Composites: Part B 43 (2012) 302–308

Contents lists available at SciVerse ScienceDirect

Composites: Part B

journal homepage: www.elsevier .com/locate /composi tesb

Production and wear characterisation of AA 6061 matrix titanium carbideparticulate reinforced composite by enhanced stir casting method

S. Gopalakrishnan a,⇑, N. Murugan b,1

a Department of Mechanical Engineering, K.S. Rangasamy College of Technology, Tiruchengode 637 215, Tamil Nadu, Indiab Department of Mechanical Engineering, Coimbatore Institute of Technology, Coimbatore 641 014, Tamil Nadu, India

a r t i c l e i n f o

Article history:Received 12 March 2011Received in revised form 12 May 2011Accepted 22 August 2011Available online 7 September 2011

Keywords:A. Metal matrix composites (MMCs)B. WearC. Analytical modellingD. Mechanical testingE. Stir casting

1359-8368/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compositesb.2011.08.049

⇑ Corresponding author. Tel.: +91 4288 274710, m+91 4288 274860.

E-mail addresses: gopalasaravanan@gmail.com (S.cit.edu.in (N. Murugan).

1 Tel.: +91 422 2574071, mobile: +91 9751824123;

a b s t r a c t

Metal matrix composite (MMC) focuses primarily on improved specific strength, high temperature andwear resistance application. Aluminium matrix reinforced with titanium carbide (Al–TiCp) has goodpotential. The main challenge is to produce this composite in a cost effective way to meet the aboverequirements. In this study Al–TiCp castings with different volume fraction of TiC were produced in anargon atmosphere by an enhanced stir casting method. Specific strength of the composite has increasedwith higher % of TiC addition. Dry sliding wear behaviour of AMC was analysed with the help of a pin ondisc wear and friction monitor. The present analyses reveal the improved specific strength as well as wearresistance.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

1.1. Metal matrix composite

Researchers in aerospace, marine and defence applications re-quire a material which offers good specific strength and wear resis-tance. Composite plays a vital role in modern material science,particularly in all types of transportation, military and structuralapplications [1]. Researchers, especially in the defence application,are continuously striving hard to find the materials that suit theirspecific requirements. Improvement in production methods andfinding the alternate materials are a few options to meet the aboverequirement.

1.2. Aluminium matrix composite

Aluminium and its alloys play an important role in the produc-tion of MMC. AMC materials have greater advantages in a widenumber of specific fields due to their high specific strength, stiff-ness, wear resistance and dimensional stability. AMC can be fabri-cated in several ways based on its end use. Among aluminiumalloys AA6061 is quite a popular choice as matrix material. It is

ll rights reserved.

obile: +91 9344152299; fax:

Gopalakrishnan), murugan@

fax: +91 422 2575020.

primarily due to its better formability characteristics and optionof modifying strength of the composite by employing optimal heattreatment. Al–TiCp composite is not readily available in the marketand it is costly. This is due to the difficulty in producing this com-posite. Few attempts were made to produce it by in-situ process.

Stir casting technique is the conventional and economical wayof producing AMC. But, with the conventional stir casting tech-nique, it is difficult to produce a particulate reinforced composite[2]. In this present method (Fig. 1) suitable modifications were car-ried out on conventional stir casting method to take care of thereaction of molten aluminium with atmosphere, segregation ofreinforcing particles and wettability. Controlled bottom pouringarrangement helps to regulate the molten metal flow. Earlier [3],Magnesium alloy–SiCp reinforced infiltrated composite was pro-duced by infiltration process. But, when compared to the presentmethod, particle infiltration is relatively a difficult process. Wearperformance of TiC as reinforcement of a magnesium alloy com-posite is recently reported. [4].

1.3. Reinforcement

In particulate reinforced MMC, reinforcement is added to thematrix of the bulk material to increase its stiffness and strength[5–9]. Furthermore, the use of discontinuous reinforcement mini-mizes the problems associated with the fabrication of continuousreinforced MMC such as fibre damage, microstructural heterogene-ity, fibre mismatch and interfacial reactions. Applications whichare subjected to heavy loads, or extreme thermal fluctuations, such

Furnace

Portable stirrer

Argon cylinder

C.I. Die placed inside the pre-heater

Flow of molten AMC

(a)

(b)

Fig. 1. Enhanced stir casting facility used for the production of Al–TiCp composite.(a) Overall view and (b) bottom pouring arrangement.

S. Gopalakrishnan, N. Murugan / Composites: Part B 43 (2012) 302–308 303

as, in automotive components, the discontinuously reinforcedMMC is shown to offer near isotropic properties with substantialimprovements in strength as well as stiffness, relative to thoseavailable for the monolithic materials [10–12]. It was reported thatseveral key factors such as type, size and volume fraction of parti-cles as well as the interfacial particle/matrix bonding had a pro-nounced influence on the wear behaviour of the reinforcedcomposite [13].

Fig. 2. Crucible used for the production of Al–TiCp composite.

1.4. Wear characterisation on Al–TiCp composite

Wear study conducted by Tyagi [14] on Al–TiCp composite pro-duced by in situ process revealed that the wear rate decreased line-arly as the volume fraction of TiC increased from 7% to 18%. Ma et al.[15] reported that the wear resistance of the composites was abouttwo times of the unreinforced matrix alloy and increased withincreasing SiC particle content and size. Ramachandra and Rad-hakrishna [16] analysed the sliding wear patterns with the help ofoptical microscopy namely abrasion, oxidation, delamination, ther-mal softening, adhesion, etc. Yao et al. [17] reported that the wearrate of spray-deposited TiC/AZ91 Mg composite decreased withincreasing TiC content and the dominant wear mechanisms wereoxidative and delamination respectively at low and high loads.

Only a few works on the wear characterisation of Al reinforcedwith TiC composite have been reported so for. Casting defects arequite common in the above method. An attempt was made to pro-duce the AMC with different % of TiC reinforcement in an effectiveand economical method called enhanced stir casting process. Afterthe production of AMC, mechanical properties such as weldabilityand wear characteristics were analysed. Box-Behnken design wasadopted to predict the wear rate. Solid state welding processnamely, Friction stir welding (FSW) was done successfully on theAl–TiCp composite [18]. Joint efficiency was more than 95% forthe most of the joints produced. Weldability of the Al–TiCp

composite by FSW is better than any other fusion welding process.This increases the opportunity for designers to utilise this material.

2. Experimental procedure

2.1. Production of Al–TiCp composite by an enhanced stir castingmethod

2.1.1. MaterialAluminium AA6061 (Mg-1.2, Si-0.8, Fe-0.7, Cu-0.4, Mn-0.15,

Cr-0.35, Z-0.25 and Ti-0.15 by weight %) supplied by M/s. HindustanAluminium Company Ltd., India was selected as matrix material andTiC supplied by M/s. Alfa Aesar (India) was used as reinforcement.

2.2. Enhanced stir casting process

Al–TiCp composite was produced by an indigenously developedenhanced stir casting process as shown in Fig. 1. This process helpsto minimise the production related problems associated with theconventional stir casting process. Cleaned AA6061 was placed in-side the crucible. Temperature was set at 30 �C above the meltingpoint of AA6061. After the complete melting of aluminium, onceagain the temperature was raised to 50 �C further to compensatethe cooling effect produced due to stirring. TiC was introduced inthe form of capsule (TiC wrapped in aluminium foil) during themechanical stirring. Two step stirring method was adopted to en-sure a thorough mixing of reinforcement. Magnesium (1%) wasadded during the stirring to improve the wettability of TiC withthe molten aluminium. Argon gas was used to prevent reactionof molten aluminium with an atmosphere after the addition ofMagnesium. Difficulty associated with material pouring was alsodrastically reduced in this process.

The crucible used for the composite production is shown inFig. 2. Pre-heated cast iron mould was placed at the bottom ofthe furnace so that the molten material could be poured by a bot-tom tapping arrangement. Manual stirring was done during thepouring of molten material. The reason for the selection of low %of addition (3–7) is to have good weldability. Otherwise, formationof fine particles in the weld zone may offer brittle interface afterwelding. Visual inspection was carried out on the AMC to findout the surface defects. No surface defects were found. Presenceand distribution of TiC was checked and confirmed by doing XRDtests and Optical Microscopy [2].

2.3. Metallurgical and mechanical characterisation

Mostly AMC have very high wear resistance and are brittle tocut using standard saw blades and steel cutting tools. AMC plates

Fig. 3. AMC castings produced with different percentage of TiC addition.

304 S. Gopalakrishnan, N. Murugan / Composites: Part B 43 (2012) 302–308

of size 100 mm � 50 mm � 50 mm were produced using EDM asshown in Fig. 3. Scanning electron microscope (SEM) analysiswas carried out and it is displayed in Fig. 4. Samples for the tensiletests were prepared from the plates. From the plates the tensiletest specimens were prepared as per the ASTM E8 standards usinga milling machine. Three standard samples were tested by a uni-versal testing machine. From the observations, the tensile strengthand ductility were estimated. Effect of % TiC content on mechanicalproperties is shown in Fig. 5.

2.4. Wear characterisation on AMC

Effects of % TiC content on friction and the wear behaviour wasinvestigated at different sliding velocities and loads as per the De-sign of experiment (DoE). Cylindrical pins of size £ 6 mm andlength 50 mm were prepared from the castings to conduct thedry sliding wear test. Pins were prepared with flat contact regionand rounded corner. It was held stationary in the pin holder againstthe counter face of a 100 mm diameter rotating disc. Disc is madeof En-32 steel and has hardness value of 65 HRC. The pins werepolished using an emery paper of fine grade to remove surfaceirregularities. Both the disc and the pins were cleaned before car-rying out the test. The dry sliding wear behaviour of the AMCwas investigated by varying normal load, % TiC content variation

Fig. 4. SEM Photographs of Al–TiCp composite p

and track velocity. The wear tests were carried out as per the de-sign matrix provided in Table 1 using a Pin on disc machine asshown in Fig. 6.

The wear loss of the pin was recorded during each wear testusing an LVDT transducer of accuracy 1.0 lm. The pin was re-moved from the pin holder after each trail and cooled to room tem-perature. It was brushed lightly to remove loose wear debris andweighed in the balance having accuracy of 0.0001 g. It was fixedagain in the same position in the pin holder so that the orientationof the sliding surface remained unchanged. The measurement ofwear loss of the pin was taken to evaluate the volumetric wear loss(VL) during the wear test. Each trail was repeated three times andthe average value of volume wear loss after each interval of timewas considered for analysis of wear behaviour. The wear ratewas calculated by using the formula [19]

W ¼ V=qD; ð1Þ

where q is the density of material, V is the weight loss and D is thesliding distance. Specific wear rate w = W/L, where L is the appliedload. The calculated wear rates are presented in Table 1. The wornsurfaces of the specimens were examined under an optical micro-scope to understand the wear pattern of AMC.

3. Mathematical model on wear rate

The function of wear rate can be expressed as

W ¼ f ðP; F;VÞ ð2Þ

where P is % TiC, F is normal load, and V is track velocity.The second order polynomial (regression) equation used to rep-

resent the response surface ‘Y’ for k parameter is given by

Y ¼ Y ¼ b0 þXk

i¼1

biXi þXk

i¼1i–j

biiX2i þ

Xk

i–j

bijXiXj ð3Þ

The selected polynomial for three factors could be expressed as

roduced by enhanced stir casting method.

Fig. 5. Effect of TiC addition on the mechanical properties of AMC. (a) Tensile strength and (b) ductility.

Table 1Parameters used in mathematical model for wear analysis.

S. No. P (%) F (N) V (m/s) Average weight loss (mg) Wear rate � 10�6 mm3/m Specific wear rate � 10�6 mm3/m N

1 3 9.81 3 0.01195 2.159 0.222 7 9.81 3 0.30000 52.5 5.353 3 49.1 3 0.34020 61.47 1.254 7 49.1 3 0.36160 63.28 1.295 3 29.44 1.5 0.22995 41.55 1.416 7 29.44 1.5 0.26655 46.65 1.587 3 29.44 4.5 0.25090 45.34 1.548 7 29.44 4.5 0.29655 51.9 1.769 5 9.81 1.5 0.00040 0.071 0.007

10 5 49.1 1.5 0.28415 50.52 1.0311 5 9.81 4.5 0.24905 44.28 4.5112 5 49.1 4.5 0.29320 52.13 1.0613 5 29.44 3 0.29820 53.02 1.814 5 29.44 3 0.27500 48.89 1.715 5 29.44 3 0.29885 53.14 1.8

S. Gopalakrishnan, N. Murugan / Composites: Part B 43 (2012) 302–308 305

Wear rate ¼ b0 þ b1P þ b2F þ b3V þ b11P2 þ b22F2 þ b33V2

þ b12PF þ b13PV þ b23FV ð4Þ

where b0 is the average of responses and b1, b2, b3, b12, b13, and b23

are the response coefficients that depend on respective main andinteraction effects of parameters. SYSTAT software was used to ob-tain the coefficients from the data provided in Table 1. The ade-quacy of the regression model was checked by calculating F ratioat 95% confidence level and was compared with the correspondingstandard tabulated value. The model was found to be adequate. Thecalculated value of correlation coefficient R2 is 0.836 which shows a

good fit of the experimental data. The developed regression modelonly with significant coefficients determined from student t test isgiven below:

Wear rate ¼ 44:46þ 7:976P þ 16:049F þ 6:857V

� 12:133FP ð5Þ

4. Results and discussion

Investigation was carried out to find the effect of TiC content onspecific strength and wear rate. The average tensile strength and

Fig. 6. Pin on disc arrangement used for the surface wear study.

4045505560657075808590

1.5 3 4.5

Velocity , m/s

Velocity Vs Wear rateF- 29.4 N, P - 5%

Fig. 8. Effect of sliding velocity on the wear rate.

306 S. Gopalakrishnan, N. Murugan / Composites: Part B 43 (2012) 302–308

ductility of AMC with different TiC additions are shown in Fig. 5a. It isevident from the figure that the specific strength of AMC has im-proved considerably with more addition of TiC. This is due to resis-tance offer by TiC during plastic deformation. Fig. 5b shows %elongation reduces slightly with more addition of TiC in the matrix.

4.1. Effect of content of TiC reinforcement on the wear rate

Fig. 7 shows the effect of volume fraction of TiC reinforcementon the wear rate. The wear rate marginally increases as the % addi-tion of TiC addition increases. Wear studies conducted by Tyagi[14] on Al–TiCp composite produced by melt reaction methodshowed that the wear rate was lower when compared to pure alu-minium and wear rate also decreased linearly with increasing vol-ume fraction of titanium carbide. Tyagi explained that it may bedue to the presence of hard TiC particles.

4.2. Effect of sliding velocity on the wear rate

Fig. 8 indicates the effect of sliding velocity on the wear rate. Asthe velocity increases the wear rate also increases. This may be dueto quick plastic deformation of material at surfaces and sub sur-faces when it is subjected to the sliding wear. At higher velocitiestemperature increases and thermal softening of matrix led to morewear rate.

4.3. Effect of normal load on wear rate

Fig. 9 shows the effect of normal load on the wear rate. Thewear rate increases steeply with the addition of normal force. Asthe normal load increases, the coefficient of friction also increasedwhich led to more wear rate. This is quiet a common phenomenon

40

50

60

70

80

90

100

110

3 5 7

% TiC

% TiC Vs Wear rateF-29.4 N, V- 3 m/s

Fig. 7. Effect of TiC reinforcement on the wear rate.

in most of the material. Further, the size of the wear debrisincreased with increasing loads and thereby, resulting in largewear loss at higher loads. Out of three parameters such as % TiC,sliding velocity and normal load, the sliding velocity has relativelylittle effect on the sliding wear rate. Normal load has the highesteffect on the sliding wear rate.

4.4. Response surface for interaction effect of normal force, % TiC onspecific wear rate

Effect of % TiC, normal force on wear rate is depicted in Fig. 10. Itis clear that the wear rate increases with the increase in normalload and % TiC. Maximum wear rate of 53 � 10�6 mm3/m was ob-served at 7% TiC (+1) and at a maximum load (+1) of 49.1 N. Com-parison on wear rate was made between the Al–TiC (7% TiCaddition) produced by melt reaction method and in the presentinvestigation. Wear rate in earlier study was 12 � 10�3 mm3/ m.But in the present investigation, the wear rate is very much lowerthan the earlier study. The reinforcement offers the best resultsonly beyond the threshold value [15]. Identifying this value is onceagain a big challenge to investigators. In surface wear test, smallersized particles in particulate reinforced composite will fracturequickly and come out of the matrix.

4.5. Optical microscopy

Examinations of wear surfaces of the pins reveal the wear type.Optical microscopy was carried out on an Olympus microscope ofmodel BX51 M to observe the microstructure of wear pin speci-mens using standard colour metallographic techniques. The brightfield was adopted for this study. The wear pattern of 3% TiC indi-cates that the sliding wear is due to plastic deformation. Fig. 11 re-veals that the scratches found on AMC containing 3% TiC arerelatively lesser.

050100150200250300350400450

9.81 29.44 49.1Normal load, N

Normal load Vs Wear rateP - 5%, V - 3m/s

Fig. 9. Effect of normal load on the wear rate.

Fig. 10. Response surface for interaction effect of normal force and % TiC on the specific wear rate.

10µm 10µm

Fig. 11. Optical micrograph of the wear test specimens (3% TiC).

10µm

10µm

Fig. 12. Optical micrograph of the wear test specimens (7% TiC).

S. Gopalakrishnan, N. Murugan / Composites: Part B 43 (2012) 302–308 307

Optical micrograph of wear test specimen after a wear test (7%TiC) is shown in Fig. 12. Parallel grooves and scratches were ob-served in the sliding direction. This was due to abrasion. It oc-curred primarily due to ploughing and wedge formation. Deepgrooves and delamination were also observed. Adhesion wear

was also noticed in the material at higher sliding speeds. But therewas less oxidation wear which was predominant in earlier studieson composites [14].

5. Conclusions

1. Defect free aluminium matrix TiCp reinforced composite wasproduced in improved stir casting method.

2. Specific strength of the material improved appreciably withmore addition of TiC. Percentage elongation was also main-tained at appreciable level even though specific strength wasincreased. Hence, this method is the most economical and effec-tive way of producing Al–TiCp composite.

3. It was clear from the mathematical model that the wear lossincreased linearly with the normal load. But, the wear rateincreased marginally with increased TiC addition.

4. Wear losses in terms of volume were found to be very less whencompared with earlier studies conducted on Al–TiCp producedby in situ process. Hence, it may be concluded that the presentmethod is better than any other method which is used for theproduction of Al–TiCp composite.

Acknowledgements

The authors wish to place their sincere thanks to Naval Re-search Board, DRDO, Govt. of India for funding to produce AMC,(vide funded project; Ref. No. DNRD/05/4003/ NRB/85 Dated30.10.2006,) and All India Council for Technical Education (AICTE),New Delhi for funding the material purchase and purchase of Wearand friction monitor (vide funded Project; File No. 8023/BOR/RID/RPS-195/2007-08.Dated March 5 2008). The authors are gratefulto The Department of Mechanical Engineering at Coimbatore

308 S. Gopalakrishnan, N. Murugan / Composites: Part B 43 (2012) 302–308

Institute of Technology, Coimbatore, India for extending the facili-ties of Welding Research Laboratory to carry out this investigation.They are also thankful to Prof. C. Narayanan, Mr. S.J. Vijay, and Mr.L. Ashokkumar for their assistance.

References

[1] Ceschini L, Minak G, Morri A. Tensile and fatigue properties of the AA6061/20Vol.% Al2O3p and AA7005/10 Vol.% Al2O3p composites. Compos Sci Technol2006;66:333–42.

[2] Gopalakrishnan S, Murugan N. Production of aluminium matrix TiCp compositeby modified stir casting process. World J Eng 2009;6(4):7–13.

[3] Muthukumar S, Dhindaw DK. Magnesium alloy – SiCp reinforced infiltratedcast composite. Mater Manuf Process 2007;22:429–32.

[4] Falcon-Franco L, Bedolla-Becerril E, Lemus-Ruiz J, Gonzalez-Rodríguez JG,Guardian R, Rosales I. Wear performance of TiC as reinforcement of amagnesium alloy matrix composite. Compos: Part B 2011;42:275–9.

[5] Srivatsan TS, Meslet Al-Hajri, Smith C, Petraroli M. The tensile response andfracture behaviour of 2009 aluminium alloy metal matrix composite. Mater SciEng 2003;346:91–100.

[6] Srivatsan TS. Microstructure, tensile properties and fracture behaviour of Al2O3

particulate reinforced aluminium alloy metal matrix composites. J Mater Sci1996;31:1375–88.

[7] Seah KHW, Sharma SC, Girish BM. Mechanical properties of as cast and heattreated ZA-27/graphite particulate composites. Compos Part A1997;28A:251–6.

[8] Kim YH, Lee CS, Han KS. Fabrication and mechanical properties of aluminiummatrix composite materials. J Compos Mater 1992;26:1062–86.

[9] Ramesh CS, Anwar Khan AR, Ramachandra A. Heat treatment of Al6061-10 wt.% SiC composites. In: Anwarul Haque AFM, Ahmed M, Mustafizul KarimAN, Dhar NR, Begum S. editors. Proceedings of the international conference onmanufacturing, ICM 2002, Dhaka 21; 2002.

[10] Surappa MK. Aluminium matrix composites: challenges and opportunities.Sadhana 2003;28:319–34.

[11] Feng CF, Froyen L. Microstructures of in situ Al/TiB2 MMCs prepared by acasting Route. J Mater Sci 2000;35:837–50.

[12] Srivatsan TS, Ibrahim IA, Mohamed FA, Lavernia EJ. Processing techniques forparticulate – reinforced metal aluminium matrix Composites. J Mater Sci1991;26. 5965–5678.

[13] Kennedy AR, Wyatt SM. The effect of processing on the mechanical propertiesand interfacial strength of aluminium/TiC MMCs. Compos Sci Technol2000;60:307–14.

[14] Tyagi Rajnesh. Synthesis and tribological characterization of in situ cast Al–TiCcomposites. Wear 2005;259:569–76.

[15] Ma ZY, Liang YN, Zhang YZ, Lu YX, Bi J. Sliding wear behaviour of SiC particlereinforced 2024 aluminium alloy composites. Mater Sci Technol1996;12:751–6.

[16] Ramachandra M, Radhakrishna K. Effect of reinforcement of fly ash on slidingwear, slurry erosive wear and corrosive behaviour of aluminium matrixcomposite. Wear 2007;262:1450–62.

[17] Yao Junping, Li Wen, Zhang Lei, Wang Fajun, Xue Mingshan, Jiang Hongliu,et al. Wear mechanism for in situ TiC particle reinforced AZ91 magnesiummatrix composites. Tribol Lett 2010;38:253–7.

[18] Gopalakrishnan S, Murugan N. Prediction of tensile strength of friction stirwelded aluminium matrix TiCp particulate reinforced composite. Mater Des2011;32:462–7.

[19] Navin Chand UK, Dwivedi SK, Acharya. Anisotropic abrasive wear behaviour ofbamboo (Dentrocalamus strictus). Wear 2007;262:1031–7.