UDK 66.017:620.168:539.92	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 46(5)497(2012)
MECHANICAL AND TRIBOLOGICAL CHARACTERISTICS OF STIR-CAST Al-Si10Mg AND SELF-LUBRICATING Al-Si10Mg/MoS2 COMPOSITES
MEHANSKE IN TRIBOLOŠKE LASTNOSTI Z MEŠANJEM ULITIH KOMPOZITOV Al-Si10Mg IN SAMOMAZALNIH KOMPOZITOV
Al-Si10Mg/MoS2
Kannappan Somasundara Vinoth1, Ramanathan Subramanian2, Somasundaram Dharmalingam3, Balu Anandavel2
1Department of Production Engineering, PSG College of Technology, 641 004 Coimbatore, India 2Department of Metallurgical Engineering, PSG College of Technology, 641 004 Coimbatore, India 3Department of Mechanical Engineering, PSG Polytechnic College, 641 004 Coimbatore, India
vinothks@yahoo.com
Prejem rokopisa - received: 2012-03-20; sprejem za objavo - accepted for publication: 2012-05-24
The mechanical and tribological characteristics of aluminium-molybdenum-disulphide self-lubricating composites have been investigated and compared to the Al-Si10Mg alloy. Al-Si10Mg/4MoS2 display the finest microstructures due to a higher fraction of MoS2 added. The densities of Al-Si10Mg/2MoS2 and Al-Si10Mg/4MoS2 were marginally higher than in the case of the aluminium alloy by 1 % and 2 % mass fractions, respectively. The ultimate tensile strength decreases considerably due to the additions of 2 % and 4 % MoS2 by 15 % and 22 %, respectively, compared to the Al-Si10Mg alloy. It was seen that while the Al-Si10Mg alloy shows a predominantly ductile fracture (fibrous regions), the composite specimens (with an MoS2 addition) show an increase in the mixed mode (ductile and brittle regions). Al-Si10Mg/2MoS2 and Al-Si10Mg/4MoS2 show an enormous decrease in the wear rate by 55 % and 65 %, respectively, compared with the Al-Si10Mg alloy. The decrease in the wear occurs due to the presence of an MoS2 layer, which forms a film on the wear surface.
Keywords: aluminium-molybdenum-disulphide self-lubricating composites
Preiskovane so bile mehanske in triboloske značilnosti aluminij-molibden disulfidnih samomazalnih kompozitov in primerjane z zlitino Al-Si10Mg. Al-Si10Mg/4MoS2 ima najdrobnejšo mikrostrukturo zaradi večjega deleža dodanega MoS2. Gostota Al-Si10Mg/2MoS2 in Al-Si10Mg/4MoS2 je bila navidezno večja za masni delež 1 % oziroma 2 %. Končna natezna trdnost se je v primerjavi z zlitino Al-Si10Mg občutno zmanjšala za 15 % oziroma 22 % pri dodatku 2 % oziroma 4 % masnega deleža MoS2. Izkazalo se je, da pri zlitini Al-Si10Mg prevladuje žilav prelom (vlaknata področja), pri kompozitnih vzorcih (z dodatkom MoS2) pa mešan prelom (duktilna in krhka področja). Al-Si10Mg/2MoS2 in Al-Si10Mg/4MoS2 izkazujeta občutno povečanje odpornosti proti obrabi, in sicer 55 % oziroma 65 % v primerjavi z zlitino Al-Si10Mg. Zmanjšanje obrabe je zaradi sloja MoS2, ki tvori tanko plast na obrabni površini.
Ključne besede: aluminij-molibden disulfidni samomazalni kompoziti
1 INTRODUCTION	deposition and consolidation, as well as in-situ reacting
process8. Of all these processes, stir casting is the
Aluminium-silicon alloys and composites are being	simplest and the most economical method. Stir-cast
used in automotive applications like pistons, brake rotors	self-lubricating composites have been successfully
and engine-block cylinder liners1,2. Tribological beha-	developed by adding graphite particles9. It has also been
viour is an important aspect in the use of aluminium	suggested that these composite materials have the
metal-matrix composites in automotive applications. The	capacity to achieve low friction and wear of the contact
wear behaviour of Al-Si alloys can be further enhanced	surfaces without any external supply of lubrication
by adding ceramic particles. Abrasive particles like	during the sliding. However, graphite films fail in lower silicon carbide, alumina, and diamond are added to
^ . , . .	loads and shorter lifetimes compared with MoS210. improve the tribological behaviour by increasing the
hardness of a composite3-5. Nevertheless, lubricating	Self-lubricating Al-MoS2 composites have l,een prepared
particles like graphite and MoS2 have also been added to	by using the powder-metallurgy route11. However,
improve the tribological behaviour of different materials	neither the preparation of MoS2- based composites by
by providing a solid lubricating layer6,7. The additions of	stir casting nor the characterisation of Al-Si10Mg/MoS2
these particles considerably affect the mechanical	composites have been reported in literature.
behaviour of the composites.	In this investigation, two self-lubricating composites
There are various methods of producing composites	of molybdenum disulphide, namely, Al-Si10Mg/2MoS2
like blending and consolidation, vapour deposition and	and Al-Si10Mg/4MoS2 have been produced with the
consolidation, stir casting, infiltration process, spray	stir-casting route. The changes in the mechanical and
tribological properties caused by the addition of MoS2 are studied and compared with the Al-Si10Mg alloy.
2 MATERIALS AND METHODS 2.1 Preparation o^ the composite
The Al-Si10Mg aluminium alloy (Table 1) with a density of 2640 kg/m3 was used in this investigation as the matrix material. The Al-Si10Mg alloy has excellent
Figure 1: Microstructures of the materials: a) Al-Si10Mg, b) Al-Si10 Mg/2MoS2, c) Al-Si10Mg/4MoS2
Slika 1: Mikrostruktura materiala: a) Al-Si10Mg, b) Al-Si10Mg/ 2MoS2, c) Al-Si10Mg/4MoS2
resistance to corrosion in both normal atmospheric and marine environments collectively exhibiting high strength and hardness.
Table 1: Chemical composition of the aluminium alloy used (mass fractions, w/%)
Tabela 1: Kemijska sestava uporabljene aluminijeve zlitine (mas. deleži, w/%)
Mg	Si	Fe	Mn	Others*	Al
0.2 to 0.6	10.0 to 13.0	0.6 max	0.3 to 0.7	1.5 max	balance
* (Cu, Ni, Zn, Pb, Sn and Ti)
The Al-Si10Mg alloy was charged into an electrical resistance-heated furnace modified for this investigation. The melting of the Al-Si10Mg alloy was carried out under argon atmosphere at 1073 K. Molybdenum-disulphide (MoS2) solid lubricant with an average particle size of 1.5 pm and a density of 4600 kg/mm3 (Figure 1) was used as the reinforcement in this investigation. The MoS2 particulates were incorporated into the molten metal and stirred continuously for ten minutes. The molten mixture was solidified in a cast-iron die in the form of a cylindrical pin with a diameter of 14 mm and a length of 70 mm.
2.2 Testing of the materials
The density of composites was determined using a top-loading electronic balance (Mettler Toledo make) according to the Archimedean principle. The micro-structure of the composite specimens was identified using a Carl Zeiss Goettingen optical microscope. The specimens were metallographically polished to obtain an average roughness value of 0.8 pm. The tensile testing was carried out using a Hounsefield tensometer. The ultimate tensile strength of the specimens was calculated from the load at which a fracture occurred. The morphology of worn surfaces of the composite specimens was examined by using a JEOL T100 Scanning Electron Microscope (SEM). The hardness was measured by using a Zwick hardness tester at a load of 100 g. The dry-sliding wear behaviour of the composites was studied using a pin-on-disc apparatus. The disc material was made of the EN-32 steel with a hardness of 65 HRC. The difference in weights before and after the test was taken as weight loss. The wear rate was calculated on the basis of the difference in the weights of a specimen using the following formula:
Wear rate	=

9.81pö
mm3/km
(1)
where W/kg = mass loss, p/(kg/mm3) = density of the material, D = sliding distance
Table 2: Mechanical properties of the Al-Si10Mg alloy and the composites Tabela 2: Mehanske lastnosti Al-Si10Mg zlitine in kompozitov
Material	Density kg/m3	UTS MPa	Hardness HV	Elongation %	Decrease in UTS, %	Increase in hardness, %
Al-Si10Mg	2640	218.45	102	1.66	-	-
Al-Si10Mg/2MoS2	2670	185.31	145	1.22	15	42
Al-Si10Mg/4MoS2	2697	170.28	148	1.10	22	45
3 RESULTS AND DISCUSSION
3.1 Microstructures
Optical micrographs of the Al-Si10Mg alloy and of the composites (Figures 1a to c) show as-cast (dendritic) structures consisting of silicon particles in a eutectic
Figure 2: Fracture analysis of the materials: a) Al-Si10Mg, b) Al-Si10Mg/2MoS2, c) Al-Si10Mg/4MoS2
Slika 2: Analiza prelomov materiala: a) Al-Si10Mg, b) Al-Si10Mg/ 2MoS2, c) Al-Si10Mg/4MoS2
matrix. The microstructures of the composites (Al-Si10Mg/2MoS2 and Al-Si10Mg/4MoS2) are significantly finer, affected probably by the heterogeneous nucleation caused by MoS2 particles. Al-Si10Mg/4MoS2 exhibits the finest microstructure due to the higher fraction of MoS2 added. Figures 1b and 1c show that MoS2 particles were uniformly distributed in the matrix.
3.2 Mechanical properties
The mechanical properties of the composites (density, hardness, and tensile strength), given in Table 2, show the average properties of various test specimens at different positions.
The density of MoS2 is higher than that of the aluminium alloy and hence an increase in the MoS2 content will raise the density of the composite. The densities of Al-Si10Mg/2MoS2 and Al-Si10Mg/4MoS2 were marginally higher than the density of the aluminium alloy by 1 % and 2 %, respectively. A similar increase in the density of the composites was achieved by adding SiC12 and Al20313 by various authors.
The ultimate tensile strength [UTS] of Al-Si10Mg was approximately 218 MPa. It was reported in previous researches that an addition of alumina to AA6061 and AA7005 causes an increase in the tensile strength14. Similar results were reported for SiCp/aluminium-alloy composites15 and aluminium-alumina, aluminium-illite and aluminium-silicon carbide particle composites5. In contrast to this, the studies on an addition of alumina to the 2024 Al alloy have shown a decrease in UTS16. Similar results were obtained for an addition of graphite to aluminium17. The tensile strength decreases considerably due to the additions of 2 % and 4 % by mass MoS2 by 15 % and 22 %, respectively. The observed decrease in UTS may be due to various mechanisms like the particle pull-out and crack propagation, which are initiated by the presence of MoS2.
The elongation of the composites decreases slightly less than in the case of the Al-Si10Mg alloy indicating that an addition of MoS2 lowers the ductility of a composite. A similar result was observed in the SiC reinforcement of the 2124, 7075 alloys and monolithic aluminium1819.
3.3 Fracture surface
Figures 2a to c show the SEM fractographs of Al-Si10Mg, Al-Si10Mg/2MoS2 and Al-Si10Mg/4MoS2,
Figure 3: Wear behaviour of the Al-Si10Mg alloy and the composites Slika 3: Obraba Al-Si10Mg-zlitine in kompozitov
Figure 4: Wear-surface SEM micrographs at a load of 50 N and a speed of 5 m/s of: a) Al-Si10Mg, b) Al-Si10Mg/2MoS2 Slika 4: SEM-posnetek obrabljene povr{ine pri obremenitvi 50 N in hitrosti 5 m/s za: a) zlitine Al-Si10Mg, b) Al-Si10Mg/2MoS2
respectively. From the fractographs of the tensile-test specimens (Figure 2) it can be seen that, in the aluminium-matrix alloy, the fracture was primarily transgranular with a microscopic void formation; later the progressive growth and the final coalescence around the reinforcement particles can be observed. It can also be seen that while the Al-Si10Mg alloy shows a predominantly ductile fracture (fibrous regions), the composite specimens show an increase in the mixed mode [ductile and brittle regions]. In addition, the composite samples also show the features like particle pullout, crack growth, and propagation typical of a brittle fracture.
3.4 Wear behaviour
Dry-sliding wear tests were conducted according to ASTM G-99 using a pin on a disc apparatus under an applied load of 50 N for a sliding speed of 5 m/s. The wear rate plotted against the sliding distance is shown in Figure 3.
The Al-Si10Mg alloy experiences the maximum wear rate. The wear mechanism was studied using a SEM micrograph of the worn surface of the Al-Si10Mg alloy (Figure 4a), revealing severe delamination that is an indication of an adhesive wear. Al-Si10Mg/2MoS2 and Al-Si10Mg/4MoS2 show an enormous decrease in the wear rate by 55 % and 65 %, respectively, compared with the Al-Si10Mg alloy. The decrease in the wear is due to the presence of the MoS2 layer, which forms a film on the wear surface. This in evident in the SEM micrograph of Al-Si10Mg/2MoS2 (Figure 4b) where the MoS2 particles form a film in certain regions, partially reducing the ploughing and delamination.
4 CONCLUSION
In this research work, Al-Si10Mg/MoS2 composites were fabricated using the stir-casting technique and the mechanical and tribological characteristics were studied. The following important observations can be noted:
1.	UTS, elongation percentage and hardness decrease with an addition of MoS2 particles to Al-Si10Mg. However, the densities of the composites are higher than the density of the Al-Si10Mg alloy.
2.	A uniform distribution of MoS2 is observed on the optical micrographs.
3.	The improved wear resistance of Al-Si10Mg/MoS2 composites is better than the wear resistance of the Al-Si10Mg alloy.
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