240 ns with 35.7 mm/s 520 ns with 35.7 mm/s 240 ns with 35.7 mm/s 240 ns with 3.6 mm/s
Single pass
350 ns with 35.7 mm/s 350 ns with 3.6 mm/s 520 ns with 35.7 mm/s 520 ns with 3.6 mm/s
Figure 8 Optical microscope images of the welds made with the G4 series Z Type laser
The visual inspection of the welds was misleading since they resembled simple surface laser markings.
However, the observation of these welds with an optical microscope revealed that single pass welds had high
aspect ratios, reached up to 1 mm of penetration at 99.9% of overlap factor and had no hot cracks (Figure 9).
It is known that this type of equipment is usually intended to perform laser ablation which is the removal
of material from a substrate by direct absorption of laser energy [25]. In ablation with pulsed laser radiation,
depending on the respective pulse length range, different beam-matter interaction mechanisms become
dominant. For short laser pulses (µs and ns range) the ablation process is dominated by heat conduction, melting,
evaporation and plasma formation. In the ablation processes involving nanosecond lasers the absorbed laser
energy first heats the target surface to the melting point and then to the vaporization temperature [26].
Additionally, metals require much more energy to vaporize than to melt [27] therefore it was reasonable to
assume that adequate control of the laser parameters could allow to make welds with surface melting with
minimal vaporization. In other words, selecting parameters that would increase the surface temperature, while
keeping it just above the vaporization temperature, would perform laser welding.
With the spot size of 0.051 mm, using a high energy density of 49 J/cm
2
(above the typical ablation
threshold of metals) combined with pulse durations of 240 ns to 520 ns and pulse frequency of 70 kHz, it makes
the ablation process inefficient for metal surface vaporization but quite efficient in accumulating heat and melting
the metal through joule heating [28]. These conditions resulted in welds in the keyhole regime featuring high
aspect ratios.

9. 9
Figure 9 Weld of the G4 series Z Type laser with pulse duration of 350 ns and 3.6 mm/s of welding speed
Remarkably, no hot cracks were observed in any of the welds. Considering that even at the kilohertz
repetition rate, the coupled laser energy is not dissipated until the next laser pulse arrives [28] the accumulation
effect at 70 kHz heated every spot weld multiple times, thus creating a linearly temperature decrease that reduced
the solidification tensions. In addition these high aspect ratio welds had very small melt pools consequently the
segregation of critical elements such as oxygen or silicon was reduced compared to conventional laser welds.
This combination of soft temperature decrease and high aspect ratios welds made possible to form autogenous
pulsed seam welds in AA6082-T651 aluminium alloy without cracks.
It was also noticed that the penetration increased significantly with the overlap factor which reached up
to 1 mm of penetration at 99.9% of overlap factor. The influence of overlapping pulsed laser processing on the
melting ratio is known [28]–[30] so these result confirmed previous studies. As a final remark, it must be noted
that the main objective of this investigation specifically, the autogenous laser spot welding of AA6082-T651 with
an overlap factor over 60%, featuring 1 mm of weld penetration, was achieved without hot cracks.
Conclusions
The conclusions of this work were divided according to the pulse durations tested. With pulse durations
of 1 to 20 ms, frequencies of 8 to 18 Hz and penetrations close to 1 mm the volume of the melt pool was sufficient
for the segregation effects of silicon (and oxygen) in the fusion zone to be dominant and increase the hot cracking
susceptibility of the welds. These weakened welds were then unable to accommodate the tensions of the
solidification contraction and cracked.
With continuous welding the segregation effects are arguably much worse but the soft gradual
solidification of this process allowed more time for accommodation of the solidification contractions and thus
developed less tensions. Hence, adequate welding speed of 25 mm/s did not show any sign of hot cracking while
reaching 2.3 and 2.73 mm of weld penetration, with 2.8 and 3.4 kW respectively.
Finally, pulse durations of hundreds of nanoseconds coupled with multiple kilohertz frequencies gave
promising results. The short pulse duration restricted the melt pool to a highly localized spot which made
irrelevant the segregation of silicon or oxygen in the fusion zone and the very high frequencies caused heat

10. 10
accumulation and created a linear heating profile much like the heating profile of continuous welding. The
combination of these effects eliminated the hot cracking problem while achieving 1 mm of weld penetration, with
adequate overlap factor.
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