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.

References

[1] Qi Yunlian *, Deng Ju, Hong Quan, Zeng Liying,

Electron beam welding, laser beam welding and gas

tungsten arc welding of titanium sheet, Materials

Science and Engineering A280 (2000) 177–181

[2] Zhang Li,SL Gobbi, I Norris, S. Zolotovsky, KH

Richter, Laser welding techniquesfor titanium alloy

sheet, J. of Material Processing Technology 65,(1997)

203-208

[3] Avanish Kr. Dubey, Vinod Yadava, Experimental

study of Nd:YAG laser beam machining—An

overview, journal of materials processing technology

195 (2008) 15–26

[4] O. Perret, M. Bizouard, Ph. Naudy and G. Pascal, D.

Nore´ , Y. Horde´ , and Y. Delaisse, Characterization

of the keyhole formed during pulsed Nd–YAG laser

interaction with a Ti–6Al–4V metallic target, Journal

of Applied Physics 90, Number 1, 1 July 2001

[5] R Rai1, JW Elmer2, T A Palmer2 and T DebRoy1,

Heat transfer and fluid flow during keyhole mode laser

welding of tantalum, Ti–6Al–4V, 304L stainless steel

and vanadium, J. Phys. D: Appl. Phys. 40 (2007)

5753–5766

[6] Hong Wang a,b,∗ , Yaowu Shi a, Shuili Gongb, Aiqin

Duanb, Effect of assist gas flow on the gas shielding

during laser deep penetration welding, Journal of

Materials Processing Technology 184 (2007) 379–385

[7] Y. Fan P. Cheng, and Y. L. Yao, Z. Yang and K.

Egland, Effect of phase transformations on laser

forming of Ti–6Al–4V alloy, Journal of Applied

Physics 98, 013518 (2005)

[8] G. Casalino, A.D. Ludovico, Finite element simulation

of high speed pulse welding of high specific strength

metal alloys, journal of materials processing

technology, 197 ( 2008 ) 301–305