In situ production of Fe–TiC surface composite coatings by tungsten-inert gas heat source

In situ production of Fe –TiC surface composite coatingsby tungsten-inert gas heat sourceX.H.Wang a,⁎,M.Zhang b ,Z.D.Zou a ,S.L.Song a ,F.Han c ,S.Y .Qu aaSchool of Materials Science and Engineering,Shandong University,Jinan 250061,China bSchool of Mechanical Engineering,Shandong University,Jinan 250061,ChinacDepartment of Mechanical and Electrical Engineering,College of Weifang,Weifang 261021,ChinaReceived 28June 2005;accepted in revised form 26September 2005Available online 2November 2005AbstractIn the present study,AISI 1045steel surfaces were alloyed with pre-placed graphite,ferrotitanium and Fe –Cr –B –Si powders by using a tungsten-inert gas (TIG)heat source.The effects of welding parameters and thickness of the pre-placed powder layers on the microstructure and properties of the coatings were also investigated.The results indicated that TiC particles can be obtained by direct metallurgical reaction between ferrotitanium and graphite during the TIG welding process.Most of TiC particles were uniformly distributed in the surface coating.The microhardness showed a gradient variation from the molten boundary to the top surface of the coatings,and it was influenced by the thickness of the pre-placed powder layer and the welding parameters.The surface composite coating exhibited a higher hardness and lower wear rate than that of the substrate due to the formation of TiC carbides.©2005Elsevier B.V .All rights reserved.Keywords:In situ TiC particles;Wear;Microstructure;In situ formation;Tungsten-inert gas1.IntroductionThe particle-reinforced metal matrix composites (MMCs)have demonstrated a high potential for manufacturing of wear and corrosion-resistant surface layers due to the ease of fabrication,lower costs,and more isotropic properties.In all of ceramic particles,TiC with high hardness and good thermal stability is a very good reinforcement in Fe-based composites,and Fe –TiC composites have been applied in wear resistance parts and high performance tooling [1–3].Traditionally,the majority of reinforced phases are directly added into coating materials [4–6].However,ceramic particles being added directly may partially dissolve during the melting process.When the surface of particles is not clean or is polluted,crack may propagate from the interface between the particles and matrix.In recent years,a novel technique,in situ formation method,has been extensively studied to produce composite materials [7–11].In this method,reinforcements are formed inthe matrix by reacting between added pure elements.The advantage is that this method eliminates interfacial incom-patibility of matrix with reinforcements.Therefore,the rein-forcements may be more compatible with the matrix.The service life of components usually relies on their surface wear resistance,and it is desirable that the surface layer of components is reinforced by ceramic particles to offer high wear resistance whilst retaining the high toughness and strength.Recently,surface modification techniques,such as laser beam [12],plasma spraying [13]and flame spraying [14],have been used to produce Fe –TiC surface composite coatings.However,for laser cladding,problems always exist owing to differences in the laser beam absorption rates of different cladded powders [15].Furthermore,comparing complex components using the tungsten inert gas (TIG)welding process is expensive and difficult.Interestingly,TIG heating source has a potential to be used for surface modification,however,very limited scientific information is available pertaining to the preparation of Fe-based surface composite coatings by this process [16,17].The aim of the present investigation was to prepare a Fe-based alloy surface composite coating reinforced byTiC⁎Corresponding author.Tel.:+8653188392208;fax:+8653182616431.E-mail address:xinhongwang@.ch (X.H.Wang).0257-8972/$-see front matter ©2005Elsevier B.V .All rights reserved.doi:10.1016/j.surfcoat.2005.09.021particles on AISI1045steel by means of a TIG heat source under a non-oxidizing atmosphere.TiC particles were synthesized by an in situ reaction of graphite and ferrotitanium powders during the TIG welding process,rather than the TiC particles being directly added into the welding pool.2.Materials and experimental procedureAISI1045steel specimens with dimensions of100×25×10 mm were used for substrate.The surfaces of the samples were thoroughly cleaned dried and finally rinsed by acetone.A powder mixture of Fe-based self-fluxing alloy,ferrotitanium (FeTi)alloy and graphite(99.5%purity)was used as the coating alloy.The ferrotitanium and graphite mixed powder was40vol.%.The ratio of FeTi alloy to carbon powder corresponds to that of stoichiometric TiC.The average size of the FeTi and graphite particles was less than10μm.The Fe-based alloy powder possessed an average size of20μm.The main chemical compositions of the Fe-based alloy,FeTi and substrate steel are listed in Table1.The first step of the cladding process was to mix powders to prepare the cladding materials in an attrition-milled machine for 1h.Then,the blended powders were mixed with a small amount of sodium silicate to keep the powders on the surface under the flow of argon during arc-melting.Finally,the powders mixed with sodium silicate were pre-placed on the substrate and dried in hot air.The pre-placed thickness of the coatings(δ)was about0.6–2.0mm.Cladding was carried out using a TIG heat source to produce a series of single clad tracks without overlap.A tungsten electrode with a diameter of2mm was used to create an arc between the tip of the electrode and the specimen surface.The TIG arc used for glazing action was produced with an operation current(I)ranging from90to180A.The voltage(U)and welding speed(v)were chosen as15–17V and70–85mm/min, respectively.The electrode height from the substrate surface was about2mm and the electrode polarity was chosen as DCSP. During the welding,surface oxidation was prevented by pure argon with the flowing rate of8l/min.The microstructure and compositions were analyzed by using a JXA-840scanning electron microscopy(SEM)and a JXA-8800R electron microprobe microanalysis(EPMA), respectively.Phase identification was carried out via D/max-Rc X-ray diffractometer with Cu-Kαradiation operated at60kV and40mA.The volume fraction of TiC particles in the MMC coatings was measured by an XQF-2000computerized image analyzer.The block-on-ring wear testing was carried out without lubrication at room temperature using a friction and wear tester (model MM200,China).The tester is composed of transfer motion device,frictional torque measuring device,and loading device in which a lever system(the ratio of the lever is1:3)was adopted for the sake of relatively stable loading force(as shown in Fig.1).The contact pressure of the two samples was applied by the dead weight.The frictional torque was measured by the swing-lever weight regulating system.The ring material of the wear couple was a cermet containing 92wt.%WC and8wt.%Co.The outer radius of the circular test ring is20mm,and its width is10mm.The test specimens were machined to block with size of30×10×10mm.Before the wear testing,the surfaces of specimens were ground with silicon carbide paper,and then polished with cool polishing powder solution.The alumina slurry was used for the specimen polishing.The specimens were then ultrasonically cleaned in alcohol and acetone until the surfaces were spotless.The wear conditions were a normal load of100N,a sliding speed of0.84 m s−1and a sliding distance of750m.The average width of the wear track was measured with the help of a microscope,and the wear volume was calculated using the following formula[18]:V¼w r2sin−1b2r−b2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2−b24s()ð1Þwhere,w is width of the specimens(mm),b is width of the wear track(mm)and r is out radius of wear ring(mm).The wear volume loss and friction coefficient were calculated after aTable1Chemical composition(wt.%)of the specimen and powders for surface alloying Material Chemical composition1045steel0.45C,0.25Si,0.66Mn,bal.Fe Ferrotitanium41.5Ti,0.08C,0.035P,0.025S,bal.Fe Fe-based alloy0.1∼0.2C,8.0Ni,18Cr,2.5B,3.0Si,1.0Mo,1.0Mn,1.0V,bal.FeFig.1.A schematic diagram of wear testing:(a)scheme of MM200model friction and wear tester;(b)worn surface of specimen;(1—specimen;2—cermet ring;3—support axes;4—support;5—lever;6—spring;7—weight).TiCγ-Feα-FeM7C3FeB1.00k204060801001202 / degreeθCPSMMMMMGGGGNNNNNNLL L LIIIFig.2.XRD spectrum of the TIG processed composite coating(I=150A,U=15 V,v=78mm/min,δ=1.2mm).6118X.H.Wang et al./Surface&Coatings Technology200(2006)6117–6122certain time interval.The coefficient of friction was calculated according to the following formula:l¼TrPð2Þwhere,μrepresents the friction coefficient,T stands for the average frictional torque(N mm),P is the normal load(N) applied to wear specimens,and r is the outer radius of the ring(mm).3.Results and discussion3.1.Microstructure of coatingThe X-ray diffraction pattern of the composite coating isshown in Fig.2.It shows that the phases of the composite coating are mainly TiC,α-Fe,γ-Fe,(Fe,Cr)7C3and FeB.It clearly confirms that TiC phase can be synthesized by using TIG heat source.Fig.3(a)–(d)shows the phase distribution in the top surface of the composite coating based on the results of EPMA analysis for elements[Ti],[C]and[Cr].Fig.3(e)shows the energy dispersive spectrograph(EDS)result of[A]carbide in Fig.3(a). EPMA results revealed that carbides contained mainly C and Ti elements;however,Cr was mostly dissolved in the matrix.It is thus deduced that most of the Ti combined with carbon to form carbides.Fig.4shows the distribution of carbides in the top surface of the composite coatings produced by TIG welding process.It is found that TiC particles with average size1–4μm are distributed uniformly in the matrix.These fine TiC particles act as reinforcement and are expected to improve the tribological properties of the coatings significantly.The uniform distribution of the particles may attribute to the interaction between the particles and the advancing solid/liquid interface.This interac-tion may lead to trapping or pushing of particles by the solid/ liquid interface.Trapping may bring about a uniform distri-bution of particles within interdendritic zones whereas pushing results in the segregation at zones finally solidified.Addi-tionally,rapid solidification can also effectively decrease coarsening of particle which is detrimental for the structure-sensitive properties of the coatings.Fig.5shows the volume fraction of TiC particles distribution along the coating depth based on the observation of image analyzer.It indicates that the volume fraction of TiC particles possesses a gradient distribution as a function of the coating depth.The average volume fraction of TiC in the whole coatings is about13.4%.This distribution may be attributed to the differential—pared with the Fe-based(a)(b)10μm10μm A(c)(d)10μm10μm(e)Fig.3.EPMA area for scanning of elements in the clad coating:(a)EPMA morphology;(b)Ti;(c)C;(d)Cr;(e)the EDS result of[A]carbide phase(I=150 A;U=15V;v=78mm/min;δ=1.5mm).Fig.4.Distribution of carbides in the top surface of the composite coatings (I=150A,U=15V,v=78mm/min,δ=1.5mm).VolumefractionofTiCCoating depth /mmFig.5.Gradient distribution of TiC particles with the coating depth(I=150A, U=15V,v=78mm/min,δ=1.5mm).6119X.H.Wang et al./Surface&Coatings Technology200(2006)6117–6122alloy,the relative lower density of TiC particles (4.94g/cm 3)tended to segregate to the upper regions in the coating,which causes a gradient distribution on a macro-scale.Microhardness of the clad coatings was measured along the coating depth from the surface;all of the data were an average of three measurements.The typical microhardness profile of the coating along the coating depth is shown in Fig.6.It can be seen that the microhardness of the coating gradually increased with an increase of distance from the molten boundary of the coatings.The gradient distribution of TiC particles led to a gradual hardness distribution of the coating.3.2.Effect of thickness of the pre-placed powder layer Fig.7shows the general appearance of the coatings under the condition of welding current 150A and welding speed 78mm/min.The melted tracks were found to be free from gas porosity,crack and inclusions,when the thickness of pre-placed mixture powders layer is less than 1.5mm (as shown in Fig.6(a)–(c)).Melted tracks gave a smooth rippled surface topography.However,for the same heat input,when the pre-placed thickness is beyond 1.5mm,the formation of the tracks becomes poor and gas porosity and incomplete fusion can be found (as shown in Fig.7(d)and (e)).Fig.8shows the effect of the thickness of the pre-placed powder layer on the hardness.It can be seen that the hardness increases with an increase of the thickness.This may be ascribed to the dilution of the substrate.Dilution is defined as the change in chemical composition of a deposited metal caused by the admixture of the substrate in the weld bead.In general,the substrate is of less carbon and alloy content than the pre-placed powder layer.For the same heat input,the amount of elements in clad coating directly depends on the thickness of the pre-placed powder layer.As the thickness of the pre-placed powder layer increases,the amount of the elements in the clad coating coming from the powder layer increases.As a result,the hardness of the coating increased.However,due to the gas porosity and incomplete fusion in the coating,the hardness reduces when the thickness of the pre-placed powder layer is over 1.5mm.3.3.Effect of welding parametersFor the same thickness of the pre-placed powder layer,dilution rate is essentially governed by welding currents.0.10.30.70.50.9 1.1 1.3 1.5M i c r o h a r d n e s s H V 0.2Coating depth /mmFig.6.Microhardness of the clad coating vs.coating depth (I =150A,U =15V ,v =78mm/min,δ=1.5mm).(b)10mm(e)10mm(c)10mm10mm(a)(d)10mmFig.7.Melted track produced on the surface of specimen with a (a)0.9mm;(b)1.2mm;(c)1.5mm;(d)1.8mm;(e)2.0mm thickness of the pre-placed powder layer (I =150A,U =15V ,v =78mm/min).M i c r o h a r d n e s s /H V 0.2Thickness of the preplaced powder layer δ/mmFig.8.Effect of thickness of the pre-placed powder layer on the hardness (I =150A,U =15V ,v =78mm/min).Fig.9.A schematic diagram of the dilution rate for bead-on-plate weld.6120X.H.Wang et al./Surface &Coatings Technology 200(2006)6117–6122Dilution rate can be measured by the percentage of the substrate in the weld bead for bead-on-plate weld (as shown in Fig.9).It was calculated according to the following formula:d ¼BB þFÂ100%:ð3ÞIn the above equation,d represents the dilution rate;B and F are the areas of the melted metal of the substrate and the clad coating,respectively.The experimental data was taken as the mean value of more than three tests in the same conditions.Statistics showed that the error was less than 8%.Fig.10shows the effect of the welding current on the dilution rate of the coatings at 1.5mm thickness ofthe pre-placed powder layers.It is obvious that the dilution rate increases in proportion to an increase of welding current.Fig.10suggests that the control of welding current is very important in order to control the dilution rate that causes chemical composition of the coatings.A small welding current is helpful to reduce the dilution,but the formation of the track became poor due to the inadequacy of heat input (calculated by using the equation:heat input=(current×voltage×0.48)/travel speed [19]).Fig.11shows the effect of the welding travel speed on the microstructure of the clad pared with Fig.4,it is found that there are a few fine TiC particles in the coatings at a 3D i l u t i o n r a t e d /%Welding current I/AFig.10.The relationship between dilution and welding current (U =15V ,v =78mm/min,δ=1.5mm).(b)(a)7.5µmFig.11.Effect of processing speed on the microstructure of composite coating:(a)3mm/s;(b)0.5mm/s (I =150A,U =15V ,δ=1.5mm).W e a r v o l u m e / m m 3sliding distance /msliding distance /m0.00.51.01.52.02.53.0W e a r r a t e x 106 m m 3/( N m m ).Fig.12.The wear volume and wear rate vs.sliding distance (a)wear volume,(b)the wear rate (I =150A,U =15V ,v =78mm/min,δ=1.5mm).sliding distance /mF r i c t i o n c o e f f i c i e n t /µFig.13.The variation of the friction coefficient with the sliding distance (I =150A,U =15V ,v =78mm/min,δ=1.5mm).6121X.H.Wang et al./Surface &Coatings Technology 200(2006)6117–6122mm s−1welding travel speed.However,there are also a few coarse TiC particles in the clad coating at a0.5mm s−1welding travel speed.Rapid welding travel speed results in the small heat input and rapid solidification,the reaction between Ti and C is incomplete.However,for sallow welding travel speed,the heat input increases,there is enough time for a reaction between Ti and C,but TiC particles are easy to get together and then grow.Therefore,the welding travel speed has a key effect on the micro-distribution of TiC particles.3.4.Wear characterization of the coatingsCompared with the substrate and Fe-based alloy TIG clad coating,the wear volume and wear rate of TiC particle reinforced Fe-based composite coating at the same processing parameters as shown in Fig.12.It can be seen that the in situ TiC reinforced Fe-based composite coating is more effective in improving wear resistance and has the smallest wear volume and also the lowest wear rate with increasing sliding distance. The friction coefficient vs.the sliding distance at a normal load of100N is shown in Fig.13.It reveals that the friction coefficient of the in situ formed TiC particles which reinforced Fe-based composite coating is a little higher than that of the Fe-based alloy clad coating.Hence,the in situ TiC reinforced composite coating may enhance the hardness and wear resistance without evidently increasing the friction coefficient of the coatings.4.Conclusions1.TiC particle reinforced Fe-based alloy surface compositecoating has been successfully produced by using tungsten-inert gas welding process.The dispersed TiC particles are formed by in situ reacting ferrotitanium and graphite during TIG welding process,instead of TiC particles being added into welding pool directly.2.The microhardness of the coatings reveals a gradientvariation.It was affected by the thickness of the pre-placed powder layer and welding parameters obviously.When welding travel speed and thickness of the pre-placed powder are certain,the hardness of the clad coatings reduces with an increase of welding current.3.The in situ TiC reinforced composite coating enhanced thehardness and wear resistance without an evident increase in the friction coefficient of the coatings. 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