Strojniški vestnik - Journal of Mechanical Engineering 64(2018)7-8, 488-495 © 2018 Journal of Mechanical Engineering. All rights reserved.
D0l:10.5545/sv-jme.2018.5362	Short Scientific Paper
Received for review: 2018-03-22 Received revised form: 2018-05-11 Accepted for publication: 2018-05-23
Influence of Laser Power on Improving the Wear Properties of Laser-Deposited Ti-6Al-4V+B4C Composite
Mutiu F. Erinosho* - Esther T. Akinlabi
University of Johannesburg, Department of Mechanical Engineering Science, South Africa
Titanium and its alloys have possessed outstanding properties such as high specific strength, good oxidation and corrosion resistance; which have made them extensively suitable for use in the aeronautical, medical, automobile, marine and chemical industries. This paper presents the impact of laser power on the microstructure and the wear properties of titanium matrix Ti-6Al-4V+B4C composites. The laser power was varied from 0.8 kW to 2.2 kW while keeping other contributing parameters constant. The microstructural effects were characterized by increasing a-Ti lamella and coarse Widmanstettan structures as the laser power was increased, with the inclusion of 20 wt % of B4C. The mechanical action during wear test has created a loop shape with inner and outer radii on the surface of the laser-deposited composites. The wear thickness, depth and COF were taken into account; with the sample C deposited at a laser power of 1.8 kW and scanning speed of 1 m/min having the lowest wear loss of 0.119 mm3. The substrate exhibited the shallowest wear depth, and the reason is attributed to the compressive nature of the material. The interlace of B4C in the titanium matrix has improved the properties of the laser-formed composites. Keywords: laser metal deposition, microstructure, wear measurement, Ti-6Al-4V+B4C composite
Highlights
•	The robustness of the a/p phase increases as the laser power is increased.
•	The mean wear thicknesses of the laser-deposited samples are varied from 0.309 mm and 0.504 mm.
•	The entire laser-deposited samples experience a rise in the COF and later maintain steadiness between 0.35 and 0.5.
•	The radii of the wear tracks and the mean wear depth have a significant effect on the wear loss.
0 INTRODUCTION
Laser metal deposition (LMD) is one of the most effective means of manufacturing complex geometrical components with high efficiency [1]. This process involves the manufacturing of a complex component directly from a three- dimensional (3D) computer-aided design (CAD) system [2], and it is characterized by an energy balance that is affected by the part being fabricated, the geometrical features of the substrate and the ambient temperature of the process [3]. This energy is however transformed into heat energy that creates the melt pool on the substrate [4]. Titanium and its alloys are metals that have been widely used in industries due to their desirable properties, including good corrosion resistance, high-strength-to-weight ratio, and retention of properties at high temperatures [2]. Despite all the fascinating properties of the alloy, they still possess poor wear resistance; however, the improvement in their properties has given room for other alloys to be integrated. TiB and TiC are widely preferred reinforcement agents for titanium due to their strength, interfacial bonding, excellent thermodynamic stability and related thermal expansion coefficient with the titanium matrix [5]. A small cylindrical bar of (Ti-6Al-4V + B) with a dimension of 9 mm by 6 mm was made to undergo a compressive test and compared
with the parent material, the Ti-6Al-4V alloy. After deformation, it was reported that a 50 % reduction in height was achieved and the microstructure of (Ti-6Al-4V + B) sample was characterized with strain-free a-colonies and more homogenous dislocation density distribution as compared with the parent material [6]. Another research was conducted on cast (Ti-6Al-4V + xB) samples; where x is 0.05 wt %, 0.10 wt % and 0.40 wt % respectively. It was discovered that the microstructural refinement of the cast samples led to an increase in the yield and the ultimate tensile strength of the casted alloys. The strain to failure ratio was also reported to increase with a small addition of boron. However, revealed further that an increase in boron content would lead to an increase in fracture toughness [7], and a general improvement in the cracking resistance [8] to [10]. Premixed titanium and boron were blended in weight ratio 8:1 with sodium silicate solution and laser-deposited onto Ti-6Al-4V alloy substrate. Hard ceramic compounds of TiB and TiB2 were revealed to be formed in the deposited alloy. Similarly, high microhardness values and excellent wear resistance of the laser deposited samples were also realized. However, the results were reported to have considerably enhanced the load-bearing capability of the substrate [11]. TiO2 was sintered with B4C powder in steel dies at 80 MPa for 1 hour after preheating to 1500 °C for 2 hours in a vacuum
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Corr. Author's Address: University of Johannesburg, Department of Mechanical Engineering Science, South Africa, 2006, mutiuerinosho1@gmail.com
Strojniski vestnik - Journal of Mechanical Engineering 64(2018)7-8, 488-495
furnace. It was discovered that the resulting hardness values of the compacted composites ranged from 800 HV to 2500 HV [12]. B4C is an extremely hard boron-carbon ceramic used for ballistic armour, tank armour, bulletproof vests etc. The friction and wear behaviour of micro-textured cemented carbide was investigated against titanium alloy grade 5 using the dry sliding means. The texture filled with lubricant was reported to effectively improve the friction and wear of the cemented carbide disk against the titanium alloy balls [13]. The effect of carbon and boron was studied in Ti-48Al alloys. The crystal structure of B4C formed was found to lessen the accumulations of boron atoms. In one study, Ti-48Al-0.2B4C was reported to have excellent mechanical properties amongst other alloys. In addition, the microstructure with uniformly distributed borides and carbides was reported to be a good fracture toughness enhancer for the Ti-48Al-0.2B4C alloys [14]. A broad approach to refurbish titanium turbine blades by direct laser deposition was investigated. The coatings involved the use of titanium carbide with boron carbide and tungsten carbide particles for an air foil shroud platform. 5 wt % of boron carbide and 5 wt % of tungsten carbide were added to TiC powder. Titanium boride obtained in the coating contains uniformly distributed large solid heat-resistant boron carbide and tungsten carbide particles. The hardness of 520 HV01 was achieved in the coating matrix [15].
Series of works have been carried out with different ceramics especially boron carbide [15] to [18], but there some paucities in the laser cladding of Ti-6Al-4V with B4C composite remain. Nevertheless, this paper presents the influence of laser power on improving the laser-deposited Ti-6Al-4V + B4C composites through the microstructure and the wear properties for nuclear applications. The cladding requires a weight percentage ratio of 4:1 of Ti-6Al-4V alloy and B4C. Here laser powers have been varied between 0.8 kW and 2.2 kW while the scanning speed of 1 m/min, powder flow rate of 3.2 rpm for Ti-6Al-4V alloy, 0.8 rpm for B4C and gas flow rate of 2.0 are kept constant.
1 EXPERIMENTAL TECHNIQUE
The primary technique used in this experiment is grouped under laser cladding. The experiment was conducted at the National Laser Centre, Council for Scientific and Industrial Research (NLC-CSIR) on a 3.0 kW Ytterbium Laser System. Both the laser system and a Kuka robot are interconnected for the operation. The substrate used is a grit-blasted, 99.6 % Ti-6Al-4V
alloy plate with dimensions of (72 x 72 x 5) mm3. The experimental matrixes used are highlighted in Table 1, and the samples are labelled from A to E.
Table 1. Experimental matrix
Sample name ABC	D	E
Laser power [kW] 0.8 1.6 1.8	2.0	2.2
Scanning speed [m/min] = 1, Gas flow rate [l/min]	= 2	
Powder flow rate [rpm]: Ti-6Al-4V = 3.2; B4C = 0.i	5	
The particle size of the Ti-6Al-4V alloy powder used for the deposition ranges between 150 ^m to 200 ^m while the particle size of the B4C powder is about 45 ^m to 90 ^m. Ten tracks with 50 % overlap were made on the substrate. During the deposition process, the clad is shielded with argon gas in order to prevent environmental oxidation. In this research work, 20 wt % of B4C powder was deposited with Ti-6Al-4V alloy powder via the laser deposition process and deposited in a single- layer-multiple-track approach to characterize the wear properties.
1.1	Microstructure
According to E3-11 ASTM standard [19], all the samples are sectioned laterally, mounted, ground and polished. The Kroll's reagent was prepared for the etchant with H2O, HF, and HNO3 in the ratio 48:1:3 following the guidelines in the Struers application note for metallurgical preparation of titanium [20]. Each sample was etched for 30 seconds before optical microscopic and SEM observations using BX51M Olympus microscope and TESCAN machine. The SEM uses a VEGA TC software and is equipped with an X-MAX instrument.
1.2	Wear
A CETR, UTM-2 tribometer, operating with linear reciprocating motion drive located at the Metallurgy and Chemical Laboratory of the Tshwane University of Technology in Pretoria West Campus, South Africa was used to test for wear. Rotary wear was conducted on the deposited surface of the entire sample for 1000 s using a constant force of 25 N and speed of 60 rpm. Prior to commencing the test, the samples were mounted tightly onto a metal plate which was placed in a reciprocating base and adjusted accordingly to ensure that the tungsten carbide ball is well located on the deposited surface. The tribometer measured the wear depth (WD) as well as the coefficient of friction (COF).
Influence of Laser Power on Improving the Wear Properties of Laser-Deposited Ti-6Al-4V+B4C Composite
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Strojniski vestnik - Journal of Mechanical Engineering 64(2018)7-8, 488-495
Fig. 1. Microstructures of laser deposited Ti-6Al-4V+B4C composite: a) sample A: 0.8 kW; b) sample B: 1.6 kW; c) sample C: 1.8 kW; d) sample D: 2 kW; e) SEM of sample B: 1.6 kW with columnar grains; f) SEM of sample D: 2 kW with martensitic phases; and g) SEM of sample E: 2.2 kW
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Strojniski vestnik - Journal of Mechanical Engineering 64(2018)7-8, 488-495
Using Eqs. (1) to (4), derived by Klenam and Chown [21], the wear loss was calculated as follows:
R2 K	(1)
\ n
Rl = R2 -1,	(2)
Vs =nx(( - R )X dmax,	(3)
i = 2nxRmean =nx(2R2 -1),	(4)
where Vs is wear volume [mm3], R2 outer radius [mm], Rj inner radius [mm], t mean track thickness [mm]. dmax maximum wear depth [mm], and s sliding distance [mm].
2 RESULTS AND DISCUSSION
The microstructural behaviour and the rotary wear properties of the laser- deposited Ti-6Al-4V+B4C composites are presented in detail in this section.
2.1 Microstructural Examination
The deposits, the fusion zones, and the heat-affected zones (HAZ) of the laser-deposited samples are taken into consideration for the microstructures. Figs. 1a to d show the microstructures of samples A, B, C, and D deposited at the laser powers of 0.8 kW, 1.6 kW, 1.8 kW, and 2.0 kW, respectively.
The microstructures of the laser deposited composites show the formation of a-Ti phase and P-Ti phase, and these were elongated towards the fusion zone and terminated at the zone. Globular microstructures were established just immediately after the fusion zone and grow epitaxially within this region and later disappear towards the HAZ(s). The robustness of the a/p phases increases as the
laser power was increased. At high laser power, the coarseness of the microstructure is more obvious than the composite deposited with the previous laser powers due to the energy density of the laser beam. The intensity of the laser beam at high laser power creates more heat input as well as a wider melt pool to accommodate the volume of metal deposit. Widmanstettan structures are also observed in all the microstructures and look harsh and rigid with the agglomeration of 20 wt % of B4C added. The microstructures of the Ti-6Al-4V alloy after boriding show more lamellar, which typically enhance the formation of (a + P)-Ti alloys [16]. The thickness of the a-Ti lamella formed is a significant feature since it influences the mechanical properties of the Ti-6Al-4V alloy [17]. Under the SEM, a fish bone-like of a-Ti martensitic phase was observed in sample B deposited with a laser power of 1.6 kW and scanning speed of 1 m/min; and this can be attributed to the stability accrued by B4C within the deposited region. Columnar grains growing perpendicularly to the heat sink direction were also observed in the deposit. There is an equiaxed transition of these grains, and these were as a result of the steep temperature gradient and the cooling rate effect during the solidification process.
2.2 Wear Properties
The wear scars of the substrate and the laser-deposited Ti-6Al-4V+B4C composite are shown in Fig. 2. A form of ring-type wear was inscribed on the surface of the samples.
The ring has an inner and outer radius which are measured from the centre and labelled as R1 and R2 respectively. The surface topographies of the wear scars of the laser-deposited Ti-6Al-4V+B4C
Deposit
m
Fig. 2. Wear track of the substrate and the Ti-6Al-4V+B4C deposit at 2.0 kW
Influence of Laser Power on Improving the Wear Properties of Laser-Deposited Ti-6Al-4V+B4C Composite
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Fig. 3. Wear track of the laser deposited samples showing the thicknesses: a) substrate, b) sample A deposited at laser power of 0.8 kW; c) sample B deposited at laser power of 1.6 kW; d) sample C deposited at laser power of 1.8 kW; e) sample D deposited at laser power of 2 kW; f) sample E deposited at laser power of 2.2 kW
composites after the dry rotary wear tests are shown in Fig. 3.
The sliding friction that exists between the tip of the tungsten carbide ball and the surface of the laser-deposited composites was instigated by the action force. The wear morphology was inscribed as a result of the mechanical action which created a ring-shaped indentation on the surface of the composites. The thicknesses of the wear scars were measured and the average is taken into consideration. This is an essential parameter for the wear volume and wear rate in order to predict the life expectancy of the composite when subjected to wear. Rubbing exists between the tungsten carbide ball and the surface of the laser-deposited composites in a circular manner; and due
to the vertically applied force, wear was initiated on the surfaces as a result of the friction. Thin grooves were generated on the scar in the rubbing direction. The substrate shows a wider thickness as compared to other wear topographies.
Fig. 4 depicts the mean wear thicknesses of the substrate and the laser-deposited Ti-6Al-4V+B4C composites from samples A to E.
Fig. 5 shows the profiling of the wear depth of the substrate and the laser-deposited Ti-6Al-4V+B4C composites.
At different points, three thicknesses of the wear scar per sample were measured with the optical microscope, and the mean was established. The mean wear thicknesses of the laser-deposited samples varied
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B	C
Samples
Fig. 4. Plot of mean wear thickness
Fig. 5. Profiling of wear depth versus time
from 0.309 mm and 0.504 mm. Sample A deposited at a laser power of 0.8 kW and scanning speed of 1 m/min depicts the lowest wear thickness of 0.309 mm whilst the substrate exhibits the highest mean wear thickness of 0.539 mm. This sample depicts the lowest wear thickness among all other deposited samples; which could be as a result of partly and inhomogeneous melting of B4C in the alloy with the low laser power used. From the scattered plot of the wear depth against the time, it can be deduced that there is a linear rise in the trend from the start time to the end time at 1000 seconds. The substrate shows the lowest average wear depth of 0.013 mm, while among
other composites, sample D deposited at a laser power of 1.8 kW and scanning speed of 1 m/min exhibits the highest average wear depth of 0.074 mm. The rolled nature and the compactness of the substrate have a greater effect on its wear depth. Sample E deposited at a laser power of 2.2 kW exhibits a constant wear depth at zero (0) time till 0.045 mm and later maintains a slow steady rise till the end.
Table 2 illustrates the wear loss (Vs) of the substrate as well as all the participating laser-deposited samples from samples A to E, respectively.
The wear loss was initiated as a result of the frictional force and the abrasive wear between the
Influence of Laser Power on Improving the Wear Properties of Laser-Deposited Ti-6Al-4V+B4C Composite
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Table 2. Wear loss (Vs) of the substrate and the laser-deposited specimen
Sample name	Substrate	Sample A	Sample B	Sample C	Sample D	Sample E
Wear loss, Vs [mm3]	0.712	0.329	0.179	0.119	0.515	0.399
tungsten ball and the top surface of the laser-deposited composite. The round thicker ridge created at the engraved path-way shows the degree of the wear loss. The Vs measured between the laser-deposited samples A to E is from 0.119 mm3 to 0.399 mm3. Among the deposited Ti-6Al-4V+B4C specimens, sample C deposited with a laser power of 1.8 kW exhibits the lowest Vs of 0.119 mm3 while the as received substrate has the Vs of 0.712 mm3 which is higher than all the deposited samples. The Vs has been improved with the addition of B4C based on the results obtained as shown in Table 2. The radii of the wear tracks and the mean wear depth have a significant effect on the wear loss. The variations between the melting points of the two participating powders as well as their densities have also contributed to the improvement in wear properties. From the values obtained, it can be deduced that, as the laser power was increased from 0.8 kW to 1.8 kW, the wear loss was found to decrease, and later showed a sudden increase from 0.119 mm3 in sample C to 0.515 mm3 in sample D at a laser power of 2 kW. For sample E, the Vs was dropped to 0.399 mm3. This behaviour can be attributed to the melting that took place at high laser power as well as the slow cooling rate.
The COF of the as-received substrate as well as the laser-deposited Ti-6Al-4V+B4C composites were also analysed and presented in Fig 6.
The COF of all the participating samples is varied between 0.382 and 0.424. Sample A deposited at a laser power of 0.8 kW and scanning speed of 1 m/ min exhibits the lowest COF of 0.382 while sample B deposited at a laser power of 1.6 kW shows the highest COF of 0.424, although the value is close to that of the substrate with a COF of 0.419. However, these do not serve as the yardstick for enhancing their wear loss. The COF of all the samples reaches a maximum before they oscillate between limits and maintain steadiness in the range of 0.25 and 0.5. All the samples experience a rise in the COF from the starting point at zero seconds and maintain steadiness in the COF between 0.35 and approximately 0.5.
3 CONCLUSION
The coagulation of titanium alloy with boron carbide through the laser deposition process was successfully achieved, and the properties were considerably enhanced. The characteristics conducted on the microstructure of the titanium alloy matrix Ti-6Al-4V+B4C composite revealed some globular structures growing epitaxially towards the heat-affected zone. It
400	600
Time [s]
Fig. 6. COF of the substrate and the laser-deposited Ti-6Al-4V+B4C composites
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was also evident that the robustness of the a/p phases is as a result of the increase in the laser power. The rubbing that exists between the hard tungsten carbide ball and the surface of the laser-deposited composites facilitated a circular wear scar with different thicknesses and wear depths. Sample A deposited at a laser power of 0.8 kW and scanning speed of 1 m/ min depicted the lowest wear thickness and COF of 0.309 mm and 0.382 but does not assure a reduced wear loss due to the wear depth and surface defect. The wear loss has been improved with the addition of 20 wt % of B4C. The metal matrix Ti-6Al-4V+B4C composites can be applied in the aerospace industries for the repair of parts due to the improved properties.
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