fabrication of a356/al2o3 composites by stir casting ...

‫ﻣﺆﺗﻤﺮ اﻷزھﺮ اﻟﮭﻨﺪﺳﻲ اﻟﺪوﻟﻲ اﻟﺜﺎﻟﺚ ﻋﺸﺮ‬

AL-AZHAR ENGINEERING THIRTEENTH INTERNATIONAL CONFERENCE December 23-25, 2014

Code: M 16

FABRICATION OF A356/AL2O3 COMPOSITES BY STIR CASTING METHOD AND THEIR CHARACTERISTICS A. A. Kandel1 and D. Saber2 1 Petroleum and Metallurgy Dept. El-Azhar University, Cairo , Egypt, 2

, Materials Engineering Dept., Zagazig Univ., Zagazig, 44519, Egypt,

ABSTRACT In this study, A356 aluminum alloy metal matrix composite (MMCs) reinforced with different weight fractions of Al2O3 particles were fabricated by stir casting method. The effect of Al2O3 particles content on density, porosity, and hardness were investigated. Good wettability between the A356 aluminum alloy and Al2O3 particles was achieved by addition 1% Mg and heat treatment of Al2O3 particles at 900◦C. The density measurements of the composites showed that, the amount of porosity in the composites increased with increasing the weight fraction of Al2O3 particles. From the hardness results, it is clear that the hardness of the composites increased with increasing the weight fraction of Al2O3 particles. © 2014 Faculty of Engineering, Al-Azhar University, Cairo, Egypt. All rights reserved. KEYWORDS: Fabrication; Stir casting; A356 aluminum alloy; MMCs; Alumina

1.INTRODUCTION In recent years, aluminum alloy based metal matrix composites (MMCs) are gaining importance in several aerospace and automobile applications. Commercial aluminum MMCs possess much higher specific strength and stiffness, improved high temperature properties, higher wear resistance and lower thermal expansion coefficient in comparison to their base alloy matrixes due to the incorporation of suitable particles or fibers into the matrix metal. The alloys most commonly studied in MMCs are Al–Si and Al–Cu based alloys [1-3]. Many investigators have focused on the commercially important Al–Al2O3 system. There are several fabrication techniques available to manufacture Al–Al2O3 composite. The fabrication methods can be divided into three types: solid phase, liquid phase and semi-solid fabrication processes [4]. The molten metal stirring method is one of the various fabrication methods of particle-reinforced MMCs. In this method, the manufacturing parameters in homogenous mixing are the temperature of molten metal, the stirring time, the partial feeding into mixture at a continuous and uniform rate and stirring speed. The specific advantages of this method are easy control of matrix structure, simplicity, low cost of processing and near net shape [5]. However a major difficulty found in this method is non wetting nature of ceramics, thus improved wetting must be achieved in order to obtain good bonding between the matrix and reinforcement. For example Al2O3 is one of most common and useful ceramics employed in MMCs and Al2O3 reinforced aluminum is a kind of MMCs with great potential but wettability of Al2O3 by molten aluminum is usually very poor [6]. Several techniques to improve wetting between matrix and reinforced particles have been developed such as metal coating of ceramic particles, heat treatment of particles and addition of reactive particles such as Mg [7]. The aim of current study was to produce the Al2O3 particle reinforced metal matrix composites by stirring casting method. The effect of weight fraction of Al2O3 on porosity and hardness of Al2O3 particles reinforced A356 aluminum alloy composites was investigated.. Al-Azhar University Engineering Journal, JAUES Vol. 9, No. 2, Dec. 2014 1

FABRICATION OF A356/AL2O3 COMPOSITES BY STIR CASTING METHOD AND THEIR CHARACTERISTICS

2.Experimental procedure 2.1.Material In this study, A356 aluminum alloy with the theoretical density of 2.68 g/cm3 was used as the matrix while Al2O3 particle with theoretical density of 3.9 g/cm3was used as the reinforcement. The chemical analysis of A356 used in this investigation is shown in Table (1). The Al2O3 particle reinforced A356 alloy-matrix composites have been produced by using stirring casting method. The composites were shaped in the form of cylinder of 10 mm diameter, and height of 100mm. Table (1): Chemical composition of A356 alloy used as the matrix in the MMCs.

Element percentage

Si 7.1

Mg 0.3

Mn 0.01

Cu 0.02

Ni 0.01

Al balance

2.2.Procedure A356 Al alloy was charged into the crucible, and heated to about 750oC. Al2O3 were added with particle addition rate equal 5g/min. The melt was stirring by stirring speed equal 700 r.p.m and stirring time 5 minutes after the completion of particle feeding. Wettability was improved by addition of 1% Mg and also by heating of Al2O3 to 900oC, for two hours, before their dispersion in the melt and. 2.3.Density and hardness measurement The experimental density of A356 alloy-matrix composites reinforced by Al2O3 particles was obtained by the Archimedian method of weighting small pieces cut from the composite cylinder first in air and then in water. On the other hand the theoretical density was determined using the mixture rule according to the weight fraction of the Al2O3 particles. The porosities were calculated from difference between the experimental and theoretical density of each sample. The hardness of the composites were measured after grinding and polishing. The hardness was determined applying a Vickers indenter using load 10kg. 3.Results and discussion 3.1.Fabrication of composites In the present work, A356 alloy reinforced by four different weight fractions (10, 15, 20 and 25%) of Al2O3 particles have been successfully fabricated using stirring cast method. As a result of various trials in the fabrication stage of investigation, the optimum process parameters were found to be as follows: pouring temperature: 750oC, stirring speed: 700 rev min−1, stirring time: 5 min after the completion of particle feeding, particle addition rate: 5 g min−1. A approximately, the same values of process parameters have been found in some previous studies [8]. Figure (1) shows the microstructure of the composite with 10 wt.% of Al2O3 after some trials of fabrication. At higher pouring temperature, Al2O3 particles tended to sink, whereas at lower pouring temperature and stirring speed, and higher particle addition rate, the particles were not incorporated into the molten metal and particle agglomeration occurred at the surface of the molten metal. In addition, at higher stirring speed some of the particles were dispersed out of the crucible by the wind of the impeller and so particle addition was not achieved and uniform dispersion of the particles was not achieved. As a result, the uniform distribution of Al2O3 particles was only achieved under the optimum process conditions given above.

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Fig.(1): Microstructure of the composite with 10 wt.% of Al2O3 after some trials of fabrication at; (a) stirring speed less than 700 rev min−1, (b) stirring speed more than 700 rev min−1, (c) pouring temperature less than 750◦C, and (d) pouring temperature more than 750◦C.

3.2.Techniques to improve wetting ofAl2O3 particles Several techniques to improve wetting have been developed in recent years based on the principle that the contact angle θ can be decreased by increasing the surface energy of the surface (γsv), decreasing the solid/liquid interfacial energy (γs1), or by decreasing the surface tension of the liquid metal (γlv). In this study, various techniques were used to improve wettability of Al2O3 by molten A356 Al alloy. These techniques were: (1) the activation of Al2O3 particles by sodium hydroxide, (2) addition of reactive elements such as Mg to the melt, (3) heat treatment of Al2O3 particles at different temperatures. Figure (2) shows the microstructure of A356 alloy-matrix composite with 10% Al2O3 after activation of Al2O3 particles by NaOH solution and addition of reactive elements such as Mg. Good wettability was achieved by addition 1% Mg, and heat treatment of Al2O3 particles to 900◦C. Addition of Mg to the melt improve wetting because of the lower surface tension of Mg (0.599 nm-1) compared with that A356 Al alloy (0.817 nm-1). Heat treatment of Al2O3 particles before their dispersion in the melt aids their transfer by causing desorption of adsorbed gases from the ceramic surface [9]. Fig.(3) shows SEM micrographs of the A356 with 25 wt.% Al2O3 that were fabricated under the best production conditions. As shown in Fig.(3-a), uniform dispersion of the particles was approximately achieved. Fig. (3-b) indicates that good bond was achieved between the matrix and Al2O3 particles.

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Fig.(2): Microstructure of A356 alloy-matrix composite with 10% Al2O3 after some trials of fabrication; (a) Activation the particles by NaOH solution and (b) Addition of reactive elements.

Fig.(3): SEM micrographs of the composite with 25 wt.% Al2O3 show; (a) particles distribution and (b) the interface between matrix and Al2O3 particle

Fig.(4) shows the metallographic specimens after etching with solution contains 75ml HCl, 25ml HNO3, 5ml HF, and 25 ml H2O. As shown in this figure, it is clear that the addition Al2O3 particles had effect on dendritic structure. The coarse dendritic structure in A356 matrix alloy transfers to fine dendritic structure by Al2O3 particles addition. 3.3.Density and porosity Theoretical density, measured density and porosity of the composites according to the weight percentage of Al2O3 particles are given in Table (2). Figure (5) shows the effect of wt.% of Al2O3 particles on the density of the A356 alloy-matrix composites. From this figure, it is clear that the theoretical density values of the composites increase linearly (as expected from the rule of mixtures). In addition the values of experimental densities, are lower than that of the theoretical densities. The density measurements showed that the composites contained some porosity. The effect of wt.% of Al2O3 particles on the porosity of the A356 alloy-matrix composites is shown in Fig.(6). As shown in this figure, it is clear that the porosity in the Vol. 9, No. 2, Dec. 2014

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composites increased with increasing weight fraction of the Al2O3 particles. The porosity in MMC with 10 wt.% Al2O3 particles was 0.7% and increased to 2.35% in MMC with 25 wt.% Al2O3 particles. M. Kok [8] reported that during the production process of the MMCs, some porosity is normal, because of the long particle feeding duration and the increase in surface area in contact with air caused by small particle size. However, in this study, the best process parameters and good treatment of wettability have reduced this porosity in the composites, and improved the bonding force between the A356 alloy and Al2O3 particles, and the wettability of the particles.

Fig (4): Optical micrographs show the microstructure of: (a) A356 alloy, (b) A356+10%Al2O3, (c) A356+ 15%Al2O3, and (d) A356+25%Al2O3 composites. Table (2): Characteristics of A256 alloy and composites with different wt.% of Al2O3 particles.

A356-10% A356-15% Al2O3 Al2O3 2.8 2.87

Properties\ materials

A356

Theoretical density

2.685

Measured density

2.67

2.78

Porosity%

0.56

0.7

A356-20% Al2O3 2. 928

A356-25% Al2O3 2.988

2.834

2.872

2. 917

1.25

1.9

2.35

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Fig.(5): Effect of wt.% of Al2O3 particles on the density of the A356 alloy-matrix composites.

Fig.(6): Effect of wt.% of Al2O3 particles on the porosity of the A356 alloy-matrix composites.

3.4.Hardness Hardness tests were performed using Vickers Hardness machine and the results of the test are shown in Fig.(7). As shown in this figure, the hardness increases with increasing wt.% of Al2O3 particles. The hardness of A356 matrix alloy was 68.24 HV, and increased to 160.2 HV, in the composite with 25 wt.% of Al2O3 particles. The hardness of the matrix alloy improve by 32%, as 10 wt.% Al2O3 particles was added. Moreover, this ratio increased to 57% in the composite with 25 wt.% of Al2O3 particles addition. This improvement was because of the hardness of pure Al2O3 and pure A356 Al-alloy are 1800HV and 68.24 HV, respectively [10].

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Fig.(7): Vickers hardness values of the composites with different wt.% of Al2O3.

3.4.Conclusion The present study aims to produce A356 alloy-matrix composites with different wt.% of Al2O3 particle by stirring casting method. The main conclusions drawn from the current study were: 1- A356 alloy metal matrix composites reinforced with different weight percentage of Al2O3 particles (up to 25%) have been successfully fabricated by stir casting method. The optimum process parameters were found to be as follows: pouring temperature: 750 ◦C, stirring speed: 700 rev min−1, stirring time: 5 minutes after the completion of particle feeding, particle addition rate: 5 g min−1. 2- Good wettability was achieved by addition of 1% Mg and heat treatment of Al2O3 particles at 900◦C. 3- Density and porosity values of the composites increased with increasing wt.% of Al2O3 particles. 4- Hardness values of the composites increased with increasing wt.% of Al2O3 particles. REFERENCES [1] A.Baradeswarana, A.Elayaperumalb, and R. Franklin Issaca, “A Statistical Analysis of Optimization of Wear Behaviour of Al-Al2O3 Composites Using Taguchi Technique”, Procedia Engineering 2013; 64: 973 [2] S. Tahamtan, A.Halvaee, M.Emamy, M.S.Zabihi, "Fabrication Of Al/A206–Al2O3 Nano/Micro Composite By Combining Ball Milling And Stir Casting Technology”, Materials and Design 2013;49: 347. [3] G.R. Li, and Y.T. Zhao, "Fabrication And Properties Of In Situ (Al3Zr+Al2O3)P/A356 Composites Cast By Permanent Mould And Squeeze Casting", Journal of Alloys and Compounds 2009; 471: 530. [4] Z. Mišković, I. Bobić, S. Tripković, A. Rac, and A. Vencl, "The Structure and Mechanical Properties of an Aluminium A356 Alloy Base Composite With Al2O3 Particle Additions", Tribology in industry, 2006;28:23. [5] Mohsen Hossein-Zadeh, Omid Mirzaee, and Peyman Saidi, “Structural And Mechanical Characterization Of Al-Based Composite Reinforced With Heat Treated Al2O3 Particles”, Materials and Design 2014; 54:245. [6] B.C.Pai, R.M. Pillai and K.G. Satyanarayana, "Stir Cast Aluminum Alloy Matrix Composites", Key Engineering Materials 1993; 79-80 : 117. [7] N.Shao, J.W.Dai, G.Y.Li and T.Hane, "Effect Of La On Wettability Of Al2O3 By Molten Aluminum", Materials letters 2004; 58:2041

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[8] M. Kok, "Production And Mechanical Properties Of Al2O3 Particle-Reinforced 2024Aluminium Alloy Composites", Journal of Materials Processing Technology 2005; 161: 381 [9] P.K.Rohatgi, R. Asthana, and S.Das, "Solidification, Structures And Properties Of Cast Metal Ceramic Particle Composites", International Metals Reviews 1986; 31;115. [10]Shang-Nan Choua, Jow-Lay Huanga, Ding-Fwu Lii, and Horng-Hwa Lu, “The Mechanical Properties Of Al2O3/Aluminum Alloy A356 Composite Manufactured By Squeeze Casting”, Journal of Alloys and Compounds 2006; 419:98

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