Tensile Properties of LBW Welds in Ti–6Al–4V Alloy at Evaluated Below 450C

8/9/2019 Tensile Properties of LBW Welds in Ti6Al4V Alloy at Evaluated Below 450C

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Tensile properties of LBW welds in Ti6Al4V alloy at evaluated

temperatures below 450 jC

S.H. Wang a,*, M.D. Wei a, L.W. Tsayb

aDepartment of Mechanical Engineering, National Taiwan Ocean University, 20224 Keelung, Taiwan, ROCbInstitute of Materials Engineering, National Taiwan Ocean University, Keelung, Taiwan, ROC

Received 5 November 2001; received in revised form 9 July 2002; accepted 19 July 2002

Abstract

The influence of temperatures below 450 jC on the tensile properties of laser beam (LB) welds in dual phase Ti6Al4V

titanium alloy was investigated. The ultimate tensile strength of the weldment is slightly superior than that of the as-received

parent materials. Conversely, the yield stress of the weldment is inferior to that of the parent metal, especially in the 150 450

jC range. The elongation of the weldment was about 5% lower than that of the parent metal for the entire temperature range.

The prominent dislocation gliding on the pyramidal planes giving (1011) < 1123> and (1122) < 1123> type slip with low

critical resolved shear stress leads to both the weldment and parent metal exhibiting the lowest ductility at a temperature range

from 200 to 350 jC. The maximum hardness in the fusion zone (FZ) corresponds to the needle-like martensite aVformed after

the postsolidification phase transformation. As the temperature increases, the dimple dimension becomes larger and deeper.D 2002 Elsevier Science B.V. All rights reserved.

Keywords:Titanium alloys; Ti 6Al 4V; Tensile properties; Ductility loss; Laser welding; Slip system

1. Introduction

A laser beam (LB) is a very concentrated energy

source that provides a high power density and results

in producing a keyhole during welding, the same as inplasma arc or electron beam welding. The keyhole

feature provides the deep penetration that gives the

weld a high depth-to-width ratio. Numerous experi-

ments [1,2] have demonstrated that laser welding

permits the manufacture of precision welded joints

with a high depth-to-width ratio and a high welding

speed. Owing to these advantages, laser beam welding

is widely applied in industrial production. It has also

been reported that laser beam welding can produce

welds of a similar quality to electron beam welds[3].The Ti6Al4V alloy is commonly used in the aero-

space industries, nuclear engineering, civil industries,

chemical industries and medically implanted materials

for its significant strength-to-weight ratio, resistance

to corrosion and high temperature creep. The mechan-

ical properties of Ti6Al4V alloy are sensitive to

both temperature and strain rate and the effect of

temperature on flow stress is greater than that of strain

rate when tests are performed at constant strain rates

ranging from 5102 to 3 103 s 1 at temperatures

0167-577X/02/$ - see front matterD 2002 Elsevier Science B.V. All rights reserved.P I I : S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 1 0 7 4 - 1

* Corresponding author. Tel.: +886-2-24622192x3221; fax:

+886-2-24620836.

E-mail address:[email protected] (S.H. Wang).

www.elsevier.com/locate/matlet

Materials Letters 57 (2003) 18151823


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ranging from room temperature to 1100 jC[4]. Low

temperature stress relieving or aging at 540 jC carried

out after the welding operation improves the tensile

properties but decreases the toughness of the fusionzone. The weldments after high temperature annealing

at 950 jC do not have an increased tensile ductility

but exhibit an improvement of the toughnessof both

the fusion zone and the heat-affected zone [5]. Aging

at 593 jC results in an increase in yield strength and a

decrease in ductility at both room temperature and 593

jC[6].There are very few reports in the literature on

the tensile properties below 450 jC. Thus, the tensile

properties of Ti 6Al 4V alloy and its welds pro-

duced by a CO2 laser were studied at various temper-

atures, room temperature, 150, 300, and 450 jC,

respectively.

2. Materials and experimental techniques

Cold rolled plates 3.3 mm thick of commercial Ti

6Al4V alloy with composition (in weight percent)

5.7% aluminum, 4.0% vanadium and balance titanium

were used. The microstructure of as-received Ti

6Al4V consisted of a small percentage of the beta

phase distributed at the elongated alpha grain boun-

daries (shown in Fig. 1). All welds were madeutilizing a Rofin-Sinar RS 850 5 KW CO2 laser set

up for bead-on-plate welding. The laser beamwelding

(LBW) process parameters are listed in Table 1.

Tensile specimens of the parent metal and of the

weldment were made parallel to the rolling direction.

A schematic configuration of the rolling and welding

direction, sampling orientation, and tensile specimen

dimension is shown in Fig. 2. The microhardness of

the weldment was measured from the base metal

(BM), across the heat-affected zone (HAZ), to the

fusion zone of the weld metal in a transverse direc-

tion, and also along the center line of the fusion zone,

using a Mitutoyo Vickers microhardness machine

under a 300-g load, maintained for 15 s. Tensile tests

with a displacement extensometer attached in the gage

length of 25.4 mm were conducted at room temper-

ature, 150, 300, and 450 jC, respectively, using a

Shimadzu AG/AGS-G machine at a strain rate of

6.6 10 4 s 1 equivalent to a cross-head rate of 1

mm/min. The fracture surfaces of each test specimen

were observed using a Hitachi S4100 scanning electr

on microscope (SEM). The fractographs were used to

identify the fracture modes. For the optical micro-

structure studies, specimens were mechanically pol-

ished and etched in a metallographic etchantcomposed of 5% HNO3, 10% HF, and 85% distilled

water.

3. Results and discussion

Due to the metal vapor in the weld from laser beam

welding (LBW), the macroscopic photograph (Fig.

3a)shows a symmetrical undercut defect on both the

top and the bottom surface of the weld. The center of

the fusion zone presented a convex shape attributed tovolume contraction, surface tension, and phase trans-

formation. This welding defect cannot be eliminated

even though the laser beam welding parameters were

changed. This undercut can be decreased by slightly

remelting the weld surface by increasing the focal

length and decreasing input power to obtain a large

heating zone using low input heat. The macrograph in

Fig. 3aalso demonstrates the growth direction of the

dendritic grains, which follow the heat flow direction

during solidification. The weldment of LBW TiFig. 1. Optical microstructure of Ti6Al4V parent metal.

Table 1

Laser welding parameters

Laser power 2500 W

Travel speed 1500 mm/min

Focal lens ZnSe

Focal length 200 mm

Shielding gas 25 lpm He

S.H. Wang et al. / Materials Letters 57 (2003) 181518231816


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6Al4V shows a narrow fusion zone about 2 mm

wide with a further small heat-affected zone (HAZ) of

about 0.5 mm. The enlarged micrograph (Fig. 3b)

shows that the microstructure rapidly transits from the

fusion zone (FZ) to the base metal (BM) through the

HAZ. The HAZ microstructure (Fig. 3c and d) con-sists of a mixture of martensitic aV, acicular a, and

primary a. This kind of microstructure corresponds to

a specimen quenched from a temperature below the

beta transus [7]. The microstructure of short marten-

sitic aV and acicular a can be observed in the HAZ

adjacent to the FZ (Fig. 3d). Further away from the

FZ, a relative increase in primary a can occur due to

the relative lower welding cooling rate. The marten-

sitic aV transformed from the h grains, which corre-

sponds to a structure quenched from the h phase

above the beta transus (980 j

C) [7], constitutes themicrostructure of the fusion zone (Fig. 4).

The microhardness distribution of the as-welded

condition in the laser weld is indicated in Fig. 5.The

hardness of the fusion zone and the heat-affected zone

is higher than that of the base metal (Fig. 5). The

fusion zone exhibits the highest hardness, and the

hardness drops rapidly as the distance increases from

the fusion line (Fig. 5b). The hardness results are

consistent with the microstructural observations. In

other words, the transformed needle-like matensite aV

present in the solidified fusion zone results in the

highest hardness in the fusion zone.

In this study, the weld was shielded with helium

gas, therefore, the fusion zone was not significantly

contaminated with oxygen, nitrogen and carbon. The

strength of the weldments should be an intrinsicmechanical property. In other words, the strength of

the weldment will depend on the microstructure,

which is a variable resulting from the choice of

welding parameters. The tensile strength and ductility

are sensitive to temperature. Normally increasing

temperature will cause a strength decrease and a

ductility increase. The flow stress curves below 450

jC for the Ti6Al4V parent metal and its weldment

are displayed in Fig. 6. The flow stress trend at

different temperatures (RT, 150, 300 and 450 jC)

for Ti6Al4V parent metal (Fig. 6a)is very similarto that for the weldment (Fig. 6b). Tensile strength,

yield strength and elongation at different temperatures

for both the parent metal and the weldment are

summarized in Fig. 7. The behavior of the ultimate

tensile stress, yield stress and elongation as a function

of the temperature shown inFig. 7is almost identical

to that reported in the literature [8],in which a tensile

test of Ti 6Al 4V material was performed in a

vacuum furnace with a vacuum approximate 10 4

Pa at a strain rate of 3 10 4 s 1.

Fig. 2. Schematic diagram of welding direction, weldment tensile specimen sampling orientation and the dimension of specimens.

S.H. Wang et al. / Materials Letters 57 (2003) 18151823 1817


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The ultimate tensile strength of the weldment is

slightly higher than that of the parent metal. This

could be caused by the hardened weld metal in the

fusion zone as shown inFig. 5b.As a consequence of

the presence of the hard fusion zone and the heat-

affected zone in the gage length of the weldment

tensile test specimen, the tensile load was actually

applied to a specimen with a composite zone. The

load direction was perpendicular to the analogous

lamellate composite structure consisting of layers of

base metal (BM), heat-affected zone (HAZ) and

fusion zone (FZ) generated from LBW. Therefore,

the deformation behavior of the weldment under iso-

stress condition is determined by the soft base metal

zone. In contrast to the weldment specimen, the

deformation behavior of the parent metal specimen

comes from the entire gage length filled with parent

metal. Furthermore, the relative uniform microhard-

Fig. 3. (a) Macrograph of laser beam welded Ti6Al4V. (b) Enlarged micrograph of both the interface of (right) the fusion zone/the heat

affected zone and the interface of (left) the heat affected zone/the base metal. (c) Microstructure of the interface of the heat affected zone/the

base metal. (d) Microstructure of the interface of the fusion zone/the heat affected zone.



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ness distribution of the weldment, shown in Fig. 8

after testing at 300 jC, which acted like aging for an

extended period of time (about 2 h) reveals a com-

paratively higher hardness of the fusion zone, the

heat-affected zone and the base metal than that of

the as-welded sample shown in Fig. 5. The phenom-

enon of increasing hardness in the parent metal zone

shown inFig. 8, as well as parent metal sample after

testing at 300 jC, could be attributed to the formation

Fig. 4. Microstructure of the fusion zone, martensitic aV (dark)

needle-like precipitate in h grains (light).

Fig. 5. As-welded condition. (a) The black dots and numbers

indicate the measured location and hardness characteristic. (b) A

plot of the distributed microhardness profile.

Fig. 6. (a) Temperature-dependent flow stress of Ti 6Al 4V parent

metals. (b) Temperature dependent flow stress of the Ti6Al4V

weldments.



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of Ti3Al[6]due to an aluminum content in the range

of 48 wt.% in the Ti6Al4V alloy. Fig. 7 also

illustrates the trends in ductility loss that accompany

aging in Ti 6Al 4V for both the parent metal and the

weldment. This implies that Ti3Al is a major contrib-

utor to the ductility loss with aging effect. The furtherincrease in microhardness at the fusion zone (FZ) of

the weldment after testing (Fig. 8) has clearly been

shown by Chestnutt et al. [9] to result from the

formation of an irregular shape of ellipsoidal h

precipitate in the tempered martensite. It brings about

a deterioration in the impact/fracture toughness and

yield strength of the FZ [5].Therefore, the weldment

during the elevated temperature test could exhibit a

lower yield strength and ductility than that of the

parent material(Fig. 7)in the temperature range from

150 to 450 j

C. These precipitates result in a prema-ture yield in the weldment sample and the trend in

weldment elongation is about 5% lower in strain than

the parent metal for the entire range of temperatures.

In addition, the drawback in ductility, a dip in the

ductility, for both the parent metal and the weldment

rapidly drops to the minimum around 250 to 300 jC,

after that, the ductility increases again with temper-

ature. The decrease in ductility phenomenon around

300 jC in Ti6Al4V for both the parent metal and

the weldment may be explained as follows: first the

Fig. 7. Tensile strength and elongation as a function of temperature for both the parent metal and the weldment in Ti6Al4V titanium alloy.

Fig. 8. The microhardness profile of the weldment and the parent

metal after tensile test at 300 jC exposed for about 2 h.



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effect of the oxide film and second the temperature

dependence of the deformation slip systems. The

color of the specimen surface changes from the shiny

gray of titanium metal to blueas the test temperatureis elevated. Fukuzuka et al. [10] proposed that the

thickness of titanium oxide film could be estimated

via the various specimen surface colors [10] due to

light interference. The colors of the specimen surfaces

after testing at 150, 300 and 450 jC were colorless,

yellow and blue, which indicates that the thickness of

the oxide film was less than 10, 1025 and 2570

nm, respectively. Because the maximum estimated

thickness via the surface color of the oxide film based

on the aforementioned of 70 nm at 450 jC occupies

only 0.0023% of the tested specimen thickness, thus

the first factor, the influence of the oxide film on the

tensile behavior can be ignored intuitively. A rational

explanation for the ductility loss is the local slip

systems associated with the deformation varies with

temperature. At room temperature, most dislocations

in a Ti are of (1010) < 1120>type on the prismatic

planes [11]. The slip system is a secondary (0001)

< 1120>type on the basal plane ora (1010)

type on the pyramidal planes [11]. In the range from

200 to 300 jC, the secondary slip systems are the

most active for dislocation motion. There is prominent

dislocation gliding on the pyramidal planes of (1011)< 1123> and (1122) < 1123> type slip systems. The

latter {1122} planes is unusual in Ti alloys because

twinning is commonly activated in this plane [11].

The literature reports that the deformation of a Ti

6Al4V alloy containing 0.22 wt.% oxygen at room

temperature produced a predominant planar pyramidal

slip after the samples were aged at 227 jC (500 K)

[12]. On the basis of Churchmans proposition [13],

the oxygen (2.83 wt.%) and nitrogen (0.32 wt.%)

atoms listed inTable 2occupying octahedral positions

interfere with < 1

21

0> type slip more severely on thebasal (0001) and the prismatic {1010} slip planes than

on the {1011} pyramidal slip planes in a-Ti. The

interstitial sites can also be coplanar with one of the

two possible pyramidal slip planes in a-Ti [11].

Similar to the work of Lecomte et al.s [11], in the

range 150 300 jC of this investigation, the most

common slip system is definitely the pyramidal sys-tem (i.e. (1011) and (1122) ) indi-

cating that the other glides (i.e. (1010) < 1120>,

(0001) < 1120> and (1011) < 1120>) have higher

critical resolved shear stresses. Therefore, the twinned

slip system activated in the pyramidal planes may

cause the drop in ductility between 150 and 300 jC to

occur for both the parent metal and the weldment. At

higher temperatures above 300 jC, prismatic slip is

the common glide but cross slip is very frequent and

all types of slip are activated [11]. As the stacking

faultenergy of titanium alloy is low at 15.4 erg/cm2

[14], partial dislocations recombine more easily and

form a prefect dislocation with thermal assistance in

order to cross slip. The subsequent by-pass disloca-

tion Ti3Al particles interaction is more homogeneous

[6], which means that deformation process proceeds

smoothly without difficulty due to recombination of

partial dislocations and thermal assistance to cross

slip. As a consequence, the ductility increases again

after 300 jCandup to 450 jC. From the limited work

of others [15], there is another ductility loss in

titanium alloys at higher temperature about 900 jC,

which falls in the HCP alpha (a) to BCC beta (h)phase transformation temperature range.

The gage length of tensile specimen in the weld-

ment (Fig. 2) is a composite structure, consisting of

zones of the base metal, the heat-affected zone (HAZ)

and the fusion zone of weld metal (Fig. 3).However,

the gage length of tensile specimen in the parent metal

is a single metal structure. Therefore, tensile test

results on samples incorporating the stronger weld

differs from tests carried out in the parent material.

From the results of the measured microhardness

profile (Fig. 5), the maximum hardness is presentedin the fusion zone and the minimum hardness in the

parent metal. The hardened fusion zone leads to a

higher tensile strength, lower yield strength and elon-

gation in the weldments at the evaluated temperatures.

Therefore, it is expected that necking instability and

fracture should occur in the relative soft parent metal

region of both the weldment and the parent material.

In a word, all specimens ruptured in the parent metal

after tensile fracture, whether it is from weldment or

parent metal. The fracture surface shows a typical cup

Table 2

Chemical analysis of parent (received) metal in Ti 6Al 4V alloy

Ti Al

(wt.%)

V

(wt.%)

Fe

(wt.%)

H

(wt.%)

O

(wt.%)

N

(wt.%)

Ti 6Al 4V Bal. 5.70 4.00 0.31 0.16 2.83 0.32



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Fig. 9. The SEM fractographs of parent metal and weldment after tensile failure at various temperatures.



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and cone ductile feature. The SEM fractographs of the

parentmetal plates and LBW weldments are shown in

Fig. 9. The dimple feature is clearly displayed in all of

the fractographs sampled from different temperatures.The dimensions of the dimple become larger and

deeper as the temperature increases.

4. Conclusions

The conclusions of this study in laser beam weld-

ing (LBW) of Ti6Al4V alloys are summarized as

follows.

(1) The microstructure of the fusion zone reveals a

needle-like martenstic aVstructure formed from trans-

formed h. The microstructure of the heat-affected

zone is a mixture of martenstic aV, acicular a and

primary a.

(2) The microhardness profile across the weldment

indicates that the hardness of the fusion zone is higher

than both the HAZ and parent metal.

(3) The ultimate tensile strength of the weldments

is slightly higher than that of the parent metal for all

testing temperatures because of the harder fusion

zone.

(4) In samples aged at the elevated test temper-

atures, a selected example of increasing hardness at300 jC may explain that the hardened weldment due

to the precipitation may be attributed to the degrada-

tion of yield strength in the weldment between 150

and 450 jC. The tensile yield strength decrease at 300

jC in the weldment is caused by the h precipitation in

the tempered martensite of the fusion zone.

(5) The minimum ductility for both the parent

metal and the weldment is caused by the twinning

slip system activated on pyramidal planes at 300 jC.

(6) The SEM fractographs of the parent metal and

the LBW weldment are characterized by a ductiledimple feature. The dimple size becomes larger and

deeper with increasing temperature.

Acknowledgements

The authors are grateful to the National Science

Council, Taiwan (ROC) for the financial supportthroughout this work under contract number NSC 89-

2623-7-019-003.

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Tensile Properties of LBW Welds in Ti–6Al–4V Alloy at Evaluated Below 450C