A detailed study on the  processing  of Al-metal matrix comp, cites with the reinforcement  of  different parti-culates such as SiC, TiN and Ti02  was  carried out. The 

Al-SiC composite  powder  prepared  through atomisation 

route is expected to limit the interfacial reaction bet-ween the matrix and the ceramic  particulate and help in making customer tailored components by powder metallurgy route. An attempt to  avoid the interfacial reaction of matrix and ceramic is investigated by impregnating the Al matrix with TiN particulates. Similarly, Al-Ti02.composite is investigated by new spray forming technique, where the interfacial reaction between Al and Ti02 is  expected  to form intermetallic compounds and improve the bonding and

mechanical properties. The results of the present studies show that the Al based composites prepared through various techniques exhibits  excellent   mechanical,  physical and tribological properties  and  could emerge  as  promising materials for defence, aerospace  and  other  engineering

applications

Chaudhury, S K

Pathak, L C

Ray, Ajoy K

Venkateswarlu, K

English

eprints@NML

K. Venkateswarlu , A. K. Ray, S. K. Chaudhury and L. C. PathakDEVELOPMENT OF ALUMINIUM BASED METALMATRIX COMPOSITESK. Venkateswarlu , A. K. Ray, S. K. Chaudhury and L . C. PathakNational Metallurgical Laboratory, Jamshedpur - 831007, IndiaABSTRACTA detailed study on the processing of Al-metal matrix comp, cites with thereinforcement of d ferent particulates such as SiC, TiN and Ti02 was carriedout. The Al-SiC composite powder prepared through atomisation route isexpected to limit the interfacial reaction between the matrix and the ceramicparticulate and help in making customer tailored components by powdermetallurgy route. An attempt to avoid the interfacial reaction of matrixand ceramic is investigated by impregnating the Al matrix with TiNparticulates. Similarly, Al-Ti02.composite is investigated by new sprayforming technique, where the interfacial reaction between Al and Ti02 isexpected to form intermetallic compounds and improve the bonding andmechanical properties. The results of the present studies show that the Albased composites prepared through various techniques exhibits excellentmechanical, physical and tribological properties and could emerge aspromising materials for defence, aerospace and other engineeringapplications.Key Words : Al-Metal matrix composites, Atomisation, Powder metallurgy,Spray forming.INTRODUCTIONMaterials that have superior physical, mechanical and tribological properties aredesired for various engineering applications . The materials with less weight and highstrength are considered as important materials for their potential applications andcomposites are no exception . In this regard, extensive research work is being carried outfor the past two decades to develop wide range of metal matrix composites (MMCs) asalternatives to the conventional engineering alloys and the same has showed tremendouspromise and phenomenal growth ( 1-4). Moreover, introducing the brittle ceramic materialsas reinforcement showed improved mechanical properties of MMCs (5-7). The particulatereinforced MMCs are of more interest due to simple preparation techniques , lower costand more isotropic properties . In addition , it is also shown that the properties of MMCsare controlled by the size and volume fraction of the reinforcement phases as well as bythe nature of the matrix - reinforced interface. A fine and thermally stable ceramicparticulates distributed uniformly in the matrix has led to the optimum set of mechanicalproperties (3,7). Among the various MMCs that are being developed , aluminium basedmetal matrix composites have shown a continuous improvement of properties and emerging171Development of Aluminium Based Metal Matrix Compositesas promising futuristic materials for various engineering applications . Most of the researchon MMCs is being carried out to improve its mechanical properties by the incorporationof hard ceramic oxides / carbides / nitrides into the metal matrices . In this paper, thepreparations of SiC, TiN and Ti02 particulates have been incorporated to the Al matrixthrough air atomisation, powder metallurgy and spray deposition techniques respectively.EXPERIMENTAL DETAILSAl-SiC composites by air atomisation techniqueCommercial grade aluminium (99.8% pure) was taken as the base material and 10wt% of silicon was added to aluminium during melting. The addition of silicon toaluminium serves dual purposes that included the retardation of the reaction betweenliquid aluminium and silicon carbide at elevated temperatures (>750°C) and improvementof the flowability of liquid aluminium. It was understood that high flowability of liquidwas essential for its better atomisation. In order to prepare aluminium-silicon carbidecomposite, commercial grade(Grindwel Norton, India) a-SiC having particle size of lessthan 60 mm was used. Silicon carbide powder (I Owt%) was initially mixed with aluminiumpowder through mechanical mixing technique to give coating of Al on SiC powder forbetter wettability.The required quantities of aluminium was initially melted in an induction furnaceand 10 wt% silicon metal was added to the liquid aluminium. The temperature of themelt was maintained at --900°C. The Al and SiC powder mixture was preheated at --200°Cand subsequently added slowly to the molten Al-Si. During addition of powder mixturethe liquid was also mechanically stirred. A liquid metal temperature was maintaineddown to 900°C as high flowabiltiy is required for atomisation. The tundish along with thewhole nozzle assembly was heated to avoid the sticking and minimise the loss of heatduring pouring. The heat loss during pouring could lead to solidification and hinderedthe atomisation process by choking the orifice of the nozzle. A 4 mm diameter graphitenozzle was used for atomising the composite powder. The air pressure was maintained at50 psi during atomisation. The atomised powder was collected in a water-receiving tankand major portion of water in the composite powder was removed by the centrifugalfiltration. Rest (-5-7%) of water was removed during drying by a vacuum oven at -80°C.The atomisation technique is schematically shown in figure 1.Al-TiN composite prepared through powder metallurgy routeTi sponge (>98% pure) was nitrided at 1550°C for 45 min using nitrogen gas andthe lumpy mass is ground. The process of nitridation and grinding were repeated for 3times to obtain single phase TiN powder. Finally the TiN powder was pulverised in a cupmill using agate jar to reduce the particle sizes and again milled with copper powder (-300 mesh and 99.5% pure), which is added to increase the wettability of Al and TiN. Theatomised Al powder (99% pure) having a particle size of less than 53 .im was mixed withTiN and Cu. The powder mixture was ball milled for 12h and subsequently compacted inthe form of cylindrical pellets (2-cm diameter and I- cm height) by applying a load of 250MPa. The pellets were sintered at 450°C for lh and subsequently hot pressed at 450°C for15 min. The details of the experimental conditions are given in Table-I.172•K. Venkateswarlu , A. K. Ray, S. K. Chaudhury and L . C. PalhakTable : 1 : Composition and Heat-treatment condition for the fabricationof Al-TiN compositesSample Composition (Wt %) Processing ConditionAl TiN Cu Sintering Hot pressingA 86 10 4 723 K for 30 ins. 723 K for 15 min.in N2 atmosphere under a load of 250-MPaB 66 30 4 723 K for 30 min. in 723 K for 15 min. underN2 atmosphere a load of 250 MPaPreparation of Al-TiO2 composite by spray forming techniqueThe schematic diagram of the spray forming technique employed is shown in figure 2.The procedure for melting was similar to the atomisation technique. The atomisation unitconsisted of two metallic tubes being placed concentrically inside the nozzle. The outertube was fitted to the atomiser, while the inner tube was suspended by fixing it on thesteel frame with a screw and jack arrangement, so that the tube could be concentricallymoved to the desired position. Mild steel and inconel tubes were used as outer and innertubes respectively. Metallic tubes were selected due to their high thermal shock resistanceas compared to the commonly used ceramic nozzles. Further, the same set of tubes couldbe reused. Care was taken to prevent the reaction of melt with the tubes, by providingboron nitride coating on the surfaces of the tubes. It was found that this coating not onlyhelped in preventing any reaction between the melt and the tubes, but also erosion has notbeen taken place during the preparation of the alloy. This was due to poor wettabilitybetween Al melt and the boron nitride coating. The end of the tubes was narrowed in theform of cone at the orifice. Two sets of such concentric tubes were made with varyingouter and inner diameter, so that the flow rates of the melt and particles could be varied.This in turn helped in varying the volume fraction of the material passing through thetube. A funnel type arrangement was made on the inner tube so that the ceramic particles(rutile in the present case) could be easily pass . The deposition was carried out on acopper substrate, which was fixed at 30cm below the nozzle. The substrate was rotated atdifferent speed and the speeds of rotation were optimised by measuring the density of thecomposite preforms for one set of tube.The average size of the rutile particles was 113µm with a standard deviation of38µm. About 1.5 Kg of aluminium was melted in a resistance furnace and degassed byadding hexachloroethane. Prior to atomisation Mg (2wt%) was added to the melt to increaseits flowability and the wettability of the melt with the ceramic particles (Ti02). The meltand the preheated (200°C) rutile particles were atomised simultaneous through outer andinner tubes respectively. The melt was atomised by using argon gas, so that fine dropletsand the rutile particles were dispersed uniformly. The resultant composite mixture (mushydroplet and rutile particles) was subsequently deposited on a rotating copper substrate,placed vertically below the atomiser. The whole atomising assembly (graphite crucible,atomiser and nozzle) was preheated to 600°C to avoid choking of melt at the orificeduring atomisation.173Development of Aluminium Based Afetal Matrix CompositesLIQUID METALNOZZLEVACUUMDRIERACENTRIFUGEFig. I : Schematic diagram of air atomisation process► Inner concentric tube► Ceramic powder► Crucible► Liquid metalFig. 2 : Schematic diagram of spray forming set-up for preparation of Al-Ti02composites174K. Venkateswarlu , A. K. Ray, S. K. Chaudhury and L. C. PathakRESULTS AND DISCUSSIONAI-SiC compositesThe particle size analyses of the mechanically mixed SiC and aluminium (50:50)powder (before addition to molten aluminium ) was found to be in the range of 30 to50 µm. The XRD results confirmed the presence of SiC and aluminium in the mixedpowder, Since SiC was coated with aluminium , the apparent intensity of SiC peaks werelower than expected (Fig. 3).The atomised powders (composite mixture of aluminium and silicon carbide) aswell as cold compacted sample were examined by using scanning electron microscopeand EDX. During SEM studies, it was observed that most of the silicon carbide particleswere uniformly distributed. The presence of Si was detected by the EDX analysis, whereas,X-ray diffraction analyses indicated the presence of SiC in the powder (Fig. 3). The EDXobservation in conjunction with the X-ray phase analyses confirmed the dispersed blackparticles as SiC. The average particle size of the atomised powder was observed to bearound 20-60 micrometer.400300100020JLJI I I I40 602680Fig. 3 : XRD pattern of Al-SiC Composite powder175IlotDevelopment of Aluminium Based Metal Matrix CompositesThe microstructural examination of the sintered composite samples showedthat the SiC particles were well distributed in the matrix. It was also been observedthat the bonding between the aluminium particles was not proper in the samplessintered at 450°C, whereas the silicon carbide particles were well bonded with thealuminium particles. To improve the bonding among the aluminium particles, thesamples were hot pressed at 450°C. This leads to improvement of the hardness anddensity values (Table 2).Table 2 : Density and hardness values of various stages of Al-SiC composite.Item GreencompactSintered(450°C)Hot Pressed(450°C, 260 MPa)Density 2 .4 gm/cc 2 . 3 gm/cc 2 . 7 gm/ccHardness 48 VPN 50 VPN 55 VPNIt was observed from the results that the density of sintered sample was apparentlylower compared to the green compact sample. This could be probably due to the presenceof binding material e.g. PVA, which might have covered the open porosity in the case ofgreen compacted material. During sintering at 450°C PVA was removed and left highporosity in the sintered sample. As a result, density of the sintered sample was loweredthan the green compact samples. However, hardness of these samples was more or lesssame while those in case of hot pressed sample increased substantially.Al-TiN compositesThe fabrication of SiC reinforced Al composites is also beset with the problem ofinterfacial reaction between Al and SiC forming A14C3. The aluminium carbide has poormechanical properties and severe corrosion problem (8). If long periods of liquid metalcontact with the reinforcing SiC phase were avoided, Al-SiC composites was found toexhibit excellent properties. However, often-long periods of processing were required atelevated temperatures during composite fabrication and resulted in interfacial reactionsand mechanical failure. To avoid this, Al-MMCs by reinforcing Al with TiN using PMroute was prepared at NML. The titanium nitride (TiN) is a technologically importantmaterial due to its high hardness, excellent stability at high temperatures, good electricaland optical properties and inertness towards Al metal (9). The titanium nitride basedalloys were presently being used in cutting tools, solar-control films and othermicroelectronic applications. Titanium nitride is thermally stable upto 3300 K andchemically stable to the most of the common etching solutions. Titanium nitride has anexcellent diffusion barrier against most of metals.The XRD analyses of the hot pressed samples A and B revealed the presence of Al,TiN and AI2Cu in the compacts and indicated that there were no significant reactionbetween TiN and Al or TiN and Cu. The density of sample `A' increased from 2.7 to 2,9gcm'3 after hot pressing of the sintered pellets, Whereas the density of the sintered sampleB increased from 2.8 to 3.0 gcm-3 after hot pressing. The maximum densities attained by176K. Venkateswarlu , A. K. Ray, S. K. Chaudhury and L .C. Pathakthe sample A and B after hot pressing are 98% and 91% respectively . The observed lowdensities in the sample B was possible due to the presence of excess amount of secondaryphases in the matrix , which was not participating during sintering and also hindered themass transport processes during sintering . The microstructural analyses of the samplesexhibited that the TiN particles were uniformly distributed along the grain boundaries inthe hot pressed AI - IOTiN samples ( Fig. 4a). Needle shaped precipitations were observedin the AI-30TiN samples ( Fig. 4b ) and that the composition of these grains were detectedto be A12Cu from the EDX analyses. The fabricated composites also exhibited excellentmechanical properties ( 10). The higher volume fraction of Al-30TiN exhibited betterwear properties as compared to Al-IOTiN . Similarly, both the composites showed betterwear response than pure Al (Fig. 5).(a) (b)Fig. 4 : SEM images of the hot pressed sample; (a) showing the distribution of TiNparticles at the grain boundaries and (b) showing the needle shaped Al2Cu particles3.02.5.,E 2.0E-.-- Al- 0 al30tinA_ #1 Otin(A)(B)0.00 200 400 600 B00 1000 1200 1400 1600Distance (m)^T. T.T.TT TT TFig.S : Wear rate of (a) Al, (b) At-10771N and (c) AI-30TW177Development of Aluminium Based Metal Matrix CompositesAI-TiO2 CompositesThe microstructural investigations of over sprayed powder showed that some ofrutile particles were engulfed by the liquid aluminium during atomisation and resulted inthe formation of a composite powder. One of such composite powder is shown in figure6a. Both the over sprayed powder Al-2Mg powder (Fig. 6b) and composite powder haverevealed dentritic structure. This was in contrast with the as spray formed composite,which revealed fine equiaxed structure observed during macrostructural observations (1 1 ).(a) (b)Fig. 6: Optical micrographs of over sprayed powder (a) Al-rutileand (b) AI-2Mg alloy X500The evolution of the fine equiaxed structure in as-sprayed deposited perform couldbe attributed to the generation of large number of nucleation sites owing to the turbulentfluid flow condition during deposition of the mushy droplets (12). The microstructure ofas spray formed composite at two different locations (center and periphery of the preformrespectively ) is shown in fgure 7a and 7b . This shows the uniform distribution of dispersoidwith good bonding between the matrix and the particles . Presence of voids in the range of40-70 µm in size was also observed . This is one of the major limitations of the sprayforming process (12).(a) (b)Fig. 7: SEMphotomicrograph of as spray formed A1-2Mg-11 Ti02 composite atlocations a) centre and h) at periphery178K. Venkateswarhi , A. K. Ray, S. K. Chaudhury and L . C. PathakBack scattered elctron image and lines of Al, Ti and Mg of spray formed compositeare shown in figure 8 a,b. At the particle/matrix interface, A1203 phase was formed in theform of a thin layer. This was probably due to the ex-situ reaction between the Al meltand Ti02 particles. Further distribution of Mg was observed to be uniform through outthe matrix. Higher cooling rates involved in the spray forming process might be preventedthe preferential segregation of Mg at the interface. In all the three line profiles, deflectionin the concentration of the constituents was noted at some places due to the presence ofporosity.TiK2( ctrlavICVil-Ci7^7KID XL)amAW a.1199Mg7,a1., U(a) (b)Fig. 8 : (a) Back scattered image of Al-2Mg-7Ti02 spray formed and line profile of Tiand (b) line profile of Al (top) and Mg (bottom)CONCLUSIONSI Al-SiC composite powder has been successfully prepared by atomisation technique.2. Aluminium coating on silicon carbide particles was responsible for the betterwettability.3. Preparation of the Al-SiC composite by the atomisation technique can eliminatenumber processing steps as compared to other methods.4. The SiC particles are almost uniformly distributed through out the matrix.5. Al-TiN composites were successfully fabricated by powder metallurgical route6. No significant interaction between the matrix and reinforcement is observed duringsintering and hot pressing7. Fabricated Al-TiN composites show improved densification only after hot pressing,which also increase the hardness8. Incorporation of TiN particles in the Al matrix improves the mechanical propertiesand wear resistance9. Spray forming of Al-2M b 7TiO2 showed better particle/matrix bonding and higherdegree of uniformity in the distribution of rutile particles in the matrix10. cold rolling of spray formed AI-TiO2 composites exhibited improved hardness values11. The ultimate strngth of as-sprayed composite increased considerably on cold rolling.179Development of Aluminium Based Metal Matrix CompositesACKNOWLEDGEMENTSThe authors would like to thank Director, National Metallurgical Laboratory,Jamshedpur, for his kind permission to publish this paper.REFERENCES[1] P. Canon : J. Metals, May, 50 (1998) 10.[2] P. Niskanen and W. R. Mohn : Adv Mater. & Process. Inc. Metal Progress, 3/88 (1988) 39[3] D. J. Lloyd : Intl. Mater. Rev., 39 (1988) 1.[4] 1. B. Jeong, K. H. Oh, H. I. Lee, D. N. Lee, Y. K. Beeck : Proc. Conf of Interface scienceand engineering, Lake placid, New York, USA (July 1987) 13-16.[5] P. K. Rohalgi, R. Asthana and S. Das, Inter. Met. Rev., 31 (1986) 115.[6] S. Kohara : Mater. Manuf Process, 5(1) (1990) 51-62.[71 R. Asthana and S. N. Tewari, Composite Manufacturing (1993) 43.[8] D. J. Lloyd : Compos. Sci. Technol, 35 (1989) 159.[9] J. F. Lynch, C. G. Ruderer and W H. Duckworth: "Engg. Prop. of Selective CeramicMaterials" (1966).[10] A. K. Ray, K. Venkateswarlu, S. K. Chaudhury, S. K. Das, B. Ravi Kumar and L. C. Pathak,accepted by Mat. Sci. Engg., 338A (2002) 160.[11] S. K. Chaudhury, Ph.D thesis, Dept. of Metal. & Mater. Engg., Kharagpur (2002).[121 Laveria E. J, Inter. J. of rapid solidifcation, 5 (1989) 47.

Development of Aluminium based Metal Matrix Composites

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Development of Aluminium based Metal Matrix Composites

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