Wear resistance of a laser alloyed A-356 aluminum/WC composite

Wear 251 (2001) 1459–1468

Wear resistance of a laser alloyed A-356 aluminum/WC composite

M.H. Staiaa,∗, M. Cruza, N.B. Dahotreba School of Metallurgy and Materials Science, Central University of Venezuela, Apartado 47283,

Los Chaguaramos, Caracas 1041-A, Venezuelab Laser Materials Processing Group, The University of Tennessee, 10521 Research Drive, Suite 400 Knoxville, TN 37932, USA

Abstract

In this study, the wear resistance of an A-356 aluminum alloy/WC composite obtained by a laser alloying technique has been analyzed.Different laser traversing velocities of 100, 200, 300 and 400 cm min−1 have been used to establish the optimum processing conditions. Theinteraction time between laser beam and the sample surface, depending on the traversing speed, has a noticeable influence on microstructuralmorphology and superficial composition, and, hence, on both wear resistance and wear mechanism. Optical microscopy (OM) and scanningelectron microscopy (SEM) techniques were used to examined the morphology of treated samples and the corresponding microstructuralchanges. Wear properties were studied under a load of 5 N by using the ball-on-disk tribometer. The static partners were WC+6% Co ballsof 6 mm diameter and the sliding speed used during the test was 0.1 m s−1. Although the friction coefficient values were approximately0.6 for all samples, their dependence on the sliding distance was quite different for each set of processing parameters. The OM togetherwith SEM techniques were used to elucidate the wear mechanism. It was shown that the WC particles from the composite material had anabrasion action on its counterpart, and this action was observed to decrease with decreasing particle size, which again was a consequenceof the increased interaction time between laser beam and substrate. It was concluded that the incorporation of WC hard particles in theA-356 aluminum base alloy was not beneficial from the point of view of wear resistance. For example, for traversing velocities between 300and 100 cm min−1, there was no significant difference between the wear rates of both unalloyed and WC-laser alloyed A-356 aluminumalloy against a WC ball. For the samples laser treated with 400 cm min−1, a smaller wear resistance was obtained when compared to theuntreated samples. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Laser alloying; Aluminum; WC; Ball-on-disc test

1. Introduction

In the past two decades, surface strengthening techniqueshave been developed to improve the wear resistance behaviorof aluminum alloys without affecting their bulk properties.

Laser surface melting can produce a rapidly solidifiedlayer, in which both the microstructure and the distributionof the alloying elements could be tailored as a function ofoperating parameters. For example, in order to improve wearresistance elements such as Cr, Cu, Mn, graphite and hardparticles such as SiC, TiC, WC, etc. have been added to themelt pool of aluminum alloys [1–4].

Different methods were used to deposit the alloy layeronto the substrate: pre-placing the alloy layer by electro-plating, vapor deposition, ion implantation, sol–gel, thermalspraying or mechanical procedure; binding the alloying el-ements by an organic resin or by powder/gas feeding [5,6].

∗ Corresponding author.E-mail addresses: [email protected] (M.H. Staia), [email protected](N.B. Dahotre).

Experimental results on the corrosion and wear resis-tance of laser alloyed aluminum and aluminum alloys withhard phases have been reported [2,6–9]. However, it wasmentioned [7] that for the unlubricated sliding situation,conflicting reports exist in the literature regarding the role ofcarbide and oxide particles. A literature survey has indicatedthat data on laser alloying of aluminum alloys with WC par-ticles and their subsequent wear resistance are scarce [10].

This investigation is part of a large program in which sandcast aluminum alloy A-356 surface is modified by laser,with the aim of improving the surface wear and avoiding adelamination problem, which may arise when this alloy isused in cylinders bores. Results from the in situ synthesisof MoSi2 in the laser-melted pool of this alloy [11] andalloying with WC by using a precursor coating [12] havebeen reported recently. In the latter investigation, the resultsof the wear behavior of a tribological pair of laser alloyedaluminum/AISI 52100 steel were reported under the sameoperational conditions as in the present study. Due to largedifferences in hardness between the materials of the matingpair, the pin material underwent high abrasive wear and wastransferred to the laser treated disc. In these conditions, the

0043-1648/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0043-1648(01)00789-X

1460 M.H. Staia et al. / Wear 251 (2001) 1459–1468

recorded friction coefficient corresponded to the steel/steelcontact and the wear scar morphology of the discs indicatedthe presence of a thick transfer film of iron oxide, whichsuffered fatigue wear. The wear data have indicated that thelaser treated samples exhibited a better wear resistance thanthe untreated alloy, and the extent of the wear volume wasdependent on the interaction time between the laser beamand the substrate.

The purpose of the present investigation was to explore theeffect on the sliding behavior of the alloyed sample when aharder material, such as WC+6% Co, is used as counterpart.The role of the laser transverse velocity on the wear responseof this tribological pair has been also investigated.

2. Experimental details

Sand-cast Al-356 alloy (Al–7.0Si–0.3Mg, in wt.%) sam-ples were metallographically prepared by using standardprocedure. Powder with a composition of 96 wt.% WC,2 wt.% Ti and 2 wt.% Mg with an average grain size of1 mm was used. Experimental details of precursor appli-cation are presented elsewhere [12]. Nd-YAG (wavelength∼1.06�m) laser with a power of 2000 W was employed totreat the samples at different traversing speeds from 100 to400 cm min−1, with increments of 100 cm min−1. Duringprocessing the pre-coated film will mix with the moltenregion at the substrate surface by a convective fluid flowmechanism [13], and the degree of mixing will depend onthe interaction time between the laser and the substratesurface.

Microstructural characterization of treated samples wasperformed on their cross-sections by using both optical mi-croscopy (OM) and scanning electron microscopy (SEM)accompanied by energy dispersive X-ray spectroscopy(EDX). The samples were etched prior to microstructuralexamination using Keller’s reagent.

The arithmetic mean roughness value (Ra) of the sur-face of the laser treated samples was evaluated by using aNew View 200 Zygo profilometer. The average value of fivemeasurements was reported.

Microhardness measurements were taken on polishedcross-sections of the coatings, using a Knoop indenter,applying a load of 100 g during 15 s, starting from the

Table 2Knoop microhardness values along laser-treated samples cross-section at different traversing velocity of laser

Distance from the surface (�m) 400 cm min−1 300 cm min−1 200 cm min−1 100 cm min−1

20 432.21± 167 590.75± 175 416.10± 112 526.24± 16950 186.87± 43 187.01± 26 167.13± 23 193.51± 40

130 121.24± 6 114.27± 13 112.47± 9 121.54± 14350 132.88± 14 113.70± 5 112.55± 7 100.67± 10550 116.43± 5 109.80± 8 118.74± 9 98.24± 1750 96.73± 3 88.01± 10 101.70± 9 105.73± 2

1200 94.08± 2 80.10± 4 97.90± 3 90.22± 3

Table 1Characteristics of laser treated samples

Traversing velocity oflaser (cm min−1)

Average surfaceroughness (Ra)

400 14.66300 12.64200 9.22100 6.30

sample surface until the unmodified substrate was reached.A mean value of 20 measurements and the standard devia-tion were reported in each case.

The wear behavior of the samples was evaluated by usinga ball-on-disc tribometer (CSEM, Switzerland) with ballsof 6 mm diameter, made of WC+ 6% Co. The experimentswere performed in air, without lubrication, at a temperatureof 298 K and a relative humidity of 65± 5%. A load of 5 Nwas used with a linear velocity of 10 cm s−1, for a slidingdistance of 1000 m. The radius of the traveling circle of theball on the disc was 0.3 cm. For comparison purposes, thewear behavior of the substrate against WC balls was alsodetermined.

Irregular wear scar morphologies of both treated discsand balls did not allow quantitative measurements of thewear volume in accordance with ASTM standard G99. In-stead, the relationship between the maximum depth of thewear track and its surface area,Dtrack/Atrack, was used asa measure of the wear behavior of all the samples underinvestigation. The surface area of the wear track was mea-sured by a combination of optical microscopy and imageanalysis. The maximum depths of the wear tracks wereevaluated by using a Zygo profilometer. The wear mech-anism was determined by using SEM in conjunction withEDX and elemental X-ray mapping.

3. Experimental results

3.1. Surface roughness andmicrohardness measurements

Results from surface roughness measurements arepresented in Table 1. It is shown that a relative decrease of

M.H. Staia et al. / Wear 251 (2001) 1459–1468 1461

the substrate roughness,Ra, was obtained with an increasedinteraction time between laser beam and substrate material.

Knoop microhardness data are shown in Table 2 and themicrohardness profiles, along the depth of the cross-sectionsof the laser treated tracks, are presented in Fig. 1. It is ob-served that a considerable increase in the average hardnesshas occurred, from an average value of 80± 5 HV100 forthe substrate to 590.75 ± 175 HV100 in the middle of thecompound zone, the latter value corresponding to samplestreated with a laser traversing velocity of 300 cm min−1. Itmay be noted that the microhardness average value deter-mined for each velocity, in the region corresponding to theinterface between the compound zone and the refined zone,was nearly the same. However, an improvement of nearly

Fig. 2. (a) Optical micrograph of sample cross-section laser treated by using 300 cm min−1, where three distinct zones are observed: subzone I, subzoneII and the substrate; (b) detail of the region marked A, where a needle-like structure is noticed.

Fig. 1. Microhardness profiles along laser treated sample cross-section.


two times in the average microhardness value was obtainedas compared to the microhardness measured in the substrateregion.

3.2. Microstructural characterization

Fig. 2a shows the optical micrograph of a metallographiccross-section from the sample laser treated at a traversingvelocity of 300 cm min−1. In the micrograph three distinctzones can be observed: subzone I, where all the WC particlesare embedded in the aluminum matrix; subzone II, whichcorresponds to a refined aluminum microstructure and thesubstrate. Detailed examination of the region marked “A” inFig. 2a is shown in Fig. 2b. The microstructure consisted ofa needle-like structure of aluminum carbide (Al4C3), whichprobably formed due to decarburization of WC and subse-quent carbon dissolution. Similar microstructures attributedto the presence of Al4C3 were reported in the literaturewhen SiC was dissolved in the aluminum matrix [4,9]. X-raydiffraction (XRD) results reported previously by the authors

Fig. 3. Back-scatter SEM micrographs of the sample cross-section corresponding to subzone I: (a) for sample laser treated with a traversing velocityof 400 cm min−1; (b) for sample laser treated with a traversing velocity of 300 cm min−1; (c) for sample laser treated with a traversing velocity of200 cm min−1; (d) for sample laser treated with a traversing velocity of 100 cm min−1.

Table 3Roughness of the laser treated samples

Traversing velocity oflaser (cm min−1)

Maximum depth ofthe molten pool (mm)

Cross-section area ofa single pass (mm2)

400 704 1.9300 778 2.0200 813 2.2100 1108 3.1

[12] have supported the presence of Al4C3 and other typesof carbides such as W2C, W6C2.54 and WCx in addition tothe initial WC.

An image analyzer was used to quantify the extent of thelaser-affected zone (LAZ) as a function of the traversingvelocities employed. Table 3 reports the results of the max-imum depth of the molten pool and the cross-section areaof a single pass.

As expected, with increasing interaction time betweenlaser and the metal surface, both maximum depth and the


Table 4Wear test results

Laser traversingvelocity (cm min−1)

Maximum frictioncoefficient,µmax

Average frictioncoefficient,µav

Surface area of the weartrack, Atrack (mm2)

Maximum depth of thewear track,Dtrack (mm)

Dtrack/Atrack

(mm−1)

400 0.71 0.61± 0.06 23.4± 3.48 0.053± 0.019 0.00226300 0.64 0.60± 0.05 17.81± 2.75 0.011± 0.001 0.00062200 0.78 0.69± 0.06 17.47± 1.96 0.017± 0.003 0.00097100 0.64 0.56± 0.03 21.81± 2.09 0.022± 0.002 0.00100Substrate 0.51 0.46± 0.05 32.52± 2.91 0.039± 0.004 0.00119

cross-sectional area of the molten pool were increased. Inthe samples, which have been treated at a laser traversing ve-locity of 100 cm min−1 maximum depth and cross-sectionalarea are approximately 60% larger than those produced at400 cm min−1.

Back-scattered electron micrographs showing a cross-section of the subzone I for the sample treated at 100–400cm min−1 are shown in Fig. 3a–d. Similar morphologicalfeatures are observed for all samples, although WC parti-cle agglomeration, which resulted from the presence of asurface tension gradient, can be observed to a lesser extentat the lower laser velocity where a more rapid dissolutionof WC particles has taken place. It is interesting to note,however, that the WC particle agglomeration was not ob-served at the surface of the sample treated at the lowest

Fig. 4. Surface area of the wear track andDtrack/Atrack for different traversing laser velocities.

velocity of 100 cm min−1, where only a few WC particleswere present (Fig. 3d). The majority of them have sunkbecause the high heat input has resulted in the formationof a much larger molten pool than in the samples treated atother laser velocities, thus leaving the surface of the samplewith only a limited distribution of hard particles.

3.3. Wear results

Wear test results are presented in Table 4 and plotted inFig. 4. The relationship between the wear track depth andwear track surface area,Dtrack/Atrack, gives an indication ofthe relative wear resistance of the samples under investiga-tion. A maximum value of 0.0027 mm−1 was obtained forthe worn sample produced at the highest traversing velocity,


i.e. 400 cm min−1, indicating that under these processingconditions the wear resistance is inferior to that of the un-alloyed substrate. These results are in agreement with thosereported by Wang and Rack [14] for ball-on-disc wear testsperformed at both low sliding velocity and loads on SiC re-inforced aluminum metal matrix composite. They found thatSiC reinforcement did not improve the wear resistance andattributed the low wear resistance to higher friction coeffi-cients and third body abrasion at low speed.

Apparently, the best wear resistance was obtained forthe sample laser treated with a traversing velocity of

Fig. 5. Friction coefficient evolution of the sample against WC ball for: (a) sample laser treated with a traversing velocity of 400 cm min−1; (b) samplelaser treated with a traversing velocity of 300 cm min−1; (c) sample laser treated with a traversing velocity of 200 cm min−1; (d) sample laser treatedwith a traversing velocity of 100 cm min−1; (e) substrate.

300 cm min−1. In this case, the ratioDtrack/Atrack is nearly3.7 and two times lower than for the sample treated with400 cm min−1 and the untreated substrate, respectively.Samples processed at 200 and 100 cm min−1 traversing ve-locities have similar values for this relationship with similarwear track morphologies, although the amount of materialtransferred to the WC ball is greater for the sample treatedat lower velocity.

Fig. 5a–e present the friction coefficients for the fourprocessing conditions used. Results on the frictional behav-ior of the mating pair substrate/WC ball are also included,


Fig. 6. Morphology characteristics of a severe adhesive wear observed on the wear track of sample treated at 400 cm min−1.

Fig. 7. (a) Overview of the wear track morphology of sample treated at 400 cm min−1; (b) Al X-ray elemental mapping; (c) W X-ray elemental mapping.


for comparison purposes. For the sample treated at a laservelocity of 400 cm min−1, the steady state is achieved after660 m sliding distance. In the transient period, the frictioncoefficient reaches a maximum value of 0.7. This value isthe result of the formation of large quantities of WC de-bris generated during sliding wear. This debris had formedfrom fragmentation under high contact pressure of the ag-glomerated WC particles at the substrate surface, and fromWC grains pulled out from the WC/Co ball material, whichsuffered brittle fracture at the beginning of the experiment.These particles are likely to act as the third body abradersand could be responsible for the higher wear rate of the sam-ple. During the test part of this debris is removed from thewear track towards the sides, and the ball comes in contactwith the aluminum matrix, which corresponds to a decreasein friction coefficient to a value of 0.6. A morphology thatis characteristic of a severe adhesive wear on the disc isobserved in Fig. 6, and an overview of the wear track mor-phology together with the X-ray maps of elements Al andW is presented in Fig. 7a–c.

The highest friction coefficient was found for the samplelaser treated at 200 cm min−1. Here, the friction coefficientvalue is 0.7, and this value is maintained until the end ofthe test. These samples have presented a more compactedlayer of wear debris, and therefore, the layer loses its abil-ity to plastically deform. A higher quantity of cracks isproduced due to the presence of shear stresses during thewear test.

For the samples treated at 100 cm min−1 an averagefriction coefficient value of 0.56 was determined, whichwas the lowest average friction coefficient obtained for

Fig. 8. Wear track of the unalloyed substrate sample.

the treated samples. This reduction in friction coefficientwas attributed [15] to the fact that alloys with smalleraverage size particles display lower frictional forces thanthe alloys containing the same volume fraction of largerparticles. As indicated previously, a limited distribution ofhard particles exists at the surface. This ball will have anabrading action on the treated sample and a high mate-rial transference will take place towards its surface. Withincreasing sliding distance, the ball is in contact with thecompound layer and more WC particles, as debris, are pro-duced at the tribo-contact surface, thus contributing to anincrease in the friction coefficient value towards the end ofthe test.

The average value for the friction coefficient for thesubstrate was determined to be 0.45, and this value is char-acteristic of the friction coefficient in sliding wear of Al–Sialloys against harder materials. The pin has a cutting actionon the substrate, and an appreciable amount of material istransferred to the ball and, subsequently, oxidized. The for-mation of a compacted layer containing aluminum oxidestakes place towards the external side of the substrate weartrack (Fig. 8).

The counterpart morphologies are presented in Fig. 9a–e.It is observed that the WC particles from the composite ma-terial had an abrasion action on the WC ball, and this actiondecreased as the particle sizes decreased, as consequence ofthe increase of interaction time of the laser beam and sub-strate. An increase in contact area between the pin and thealloy matrix was also observed, and the amount of materialtransferred from the aluminum matrix to the pin increasedas the traversing velocity was decreased.


Fig. 9. Scanning electron micrographs of the WC balls against: (a) sample laser treated with a traversing velocity of 400 cm min−1; (b) sample lasertreated with a traversing velocity of 300 cm min−1; (c) sample laser treated with a traversing velocity of 200 cm min−1; (d) sample laser treated with atraversing velocity of 100 cm min−1; (e) substrate.

4. Conclusions

Powder with a composition of 96 wt.% WC, wt.% Ti and2 wt.% Mg has been alloyed on the surface of a sand castAl-356 alloy (Al–7.0 Si–0.3 Mg) by using Nd-YAG laser.

Different traversing velocities of 100, 200, 300 and400 cm min−1 have been used in order to establish the

optimum processing conditions. It was shown that changesin traversing velocity produced a noticeable difference inthe extent of the LAZ, in microstructural morphology ofthe treated samples and in the distribution of the hard par-ticles inside the molten pool. These operational parametershave resulted in differences in the wear behavior of thetreated samples. The worst wear behavior was exhibited


by the samples that have been treated using 400 cm min−1

traversing speed. The relative wear resistance of thesesamples, taking the relationship between wear track depthto wear track surface area,Dtrack/Atrack, as a comparativeparameter, is less than half that of untreated substrate.Therefore, WC particles, which would have been expectedto act as load-supporting elements against the WC–Coball, have a completely different wear behavior, thus con-tributing to an appreciable decrease in the sample wearresistance.

Comparable wear resistance behavior was found betweenthe untreated substrate and the sample laser treated by usingtraversing velocities of 200 and 100 cm min−1. The bestwear resistance seems to have been exhibited by the sam-ple treated at 300 cm min−1, although the aluminum matrixtook part in the transference process, i.e. some of the ma-terial from the laser treated surface has been transferred tothe WC ball.

Therefore, it was concluded that it is not beneficial to in-corporate WC in the base alloy since there was no significantdifference between the wear rates of unalloyed substrate andWC-laser alloyed A-356 aluminum alloy against a WC ball.

These results are totally different from the results pre-sented by the authors when the counterpart was a standardAISI 52100 ball. Therefore, it is important to mention thatthe effect of the counterface material on the wear resistanceof laser alloyed aluminum with hard particles has to be takeninto account.

Acknowledgements

The authors wish to acknowledge the financial supportreceived from the Venezuelan National Council for Sci-entific and Technological Research (CONICIT) throughproject S1-96001366.

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Wear resistance of a laser alloyed A-356 aluminum/WC composite