Investigation of microstructure and mechanical properties of Al6061-nanocomposite fabricated by stir casting

doi:10.1016/j.matdes.2013.10.060

Materials & Design

Volume 55, March 2014, Pages 921-928

https://doi.org/10.1016/j.matdes.2013.10.060 Get rights and content

Highlights

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Nano-Al2O3/Al composite is fabricated by developed stir-casting process.
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Injection Al/Al2O3 powder is benefit way for improving nanoparticles distribution.
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Nanocomposites present a fine grain microstructure with high porosity.
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Extrusion process improves significantly mechanical properties of nanocomposites.

Abstract

In comparison to unreinforced alloy, Al6061 reinforced with hard particles possesses higher strength, stiffness, and wear and creep resistance in a wide temperature range. They can be used for automotive components and aircraft structures. The aim of the present work is to investigate the effects of adding nano-alumina particles to Al6061 alloy and extrusion process on the mechanical and microstructure properties of the composites. For this purpose, Al6061/nano Al₂O₃ composites were fabricated by stir casting process through injection of Al₂O₃ particles into the molten Al alloy by argon gas along with stirring using a mechanical stirrer. The nanocomposites were then extruded at 550 °C. The microstructure results revealed that injection of powders in the form of Al/Al₂O₃ and application of extrusion process led to fabrication of nanocomposites with reasonable distribution of Al₂O₃ in the matrix alloy with low agglomeration and low porosity. In addition, the extruded samples showed strength and ductility values superior to those of the as-cast samples. It was also found that for both as-cast and extruded samples, with increasing amount of Al₂O₃ nanoparticles, yield strength and tensile strength increased but elongation decreased.

Introduction

Al6061, originally called “Alloy 61S”, is a precipitation hardening aluminum alloy containing magnesium and silicon as its major alloying elements. The alloy has good mechanical properties and exhibits good weldability [1]. It is one of the most common aluminum alloys for general-purpose use. Metals reinforced with hard particles are very interesting in achieving enhanced mechanical properties such as: hardness, Young’s modulus, yield strength and ultimate tensile strength [2]. Generally, regarding to the mechanical properties, the reinforcements result in higher strength and hardness, often at the expense of some ductility.

In metal-matrix composites (MMC), relatively soft alloy like aluminum can be made highly resistant by introducing predominantly hard but brittle particles such as: Al₂O₃, SiC, and B₄C. Commonly, micro-sized particles are used to improve yield and ultimate strength of alloys. However, ductility of MMCs decreases significantly with increasing ceramic particle concentration. Although, application of nano-sized ceramic particles strengthens the metal-matrix composites while maintaining good ductility, high temperature creep resistance and better fatigue strength [3], [4].

The fabrication methods of the composites can be categorized into three processes: solid-state methods, semi-solid-state methods and liquid-state methods. Stir casting is a liquid-state method of composite materials fabrication, in which a dispersed phase (ceramic particles or short fibers) is mixed with a molten matrix metal by means of mechanical stirring. Compared with solid-state methods, melt processing which involves stirring of ceramic particles into melt, has some important advantages: better matrix-particle bonding, easier control of matrix structure, simplicity, low cost of processing and nearer net shape and the wide selection of materials for this fabrication method [5], [6]. Depending on the temperature at which the particles are introduced into the melt, there are two types of melting methods for making composites. In the liquid metallurgy process the particles are incorporated above the liquid temperature of the molten alloy, while in compocasting method the particles are incorporated into the semi-solid slurry [7].

Amongst the various classical metal-forming procedures, extrusion process is generally used as the secondary processing of discontinuously reinforced composites which can lead to the breakup of particle (or whisker) agglomerates, to the reduction or elimination of porosity, and to improved bonding. All of them contribute to improve the mechanical properties of MMCs [8], [9], [10].

In this research, in order to improve the wettability and distribution of nano-sized Al₂O₃ particles within the matrix with low agglomeration and to achieve the good mechanical properties, a new fabrication method was used. The method included injection of the milled nano-Al₂O₃/Al composite powder within the molten by inert gas and mechanical stirring. Finally, the composites were extruded at constant temperature. The microstructure and mechanical properties of the fabricated nanocomposites were studied.

Section snippets

Raw materials

For fabrication of A6061–Al₂O₃ nanocomposites, A6061 alloy, with the chemical composition presented in Table 1, was used as the matrix. Nano-Al₂O₃ particles (supplied by Merck Company) with the average size of 40 nm and aluminum powders (supplied by Merck Company) with the average size of 50 μm were used as components of reinforcement powders.

Mixing of nano-Al₂O₃/Al composite powder by ball-milling

Mechanical milling was performed in a planetary ball mill machine. To prevent severe cold welding, ethanol alcohol (3 wt.%) was added into the mixture during

Density and porosity

The experimental density of the composites was measured by the Archimedean method of weighing small pieces cut from the composite cylinder first in air and then in water. In addition, the theoretical density was calculated using the mixture rule based on the known densities of the matrix alloy and Al₂O₃ particles which are 2.7 and 3.6 g/cm³, respectively [7], [11].

The effect of the reinforcement content and extrusion process on the observed porosity of the composite samples is illustrated in

Conclusions

Al6061–Al₂O₃ nanocomposites were fabricated by injection of nano-Al₂O₃ powder into the melt and stir casting process. Then composites were extruded at an extrusion ratio of 2.8 and at 550 °C. For investigation of the effects of the addition of mixed powders and hot extrusion process, microstructure and mechanical properties of the samples in as cast state and in extruded condition were investigated. The following results were obtained:

1.
Nanocomposites exhibited a fine grain microstructure with

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Investigation of microstructure and mechanical properties of Al6061-nanocomposite fabricated by stir casting

Highlights

Abstract

Introduction

Section snippets

Raw materials

Mixing of nano-Al2O3/Al composite powder by ball-milling

Density and porosity

Conclusions

J Alloys Compd

J Mater Des

J Alloys Compd

J Mater Process Technol

J Mater Process Technol

J Alloys Compd

J Mater Proc Technol

Trans Nonferr Met Soc China

J Mater Sci Technol

J Mater Sci Eng A

Mixing of nano-Al₂O₃/Al composite powder by ball-milling