Laser metal deposition of functionally graded Ti6Al4V/TiC
R. M. Mahamood1.2,* and E. T. Akinlabi1.
1
Department of Mechanical Engineering Science, University of Johannesburg, Auckland Park
Kingsway Campus, Johannesburg, 2006, South Africa.
2
Department of Mechanical Engineering, University of Ilorin, Nigeria.
*Corresponding Author: Email address- mahamoodmr2009@gmail.com, Tel.-+27781112301
ABSTRACT
Functionally graded materials are advanced materials with improved properties that enable them to withstand severe
working environment which the traditional composite materials cannot withstand. FGM found their applications in
several areas which include: military, medicine and aerospace. Various manufacturing processes are used to produce
functionally graded materials that include: powder metallurgy, physical vapour deposition, chemical vapour
deposition process and laser metal deposition process. Laser metal deposition (LMD) process is an additive
manufacturing process that can be used to produce functionally graded material directly from the three dimensional
(3D) computer aided design (CAD) model of the part in one a single process. LMD process is a fairly new
manufacturing process and a highly non-linear process. The process parameters are of great importance in LMD
process and they need to be optimized for the required application. In this study, functionally graded Titanium alloy
composite was produced using optimized process parameters for each material combination as obtained through a
model that was developed in an initial study and the FGM was characterized through metallurgical, mechanical and
tribological studies. The results show that the produced FGM has improved properties when compared to those
produced at constant processing parameters for all material combinations.
Keywords
Deposition; Laser; Microstructure; Titanium Wear.
INTRODUCTION
Functionally Graded Material (FGM) belongs to a class of advanced materials with varying
properties over its changing dimension [1, 2]. Functionally graded materials are seen in nature as
bones, teeth etc. [3]. Nature designed these materials to meet their expected service requirements.
Functionally graded material is produced in order to eliminate the sharp interfaces that exist in the
traditional composite material where failure is always initiated [4]. These sharp interfaces are
replaced with a gradient interface that produces smooth transition from one material to the other
[5]. One unique characteristics of FGM is their ability to be tailored for a specific application [1].
Functionally graded materials find their applications in aerospace, automobile, medicine, sport,
energy, sensors, optoelectronic etc. [6, 7]. There are different kinds of fabrication processes for
producing functionally graded materials, they include: Physical or Chemical Vapour Deposition
(PVD/CVD), Plasma Spraying, Self-propagating High temperature Synthesis (SHS), powder
metallurgy technique, centrifugal casting method, and laser metal deposition process [3, 8]. Laser
metal deposition process is a class of additive manufacturing process that is capable of producing
a functional part directly from its three dimensional (3D) computer aided model (CAD) of the part
[9]. The part can also be made with functionally grade material because LMD can use more than
one material simultaneously.
1
Many researches have been conducted in the literature on the laser metal deposition process
[10-19] and specifically on functionally graded Ti6Al4V/TiC composite [20-22]. The importance
of producing functionally graded parts directly from the 3-D CAD model in one single step has
been the driving force for the research interest in FGM, by using LMD. The earlier works tried to
establish the feasibility of making FGM through the LMD process [23, 24]. In some other works,
FGM were built using the LMD process, and then characterized. For example, in a study conducted
by Zang et al. [22], they deposited functionally graded Ti/TiC on a Ti6Al4V substrate. They first
established the processing parameters for various volume fractions of Ti/TiC composite in their
preliminary works. They used the results from their preliminary work to successfully deposit a thin
wall of functionally graded material – by adjusting the processing parameters during the deposition
process. They showed that the wear-resistance performance of the Ti6Al4V substrate was
improved with the addition of TiC. They also showed that FGM can be produced with LMD
without a discrete interface. In their preliminary studies, the method used to obtain the optimal
process parameters for the various premix of Ti and TiC was not clear. Also, premixing the powder
before deposition can result in having the powder with higher density to be deposited first before
the less dense powder. And this could affect the results (density of Ti is 4.5 g/cm3 and that of TiC
is 4.93 g/cm3). This could be one of the reasons why they observed cracking in the deposit at 40%
TiC. In another study performed by Wang et al. [21], a functionally graded material of
Ti6Al4V/TiC was deposited. They used Ti6Al4V wire and TiC powder; and the two materials
were fed simultaneously. They achieved the compositional grading by keeping the wire feed rate
of Ti6Al4V constant; while they varied the TiC powder feed rate. Also, other processing
parameters were kept constant. They did not consider the effect of processing parameters on the
resulting deposit properties. Liu and DuPont [25] successfully deposited functionally graded
material of Ti/TiC composite using LMD. They relied on the controller in LENS, which monitors
the melt pool area and controls the laser power to achieve a constant melt pool area, for the
deposition of their FGM. The melt pool area control in LENS is intended to control the dimensional
accuracy in the deposited part. Hence it does not control other properties of the deposited part.
Shah [26] deposited functionally graded material of Inconel 718 Nickel alloy and Ti6Al4V using
the laser-metal deposition process. The effect of the laser pulse parameters and the powder flow
rate on residual stress was studied. The study found that the layer thickness plays an important role
in the crack behaviour of the functionally graded material produced. The effect of the powder flow
rate on the melt pool size was also studied; increasing the powder flow rate was found to increase
the melt pool size. Lin et al. [27] used the laser-metal deposition process to produce functionally
graded material of stainless steel-SS316L/super alloy-Rene88DT. They investigated the
solidification behaviour and the microstructural evolution of the FGM. Epitaxial growth and
columnar dendrites’ microstructure were also observed. Insitu functionally graded material using
the laser-metal deposition process was also studied in the literature. Qin et al. (2011) [28] produced
in situ functionally graded TiC reinforced titanium matrix from Ti and Cr3C2 powder using laser
metal deposition process. The functionally graded material was achieved by changing the powder
flow rate of the Ti and the Cr3C2 powder. The microhardness and the wear-resistance properties
were studied. The microhardness and the wear resistance were found to be greatly improved. The
problem with functionally graded material produced in situ is that the magnitude of reinforcement
achieved will largely depend on the reactions taking place during the deposition and cooling
process. It would be very difficult to achieve a desired percentage ratio of the reinforcement and
the matrix.
2
There is no doubt that different Ti6Al4V/TiC ratios would have different optimal process
parameters; but these were not considered in any of the above studies. It has also been
demonstrated that having powders placed in separate hoppers is an effective way of producing
composite, with proper composition control, and without segregation due to difference in the
densities of the powders [29]. This method was employed in this study.
In this study, functionally graded Ti6Al4V/TiC was developed using laser metal deposition
process. The process parameters used to deposit each layer were obtained using a model that was
developed in our initial study [30]. Trial experiments were first performed in order to establish the
process window for the deposition of the porous free and fully dense deposit of TiC/Ti6Al4V
composites. The results from these trial runs formed the basis for the selection of the lower levels
selected for the design of experiment. Screening experiment was then performed using full
factorial design of experiment. The parameters screened were: laser power, scanning speed,
powder flow rate and gas flow rate. The gas flow rate was found to be the least significant process
parameter. The percentage TiC was then added to the remaining process parameters for the RSM
that was used to develop the model. The model was used to generate the optimized process
parameter for the each of the layers in the functionally graded Ti6Al4V/TiC composite. The
developed FGM was characterized metallurgically, mechanically and tribologically. The
developed FGM produced at optimized process parameters was compared to the one produce at
constant process parameters and the substrate. The results are presented and discussed in detail.
MATERIALS AND METHODS
Annealed 5mm thick sheet of 99.6% pure Ti6Al4V was used as the substrate for the laser metal
deposition of the functionally graded Ti6Al4V/ TiC composite samples. The sheet was supplied
by supplied by the VSMPO-AVISMA Corporation, Russia. The sheet was cut into square shape
of 72 x 72 mm and sandblasted and cleaned with acetone prior to the deposition process. The
purpose of the sandblasting is to roughen the surface of the substrate in order to aid the laser-power
absorption process. Shining surface would reflect most of the laser beam. The Ti6Al4V and the
TiC powder used in this study were supplied by F.J. Brodmann and Co., L.L.C., Louisiana. The
Ti6Al4V powder use is of 99.6% purity, with particle size range of between 120 and 350 μm. The
TiC powder used is a ball-milled powder of particle size range below 60 μm. A 4.0 kW Nd: YAG
laser was used for the deposition process and it was attached to a kuka robot. The laser spot
diameter was maintained at 2mm at a focal distance of 195mm above the substrate. A co-axial
nozzle was attached to the robot’s end effector for the powder delivery into the melt-pool. The
shield gas used for carrying the powder as well as to protect the deposited samples from the
environmental attack was Argon gas. The experimental set-up is available at the CSIR National
Laser Center, Pretoria and it is shown in Figure 1a. The laser metal deposition process was
achieved by creating a melt pool on the surface of the substrate by the laser and the powders were
delivered into the melt pool by a powder feeder. Each of the powders was placed in each hopper
of the powder feeder. The nozzles from the feeder were attached coaxially to the robot. The
substrate was fixed in place and the robot was moved to perform the deposition process. The
schematic of the LMD process is shown in Figure 1b.
<insert Figure 1>
The functionally graded material was produced from 100 % Ti6Al4V: 0 % TiC to 50 % Ti6Al4V:
50 % TiC. The optimized process parameters that were used to deposit each of the functionally
3
graded layers are presented in Table 1. To be able to compare the results, another functionally
graded Ti6Al4V/TiC was deposited at constant process parameters as follows: laser power 2.5
kW, scanning speed 0.01 m/s, powder flow rate 2 rpm and gas flow rate 2l/min. Four layers each
was deposited at 50 % overlap percentage.
<Inset Table 1>
After the deposition process, the samples for the metallurgical examination were sectioned to
reveal the cross sections. The cut samples were mounted in hot resin; the mounted samples were
ground and polished according to the standard metallurgical preparation of Titanium and its alloys.
The polished samples were etched using Kroll’s reagent. The microstructures were studied using
Olympus Optical microscope (BX51M; Olympus) and the Scanning Electron Microscope (SEM)
(by TESCAN) equipped with the Oxford Energy Dispersion Spectrometry (EDS). The
microhardness profiling was taken on the cross section of the samples (polished) using the MH-3
Vickers Microhardness indenter (by Metkon) with a load of 500 g and a dwell time of 15 seconds.
The distance between the indentations was maintained at 12 μm which is more than twice the
indentation diameters. The X-ray diffraction analysis was performed using the X-ray
diffractometer (by Ultima IV) which was used to study the phases present. The tribological
property was conducted on the surface of the samples after grinding to 1000 µm using the ball on
disk tribotester (by Cert) with tungsten carbide ball of diameter 10 mm. A load of 25 N, a sliding
distance of 2000 m and the sliding speed of 0.02 m/s were employed during the wear test. The
tests were conducted in the dry air without lubrication. The wear volumes were calculated using
the equation proposed by Sharma et al. [32] in equation 1.
(1)
Where v is the wear volume in mm3, r is the ball radius, w is the wear-track width and L is the
stroke length.
RESULTS AND DISCUSSION
The micrographs of the substrate is shown in Figure 2a, the morphology of the Ti6Al4V powder
is shown in Figure 2b and that of the TiC powder is shown in Figure 2c. The microstructure of the
substrate (see Figure 2a) shows white parts and the dark parts. The white part is the alpha grains
while the dark parts are the beta grains which are the characteristic of a typical Ti6Al4V. The
Ti6Al4V powder is spherical in shape (see Figure 2b) which is typical of a gas atomized powder.
Spherically shaped powder is more favoured in the laser deposition process because they maximize
laser absorption. The TiC powder is an irregular shaped powder which is the characteristic of a
ball milled powder. The photographs of the deposited functionally graded materials are shown in
Figure 3. The sample labeled 1 is the FGM produced at constant or fixed processing parameters,
i.e. for material combinational ratio from 100% Ti6Al4V: 0% TiC to 50% Ti6Al4V: 50% TiC, the
same processing parameters (laser power 2.5 kW, scanning speed 0.01 m/s, powder flow rate 2
rpm and gas flow rate 2l/min.) were used to deposit all these material compositions in the FGM
sample. The micrograph labeled 2 in Figure 3 is the one in which each layer of the FGM was
produced at optimized process parameters as obtained in Table1. The macrographs of the FGM
4
sample produced at produced with the optimized process parameters is shown in Figure 4a and the
one produced at a fixed-process parameter is shown in Figure 4b. It can be seen from Figure 4 that
the FGM produced at optimized process parameter is taller than the FGM produced at constant
process parameters.
<Insert Figure 2>
<Insert Figure 3>
Whereas the width of the sample produced at constant process parameters is larger than that
produced using optimized process parameters. This can be attributed to higher dilution seen in the
sample produced at constant process parameter that makes is to be shorter and wider. Dilution rate
has been established to be proportional to the melt pool size produced [33]. The bigger the melt
pool, the longer it takes to solidify and hence the more the degree of mixing of the base material
and the deposited powders.
The microstructures of the topmost parts of the two samples are shown in Figure 5. The sample
produced at the optimized process parameters is shown in Figure 5a; the unmelted Carbides
(UMC) is seen in the microstructure. The higher magnification of the sample in Figure 5a is shown
in Figure 5b. The sample produced at fixed-processing parameters is shown in Figure 5c; and fewer
UMCs are seen in the microstructure. This shows that most of the TiC powder had fully melted;
and the higher magnification micrograph of the sample in Figure 5c shown in the Figure 5d
displayed mostly dendritic TiC.
<Insert Figure 4>
Comparing Figure 5b and Figure 5d, it can be seen that that there are a larger number of UMCs in
the sample that was produced at optimized process parameters (see Figure 5b) compared to the
sample produced at fixed processing parameters. The SEM micrograph of the sample prepared at
optimized process parameters is shown in Figure 6a, showing some of the UMCs.
<Insert Figure 5>
To confirm the UMC particles, an EDX analysis was performed on the UMC, using the Oxford
EDX equipped with the TESCAN SEM. The EDX analysis of the label ‘A’ in the Figure 6a is
shown in Figure 6b. The EDS confirms that those points are unmelted carbide particles. The SEM
micrograph of the sample produced at optimized process parameters is shown in Figure 7, showing
the different layers.
<Insert Figure 6>
<Insert Figure 7>
It can be seen how the how the microstructure changes with the height of the functionally graded
material. The point D, which contains the lowest percentage of the TiC displayed a microstructure
that is typical of laser deposited Ti6Al4V, which is columnar in nature that grows epitaxially on
5
the globular microstructure of the heat affected zone. This shows that the low percentage of the
TiC is not seen as a different material, which is the aim of the functionally graded material: That
is, the eradication of the sharp interface that shows the distinction between the different materials.
An XRD analysis was performed on the optimized FGM sample; and it revealed three different
phases, as shown in Figure 8.
<Insert Figure 8>
The phases are: alpha Ti, TiC and Ti3Al. The TiC has the highest percentage of 49%. The
intermetallic compound Ti3Al formed is 22%. The Ti3Al is shown in Figure 9 as indicated by EDX
analysis of the SEM micrograph of the optimised FGM sample.
<Insert Figure 9>
The average wear volume obtained using equation 1 for the optimized functionally graded
Ti6Al4V/TiC composite, designated as sample A, and that of the one produced at fixed-process
parameters (sample B) are presented in Table 2. The wear volume of the substrate, designated as
PM, is also presented in the Table for comparison.
<Insert Table 2>
From Table 2, it may be seen that the sample produced with the optimized-process parameters
(sample A) has the lowest wear volume. The substrate material (PM) has the highest wear volume.
This is expected because of the property of Titanium that makes it chemically reactive to itself, or
to other materials with which it comes into contact in sliding or rubbing action. The wear
mechanism that takes place on the PM sample is a combination of abrasion, adhesion and plastic
deformation. The abrasion wear mechanism starts the moment the counter body is engaged, as a
result of the rubbing action of the two surfaces in contact. As the rubbing action of the two surfaces
in contact continues, the frictional force between the two surfaces increases. The increase in the
frictional force resulted in strong adhesion of the two surfaces in contact. This causes the
coefficient of friction (COF) to increase. The COF of the substrate and that of the optimized FGM
sample are compared, and shown in Figure 10.
<Insert Figure 10>
This strong adhesion and the sliding action of the two surfaces increases the temperature of the
surfaces, which resulted in plastic deformation, which eventually leads to shearing and tearing of
the surface of the Ti6Al4V. Some of the debris produced from the tearing and shearing of the
Ti6Al4V surface adheres to the surface of the tungsten carbide ball; while the rest stays in between
the two surfaces. This debris worsens the wear action, as it changes the two-body wear mechanism
to a three-body wear mechanism.
As the sliding action continues, the debris becomes work-hardened and it cuts deeper and deeper
into the substrate. The debris cutting into the substrate results in the characteristic parallel grooves,
as described by Wu et al. (2013), and this is shown in Figure 11. Figure 11a shows the low
magnification of the wear track of the substrate; and the higher magnification is shown in Figure
11b.
6
<Insert Figure 11>
The wear track of the sample produced at a constant process parameter is shown in Figure 12a.
The wear resistance of this sample is far better than that of the substrate shown in Figure 11b
because of the TiC content of the FGM sample. Although, the sample was not produced at
optimized-process parameters, yet the wear behaviour is better than that of the substrate. The wear
track of the optimized sample is shown in Figure 12b.
<Insert Figure 12>
The sample showed the best wear resistance behaviour, because there were reasonable quantities
of UMC retained in the sample, which then forms powder lubrication between the surfaces during
the sliding action [33]. The few UMC found in the sample produced at fixed-process parameters
also helped in the wear resistance behaviour of that sample; but not as much as that of the sample
produced at optimized-process parameters. The process parameter optimization using the
developed model was really helpful in producing the low wear volume observed in the optimized
sample. Although intermetallic compounds are present in the sample, which are very hard, but it
was found that this compound is not detrimental to the wear property of this composite. Instead, it
served as reinforcement, and helped to improve the wear-resistance property of the FGM sample.
The Ti3Al has been proved in the literature to improve the wear properties of some metals [34, 35].
The microhardness test was performed on the optimized FGM sample, and the substrate using the
Vickers hardness tester with a load of 500g. Figure 13a shows the microhardness graph of the
optimized FGM sample. Figure 13b shows the bar chart of the average microhardness value of the
optimized FGM sample and compared to that of the parent material.
<Inset Figure 13>
It may be seen that the microhardness value of the topmost part of the FGM sample is as high as
1200 HV, which is as high as four times that of the substrate as shown in Figure 13b. The high
hardness is as a result of the TiC content; and it is responsible for the improved wear resistance
behaviour of the optimized FGM sample.
CONCLUSION
Functionally graded Ti6Al4V/TiC composite has been developed using laser metal deposition
process and characterized in terms of microstructure, microhardness and wear resistance
behaviour. The functionally graded material was produced using optimized process parameters
obtained from earlier developed empirical model. The optimized functionally graded Ti6A4V/TiC
composite produced was compared with a functionally graded Ti6Al4V/TiC which was produced
at fixed-process parameters as well as the substrate. The wear behaviour of the two functionally
graded samples was compared with that of the substrate material (Ti6Al4V). The results showed
that the optimized functionally graded produced sample has the best wear-resistance behaviour.
Also the microhardness is found to be improved and as high as 1200 VHN which is four times that
of the substrate. The high microhardness also contributed to the improved wear resistance. In the
literature, functionally graded materials are usually produced using constant processing parameters
for each of the compositional ratio in the FGM. The same setting for each of the layer in the FGM
7
does not produce each layer optimized for the property of interest. This study has shown that
producing functionally graded material using optimized process parameters for each compositional
material ratio in the gradient has a better property.
because each of the layers of the functionally graded material is able to withstand extreme working
condition even if the succeeding layer has been lost to wear.
ACKNOWLEDGMENTS
This work is supported by the Rental Pool Programme of National Laser Centre, Council of
Scientific and Industrial Research (CSIR), Pretoria, South Africa. The Department of Chemical
and Metallurgical Engineering is appreciated for the facilities used at the materials lab.
REFERENCES
[1] Shanmugavel, P.; Bhaskar, G. B.; Chandrasekaran, M.; Mani P. S. and Srinivasan, S. P.
An overview of fracture analysis in functionally graded materials. European Journal of
Scientific Research 2012, vol.68 (3), 412-439.
[2] Atai, A. A.; Nikranjbar, A. and Kasiri, A. Buckling and post-buckling Behaviour of
semicircular functionally graded material arches: a theoretical study. Proceedings of the
Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science
(2012), 226, 607-614.
[3] Knoppers, R.; Gunnink, J. W.; Van den Hout, J. and Van Vliet, W. The reality of
functionally graded material products, TNO Science and Industry, The Netherlands, 2004,
38-43.
[4] Wang, S. S. Mechanics for delamination problems in composite materials. Journal of
Composite Materials 1983, 17(3), 210-223.
[5] Niino, M.; Hirai, T. and Watanabe, R. The functionally gradient materials, J Jap Soc
Compos Mat 1986, 13, 257-264.
[6] Pompe, W.; Worch, H.; Epple, M.; Friess, W.; Gelinsky, M.; Greil, P.; Hempel, U.;
Scharnweber, D and Schulte, K. Functionally graded materials for biomedical applications.
Materials Science and Engineering: A 2003, 362(1), 40-60.
[7] Miyamoto, Y.; Kaysser, W. A.; Rabin, B. H.; Kawasaki, A. and Ford, R. G. (Eds.).
Functionally graded material design, processing and applications. Materials Technology
Series 1999, Springer.
[8] Ivosevic, M.; Knight, R.; Kalidindi, S. R.; Palmese G. R. and Sutter, J. K. Solid particle
erosion resistance of thermally sprayed functionally graded coatings for polymer matrix
composites, Surface Coat Tech 2005.
[9] Scott J.; Gupta N.; Wember C.; Newsom S.; Wohlers T. and Caffrey T. Additive
manufacturing: status and opportunities, Science and Technology Policy Institute 2012,
Retrieved
11th
July
2012,
from
https://www.ida.org/stpi/occasionalpapers/papers/AM3D_33012_Final.pdf
[10]
K. Shah, I. Haq, A. Khan, S. A. Shah, M. Khan, A. J. Pinkerton, 2004, Parametric
study of development of Inconel-steel functionally graded materials by laser direct metal
deposition, Materials & Design, Volume 54: 531-538.
[11]
T. Durejko, M. Ziętala, W. Polkowski, T. Czujko, 2014, Thin wall tubes with
Fe3Al/SS316L graded structure obtained by using laser engineered net shaping
technology, Materials & Design, Volume 63: 766-774.
8
[12]
K. Zhang, S. Wang, W. Liu, X. Shang, 2014, Characterization of stainless steel
parts by Laser Metal Deposition Shaping, Materials & Design, Volume 55: 104-119.
[13]
F. Weng, C. Chen, H. Yu, Research status of laser cladding on titanium and its
alloys: A review, Materials & Design, 2014. Volume 58: 412-425.
[14]
S. Kumar, J.-P. Kruth, 2010, Composites by rapid prototyping technology,
Materials & Design, Volume 31, Issue 2: 850-856.
[15]
S. H. Riza, S.H. Masood, C. Wen, D. Ruan, S. Xu, 2014, Dynamic behaviour of
high strength steel parts developed through laser assisted direct metal deposition, Materials
& Design, Volume 64: 650-659.
[16]
G. Bi, G. L. Ng, K. M. Teh, A. E.W. Jarfors, 2010, Feasibility study on the Laser
Aided Additive Manufacturing of die inserts for liquid forging, Materials & Design,
Volume 31: S112-S116.
[17]
C. Meacock, R. Vilar, 2008, Laser powder microdeposition of CP2 Titanium,
Materials & Design, Volume 29, Issue 2: 353-361.
[18]
H.P. Qu, P. Li, S.Q. Zhang, A. Li, H.M. Wang, 2010, Microstructure and
mechanical property of laser melting deposition (LMD) Ti/TiAl structural gradient
material, Materials & Design, Volume 31, Issue 1: 574-582.
[19]
P. K. Jain, P.M. Pandey, P.V.M. Rao, 2010, Tailoring material properties in layered
manufacturing, Materials & Design, Volume 31, Issue 7: 3490-3498.
[20]
Obielodan, J. and Stucker, B. Characterization of LENS-fabricated Ti6Al4V and
Ti6Al4V/TiC dual-material transition joints, International Journal of Advanced
Manufacturing Technology 2013, 66(9-12), 2053-2061.
[21]
Wang, F.; Mei, J. and Wu, X. Compositionally graded Ti6Al4V + TiC made by
direct laser fabrication using powder and wire. Materials and Design 2007, 28, 2040–2046.
[22]
Zhang, Y.; Wei, Z.; Shi, L. and Xi, M. Characterization of laser powder deposited
Ti–TiC composites and functional gradient materials. Journal of Materials Processing
Technology 2008, 2 0 6, 438–444.
[23]
Balla, V. K; De Vas ConCellos, P. D.; Xue, W.; Bose, S. and Bandyopadhyay, A.
Fabrication of compositionally and structurally graded Ti–TiO2 structures using laser
engineered net shaping (LENS). Acta Biomater 2009, 5(5). 1831–1837.
[24]
Thivillon, L.; Bertrand, P. H.; Laget, B. and Smurov, I. Potential of direct metal
deposition technology for manufacturing thick functionally graded coatings and parts for
reactors components. Journal of Nuclear Materials 2009, 385(2), 236–241.
[25]
Liu, W. and DuPont, J. N. Fabrication of functionally graded TiC/Ti composite by
laser engineered net shaping. Scripta Materialia 2003, 48(9), 1337-1342.
[26]
Shah, K. .Laser Direct Metal Deposition of Dissimilar and Functionally graded
alloys, PhD Thesis, The University of Manchester 2011.
[27]
Lin, X.; Yue, T.M.; Yang, H.O. and Huang, W.D. Laser rapid forming of
SS316L/Rene88DT graded material. Materials Science and Engineering: A 2005, 391(1–
2), 325–336.
[28]
Qin, L. Y; Wang, W. and Yang G. Fabrication of in situ reinforcement Ti matrix
wear- resistant and its wearing performance at 5000C. Advanced Materials Research 2011,
189-193, 3731-3735.
[29]
Schwendner, K.I.; Banerjee, R.; Collins, P. C.; Brice, C.A. and Fraser, H.L. Direct
laser deposition of alloys from elemental powder blends. Scripta Materialia 2001, 45(10),
1123-1129.
9
[30]
Mahamood, R. M.; Akinlabi, E. T.; Shukla, M. and Pityana, S. Process for
manufacture of Titanium based composite. SA Patent No. 2014/03117. 2014.
[31]
Mahamood, R. M.; Akinlabi, E. T.; Shukla, M. and Pityana, S. Characterization of
laser deposited Ti6A4V/TiC composite. Lasers in Engineering 2014. 29(3-4), 197-213.
[32]
Sharma, S.; Sangal, S. and Mondal, K. On the optical microscopic method for the
determination of ball-on-flat surface linearly reciprocating sliding wear volume, Wear
2013, 300(1–2), 82-89.
[33]
Mahamood, R. M.; Akinlabi, E. T.; Shukla, M. and Pityana, S. Scanning velocity
influence on microstructure, microhardness and wear resistance performance on laser
deposited Ti6Al4V/TiC composite. Materials and Design 2013, 50, 656-666.
[34]
Shimaa, E.; Hisashi, S.; Eri, M. and Yoshimi, W. Fabrication of Al-Al3Ti/Ti3Al
Functionally Graded Materials under a Centrifugal Force. Materials 2010, 3, 4639-4656.
[35]
Li, J.; Chen, C. and Zhang, C. Phase Constituents and Microstructure of
Ti3Al/Fe3Al + TiN/TiB2 Composite Coating on Titanium Alloy. Surface Review and
Letters 2011, 18, 103-108.
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11
Table 1. - Optimized process parameters
Sample
% TiC
Designation
Laser Power Scanning
Powder
(kW)
flow
Speed (m/s)
Gas
rate rate (l/min)
(rpm)
A
0
2.0
0.01
2.00
2.00
B
5
2.0
0.0075
3.00
2.00
C
10
2.2
0.0075
2.00
2.00
D
15
2.26
0.0075
2.00
2.00
E
20
2.21
0.0077
2.00
2.00
F
25
2.19
0.0078
2.00
2.00
G
30
2.16
0.0079
2.00
2.00
H
35
2.14
0.0080
2.00
2.00
I
40
2.12
0.0081
2.00
2.00
J
45
2.10
0.0082
2.00
2.00
K
50
2.07
0.0083
2.00
2.00
12
flow
Table 2. - Wear volume for samples A, B and PM
Sample designation Wear
(mm3)
A
0.021
B
0.033
PM
0.120
13
volume
(a)
(b)
Figure 1- (a) Experimental set-up (b) schematics of the LMD process [31]
14
(a)
(b)
(c)
FIGURE 2- (a) The micrograph of the substrate (Ti6Al4V) [33] (b) The SEM micrograph of the
Ti6Al4V powder [33] (c) The SEM micrograph of the TiC powder [33].
15
Figure 3- Photograph of the functionally graded Ti6Al4V/TiC
16
(a)
(b)
Figure Error! No text of specified style in document.- Optical micrograph of the functionally
graded Ti6Al4V/TiC at: (a) optimized parameters settings (b) fixed parameters settings
17
(a)
(b)
(c)
(d)
Figure 5- Micrograph of the top layer of the functionally graded sample at: (a) optimized process
parameters; (b) Figure 5a at higher magnification; and (c) at fixed-process parameters (d) Figure
5c at higher magnification
18
(a)
(b)
Figure 6- (a) The SEM microstructure of the optimized FGM sample showing the UMC; and (b)
the EDS analysis of the point A label in Figure 6a
19
Figure 7- SEM micrograph of the sample at produced at optimized-processing parameters,
showing the top (A); middle (B); lower level (C); and the dilution region (D)
20
Figure 8- XRD analysis showing the phases present in the optimized FGM sample
21
(a)
(b)
Figure 9- (a) The SEM microstructure of the optimized FGM sample showing the intermetallic
compound; and (b) the EDS analysis of the spectrum labelled 2 in (a)
22
Figure 10- The plot of coefficient of friction versus sliding time of the substrate and the
optimized FGM
23
(a)
(b)
Figure 11- The SEM micrograph of the wear track of the substrate at (a) low magnification [33]
(b) higher magnification [33]
24
(a)
(b)
Figure 12- SEM micrograph of the wear track of: (a) FGM at fixed-process parameter; (b) FGM
at optimized-process parameters
25
(a)
(b)
Figure 13- (a) the microhardness of the optimized FGM across the section of the sample (b) the
bar chart of the average microhardness of the optimized FGM and that of the substrate material
26