Dry Sliding wear behavior of silicon carbide particulate reinforced AA6061 aluminum alloy composites produced via stir casting

J.Jebeen Mosesa*, S.Joseph Sekharb a*,bDepartment of Mechanical Engineering, St. Xavier's Catholic College of Engineering, Nagercoil

629003, Tamilnadu, India

a*jebeenmoses@gmail.com, b*josephsekhar@hotmail.com

Keywords: Metal Matrix Composites; Stir Casting; Microstructure; Wear.

Abstract

Stir casting is an economical method to produce aluminum matrix composites (AMCs). In

the present work, Aluminum alloy AA6061 reinforced with various amounts (0, 5, 10 and 15wt. %)

of SiC particles were prepared. The matrix alloy was melted in a furnace and stirred to form a

vortex. SiC particles were added to the periphery of the vortex and the composite melt was

solidified in a permanent mold. The microstructures of the AMCs were studied using optical and

scanning electron microscopy. SiC particles were observed to refine the grains and were distributed

homogeneously in the aluminum matrix. SiC particle clusters were also seen in a few places. SiC

particles were properly bonded to the aluminum matrix. Dry sliding wear behavior was analyzed by

Pin on Disc apparatus. The reinforcement of SiC particles improved the wear resistance of the

AMCs.The details of worn surface and wear debris are also presented in this paper.

1. Introduction

Aluminum alloys reinforced with various particulate ceramic particles are universally known as

aluminum matrix composites (AMCs). AMCs display superior properties such as high specific

strength, high elastic modules, excellent friction and wear resistance and low thermal expansion

coefficients etc. The excellent properties made them as a demandable material in several industries,

particularly in aircraft, automotive and marine industries [7, 8]. The development in production

techniques of AMCs caused the progressive replacement of aluminum alloys in many applications

to enhance product performance.

The production of AMCs with proper distribution and bonding of ceramic particles has been a

challenge over the years. Development and optimization of production methods will improve the

properties of AMCs to fulfill the requirement of various industries. AMCs are presently produced

using several methods which can be classified into solid state processing and liquid state processing.

The latter is preferred due to its simplicity, lower cost, near net shape and mass production [13].

Stir casting is the widely adopted liquid method to produce AMCs[4]. The matrix material is

melted in a furnace. The melt is stirred to form a vortex. An inert gas is passed to prevent the

formation of oxides. The ceramic particles are fed at a predetermined rate to the periphery of the

vortex. The stirring is continued till all the particles are added. The composite melt is then poured

into a mold. The solidified composite can be further subjected to heat treatment or other secondary

processing to improve the properties. The critical process parameters are temperature of the melt,

stirrer speed, stirring time, particle feed rate and temperature of the mold. The limitations of stir

casting are poor distribution, wet ability, porosity, interfacial reactions and moderate volume

fraction[6]. Proper selection of process parameters can help to produce sound AMCs [10].

Some studies on the production and characterization of aluminum alloy reinforced SiC particles

using stir casting were reported in the literatures. [6].prepared A359/SiC AMC using stir casting and

enhanced the wettablity between SiC particles and the aluminum matrix by adding magnesium. [12]

formed AA2014/SiC using stir casting and recorded the tool during machine of the AMC. [1]

produced A356/Sic using stir casting and compo casting and analyzed the effect of casting

temperature on the distribution of SiC particles. [9] fabricated AA6061/SiC AMC using stir casting

Advanced Materials Research Vols. 984-985 (2014) pp 221-226Online available since 2014/Jul/16 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.984-985.221

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and studied the effect of heat treatment on the fatigue properties of the AMC. [2] developed

A356/SiC AMC by injecting SiC particles into the molten aluminum alloy and enhanced the

distribution of SiC particles. [14] observed the dispersion of SiC particles all over the aluminum

matrix provides Orowan strengthening.

In this work, an attempt is made to fabricate aluminum alloy AA6061 reinforced with SiC

particles by stir casting and study the effect of SiC content on microstructure and wear behavior of

AA6061/SiC AMC.

2. Experimental Procedure

AA6061 rods were placed in a graphite crucible and was heated using an electrical

resistance furnace. The chemical composition of AA6061 aluminum alloy is presented in Table 1. A

coating was applied inside the crucible to avoid contamination. The temperature of the furnace was

set at 7500C. The mechanical stirrer was lowered into the aluminum melt after all the rods were

melted completely. The stirrer was rotated at a constant speed of 300 rpm to form a vortex.

Measured quantity of SiC particles of size 30-40 µm was gradually fed to the vortex at a feed rate of

approximately 15 g/min. Stirring of the melt and feeding of SiC particles was continued

intermittently for 30 minutes. The composite melt was then poured into a die preheated to 3000C.

Castings were taken with various amounts of (0, 5, 10 and 15wt. %) of SiC particles.

Specimens of size 6 mm x 6 mm x 6 mm were prepared from the castings to carry out

microstructure and wear behavior. The specimens were polished using standard metallographic

technique and etched with Keller’s reagent. The etched specimens were observed using an optical

microscope and a scanning electron microscope (SEM). The sliding wear behavior of AA6061/SiC

AMCs was measured using a pin-on-disc wear apparatus (DUCOM TR20-LE) at the room

temperature according to ASTM G99-04 standard. The wear test was conducted at a normal force

of 25 N, a sliding velocity and distance of 1 m/s and 2500m respectively. The polished surface of

the pin was slid on a hardened chromium steel disc. A computer aided data acquisition system was

used to monitor the loss of height. The volumetric loss was computed by multiplying the cross

sectional area of the test pin with its loss of height. The wear rate was obtained by dividing

volumetric loss to sliding distance. The worn surfaces of the test specimens were observed using

SEM. The wear debris which were scattered on the face of the counterface were carefully collected

and characterized using SEM.

3. Results and Discussion

3.1 Microstructure of AA6061/SiC AMCs

The optical and SEM micrographs of the fabricated AA6061/SiC AMCs are shown in Fig. 1

and 2. Fig. 1a reveals the optical micrograph of as cast AA6061. It depicts the typical dendritic

structure of aluminum. The dendritic structure exhibits elongated primary α-Al dendritic arms

having a high aspect ratio The high rate of cooling during the solidification of the casting results in

the formation of dendritic structure. The secondary precipitation phase Mg2Si is observed along the

dendritic boundaries due to the high solubility limits of the alloying elements such as Mg and Si.

Fig. 1b-d reveals the optical micrographs of the fabricated AA6061/SiC AMCs. The

dendritic structure of the cast matrix is completely disappeared. SiC particles refined the dendritic

structure into a grainy structure. SiC acts as a grain refiner. SiC particles play a significant role

during the solidification of the composite melt, which results in grain refinement. The following

two factors may contribute to the grain refinement. The presence of distributed SiC particles in the

melt offers resistance to the freely growing aluminum grains. The number of grain nucleation sites

increases with the distribution of SiC particles in the melt. Several SiC particles act as grain

nucleation sites due to constitutional under cooling zone in front of the particles. More the content

of SiC particles more will be the resistance to grain growth and more nucleation sites. As a result,

formation of finer grains takes place due to enhanced grain refinement.

222 Modern Achievements and Developments in Manufacturing and Industry

It is evident from the optical (Fig. 1b-d) and SEM micrographs (Fig. 2) that the distribution of

SiC particles in the aluminum matrix is fairly homogeneous. There is no segregation of SiC

particles along the grain boundaries. Most of the particles are located within the grains i.e. the

distribution is intra-granular. Intra granular distribution of ceramic particles is preferred in AMCs

over inter granular distribution to obtain higher mechanical and tribological properties. The

distribution of reinforcement particles in the matrix material takes place in three stages as far as stir

casting is concerned; (a) distribution of particles in the melt as a result of mixing, (b) distribution of

particles in the melt before pouring or solidification and (c) redistribution of particles as a result of

solidification [5] Adequate stirring causes the particles to be dispersed effectively in the melt. The

mechanical stirring action drives the particle into the melt. Suspension of SiC particles in the melt

throughout the stirring period before pouring is needed to achieve proper distribution. The wetting

action between SiC particle and the aluminum melt provides resistance to the free movement of SiC

particle within the melt. Further, the incorporation of SiC particles into the aluminum melt increases

the viscosity of the melt. The increase in viscosity leads to retard the movement of SiC particle.

More the content of SiC particle, more will be the viscosity and resistance to the movement of

particles. The above listed factors results in better distribution of SiC particles in the aluminum

matrix.

Fig.1. Optical micrograph of AA6061/ SiC AMC with SiC content wt. of; (a) 0%; (b) 5 %; (c) 10 %(d) 15 %.

Fig. 2. SEM micrograph of AA6061/15 wt.%. SiC at magnification; (a) 100x wt.%; (b) 400x; (c) 750x and (d) 1000x.

The solidification pattern plays a role in the redistribution of particles after pouring. During

solidification, several factors such as convection current, movement of the solidification front

against particles and buoyant motion of particles influence the distribution of particles in AMCs [3]

If the solidification front pushes the particles, the distribution will be inter granular. Otherwise, the

intra granular distribution will occur if the solidification front engulfs the particles. Observing the

distribution of SiC particles, it appears that the SiC particles were engulfed by the solidification

front which led to intra-granular distribution.

The SEM micrograph of AA6061/15wt. % SiC AMC at higher magnification is presented in Fig.

2d. The figure reveals the details of the interface existing between the aluminum matrix and SiC

particle. The interface is clear without the presence of any reaction products. SiC did not react with

the aluminum to produce any undesirable compounds. The absence of any interfacial reaction can

be attributed to combination of process parameters selected for stir casting. SiC particles are

thermodynamically stable under the experimental conditions. SiC particle did not decompose during

casting. There are no pores or voids are seen around SiC particles which are properly bonded to the

matrix alloy. A clear interface and proper bonding enhance the load bearing capacity of the AMC.

Advanced Materials Research Vols. 984-985 223

3.2 Sliding wear behavior of AA6061/SiC AMCs

The effect of weight percentage of SiCparticles on the wear rate of AA6061/SiC AMCs is

depicted in Fig. 3. It is evident from Fig. 3 that the SiC particles reduces the wear rate considerably.

AA6061/15 wt.% SiC AMC exhibits 30.5% lower wear rate compared to unreinforced

AA6061.This is attributed to the high strength and hardness of composites with SiC. The

relationship between material hardness and material removal during sliding is described as [11].

(1)

Equation (1) shows that higher the hardness of the material, lower will be the wear rate. The

enhanced hardness of the composite offers resistance to the cutting action of the counter surface

during sliding. A good interfacial bonding between the SiC particle and the aluminium matrix

retards the removal of particles during sliding. When the weight percentage of SiC particle

increases, the contribution of above factors further increases. As a result, the wear rate is further

reduced.

The effect of weight percentage of SiC particles on the morphology of the worn surface of

AA6061/SiC AMCs is shown in Fig.4 The worn surface of the matrix alloy in Fig.4a shows a large

number of parallel grooves. But the grooves are blended at the edges due to piled up plasticized

matrix. The frictional heat due to sliding between matrix alloy and counter surface causes plastic

deformation. The wear mode is observed to be adhesive. The worn surfaces of AA6061/SiC AMCs

(Fig.4b-d) show distinct parallel grooves which bear evidence to abrasive wear mode. The edges of

the grooves are sharper due to the ploughing action of the counter surface. Loose wear debris are

also seen on the worn surface. As the weight percentage of SiC particle increases, the number and

size of pits on the worn surface decreases.

Fig. 3 Effect of SiC content on wear rate of AA6061/SiC stir cast composites

Wear coefficient x Applied load x Sliding distance

Volume loss =

Hardness of material

(1)

224 Modern Achievements and Developments in Manufacturing and Industry

Fig.4 Worn surface of AA6061/SiC stir cast

composites containing SiC: (a) 0wt.%, (b)

5wt.%, (c) 10wt.% and (d) 15wt.%

Fig.5 Wear debris of AA6061/SiC stir cast

composites containing SiC: (a) 0wt.%, (b)

5wt.%, (c) 10wt.% and (d) 15wt.%

Table 1 The chemical composition of AA6061-T6 alloy

Element Mg Si Fe Mn Cu Cr Zn Ni Ti Aluminium

wt.% 0.95 0.54 0.22 0.13 0.17 0.09 0.08 0.02 0.01 Balance

The effect of weight percentage of SiC particles on the morphology of wear debris of

AA6061/SiC AMCs is shown in Fig.5 It reveals that the increase in weight percentage of SiC

particle results in finer wear debris. The wear debris of the matrix alloy in Fig.5a exhibits large thin

plate like morphology, which is due to adhesive wear between matrix and counter surface. Local

welding or adhesion takes place between the plasticized asperities of matrix and counter surface. As

sliding continues, the local adhesion spreads and it is removed in the form of thin plates. When a

SiC particle is added to the matrix alloy, the local adhesion is prevented and the wear mode shifts

from adhesion to abrasive. During the initial stages of sliding of AA6061/5 wt. % SiC AMC, some

of SiC particles are removed due to cutting action of counter surface. When the matrix surrounding

the SiC particle is removed, SiC particle is eventually pulled out. These SiC particles are trapped

between the specimen and the counter surface converting two body abrasion into three body

abrasion. Rolling is promoted to over sliding which reduces the wear rate. The three body abrasion

generates finer wear debris. Fig.4c and d show that the weight percentage of SiC increases the

milling action. As a result it increases the forms of the finer spherical debris.

4. Conclusions

In the present work, AA6061/SiC AMCs were successfully fabricated using stir casting. The

effect of SiC content on microstructure and wear behavior was analyzed. The distribution of SiC in

the aluminium matrix was fairly homogeneous. The distribution of SiC particles was observed to be

intra granular. SiC particles refined the grains of matrix alloy and were properly bonded to the

matrix. The interface between the SiC particle and the aluminium matrix was clean without the

presence of reaction products, pores and voids. SiC particles enhanced the wear resistance of the

composite. But reduced the ductility of the composite. AA6061/15 wt. % SiC AMC exhibited

30.5% lower wear rate when compared with unreinforced AA6061 alloy. The increase in SiC

content shifted the wear mode from adhesive to abrasive. The size of wear debris became finer

when the content of SiC particle was increased due to three body abrasion.

Advanced Materials Research Vols. 984-985 225

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