Mechanical characterization of Nd:YAG laser welding on Ti-6Al-4V
G. Serroni a, C. Bitondo
a, U.Prisco
a, A. Squillace
a, A.Favi
b and A.Prisco
c
a Department of Material Engineering and Production, University of Naples “Federico II”, Naples, Italy
b RTM S.p.A., Pomigliano D'Arco (NA)
c Alenia Aeronautica., Pomigliano D'Arco (NA)
Abstract
Since 10 years it has been trying to develop an innovative welding technique for titanium. The difficulties
encountered are mainly related to the need of developing a joint free from defects (e.g. undercut and porosity).
The attention of the researchers passed from the traditional TIG welding to Laser Beam Welding [1]. The laser
technology, while achieving a better coupling, requires a great deal of energy, especially the high-power
Nd:YAG lasers [2]. Adjusting the parameters of the laser, welds almost free of defects can be obtained, so it
would be necessary to plan the experiment using a DoE [3]. This is not possible for laser welding because this
type of technology has limitations that do not allow for proper planning. The aim of this study is to provide a
welding process that could produce high quality joints. Static tensile tests showed a high resistance joints, fatigue
strength is strongly influenced by the presence of undercut, so it is important to work in conditions where such
defect is minimized.
Keywords: Laser Welding, titanium, stress test, fatigue test.
1. Introduction
Development of aircraft product is marked by
leaps in technology that have allowed its greater use.
The introduction of the technology of
composites in all primary structures of aircraft has
been the last, in order of time. The criteria for the
design, manufacture and operation, were changed to
obtain structures lighter and more efficient than the
ones made of aluminum. However, the structures in
CFRP, although they show greater efficiency for
loads in the airplane, require the use of metal
structures, especially in areas of great concentration
of loads. For highest electrochemical compatibility
with the CFRP, titanium structures are preferred, as
they are more resistant to corrosion. On the other
hand, this implies excessive manufacturing costs
related to the cost of raw materials, the high volumes
of waste and the complex and expensive finishing.
The innovative solutions are designed to
manufacture semi-finished products not obtained
through solidification of molten and subsequent
plastic deformation, but through welding simpler
parts. This allows, with the same performance of the
final product, to obtain more complex near net-
shapes, with a reduction of scrap (up to a ratio of
30:1) and of energy processing.
The considerable advantages, in terms of
productivity, and the high quality end-product after
the application of lasers in the machining field,
encourage the growing use of new technology in the
most advanced processing. For these reasons, it was
necessary, since its initial application, a detailed
study of laser-material interaction, to optimize the
welding parameters as a function of the heat
treatment of the material to be welded and to get
working procedures, able to produce joints
responding to the design requirements.
This work was focused on the study of Nd:YAG
laser welding of Ti-6Al-4V plates. In particular, we
tried to optimize the welding parameters to assure a
good penetration [4-5] and the absence of defects
and of oxidation. It is provided to check the
effectiveness of shielding gas, as has been shown [6]
that it can stabilize the welding process. Adjusting
the heat input is important to define the final
microstructure of the joint and, as a consequence, its
mechanical properties [7]. FEM simulations have
been carried out recently able to adequately describe
the laser welding process [8].
2. Materials and methods
The welding parameters (speed, laser power of
phasic, inert gas and the reverse) were selected on
the basis of form and dimension of the cord cross-
section according to UNI EN ISO 13919-1:1997. To
get an overview on the weldability of the alloy,
preliminary tests were carried out on sheets of
Titanium Grade 5 (Ti-6Al-4V) mm 100x100x2.5.
We proceeded, therefore, on welding of titanium
with rolled-sized 503x105x2.5 mm in butt-joint
configuration.
For the analysis of factors influencing the
process, steps were taken to carry out non-
destructive testing, such as visual analysis and
destructive tests such as tensile test, bending test and
fatigue test, and finally microhardness. The welding
system consists of a robot, with a maximum power
of the laser beam of 2 kW, with a welding head
equipped with optic fibers ( 0.6 mm). An important
trick used was to provide the focusing head of a shoe
in which it was applied the supply ducts of the
shielding gas. This shoe was fitted into the lower
edges of a board made of fire resistant fabric to
minimize the loss of shielding gas. The diameter of
the fiber has a significant impact on spot
size. Reducing the size of the spot is related to the
relationship between the focal lengths of re-
collimating and focusing lenses, as shown in Figure
1. The instrumentation dimensions are: d0=0.6mm,
Lf=80mm e Lc= 120mm so focus spot was set to
0.4mm.
The system was equipped with two shielding
gas: Helium, which because of its high ionization
potential, has been used in coaxial and laminar
direction so as to create a cloud of protection, and
Argon, with low-threshold ionization, which is used
in the lower area of the specimen as protective gas in
reverse.
In the first phase of the research, some
penetration tests have been carried out to define what
were the changes to be applied to the process
parameters. An index of the goodness of penetration
is the colour of the welds. (Fig. 2) The color scale is
associated with the following general behaviors:
1) shiny silver - indicates a good coverage and a
weld is not infected, 2) pale yellow acceptable
Fig. 1 – focus – collimating lens
Fig. 2 – The color, due to
oxidation, is an indication of the
quality of the weld
Fig. 3 – Misalignment of weld sheets.
Table 1
Process Parameter Spec. Laser
Power [W]
rate.
[mm/s]
Coax
Gas
Assis
. Gas
Rate
He [Nl/h]
Prot.
Gas
A 1 1700 28 He none 250 Ar
A 2 1700 28 He none 250 Ar
A 3 1700 28 He none 250 Ar
B 1 1700 28 He none 250 Ar
B 2 1700 28 He none 250 Ar
B 3 1700 28 He none 250 Ar
C 1 1500 22 He He 250 Ar
C 2 1200 18 He He 250 Ar
C 3 1200 16 He He 250 Ar
D 1 1500 22 He He 250 Ar
D 2 1200 18 He He 250 Ar
D 3 1200 10 He He 250 Ar
E 1 1200 13 He He 250 Ar
E 2 1200 13 He He 750 Ar
E 3 1200 13 He He 750 Ar
F 1 1500 24 He He 500 Ar
F 2 1500 20 He He 500 Ar
F 3 1500 16 He He 500 Ar
G 1 1500 32 He He 500 Ar
G 2 1500 28 He He 500 Ar
G 3 1700 20 He He 500 Ar
H 1 1700 32 He He 500 Ar
H 2 1700 28 He He 500 Ar
H 3 1700 24 He He 500 Ar
I 1 1700 36 He He 500 Ar
I 2 1200 16 He He 500 Ar
I 3 1200 12 He He 500 Ar
L 1 1200 28 He He 500 Ar
L 2 1200 24 He He 500 Ar
L 3 1200 20 He He 500 Ar
welding with reduced ductility, and 3) blue, purple -
low ductility, unsuitable for high performance
applications , 4) gray - not acceptable.
An overview of the tests is shown in Table 1. 10
samples were used (named with the letters AL) and
on each of these three penetration tests were
performed (indicated by numbers 1-3). Some tests
were carried out with the same parameters to test the
repeatability of the process.
After all the penetration tests and optimized the
process parameters, were carried out butt welds. For
the tests of visual and stereoscopic analysis was used
a metallographic microscope Leica DM4000DM.
Two samples of each specimen were
encompassed and observed after Kroll attack
(designated by the letters a and b in Table 2).
For the microhardness test was used
Microdurometer Leitz, equipped with an optical
microscope, connected via interface with a digital
meter. The load used was 500p for a time of 15
seconds imprinting
The tensile test was carried out by subjecting the
material, shape and size appropriate, to a tensile load
acting in the direction normal to the straight section
of the specimen, ASTM E8-EM8.
The bending tests were conducted in accordance
with ASTM E-190/92, choosing a test roll with a
diameter of 10mm.
Finally, the cyclic fatigue tests were carried out
using ten samples in accordance with ASTM E466.
3. Results and discussions.
In Table 3 are compared the different sections of
some representative samples, which have suffered
the attack Kroll to highlight the heat-affected zone. It
is apparent that the welds made with higher power
and lower speeds show a deeper and wider molten
zone and HAZ. Table 2 shows the measurements
taken on specimens for which was set a level of He
flow of 500 Nl/h. All measurements concerning the
cross-sectional dimension, such as undercut, excess
material in the upper and excessive penetration,
falling within the range of values imposed by the
UNI EN ISO 13919 1:1997: Whereas the grading
more stringent, in fact you can see the values of
0.125 mm to 0.575 mm for undercut and excess
material and the excessive penetration.
This reflection allows the direct selection of
parameters for subsequent butt welds excluding the
samples that do not pass through welds, those with
excessive penetration and a welding area too
wide. The information obtained from this visual
analysis, addressed the choice of the parameters for
maximum power, choosing to make the welding
parameters of the penetration test H3 (Power 1700
W; Speedr = 24 mm / s) referred to as the weld "BJ
H3”. Before proceeding with subsequent tests, was
observed under optical microscope incorporated a
section of a sample, in order to verify the mismatch
between the laminates. The result is a value of
Table 2
Geometric Measurements
Spec.
Under-
cut
[µm]
Exc.
Mat.
[µm]
Exc.
Pen.
[µm]
MHV
ZTA
L
[HV]
MHV
Bulk
[HV]
MHV
ZTA
R
[HV]
I3a 34 88 189 371 324 356
I3b 35 91 222 356 324 343
I2a 26 150 29 371 324 350
I2b 28 186 39 371 324 356
L3a 29 146 29 386 324 365
L3b 29 172 26 386 324 356
L2b 29 117 9 371 324 386
L2a 26 107 19 380 324 350
L1a 23 123 13 371 324 350
L1b 22 134 4 371 324 386
F3a 30 199 62 371 324 350
F3b 30 209 65 371 324 350
F2a 45 186 98 380 324 350
F2b 50 134 166 386 324 350
F1a 39 153 196 371 324 350
F1b 37 150 173 386 324 365
G2a 16 170 27 371 324 350
G2b 25 183 23 380 324 350
G1a 24 173 26 386 324 350
G1b 22 166 26 371 324 350
G3a 44 205 67 386 324 356
G3b 46 245 59 371 324 350
H3a 35 178 163 386 330 343
H3b 37 154 147 396 330 343
H2a 65 208 156 371 330 336
H2b 71 219 163 386 330 343
H1a 34 157 225 386 330 336
H1b 29 183 228 380 330 336
I1a 20 189 55 396 324 356
I1b 16 225 22 371 324 386
Table 3
Cross Section of weld beads
Rate mm/s
Power [KW]
1200 1500 1700
20
L3 F2 G3
24
L2 F1 H3
28
L1 G2 H1
Table 4
Hardness Vickers Report
No x y
Hardness
HV
1 -3 0 324,50
2 -2 0 329,90
3 -1 0 386,40
4 0 0 379,80
5 0 0,5 388,60
6 0 -0,5 382,00
7 1 0 373,50
8 2 0 332,80
9 3 0 324,50
10 0,8 0 369,30
11 -0,8 0 371,40
0.184mm, less than the limit imposed by the rule
(0.250mm) (Fig.3). Visual inspection of the welding
has shown no rust, with a thick cord similar to that of
the base material and good quality, although they
were found small pieces of molten material, welded,
both on the rope to the base material.
Tensile tests have given satisfactory results. All
samples under investigation showed a break away
from the weld. The appearance of the curves is
completely overlapped for four samples (Fig. 5) that
had a scratch on the surface that triggered the crack
of breaking the last sample has confirmed the trend,
however, expressed by the previous (Fig. 5). The
average values obtained were: Ultimate Tensile
Stress=1035 N/mm2, Yield Stress = 960 N/mm
2,
Maximum Force = 65KN, E=114 KN/mm2.
The specimens tested in bending were
positioned so as to have a top of the weld in tension,
resulting in a bending angle of 30 degrees with a
fracture in the weld, and the other in order to
have the bottom of the weld in tension, resulting in a
bending angle of 92°, and most importantly, the
initiation of fracture within the bulk material, not
inside the cordon.
In this regard it should be noted that the
specification SAE AMS 4911L provides, for the
specimens prepared with only the base metal, so no
welding, a bending angle of 105 degrees (Fig. 8).
The development of cyclic fatigue tests, carried
out for load levels ranging between 60% and 90%
UTS, is explained in the diagram of Fig. 9: The
breaking of all the samples, occurred between the
cord and base material, as can be seen from Fig.
10. A more careful analysis of a fracture line denotes
that the onset of the crack was caused by the
presence of the undercut.
Table 4 shows the values of microhardness
Vickers and the coordinates of the points of
measurement (where the source was considered to be
the center of the weld,), referring to the sample is not
attacked. The microhardness of the bulk material is
about 330 HV, and this value increases approaching
the HAZ, to reach the maximum of about 390 HV
Fig. 4 – Fracture on specimen after Tensile
Stress test.
Fig. 5 – Stress Test
0
300
600
900
1200
0 0,05 0,1 0,15 0,2 0,25S
igm
a, [
MP
a]
e, [mm/mm]
Table 5
Averages of 3 measurement of UnderCut (micron)
Rate
[mm/s]
Power [KW]
1200 1450 1700
20 29 47,5 45
24 27,5 38 36
28 22,5 20,5 68
Table 6
ANOVA Undercut Source gdl Seq SS Adj MS F P
KW 2 1677,78 838,89 260,34 0,000
mm/sec 2 186,11 93,06 28,88 0,000
KW*mm/sec 4 2138,89 534,72 165,95 0,000
Error 9 29,00 3,22 S = 1,79505 R-Sq = 99,28% R-Sq(adj)= 98,64%
inside the nugget zone. The diagram overlaid on the
joint image, also shows how the values, more in the
upper zone, maintaining in all measurements, a delta
of about 10 HV area between the right and left of the
weld. It can be assumed that this effect has a
dependency with the process, providing a starting
point for further studies.
The analysis was completed by a statistical study
on the geometry of the UnderCut, investigating the
influence of two factors, power and speed. The data
analysis are given in Table 5, while Table 6 shows
the results for the evaluation of the effects. A model
that included the effects of two factors and their
interaction was used:
xijk=μ + αi + βj + (αβ)ij + εijk (1)
where i=1 to 3 (levels of effect α, speed rate);
j=1 to 3 (levels of effect β, Power of the beam), k=1
to 2 (number of repetitions)
Fig. 7 allows to confirm that, excluding the
technological parameters are not significant, to
reduce the formation of undercut which influences
the process, the initial choice (speed 24 mm / s,
power 1700 W) can be considered a compromise
between quality and applicability of the process,
ever, given the technological constraints imposed by
the plant used laser.
Conclusions
Data from this study may serve as a guide for
future work. The purpose was to learn better the
Fig. 6 - Hardness Vickers
KW
mm
/se
c
170016001500140013001200
28
27
26
25
24
23
22
21
20
>
– – – < 20
20 30
30 40
40 50
50
Under
Contour Plot of Under vs mm/sec; KW
Fig. 7 - Contour Plot of Undercut vs SpeedRate/Power
Fig. 8 – Bending test (a) angle 30° (b-c)
angle 92°
Fig. 9 - Fatigue Cycles Test
Fig. 10 - Fatigue Cycles Test
Fractures
0
200
400
600
800
1000
1000 10000 100000
σ max[MPa]
Number of Cycles
interaction of laser welding process of titanium and
to wire the quality of the joints to numerical value of
process variables.
The tests allow us to observe the excellent
protection of the weld . excellent data obtained by
tensile tests highlight the potential of the process.
Results from fatigue tests showed that undercut
get a decisive role in the beginning of fatigue cracks
(due to geometric aspects) as it propagates only in
the base material, at the edge of the HAZ.
Acknowledgements
Thanks for Alenia Aeronautica to provide the
panel and RTM spa for the welds.
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