Materials and Design 24(2003) 671–679

0261-3069/03/$ - see front matter� 2003 Elsevier Science Ltd. All rights reserved.doi:10.1016/S0261-3069Ž03.00156-0

Preparation and some properties of SiC particle reinforced aluminiumalloy composites

Y. Sahin*

Department of Mechanical Education, Faculty of Technical Education, Gazi University, Ankara 06500, Turkey

Abstract

Aluminium alloy composites containing various particle sizes of 10 and 20 wt.% SiC particles were prepared by molten metalmixing and squeeze casting method under argon gas. The stirring was carried out with graphite impeller during addition ofparticle. The molten mixture was poured into a die when the stirring was completed and metal matrix composites were producedby applying the pressure. Optical microscopic examination, hardness, density and porosity measurement were carried out.Moreover, metal matrix composites were machined at various cutting speeds under a fixed depth of cut and feed rate usingdifferent cutting tools. It is observed that there was a reasonably uniform dispersion of particles in the matrix alloy. The densitydecreased with decreasing particle sizes, but porosity decreased considerably with increasing particle size. In addition, the toollife decreased considerably with increasing cutting speeds for all tests. Among cutting tools, the wear resistance of Al O coated2 3

tools showed better performance than those of the other tools without chip breaker geometries in the machining of SiCp-reinforcedcomposites.� 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Metal matrix composite; SiC particle; Molten metal mixing; Porosity; Tool wear

1. Introduction

Aluminium matrix composites possess many advan-tages such as low specific density, high strength andgood wear resistance with the development of somenon-continuous reinforcement materials, whisker, fibresor particles. In particular, the particulate reinforcedaluminium matrix composites not only have goodmechanical and wear properties, but are also economi-cally viable w1–4x. Therefore, SiC-particulate-reinforcedaluminium composites have found many applications inthe aerospace and automotive industry.There are many methods for fabrication of particulate

reinforced metal matrix composites(MMCs) such aspowder metallurgyw5x, squeeze castingw6–9x, compo-castingw10–13x and so on. For the metal matrix com-posites, molten metal mixing is a cost effective methodwhile powder metallurgy is costly, and squeeze castingprovides good infiltration quality of chopped preformsw14–16x. From the available literatures on MMCs, it isobvious that the morphology, distribution and volumefraction of the reinforcement phase as well as the matrix

*Tel.yfax: q52-777-329-3016.E-mail address: ysahin@gazi.edu.tr(Y. Sahin).

properties are all factors that affect the overall mechan-ical and cutting properties. The literature review showedthat the wear characteristics of various cutting toolsduring machining of aluminium based composites, rein-forced by SiCp, SiCw, SiCyAl O w17–25x and coated2 3

cutting toolsw25–36x were studied with respect to toolwear and surface finish. The purpose of the currentstudy, therefore, is to:(a) produce particle-reinforcedmetal matrix composites by developing the method;(b)measure the density, hardness and porosity of the com-posites; and(c) investigate the tool wear in the turningof SiCp-reinforced aluminium matrix composites.

2. Experimental procedure

2.1. Fabrication of composite

Metal matrix composites including various volumefractions of SiC particles were produced by liquidmetallurgy method. 2014 Al alloy was used as thematrix material, while SiC particles with an average sizeof 110, 45 or 29mm were used as the reinforcementmaterial. The SiCp in the experiments were supplied byNorton Co. The chemical composition of Al-2014 alloy

672 Y. Sahin / Materials and Design 24 (2003) 671–679

Table 1The chemical composition of aluminium alloy

Types of alloy Si Fe Mn Mg Zn Cr Cu Alwt.(%)

Al-2014 0.66 0.504 0.62 0.6 0.11 0.03 4.49 92.9

used in this study is presented in Table 1. The compos-ites were fabricated by a molten metal of aluminiumalloy using an electric induction furnace, which is 2 kWpower under protected argon gas. For manufacturing ofMMCs, 10 and 20 wt.% SiC particles were used.Approximately, 350 g of Al-2014 alloy part with a 110-mm diameter was charged into the crucible made fromgraphite and heated up from 200 to 7508C for melting.The inside diameter of crucible was 55 mm and thediameter of the impeller was varied from 38 to 48 mmduring the mixing process. The top of the furnace wascovered with insulation board of alumina. When themelting temperature reached to 5008C, the graphitemixer fixed on the mandreal of the drilling machine wasinserted into the crucible, and started to stir the moltenalloy at approximately 700 rev.ymin speed.Three methods were used for adding SiC particles

into the composites. First method is to use a tunnel-typepipe. Small amount of SiC particles were introducedinto the matrix during the adding time by using thismethod since some particles distributed around or someothers remained at bottom oryand top of the furnace.Second adding method, a block of alloy was used andmany holes on the matrix were drilled with a 10-mmdiameter drill. The particles were filled in the holeaccording to the desired amount. After completed thefilling operation, these blocks were put into the crucibleand remained to be melt. After melting was completed,mixing process started. However, the mixing processcould not be achieved properly and some of particleswere agglomerated or chemical reaction between theparticle and matrix was more visible. Finally, SiCparticles were oxidized at a temperature of 11008C at2 h before introducing the melting process. Approxi-mately, 5–8 g of silicon carbide particles were insertedon an aluminium foil by forming a packet. The packetwas added into molten metal of crucible when the vortexwas formed at every 15–25 s. The packet of mixturemelted and the particles started to distribute around thealloy sample. This method enabled a full and homoge-nous distribution of the particles in the matrix alloy.After completing the particle addition, the mixer wasturned off and the molten mixture was poured into thepre-heated mould.Thermo-couples were inserted into the melt and fur-

nace to measure the temperature. Argon gas is dividedinto two channels and one is sent to over the cruciblein order to prevent molten metal with atmosphere whileanother one is fixed on reinforcement unit to control the

flow rate of the reinforcements. The experimental set-up used for the production of MMCs is shown in Fig.1. The fabricated billets are of 40-mm diameter and of160-mm height, the mold made from cast iron andpouring process is done from the bottom of mold. Tominimize porosity of the MMCs, 3000 kg of force byhydraulic press was applied mechanically before takenfrom the mold. The pressure was carried out at a periodof 7 min and mold was taken from the press to cooldown at approximately 20 min. Details of the experi-mental study can be found in Ref.w37,38x.

2.2. Metallography

The samples were sectioned and examined by anoptical microscopy. Specimens for metallographic obser-vation were prepared by grinding through 800 grit papersfollowed by polishing with 6mm diamond paste.

2.3. Density and hardness measurement

The density of the composites was obtained by theArchimedian principle of weighing small pieces cutfrom the composite disc first in air and then in water.Then, theoretical density of composite and its alloy wascalculated from the chemical analysis data. The porosityof the composites was also determined. The hardnessesof the composites and matrix alloy were measured afterpolishing to a 3mm finish.

2.4. Cutting conditions

Machining tests were carried out to determine thetool wear or tool life under various cutting conditionswhen cutting three types of composites and their alloymatrix. The tests were conducted under different cuttingconditions using a Boxford 250 CNC lathe machine,which is 5 kW power. The cutting speed was derivedfrom the measured spindle speed and the diameter ofthe surface of the workpiece. All tests were carried outwithout coolant at a depth of cut equal to 0.6 mm andfeed rate of 0.12 mm rev. . The cutting conditions andy1

tool geometry used in the experiment are listed in Table2. The inserts were clamped mechanically in a rigid toolholder of CTANR 2525-M16 type. Different tools,including an Al O coating on K15 carbide grade denot-2 3

ed by the term of tool A, not having any chip breakergeometry and an Al O coated on K15 carbide grade,2 3

denoted by the term of tool B, which is a chip breakergeometry and TiN coated on P15 carbide(TP100)cutting tool denoted by tool C in this study, have beenused. All tools are commercially available inserts,according to ISO code, TNMA 160412, TNMG 160412and TNMG 160412-MF2 were supplied by Sandvickand Seco, respectively, during the machining tests. The

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Fig. 1. Schematic diagram of experimental set-up for manufacturing MMCs.

Table 2Cutting tools used in the experiments

Type of Cutting tool Chemical Cutting Cutting DOCthe tool designation composition of fluids speed (mm)

coatings (mymin)

Tool A TNMA 160412 TiCqAl O2 3 Dry 40, 60, 80 0.6Tool B TNMG 160412-23 TiCqAl O2 3 Dry 40, 60, 80 0.6Tool C TNMG160412-MF2 Al OqTi (C,N)qTiN2 3 Dry 40, 60, 80 0.6

work material to be machined is the SiC reinforcedMMCs.After each test, the worn cutting tool is measured

with the optical tool microscope to determine the degreeof flank wear. For these tests, tool life criteria weretaken as 0.6 mm.

3. Results and discussion

3.1. Microstructure

The properties of the MMCs depend not only on thematrix, particle, and the volume fraction, but also ondistribution of reinforcing particles and interface bond-ing between the particle and matrix. In practical way, toachieve a homogenous distribution is difficult. Thus, theabove process parameters should be optimized. Theoptical micrograph of the aluminium composite rein-forced with approximately 10 wt.% of SiCp is shown in

Fig. 2a, b and c, respectively. The distribution of SiCpin these composites is uniform. These results show thatthis process can be used to produce MMCs when thevolume percent was less than the 30%. For the micro-structure of these composite specimens, no pores existedin these specimens due to the improvement of wettabilitywhen the Al-2014 alloy was used. Fig. 2b shows themicrostructure of the composite reinforced with 45mmparticle size. It indicates that no evidence of the presenceof cavities neither at interfaces nor in the matrix wasfound with optical microscopy, which indicates that agood bonding between the matrix and ceramic particu-late was obtained by using the molten mixing method.Fig. 2c also shows the 110mm particulate reinforcedaluminium composite. The SiC particles are observed tobe angular in shape. A careful examination of Fig. 2cindicates that apart from the large SiC particles, fineSiC particles less than 25mm size are also present. Inthe case of 20 wt.% of SiCp reinforced composites, Fig.

674 Y. Sahin / Materials and Design 24 (2003) 671–679

Fig. 2. Optical micrographs of the metal matrix composites. 10 wt.% SiCp with 29mm particles(a); 10 wt.% SiCp with 45mm size(b); 10wt.% SiC with 110mm particles(c); 20 wt.% SiCp with 29mm (d); 20 wt.% SiCp with 45mm (e); and 20 wt.% SiCp with 110mm (f).

2d, e and f shows optical micrographs of these compos-ites reinforced with 29mm, 45 mm and 110 mmparticulate sizes, respectively. Again these micrographsreveal that the interface between the matrix and particleto be from porosity. The homogenous distribution wasobtained for the composite reinforced with a 110mmparticle when compared with other particle reinforcedcomposites. Some agglomeration is also observed whenvolume fraction and particle size is less than 110mm.However, in general, the particle size distribution wasnearly identical in all the composites. The micrographtaken under high magnification also revealed that ceram-ic particles were covered and were wetted by the matrixalloy.

3.2. Hardness and density measurement

The variations of hardness of the composites areshown in Fig. 3. The hardness of the MMCs increasedmore or less linearly with the volume fraction ofparticulates in the alloy matrix due to the increasingceramic phase of the matrix alloy. A significantimprovement in both strength and hardness of squeezecast Al–Si alloy reinforced with alumina short fibresand Al-4.4%Cu alloy reinforced with continuous boronfibes has been reportedw4,7x. The introducing aluminashort fibres into the Al matrix resulted in increase inmechanical properties such as hardness and compressivestrengthw8x.

675Y. Sahin / Materials and Design 24 (2003) 671–679

Fig. 3. Variation of hardness with volume fraction of particle.

Fig. 4. Variation of density with volume fraction of particle(a), and variation of porosity level with volume fraction particle(b).

The density of the composite is shown in Fig. 4a. Asshown in the figure, the density increased with increas-ing the volume fraction of particulates. The increase indensity indicates that particle breakage may not haveany significant influence on the composites. It isbelieved to achieve an improvement of the bondingbetween the particle and matrix. The porosities ofcomposites were evaluated from the difference betweenthe expected and the observed density of each sample.The variations of porosity level in these composites arealso shown in Fig. 4b. This figure indicates that increas-ing amount of porosity is observed with increasing thevolume fraction, especially for low particle sizes of

composites, because of the decrease in the inner-particlesspacing. In other words, with increasing the volumefraction of MMCs during the production stage, it isrequired that the longer particle addition time is com-bined with decreasing the particle size. The porositylevel increased, since the contact surface area wasincreased. It is also reported by the early workw5,8x.The porosity of the composite was found between 1 and2.1 vol.%. This is due to application of pressure on themixed slurry or oxidation of SiCp before introductionto the matrix alloy during the fabrication stage. Thosevalues indicate that the porosities of the composites areat acceptable levels for low volume fraction of compos-ites. Kok et al.w8x who demonstrated that metal matrixcomposites were produced using the same technique andporosity level was found approximately 4%. In his work,Al-2024 alloy was reinforced by Al O , but oxidation2 3

process could not be applied on the SiC particles beforeintroduction of particles into the molten alloy.

3.3. Tool wear and surface observation

The relationships between the tool life for variouscutting tools under different cutting speeds are shownin Fig. 5. Tests were carried out on the 10 wt.%composite reinforced with 45mm particle size. It canbe seen that tool life decreased with increasing thecutting speed in all cutting conditions. The result showedthat tool A sustained the least flank wear due to theextreme hardness and not having a chip breaker geom-etry or the result of chemical stability of this tool. Thetool C is found to be very unsatisfactory and sustain themost severe flank wear. This might be due to a triple-layer-coating tool and increasing the stress concentrationeffect on tool during machining of the MMCs. The toollife also depends upon the cutting conditions. For

676 Y. Sahin / Materials and Design 24 (2003) 671–679

Fig. 5. Variation of flank wear with cutting speed for the 45mm particle-reinforced composites. Tool A(a), Tool B (b) and Tool C(c).

example, tool lives for cutting tools of A, C were 47,24 s when the test conducted at a speed of 40 mmin . There was an appreciable reduction at highery1

cutting speeds for all inserts. The reason for this maybe due to high cutting temperature generated duringmachining. The increase in temperature coupled withthe high compressive stresses near the cutting edge,could accelerate tool wear and lead to the shorter toollife at high speed. The tool C lasted at 7 s at acuttingspeed of 80 m min . The reason is that the cuttingy1

ability of SiCp distribution in the composites increasedand behaved as an abrasive particle. In the machiningof conventional metals, the cutting tools are abraded bythe strain-hardened chips and the workpiece surfacegenerated in machining process. Since, the pressure

between the workpiece and the cutting tool is very high,much heat is generated and the cutting tools becomessofter and sometimes chemical diffusion occurs betweenthe cutting tool when the shape of the chip is ofdiscontinuous type. However, the chip generated in themachining of particle-reinforced composite materials isof shattered type. The hardness of these particles isapproximately 2600 Hv, while the microhardness of thealuminium alloy matrix ranges from 90 to 120 BHN.However, the hardness of carbide insert(1700 MPa) orceramic tool(2100 MPa) was lower than that of theparticles. Therefore, the coating was removed from thetools when cutting the MMCs with coated carbides andthe dominating wear occurred on the flank face of thetool.

677Y. Sahin / Materials and Design 24 (2003) 671–679

Fig. 6. Wear surface of tool when machining the 10 wt.%SiCp-reinforced composite with 45mm particle size, tested at 60 mymin speed under0.12 mmyrev. feed rate and 0.6 mm depth of cut.(a) Tool A and(b) Tool C.

Tribological characteristics of coated tools were inves-tigated experimentally by several authorsw29–36x.Tomac et al. w35x suggested that coatings with lesshardness than that of Al O and SiC offerred little or2 3

no advantage during the machining of SiCyAl compos-ites. The performance of chemical vapor deposition(CVD) inserts to that of TiN, Ti(C,N) and Al O2 3

coated tools were compared with same authors. Theseoffered better performance than that of other tools.Quigley et al. w28x found that a triple-coated carbide,having a top layer of TiN performed best in terms offlank wear, but gave the poorest surface finish whenmachining the SiCp-reinforced composite. Monaghanand O’Reilly w20x indicated that the polycrystallinediamond(PCD) tipped tools were superior to the carbidetools and they were much better than the coated anduncoated HSS drillsw22x. The PCD tools providedsatisfactory tool life compared to alumina and coatedcarbide tools, where the latter tools suffered from exces-sive edge chipping and crater wear for machining ofSiCyAl composites to select the optimum tool material,tool geometry and cutting parametersw23x. Previouswork carried out by Sahin et al.w33x showed that themachining of the 10, 20 and 30 wt.%Al O particle-2 3

reinforced composites was conducted at 100 m miny1

cutting speed using different tools, the flank wearassociated with BUE formation in addition to brokenlayer of coated material of TiN(K10) coated tool. Thissuggests that a-triple-layer coating was more resistantthan a-double-coated layer based on the tool life, whichis not the case for the present study. However, the toolhaving chip breaker geometry played a predominant rolein decreasing the tool life and wear. For the TP30 tool,however, amount of chipping on the cutting edge wasobserved, smooth flank wear and small amount of

adhering material on flank face were observed and nosuch a BUE was formed here. Similar results werereported by Brun et al.w21x, who related the tool wearrate, mainly due to abrasion as in the case of our presentstudy.As far as considering the wear surface of the com-

posite, Fig. 6a,b shows wear surfaces of the tool A andB when the test carried out at 60 m min speed iny1

machining of the same weight fraction of the composite.It can be seen that a quite smooth surface in Fig. 6acan be observed in comparison to Fig. 6b. Some particleswere removed from the rake face and abrasive groovinglengths slightly decreased for tool B. This might be dueto related to the contacting area of the tool withworkpiece specimen. Similar observations can be madebut depth and length of abrasive grooves was less andsurface also looks uniform(Fig. 6a). However, it isobserved that amount of erosion was evidenced due tocombination of high temperature and adhesion wear(Fig. 6b). It can be concluded that the main wearmechanism is the result of abrasion of SiC particles.Similar results were found by Yanming et al.w22x, forcutting SiCp-reinforced Al matrix composites. TheMMCs could not be deformed plastically since the SiCparticles are harder than carbide inserts or ceramic tools.When they cut by these tools, the SiCp particles in thecomposites also microcut these tools randomly anddensely in the formation between the workpiece and thetool.

4. Conclusions

The microstructure and machinability of SiCp-rein-forced aluminium alloy composites were investigated.

678 Y. Sahin / Materials and Design 24 (2003) 671–679

The hardness, density and porosity were also measured.The following conclusions have been drawn.

1. A liquid metallurgy route was developed for manu-facturing the metal matrix composites by adding SiCparticles as packets. The pouring process was con-ducted from the bottom of the crucible in the electri-cal furnace after mixing process completed.

2. MMCs consisting of 10 and 20 wt.% SiC particleswith various sizes could be produced successfully bymolten metal mixing method and subsequently thepressure applied.

3. Microstructural examination showed that the SiCpdistributions were homogeneous and no interfaceporosity could be observed. Hardness of the alumin-ium alloy improved significantly by addition of SiCparticles into it, while density of the composite alsoincreased almost linearly with the weight fraction ofparticles. However, porosity level increased slightlywith increasing particulate content and decreasedwith increasing particle size.

4. It is shown that the tool life decreased with increasingthe cutting speeds in all cutting conditions. The lifeof the tool A was significantly longer that that oftool B. In addition, it was observed that the majorwear form of the tool was the mild abrasion andedge chipping on the flank face of the tools.

Acknowledgments

This research project was financed by the NationalTurkish University of Gazi in Turkey. The author alsowishes to acknowledge the technical assistance by MrG. Sur for carrying out this work.

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