Oviedo University. Mechanical Engineering Area. ETS Ingenieros · 2014-05-20 · Laser cladding of tungsten carbide powder M. Cadenas, E. Fernandez, M. R. Fernandez, J. M. Cuetos

Laser cladding of tungsten carbide powder

M. Cadenas, E. Fernandez, M. R. Fernandez, J. M. Cuetos

Oviedo University. Mechanical Engineering Area. ETS Ingenieros

Industrials. Campus Viesques s/n. 33203 Gijon, Spain.

Email: [email protected]

Abstract

Cermets ofWC-Co as materials for coatings to resist wear could be considered of frequent usein industry. However, the behaviour of those coatings depends on the deposition techniqueused: welding, plasma-spray, laser cladding, etc..

This paper presents the study of 83% WC-Co coatings on the surface of AISI 1043steel. A CO: laser was used to melt the powder and a minimal part of the substrate.

The comparison of the characteristics of laser cladding coatings with plasma-spraycoatings, both of them obtained with the cermet, shows in the former a very important decreaseof the porosity, a higher coating hardness and a very good adherence layer-substrate.

In consequence, its wear behaviour also presents differences. The wear evaluation ispresented by PV diagrams and the wear rate measures. Block-on-ring in lineal contact test weredone with laser cladded layers and plasma sprayed layers versus a tempered and quenched AISI1043 steel. Tests were done under limit lubrication conditions for different sliding speeds.

PV diagrams results show that for sliding speeds over 0.4 m/s. the plasma-spraycoatings support higher contact pressures than the laser cladded layers. However to lowerspeeds, these have a better behaviour. On the other hand, it could be observed that lasercladding coating has a wear rate approximately a 34% lower than the plasma-spray one.

1 Introduction

The use of WC-Co cermets is usual in the industry of plasma sprayedcoatings to obtain abrasion, erosion, fretting and hard surfaces resistantcoatings, useful for industrial applications at temperatures below 500°C and fornon-corrosive uses (jet engine compressor blades, exhaust fans, hydraulic

valves, cam-follower contacts...) [1].The CO? laser has also been revealed like a useful tool for the realisation

of coatings starting from a great variety of materials, and particularly of WC-Cocermets, improving the characteristics of plasma sprayed coatings in someapplications. An example of this is the entry roller for a bar hardeninginstallation. Tests with WC-Co plasma sprayed coatings did not produce

Transactions on Engineering Sciences vol 17, © 1997 WIT Press, www.witpress.com, ISSN 1743-3533

248 Surface Treatment, Computer Methods and Experimental Measurements

satisfactory results, since the frequent temperature reversals caused the layersto flake off. Metallurgical bonding of the coating to the substrate was essential.Rollers coated with WC-Co using the laser cladding process have demonstrateda considerable increase of their operation life [2].

In this paper, further more describing the procedure used to obtain WC-Co coatings onto a AISI 1043 steel using a CO? laser, the most importantcharacteristics of the resulting coatings are analysed: adherence, porosity,hardness, microstructure, etc.. It also studied their tribological behaviour bymeans of PV diagrams and by means of their wear rate, when they are placedagainst a ring of hardened and tempered AISI 1043 steel, under lubricatedconditions in sliding lineal contact. These results are compared with the resultsof WC-Co plasma sprayed coatings for the same tribological demands.

2. Experimental procedure

2.1 Laser cladding by powder injection

In laser cladding by powder injection, powder of metallic alloys, ceramics,metal-ceramics or cermets... is blown in an inert gas stream into the lasergenerated melt pool. The powder is fused on the surface of a substrate, with theminimum of dilution to obtain layers, which are composed nearly exclusively ofthe injected materials.

This technique results suitable to coat small areas with a single clad trackor overlapping several tracks. In big areas, problems caused by the heating ofthe substrate and the internal efforts induced in the material during thetreatment appear [3].

For the experimental development, laser cladding was performed usingfast axial flow CO2 Rofm Sinar laser equipment DC-excited, with 10.6 jamwavelength, 1700 W nominal power and TEM 01* beam mode. Like laserfocusing head, it was used a Kugler head with a focal distance of 150 mm. Theexperimental conditions where fixed, after some initial tests, with a powerdensity within the range of 6000 to 18000 W/cnr. The processing speedstested went from 200 to 800 mm/min.

The bond material used was Metco 73F-NS-1 powder, with average grainsize of 39 pm and the following composition: 83%WC-17%Co.

Powder injection took place by means of a Metco 4MP powder feeder,using argon at 3 bar at the feeder entrance, as drifting gas. The nozzle was atube of 2.5 mm of diameter located outside the laser focusing head and inclinedan angle of 60°. Powder flows range among 5 and 20 g/min.

This system is simple, however, it presents problems of operation thataffect to the quality of final coatings:

- It is not indifferent work in any direction because there is no symmetryabout laser beam. The best results are obtained injecting the powder as figure 1


Surface Treatment, Computer Methods and Experimental Measurements 249

shows [4].- The protection gas of the laser optics modifies the trajectory of the

particles. The use of a second nozzle, that gives way a horizontal air stream of

protection, allows a low dispersion of the injected powder and a greatefficiency is obtained. It also contributes to enhance the porosity of the cladand decrease the melting of the substrate, improving its dilution.

- An important loss of powder is produced by the rebound of particles on

the surface of the substrate. To minimise the problem it was indispensable to

control the speed of the particles leaving from the nozzle.- Porosity and cracking are reduced with an adequate adjustment of

parameters of process, although their total elimination was not possible withthe available equipment.

Considering all these factors, the final lay out for laser cladding and animage of the process realisation are shown in the figure 1.

mi

%

KUGLER

Laserfocusinghead1

Clad

Substrat

L_ —

Pn

e

Direction of, movement

Air ofprotectionnozzle

Powdernozzle

Figure 1. Final final lay out used to laser cladding tests.

2.2. Characterisation of coatings

The study of the coatings, both laser cladded and plasma sprayed coatings,consisted in a microstructural analysis using optical microscopy and scanningelectron microscopy with an X-ray scanner. Hardness, microhardness, crackingand porosity were also tested.

Adherence between coating and substrate was also studied by performingadherence tests in accordance to ASTM C633.

Finally, to get the PV diagrams and wear rates, a block (the flat test piece)



of AISI 1043 steel with 22-23HRC hardness, and 15.75x10.16x6.35 mm ofdimensions (according ASTM G77), has been used. Once coated, it wasrectified and placed against another ring-shaped mobile test piece whoseexternal diameter is 50 mm. The ring is made of the same steel as the block, butit is hardened and tempered and has a hardness equal to 46-50 HRC. Every testwas carried out under lubrication with oil SAE 20 of 8,3 cSt at 100°C and 61,4cSt at 40°C.

3 Results and discussion

3.1 Coatings

Figure 2 illustrates how remarkably coating morphology varies depending onthe cladding technique applied.

The plasma sprayed coating (figure 2.A) shows abundant porosity. Smallrounded carbides (WC) are homogeneously distributed throughout the cobaltmatrix. The interface has irregular profile and the steel substrate is deformed byshot blasting before it is sprayed.

On the other hand, (figure 2.B) laser cladded coatings build up bysuccessively overlapping tracks. Thus, part of the material from the first tracksmelts again as the second tracks deposits. This causes the overlapping zone tohave a structure different from the rest of the coating.

In the zone where the laser beam has acted just once, tungsten carbides arebig, are prism-shaped and spread along the matrix. At the overlapping zone,however, carbides are smaller and rounded, similar to those found in the plasmasprayed coatings. There are also some quite big pores and several crackingslaying perpendicularly to the surface and to the tracks trend which are difficultto prevent.

The round shapes and small size of the carbides evidence that theydissolve partially due to the heat generated during their deposition [5]. It is alsopossible for brittle particles such as (CogWg)C to form when there is abundant

cobalt [6].On the other hand, Figure 3 shows the laser cladded coating interface. This

zone exhibits dendritic structures derived from the partial melting of thesubstrate (the dilution phenomenon) and Figure 4 presents the ED AX spectralanalyses of this zone and of the matrix zone of laser cladded coating. Theyshow the presence of iron during the matrix phase (figure 4. A) remarkably moreabundant at the interface (figure 4.B). Xi-Chen [7] claims that the iron in thecoating gives rise to a eutectic, forming even eutectic ledeburite if the amount ofiron is large.


ouna.ce ircaiinein, v_,umpuici

B)

100

Figure 2. Microstructure of 83% WC-Co coatings with different techniques.A) Plasma sprayed. B) Laser cladded.

100

Figure 3. 83%WC-Co laser cladded coating. Layer and interface structure.



F

W hoc

, J

CoK«

W fte

IJL̂ . L .. ,„

Fe*U

f.r,•4.3ZKEV 10»V/ch A EDRX

A) Matrix phase

23CNT 4.3ZKEV Ift-V/ch A EDRX

B) Interface zone

Figure 4. ED AX spectral analyses of the 83%WC-Co laser cladded coating.

Vickers Hardness HV200

1400

1200

1000

800

600

400

200

n

83%WC-Co Li — * — Dilution~ — 0 — Dilution___ — 0 — Dilution

SUBST

0*

aser15%30%62%

RATE

£̂ dg825fl

o'

LAYER

-1,6 -1,2 -0,8 -0,4 0,0 0,4 0,8 1,2Distance from the interface (mm)

Figure 5. Microhardness in 83%WC-Co laser cladded coatings, with differentdilutions: 15% dilution (12889 W/cnf, 600 mm/min, 0,41 s., 4,9 gr/min); 30%dilution (12889 W/cnf, 700 mm/min, 0,35 s., 9,4 gr/min); 62% dilution (9666W/cnf, 400 mm/min, 0,71 s., 4,9 gr/min).

The parameters used for the laser cladding process influence dilutionstrongly. Figure 5 compares microhardness in three coatings for whose claddingdifferent technical parameters were used, giving rise to different degrees ofdilution. It can be seen that the smaller the dilution the bigger the hardness.Finally, figure 6 compares microhardness in plasma sprayed and laser cladded83%WC-Co coatings. The stronger dilution of the WC carbides in the plasmasprayed coatings accounts for their lower hardness.



1200 •—-—*—83%WC-Co Laser

1000 - —6—83%WC-Co Plasma

800 -

600 - / / "SUBSTRATE /£ LAYER

400 -

-1200 -900 -600 -300 0 300 600 900Distance from the interface (pm)

Figure 6. Microhardness of 83%WC-Co plasma sprayed coatings and lasercladded coatings (dilution 30%).

3.2 Results of wear tests

3.2.1 PV DiagramsSet out the PV diagrams, from the experimental results, as shown in figure 7.certain variability is observed in the clouds of points for the pairs of friction(83% WC-Co laser/ AISI 1043 steel), (83% WC-Co plasma/ AISI 1043 steel)and (AISI 1043 steel/ AISI 1043 steel), under lubricated condition in slidinglineal contact. For this reason it has been carried out a statistical analysisapplying the Extreme Value Theory [8], due to pressures distribution thatcould be supported in a contact between two surfaces, for a determined speed.can be perfectly adapted to a problem of minimal extreme values. Thedevelopment of this theory, according to the procedure indicated by Castillo[9], allows to associate to every level of speed, a function of distribution ofWeibull for minimum's of three parameters. This functions proportions themaximal pressure that supports the contact according to different probabilitiesof failure.

With it, a family of curves for the different probabilities is obtained.included the curve of probability of failure of 5%, that is selected like PVdiagram. Moreover, figure 7 shows PV diagrams for all pairs of friction studied.

For sliding speeds higher to 0.4 m/ s, WC-Co plasma sprayed coatings incontact with steel are capable of supporting pressures that are almost thedouble of those that could resist WC-Co laser cladded coatings, and fromsixteen to twenty times superior to them accepted by the contact steel-steel.However, for sliding speeds inferior to 0.4 m/s the behaviour of laser claddedcoatings is higher than plasma sprayed coatings.

For the range of higher speeds, plasma sprayed coatings have a betterbehaviour, probably due to their porosity, since it could make the functions ofmicrodeposits of the lubrication oil. The lubrication of the contact zone is



favoured thereby. Also the sliding speed favours the hydrodynamic lubrication.However low speed ranges, the lubrication loses effectiveness and the

effect of the porosity is annulled. In this situation, laser cladded coatings have abetter behaviour than plasma sprayed coatings as consequence of their greaterhardness.

1000 :

o 83%WC-Co Plasman 83%WC-Co LaserA AISI 1043~~—PVWC-Co Plasma— PV WC-Co Laser

PV AISI 1043

0,10 1,00 10,CSliding speed (mis)

Figure 7. Experimental data and PV diagrams of the 83% WC-Co coatings andof the AISI 1043 steel when they are been placed against AISI steel 1043hardened and tempered under lubricated conditions in sliding lineal contact.

3.2.2 Wear rate

Using the previous conditions of test, they were carried out tests with severalpairs of loads and speeds, (C, V), to measure the rate of wear for both types oflayers. In the case of coatings, this information is important, because theirduration under work conditions described could be anticipated. The coefficientof friction [i was also measured and it resulted lightly upper in plasma sprayedcoatings.

The graph of figure 8 relates the average wear rate of each test piece andthe product ji.C.V, which it is constant in every test.

From the results, it can be deduced that 83%WC-Co laser cladded coatingspresents a lower dispersion of results, and their average value of wear rate is



34% below than value in 83%WC-Co plasma sprayed coatings, for the sameconditions of load and speed.

To understand this behaviour, it is important to consider the higherhardness of the laser cladded coatings. In addition, their stronger surfaceroughness and their granular structure causes plasma sprayed coatings to loseparticles more easily. This may explain the wider range of results obtained fromthe plasma sprayed coatings.

It was also observed that wear rate of AISI 1043 steel is 50 to 300 timeshigher than wear rate of coatings.

2.0E-5O Laser Vd'= 5,122E-8*pCV

R2 = 0,9803+ Plasma vd' = 7,732E-8*pCV

1,5E-5 - Ff = 0,593

1,OE-5 -

5,OE-6 -

0,OE+050 100 150 200

MCV (Nm/s)

Figure 8. Wear rate of the 83%WC-Co coatings in lineal contact with AISI 1043as a function of product jiCV.

4 Conclusions

After resolving different problems in the powder injection technique, coatingsof 83%WC-17%Co over AISI 1043 have been obtained by smelting the formerwith a CO: laser. By comparing with 83%WC-17%Co plasma sprayedcoatings, it can be seen how in the former, porosity decreases remarkably,hardness increases and adherence between coating and substrate improve.

Analysing wear rate in laser cladded and plasma sprayed WC-Co coatings,it is found that the wear rate of the former is 34% slower than wear rate ofplasma sprayed WC-Co coatings, when they placed against AISI 1043 steel inlubricated sliding lineal contact. It was also observed that wear rate of AISI1043 steel is 50 to 300 times higher than wear rate of coatings.

In the same test conditions, PV diagrams of both types of layers and ofthe substrate have also been obtained. WC-Co plasma sprayed coatings



support pressures which are almost the double of those that could resist WC-Co laser cladded coatings, and from sixteen to twenty times superior to themaccepted by the contact steel-steel. However for lower speeds to 0.4 m/s, thebehaviour of laser cladded coatings is upper to plasma sprayed coatings.

5 Acknowledgements

We thank Fomento en Asturias de la Investigation Cientifica Aplicada y laTecnologia (FICYT) for funding our project PA-MAT92-04 "Microstructuraland tribological characterisation of laser cladded and plasma sprayed ceramiccoatings".

References

[1] Metco Inc. Technical Bulletin. 1988.[2] Ritter, U.; Kahrmann, W.; Kupfer. R.; Glardon, R..Laser coating proven in

practice. Sulzer Technical Review, n°. 3. 1991.[3] Ramous, E.; Giordano, L.; Tiziani, A.; Badan, B.; Cantello, M.. 1990. Laser

cladding of ceramic and metallic coatings on steel. Key EngineeringMaterials Vols. 46 & 47 pp. 425-434.

[4] Fouquet, F.; Pelletier, J.M.; Pilloz, M.; Vannes,A.B.. 1991. Alliages desurface avec predepots revetementes et alliages avec projection. Laser depuissance et Trait. Mat.. Presses Polytech. Univ. Romandes.

[5] Nerz, T. C.; Nerz, J. E.; Kushner, B. A.; Riggs, W. L. Evaluation of highenergy plasma sprayed WC-Co coatings using experimental design. SurfaceEngineering. Vol 9 N°3. 1993.

[6] Metals Handbook. Vol. 9. Metallography and microstructures. AmericanSociety for Metals. 1985.

[7] Xi-Chen, Y.; Tian-Xi, Z.; Nai-Ken, Z.. Laser cladding of WC-Co powder.Proceedings 1C ALEO. 1990.

[8] Castillo, E.. Extreme value Theory in Engineering. Academic Press, Inc.1988.

[9] Castillo, E.; Galambos, J.; Sarabia, J.M.. The selection of the domain ofattraction of an extreme value distribution from a set of data. Lecture Notesin Statistics. 1989.

[lOJNowotny, S.; Techel, A.; Luft, A. Reitzenstein, W.. 1993. Microstructureand wear properties of laser clad carbide coatings. ProceedingsICALEO'93, pp 985-993.


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Oviedo University. Mechanical Engineering Area. ETS Ingenieros · 2014-05-20 · Laser cladding of tungsten carbide powder M. Cadenas, E. Fernandez, M. R. Fernandez, J. M. Cuetos