Process Parameter Optimization for Laser Metal
Deposited Ti6Al4V/TiC Composites
Rasheedat M. Mahamooda,b, Esther T. Akinlabia
a
University of Johannesburg, Auckland Park, Kingsway Canpus, Johannesburg, 2006, South Africa
b
University of Ilorin, Tanke road, 23400003, Nigeria.
Abstract
Laser material deposition process is an additive
manufacturing technology that is used to produce functional
parts directly from the three dimensional (3D) model of the part.
It offers a lot of advantages in the surface modification of
components, in the repair of existing worn parts, as well as for
building parts that is made up of composites and functionally
graded materials. This is possible because the laser metal
deposition process can handle more than one material
simultaneously. Processing parameters are of great importance
in achieving the desired properties. Ti6Al4V is the most widely
used titanium alloy in the aerospace industry. This is because of
its excellent properties. However, the wear resistance behavior
of these materials is not impressive because of the surface
damage that occurs when they are used in applications that
involves contact loadings. In this study, the effect of laser power
and scanning velocity on the microstructure, the microhardness
and the wear resistance properties of Ti6Al4V/TiC composites
has been thoroughly investigated in order to optimize these
process parameters. The Ti6Al4V/TiC composites were laser
deposited with a composition ratio of 50 W% Ti64 and 50 W%
TiC and at 50% overlap percentage. The laser power was varied
from 1 to 3.8 kW and the scanning speed was varied between
0.03 and 0.1 m/s. The results shows that the optimum process
parameters is at a laser power of about 2.0 kW and the
scanning speed of about 0.055 m/s.
Titanium and its alloys in the literature (Xiang et al., 2012;
Rastegariet al., 2011).
TiC is compatible with titanium because of its good wettability
properties. Ti6Al4V/TiC is a promising hybrid material for high
performance engineering systems because of their ability to form
composites with well refined microstructure and better wear
resistance properties (Popoola and Adebiyi, 2011). Laser metal
deposition process is an important manufacturing additive
manufacturing process that can be used to produce Titanium alloy
composites.
Laser metal deposition (LMD) process belongs to the class of
additive manufacturing process named Directed Energy
Deposition (DED) (Scot et al., 2012). It is a highly flexible
process that is capable of handling more than one material
simultaneously, making it suitable to produce composites and
functionally graded materials. LMD like any additive
manufacturing technology builds up a three dimensional dense
metallic components directly from computer aided design (CAD)
model of the part by adding materials layer by layer (Wu et al.,
2004). LMD is also useful in repairing high valued worn
component parts which were prohibitive to repair in the past
(Bergan, 2000). Composite materials have received great attention
because they offer superior strength, and good wear resistance
when compared to their monolithic counterparts (Majumdar et al.,
2008).
A number of research works has appeared in the literature on
microstructure;
laser metal deposited Titanium alloy composites (Wang et al.,
sliding wear;
2007; Zhang et al., 2010; Mahamood et al., 2014a; Mahamood
et al., 2013). Wang et al., 2007, investigated the suitability of
using the LMD process to produce composite and functionally
1. INTRODUCTION
graded material of Ti6Al4V and TiC. Zang et al., 2010, also
studied surface modification of Ti6Al4V with Ti-TiC using the
Titanium and its alloys find their applications in industries
laser. They reported that the sliding wear tests conducted on the
such as aerospace, electronics, biomedical, and defense due to
samples revealed that the tribological properties of the samples
their excellent properties (Nasin-Abarbekoh et al., 2012;
were improved by the reinforced TiC particles (Wang et al., 2007;
Milovanovic et al., 2010). However, the wear property is poor.
Zhang et al., 2010). Mahamood et al., 2014, studied the effect of
Hence, there is a need for surface modification to improve the
laser power on the wear resistance property of laser metal
wear property of this important material. Composites of titanium
deposited Ti6Al4V/TiC composites. The study revealed that laser
alloy (Ti6Al4V) and Titanium Carbide (TiC) have been proved to
power has a strong influence on the wear behaviour of the
provide better surface enhancements for
deposited composites. Mahamood et al., 2013, also studied the
effect of scanning speed on the
Keywords: laser material deposition;
microhardness; process parameters;
Ti6Al4V/TiC composites.
Proceedings of the 3rd International Conference on Laser and Plasma Application in Materials Science
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Process Parameter Optimization for Laser Metal Deposited Ti6Al4V/TiC Composites
resulting wear resistance behaviour of laser deposited Titanium
alloy composite. A non-linear behaviour was established, as the
scanning speed was initially increased, the wear resistance was
found to improve. The wear resistance decreases as the scanning
speed was further increased. These studies show that processing
parameters are very important in the resulting properties of the
laser deposited composites. There is a need to further establish the
optimum process parameters for effective deposition of improved
wear resistant Titanium alloy composites.
robot co-axially. Attached to the end effector of the Kuka robot is
also the 4 kW Nd: YAG laser. The beam diameter was kept at 2
mm at a focal length of 195 mm from the substrate. Six tracks
each was deposited at 50% overlap percentage. The laser metal
deposition process was achieved ass the laser beam creates a meltpool on the surface of the substrate and the powders are deposited
into the melt-pool, upon solidification, a track of the Ti6Al4V/TiC
composite is seen on the laser path. The schematic diagram of the
laser metal deposition process is presented in Figure 1.
In this study, an attempt is made to optimize the process
parameters: - the laser power and the scanning speed for
producing Ti6Al4V/ TiC composite with the aim of enhancing the
hardness and the wear resistance using laser metal deposition
process. The results are presented and discussed in detail.
Nomenclature
UMCA unmelted carbide
SEM
scan electron microscopy
HV Vickers Hardness
2.
EXPERIMENTAL PROCEDURE
The experimental procedures are presented in five subsections
namely: the materials, laser metal deposition process, sample
preparation, microhardness test and wear test.
Table 1. Processing parameters for samples
at varying laser power
Scanning
Speed (m/s)
Laser
Power
(kW)
Powder
Flow
Rate
(g/min)
Ga s
Flow
Rate
(l/min)
A
B
C
0 .05
0 .05
0 .05
1 .0
1 .4
1 .8
2 .88
2 .88
2 .88
2
2
2
D
E
F
G
0 .05
0 .05
0 .05
0 .05
2 .2
2 .6
3 .0
3 .4
2 .88
2 .88
2 .88
2 .88
2
2
2
2
H
0 .05
3 .8
2 .88
2
Sample
Number
Table 2. Processing parameters for samples
at varying scanning speed
2.1 Materials
The materials used in this study are Ti6Al4V, TiC, and Argon
gas. The Ti6Al4V sheet was used as the substrate and it is 99.6%
pure 72 x 72 x 5 mm sheet in annealed form supplied by VSMPOAVISMA Corporation, Russia. The Ti6Al4V powder used is also
99.6% pure and it is of between 120 and 350 µm particle size. The
TiC powder is of particle size range below 60 µm. The two
powders were supplied by F. J. Brodmann and Co., L. L. C.,
Louisiana. The argon gas was used both as powder carrying gas
and the shield gas for the deposited samples to prevent the
deposited samples from environmental attack. The substrate was
sandblasted and washed with acetone before the laser metal
deposition process.
Sample
Number
A
B
C
D
E
F
G
H
Scanning
Speed
(m/s)
Laser
Power
(kW)
0 .03
0 .04
0 .05
0 .06
0 .07
0 .08
0 .09
0 .10
3 .2
3 .2
3 .2
3 .2
3 .2
3 .2
3 .2
3 .2
Powder
Flow
Rate
(g/min)
2 .88
2 .88
2 .88
2 .88
2 .88
2 .88
2 .88
2 .88
Gas
Flow
Rate
(l/min)
2
2
2
2
2
2
2
2
2.2 The Laser Metal Deposition Process
The laser metal deposition process was employed to deposit
different Ti6Al4V/TiC composites made up of 50 W% Ti6Al4V
and 50 W% TiC powders on the Ti6Al4V substrate. The
deposition process was achieved by using two powder feeder
hoppers each containing the Ti6Al4V and the TiC powders
respectively. Tables 1 and 2 present the processing parameters
employed to produce the samples at varying laser power and
varying scanning speed respectively. The nozzles
from these hoppers were fixed to the end effector of a Kuka
Fig. 1. Schematic diagram of the laser metal deposition process
(Mahamood et al., 2013)
Proceedings of the 3rd International Conference on Laser and Plasma Application in Materials Science
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Process Parameter Optimization for Laser Metal Deposited Ti6Al4V/TiC Composites
2.3 Sample Preparations
After the deposition process was completed, the samples for
microstructural and microhardness tests were cut perpendicular to
the deposition direction with a Mecatome T300 cutting machine.
The cut samples were mounted in hot resin. The mounted
samples were ground and polished following the standard
metallurgical preparation of the Titanium and its alloys. The
microstructural samples were etched using the Kroll's reagent.
The microstructural analysis was carried out using the TESCAN
Scanning Electron Microscopy (SEM) which has been equipped
with Oxford Energy Dispersion Spectrometry (EDS).
2.4 Hardness Tests
Fig. 2. The morphology of the (a) ti6al4v powder (Mahamood et
al., 2014a) (b) tic powder (Mahamood et al., 2014a)
Microhardness measurements were conducted on the crosssectional surface of the samples using a Metkon Vickers hardness
indenter according to ASTM E384-11e1 standard. The
indentations were started from the surface of the deposited layers
through to the substrate. The microhardness of the samples was
obtained using a load of 500 g for 15 s dwell time. The spaces
between the indentations were maintained at 12 µm.
2.5 Wear Tests
The wear test was performed using a universal material tester
UMT 2 CETR tribotester with ball-on-plate arrangement under dry
condition (no lubrication). The ball is a Tungsten Carbide of 10
mm diameter, at a load of 25 N, reciprocating frequency of 20 Hz
and at a sliding distance of 2000 m. The wear test was performed
according to the ASTM G133 - 05 (2010) Standard. The
morphology of the worn surface was examined using the
TESCAN SEM. The wear volume was determined from the wear
track information according to Sharma et al. (Sharma et al., 2013).
3.
RESULTS AND DISCUSSION
The SEM photograph of the Ti6Al4V and the TiC powders are
shown in Figure 2a and 2b respectively. The micrograph of the
Ti6Al4V substrate is shown in Figure 3.
Fig. 3. The micrograph of the ti6al4v substrate
The Ti6Al4V powder is characterized by spherically shaped gas
atomized powder with smooth surfaces. Spherical shaped powders
exhibit low surface oxidation due to the reduced surface area and
they are most preferred in laser processing (Schade et al., 2014).
The TiC powder is characterized by the irregular shaped ball
milled powder which is typical of any ball milled powder. The
micrograph of the Ti6Al4V substrate is characterized by alpha
(lighter parts) phase and beta (darker) phase microstructure that is
typical of any Ti6Al4V alloy.
The results are presented in Table 3 for the samples produced
at varying laser power.
Table 3. Results for samples at varying laser power
Sample
Number
A
B
C
D
E
F
G
H
Laser
Power
(kW)
1 .0
1 .4
1 .8
2 .2
2 .6
3 .0
3 .4
3 .8
Proceedings of the 3rd International Conference on Laser and Plasma Application in Materials Science
Microhardness
(HV)
500
510
500
430
380
350
330
315
Wear
volume
(mm3)
0 .065
0 .045
0 .037
0 .08
0 .125
0 .14
0 .15
0 .155
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Process Parameter Optimization for Laser Metal Deposited Ti6Al4V/TiC Composites
From Table 3, it can be seen that the microhardness and the wear
resistance increases initially as the laser power was increased and
then experienced a decrease as the laser power
was further increased. This is consistence with what was
observed in the earlier study (Mahamood et al., 2014a). The
reason for this behaviour was attributed to the fact that at low
laser power, the quantity and size of unmelted powder was high
and these unmelted carbides due to their sizes aggravate the wear
action cutting grooves on the wear track as the sliding action
progresses. As the laser power was increased, more carbide
particles were melted and the sizes of the unmelted carbides were
reduced. As the sliding action progresses, these unmelted carbides
are grounded and form a powder lubricant between the sliding
surfaces thereby reducing the wear on the sliding surfaces. The
SEM micrograph of the worn surface of a
sample produced at the laser power of 1 kW is shown in Figure 4a
and the micrograph of the sample produced at the laser power of
2.2 kW is shown in Figure 4b. It can be seen that there is an
improvement in the wear behaviour of the sample produced at the
laser power of 2.2 kW when compared to the worn surface of the
sample produced at the laser power of 1 kW. When the laser
power was further increased, most of the carbides were fully
melted and the few unmelted carbides caused deeper scratch and
aggravate the wear on the sliding surface.
The microhardness and the wear volume loss results for samples
produced at varying scanning speed are presented in Table 4.
From Table 4, the effect of the scanning speed on the
microhardness and wear volume loss also revealed that as the
scanning velocity was increased, the microhardness was found to
increase. This result is consistence with the earlier result
(Mahamood et al., 2013). The wear volume loss of the samples
decreased initially as the scanning speed was increased and then
increase as the scanning speed was further increased. At low
scanning speed, there was less quantity of the unmelted carbide in
the microstructure because more melting of the TiC powder
occurred. These result in high wear volume loss at lower scanning
speed.
Table 4. Results for samples at varying scanning speed.
W ear
Sample
Number
Scanning
Speed (m/s)
Microhardness
( HV)
A
B
C
D
E
F
G
H
0 .03
0 .04
0 .05
0 .06
0 .07
0 .08
0 .09
0 .10
400
420
445
440
460
470
490
500
volume
(mm3)
0 .085
0 .07
0 .065
0 .05
0 .0650
.075
0 .09
0 .1
As the scanning speed was increased, the quantity of the
unmelted carbide (UMC) is increased thereby reducing the wear
volume loss similar to that observed when the laser power was
slightly increased. This result is also similar to what was observed
in the earlier study (Mahamood et al., 2013). Also at the highest
scanning speed, there were larger UMC particle sizes seen and in
fewer quantity because the laser material interaction time is low at
high scanning speed. These large unmelted carbides cut deeper
grooves on the wear track. The SEM micrograph of the wear track
of the sample produced at the scanning speed of 0.03 m/s is shown
in Figure 5a and the micrograph of the sample at the scanning
speed of 0.05 m/s is shown in Figure 5b. It can be seen that the
wear track in Figure 5b is improved compare to 5a because the
unmelted carbide particles has formed a powdered protective layer
on the wear track which tend to inhibit the wear action.
(a)
(b)
Fig. 5. The SEM micrograph of the wear track of a sample at the
scanning speed of (a) 0.03 m/s (b) 0.05 m/s
(a)
(b)
Fig. 4. The SEM micrograph of the worn surface of a sample at
the laser power of (a) 1 kW (b) 2.2 kW
To find the optimum process parameters for the microhardness and
the wear volume loss, the effect of laser power and scanning
speed are plotted together. The plot of effect of the laser power
and the scanning speed on microhardness is shown in Figure 6; the
microhardness is seen to reduce as the laser power was increased.
On the other hand, the microhardness
wass found to increase as the scanning speed was increased.
The optimum processing parameters for the micro hardness
are: - the laser power is about 2.0 kW and the scanning speed is
about - 0.055 m/s. The graph of the laser power and the
Proceedings of the 3rd International Conference on Laser and Plasma Application in Materials Science
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Process Parameter Optimization for Laser Metal Deposited Ti6Al4V/TiC Composites
scanning speed against the wear volume loss is shown in Figure
7. The wear volume loss is seen to initially decrease as the laser
power and the scanning speed were increased and then increase
as the laser power and the scanning speed were further increased.
The optimum process parameters are found from the graph to be:
laser power is about 2 kW and the scanning speed is about 0.055
m/s. This is similar with the optimum parameters observed for the
microhardness.
scanning speed was increased, and decreased as the laser power
was increased. The wear volume loss was found to initially
decrease as the laser power and the scanning speed were increased
and then increases as the laser power and the scanning speed were
further increased. The optimum process parameters for the
microhardness and the wear volume loss were found to be at about
2.0 kW of laser power and 0.055 m/s of scanning speed for the
processing parameters employed in this study.
5.
ACKNOWLEDGEMENTS
This work is supported by the Rental Pool Grant of the National
Laser Centre - Council of Scientific and Industrial ResearchNational Laser Center (NLC-CSIR), Pretoria South Africa.
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