Ni

O: Cgy, P

Article history:Received 4 January 2013

Accepted 6 March 2013Available online 9 April 2013

Keywords:WC-Ni claddingLaser claddingErosion wear

We investigated the solid particle erosion behaviour of WC-reinforced Ni-matrix based laser clad layersto improve the performance of engineering components for potential power plant applications.

erosion because it is the consequence of a series of essentially

contact the component surface for a very short duration [2].

ayerss aref thesition

with a dened distribution of ingredients within the material can

properties [5]. These useful engineering properties (such as hard-

Contents lists available at SciVerse ScienceDirect

lse

Optics & Laser

Optics & Laser Technology 50 (2013) 155162strength of these materials can be tuned by adding an adequateE-mail address: paulcp@rrcat.gov.in (C.P. Paul).Recently, metal matrix composites (MMCs), which consist of hard ness, specic modulus, strength, thermal stability and wearresistance) are extending the list of engineering applications forMMCs. In these materials, the bulk hardness is governed by thehard particles (such as WC, TiC or Cr3C2) while the toughness and

0030-3992/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.optlastec.2013.03.002

n Corresponding author. Tel.: 91 731 248 8384; fax: 91 731 248 8380.independent but similar impact events in which hard particles be used to make MMCs with a tailored set of useful engineeringengineering systems, including steam and jet turbines, pipelines,valves used in slurry transportation of materials and uidised bedcombustion systems [1]. SPE is different from other forms oferosion such as liquid impact erosion, slurry erosion and cavitation

been widely investigated in an effort to obtain protective lagainst erosive wear [4]. In principle, an innite range of MMCpossible with variations in the types and compositions ometal and ceramic. A suitable process of controlled compoSolid particles entrained in a uid stream can strike a surfacewith high-velocity, which results in material loss known as solidparticle erosion (SPE). The phenomenon is useful in sand-blasting,high-speed abrasive water jet cutting, shot peening of rotatingcomponents and cutting of hard and brittle materials such as rockby abrasive jets. However, SPE is considered undesirable in many

of wear [3]. The erosion resistance of the surfaces is not onlyinuenced by the impact angle, particle velocity and environ-mental temperature but also depends strongly on the depositiontechnique. Many deposition techniques such as thermal spraying,sputtering, cladding, physical vapour deposition, chemical vapourdeposition, detonation spraying and electro-spark detonation have1. IntroductionWC-reinforced Ni-matrix based laser clad layers having various compositions of WC (5, 10 and 15 wt%)were deposited on austenitic stainless steel substrates. The laser clad layers were characterised usingoptical and scanning electron microscopy, microhardness testing and air-jet erosion testing. In solidparticle erosion studies using the air-jet erosion tester, the set of testing parameters, including air-erodent compositions, erodent particle velocities and impact angles, was selected by using the Taguchitechnique. The morphologies of the worn surfaces were used to predict the wear mechanisms. Theresults of a microstructural examination of the cross-sections of laser clad revealed a good metallurgicalbond between the WC-reinforced Ni matrix and the austenitic stainless steel substrate. Dissociation/partial melting/full melting of WC particles was not observed in the laser clad layers. The microhardnessvalue in the laser cladding zone was between 9002400 VHN, while it was 230270 VHN on thesubstrate. The results of erosion wear studies of the WC-Ni laser clad surface revealed that the erosionbehaviour of the WC-Ni laser clad is primarily governed by erodent jet velocity followed by impact angle.The erosion does not much depend on the Ni-concentration in the MMC or the erodent feed rate. Thewear signature at the erosion wear surface indicated that the erosion was primarily governed by a ductileerosion mechanism followed by the removal of WC particles from the matrix. The erosion resistance ofthe Ni-clad layer with WC was found to be at least four times higher than that without WC particles. Thequantied contribution of various erosion parameters is useful for function-based design of componentswith extended service life.

& 2013 Elsevier Ltd. All rights reserved.

and brittle particles or bres embedded in a tough metal binder,are nding increasing attention for the control and minimisationReceived in revised form16 February 2013Solid-Particle Erosion Behaviour of WC/Different Contents of WC Particles

C.P. Paul a,n, S.K. Mishra a, P. Tiwari b, L.M. Kukreja a

a Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Pb Indus Synchrotrons Utilisation Division, Raja Ramanna Centre for Advanced Technolo

a r t i c l e i n f o a b s t r a c t

journal homepage: www.eComposite Clad layers with

AT, Indore (M.P.) 452 013 INDIAO: CAT, Indore (M.P.) 452 013 INDIA

vier.com/locate/optlastec

Technology

identify the zone of optimal processing parameters, a number of

C.P. Paul et al. / Optics & Laser Technology 50 (2013) 155162156amount of metallic matrix/binder (such as Co, Ni, Fe, Ni-Fe orNi-Fe-Co). For a WC reinforced metal matrix, Co is preferred as abinder due to its excellent wettability, and the WC-Co combinationis known for high hardness and outstanding toughness [6]. Nickelis employed as a binder for specialised applications where highhardness and resistance against thermal cracking or corrosion/oxidation resistance are required along with wear resistance[711]. To date, many researchers have reported the laser claddingof Ni-based alloys with WC to produce a metal matrix composite(MMC) [1215]. Hidouci et al. [16] tried to pre-heat the substrateduring laser cladding of 50 wt% Ni50 wt% WC to avoid cracking.Zhong et al. [17] studied the formation of a WC-Ni hard-alloy lasercladding using a W-C-Ni powder blend, and they observed step,twist and cross growth morphologies of the WC. The abrasionwear of cemented carbide laser-clad surfaces is widely repre-sented in the literature [18,19]. Acker et al. [20] investigated theinuence of the concentration and size of WC/W2C on the abrasionwear resistance of a laser-clad MMC. They found that an increasein concentration of the carbides and/or a decrease in their sizewere favourable trends for the two-body abrasive wear resistance.A small concentration of ne carbides was sufcient to reducewear considerably, and the wear coefcient decreased logarith-mically with the concentration of carbides. Investigations byBonny et al. [21] indicated that WCNi cemented carbide hadpromising friction and wear properties under dry sliding contact.They found that cemented carbide with a reduced binder phaseenhanced the abrasion wear resistance considerably. Huang et al.[22] studied the abrasion wear behaviour of WC-Ni clad layersproduced with a pulsed Nd:YAG laser and found that the abrasionwear of the laser clad layers was 23 times higher than that of anunclad H13 substrate. In general, measurements of abrasion wearare relatively straightforward in terms of experimental procedure,material response to testing and test rig congurations. Moreover,abrasion wear behaviour can be understood in terms of mechan-ical properties of the materials [23]. However, the erosion wearbehaviour requires a deeper understanding beyond the mechan-ical properties of the materials. Desale et al. [24] investigated theslurry erosion behaviour of various laser hard-faced materials onlow-carbon austenitic stainless steel. Kleis and Kulu [5] studiedvarious material properties such as hardness, Young's modulus,porosity, phase composition and microstructure of a hard metalcoating and could not establish a detailed interpretation of theerosion wear resistance of each composition. Classical brittlematerials (such as glasses and ceramics) have been investigatedfor lateral, cone and median cracks created by the impact of asingle particle, and it was found that erosion was due to theformation of lateral cracks on the target surface leading to removalof the material [25]. Recently, Jana and Stack investigated thethreshold velocity criteria in the modelling of the solid particleerosion of WC/Co MMCs [26]. In ductile materials, erosion initiateswith a localised deformation near the surface region that leads tolip formation, and when the inertial stress exceeds the ultimatestrength of the material, the lip is removed from the material [27].In contrast, cemented carbides did not behave in a classical brittlemanner when subjected to an erosive uid jet at least on themicroscopic scale. At this scale, they could sometimes behave in aductile manner or display some attributes of both ductile andbrittle behaviour. The combination of test conditions and materialproperties/microstructure determined the response of a particularcemented carbide to a uid jet in terms of severity and the nature(brittle or ductile) of the erosion [26]. The erosion behaviour ofWC-reinforced Ni-matrix laser-clad layers has not been widelyreported. Hence, we have undertaken the deposition of a WC-reinforced Ni-matrix using laser cladding and a systematic inves-tigation of the erosion behaviour of these layers. In the present

study, multi-layer overlapped laser cladding of WC-Ni was carriedsingle tracks with different processing parameters were deposited.The deposited tracks were rst examined visually. Uniform trackswithout visual defects (such as pores, an irregular surface or carbondeposition adjacent to the tracks) on and near the track surface wereselected, and their cross sections were examined more fully. Processparameters that led to laser cladding with a track aspect ratio (i.e.,the ratio between clad width and clad height) greater than ve werechosen because it is well-established that use of laser cladding withan aspect ratio less than ve leads to inter-run porosity [32]. Thechosen process parameters have the following ranges: laser powerout for three different Ni compositions (5, 10 and 15 wt%) on anaustenitic stainless steel substrate using a 3.5 kW continuous-wave CO2 laser-based rapid manufacturing system. The micro-structure of the clad layers was investigated using optical micro-scopy. The microhardness of the clad layers was measured using aVickers microhardness tester. The solid particle erosion behaviourof the laser clad specimens was examined using an air-jet erosionsetup at various air-erodent compositions, erodent particle velo-cities and impact angles.

2. Experimental Procedure

The present study was carried out using a CO2 laser basedcladding system consisting of an indigenously developed 3.5 kWcontinuous-wave CO2 laser system [28], a co-axial powder-feedingnozzle with a volumetrically controlled powder feeder [29] and a5-axis CNC laser workstation. The CO2 laser beam was transferredto the 5-axis CNC laser workstation by steering the beam withwater-cooled gold-coated plane copper mirrors. A concave mirror(radius of curvature600 mm) at an inclination angle of approxi-mately 221 was used to focus the laser beam at the laser work-station, and a defocused beam with a diameter of approximately1.2 mm was delivered at the substrate. The beam intensity proleof the CO2 laser used was nearly a at-top due to the multi-modelasing [30]. The at-top beam intensity prole is preferred becauseit leads to nearly uniform heat input at the irradiated surfaceacross the beams cross-section. Argon gas was used as a shieldingand carrier gas. The particle size range of the powder used in thepresent study was 45106 mm and 2575 mm for WC and Ni,respectively. WC particles were agglomerated and had nearlyspheroidal morphology. This particular range of particle size wasselected as smaller sized particles tended to y due to the highsurface-area-to-weight ratio while larger sized particles sufferedpoor absorption of the laser energy and subsequent poor wettingduring laser cladding. WC and Ni powders were pre-mixed inthree different ratios (5, 10 and 15 weight % of Ni). In the followingsection, the ratios of WC and Ni are designated as WC-5Ni, WC-10Ni and WC-15Ni for 5, 10 and 15 wt% of Ni-concentration inMMC, respectively. The present investigation was limited to WC-15Ni because there was a dip in the erosion loss for the MMC with10% matrix phase [6]. The laser cladding process was carried outon austenitic stainless steel AISI 316 L substrates of diameter75 mm and thickness 12 mm. Prior to the experiments, thesubstrates were sand-blasted to roughen the machined surfacein order to increase the laser absorption [31].

To optimise the process, a number of tracks were deposited atdifferent process parameters. This choice was critical as excessiveheating or partial melting of the WC resulted in a carbondeciency in the MMC due to precipitation of carbon as graphite.

2WCW2CC 1This graphite reacted with atmospheric oxygen and formed CO

and CO2, which often appeared as gas porosity in the MMC. Tofrom 1.01.5 kW; scan speed from 0.20.5 m/min; and powder feed

rate from 58 g/min with an argon gas ow rate at 10 lpm.Subsequently, multi-layer overlapped tracks were deposited withthe chosen process parameters. The scheme of the overlap isdescribed in Fig. 1. Generally, the overlap between two adjacentclads is kept between 50% and 60% to reduce post-cladding machin-ing work [33]. A 60% overlap was used in our present experiment.The deposited multi-layer overlapped tracks were cut, polished andprepared for various material characterisations. The microstructureof the clad layer was assessed using optical microscopes (NisshoOptical TZ-240 and Olympus PME3). Vickers microhardness mea-surements were performed on the cross-sections of the laser cladsamples using a Leitz Mini load-2 microhardness tester with a load of200 g as per ASTM standard [34].

The erosion behaviour of the laser clad samples was evaluated

Erosion resistance, Re 1E: 4

The Taguchi method is a proven and reliable method to evaluateseveral design parameters simultaneously with a lower number ofexperiments [35]. In the present study, comprehensive experimentswere designed as per the orthogonal L9 array of the Taguchi methodto understand the effects of Ni-concentration in the MMC, erodent jetvelocity, erodent feed rate and impact angle on the erosion wearvalue. Table 1 presents the control factors and their values as used in

per unit amount of powder fed. The laser energy per unit amount of

C.P. Paul et al. / Optics & Laser Technology 50 (2013) 155162 157under simulated test conditions using an air-jet erosion tester. Thetester consisted of an erodent hopper with vibrator, a volumetricallycontrolled feeding device for the erodent, a tungsten carbide nozzleand the target mount. Alumina (Al2O3) particles with an averageparticle size of 50 mmwere used as the erodent powder. The erodentpowder was lled into the hopper and fed to the air line through avolumetrically controlled feeding device, i.e., a vertical rotatinggrooved disk. At the end of the air line, there was a tungsten carbidenozzle with a hole of 1.5 mm diameter to force out the air-erodentmixture at high velocity. The mass ow of the erodent powder wassimply controlled by manipulating the rotational speed of the feedingdevice while the airline pressure was varied to achieve the desirederodent velocity on the target. The target mount had a rotationmount that allowed the option of setting the jet impact angle () atthe target in the range of 01 - 901. The jet impact angle () is theangle between the direction of jet ow and the target. The distance(h) between the nozzle exit and target was kept to 10 mm.

The erodent jet velocity was measured using a rotating doubledisc setup where the upper disc had a slit and the lower disc hadno slit. In the double disc setup, the gap between the upper andlower disc was l. The set up was rotated at N rpm, and there was anangular offset of in erodent impact impression on the lower discwith respect to the slit. The erodent jet velocity was computedusing the following relation:

v 2Nl60

m=s 2

The erosion experiments were carried out on laser clad samplesat different erodent feed rates, erodent jet velocities and impactangles for a 2 minute duration using this air jet erosion setup. Foran erosion experiment of t minutes with a mass loss of the laserclad samples W at an F g/min erodent feed rate, the erosion valueand resistance can be calculated by the following equations:

Erosion value, E WFt

3Fig. 1. The scheme of overlapping (W - Track width, x Transverse traverse).powder fed (Ep/m) is the ratio of laser power (PL) to the powder feedrate (mp). Mathematically,

EP=m PLmp

kJ=g 6

The compilation of data illustrates that a laser energy per unitamount of powder fed in the range of 1015 kJ/g was a primaryrequirement for successful track deposition in the range of proces-sing parameters under investigation. When the laser energy per unitamount of powder fed was lower than the specied range, discon-tinuous tracks with non-uniform cross sections were observed.The value of this parameter was more critical for laser cladding of

Table 1Control factors & their levels, as per Taguchi L9 experiment.

Control Factors Levels

1 2 3

A Ni-concentration in MMC ( wt% ) 5 10 15B Erodent jet velocity (m/s) 30 50 70C Erodent feed rate (g/min) 2 3 4D Impact angle (1) 90 60 30the experiments. After conducting the experiments as per the L9orthogonal array, the results were converted into signal-to-noise(S/N) ratio data by applying the criterion that the smaller the values,the better. The the-smaller-the-better criterion is chosen becausethe quality characteristic is erosion wear value. This criterion is givenby the following:

SN 10log10

1nn

i 1yi

" #5

where n represents the total number of tests in an experi-mental trail (n3) and yi represents the erosion wear value of thespecimen corresponding to the ith test (i1, 2 or 3).

3. Results and discussion

3.1. Effect of processing parameters

As discussed in the previous section, the process window isidentied by experimental trials. First, a number of single tracks withdifferent processing parameters were deposited, and the depositedtracks were examined visually. It was observed that for a constantlaser power and powder feed rate, a lower scan speed caused themelt-pool liquid state to exist for a longer time before being bondedand solidied; this in turn resulted in the formation of a good WC-Nimetal matrix. At very low scan speeds, there was deposition of acarbon powder, which was mainly generated due to decompositionof WC particles, adjacent to the deposited track. An increase in thescan speed resulted in lower energy per unit length and higheramounts of powder fed per unit length, which led to a reduction inmelting of the binder and a narrower track width. Considering thisobservation, the effects of the processing parameters on lasercladding of WC-Ni were compiled as a function of the laser energy

WC-5Ni. When the laser power energy per unit amount of powderfed was more than the specied value, a dissociation of WC particlesand black deposits adjacent to the tracks were observed, and this wasundesirable. The observations were the same for multi-layer singletrack and multi-layer overlapped track depositions.

3.2. Microstructural examinations

The laser clad was continuous and free from any visible defectsfor all the samples under investigation. Fig. 2 (a) and (b) showtypical cross-sections of laser clad for WC-10Ni and WC-15Ni,respectively. A few cracks at isolated locations were observed forWC-5Ni and WC-10Ni samples. These cracks were mostly betweenthe adjacent tracks and one end of the crack was at the top of cladtrack while the other was at a WC particle in the Ni matrix. WCparticles were observed on the path of cracks. These are primarilydue to thermal mismatch in the MMC and specically in the regionof overlapped tracks. The top surface of the deposits was unevenand had some porosity in all samples. The clad-substrate interfacewas free from defects for all samples except for WC-5Ni samples.The defects for WC-5Ni samples were primarily due to theunavailability of the Ni matrix at the interface during lasercladding. No measurable dilution was observed in any samples.The bulk of the laser clad was mostly free from volume defects,like- porosity and lateral cracks.

A uniform dispersion of WC particles in Ni matrix was observedacross the clad cross-section in all of the samples under investiga-tion. Few WC particles could reach deep down to the substrate dueto the high density. A similar trend was observed in our earlier workon laser cladding of WC-12Co [36]. As expected, there was a visible

C.P. Paul et al. / Optics & Laser Technology 50 (2013) 155162158Fig. 2. Typical cross section of laser clad layer for (a) WC-10Ni and (b) WC-15Ni.

Fig. 3. Typical distribution of WC particles in Ni matrix for (a) WC-5Ni, (b) WC-

10Ni and (c) WC-15Ni.

difference in the distribution of WC particles in a Ni matrix for thesamples of different compositions. Fig. 3 (a), (b) and (c) present thetypical distribution of WC particles in the Ni matrix for WC-5Ni,WC-10Ni and WC-15Ni, respectively. The densities of WC particlesin Ni matrix lined up in descending order for WC-5Ni, WC-10Ni andWC-15Ni. A dendritic microstructure was observed in the Ni matrix.The orientation of the dendrites was perpendicular to the clad-substrate interface (as shown in Fig. 4 (a)), and this is primarily dueto a larger cooling contribution from the substrate. It is interestingto observe that the orientation of dendrites was almost radiallyoutward around the WC particles (refer to Fig. 4 (b)); this occursbecause the superior thermal properties of WC particles acted as aheat sink in the Ni matrix and gave rise to a radial solidicationfront around the WC particles. The dissociation/partial melting/fullmelting of WC particles was not observed in the laser clad layers asthe sharp corners of the WC particles were found intact in the laserclad layers.

3.3. Microhardness Measurement

Microhardness measurements across the transverse cross-section of the laser clad WC-Ni samples were carried out using amicrohardness tester. Fig. 5 shows the results of the microhardnessmeasurements. The substrate side and the top side are shown on

the left and right hand sides, respectively. The value of microhard-ness in the laser cladding zone was between 9002400 VHN1.961 Nwhile it was 230270 VHN1.961 N on the substrate. There were somepeaks in microhardness where the measurement was directly on ahard WC particle. A distinct gradient in the microhardness isobserved as the distance approaches near the interface. There wasnot much variation in the microhardness betweenWC-5Ni andWC-10Ni while the microhardness for W-15Ni was relatively low. Thisvalue is attributed to the higher concentration of the softer Nimatrix.

3.4. Solid Particle Erosion Studies

The Taguchi method is a statistical method that involvesinvestigating the effect of process control parameters throughthe robust design of experiments. Unlike conventional full factorialdesign of experiments, the Taguchi method uses predenedcombinations of parameters (as per an orthogonal array) togenerate the necessary data. The data from the arrays can be

C.P. Paul et al. / Optics & Laser Technology 50 (2013) 155162 159Fig. 4. Optical micrograph presenting microstructure (a) at the bulk clad layer, and

(b) at substrate-clad interface for WC-Ni deposits.analysed by plotting and performing a visual analysis, an analysisof variance (ANOVA), a bin yield with Fisher's exact test or a Chi-squared test. These analyses or tests are used to determine theeffect and contribution of various individuals/combinations ofindividual parameters on the process with a minimal amount ofexperimentation, which saves time and resources. In the presentstudy, the results of comprehensive experiments that were carriedout as per an L9 orthogonal array of the Taguchi method arepresented in Table 2. Fig. 6 presents the effect of variation of one offour parameters at a time (Ni-concentration, erodent jet velocity,erodent feed rate and impact angle) in terms of S/N ratio. It wasobserved that the width of the eroded pit (presented in column 8,Table 2) was increased with an increase in the velocity, and thesmallest width was obtained at 30 m/s. This effect occurs becausethe erodent jet diameter at the impact plane increases withincreased erodent particle velocity. The length of the eroded pit(presented in column 9, table 2) increased with a decrease in theimpact angle, and the longest length was obtained at an impactangle of 301. The effect of individual parameters was evaluatedusing the Taguchi method. The analysis of means (ANOM) test wasperformed by averaging the resulting S/N ratios for each of theparameter levels. Table 3 presents the effect of parameters on theerosion wear from one level to another as computed from theresults of the ANOM test. The analysis revealed that the contribu-tions of Ni-concentration in the MMC, erodent jet velocity, erodentfeed rate and impact angle to the erosion are 4%, 63%, 6% and 28%,Fig. 5. Value of microhardness measurement of various WC-Ni laser clad samples.

M(g/

2.94.83.31.2

C.P. Paul et al. / Optics & Laser Technology 50 (2013) 155162160Table 2Data summary of the erosion wear experiments.

Expt.No.

Ni-concentration inMMC (wt %)

Erodent jetvelocity (m/s)

Erodent feed rate(g/min)

Impactangle (1)

1 5 30 2 902 5 50 3 603 5 70 4 304 10 30 3 30respectively. Thus, the erosion behaviour of the WC-Ni laser clad isprimarily governed by erodent jet velocity followed by impactangle. It does not much depend on the Ni-concentration in theMMC or the erodent feed rate. The curve in Fig. 6 for Ni-concentration in the MMC showed that there was not muchvariation in the erosion wear value for WC-5Ni and WC-10Ni,but the value increased for WC-15Ni. As the erodent feed rate wasincreased, the result was an increase in the number of particlesimpacting on the laser clad surface, and the erosion wear value

5 10 50 4 90 4.36 10 70 2 60 9.27 15 30 4 60 1.58 15 50 2 30 2.39 15 70 3 90 6.1

Fig. 6. Effect of Ni-concentration, erodent jet velocity, erodent feed rate and impactangle on erosion wear in terms of S/N ratio.

Table 3Analysis of erosion value data.

Average n by Factor Level (dB) DegreeofFreedom

SumMeanSquares

MainEffectSquare

%Contribu-tion

F

Factor 1 2 3

A Ni-Conc.(wt%)

53.62 53.55 55.76 2 9 5 4

B Erodentjetvelocity(m/s)

59.87 53.44 49.63 2 161 80 63 17

C Erodentfeed rate(g/min)

52.72 54.44 55.78 2 14 7 6 1

D Impactangle (1)

52.17 52.49 58.27 2 71 35 28 7was increased. The trend conrms the general behaviour oferosion wear resistance [37]. The curve for erodent jet velocityshowed that the erosion value decreased with an increase in theerodent jet velocity. This may be attributed to the combinedeffects from the formation of a blanket by ricocheting erodentparticles and the fracture of erodent particles during collisionswith the MMC. When erodent particles with reasonably highvelocity strike the surface, they ricochet and interrupt the incom-ing path of the erodent jet by forming a blanket. Though theexplanation of the results is logical and closely associated withrealistic conditions, there is some apprehension about the resultsof the air jet erosion testing for very high velocities due to theblanket effect. As relevant testing and simulation is a tricky topicthat needs special attention, it is proposed that an alternative testis needed to evaluate the material performance at such highervelocities. Fracturing of the erodent particles results in a reductionof the impact pressure on the target and subsequently lowers theerosion value. The relationship between critical erodent particlesize (dF) and erodent particle velocity (V) was given by [37] as

dFKcpHp

2 Hp1=2Ht

1=6 1=3V2=3 7

where Kcp, Hp and are the fracture toughness, hardness anddensity of the erodent particles (Al2O3), respectively, and Ht is thehardness of the target material (WC). Eq. 7 indicates that smallerparticles also become fractured at higher erodent particle velocity.The size distributions of the erodent particles before and after theerosion test were also evaluated. A lesser number of fracturederodent particles was found at lower erodent velocity, and thenumber found increased for higher values of erodent velocity. Thecurve for impact angle showed an increase in the erosion value asthe impact angle was changed from 901 to 301. Change in the

ean erosion valueg x 103)

S/N(dB)

Maximum width oferoded pit (mm)

Maximum length oferoded pit (mm)

3 55.45 1.08 3.105 51.06 3.20 4.801 54.37 2.25 2.900 63.19 2.65 8.055 52.00 3.25 4.175 45.45 3.50 4.735 60.96 3.03 3.208 57.26 3.20 6.300 49.06 4.00 4.30impact angle from 901 to 601 did not inuence the erosion wearvalue much, but a similar variation from 601 to 301 resulted in alarger difference in erosion wear value. This difference occursbecause of a change in the mechanism of erosion wear from thebrittle mode to the ductile mode. A similar trend was observed forHVOF-sprayed WC-Ni coatings by Berger et al. [38].

Figs. 7(a)(c) present micrographs of the eroded pits. The depth ofthe eroded pit was reduced as the impact angle changed from 901 to301. These micrographs clearly indicate that the ow of the matrixmetal and total erosion were primarily governed by a ductile erosionmechanism. Fig. 8 presents a typical eroded pit. The presence ofmicro-pores on the eroded pit surface shows that WC particulateswere removed after the removal of the softer Ni-metal matrix. Theerosion value of the Ni-clad layer with WC was found to be at leastfour times higher than that without WC particles. This ndingreconrmed that the introduction of WC particles with a Ni matrixincreased the erosion resistance properties.

C.P. Paul et al. / Optics & Laser Technology 50 (2013) 155162 1614. Conclusions

In the present study, the laser cladding of WC reinforced Nimatrix on austenitic stainless steel was carried out for threedifferent WC-Ni ratios (5, 10 and 15 wt % of Ni). The successfuldeposition of fully dense and crack free WC-Ni clad layers for WC-15Ni with a defect-free interface and low dilution was demon-strated. During the microscopic study, dissociation/partial melting/full melting of the WC particles was not observed in the laser cladlayers as the sharp corners of WC particles were found intact in

and WC-15Ni. A dendritic microstructure was observed in the Ni

Fig. 7. Macrograph showing the eroded surface signature for solid particle erosionat various impact angles (a) 901, (b) 601 and (c) 301.matrix. The orientation of dendrites was perpendicular at the clad-substrate interface due to larger cooling from the substrate. Theorientation of dendrites was almost radially outward around theWC particles because these particles acted like heat sinks due totheir superior thermal properties. The value of microhardness inthe laser cladding zone was between 9002400 VHN1.961 N while itwas 230270 VHN1.961N on the substrate. The result of the erosionwear studies of the WC-Ni laser clad surface revealed that theerosion behaviour of WC-Ni laser clad is primarily governed byerodent jet velocity followed by impact angle. The erosion doesnot much depend on the Ni-concentration in MMC or the erodentfeed rate. The decrease in erosion wear value with increasingerodent jet velocity was observed due to the combined effect ofblanket formation by the ricocheting erodent particles and thefracture of erodent particles during impact. The signature wearseen at the erosion wear surface indicated that the erosion wasprimarily governed by ductile erosion mechanisms followed by theremoval of WC particulates from the matrix.

Acknowledgement

The authors thankfully acknowledge the technical support ofthe laser clad layers. The microscopic studies also showed that theWC particles were uniformly distributed in the Ni matrix, andtheir densities were in a descending order for WC-5Ni, WC-10Ni

Fig. 8. Macrograph showing sites of removed WC particulates from laser clad layer.Dr. Atul Kumar, Mr. P Bhargava, Mr. C H Premsingh, Mr. S K Perkar,Mr. N K Mourya and other members of the Laser Material Process-ing Division during the course of the presented study.

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C.P. Paul et al. / Optics & Laser Technology 50 (2013) 155162162

Solid-Particle Erosion Behaviour of WC/Ni Composite Clad layers with Different Contents of WC ParticlesIntroductionExperimental ProcedureResults and discussionEffect of processing parametersMicrostructural examinationsMicrohardness MeasurementSolid Particle Erosion Studies

ConclusionsAcknowledgementReferences