ap-6-10.dvi


Acta Polytechnica Vol. 50 No. 6/2010

Welding of Aluminum Using a Pulsed Nd:YAG Laser

M. Nerádová, P. Kovačócy

Abstract

This paper deals with pulsed laser welding of aluminum using an Nd:YAG laser with wavelength 1.06 μm. Technically
pure aluminum (95.50 wt. %) was used as the welded material. Eighteen welds (penetration passes) were fabricated in
the experiment. Optical microscopy was used to assess the influence of changes in the parameters of the pulsed laser on
the quality and geometry of the penetration passes of aluminumand on the hardness measurement through the interface
of the welds. The results show that the geometry of the penetration passes was influenced above all by the position of
the beam focus.

Keywords: welding parameters, laser welding, aluminum, weldability of aluminum.

1 Introduction
Laser beams can be applied in a versatile manner
in a wide range of technical and non-technical fields.
In laser welding of materials, various types of laser
devices with various power ranges are used. The au-
tomotive industry is one of many areas where laser
welding of aluminum is utilized [1]. Aluminum and
its alloys are characterized by low density, relatively
high strength and high corrosion resistance [2, 3].
They are used as structural materials in various in-
dustrial fields.
Welding ofaluminumand its alloyshas its specific

features. Aluminumoxidizes strongly above itsmelt-
ing point. The oxidic layer has a high melting point,
and it does not melt in the welding process [4]. This
layer has a strong ability to absorb gases and vapors,
which then get into the weld metal. Oxidic particle
layersmay lead to the presence of oxidic inclusions in
theweldmetal, which deteriorates the characteristics
of the welded joints [5].
When welding aluminum, it is necessary to use

a higher intensity laser beam on the surface of
the workpiece, due to the high reflectivity of alu-
minum [4]. Given the high reflectivity of the radi-
ation from the surface of aluminum, it is preferable
to use just anNd: YAG laser. Depending on the con-
figuration and the geometry of the welds, additional
materials are sometimes used for aluminum welding.
When welding aluminum and its alloys, pores often
form in the weld metal. Source of these pores is hy-
drogen. Hydrogenhas atmelting temperature of alu-
minum relatively high solubility [5].
The high thermal conductivity and the high co-

efficient of expansion of aluminum give rise to ma-
jor distortions in comparison with steel. The use of

highly concentrated laser beam welding provides the
preconditions for success in addressing these prob-
lems. In order to obtainhigh-qualitywelded joints, it
is particularly necessary to prepare the surface prior
to laserwelding. The oxidic layer along the length of
the surface has to be removed. This surface prepa-
ration minimizes the formation of defects in welding
and the presence of pores and oxidic inclusions in the
weld metal [2].

When welding aluminum alloys, it is essential to
protect the gas bathmelt from oxidation. The use of
heliumas the protective gas enablesmaximumdepth
for translating a high quality weld metal.

The basis for achieving high-qualitywelded joints
is the correct choice of the laser welding parameters.
Themicrostructure of the weldmetal joints made by
the laser beam and the optimal welding parameters
are significantly different from the microstructure of
weld joints made with the use of metal arc welding.
The weld metal has a fine dispersion structure with-
out the presence of low-eutectic, while the dendrite
dimensions are significantly smaller than in arcweld-
ing. The structural changes in the heat-affected area
of laser welding take place in a volume 5 to 6 times
smaller than in the case of arc welding. The grain
in this area increases minimally. This structure is
advantageous in terms of mechanical properties and
good resistance to hot cracks [1].

2 Experimental materials

Technically pure aluminium was used as the experi-
mental material. The dimensions of the test samples
were 76 × 30 × 1 mm. The chemical composition of
Al 99.50 % is shown in table 1.

66



Acta Polytechnica Vol. 50 No. 6/2010

Table 1: Chemical composition Al 99.50 wt. %

material Chemical composition [wt.%]

Al min Fe Si Zn Cu Ti Others

99.50 max. 0.40 max. 0.30 max. 0.07 max. 0.05 max. 0.05 max. 0.03

Mechanical characteristics

Rm (MPa) Ductility A5 [%] Modulus of elasticity E

60–100 min. 10–20 [GPa] inf. 71

Table 2: Welding parameters

Sample f (mm)* τ (ms)* U (V)* E1(J)*

1.1 5.5 20 400 69.8

1.2 5.25 20 400 70.1

1.3 5.0 20 400 70.2

1.4 4.75 20 400 69.9

1.5 4.5 20 400 70.0

1.6 4.25 20 400 69.8

1.7 4.0 20 400 70.3

1.8 3.75 20 400 70.0

1.9 3.5 20 400 69.1

1.10 3.25 20 400 70.1

1.11 3.0 20 400 70.1

1.12 2.75 20 400 69.9

1.13 2.5 20 400 70.1

1.14 2.25 20 400 72.8

1.15 2.0 20 400 74.2

2.1 2.75 20 400 69.8

2.2 2.75 20 375 63

2.3 2.75 20 350 53.1

*f (mm) – focal length, τ (ms) – pulse duration,
U (V) – pump power, E1 (J) – pulse energy

2.1 Procedure and parameters for
welding

The experiment was executed at the International
Laser Centre in Bratislava. The experimental work
was performed on a W50 Laser Welder, produced by
Solar Laser Systems, with wavelength 1.06 μm and
maximum output power 74.2 J. During the exper-
iments, 18 penetration passes were carried out, in
which we observed the impacts of the focus location,
the intensity of performance and also the impact of
the energypulse values on the geometryand integrity
of the welds. Laser welding was performed in a pro-
tective atmosphere of argonwith 5 l/min. flow. The
welding parameters are shown in table 2.

2.2 Assessment of penetration welds

Optical microscopy was used for assessing the pene-
tration welds and for measuring the hardness (HV)
through the interface of the welds. Figs. 1 to 4
document the macrostructures of penetration weld
1.2–1.5. It follows fromobserving the structures that
in the case of samples 1.1–1.4 the material was not
fully penetrated due to low power density.

Fig. 1: Surface of penetration pass 1.5

Fig. 2: Root of penetration pass 1.5

67



Acta Polytechnica Vol. 50 No. 6/2010

Fig. 3: Penetration pass 1.2

Fig. 4: Penetration pass 1.5

The surface and the root of penetration pass 1.5
are distinguished by roundness without a split. The
position of the focus in relation to the surface of the
material has caused the whole width of the mate-
rial to be penetrated. A slight depression can be
seen on the surface of the material, Fig. 4. The weld
metal does not show any non-integrities or defects.
The width of the surface of the penetration pass is
1.336 mm and the width of the root of the penetra-
tion pass is 0.872mm. Figs. 3 and 4 showdifferences
in the character of the penetration. In Fig. 3, the
material was not liquidized — this is known as con-
ductionalmodewelding. In thismode, a thin surface
layer ofmaterial ismelted down and then ismaterial
heated due to the thermal conductivity. In Fig. 4,
the parameters are used to produce a sufficient key-
hole to enable deeper penetration of the laser beam
into the material.
Fig. 5 documents the microstructure of the tran-

sition (the heat-affected zone — boundary melting
down — weld metal) on penetration weld 1.5. Fig. 6
shows the microstructure of the welding metal.
Grain (subgrain) boundarieswere observed in the

weldmetal with oxides distribution inside the grains.

Fig. 5: Microstructure theheat-affectedzone (Al99.50%)

Fig. 6: Microstructure weld metal (Al 99.50 %)

No non-integrities were present. A smelting bound-
ary can be observed between the heat-affected zone
and the weldmetal, and there is a smooth transition
between them. The orientation of the dendrites can
be observed in the weld metal.
Vickers microhardness tests were conducted on

samples 1.5, 1.9 and 2.1. Figs. 7 to 9 show the effect
of some parameters on the geometry of the penetra-
tions.
Ten measurements were made on each sample.

Fig. 10 shows that the highest microhardness values
were for sample 1.9. This may be due to the smaller
volumeof the smeltedmaterial, quicker cooling of the
material, and the formation of a finer structure.
Inwelds 1.5, 1.9, 2.1 the highestmicrohardness is

in the parentmaterial. Thismay be because the ma-
terial hasbeen cold-rolled. Thehardness drops in the
heat-affected zone due to thermal processing, and on
the smelting boundary and in the welding metal the
hardness starts to rise. The welding metal is charac-
terized by the pouring structure. The slight growth
in the hardness of the welding metal may be due to
softening of the structure.

68



Acta Polytechnica Vol. 50 No. 6/2010

Fig. 7: Effect of pulse energy on the geometry of the pen-
etration passes

Fig. 8: Dependence of thewidth anddepthof penetration
on the excitation voltage

Fig. 9: Influence of the focal length on the geometry of
the penetration passes

Fig. 10: Graphic illustration of the course of microhard-
ness on samples 1.5, 1.9, 2.1 A – parent material, B –
heat-affected zone, C – boundary of smelting, D – weld
metal

3 Conclusion
Based on the results obtained in the experiment it
canbe stated, that the values of the standoffdistance
being between 0–2mmandbetween 4.75–5.5mmare
characterized by low penetration weld form factor
(conductional welding system), which is more suit-
able for surface treatment ofmaterials. A total of 10
sampleswereused to recast thewhole thickness of the
material. The standoff distance in these cases varied
in the range from 2.5–4.5 mm. Excitation voltage of
400 V was used for the production of penetrations.
Because of the possibility of assessing the impact of
voltage-inspiring geometry, penetration welding was
carried out at voltages 350 V and 375 V. Based on
themeasureddimensions influencesof voltageenergy,
pulse duration and standoff distance on the weld ge-
ometrywereevaluated. Basedonthe resultsofmacro
and microstructural analysis, it can be considered,
that themost suitableparameterswereused forweld-
ing the sample no. 1.5. The greatest microhardness
values were measured in the base material. In the
thermally influenced area a decrease in hardness was
observed.

Acknowledgement

The paper was prepared with support from the
VEGA project, no. 1/0842/09.

References

[1] Turňa, M., Kovačócy, P.: Zváranie laserovým
lúčom. Bratislava, STU, 2003.
ISBN 80-227-1921-8.

[2] Accessible on internet:
http://www.world-aluminium.org/?pg=107

[3] Accessible on internet:
http://www.chemicool.com/elements/
aluminum.html

[4] Accessible on internet:
http://www.keytometals.com/Article12.htm

[5] Ghaini, F. M. et al.: The Relation Between Li-
quation and Solidification Cracks in Pulsed Laser
Welding of 2024 Aluminium Alloy. Accessible on
internet: http://www.sciencedirect.com

Ing. Martina Nerádová
Doc. Dr. Ing. Pavel Kovačócy
Phone: +421 335 521 007
Slovak University of Technology in Bratislava
Faculty of Materials Science and Technology
in Trnava
Institute of Production Technologies
Bottova 23, 917 24 Trnava, Slovak Republic

69