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Microstructural investigation of as cast and PREP atomised Ti-6Al-4V alloy

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A microstructure characterisation of Ti–6Al–4V is conducted for cast, extruded and micrometre sized particles. The plasma rotating electrode process is used to produce spherical Ti–6Al–4V powders from an alloy electrode. The process parameters and their impact on the material properties are described. The effects of electrode rotation speed on the particle size distribution, particle shape and crystal structure are investigated in detail. Optical microscopy and scanning electron microscopy are used for microstructural characterisation. The analysis shows that cast and extruded Ti–6Al–4V alloys have equiaxial a and azb phase structures, while plasma rotating electrode processed powder from the same alloy compositions has an acicular or martensitic (a) structure. The microstructure scale depends on the particle size. Microhardness measurements are used to assess mechanical property dependence on the microstructure of this alloy. The rapidly cooled alloy particles have much higher hardness than cast or extruded bulk alloy.
Content may be subject to copyright.
Microstructural investigation of as cast and
PREP atomised Ti–6Al–4V alloy
R. Yamanoglu*
1
, R. M. German
2
, S. Karagoz
3
, W. L. Bradbury
2
, M. Zeren
1
,
W. Li
2
and E. A. Olevsky
2
A microstructure characterisation of Ti–6Al–4V is conducted for cast, extruded and micrometre
sized particles. The plasma rotating electrode process is used to produce spherical Ti–6Al–4V
powders from an alloy electrode. The process parameters and their impact on the material
properties are described. The effects of electrode rotation speed on the particle size distribution,
particle shape and crystal structure are investigated in detail. Optical microscopy and scanning
electron microscopy are used for microstructural characterisation. The analysis shows that cast
and extruded Ti–6Al–4V alloys have equiaxial aand azbphase structures, while plasma rotating
electrode processed powder from the same alloy compositions has an acicular or martensitic (a)
structure. The microstructure scale depends on the particle size. Microhardness measurements
are used to assess mechanical property dependence on the microstructure of this alloy. The
rapidly cooled alloy particles have much higher hardness than cast or extruded bulk alloy.
Keywords: Solidification, Microstructural characterisation, Ti–6Al–4V powders, PREP atomisation
Introduction
Titanium and its alloys have been under active research
for the past 50 years. Titanium alloys have many
advantages, such as high strength/weight ratio, excellent
corrosion resistance and biocompatibility.
1,2
Casting,
forging and powder metallurgy are established techni-
ques for processing titanium. A variety of advanced
techniques have emerged in recent years: electron beam
melting, laser engineered net shaping, direct metal
laser sintering and ultrasonic consolidation; however,
wrought products continue to dominate titanium usage.
3
Powder metallurgy offers an alternative route which has
demonstrated significant gains in material utilisation
and faster conversion into final products.
Ti–6Al–4V is the most widely used titanium alloy for
industrial applications. Prior research has reported that
the mechanical properties of titanium are strongly
dependent on porosity, impurity content and pore size.
Consequently, proper processing of alloy powders enables
production of low porosity materials with uniform pore
size, or even full density with desired mechanical proper-
ties, comparable to wrought materials.
4
Pure titanium and Ti–6Al–4V have shown promise as
biomedical implant components.
5,6
Commercially pure
titanium, Ti–6Al–4V and extra low interstitial Ti–6Al–4V
can be also used for many structural applications, such as
in the aerospace industry.
7
The plasma rotating electrode process (PREP) is a
useful technique to produce rapidly solidified spherical
Ti based alloy powders with low impurity levels.
8
In the
present study, the microstructures and morphologies
of the alloy powders produced by the PREP were
investigated in detail by means of optical microscopy
and scanning electron microscopy (SEM). The effects of
the rotation speed on the particle size and material
properties are discussed.
Experimental
Material
The chemical composition of the titanium alloy Ti–6Al–
4V used in this study is 89?45Ti–6?20Al–4?14V–0?02Si–
0?01Mn–0?14Fe–0?04Nb (wt-%). Aluminum additions
stabilise the hexagonal close packed aphase, and
vanadium, being body centreed cubic, stabilises the b
phase. When Ti–6Al–4V is slowly cooled from the b
temperature, aphase regions begin to form below the
btransus temperature which is y980uC. The kinetics of
bRatransformation during cooling strongly influences
resulting properties of this alloy.
9
Potential areas of
application for titanium and its alloys depend strongly
on their phase structure.
10
Plasma rotating electrode process atomisation
To produce Ti–6Al–4V alloy powders, cast and extruded
bar was atomised by the PREP in an Ar atmosphere.
Plasma rotating electrode process is based on the
pulverisation of melted pool by rotation of metal bars
in contact with argon plasma arc within a controlled
1
Department of Metallurgical and Materials Engineering, Kocaeli
University, Umuttepe Campus, Kocaeli 41380, Turkey
2
Department of Mechanical Engineering, San Diego State University, San
Diego, CA 92182-1323, USA
3
Department of Mechatronics Education, Marmara University, Istanbul
34730, Turkey
*Corresponding author, email ryamanoglu@gmail.com
604
ß2011 Institute of Materials, Minerals and Mining
Published by Maney on behalf of the Institute
Received 8 August 2010; accepted 17 September 2010
DOI 10.1179/1743290110Y.0000000006 Powder Metallurgy 2011 VOL 54 NO 5
atmosphere chamber of 2?5m in diameter. Argon
plasma discharge was ignited between a tungsten tip
(cathode) and the alloy bar (anode) via generated
electrical arc of 13 kV A power, fueled by flowing argon
as depicted in Fig. 1. The rotation speed of the 55 mm
anode bars were either 6000 or 12 000 rev min
21
producing powder with different particle size distribu-
tions. Argon plasma was used to melt the end of a
rapidly rotating bar, and then the molten droplets were
spun off and solidified during free flight in argon. Owing
to the high centrifugal force, melted metallic droplets
from the rapidly rotating bars were transformed into
spherical particles with diameters below 0?5 mm.
11,12
To minimise potential oxidation, the atmospheric
chamber was evacuated before PREP atomisation and
then purged with shielding argon gas. Atomisation
parameters are given in Table 1.
Microstructure characterisation
The evolution of the microstructure of the starting bar
and produced particles was investigated by microscopy.
All specimens were polished and etched with Kroll’s
reagent (aqueous 3% HF and 6% HNO
3
). Etching time
was about 5–10 s. The microstructure images were
obtained using an optical microscope (Zeiss Axiophot)
and an SEM (JEOL 6060).
Results and discussion
Determination of size distribution
Alloy powders produced at different PREP rotation
speeds resulted in an average of 1500 g per experimental
run. The powder lots were then sieved in y100 g
quantities. The SEM image of the spherical Ti–6Al–4V
alloy powder is shown in Fig. 2a. The particles have
uniform size and no satellite formation was observed.
Particle size distribution was determined by sieve
analysis. The expected inverse relationship of rotational
speed and particle size can be seen in Fig. 2b. Increased
rotational speed produced smaller sized particulate
powder. The median particle size decreased from 312
(6000 rev min
21
) to 168 mm (12 000 rev min
21
).
Morphology
Scanning electron micrographs of powders produced by
PREP atomisation reveal homogenous spherical parti-
cles. The grain structure and morphology of the plasma
rotating electrode processed powders of different parti-
cle sizes are shown in Fig. 3. The powders were
spheroidised due to surface tension, and during the
atomisation, the droplets minimise their surface area by
1 Schematic of PREP set-up
2aSEM image of powders fabricated by PREP and
bsize distribution of powders depending on rotational
speed (cumulative volume curves)
Table 1 Atomisation parameters
Rotation speed, rev min
21
6000 and 12 000
Plasma gas Argon
Vacuum, mbar 10
23
Shielding gas Argon
Shielding gas pressure, atm 1.3
Plasma voltage, V 40
Plasma current, A 350
Bar diameter, mm 55
aparticle with no grain boundaries; bparticle with multiple grains
3 Images (SEM) of powder morphology and grain structure
Yamanoglu et al. AscastandPREPatomisedTi6Al4Valloy
Powder Metallurgy 2011 VOL 54 NO 5605
forming a sphere. The particle shape depends on
solidification time. Calculated solidification and spher-
oidisation parameters are shown in Table 2.
13
It is clear
that spheroidisation time is significantly shorter than the
solidification time, giving the observed spheres.
Particle size and solidification time affect the crystal
structure of an individual sphere. If the solidification rate
is sufficiently high, amorphous structure is obtained. The
SEM image of a 50 mm sized particle is shown in Fig. 3a,
giving a featureless amorphous structure. A larger
particle with a diameter of 200 mm and similar shape
required longer cooling time, and a polycrystalline
structure was obtained, as shown in Fig. 3b.
Microstructural evaluation
Optical images of typical equilibrium microstructure of
the extruded Ti–6Al–4V bar are shown in Fig. 4. The bi-
phase alloy consisting of equiaxed aand azblamellar
structures transformed from a dendritic bstructure
during solid state cooling. Bright areas are aphase
precipitated from the matrix of bphase (the dark areas).
The equiaxed aphase microstructure is more resistant
to void nucleation than the lamellar structure due to its
high ductility. Cracks nucleate within the lamellar region
more easily than in the equiaxed aphase region;
however, crack propagation is more difficult in the
lamellar structure.
14
High and low magnification SEM
images of a typical wrought alloy product are shown in
Fig. 5. This structure is composed of finely dispersed b
particles in a fine equiaxed grain matrix of aphase.
It is possible to control the microstructure of Ti–6Al–
4V by manipulation of cooling rates, generating various
structures from bphase region as shown in Fig. 6.
15
As
seen from the phase diagram, the structure will change
from lamellar to a9martensite if the cooling rate is
increased appropriately.
Powders produced through the PREP method pre-
sented microstructures reflecting rapid cooling. A cross-
sectional image of the atomised powder particle is shown
in Fig. 7, with a structure consisting of a9martensite.
Table 2 Solidification and spheroidisation parameters of Ti–6Al–4V powders
Rotation speed,
rev min
21
Electrode
diameter, mm
Measured
median size, mm
Predicted
median size, mm
Solidification
time, ms
Spheroidisation
time, ms
Particle
shape
6000 55 312 328 66 0.7 Sphere
12 000 55 138 164 33 0.3 Sphere
4 Optical images of as cast and extruded microstructures: bright areas are aand dark areas are bphase
5 Images (SEM) of microstructure of mill annealed hip implant
6 Structures depending on different cooling rates from b
transus
15
Yamanoglu et al. AscastandPREPatomisedTi6Al4Valloy
606 Powder Metallurgy 2011 VOL 54 NO 5
A typical cross-sectional SEM image of Ti–6Al–4V
alloy powder particles is shown in Fig. 8. In preparing
the particles, nickel electrocoating is used to retain
edge definition. Clearly, these particles consist of a9
martensite.
Hardness
Vickers microhardness measurements were conducted to
assess the mechanical properties of plasma rotating
electrode processed powders. The electrode hardness
was measured at 2?60 GPa, while the powders of two
different particle sizes gave 2?73 (387 mm) and 3?41 GPa
(83 mm). Rapid cooling rate achieved during PREP
clearly improves obtained hardness values compared
with the original wrought bar stock.
Conclusions
The PREP is an advantageous method to produce fully
spherical titanium powders with minimal impurities.
Excellent flowability is accompanied with the spherical
powders, which is pertinent to metal injection moulding
and hot isostatic pressing technologies. Control of the
particle size is an indirect way to further control alloy
powder microstructure. Material hardness increases with
decreasing particle size. Manipulation of the PREP
rotation speed enables particle size control, particularly
important for utilisation of this alloy in specialised and
diverse applications.
1. Solidification characteristics of PREP atomised
powders and wrought materials have been compared.
2. Different atomisation rotation speeds have been
applied to gain various microstructural characteristics.
3. High rotational speeds in the PREP produce
smaller powders. The microstructural characteristics
can be controlled according to powder size.
4. Powders produced by PREP have higher hardness,
and the hardness increases with decreasing particle size.
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7aplasma rotating electrode processed Ti–6Al–4V powder and bhigher magnification of a9martensite (DIC contrast)
8 Images (SEM) of cross-sectional view of Ti–6Al–4V alloy powders (a9martensite)
Yamanoglu et al. AscastandPREPatomisedTi6Al4Valloy
Powder Metallurgy 2011 VOL 54 NO 5607
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Porous Ti alloy compacts were fabricated and their microstructure and mechanical properties were investigated in this study. Ti alloy powders were atomized from Ti–15Mo–5Zr–3Al (wt.%) bar using the plasma rotating electrode process (PREP) in an Ar atmosphere. These alloy powders were sintered under 1–30 MPa at 1223 K for 7.2 ks by hot-pressing (HP). These compacts were solution treated at 1223 K for 1.2 ks, and then quenched into iced water (STQ). X-ray diffraction analysis revealed that a small amount of α phase appeared in the β phase of the HP compacts, while not in the STQ compacts. Young's modulus of STQ compacts is lower than that of HP compacts. It was found that the strength of porous Ti–15Mo–5Zr–3Al is higher than those of porous pure Ti and human cortical bone, as compared in the range from 10 to 30 GPa of Young's modulus for human bone.
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Microstructures of Ti50Al45Mo5 (at.%) alloy powders produced by the plasma rotating electrode process (PREP) were investigated. The powders have inhomogeneous structures, which consist of dendrites and rounded grains. The dendrites, which show a “rosettelike” morphology, are formed on the powder surface and around the rounded grains. The rosettelike dendrites are of hexagonal α 2 (D019) phase even though the dendrites have an equiaxial morphology, and a small amount of β 2 (B2) phase is also contained inside. It is suggested that the solidification to α (hcp-A3) phase occurred by the peritectic reaction between the primary β (bcc-A2) dendrites and the liquid: L+β→L+β+α. The rounded grains, on the other hand, are of β 2 phase in which acicular α or α 2 laths are precipitated with the Burgers orientation relationship. Antiphase domain boundaries in the β 2 matrix are intersected by α(α 2) laths. It is interpreted that the α(α 2) laths were formed by the solid-state transformations: β 2→β 2+α→β 2+α 2. The formation of the two different microstructures in the powder particles is rationalized in terms of the changes in local composition of the liquid phase during the rapid solidification process.
Article
The Plasma-Rotating-Electrode-Process (PREP) is based on the pulverization by rotation of metal bars in contact with a Ar/N-2 plasma are. Due to the high centrifugal forces of the rapidly rotating bars, high nitrogen steel powders are produced with diameters in the range of 0.02 to 0.5 mm. The low diameters and the high centrifugal forces, as well as the high particle velocities, cause the steel droplets to cool down rapidly in the reactor chamber. Mathematical calculations show that cooling rates of up to 10(5) K/s are attained. It is demonstrated that the cooling rate of all powder particles and the structures produced can be predicted.
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The high strain rate (220-550 s(-1)) and quasi-static (0.0016 s(-1)) compression deformation behavior of a sintered Ti6Al4V powder compact was investigated. The compact was prepared using atomized spherical particles (100-200 mu m) and contained 38 +/- 1% porosity. The deformation sequences of the tested samples were further recorded by high speed camera and analyzed as a function of strain. The failure of the compact, which was found to be similar in the studied high strain rate and quasi-static strain rate testing regimes, occurs through particle decohesion along the surface of the two cones in a ductile (dimpled) mode consisting of void initiation and growth and by void coalescence in the interparticle bond region. The effect of strain rate was to increase the flow stress and compressive strength of the compact while the critical strain corresponding to the maximum stress was shown to be strain rate independent. (C) 2007 Elsevier B.V. All rights reserved.
Article
Production of investment castings of titanium alloys was considerably increased during last years due to the significant cost savings compared to complicated machined parts. However, the disadvantage of as-cast titanium alloys is that the heat-treatment remains only a limited option for improvement of their properties. The object of this paper was to study the effect of heat-treatment of investment cast Ti–6Al–4V alloy performing X-ray diffraction analysis, light microscopy and quantitative metallography together with hardness and room temperature tensile tests. The effect of annealing temperatures (above and below β transus temperature) and cooling rates on microstructure and mechanical properties was discussed in terms of the β → α transformation. The results of this paper also show that, besides heat treatment parameters, melting and casting practice together with mold technology strongly influence the properties of castings.