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Intermetallics 18 (2010) 2124e2127

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Intermetallics

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A facile route to produce FeeAl intermetallic coatings by laser surface alloying

Garima Sharma a,*, Reena Awasthi b, Kamlesh Chandra b

aMechanical Metallurgy Section, Bhabha Atomic Research Centre, Mumbai 400 085, IndiabMaterials Science Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

a r t i c l e i n f o

Article history:Received 24 May 2010Received in revised form24 June 2010Accepted 28 June 2010Available online 11 August 2010

Keywords:A. Iron aluminides (based on FeAl)C. Coatings, intermetallics and otherwiseC. Laser processing

* Corresponding author. Tel.: þ91 22 25590457; faxE-mail addresses: garimas@yahoo.com, garimas@b

0966-9795/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.intermet.2010.06.023

a b s t r a c t

This work studies the feasibility of obtaining iron aluminide coatings on pure iron substrate by means oflaser surface alloying (LSA). Pure Al was plasma spray coated on pure Fe substrate and subsequently laseralloyed by using multi mode continuous wave Nd-YAG laser at different output power and scanningrates. The aluminum content of the alloyed layer shows gradual change from surface to the inside ofsubstrate. Detailed optical, SEMeEDX and XRD investigations were performed to characterize the phasesformed during laser alloying. At 300 W laser output power, Al rich intermetallic phases mainly FeAl3,Fe2Al5 had formed with scanning rate in the range of 100e400 mm/min. However, at high power outputs(500 W) FeeAl intermetallic phase had formed, which was found to have sound and crack free interfacewith the substrate.

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1. Introduction

Iron Aluminides (FeeAl) intermetallic compounds are nowreceiving extensive attention as materials with good potential forindustrial applications as replacement for high temperatureoxidation resisting or corrosion resisting stainless steel. Thesealloys offer lower material cost and high strength to weight ratiothan many stainless steel [1]. However, it is difficult to fabricatestructural components of FeeAl due to their brittle nature at roomtemperature. Recently, research efforts are underway to produceFeeAl coatings by various techniques like plasma spraying, packcementation, hot dipping etc for exploiting the superior hardfacingproperties of intermetallic on the surface [2e9]. These surfacemodification techniques can be used to form iron aluminides on thevarious substrate which can potentially offer benefits of iron alu-minides in term of enhanced resistance to wear, corrosion, hightemperature oxidation and sulphidation resistance [10e12]. Theformation of permeation barriers coatings by aluminizing has alsobeen reported widely. Structural materials such as stainless steelsexhibit poor oxidation resistance at high temperatures, and can beprotected by applying a thin FeAl coating [13e15]. Forcy et al. [16]have reported successful application of aluminide intermetalliccoatings for corrosion protection of stainless steel surface byforming protective oxide scales for nuclear applications. FeAlcoatings on 440C steel used for fuel injector nozzle offer a greatpotential to prevent coking of fuel injector nozzles [17]. In addition,

: þ91 22 25505151.arc.gov.in (G. Sharma).

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SS 304 exhibits high temperature strength and corrosion resistancebut possesses poor oxidation resistance. Application of a thin FeAlcoating would improve the oxidation resistance of SS 304. Lasersurface alloying (LSA) process is one such technique which can beused to improve the hardness, wear and corrosion resistances bymodifying the alloy composition andmicrostructure of thematerialsurface along with a refined structure, lower porosity and cracks.This process offers an added advantage of producing alloyed layerof required depth (micro to millimeter range) in a short time witha small heat affecting zone by using a high energy density heatsource. The alloying material can be introduced either from a pre-deposited layer or directly by co-depositing alloying material. Sofar, there had not been much research work reported in the liter-ature on the LSA of FeeAl intermetallic alloys on various substrates.The present paper is focused on the production of FeeAl interme-tallic layer on pure iron substrate by LSA technique and detailedcharacterization of the various phases formed due to variation inlaser operating conditions.

2. Experimental

A sheet of pure iron (99.9%) to be used as substrate was cut intorectangular cross section of 10 cm � 5 cm � 5 mm. The iron sheetwas cleaned with ultra sonicator. High purity aluminum powder(99.99%) was plasma coated on iron sheet. The coated samples werethen cut into smaller cross section for LSA treatment. LSA wasconducted on multi mode continuous wave Nd-YAG laser withbeam spot size of 2 mm corresponding to power density more than100W/mm2. Laser surface alloying was carried out at a laser output

Fig. 1. SEM micrograph showing a typical morphology of laser alloyed regionsobtained after laser melting of pure Al and Fe substrate.

G. Sharma et al. / Intermetallics 18 (2010) 2124e2127 2125

power of 300 W and 500 W. The scanning of the laser beam werecarried out at 100, 200, 400 mm/min at power level of 300 W and400, 600 mm/min at 500 W laser power. Area coverage wasproduced by overlapping adjacent tracks by 50%. Laser treatment

Fig. 2. (a) SEMmicrograph showing the formation of the FeAl3 (needle like) and brittleFe2Al5 intermetallic phases in the alloyed zone (b) SEMeEDS showing concentrationdepth profile of elements in the alloyed zone; at 300Wand 400mm/min scanning rate.

Fig. 3. (a) Cross section microstructure of the alloyed zone showing formation of FeAl3and FeAl intermetallic phases (b) Concentration depth profiles of the Fe/Al across thealloyed zone; 300 W and 200 mm/min.

was performed by providing argon gas shielding in order to shroudthe melt pool from outside atmosphere to minimize oxidation. Theargon gas flow was maintained at 15 L/min. After LSA, the sampleswere prepared for cross section analysis by metallographic pol-ishing technique. The cross section of the LSA samples wereexamined in detail by optical and SEMeEDX technique. X-Raydiffraction (XRD) measurements were carried out on a PhilipsInstrument, operating with Cu-Ka radiation (l ¼ 1.5406 �A) andemploying a scan rate of 0.02�/s in the scattering angular range (2q)of 10�e80�. Silicon was used as an external standard for correctiondue to instrumental broadening.

3. Results and discussions

In laser surface alloying, coating as well as the substrate meltafter absorbing a large amount of heat from the laser beam andthen release the heat during resolidification to form fully densestructure. The typical microstructure of the alloyed layer obtainedafter laser treatment is shown in Fig. 1 The microstructure,composition and the thickness of the alloyed layer obtained afterlaser treatment was found to be a strong function of the power andscanning rate of the laser used. Laser tracks on pre-coated Al onpure Fe substrate produced with different scanning speeds resultedin FeeAl binary alloys of different compositions. According to FeeAl

Fig. 4. Cross sectionmicrostructure showing formation of FeAl intermetallic phase along with superimposed elemental profiles in the alloyed zone, (b) Qualitative EDS analysis of theFeAl phases formed (c) XRD of the alloyed zone showing peaks corresponding to FeAl phase (d) Optical micrograph of the alloyed zone showing FeAl grains; at 500 W, 600 mm/min.

G. Sharma et al. / Intermetallics 18 (2010) 2124e21272126

binary phase diagram, five types of intermetallic compounds(Fe3Al, FeAl, FeAl2, Fe2Al5 and FeAl3) might form during laseralloying. The surface modified zones under these laser tracksappeared with different case depth and width. Fig. 2(a) showsalloyed layer obtained at 300 W (power) and 400 mm/min (scan-ning rate) of laser. The alloyed region showed deep cracks at thecoating/substrate interface. When concentration profiles of theconstituent elements were measured by EDS along the directionnormal to the interface, the interfacial compositions of the relevantphases can be readily determined. Detailed SEMeEDS analysis ofthe alloyed region identified needle like structure as FeAl3 inter-metallic and cracked lumpy structure as Fe2Al5 intermetallic alongwith unreacted a-Al close to surface. The brittle nature of Fe2Al5intermetallic compound is responsible for the cracks formation inthe alloyed layer. A reduction in scanning rate to 200 mm/min, alsoshows the presence of a-Al and FeAl3 þ Fe2Al5 intermetallic phasesin the alloyed region. Fig. 2(b) shows the concentration depthprofile of Fe and Al along the laser alloyed region. However, withthe decrease in scanning rate to 100 mm/min, a crack free alloyedlayer was obtained (Fig. 3(a)). The alloyed layer showed two distinctzones, one consists of intermetallic FeAl3 phase and other consistsof intermetallic FeAl phase. It is evident that when laser beamtraversed at high scanning speed (400 mm/min), it resulted in Alrich phases and lower speeds produced alloys richer in Fe. It couldbe due to the fact that slow scanning had more time to dissipatemore heat and hence could involve relativelymore base (pure Fe) tomelt and mix with the same available amount of Al contentresulting in relatively low wt% of Al in the melt. In addition, an

increase in laser power from 300 W to 500 W, the melt depth ofthe alloyed region was also found to increase from approx.200 mme350 mm. This result has been found to be in good agree-ment with the results reported earlier that melt depth increasewith the increase in laser power density. According to the binaryphase diagram of Fe and Al, there are two intermetallic phases onthe Fe rich part of the phase diagram, FeAl (B2 structure, 22e32wt%Al,) and Fe3Al (DO3 structure, 12e22 wt% Al). Fig. 4(a) shows theformation of pure FeAl phase at 500 W with a scanning rate of600 mm/min along with the superimposed elemental concentra-tion profile in the alloyed region. The detailed SEMeEDS analysis ofthe alloyed zone confirmed the presence of FeAl phase (Fig. 4(b)).The XRD pattern of the alloyed region showed peaks correspondingto FeAl (B2) phase (Fig. 4(c)). As laser surface alloying is associatedwith very fast cooling of the melted zone, this method usuallyproduces fairly small grain sizes in the alloyed region. The opticalmicroscopy of the alloyed zone showed relatively small FeAl grainsas shown in Fig. 4(d). The wide variation in grain size was expectedand was due to variation in cooling rate from surface to the core ofthe melted zone. These small grains have an additional advantageon the properties of FeAl coatings in terms of ductility and strengthunder service conditions. The conventional casting and extrusionsprocess always produce FeAl alloys with fairly large grain sizeswhich can severely affect the ductility and strength of these alloys.Detailed research efforts have shown that a reduction in grain sizeand controlling Al content in FeAl alloys can improve ductility aswell as strength in these alloys [18e20]. Morris et al. [19] haveshown that a reduction in grain size from 100 mm to 1 mm can

Fig. 5. (a) SEM image showing FeAl phase formed along with Fe3Al phase with cor-responding EDS analysis (inset) (b) XRD showing peaks corresponding to FeAl phase;at 500 W, 400 mm/min.

G. Sharma et al. / Intermetallics 18 (2010) 2124e2127 2127

improve the ductility of FeAl by w10%. In addition, the strength ofFeAl alloys has been found to increase by solute additions like Cr,Nb, Zr, C, B etc. Kratochvil et al. [21] have shown an improvement inthe strength of FeAl weld by the carbide precipitation in theweldedzones. However, in the present study, pure FeAl is producedwithout any solutes or precipitates, but with a small grain size.These refined grains are expected to show high strength andductility under service conditions.

With the reduction in the scanning rate to 400 mm/min, inaddition to mostly FeAl phase, few grains of Fe3Al were also foundto be present andwhich were confirmed by the EDS analysis shownas inset in Fig. 5(a). The elemental quantitative analysis of most ofthe grains showed Al in the range of 25e30 wt% corresponding toFeAl composition but few grains shows a dip in Al concentration upto 20e24 wt% corresponding to Fe3Al composition. Although thevol. fraction of these Fe3Al grains were much less and widelydistributed in the otherwise alloyed FeAl region. These Fe3Al grainscould be formed due to composition fluctuations during alloying.Fig. 5(b) shows the XRD pattern of the alloyed region shown in

Fig. 5(a). The Fe3Al (DO3) phase is nothing but long range orderingof FeAl (B2) phase and all the peaks from B2 structure in XRDpatterns overlap with the peaks from DO3, apart from few charac-teristic peaks like (111) from DO3 structure does not overlap withthe peaks from B2. However, the (111) peak from DO3 could not beidentified from XRD pattern (Fig. 5(b)), indicating that the volumefraction of Fe3Al phase formed during alloying was very small andalso it may not be completely ordered from B2. The present studyshows the feasibility of laser surface alloying process to producepure and crack free FeAl coatings on the substrate at optimum laserprocessing conditions.

4. Conclusions

Laser surface alloying technique was used to produce FeeAlcoating on pure iron substrate coated with pure aluminum. Byvarying output power of laser, FeAl3, Fe2Al5 and FeAl intermetallicphases were found to be present. At 300 W power with 400 and200mm/min scanning speed, FeAl3 and Fe2Al5 intermetallic phaseswere formed with deep cracks at the interface due to brittle natureof Fe2Al5. However, at 300 W power and 100 mm/min scanningspeed, FeAl3 and FeAl intermetallic phases were formed. At 500 Wpower, a pure and uniform FeAl phase was formed at 600 mm/minwhereas at 400 mm/min FeAl along with few grains of Fe3Al wasfound to be present.

Acknowledgements

The authors would like to thank Dr. J. K. Chakravartty, Head,Mechanical Metallurgy Section, Dr. G.K. Dey, Head, Material ScienceDivision and Dr. A.K. Suri, Director Materials Group for theencouragements and support in carrying out this work.

References

[1] McKamey CG, DeVan JH, Tortorelli PF, Sikka VK. Journal of Materials Research1999;6:1779.

[2] Houngninou C, Chevalier S, Larpin JP. Applied Surface Science 2004;236:256.[3] Sanchez L, Bolıvar FJ, Hierro MP, Perez FJ. Intermetallics 2008;16:1161.[4] Qureshi KA, Hussain N, Akhter JI, Khan N, Hussain A. Materials Letters

2005;59:719.[5] Naoi D, Kajihara M. Materials Science and Engineering A 2007;459:375.[6] Shahverdi HR, Ghomashchi MR, Shabestari S, Hejazi J. Journal of Material

Processing Technology 2002;124:345.[7] Xiao-lin Z, Zheng-Jun Y, Xue-Dong G, Wei C, Zhang, Ping-Ze. Transactions of

Nonferrous Metals Society of China 2009;19:143.[8] Pal Dey S, Deevi SC. Materials Science and Engineering A 2003;355:208.[9] Abound JH, Rawlings RD, West DRF. J Mat Sci 1995;30:5931.

[10] Zhang Y, Pint BA, Garner GW, Cooley KM, Haynes JA. Surface and CoatingsTechnology 2004;188e189:35.

[11] Kwok CT, Cheng FT, Man HC. Surface and Coatings Technology 2006;200:3544.

[12] Xiang ZD, Rose SR, Datta PK. Intermetallics 2009;17:387.[13] Stringer J. Materials Science and Engineering 1987;87:1.[14] Sivakumar R, Rao EJ. Oxidation of Metals 1982;17:391.[15] Wang TH, Seigle LL. Materials Science and Engineering A 1989;108:253.[16] Forcy KS, Ross RK. Journal of Nuclear Materials 1991;182:35.[17] S.C. Deevi, S. Gedevanishvili, S. Pal Dey, US Patent 6489043, Dec 2002.[18] Morris DG, Morris-Munoz MA. Intermetallics 1999;7:1121.[19] Chao J, Morris DG, Morris-Munoz MA, Gonzalez Carrasco JL. Intermetallics

2001;9:299.[20] Morris DG, Morris-Munoz MA, Chao Jesus. Intermetallics 2004;12:821.[21] Kratochvıl P, Neumann H. Intermetallics 2009;17:378.