effect of built orientation on direct metal laser...

Indian Journal of Engineering & Materials Sciences

Vol. 25, February 2018, pp. 69-77

Effect of built orientation on direct metal laser sintering of Ti-6Al-4V

P Chandramohana*, Shepherd Bhero

a, Babatunde Abiodun Obadele

b,

Peter Apata Olubambib & B Ravisankar

c

aDepartment of Metallurgy, University of Johannesburg, South Africa 2028 bDepartment of Chemical Engineering, University of Johannesburg, South Africa 2028

cDepartment of Metallurgical and Materials Engineering, National Institute of Technology, Trichy 620 015, India

Received 26 December 2016; accepted 23 June 2017

The use of direct metal laser sintering for fabrication of three-dimensional engineering parts is fast gaining

momentum in the engineering industries. This process consolidates metallic powders by using a laser source. In this

study, the effect of built direction (horizontal and vertical) on the microstructure, mechanical properties and corrosion

resistance of Ti-6Al-4V alloy printed using direct metal laser sintering (DMLS) technology was investigated. Results

show that microstructure of Ti-6Al-4V surface have few pores. The melt pool of vertically built parts revealed the

presence of fine cellular-dendritic martensite. Microhardness of vertical built specimens was relatively higher than the

horizontal built specimens which could be due to the presence of vanadium carbide. Fractography analysis revealed

that horizontal built specimens displayed higher ductility than the vertical built. For horizontal built sample, the

corrosion current densities in 3.5% NaCl and 1 M HCl solutions are significantly lower when compared with that of 1

M H2SO4, which might be because of stable oxide layer formation. Generally, horizontal build parts yield relatively

better mechanical properties and corrosion resistance supported by ideal microstructure. In case of vertical built-up, it

is better to limit to a maximum height of 40 mm for the set parameters.

Keywords: Titanium, Laser, Sintering, Corrosion, Microhardness, Microstructure, Manufacturing, Morphology, Fractography

Additive manufacturing (AM) or 3D printing as it is

often termed, forms complex shapes and structures in

different alloys like stainless steels, titanium, copper

using different technologies like laser metal deposition

(LMD) or cladding, direct metal laser sintering

(DMLS), selective laser melting (SLM)1,2

. DMLS, also

termed as selective laser melting is reverse to that of

erosive manufacturing or material removal process that

builds near net shaped components. These components

can be directly fit for usage in aircraft industry as well

as in biomedical industry3. The basic process of this

DMLS process consists of coating the building

platform with metallic powders such as titanium,

magnesium etc, followed by partially fusing the

powder using fiber laser beam source and lowering the

platform to remove the excess powder4-6

.

Ti–6Al–4V (Ti64), categorized as α+β Ti alloy is a

workhorse in Ti alloy family, which is attributed to its

high specific strength and excellent biocompatibility.

Studies on modification of mechanical properties and

metallography with respect to process parameters

have been carried out in this alloy by few

researchers7-9

. Uniform tensile strength have been

reported at different locations of a laser formed Ti64

plate which is developed through pre-alloyed powder

deposition on a moveable substrate using free-form

fabrication process.10

There was no significant effect

on microstructure as well as the mechanical properties

of sample layer closer and farther from the build plate

in electron beam melted (EBM) built structures

without changing the electron beam parameters11

. At

the same time, another researcher who studied the

same effect of distance from the build plate by

changing the electron beam parameter reports a

change in microhardness and microstructure12

. Built

orientation also has considerable effect on the

component properties. Horizontally manufactured

samples differ from vertically manufactured samples

in terms of mechanical properties due to its grain

orientation which in turn is decided by solidification

pattern. Elongated grain growth occurs from the

cooler surface (substrate) to the hot surface (top

surface) and hence samples built with different

building direction vary with its tensile properties13

.

Fracture analysis made on the ductile and brittle

Ti6Al4V cylindrical and cubical parts made through _______________

*Corresponding author (E-mail: [email protected])

INDIAN J. ENG. MATER. SCI., FEBRUARY 2018

70

DMLS technology reveals cup and cone failure with

staircase like feature. The ductile part contains

ultrafine lamellar (α + β) structure and brittle part

contains α’ martensite in its microstructure

14.

In the presented literature, the aim is to achieve

better metallography and mechanical characteristics

of Ti-6Al-4V components without any necessity for

post processing. Even though certain investigations

have been carried out, there is still a need for further

research with respect to build orientation and its

influence on corrosion behaviour of Ti6Al4V. It is

imperative to mention that there are very few studied

reported on corrosion performance of DMLS

processed Ti-6Al-4V and no literature noticeable on

its behaviour in 3.5% NaCl and 1 M H2SO4 solutions.

This paper presents the effect of built orientation

(horizontal and vertical) of Ti-6Al-4V parts using

DMLS technology. Discussions are made related to

the machining of test specimens from the parts, surface

characteristics, microstructure, hardness, tensile testing,

and fractography and corrosion studies.

Materials and Methods

Direct metal laser sintering process (DMLS)

Ti-6Al-4V alloy powder of particle size 45 µm with

spherical shape was supplied by TLS Technik GmbH &

Co. Germany and used for manufacturing rectangular

parts of size 100 × 30 × 15 mm. Layers were deposited

vertically as well as horizontally using DMLS

technology in EOSINT M270 machine with processing

parameters of laser power170 W, scanning speed 1400

mm/s, layer thickness 30 µm, laser spot size 140 µm in

high purity argon atmosphere to avoid oxidation of Ti.

Machining and specimen preparation

Wire cutting machine was used to cut specimens

from as-sintered horizontal built (HB) and vertical

built (VB) blocks for tensile, microstructure and

micro-hardness testing as shown in Fig 1.

Bulk tensile samples were cut out from as-sintered

HB and VB blocks as per ASTM E8 standard: total

length 100 mm, gauge length 25 mm, width 6 mm and

thickness 3 mm. The leftover material was machined

into 5 pieces from top to bottom in case of VB and

sideways in case of HB parts to characterize its

mechanical properties and metallography with respect

to the distance from built platform.

Macrostructural and microstructural characterization

Samples were characterized using field emission

scanning electron microscopic technique (Jeol,

FESEM, JSM-7600F) along with energy dispersive

X-ray spectrometer (EDS). The specimens were

finally polished using fumed silica and etched for 30 s

using Kroll’s reagent. The microstructures were then

examined under the optical microscope attached to the

microhardness tester and also under scanning electron

microscope. XRD study was carried out using a

Philips diffractometer (PW1710) and X-Pert High

Score Plus software.

Mechanical properties tests

Tensile testing was carried out using MTS (make)

C64.605 (model) 600 kN capacity machine keeping

with the ASTM E8 standards for all specimens at a

strain rate of 2.5×10-4

s−1

. The engineering stress–

strain curve of each specimen was constructed to

obtain the ultimate tensile strength (UTS), yield

strength (YS- 0.2% off set method) and % elongation.

Fractography was studied using SEM and discussed.

A Future-tech Vickers microhardness tester was used

at a load of 0.98 N (100 gf) and dwelling time was

15 s. The average of five indentations was considered

for all the specimens.

Corrosion test

Open circuit potential and potentiodynamic

polarization tests were carried out using Autolab

PGSTAT 302 supplied by Metrohm, South Africa.

Fig. 1. — Schematic illustration of the horizontal, vertical Ti–6Al–4V ELI parts (100 mm*30mm*15mm) built by DMLS with layout of

standard machined specimens for metallography and mechanical testing. The built direction is indicated by an arrow.

CHANDRAMOHAN et al.: DIRECT METAL LASER SINTERING OF Ti-6Al-4V

71

The system is equipped with general purpose

electrochemical system (GPES) software used for the

Tafel extrapolation. Counter electrode material was

chosen as graphite rod, reference electrode as

saturated silver/silver chloride (Ag/AgCl) and

working electrodes as samples. Scan rate of 2 mV s−1

was chosen to carry out polarization tests from -0.3 V

to 0.7 V. Three different media; 3.5% NaCl,

1 M H2SO4 and 1 M HCl were used to carry out the

tests at a temperature of 20±2°C.

Results and Discussion

Surface characteristics

Surface finish of the part manufactured could play

significant role on its mechanical properties. Hence, it

is imperative to make a comparative surface

characteristic study on both the HB and VB parts.

Figure 2 shows the surface morphology of VB and

HB parts of Ti-6Al-4V alloy. The top layer of VB part

(Fig. 2a) is characterized by a mixed mode

consecutive steps and wavy appearance which is

different from the findings of Rafi et al.15

who have

reported wavy appearance without any discontinuity

in circular specimens built using EOS M270 SLM

machine. The bottom layer of VB part (Fig. 2b) is

observed to have even hatch lines with appropriate

overlap which is in agreement with the observations

of Gong et al.16

, which states the effect of scan speed

and energy density on hatch lines and melt pool

overlap. The side surface layer (Fig. 2c) is seen to

possess circular solid powder particles which implies

that, subsequent deposition of layers retain more heat

at the central region whereas in the extreme outer

surface, the heat is not sufficiently retained. The top

layer of HB part (Fig. 2d) is found to have a wavy

appearance with some unmelted circular powder

particles which is different from the findings reported

by Rafi et al.15

They observed the formation of

consecutive steps in circular specimen. The bottom

layer of HB part (Fig. 2e) is characterized by even

hatch lines and appropriate overlap which is in

agreement with the reporting of Gong et al.16

. The

side surface (Fig. 2f) developed circular powder

particles without melting just like the VB parts.

Altogether, certain surface characteristics differ from

previous studies which are due to the variation in

process parameters and built orientation.

Microstructure and microhardness of as sintered Ti-6Al-4V alloy

Figure 3 shows the microstructure of HB specimen

(Fig. 3a) with acicular α phase finely dispersed in

β phase matrix and VB specimen (Fig. 3b) with

martensite needles. In case of HB specimen, the

length (100 mm) and width (30 mm) of the deposited

layers are more when compared to the length (30 mm)

and width (15 mm) of VB specimen. This leads to

more heat retention in HB and relatively higher

cooling rate in VB specimens. Hence, martensite

could have formed in the VB specimens, but not in

HB specimens. Martensite formation in laser sintered

Ti-6Al-4V has been reported by Knowles et al.17

which is due to the extreme temperature change as the

laser passes across the powder bed. However,

Vrancken et al.18

reported the presence of only

α phase in the SLM manufactured Ti-6Al-4V matrix.

Similar observation of martensite existence in the

microstructure of VB specimens and its

nonappearance in HB specimens were reported by

Yang et al.19

The sintered microstructure reveal

imperfections such as inclusions and pores. Similar

observations have been reported Gong et al.20

, stating

that laser sintered Ti-6Al-4V part shows some of

defects or pores in the as-polished condition. The type

and the amount of defects vary on the basis of energy

density. Low energy density could result into partial

Fig. 2 — Surface morphology of (a) top, (b) bottom and

(c) side layer of vertical built. Surface morphology of (d) top,

(e) bottom and (f) side layer of horizontal built


72

powder melting or inadequate fusion between

successive layers and high energy density causes gas

entrapment due to the increased melt pool depth

thereby forming pores.

Figure 4 shows the hardness profile made in the

horizontal (H-Top and H-Bottom) and vertical built

specimens (V-Top and V-Bottom). The micro

hardness of HB part is measured at the top and bottom

of the specimen and recorded as H-Top and

H-Bottom. In the VB part, five specimens were made

from bottom to top (moving away from build plate)

and microhardness values were recorded. It can be

noticed that hardness values are quite higher

(365, 463 HV0.1) in H-Top and V-Middle (V-3 Top)

as compared to the other sections due to the presence

of vanadium carbide. To ensure its presence, EDS

analysis (composition) have been carried out in these

specimens and a report is shown in Fig. 5. It contains

higher carbon with Al, Ti and V. Al will not form

carbides. Gibbs free energy for the formation of both

TiC and VC is negative at the melting temperature,

indicating the possibility of the formation of TiC and

VC (Wang et al.21

). But, solubility of carbon is low in

Ti. Therefore, more possibility is there for the

formation of VC. The suspected particle is analysed

using SEM-EDS for identification. Since the particle

is too fine for resolving in SEM-EDS, exact size,

distribution could not be attained. However, XRD

results shown in Fig. 6, supports the presence of VC

particles along with α and β phases. The mechanism

underlying here is that due to more heat retention,

some carbon from VC might have come out to form

carbon rich zone thereby increasing hardness.

El‐Labban et al.22

have reported similar observation.

Fractography and tensile testing

Fractography study was carried out

macroscopically and microscopically using a scanning

electron microscope after subjecting the specimens to

tensile testing. The macro scale fracture surface is

shown in Fig. 7(a). It can be observed that both

specimens did not show a 45o angle failure but a

channel type failure which indicates a mixed mode of

ductile and brittle fracture. On one hand, it can be

visualized that the specimens made from HB part

show a visual reduction in area (necking) while on the

other hand, the VB part does not display any such.

Fig. 3 — SEM images of (a) horizontal built and (b) vertical built specimen

Fig. 4 — Microhardness profile of (a) horizontal and vertical part

top layers and (b) horizontal and vertical part bottom layers


73

This infers that HB specimens are relatively ductile

than the VB specimens.

The micro scale fractography shown in Figs 7

(b, c and d) justify the macro observation with more

number of dimples in HB specimens indicating

ductile fracture and river type pattern in VB

specimens, indicating a relatively brittle fracture.

Also the fractography of some vertical specimens

(Figs 7e and f) show small areas without

coalescence. The quantity of dimples remain more

or less the same in all the fractured tensile

specimens sliced from the base to top of the HB

part. This observation is well supported by the

tensile results shown in Fig. 8.

Tensile results of HB and VB parts are represented

in Fig. 8. The UTS of HB part remains in the range of

1336-1370 MPa with an elongation of 6.5% and yield

strength in the range of 1151-1275 MPa. At the same

time, VB part shows a drop in UTS of 1263 MPa,

4.87% elongation and yield strength of 1201 MPa.

The difference in the slip length normal to α colony is

attributed to the cause for variation in strength and

elongation between VB and HB specimens. Slip

length of HB specimens could be more than VB

Fig. 6 — XRD analysis of (a) horizontal built part at top and

bottom layer and (b) vertical built part in various layers

Fig. 7 — (a) Macro scale fractography of horizontal and vertical

built specimens (b, c, d) micro scale fractography of horizontal

built and (e, f) vertical built specimens

Fig. 5 — EDS analysis supporting the presence of VC particles


74

specimens which lead VB specimens to intergranular

fracture. This trend is not observed in the recent

research published by Zhao et al.6 wherein it is found

that both tensile strength and elongation are better in

vertical orientation than in horizontal orientation.

Knowles et al.17

pointed that the residual stresses that

remain in the as-sintered parts will affect the yield

strength whereas in both the parts built, there is no

such drop in yield strength. This indicates that

residual stress is not significantly induced and hence

no stress relieving is needed. However, beta annealing

is necessary to increase ductility higher than 10% for

medical applications. The difference in elongation

could be due to varying VC content in the HB and VB

parts. Also, there exists a difference in the yield

strength between the HB top layer and bottom layer

due to the presence of VC in H-Top.

Altogether, HB parts yields relatively better

mechanical properties supported by ideal

microstructure. In spite of few porosities and partially

melted particles observed at the surface and side of

the manufactured parts, the tensile properties are not

much affected which shows that the quantum of

porosity has not exceeded the permissible limit of

1% as reported by Rafi et al.15

. In case of unavoidable

design strategy, parts can be built vertically but

limited to a maximum height of 40 mm for this

selected thickness and width.

Corrosion tests

The open circuit potential (OCP) curves versus

time of immersion in 3.5% NaCl, 1 M H2SO4 and 1 M

HCl solutions for HB and VB are presented in Fig. 9.

At the end of immersion for 2 h, the OCP of HB

(Fig. 9a) in all the three media were about -0.27, -0.6

and -0.2 V, respectively. It could be observed that

upon immersion in 3.5% NaCl, the OCP increases to

more noble values with less fluctuations and a steady

curve was recorded after 3600 s suggesting a passive

film formation on the surface of HB of a quite stable

nature. On the other hand, a steady decrease in OCP

was observed upon immersion in 1 M H2SO4 solution.

The OCP further decreased sharply between 1400 s

and 2000 s and stabilizes thereafter until the end of

immersion reaching a value of -0.6 V. The sharp

decrease in potential could be attributed to an increase

of the chemical reactivity of titanium in sulphuric

acid. Generally, OCP curves recorded in Fig. 10a

indicate that Ti exhibit least potentials when subject

to sulphuric acid environment. In other words, Ti

would perform better in NaCl and HCl environments

as compared to H2SO4 environment.

Figure 9b shows the OCP for VB sample after

immersion in 3.5% NaCl, 1 M H2SO4 and 1 M HCl

solutions. It can be observed that at the early stage

of immersion, the potentials of VB sample

immersed in 3.5% NaCl and 1 M HCl increases

steadily with small fluctuations to more noble

potential values until steady state is reached till the

end of immersion period. This is attributed to oxide

film formation. On the other hand, continuous

potential drop was observed in the 1 M H2SO4 plot

Fig. 8. — Tensile results of horizontal and vertical parts.

Fig. 9 — OCP measurements for sample (a) horizontal built (HB)

and (b) vertical built (VB) Ti-6Al-4V alloy in three different

media


75

until after 6000 s. The oxide layer formed in 1 M

H2SO4 is generally porous and allows SO4-

ions

penetration which cause local breakdown of the

passive film, as a result, formation of pits are

initiated. After 6000 s, a steady potential was

recorded which indicate the formation of stable

oxide layer. Generally, OCP value of -0.18 V was

attained for VB in 3.5% NaCl while that of 1 M

H2SO4 and 1 M HCl were -0.6 and -0.24 V,

respectively. Indeed, both samples exhibit close

potentials in the three different media. This

suggests that the samples irrespective of the build

direction (vertical or horizontal) will display

similar potentials. Nevertheless, the OCP

measurement will only provide the tendency for

corrosion to occur.

Figure 10 depicts the polarization curves for HB

and VB samples where the sweep rate was of 2 mV/s

in 3.5% NaCl followed by 1 M H2SO4 and

subsequently 1 M HCl solutions. Fig. 10a shows that

HB immersed in both NaCl and HCl doesn’t have any

active-passive region in the polarization plots

constructed in accordance with the Tafel region.

However, in 1 M HCl, HB fell into the passive region

quite stably while a pseudo-passive behaviour was

recorded in 3.5% NaCl. Furthermore, the anodic

polarization curves of HB in 1 M H2SO4 exhibited

active-passive region followed by a narrow passive

region beginning from -0.24 V to 0.06 V. Table 1

portrays both the anodic and cathodic branches of the

polarization plots constructed using Tafel analysis in

order to determine the current density (Icorr) values.

Quite a low value of corrosion current density was

obtained in case of 3.5% NaCl and 1 M HCl which

were 0.022 µA/cm2 and 1.938 µA/cm

2 respectively in

comparison to 1 M H2SO4, which was found to be

17.450 µA/cm2. It can be inferred in accordance with

the results obtained that the oxidic HB surface formed

as an action of NaCl solution has improved corrosion

protection features when compared with the one

formed when 1 M HCl and 1 M H2SO4 solutions were

put to action. It is reported that Ti and Ti alloys do not

exhibit better corrosion performance in reducing acids

such as H2SO4 due to chemical dissolution of the

surface oxide film.

Figure 10b shows the polarization curves of VB

sample in three different media. All the

potentiodynamic anodic polarization curves in 3.5%

NaCl, 1 M H2SO4 and 1 M HCl solutions

respectively show no significant difference in shape

when compared with HB except for 1 M HCl which

display a current plateau after the Tafel region

followed by narrow transpassive region. The

existence of current fluctuations was recorded on the

pseudo-passive region in 3.5% NaCl indicating a

fairly stable oxide film formation (this is also

evidenced in Fig. 9b).

Figure 11 clearly display the SEM morphology of

the HB sample after the corrosion test with corrosion

products present on the sample surfaces, however, no

micro cracks was detected. Figure 11b showed

corrosion products in form of flower-like structure on

the surface of sample immersed in 1 M H2SO4. The

presence of acicular martensitic structure also

Fig. 10 — Polarization curves for DMLS Ti-6Al-4V solid

(a) horizontal built and (b) vertical built (HB) samples immersed

in three different media

Table 1 — Current density values for HB and VB samples in three

different solutions.

Solutions Ecorr (V) Icorr (µA/cm2) Corrosion rate (mm/yr)

Horizontal built (HB)

3.5% NaCl -0.374 0.022 1.97 x 10-4

1 M HCl -0.444 1.938 1.70 x 10-2

1 M H2SO4 -0.613 17.450 1.53 x 10-1

Vertical built (VB)

3.5% NaCl -0.369 0.050 4.46 x 10-4

1 M HCl -0.420 1.676 1.47 x 10-2

1 M H2SO4 -0.615 17.090 1.50 x 10-1


76

demonstrates the etching effect of sulphuric acid. In

all, sample immersed in 1 M H2SO4 displayed severe

corrosion attack. This is an indication that titanium

has relatively poor corrosion resistant in reducing

acids such as H2SO4. Surface textures were similar

among titanium samples immersed in 3.5% NaCl and

1 M HCl solutions.

Conclusions

Ti-6Al-4V has been successfully fabricated using

DMLS technique and the effect of built direction on

surface morphology, microstructure, microhardness,

tensile properties and corrosion have been

investigated. The main conclusions are:

(i) Surface morphology of as-sintered parts

revealed a mixed mode consecutive steps and wavy

appearance at its top and bottom surfaces with

circular powder particles without melting at its side

surface.

(ii) Metallography of horizontally build

specimens revealed α+β phases and vertically build

specimens revealed fine martensite lath along with

α+β phases.

(iii) Vanadium carbide formation at varying

contents in horizontal-top and vertical-middle sections

increased the microhardness with a drop in its

ductility in vertical built specimens.

(iv) HB sample immersed in 1 M H2SO4 solution

displayed least corrosion potential, highest current

density and severe corrosion attack. This is an

indication that titanium has relatively poor corrosion

resistance in H2SO4.

Acknowledgment

The authors are grateful to Mr Farouk Varachia for

funding DMLS component making through Metal

Casting Station, University of Johannesburg, South Africa.

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