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Monitoring and assessment of tungsten carbide wettability in laser cladded metal matrix composite coating using an IR pyrometer
Accepted Manuscript Monitoring and assessment of tungsten carbide wettability in laser cladded metal matrix composite coating using an IR pyrometer Muvvala Gopinath, Debapriya Patra Karmakar, Ashish Kumar Nath PII:
S0925-8388(17)31464-0
DOI:
10.1016/j.jallcom.2017.04.254
Reference:
JALCOM 41658
To appear in:
Journal of Alloys and Compounds
Received Date: 11 January 2017 Revised Date:
19 April 2017
Accepted Date: 23 April 2017
Please cite this article as: M. Gopinath, D.P. Karmakar, A.K. Nath, Monitoring and assessment of tungsten carbide wettability in laser cladded metal matrix composite coating using an IR pyrometer, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.254. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Monitoring and assessment of tungsten carbide wettability in laser cladded metal matrix composite coating using an IR Pyrometer Muvvala Gopinath, Debapriya Patra Karmakar, Ashish Kumar Nath Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur, 721302, India
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Abstract Laser cladding technique was applied to deposit a metal matrix composite coating of Inconel 718 with 30 wt% WC as a ceramic phase. One of the major problems associated with obtaining fully dense MMC coating is the poor wetting/bonding between the ceramic
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particles and the metal matrix. Further, a large number of process parameters involved in the laser cladding process makes it more complex to optimise the process window. Therefore, the current study focuses on the monitoring of molten pool thermal history which is an outcome
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of the combination of all process parameters, and identifies the molten pool lifetime and cooling rates that are favourable for the formation of interfacial layer improving the wetting characteristics. The thermal history of molten pool was recorded using an IR pyrometer. The molten pool lifetime, solidification shelf time and cooling rates were assessed from the data acquired and effect of these on wetting characteristics and formation of interfacial layer
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between WC particles and metal matrix was analysed using SEM analysis. Molten pool lifetime greater than 0.68 s exhibited proper wetting of WC particles with the matrix. Fractured surface of coatings that experienced relatively fast cooling revealed the delaminating of ceramic particles from the metal matrix under the tensile load, whereas in
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one with slow cooling particles found to stay intact with the metal matrix improving the wear properties of the coating. However, too slow cooling resulted in settlement of WC particles at
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the bottom of molten pool reducing the wear resistance of the coating.
Keywords: Laser cladding, IR pyrometer, monitoring, molten pool thermal history, Wetting. ∗
Corresponding Author: Tel: +91 - 3222 – 281784; fax: +91-3222-25530 E-mail address: [email protected] Postal address: Department of Mechanical Engineering, IIT Kharagpur, Kharagpur-721302, India
ACCEPTED MANUSCRIPT 1. Introduction Failure of mechanical components is generally caused by fatigue, wear, erosion and corrosion at the surface. With the advancement of technology in past few decades, several surface engineering techniques came into light which could improve the hardness and wear resistance of surface by developing metal matrix composite coatings on the surface of the
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component. Among them, laser cladding technique for development metal matrix composite coatings (MMC) and functionally graded coatings became more popular because of several advantages over conventional techniques like arc welding, TIG, plasma and thermal spry coating etc [1-3]. This includes localized treatment with minimum heat input, dilution and
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distortion, fast cooling rates experienced from self quenching mechanism resulting in fine microstructure enhancing the surface properties along with the process flexibility and automation. The MMC coatings involve addition of carbide, nitride and oxide ceramic
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particles to the relatively softer metal matrix to enhance the wear characteristics of the surface. Further, addition of ceramic particles enhances the specific strength, stiffness, toughness of the material and exhibits good stability at high operational temperatures [4-7]. MMC coating finds application in mining and mineral industries for protection over high value down-hole drilling tools [8], to improve the lifetime of components such as rollers,
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rails, wearing plates, piston rods, cylinders and turbines etc [9-11]. In laser cladding process of developing MMC coatings, the high energy laser beam melts the powder particle with low melting point which forms the metal matrix. The liquid metal flows around the ceramic particles consolidating them which generally have very high
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melting point and stays in solid state as the molten pool solidifies. The interface of metal matrix and ceramic particles plays a vital role as improper wetting/bonding may result in
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pull-out of ceramic particles during abrasion through ploughing resulting in decreased wear resistance properties of the coating and premature failure of component under mechanical loading as the weak interface acts as the sight for crack initiation [12-14]. Thus it is very essential to tailor the ceramic particles to the metal matrix. Bartkowski et al. [15] studied the effect of laser power on tailoring of WC particle with stellite matrix. It was observed that with the increase of laser power (400 to 700 W), the surface of WC particle started melting increasing the wetting/bonding with the metal matrix. However, Abioye et al. [16] demonstrated that excess dissolution of WC may lead to decrease of corrosion resistance of the coatings. Rong et al [17] studied the formation of diffusion layer between WC1-x and Inconel 718 as a function of applied laser energy. Hong et al. [18] studied the effect of line energy (80 to 160 kJ/m) on formation of interfacial layer between the TiC particles and the
ACCEPTED MANUSCRIPT Inconel 718 matrix. It was observed that with increase of line energy, the interfacial layer thickness was found to increase. However at higher line energy TiC started decomposing. Authors [19] further extend the work with fine (2-7 µm) TiC particles where similar results were observed. Liu et al. [20] carried out a comparative study of formation of interfacial reaction layer in Ti-6Al-4V matrix with cast and single crystal WC particles. The reaction
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layer found to have W2C and a mixed layer of W and TiC which are effective in load transfer under an external load. It was reported that cast WC particles would perform better because of low content of carbon (3.9 ± 0.1 ) when compared to single crystal WC (6.8 ± 0.05) as titanium has high affinity towards carbon, forming excessive TiC dendrites which has poor
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plasticity and toughness compared to Ti matrix. Therefore it is apparent that formation of reaction layer is necessary in order to enhance the mechanical properties of the coating, while too much of reaction may lead to deterioration of the same as it results in the formation of
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brittle metal matrix. Thus, it is very vital to understand the favorable conditions for the formation of reaction layer and its dependence on process parameters. The process parameters window of laser cladding process is too large containing more than 19 parameters which are going to affect the metallurgical as well as geometrical characteristics of the formed clad layer directly [21-23]. Many of these process parameters
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are system and material dependent like the laser beam intensity profile (TEM mode), operating wavelength, laser cladding head design (lateral/coaxial), substrate dimension and thermo-physical properties etc. Thus, the optimized process window for a given system and material may not work elsewhere. However, it may be noted that the formation of reaction
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layer mainly depends on the temperature to which the ceramic particles are exposed and its duration as it dictates the formation of reaction layer and its thickness. Therefore, the current
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study focuses on monitoring the thermal history of the molten pool which includes the surface peak temperature, molten pool lifetime and cooling rate using an IR pyrometer, and their effect on formation of reaction layer and wear characteristics of the coating.
2. Experimental setup
The gas atomized, spherical Inconel 718 powder particles (MetcoClad 718) (Fig. 1a) with powder size distribution of 43-100 µm and irregularly shaped WC particles (Fig. 1b) were used in the current study as metal matrix and ceramic particle phase respectively. AISI 304 austenitic steel plates of 50 mm×50 mm×8 mm dimensions were used as substrates. The chemical compositions of Inconel 718 and AISI 304 substrate are as presented in Table. 1. The surface of as received substrates were polished with emery paper (220 mesh) and cleaned
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Fig. 1 Typical morphologies of (a) Inconel 718 and (b) WC particles Table 1 Chemical composition (Wt%) of cladding and substrate materials
IN 718 AISI 304
Cr
Fe
Nb
Mo
Al
Ti
Mn
Si
B
Ni
C
P
19.02
18.1
4.92
3.19
0.54
0.97
0.04
0.20
0.004
Bal.
-
-
18.97
Bal.
-
0.224
-
-
1.731
0.753
-
8.554
0.067
0.045
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(Wt%)
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Element
Table 2 Laser cladding process parameters CW mode
S
0.031
Scan speed
Thickness of preplaced powder coating
Beam spot diameter
(W)
(mm/min)
(mm)
(mm)
1200
200 - 1200
1
3
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Laser beam power
with acetone. Laser cladding was carried out using a two step process. Initially Inconel 718
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(70%) and WC (30%) powder particles were mixed with aqueous solution of 2% polyvinyl alcohol (Alfa Aesar, 87–89% hydrolysed) which was used as a binder. Mixing was done
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mechanically by alternate cycles of mechanical stirring (Spinot Model MC-02, Tarsons). After proper mixing, a predefined thickness (1 mm) of the powder-PVA blend was applied on to the substrate surface using a coating machine (Model no: K101 control coater, RK Print Coat Instruments Ltd., UK,). The coated sample were baked in furnace at 100 °C in Argon environment for 15 min in order to remove the moisture content from the coating and to obtain a temporary bonding. A 2kW Yb-Fibre laser (IPG photonics, Model no. YLR 2000) operating at 1.07 µm wavelength is used for cladding the prepared sample. It can be operated in CW as well as in modulated mode at 50–1000 Hz frequency range with 5–100% duty cycle with rectangular shaped temporal power profile. The laser has multimode beam intensity profile with high intensity at the centre and two annular rings of relatively low intensity [24]. The laser beam
ACCEPTED MANUSCRIPT effectively. Table 2 shows the process parameters that were used to carry out the laser cladding. Single spot monochromatic IR pyrometer operating at 1.6 µm wavelength with 0.7 mm vision zone and 1 ms acquisition time with temperature measuring range of 385 °C 1600 °C has been used to record the thermal history of the molten pool. Though the pyrometer was specified to operate at 1.6 µm wavelength, laser radiation was found to affect
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the recorded signal. Therefore, a 1064 ± 25 nm notch filter with optical density of 3 was mounted in front of the optical components of the pyrometer to isolate it from the laser radiation. The filter attenuated a certain fraction of the IR radiation also, necessitating recalibration of the detector. Proper calibration was carried out by melting various metals
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like Al, SS and Cu, and correlating the obtained temperature signal with the known melting point of the respective metals. The recalibrated temperature measuring range with the notch filter is ~785 °C - 3260 °C. Thermal history of the molten pool was recorded keeping the
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pyrometer stationary and focused at a point along which the laser beam scans forming the clad tracks. The obtained clad tracks were cross-sectioned using a wire-cut EDM, mirror polished and analysed under SEM to study the interfacial reaction between ceramic particles and the metal matrix. Fracture test was carried out to study the effect of wetting on bonding characteristics of the ceramic particles with metal matrix. For carrying out the fracture test,
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single tracks were cut along the length on either side of the clad track using wire cut EDM and the obtained samples were cut at the middle from the bottom of substrate till the clad track is approached, thus exposing the clad track alone to the tensile load applied using a universal testing machine (Tinius Olsen H50KS). Wear test was also carried out on the clad
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surface developed by 30% overlapping of tracks using a ball on disk setup (Make: DUCOM, TR-201-M3). WC ball was used as a counter body and wear test was carried at 300 RPM
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with 19.62 N load for 15 min with a track diameter of 5 mm.
3. Results and discussion
3.1 Molten pool thermal history and its variation with process parameters Fig. 2 depicts the typical molten pool thermal history recorded during the laser
cladding of Inconel 718 with 30% WC carbide at 1200 W laser power and 600 mm/min scan speed. The thermal history of molten pool consists of heating cycle 0A and cooling cycle AD. Slope of these lines represent the heating and cooling rates of the molten pool. It can be observed that heating cycles 0A has a steep change in its slope at around 300 ms seconds which is due to the advancement of centre of laser beam spot which is having peak intensity
ACCEPTED MANUSCRIPT [24] towards the centre of zone of interest where pyrometer is focused. The cooling cycle has several stages as it changes its phase from liquid to solid. AB represents the drop in molten
1385
TL
B
TS
1185
L
985
L+S
S
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C
D
785 0
500
1000
1500
2000
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Temperature (°C)
TL – Liquidus Temperature TS – Solidus Temperature L – Liquid S – Solid
A
1585
Time (ms)
Fig. 2 Typical molten pool thermal history (1200 W, 600 mm/min)
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pool temperature when it is still in liquid state. At point B, solidification initiates and ends at point C which is nothing but the liquidus and solidus temperature of the material under study respectively. It may be clearly observed that the slope or the cooling rate (∂T/∂t) during the period BC is quite low compared to that of AB which is because of the release of energy in the form of latent heat of fusion in the zone BC. Zone BC is generally termed as solidification
1600 1550 1500
Cooling rate (103 °C/s)
1650
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Temperature (°C)
1700
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shelf or freezing zone where liquid and solid co-exists followed by solid phase cooling CD.
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Scan speed (mm/min)
Solidification shelf time (s)
Melt pool lifetime (s)
0.4 0.2
200 400 600 800 1000 1200 Scan speed (mm/min) 1
1.6 1.2 0.8 0.4
0 200 400 600 800 1000 1200 Scan speed (mm/min)
0.6
(b)
2
(c)
0.8
0
200 400 600 800 1000 1200
(a)
1
(d)
0.8 0.6 0.4 0.2 0 200 400 600 800 1000 1200 Scan speed (mm/min)
Fig. 3 Effect of laser scan speed on (a) surface peak temperature, (b) cooling rate, (c) melt pool lifetime and (d) solidification shelf time
ACCEPTED MANUSCRIPT Further, the zones AC and BC represent the molten pool lifetime and the solidification shelf time respectively. Fig. 3 shows the variation of various aspects of molten pool thermal history with the scan speed. It may be observed that with the decrease of laser scan speed from 1200 mm/min to 200 mm/min, keeping the laser power and spot diameter constant at 1200 W and 3 mm
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respectively, the surface temperature showed an increasing trend as shown in the Fig. 3(a). However, the increase in surface temperature is not so significant in case of 200 mm/min and 400 mm/min which may be due to the initiation of metal vaporization from the surface that restricts further increase of surface temperature because of consumption of energy in the form
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of latent heat of vaporization. With the decrease of laser scan speed, cooling rate is found to decrease (Fig. 3b) as slower scan speeds result in increased line energy as well as substrate temperature, decreasing the temperature gradients. This also resulted in increase of molten
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pool lifetime as well as solidification shelf time as shown in Fig. 3(c) and (d) respectively.
3.2 Effect of molten pool thermal history on wetting of ceramic particle The wetting condition of ceramic particles in the metal matrix composite coating developed by laser cladding process at different scan speeds was as shown the Fig. 4, which
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was analysed using BSE detector. A distinct boundary between the ceramic particles and metal matrix was clearly observed in case of cladding with scan speed of 1200 mm/min and 1000 mm/min, corresponding to which the molten pool lifetime is 0.541 s and 0.605 s, as
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shown in the Fig. 4(a) and (b) respectively. A partial wetting between ceramic particle and
Fig. 4 Variation in wetting condition of ceramic particles with laser scan speed (a) 1200 mm/min, (b) 1000 mm/min, (c) 800 mm/min, (d) 600 mm/min, (e) 400 mm/min, (f) 200 mm/min (1200 W)
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Fig. 5 BSE images of the bonding layer between ceramic particles and the metal matrix (a) 800 mm/min, (b)
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600 mm/min, (c) 400 mm/min and (d) 200 mm/min (1200 W)
metal matrix was observed in case of 800 mm/min scan speed (Fig. 4c). However, this was limited to the particles at the centre of the molten pool where cooling rate is expected to be
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relatively slow. A proper wetting was observed in case of cladding with 600 mm/min, 400 mm/min and 200 mm/min as shown in the Fig. 4 (d), (e) and (f) respectively. It may be noted
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that molten pool life above ~0.68 s favoured the formation of reaction layer resulting in proper wetting between the ceramic particles and the metal matrix. Fig. 5 shows the interface formed between the ceramic particle and the metal matrix
cladded at a constant laser power and beam spot diameter of 1200 W and 3 mm respectively, at different scan speeds. A clear increase in bond layer thickness with decrease of laser scan speed is evident. Fig. 6 shows the variation in bond layer thickness with respect to molten pool lifetime as well as the cooling rate of the clad layer. It was observed that with the increase of molten pool lifetime, the ceramic particles surface started melting causing the diffusion between the WC particles and the matrix whose thickness increased with increase in molten pool lifetime and decreased with increase in cooling rate. The diffusion between the
ACCEPTED MANUSCRIPT Molten pool lifetime (s)
2
800
1.6
600
1.2 400 0.8
0 0
200 400 600 800 Scan speed (mm/min)
0 1000
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200
0.4
Cooling rate (°C/s)
Reaction layer thickness (µm) & molten pool life time (s)
Bond layer thickness (µm) Cooling rate (°C/s)
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Fig. 6 Variation in bonding layer thickness with respect to molten pool lifetime
Table 3.
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Fig. 7 Elemental analysis (EDS) of various phases (1200 W, 600 mm/min)
Gibbs free energies of formation of solid carbides
Nb2 C TiC W2 C Mo2 C
Gibbs free energy (∆G Cal/mole) -46,000 + 1.0T -44,600 + 3.16T -7300 - 0.5T -11,710 - 1.83T
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Nb Ti W Mo
Compound
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Element
WC particle and metal matrix was found to be more dominant in case of laser cladding with 200 mm/min as shown in Fig. 5d. The diffusion also caused change in metal matrix composition as well as the microstructure. Fig. 7 shows the results of EDS analysis of various phases in the microstructure around the ceramic particle. The characteristic bright zone around the ceramic particle was found to be rich in Nb, Ti, Mo, C and W, where as the dark matrix was rich in Ni, W, Fe, C and Cr with traces of Nb, Ti and Mo. These characteristic bright zones are expected to be complex secondary carbides of Ti, Nb, Mo and W which have very high affinity towards carbon as per the Gibbs free energy shown in the Table 3 [25]. In
ACCEPTED MANUSCRIPT case of 200 mm/min, Nb and Ti were found to segregate around the WC particle forming another layer of secondary carbides which is clearly visible in Fig. 5d.
3.3 Fracture surface and wear test analysis Fracture test was carried on the single clad tracks in order to analyze the effect of
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molten pool lifetime on the wetting characteristics as well as on the bonding strength between WC particles and the metal matrix. Fig. 8 shows the fractured surfaces of the samples cladded at different scan speeds whose molten pool lifetime was specified in Fig. 4. A clear debonding of ceramic particles from metal matrix under the applied tensile load is evident in
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case of MMC clad tracks deposited with scan speed of 1200 mm/min and 1000 mm/min as shown in Fig. 8(a) and (b) respectively. As mentioned earlier, a partial bonding was observed in case of cladding with 800 mm/min, yet WC particles found to debond from the metal
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matrix as shown in Fig. 8(c). Clad tracks deposited with scan speeds of 600 mm/min, 400 mm/min and 200 mm/min showed a strong bonding with the matrix as shown in Fig. 8(d), (e) and (f) respectively. Similar to Fig. 5, diffusion of WC into metal matrix and formation of a secondary carbide layer around the WC at 400 mm/min and 200 mm/min respectively, was observed which strongly shows the reproducibility of results. Thus, the WC particles in the
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metal matrix cladded at relatively slow scan speed exhibited strong bonding as slow scan speed results in relatively longer molten pool life (>0.68 s) which allows the melting and
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diffusion of WC surface into the metal matrix enhancing the bonding characteristics.
Fig. 8. Fracture surfaces of clad layer showing the bonding condition between the ceramic particles and the metal matrix deposited with 1200 W laser power and scan speed of (a) 1200 mm/min, (b) 1000 mm/min, (c) 800 mm/min, (d) 600 mm/min, (e) 400 mm/min and (f) 200 mm/min
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Fig. 9. Cross-section of multi-tracks cladded with 50% overlap at 1200 W laser power and 600 mm/min scan
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speed
Wear test was carried out on the samples with multiple tracks deposited with 50%
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overlap as shown in Fig. 9, in order to study the effect of bonding on wear characteristics of the MMC coating developed. During overlapping, the surface temperature was always maintained at less than 40 °C which was monitored using a hand held digital non contact type infrared thermometer (EEE-Tech IRT-4). Fig. 10 shows the effect of molten pool lifetime on the wear rate of the coatings developed. Three replicates were taken for each case. It was observed that the wear rate increased with decrease of scan speed from 1200 mm /min till 800
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mm/min and with further decrease of scan speed to 600 mm/min the trend got reversed and wear rate reduced to a great extent. However, the wear rate again increased with the decrease of scan speed from 400 mm/min to 200 mm/min. Fig. 11 shows the variation in wear track dimension with the scan speed which conforms to the trend observed in Fig. 10. It has been
Wear rate (10-6 g/s)
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mentioned in a previous section that claddings developed with molten pool lifetime less than
Molten pool lifetime (s) 12000.605 0.680 0.845 1.148 1.753 0.541 35 30 25 20 15 10 5 1200 1000 800 600 400 Scan speed (mm/min)
200
Fig. 10 Effect of molten pool lifetime on wear rate
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Fig. 11 Variation in wear track dimension with scan speed (a) 1200 mm/min, (b) 800 mm/min and (c) 400
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mm/min
Fig. 12 Distribution of WC particles in the clad layer (a) 600 mm/min, (b) 400 mm/min and (c) 200 mm/min
~0.68 s, corresponding to scan speed greater than 800 mm/min, showed a weak bonding between the ceramic particles and metal matrix which could have resulted in pull-off of WC
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particles during the wear test, where as in case of molten pool lifetime greater than ~0.68 s, WC particles showed strong bonding resulting in enhanced wear properties. Thus, WC particles does not contribute in improving the wear characteristics of the metal matrix for scan rates that result in molten pool lifetime shorter than ~0.68 s.
The increasing trend in
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wear rate with decrease of scan speed from 1200 mm/min to 800 mm/min could be attributed to the elemental segregation in the Inconel 718 metal matrix. Elemental segregations depend
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mainly on the rate at which liquid metal is solidifying and reaching to the room temperature. It can be observed from Fig. 3(b) that, with decrease of scan speed from 1200 mm/min to 800 mm/min, cooling rate decreases from ~860 °C/s to ~606 °C/s. Gopinath et al. [26] and Zhang et al. [27] showed that with decrease of cooling rate, the volume fraction of Laves phase which is basically (Ni,Fe,Cr)2 (Nb,Mo,Ti) increases depleting Nb in the matrix which is a phase strengthening element. This depletion of Nb and formation of Laves phases results in decrease of hardness and wear resistance of the Inconel 718 matrix [28]. Fig. 12(a), (b) and (c) shows the distribution of WC particles in the metal matrix of the samples cladded at 600 mm/min, 400 mm/min and 200 mm/min respectively. It can be seen that in case of 600 mm/min and 400 mm/min, the WC particles were uniformly distributed over the cross
ACCEPTED MANUSCRIPT section. The uniform distribution of WC particles in the coating and their sound bonding within the matrix for scan speeds resulting in molten pool lifetime more than ~0.68 s could be the reasons for the observed decrease of wear rate when the scan speed was reduced from 800 mm/min to 400 mm/min. However, in case of 200 mm/min, the WC particles found to settle down at the bottom of the clad layer [29], exposing the matrix alone during the wear test,
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resulting in increased wear rate as shown in Fig. 10. Further, excess dilution in case of 200 mm/min, which is evident from the Fig. 11(c) could also be responsible for the high wear rate. Thus, by ensuring proper bonding of WC particles and their uniform distribution in metal matrix through optimizing the laser process parameters high wear resistant metal-
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matrix laser clad layers can be deposited.
Conclusion
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Molten pool thermal history and its dependency on process parameters was monitored in laser cladding of Inconel 718 and WC MMC coating and correlated with the wetting characteristics of WC with metal matrix and wear characteristics of the coatings deposited. The following conclusions are drawn based on the observations:
1. Molten pool lifetime and solidification shelf time increase with decreasing scan speed,
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whereas cooling rate decreases.
2. WC particles exhibits proper wetting and bonding with the metal matrix for molten pool lifetime greater than 0.68 s.
3. Precipitates of secondary carbides are formed around the WC carbide particles at
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relatively long molten pool lifetime.
4. Debonding of WC particles from metal matrix under tensile load occurs in clad tracks
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with relatively short molten pool lifetime. 5. There is an optimum molten pool lifetime which yields highest wear resistance of the coating. While shorter lifetime prevents proper wetting of WC particles with the matrix, longer lifetime causes settling down of ceramic particles at the bottom of molten pool due to their weight, reducing the wear resistance of the coating.
Acknowledgement Authors gratefully acknowledge the financial support from the Department of Science and Technology, Ministry of Science and Technology, Government of India, under the FIST Program-2007 (SR/FIST/ETII-031/2007).
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[19] C. Hong, D. Gu, D. Dai, M. Alkhayat, W. Urban, P. Yuan, S. Cao, A. Gasser, A. Weisheit, I. Kelbassa, M. Zhong, R. Poprawe, Laser additive manufacturing of ultrafine TiC particle reinforced Inconel 625 based composite parts: tailored microstructures and enhanced performance, Mat. Sci. Eng. A-Struct., 635 (2015) 118-128. [20] D. Liu, P. Hu, G. Min, Interfacial reaction in cast WC particulate reinforced titanium
(2015) 180–186.
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metal matrix composites coating produced by laser processing, Opt. Laser Technol., 69
[21] E. Toyserkani, A. Khajepour, S. Corbin, Laser cladding, CRC press, New York (2004). [22] M. M. Quazi, M. A. Fazal, A. S. M. A. Haseeb, Farazila Yusof, H. H. Masjuki, A.
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Arslan, Effect of rare earth elements and their oxides on tribo-mechanical performance of laser claddings: A review, J. Rare. Earth., 34 (2016) 549-564.
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[23] F. J. Kahlen, A. Kar, Tensile strengths for laser-fabricated parts and similarity parameters for rapid manufacturing, J. Manuf. Sci. Eng., 123(2001) 38-44. [24] Y. K. Madhukar, S. Mullick, D. K. Shukla, S. Kumar, A. K. Nath. Effect of laser operating mode in paint removal with a fiber laser. Appl. Surf. Sci. 264 (2013) 892–901. [25] Stephen R. Shatynski, The Thermochemistry of transition metal carbides, Oxid. Metal., 13 (1979) 105–118. [26] M. Gopinath, D. P. Karmakar, A. K. Nath, Online monitoring of thermo-cycles and its correlation with microstructure in laser cladding of nickel based super alloy, Opt. Laser. Eng., 88 (2017) 139–152. [27] Y. Zhang, Z. Li, P. Nie, Y. Wu, Effect of cooling rate on the microstructure of laserremelted INCONEL 718 coating, Metall. Mater. Trans. A, 44 (12) (2013), 5513–5521.
ACCEPTED MANUSCRIPT [28] E. L. Stevens, J. Toman, A. C. To, M. Chmielus, Variation of hardness, microstructure, and Laves phase distribution in direct laser deposited alloy 718 cuboids, Mater. Design., 119 (2017) 188–198. [29] T. Liyanage, G. Fisher , A.P. Gerlich, Microstructures and abrasive wear performance of PTAW deposited Ni–WC overlays using different Ni-alloy chemistries, Wear, 274–275
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(2012) 345–354.
List of figures
Fig. 1. Typical morphologies of (a) Inconel 718 and (b) WC particles
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Fig. 2. Typical molten pool thermal history (1200 W, 600 mm/min)
Fig. 3. Effect of laser scan speed on (a) Surface peak temperature, (b) Cooling rate, (c) Melt pool lifetime and (d) Solidification shelf time
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Fig. 4. Variation in wetting condition of ceramic particles with laser scan speed (a) 1200 mm/min, (b) 1000 mm/min, (c) 800 mm/min, (d) 600 mm/min, (e) 400 mm/min, (f) 200 mm/min (1200 W)
Fig. 5. BSE images showing the bonding layer between ceramic particles and the metal matrix (a) 800 mm/min, (b) 600 mm/min, (c) 400 mm/min and (d) 200 mm/min (1200
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W)
Fig. 6. Variation in bonding layer thickness with respect to molten pool lifetime Fig. 7. Elemental analysis (EDS) of various phases (1200 W, 600 mm/min) Fig. 8. Fracture surfaces of clad layer showing the bonding condition between the ceramic
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particles and the metal matrix deposited with 1200 W laser power and scan speed of (a) 1200 mm/min, (b) 1000 mm/min, (c) 800 mm/min, (d) 600 mm/min, (e) 400
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mm/min and (f) 200 mm/min Fig. 9. Cross-section of multi-tracks cladded with 50% overlap at 1200 W laser power and 600 mm/min scan speed
Fig. 10. Effect of molten pool lifetime on wear rate Fig. 11. Variation in wear track dimension with scan speed (a) 1200 mm/min, (b) 800 mm/min and (c) 400 mm/min Fig. 12. Distribution of WC particles in the clad layer (a) 600 mm/min, (b) 400 mm/min and (c) 200 mm/min
List of Tables Table 1. Chemical composition (Wt%) of cladding and substrate materials
ACCEPTED MANUSCRIPT Table 2. Laser cladding process parameters Table 3. Gibbs free energies of formation of solid carbides Vitae Muvvala Gopinath is currently a Ph.D. student in the Department of Mechanical Engineering of Indian Institute of Technology, Kharagpur. He is working in the area of ‘Laser additive
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manufacturing’ and also on ‘Laser surface modification’. He obtained his M.Tech. degree in Manufacturing Science and Engineering in 2013 from the same institute, and B.Tech degree in Mechanical Production and Industrial Engineering from GITAM
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University, Visakhapatnam, India in 2011. He has co-authored several conference papers in the area of laser cladding.
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Debapriya Patra Karmakar is currently a Ph.D. student in the Department of Mechanical Engineering of Indian Institute of Technology, Kharagpur. He is working in the area of ‘Laser cladding of hardfacing alloy on hot-work tool steel’ and also on ‘Laser surface modification’. He obtained his M. Tech degree in Manufacturing Science and Engineering in 2013 from the same
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institute, and B.E. degree from Bengal Engineering and Science University, Shibpur (currently known as IIEST, Shibpur), India in 2011.
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Dr. Nath started his scientific career in BARC, Mumbai, India after graduating from BARC Training School in 1972. He worked mainly on the development of high power lasers and their scientific and
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industrial applications. He obtained Doctoral degree in 1982. He was a PDF in University of Alberta, Canada during 1982-84. During 1986-‘07 he headed Industrial CO2 laser Program and also Solid
State and Semiconductor Lasers Program (2005-‘07) in RRCAT, Indore. Since 2008 he is a Professor in Mechanical Engineering Department, IIT Kharagpur. He has co-authored more than 135 journal papers. His present research interests include laser additive manufacturing and process monitoring, laser surface engineering, and underwater laser material processing.
ACCEPTED MANUSCRIPT Highlights
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Molten pool thermal history in laser cladding of Inconel 718/WC is reported. Variation in molten pool thermal history with laser scan speed is discussed. Process parameters favouring wetting of WC particle with metal matrix is presented. Effect of WC particle wetting in metal matrix on mechanical properties is presented
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• • • •
S0925-8388(17)31464-0
DOI:
10.1016/j.jallcom.2017.04.254
Reference:
JALCOM 41658
To appear in:
Journal of Alloys and Compounds
Received Date: 11 January 2017 Revised Date:
19 April 2017
Accepted Date: 23 April 2017
Please cite this article as: M. Gopinath, D.P. Karmakar, A.K. Nath, Monitoring and assessment of tungsten carbide wettability in laser cladded metal matrix composite coating using an IR pyrometer, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.254. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Monitoring and assessment of tungsten carbide wettability in laser cladded metal matrix composite coating using an IR Pyrometer Muvvala Gopinath, Debapriya Patra Karmakar, Ashish Kumar Nath Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur, 721302, India
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Abstract Laser cladding technique was applied to deposit a metal matrix composite coating of Inconel 718 with 30 wt% WC as a ceramic phase. One of the major problems associated with obtaining fully dense MMC coating is the poor wetting/bonding between the ceramic
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particles and the metal matrix. Further, a large number of process parameters involved in the laser cladding process makes it more complex to optimise the process window. Therefore, the current study focuses on the monitoring of molten pool thermal history which is an outcome
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of the combination of all process parameters, and identifies the molten pool lifetime and cooling rates that are favourable for the formation of interfacial layer improving the wetting characteristics. The thermal history of molten pool was recorded using an IR pyrometer. The molten pool lifetime, solidification shelf time and cooling rates were assessed from the data acquired and effect of these on wetting characteristics and formation of interfacial layer
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between WC particles and metal matrix was analysed using SEM analysis. Molten pool lifetime greater than 0.68 s exhibited proper wetting of WC particles with the matrix. Fractured surface of coatings that experienced relatively fast cooling revealed the delaminating of ceramic particles from the metal matrix under the tensile load, whereas in
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one with slow cooling particles found to stay intact with the metal matrix improving the wear properties of the coating. However, too slow cooling resulted in settlement of WC particles at
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the bottom of molten pool reducing the wear resistance of the coating.
Keywords: Laser cladding, IR pyrometer, monitoring, molten pool thermal history, Wetting. ∗
Corresponding Author: Tel: +91 - 3222 – 281784; fax: +91-3222-25530 E-mail address: [email protected] Postal address: Department of Mechanical Engineering, IIT Kharagpur, Kharagpur-721302, India
ACCEPTED MANUSCRIPT 1. Introduction Failure of mechanical components is generally caused by fatigue, wear, erosion and corrosion at the surface. With the advancement of technology in past few decades, several surface engineering techniques came into light which could improve the hardness and wear resistance of surface by developing metal matrix composite coatings on the surface of the
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component. Among them, laser cladding technique for development metal matrix composite coatings (MMC) and functionally graded coatings became more popular because of several advantages over conventional techniques like arc welding, TIG, plasma and thermal spry coating etc [1-3]. This includes localized treatment with minimum heat input, dilution and
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distortion, fast cooling rates experienced from self quenching mechanism resulting in fine microstructure enhancing the surface properties along with the process flexibility and automation. The MMC coatings involve addition of carbide, nitride and oxide ceramic
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particles to the relatively softer metal matrix to enhance the wear characteristics of the surface. Further, addition of ceramic particles enhances the specific strength, stiffness, toughness of the material and exhibits good stability at high operational temperatures [4-7]. MMC coating finds application in mining and mineral industries for protection over high value down-hole drilling tools [8], to improve the lifetime of components such as rollers,
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rails, wearing plates, piston rods, cylinders and turbines etc [9-11]. In laser cladding process of developing MMC coatings, the high energy laser beam melts the powder particle with low melting point which forms the metal matrix. The liquid metal flows around the ceramic particles consolidating them which generally have very high
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melting point and stays in solid state as the molten pool solidifies. The interface of metal matrix and ceramic particles plays a vital role as improper wetting/bonding may result in
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pull-out of ceramic particles during abrasion through ploughing resulting in decreased wear resistance properties of the coating and premature failure of component under mechanical loading as the weak interface acts as the sight for crack initiation [12-14]. Thus it is very essential to tailor the ceramic particles to the metal matrix. Bartkowski et al. [15] studied the effect of laser power on tailoring of WC particle with stellite matrix. It was observed that with the increase of laser power (400 to 700 W), the surface of WC particle started melting increasing the wetting/bonding with the metal matrix. However, Abioye et al. [16] demonstrated that excess dissolution of WC may lead to decrease of corrosion resistance of the coatings. Rong et al [17] studied the formation of diffusion layer between WC1-x and Inconel 718 as a function of applied laser energy. Hong et al. [18] studied the effect of line energy (80 to 160 kJ/m) on formation of interfacial layer between the TiC particles and the
ACCEPTED MANUSCRIPT Inconel 718 matrix. It was observed that with increase of line energy, the interfacial layer thickness was found to increase. However at higher line energy TiC started decomposing. Authors [19] further extend the work with fine (2-7 µm) TiC particles where similar results were observed. Liu et al. [20] carried out a comparative study of formation of interfacial reaction layer in Ti-6Al-4V matrix with cast and single crystal WC particles. The reaction
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layer found to have W2C and a mixed layer of W and TiC which are effective in load transfer under an external load. It was reported that cast WC particles would perform better because of low content of carbon (3.9 ± 0.1 ) when compared to single crystal WC (6.8 ± 0.05) as titanium has high affinity towards carbon, forming excessive TiC dendrites which has poor
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plasticity and toughness compared to Ti matrix. Therefore it is apparent that formation of reaction layer is necessary in order to enhance the mechanical properties of the coating, while too much of reaction may lead to deterioration of the same as it results in the formation of
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brittle metal matrix. Thus, it is very vital to understand the favorable conditions for the formation of reaction layer and its dependence on process parameters. The process parameters window of laser cladding process is too large containing more than 19 parameters which are going to affect the metallurgical as well as geometrical characteristics of the formed clad layer directly [21-23]. Many of these process parameters
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are system and material dependent like the laser beam intensity profile (TEM mode), operating wavelength, laser cladding head design (lateral/coaxial), substrate dimension and thermo-physical properties etc. Thus, the optimized process window for a given system and material may not work elsewhere. However, it may be noted that the formation of reaction
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layer mainly depends on the temperature to which the ceramic particles are exposed and its duration as it dictates the formation of reaction layer and its thickness. Therefore, the current
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study focuses on monitoring the thermal history of the molten pool which includes the surface peak temperature, molten pool lifetime and cooling rate using an IR pyrometer, and their effect on formation of reaction layer and wear characteristics of the coating.
2. Experimental setup
The gas atomized, spherical Inconel 718 powder particles (MetcoClad 718) (Fig. 1a) with powder size distribution of 43-100 µm and irregularly shaped WC particles (Fig. 1b) were used in the current study as metal matrix and ceramic particle phase respectively. AISI 304 austenitic steel plates of 50 mm×50 mm×8 mm dimensions were used as substrates. The chemical compositions of Inconel 718 and AISI 304 substrate are as presented in Table. 1. The surface of as received substrates were polished with emery paper (220 mesh) and cleaned
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Fig. 1 Typical morphologies of (a) Inconel 718 and (b) WC particles Table 1 Chemical composition (Wt%) of cladding and substrate materials
IN 718 AISI 304
Cr
Fe
Nb
Mo
Al
Ti
Mn
Si
B
Ni
C
P
19.02
18.1
4.92
3.19
0.54
0.97
0.04
0.20
0.004
Bal.
-
-
18.97
Bal.
-
0.224
-
-
1.731
0.753
-
8.554
0.067
0.045
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(Wt%)
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Element
Table 2 Laser cladding process parameters CW mode
S
0.031
Scan speed
Thickness of preplaced powder coating
Beam spot diameter
(W)
(mm/min)
(mm)
(mm)
1200
200 - 1200
1
3
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Laser beam power
with acetone. Laser cladding was carried out using a two step process. Initially Inconel 718
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(70%) and WC (30%) powder particles were mixed with aqueous solution of 2% polyvinyl alcohol (Alfa Aesar, 87–89% hydrolysed) which was used as a binder. Mixing was done
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mechanically by alternate cycles of mechanical stirring (Spinot Model MC-02, Tarsons). After proper mixing, a predefined thickness (1 mm) of the powder-PVA blend was applied on to the substrate surface using a coating machine (Model no: K101 control coater, RK Print Coat Instruments Ltd., UK,). The coated sample were baked in furnace at 100 °C in Argon environment for 15 min in order to remove the moisture content from the coating and to obtain a temporary bonding. A 2kW Yb-Fibre laser (IPG photonics, Model no. YLR 2000) operating at 1.07 µm wavelength is used for cladding the prepared sample. It can be operated in CW as well as in modulated mode at 50–1000 Hz frequency range with 5–100% duty cycle with rectangular shaped temporal power profile. The laser has multimode beam intensity profile with high intensity at the centre and two annular rings of relatively low intensity [24]. The laser beam
ACCEPTED MANUSCRIPT effectively. Table 2 shows the process parameters that were used to carry out the laser cladding. Single spot monochromatic IR pyrometer operating at 1.6 µm wavelength with 0.7 mm vision zone and 1 ms acquisition time with temperature measuring range of 385 °C 1600 °C has been used to record the thermal history of the molten pool. Though the pyrometer was specified to operate at 1.6 µm wavelength, laser radiation was found to affect
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the recorded signal. Therefore, a 1064 ± 25 nm notch filter with optical density of 3 was mounted in front of the optical components of the pyrometer to isolate it from the laser radiation. The filter attenuated a certain fraction of the IR radiation also, necessitating recalibration of the detector. Proper calibration was carried out by melting various metals
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like Al, SS and Cu, and correlating the obtained temperature signal with the known melting point of the respective metals. The recalibrated temperature measuring range with the notch filter is ~785 °C - 3260 °C. Thermal history of the molten pool was recorded keeping the
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pyrometer stationary and focused at a point along which the laser beam scans forming the clad tracks. The obtained clad tracks were cross-sectioned using a wire-cut EDM, mirror polished and analysed under SEM to study the interfacial reaction between ceramic particles and the metal matrix. Fracture test was carried out to study the effect of wetting on bonding characteristics of the ceramic particles with metal matrix. For carrying out the fracture test,
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single tracks were cut along the length on either side of the clad track using wire cut EDM and the obtained samples were cut at the middle from the bottom of substrate till the clad track is approached, thus exposing the clad track alone to the tensile load applied using a universal testing machine (Tinius Olsen H50KS). Wear test was also carried out on the clad
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surface developed by 30% overlapping of tracks using a ball on disk setup (Make: DUCOM, TR-201-M3). WC ball was used as a counter body and wear test was carried at 300 RPM
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with 19.62 N load for 15 min with a track diameter of 5 mm.
3. Results and discussion
3.1 Molten pool thermal history and its variation with process parameters Fig. 2 depicts the typical molten pool thermal history recorded during the laser
cladding of Inconel 718 with 30% WC carbide at 1200 W laser power and 600 mm/min scan speed. The thermal history of molten pool consists of heating cycle 0A and cooling cycle AD. Slope of these lines represent the heating and cooling rates of the molten pool. It can be observed that heating cycles 0A has a steep change in its slope at around 300 ms seconds which is due to the advancement of centre of laser beam spot which is having peak intensity
ACCEPTED MANUSCRIPT [24] towards the centre of zone of interest where pyrometer is focused. The cooling cycle has several stages as it changes its phase from liquid to solid. AB represents the drop in molten
1385
TL
B
TS
1185
L
985
L+S
S
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C
D
785 0
500
1000
1500
2000
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Temperature (°C)
TL – Liquidus Temperature TS – Solidus Temperature L – Liquid S – Solid
A
1585
Time (ms)
Fig. 2 Typical molten pool thermal history (1200 W, 600 mm/min)
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pool temperature when it is still in liquid state. At point B, solidification initiates and ends at point C which is nothing but the liquidus and solidus temperature of the material under study respectively. It may be clearly observed that the slope or the cooling rate (∂T/∂t) during the period BC is quite low compared to that of AB which is because of the release of energy in the form of latent heat of fusion in the zone BC. Zone BC is generally termed as solidification
1600 1550 1500
Cooling rate (103 °C/s)
1650
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Temperature (°C)
1700
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shelf or freezing zone where liquid and solid co-exists followed by solid phase cooling CD.
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Scan speed (mm/min)
Solidification shelf time (s)
Melt pool lifetime (s)
0.4 0.2
200 400 600 800 1000 1200 Scan speed (mm/min) 1
1.6 1.2 0.8 0.4
0 200 400 600 800 1000 1200 Scan speed (mm/min)
0.6
(b)
2
(c)
0.8
0
200 400 600 800 1000 1200
(a)
1
(d)
0.8 0.6 0.4 0.2 0 200 400 600 800 1000 1200 Scan speed (mm/min)
Fig. 3 Effect of laser scan speed on (a) surface peak temperature, (b) cooling rate, (c) melt pool lifetime and (d) solidification shelf time
ACCEPTED MANUSCRIPT Further, the zones AC and BC represent the molten pool lifetime and the solidification shelf time respectively. Fig. 3 shows the variation of various aspects of molten pool thermal history with the scan speed. It may be observed that with the decrease of laser scan speed from 1200 mm/min to 200 mm/min, keeping the laser power and spot diameter constant at 1200 W and 3 mm
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respectively, the surface temperature showed an increasing trend as shown in the Fig. 3(a). However, the increase in surface temperature is not so significant in case of 200 mm/min and 400 mm/min which may be due to the initiation of metal vaporization from the surface that restricts further increase of surface temperature because of consumption of energy in the form
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of latent heat of vaporization. With the decrease of laser scan speed, cooling rate is found to decrease (Fig. 3b) as slower scan speeds result in increased line energy as well as substrate temperature, decreasing the temperature gradients. This also resulted in increase of molten
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pool lifetime as well as solidification shelf time as shown in Fig. 3(c) and (d) respectively.
3.2 Effect of molten pool thermal history on wetting of ceramic particle The wetting condition of ceramic particles in the metal matrix composite coating developed by laser cladding process at different scan speeds was as shown the Fig. 4, which
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was analysed using BSE detector. A distinct boundary between the ceramic particles and metal matrix was clearly observed in case of cladding with scan speed of 1200 mm/min and 1000 mm/min, corresponding to which the molten pool lifetime is 0.541 s and 0.605 s, as
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shown in the Fig. 4(a) and (b) respectively. A partial wetting between ceramic particle and
Fig. 4 Variation in wetting condition of ceramic particles with laser scan speed (a) 1200 mm/min, (b) 1000 mm/min, (c) 800 mm/min, (d) 600 mm/min, (e) 400 mm/min, (f) 200 mm/min (1200 W)
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Fig. 5 BSE images of the bonding layer between ceramic particles and the metal matrix (a) 800 mm/min, (b)
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600 mm/min, (c) 400 mm/min and (d) 200 mm/min (1200 W)
metal matrix was observed in case of 800 mm/min scan speed (Fig. 4c). However, this was limited to the particles at the centre of the molten pool where cooling rate is expected to be
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relatively slow. A proper wetting was observed in case of cladding with 600 mm/min, 400 mm/min and 200 mm/min as shown in the Fig. 4 (d), (e) and (f) respectively. It may be noted
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that molten pool life above ~0.68 s favoured the formation of reaction layer resulting in proper wetting between the ceramic particles and the metal matrix. Fig. 5 shows the interface formed between the ceramic particle and the metal matrix
cladded at a constant laser power and beam spot diameter of 1200 W and 3 mm respectively, at different scan speeds. A clear increase in bond layer thickness with decrease of laser scan speed is evident. Fig. 6 shows the variation in bond layer thickness with respect to molten pool lifetime as well as the cooling rate of the clad layer. It was observed that with the increase of molten pool lifetime, the ceramic particles surface started melting causing the diffusion between the WC particles and the matrix whose thickness increased with increase in molten pool lifetime and decreased with increase in cooling rate. The diffusion between the
ACCEPTED MANUSCRIPT Molten pool lifetime (s)
2
800
1.6
600
1.2 400 0.8
0 0
200 400 600 800 Scan speed (mm/min)
0 1000
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200
0.4
Cooling rate (°C/s)
Reaction layer thickness (µm) & molten pool life time (s)
Bond layer thickness (µm) Cooling rate (°C/s)
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Fig. 6 Variation in bonding layer thickness with respect to molten pool lifetime
Table 3.
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Fig. 7 Elemental analysis (EDS) of various phases (1200 W, 600 mm/min)
Gibbs free energies of formation of solid carbides
Nb2 C TiC W2 C Mo2 C
Gibbs free energy (∆G Cal/mole) -46,000 + 1.0T -44,600 + 3.16T -7300 - 0.5T -11,710 - 1.83T
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Nb Ti W Mo
Compound
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Element
WC particle and metal matrix was found to be more dominant in case of laser cladding with 200 mm/min as shown in Fig. 5d. The diffusion also caused change in metal matrix composition as well as the microstructure. Fig. 7 shows the results of EDS analysis of various phases in the microstructure around the ceramic particle. The characteristic bright zone around the ceramic particle was found to be rich in Nb, Ti, Mo, C and W, where as the dark matrix was rich in Ni, W, Fe, C and Cr with traces of Nb, Ti and Mo. These characteristic bright zones are expected to be complex secondary carbides of Ti, Nb, Mo and W which have very high affinity towards carbon as per the Gibbs free energy shown in the Table 3 [25]. In
ACCEPTED MANUSCRIPT case of 200 mm/min, Nb and Ti were found to segregate around the WC particle forming another layer of secondary carbides which is clearly visible in Fig. 5d.
3.3 Fracture surface and wear test analysis Fracture test was carried on the single clad tracks in order to analyze the effect of
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molten pool lifetime on the wetting characteristics as well as on the bonding strength between WC particles and the metal matrix. Fig. 8 shows the fractured surfaces of the samples cladded at different scan speeds whose molten pool lifetime was specified in Fig. 4. A clear debonding of ceramic particles from metal matrix under the applied tensile load is evident in
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case of MMC clad tracks deposited with scan speed of 1200 mm/min and 1000 mm/min as shown in Fig. 8(a) and (b) respectively. As mentioned earlier, a partial bonding was observed in case of cladding with 800 mm/min, yet WC particles found to debond from the metal
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matrix as shown in Fig. 8(c). Clad tracks deposited with scan speeds of 600 mm/min, 400 mm/min and 200 mm/min showed a strong bonding with the matrix as shown in Fig. 8(d), (e) and (f) respectively. Similar to Fig. 5, diffusion of WC into metal matrix and formation of a secondary carbide layer around the WC at 400 mm/min and 200 mm/min respectively, was observed which strongly shows the reproducibility of results. Thus, the WC particles in the
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metal matrix cladded at relatively slow scan speed exhibited strong bonding as slow scan speed results in relatively longer molten pool life (>0.68 s) which allows the melting and
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diffusion of WC surface into the metal matrix enhancing the bonding characteristics.
Fig. 8. Fracture surfaces of clad layer showing the bonding condition between the ceramic particles and the metal matrix deposited with 1200 W laser power and scan speed of (a) 1200 mm/min, (b) 1000 mm/min, (c) 800 mm/min, (d) 600 mm/min, (e) 400 mm/min and (f) 200 mm/min
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Fig. 9. Cross-section of multi-tracks cladded with 50% overlap at 1200 W laser power and 600 mm/min scan
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speed
Wear test was carried out on the samples with multiple tracks deposited with 50%
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overlap as shown in Fig. 9, in order to study the effect of bonding on wear characteristics of the MMC coating developed. During overlapping, the surface temperature was always maintained at less than 40 °C which was monitored using a hand held digital non contact type infrared thermometer (EEE-Tech IRT-4). Fig. 10 shows the effect of molten pool lifetime on the wear rate of the coatings developed. Three replicates were taken for each case. It was observed that the wear rate increased with decrease of scan speed from 1200 mm /min till 800
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mm/min and with further decrease of scan speed to 600 mm/min the trend got reversed and wear rate reduced to a great extent. However, the wear rate again increased with the decrease of scan speed from 400 mm/min to 200 mm/min. Fig. 11 shows the variation in wear track dimension with the scan speed which conforms to the trend observed in Fig. 10. It has been
Wear rate (10-6 g/s)
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mentioned in a previous section that claddings developed with molten pool lifetime less than
Molten pool lifetime (s) 12000.605 0.680 0.845 1.148 1.753 0.541 35 30 25 20 15 10 5 1200 1000 800 600 400 Scan speed (mm/min)
200
Fig. 10 Effect of molten pool lifetime on wear rate
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Fig. 11 Variation in wear track dimension with scan speed (a) 1200 mm/min, (b) 800 mm/min and (c) 400
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mm/min
Fig. 12 Distribution of WC particles in the clad layer (a) 600 mm/min, (b) 400 mm/min and (c) 200 mm/min
~0.68 s, corresponding to scan speed greater than 800 mm/min, showed a weak bonding between the ceramic particles and metal matrix which could have resulted in pull-off of WC
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particles during the wear test, where as in case of molten pool lifetime greater than ~0.68 s, WC particles showed strong bonding resulting in enhanced wear properties. Thus, WC particles does not contribute in improving the wear characteristics of the metal matrix for scan rates that result in molten pool lifetime shorter than ~0.68 s.
The increasing trend in
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wear rate with decrease of scan speed from 1200 mm/min to 800 mm/min could be attributed to the elemental segregation in the Inconel 718 metal matrix. Elemental segregations depend
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mainly on the rate at which liquid metal is solidifying and reaching to the room temperature. It can be observed from Fig. 3(b) that, with decrease of scan speed from 1200 mm/min to 800 mm/min, cooling rate decreases from ~860 °C/s to ~606 °C/s. Gopinath et al. [26] and Zhang et al. [27] showed that with decrease of cooling rate, the volume fraction of Laves phase which is basically (Ni,Fe,Cr)2 (Nb,Mo,Ti) increases depleting Nb in the matrix which is a phase strengthening element. This depletion of Nb and formation of Laves phases results in decrease of hardness and wear resistance of the Inconel 718 matrix [28]. Fig. 12(a), (b) and (c) shows the distribution of WC particles in the metal matrix of the samples cladded at 600 mm/min, 400 mm/min and 200 mm/min respectively. It can be seen that in case of 600 mm/min and 400 mm/min, the WC particles were uniformly distributed over the cross
ACCEPTED MANUSCRIPT section. The uniform distribution of WC particles in the coating and their sound bonding within the matrix for scan speeds resulting in molten pool lifetime more than ~0.68 s could be the reasons for the observed decrease of wear rate when the scan speed was reduced from 800 mm/min to 400 mm/min. However, in case of 200 mm/min, the WC particles found to settle down at the bottom of the clad layer [29], exposing the matrix alone during the wear test,
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resulting in increased wear rate as shown in Fig. 10. Further, excess dilution in case of 200 mm/min, which is evident from the Fig. 11(c) could also be responsible for the high wear rate. Thus, by ensuring proper bonding of WC particles and their uniform distribution in metal matrix through optimizing the laser process parameters high wear resistant metal-
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matrix laser clad layers can be deposited.
Conclusion
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Molten pool thermal history and its dependency on process parameters was monitored in laser cladding of Inconel 718 and WC MMC coating and correlated with the wetting characteristics of WC with metal matrix and wear characteristics of the coatings deposited. The following conclusions are drawn based on the observations:
1. Molten pool lifetime and solidification shelf time increase with decreasing scan speed,
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whereas cooling rate decreases.
2. WC particles exhibits proper wetting and bonding with the metal matrix for molten pool lifetime greater than 0.68 s.
3. Precipitates of secondary carbides are formed around the WC carbide particles at
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relatively long molten pool lifetime.
4. Debonding of WC particles from metal matrix under tensile load occurs in clad tracks
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with relatively short molten pool lifetime. 5. There is an optimum molten pool lifetime which yields highest wear resistance of the coating. While shorter lifetime prevents proper wetting of WC particles with the matrix, longer lifetime causes settling down of ceramic particles at the bottom of molten pool due to their weight, reducing the wear resistance of the coating.
Acknowledgement Authors gratefully acknowledge the financial support from the Department of Science and Technology, Ministry of Science and Technology, Government of India, under the FIST Program-2007 (SR/FIST/ETII-031/2007).
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List of figures
Fig. 1. Typical morphologies of (a) Inconel 718 and (b) WC particles
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Fig. 2. Typical molten pool thermal history (1200 W, 600 mm/min)
Fig. 3. Effect of laser scan speed on (a) Surface peak temperature, (b) Cooling rate, (c) Melt pool lifetime and (d) Solidification shelf time
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Fig. 4. Variation in wetting condition of ceramic particles with laser scan speed (a) 1200 mm/min, (b) 1000 mm/min, (c) 800 mm/min, (d) 600 mm/min, (e) 400 mm/min, (f) 200 mm/min (1200 W)
Fig. 5. BSE images showing the bonding layer between ceramic particles and the metal matrix (a) 800 mm/min, (b) 600 mm/min, (c) 400 mm/min and (d) 200 mm/min (1200
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W)
Fig. 6. Variation in bonding layer thickness with respect to molten pool lifetime Fig. 7. Elemental analysis (EDS) of various phases (1200 W, 600 mm/min) Fig. 8. Fracture surfaces of clad layer showing the bonding condition between the ceramic
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particles and the metal matrix deposited with 1200 W laser power and scan speed of (a) 1200 mm/min, (b) 1000 mm/min, (c) 800 mm/min, (d) 600 mm/min, (e) 400
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mm/min and (f) 200 mm/min Fig. 9. Cross-section of multi-tracks cladded with 50% overlap at 1200 W laser power and 600 mm/min scan speed
Fig. 10. Effect of molten pool lifetime on wear rate Fig. 11. Variation in wear track dimension with scan speed (a) 1200 mm/min, (b) 800 mm/min and (c) 400 mm/min Fig. 12. Distribution of WC particles in the clad layer (a) 600 mm/min, (b) 400 mm/min and (c) 200 mm/min
List of Tables Table 1. Chemical composition (Wt%) of cladding and substrate materials
ACCEPTED MANUSCRIPT Table 2. Laser cladding process parameters Table 3. Gibbs free energies of formation of solid carbides Vitae Muvvala Gopinath is currently a Ph.D. student in the Department of Mechanical Engineering of Indian Institute of Technology, Kharagpur. He is working in the area of ‘Laser additive
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manufacturing’ and also on ‘Laser surface modification’. He obtained his M.Tech. degree in Manufacturing Science and Engineering in 2013 from the same institute, and B.Tech degree in Mechanical Production and Industrial Engineering from GITAM
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University, Visakhapatnam, India in 2011. He has co-authored several conference papers in the area of laser cladding.
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Debapriya Patra Karmakar is currently a Ph.D. student in the Department of Mechanical Engineering of Indian Institute of Technology, Kharagpur. He is working in the area of ‘Laser cladding of hardfacing alloy on hot-work tool steel’ and also on ‘Laser surface modification’. He obtained his M. Tech degree in Manufacturing Science and Engineering in 2013 from the same
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institute, and B.E. degree from Bengal Engineering and Science University, Shibpur (currently known as IIEST, Shibpur), India in 2011.
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Dr. Nath started his scientific career in BARC, Mumbai, India after graduating from BARC Training School in 1972. He worked mainly on the development of high power lasers and their scientific and
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industrial applications. He obtained Doctoral degree in 1982. He was a PDF in University of Alberta, Canada during 1982-84. During 1986-‘07 he headed Industrial CO2 laser Program and also Solid
State and Semiconductor Lasers Program (2005-‘07) in RRCAT, Indore. Since 2008 he is a Professor in Mechanical Engineering Department, IIT Kharagpur. He has co-authored more than 135 journal papers. His present research interests include laser additive manufacturing and process monitoring, laser surface engineering, and underwater laser material processing.
ACCEPTED MANUSCRIPT Highlights
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Molten pool thermal history in laser cladding of Inconel 718/WC is reported. Variation in molten pool thermal history with laser scan speed is discussed. Process parameters favouring wetting of WC particle with metal matrix is presented. Effect of WC particle wetting in metal matrix on mechanical properties is presented
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