Preparation, Characterization and Mechanical Properties of Al₂O₃ Reinforced 6061Al Particulate MMC’s

Preparation, Characterization and Mechanical Properties of Al₂O₃ Reinforced 6061Al Particulate MMC’s

Bharath V, Mahadev Nagaral and V. Auradi*

R & D Centre, Department of Mechanical Engineering, Siddaganga Institute of Technology, Tumk ur-572102, Karnatak a, India

Abstract

Aluminum MMCs are preferred to other conventional materials in the fields of aerospace, automotive and marine applications owing to their improved properties like high strength to weight ratio, good wear resistance etc. In the present work an attempt has been made to synthesize Al6061-Al2O3 particulate metal matrix composites by liquid metallurgy route (stir casting technique). The addition level of reinforcement was 0, 6 and 9wt%. For each wt%, reinforcement particles were dispersed in steps of three into molten Al6061 alloy. Microstructural analysis was carried out for the above prepared composites to reveal distribution of particles. The prepared composites are subjected to the mechanical testing as per the ASTM standards. Microstructural characterization revealed fairly uniform distribution and some amount of grain refinement in the specimens. The Micro -Vick ers hardness of the composite was found to increase with increase in filler content in the composite. The tensile strength of the composites was also found to increase confirming the enhancement of the mechanical properties.

Ke y Wor ds: MMCs, Al2O3 particulates, 6061A l, stir- casting

1. Introduction

   Metalmatrix co mposites (MMCs) are most promising in achieving enhanced mechanical properties such as: hardness, Youngs modulus, 0.2% yie ld strength and ultimate tensile strength due to the presence of mic ro- sized re inforce ment partic les into the matrix. Generally, regards to the mechanical properties, the reinforce ments result in h igher strength and hardness, often at the expense of some ductility [1]. Alu minu m- matrix co mposites (AMCs) reinforced with partic les and whiskers are widely used for high performance applications such as in automotive, military, aerospace

   and electricity industries because of their improved physical and mechanical properties [2]. In the composites relatively soft alloy like a lu minum can be made highly resistant by introducing predominantly hard but brittle partic les such as Al2O3 and SiC.

   Among Al-a lloys, 6061Al-alloy is widely used in numerous engineering applications including transport and construction where superior mechanical properties such as tensile, strength, hardness etc., are essentially required [1]. Hard partic les such as B4C, Al2O3 and SiC are commonly used as reinforce ment phases in the composites. The application of Al2O3 or SiC partic le reinforced alu minu m alloy matrix co mposites in the automotive and aircraft industries is gradually increasing for pistons, cylinder heads, connecting rods etc. where the tribological properties of the materia ls are very important [14]. In addition, the mechanical properties of MMCs are sensitive to the processing technique used to fabricate the materia ls. Considerable improve ments may be achieved by applying science- based modelling techniques to optimize the processing procedure. Several techniques have been employed to prepare the composites including powder metallurgy, me lt techniques and squeeze casting [2, 3]. Ho wever, powder metallurgy appears to be the preferred process in vie w of its ability to give more uniform dispersions. Hot ext rusion is generally used as post-treatment to take the advantages of applying compressive forces and high temperatures, simultaneously [5].

   Liquid state fabrication of MMCs include two methods depending on the temperature at which the particles are introduced into the melt. In melt stirring process, the particles are incorporated above the liquidus temperature of the mo lten alloy, wh ile in compo-casting method the particles are incorporated at the semi-solid slurry te mperature of the alloy. In both processes, the vortex is used for introducing reinforce ment part icles. Ho wever, the me lting process has two ma jor proble ms firstly, the ceramic partic les are generally not wetted by the liquid metal matrix, and

   secondly, the particles tend to sink or float according to their density relative to the liquid metal. Consequently, the dispersion of the ceramic part icles is not uniform. In order to decrease the porosity in the composite materia l, the pressure casting such as die and squeeze casting methods is needed [6]. Although powder meta llurgy produces better mechanical properties in MMCs, me lt processing has some important advantages. They are as: better matrix-part icle bonding, easier control of matrix structure, simplicity, low cost of processing, nearer net shape and the wide selection of materia ls [1 4]. Investigation of mechanical behavior of aluminu m alloys reinforced by micro hard particles such as Al2O3 and SiC is an interesting area of research. Therefore, the aim of this study is to investigate the effects of different factors such as: particle size, we ight percentage of the particles , processing method on the microstructure and mechanica l properties of the co mposites.

2. Experime ntal Details

   The following section highlights the material, its properties and methods of composite preparation and testing.

   1. Materials used

      The matrix material for the present study is Al6061. Al2O3 is used as reinforce ment materia l in the preparation of composites. The chemical co mposition of matrix materia l is as shown in Table1. The partic les size of the reinforc ing material was about 125 µm. Table 2. gives the properties of Matrix and Re inforc ing materia ls used in the present study taken from the literature.

      Table.1 Shows the Che mical Composition of 6061 Al alloy by wt%

      Components	Amount (wt %)
      Aluminium	Balance
      M agnesium	0.8-1.2
      Silicon	0.4-0.8
      Iron.	M ax 0.7
      Copper	0.15-0.40
      Zinc	M ax 0.25
      Titanium	M ax 0.15
      M anganese	M ax 0.15
      Chromium	0.04-0.35
      Others	0.05

      Table.2 shows the pr operties of matrix and reinforcing materials use d in the study

      Material/ Properties	Density gm/cc	Hardness (HB500)	S trength (Tensile/Co mpressive) (MPa)	Elastic modulus (GPa)
      Matrix 6061 Al	2.7	30	115 (T)	70-80
      Reinforcem ent Al2O3 Particle	3.69	1175	2100 (C)	300

   2. Preparation of composites

      The liquid meta llurgy route (stir casting technique) has been adopted to prepare the cast composites as described below. Preheated Al2O3 particles of laboratory grade purity of particle size 125 m were introduced into the vortex of the mo lten alloy after effective degassing using solid hexachloroethane (C2 Cl6). Before introducing reinforce ment particles into the me lt they were p reheated to a temperature of 2500 C. The extent of incorporation of Al2O3 particles in the matrix alloy was achieved in teps of 3. i. e Total amount of reinforce ment required was calculated and is being introduced into melt 3 times rather than introducing all at once. At every stage of before and after introduction of reinforcement particles, mechanica l stirring of the molten alloy for a period of

      10 min was achieved by using Zirconia-coated steel impeller. The stirrer was preheated before imme rsing into the melt, located approximately to a depth of 2/3 height of the molten meta l fro m the bottom and run at a speed of 200 rp m. A pouring temperature of 7500 C was adopted and the molten co mposite was poured into permanent cast iron moulds. Thus composites containing particles 0, 6 and 9wt % were obtained in the form of cylinders of dia meter 12.5mm and length 125mm.

   3. Testing of composites

      To study the microstructure of the specimens the central portion of the casting was cut by an automatic cutter device. The specimen surfaces were prepared by grinding through 300, 600 and 1000 grit papers and then by polishing with 3 m dia mond paste. Microscopic exa mination of the composites was carried out by optical microscopy. To investigate the mechanica l behavior of the compos ites the hardness and tensile tests were carried out using Zwick and computerized uni-a xia l tensile testing machine as per ASTM standards. Fig. 1. Shows the dimensions of the

      mould and specimen used for tensile studies. The Micro-Vic kers hardness values of the samples were measured on the polished samples using diamond cone indentor with a load of 20N. Hardness value reported is the average value of 100 readings taken at different locations on the polished specimen. For tensile results, test was repeated three times to obtain a precise average value.

      Figure (a)

      Figure (b)

      Figure. 1 showing the details of (a) Per manent moul d for pr oducing c omposites (b) Dime nsions of the te nsile specimen.

3. Results and Discussions

   1. Microstructural studies:

      Fabrication of metal matrix co mposites with alu mina particles by casting processes is usually difficult because of the very low wettability of alu mina particles and agglomeration phenomena which results in non – uniform distribution and weak mechanical properties. In the current work, Al6061 alu minu m alloy matrix composites with mic ro size a lu mina partic les were produced by stir casting method. The magnitude of alu mina powder used in the composites were 6 and 9wt.% . The optical micrographs of the 6061-A l alloy with 0, 6 and 9wt. % alu mina particu lates were shown in Fig 3(a-d).

      Fig. 3a-e shows microstructure of as cast 6061A l and 6061Al with 6 wt% (Fig. 3b-c) and 9wt% (Fig.3d -e)

      Al2O3 particulates. The mic rostructure of the prepared composites contains primary -A l dendrites and eutectic silicon, wh ile A l2O3 particles are separated at inter-dendritic regions and in eutectic silicon. The stirring of melt before and after introducing particles

      has resulted in breaking of dendrite shaped structure into equia xed form, it improves the wettability and incorporation of particles within the me lt and also it causes to disperse the particles mo re uniformly in the matrix. Fig. 3b-e reveals the distribution of alumina particles in different specimens and it can b e observed that there is fairly uniform distribution of part icles and also agglomeration of particles at few places were observed in both the composites reinforced with 6 wt.% and 9wt%Al2O3. The mic rophotographs also indicate that the Al2O3 particles have tendency to segregate and cluster at inter-dendritic regions which are surrounded by eutectic silicon (Fig. 3be). Further, the mic rographs show that grain size of the re inforced co mposite (Fig.3.a-e) is s maller than the alloy without alu mina particles (Fig. 3a) because, Al2O3 particles added to me lt also act as heterogeneous nucleating sites during solidification.

   2. Density measurements:

      Table 3.1 is presented with the comparison of theoretical density obtained by rule of mixture and measured density values by experiment for both the composites studied for diffe rent wt% of re inforce ments. Expe rimentally, the density of a composite is obtained by displacement techniques [7] using a physical balance with density measuring kit as per ASTM: D 792-66 test method. Further, the density can also be calculated fro m porosity and apparent density values (sample mass and dimensions) [8].

      Table 3.1: showing the theore tical and me asure d de nsities of as cast 6061Al and with 6 wt% of Al2O3p respecti vely

      Composition	Theoretical Density (g/cm3)	Measured Density(g/cm3)
      6061Al+6% Al2O3p	2.74	2.57
      6061Al+9% Al2O3p	2.76	2.58

      Fro m the table 3.1 it can be concluded that the e xperimental density of composite containing 6 and 9wt% Al2O3p is less when compared to the theoretical density. Further, measured density of co mposites is lesser than theoretical density, could be due to the presence of porosity. The porosity is probably due to

      Al2O3

      50X

      (e )

      100X

      Al2O3

      Mg2Si

      (a)

      (b)

      Al2O3

      Porosities

      (c)

      (d)

      50X

      100X

      50X

      Fig.3-a-d Showing the optic al microphotogr aphs of 6061Al with and without Al2O3 partic ulates (a) As- cast 6061Al (b) wi th 6 wt% of Al2O3 p at 50X (c) wi th 6 wt% of Al2O3 at 100X (d) wi th 9 wt% of Al2O3p at 50X (e ) wi th 9 wt% of Al2O3p at 100X

      1. increase in surface area in contact with air (ii) gas entrapment during stirring; (iii) gas injection of particles introduces a quantity of gas into the melt; (iv) hydrogen evolution; (v) the pouring distance from the crucible to the mold and (vi) shrin kage during solidification [9].

   3. Hardness measurements:

      Fig.3.2 shows the results of mic ro hardness tests conducted on Al6061 alloy and the 6061Al co mposite containing different weight percentage of Al2O3 particles. The M icro-Vicke rs hardness were measured on the polished samples using diamond cone indentor with a load of 20N and the value reported is average of 100 readings taken at different locations. A significant increase in hardness of the alloy matrix can be seen with addition of A l2O3 particles. A hardness reading showed a higher value of hardness indicating that the e xistence particulates in the matrix have improved the overall hardness of the composites. This is true due to the fact that aluminu m is a soft materia l and the reinforced partic le especially cera mics material be ing hard, contributes positively to the hardness of the composites. The presence of stiffer and harder Al2O3 reinforce ment leads to the increase in constraint to plastic deformation of the matrix during the hardness test. Thus increase of hardness of composites could be attributed to the relatively high hardness of Al2O3 itself.

   4. Tensile Properties

      To investigate the mechanical behavior of the composites the tensile tests were ca rried out using

      200

      VHN

      100

      0

      93.67 104.7

      145.67

      Table 3.2: showing the tensile test results of as cast 6061Al, with addi tion of 6 and 9 wt% of Al2O3 particul ates to 6061Al

      Sl No	Weight percentage of Al2O3 particles (%)	Yield S tress (N/mm2)	Percentage Elongation (%)	Ultimate Tensile strength (N/mm2)
      1	0	138.06	15.16	149.76
      2	6	145.51	10.63	167.93
      3	9	155.94	8.34	173.61

      0 6 9

      % of Al2O3 in6061Al

      Figure.3.2 showing the variati ons in har dness of 6061Al before and after additi on of different wt% of Al2O3 particul ates

      computerized uni-a xia l tensile testing machine as per ASTM standards. Three test specimens were used for each run. The tensile properties, such as, Tensile strength, Yie ld strength and Percentage elongation, were e xtracted fro m the stress -strain curves and are represented in Table 3.2. Fro m Table it is clear that fracture strength incase of composites (both 6 and 9wt

      %) is greater when co mpared to as cast 6061Al. It is also clear fro m the tensile results that with increase in amount of reinforce ment tensile strength increases, while there is decrease in ductility. As mentioned above, a thermal mis match between the metal matrix and the reinforce ment is a ma jor mechanis m for increasing the dislocation density of the matrix and therefore, increasing the composite strength. However, the composite materia ls exh ibited lower elongation than that of unreinforced specimens. It is obvious that plastic deformation of the mixed soft metal matrix and the non-deformable reinfo rce ment is more difficult than the base metal itself. There fore, the ductility of the composite must be lower than that of unreinforced materia l.

4. Conclusions

The present work on synthesis and characterizat ion of 6061Al-A l2O3 composites led to following conclusions

1. The composites containing 6061Al with 6 and 9wt% of Al2O3 particulates were successfully synthesized by melt stirring method.

2. The optical mic rographs of composites produced by stir casting method shows fairly uniform d istribution of Al2O3 particulates in the 6061A l meta l matrix.

3. The microstructure of the composites contained the primary -Al dendrites and eutectic silicon. While Al2O3 partic les were separated at inter-dendritic regions and in the eutectic silicon.

4. It was revealed that the hardness of composite samples increased with increasing the weight percentage of Al2O3 particles.

5. Strength of prepared composites both tensile and yield was higher incase of composites , while ductility of composites was less when compared to as cast 6061A l. Further, with increasing wt% of Al2O3, the tensile strength shows an increasing trend.

5. References

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