Microstructural and mechanical properties of Al-4.5 wt% Cu reinforced ...

International Journal of Minerals, Metallurgy and Materials V olume 20 , Number 10 , October 2013 , P age 978 DOI: 10.1007/s12613-013-0824-2

Microstructural and mechanical properties of Al-4.5wt% Cu reinforced with alumina nanoparticles by stir casting method N. Valibeygloo1), R. Azari Khosroshahi1), and R. Taherzadeh Mousavian2) 1) Faculty of Materials Engineering, Sahand University of Technology, Tabriz 51335-1996, Iran 2) Department of Metallurgy, Zanjan Branch, Islamic Azad University, Zanjan 45156-58145, Iran (Received: 3 February 2013; revised: 8 March 2013; accepted: 14 March 2013)

Abstract: The microstructure and mechanical properties of Al-4.5wt% Cu alloy reinforced with different volume fractions (1.5vol%, 3vol%, and 5vol%) of alumina nanoparticles, fabricated using stir casting method, were investigated. Calculated amounts of alumina nanoparticles (about φ50 nm in size) were ball-milled with aluminum powders in a planetary ball mill for 5 h, and then the packets of milled powders were incorporated into molten Al-4.5wt% Cu alloy. Microstructural studies of the nanocomposites reveal a uniform distribution of alumina nanoparticles in the Al-4.5wt% Cu matrix. The results indicate an outstanding improvement in compression strength and hardness due to the effect of nanoparticle addition. The aging behavior of the composite is also evaluated, indicating that the addition of alumina nanoparticles can accelerate the aging process of the alloy, resulting in higher peak hardness values. Keywords: metallic matrix composites; nanocomposites; aluminum; alumina; nanoparticles; casting; mechanical properties; microstructure

1. Introduction Aluminium matrix nanocomposites (AMNCs) having the grain size or particulate size less than 100 nm are gaining the significant attention in recent years. This is primarily due to their lightweight, low coefficient of thermal expansion, machinability, and mechanical properties, such as 0.2% yield strength (YS), ultimate tensile strength (UTS), and hardness. Because of these advantages, they are used in aerospace industries (airframe and aerospace components), automobile industries (engine pistons), and electronic components. Extensive works have been carried out to evaluate various ceramic particles as the reinforcement materials for AMNCs. Therefore, alumina is a suitable choice as the reinforcement due to its outstanding mechanical properties and thermodynamical stability with aluminum, as no detrimental reaction occurs at high temperature [1-10]. Many techniques have been developed for producing the particulate reinforced AMNCs, such as powder metallurgy and squeeze casting. Besides the fact that each of these methods has its own advantages and disadvantages, Corresponding author: R. Taherzadeh Mousavian

they are all relatively expensive. Nowadays, researchers are focusing on producing low-cost composites. Stir casting (vortex technique) is generally accepted as a commercial low-cost method. Its advantage lies in its simplicity, flexibility, and applicability to the large volume production. This process is the most economical of all the available routes for AMNCs production and allows very largesized components to be fabricated. However, several difficulties in stir casting are of concern as follows [3, 11-23]: (1) chemical reactions between the reinforcement material and matrix alloy, (2) porosity in the cast AMNCs, (3) wettability between the two main substances, and (4) difficulty in achieving a uniform distribution of the reinforcement material. It is an extremely challenging mechanical stirring method to distribute and disperse nanoparticles uniformly in molten metals because of the poor wettability and higher specific surface areas of nanoparticles, which lead to agglomeration and clustering, especially in the case of alumina ceramic in molten aluminum [10]. Recently, some authors [3, 21-23] reported that the mixing of ceramic re-

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N. V alibeygloo et al., Microstructural and mechanical properties of Al-4.5wt% Cu reinforced with alumina ... 979

inforcement with aluminum powders by milling process would lead to an increase in the wettability of ceramic materials with molten aluminum. In this study, nano alumina powders were used as the reinforcement, leading to no reaction between these powders and molten aluminum; the wettability increment was investigated using a high-energy planetary ball mill to mix and mill nano alumina with aluminum powders, and the packets of milled powders were introduced in the Al-4.5wt% Cu molten metal.

2. Experimental An aluminum ingot with 99.8% commercial purity was alloyed with 4.5wt% copper. Analysis of Al-4.5wt% Cu ingot casting is given in Table 1. Table 1. Al 0.013

Chemical composition of Al-4.5wt% Cu wt% Cu 0.002

Si 0.25

Fe 0.36

Mg 4.47

Zn 94.87

Mixtures of Al powders (about φ35 μm in size) and 50wt% nano alumina powders (about φ50 nm in size) were ball-milled in a high-energy planetary ball mill for 5 h with Table 2.

∼1wt% stearic acid solid (CH3 (CH2 )16 ·COOH) as the process control agent at the rotating speed of about 300 r/min. The weight ratio of ball to powder was 10:1. To increase the mechanical bonding between aluminum and alumina powders, the milled powders were cold and pressed into the cylindrical specimen with a diameter of 4 cm under a pressure of 180 MPa for 10 min. These bulk samples were then grinded and passed through 60-mesh screen. About 1 g powder mixture was inserted in an aluminum foil to form a packet of composite with 1.5vol%, 3vol%, and 5vol% nano Al2 O3 as reinforcement. The Al-Cu alloy was heated up to 700◦ C using a bottom-pouring system. The graphite stirrer was placed below the surface of melt and rotated at a speed of ∼600 r/min, and simultaneously, the argon gas with high purity was blown to the melt surface. The packets were added to the vortex center. The stirring was continued for about 5 min, and temperature was decreased to 670◦ C for casting. Composite slurry was poured into a preheated cast iron mould. For comparison, the composite samples were produced with nano size Al2 O3 without addition of Al powders. Table 2 shows the characteristic of produced samples.

Characteristic of the produced nanocomposite samples

Production method Casting at 670◦ C Production of the composite with nano Al2 O3 powders without Al powders, casting at 670◦ C Production of the composite with the mixing of Al2 O3 Al powders, casting at 670◦ C Production of the composite with the mixing of Al2 O3 Al powders, casting at 670◦ C Production of the composite with the mixing of Al2 O3 Al powders, casting at 670◦ C

Microstructural investigations were performed by the scanning electron microscopy (SEM, Cam Scan Mv2300) with energy dispersive X-ray (EDX) analysis and optical microscopy. For this purpose, the samples were polished and etched with Keller’s reagent (190 mL water, 5 mL HNO3 , 3mL HCl, and 2 mL HF). Phase analysis of Al4.5wt% Cu samples was obtained using a Philips X Pert powder diffractometer with Cu Kα (λ=0.15405 nm) radiation. To estimate the amount of rejected particles (due to the insufficient wettability), the remained powders in the crucible were collected and weighted. The relative density of the samples was evaluated by the Archimedes method to study on the effect of alumina addition on the porosity formation. Aging studies were carried out to obtain the peak hardness values with respect to time. Specimens were solutionized at 540◦ C in a furnace for 2 h, followed by water quenching. The samples were then immediately kept in a refrigerator to avoid the diffusion of solute atoms at room temperature. Aging treatment was done at 180◦ C

Reinforcement / vol% No reinforcement

Sample code M

1.5

NA

1.5

NB

3

NC

5

ND

for various intervals of time (2.5, 3, 5, and 8 h). Mechanical properties of the samples were determined using a compression testing machine (DY-26 model) according to ASTM E9-89aR00 with the strain rate of 0.2 mm/min. Microhardness test was done based on ASTM E384 using an applied load of 490 mN for 10 s duration, and the results were reported for at least five parts of samples.

3. Results and discussion 3.1. Evaluation of rejection particles Evaluation results of rejection particles for samples are shown in Fig. 1. The particle rejection percentage of sample NA is the highest, wherein nano alumina powders directly incorporate into the molten metal without aluminum powders. By comparing samples NA and NB, it can be observed that milling of aluminum with nano alumina powders is so beneficial, leading to a considerable increase in the wettability of ceramic particles with the molten metal. Strong mechanical connections, which are created between aluminum and alumina particles during

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Int. J. Miner. Metall. Mater., V ol. 20 , No. 10 , Oct. 2013

the milling, give a better incorporation of reinforcements. Because of milling process, Al2 O3 particles are stuck between Al particles, and actually, Al particles, as the role of Al2 O3 particles carrier, improve the wettability. The same results were obtained in previous studies [3, 21-23]. Based on our previous investigations, the temperature at which the packets were incorporated in the vortex was very important, meaning that at high temperature, the aluminum powders were quickly melted, and no considerable increase was obtained in the wettability of ceramic powders. In this study, the temperature was set between 670◦ C and 700◦ C.

Fig. 1.

Particle rejection percentage of different sam-

ples.

The surface of ceramic particles is normally covered by layers of gas [18]. This matter and the oxide layer on the surface of molten metal are the main reasons for the poor wettability of ceramic particles with molten aluminum. The highest rejection percentage of particles is for sample NA. Mechanical milling of nanoparticles with Al powders, creating a cover of ductile aluminum with a large portion of particles, causes a significant reduction in levels of reinforcement contact with gas layers, and causes an appropriate surface between the nanoparticles and aluminum. The temperature of melt is not much higher than the melting point of aluminum, resulting in a low-speed melting of aluminum powders. Therefore, the aluminum powders can carry the reinforcement particles into the melt. The rejection percentage of particles is less than 20% for sample NB, while it is revealed that, by increasing the amount of reinforcement to 5vol%, more rejection occurs.

3.2. Microstructural characterization Because of the fine particle size of reinforcement, the particles observation is impossible using an optical microscope. The solidification structure of nanocomposite samples is shown in Fig. 2. Nano particles lead to the refining of grains, making a coaxial finer grain structure than the matrix alloy, which is attributed to the good efficiency of particles entering, as shown in Figs. 2(a) and (d) [24]. In fact, the growth is restricted by ceramic particles, because they can slow down the velocity of the solidification front, so the local solidification time increases and more nuclei form, leading to grain refinement [24]. In sample NA, no particles were entered into the melt.

As shown in Fig. 2(b), the considerable amounts of gas holes are observed due to the sticking of environmental gasses and suction of air bubbles into the melt during casting. Compared with Figs. 2(c) (sample NB) and (d) (sample NC), it can be concluded that the particles act as the homogeneous nucleation sites, and when the volume fraction of particles increasing, this effect becomes obviously. On the other hand, an increase in the volume fraction of particles leads to an increase in the amount of clustering and a decrease in the efficiency of particle entrance. Therefore, by comparing samples NC and ND (Figs. 2(d) and (e)), the growth of grains can be observed. In Fig. 2(e) (sample ND), porosities can be observed due to the trapped gases in the melt. Fig. 3 shows the SEM image of the matrix alloy (Al4.5wt% Cu). Based on the X-ray diffraction (XRD) pattern in Fig. 4 and EDX analysis in Fig. 5, the white phase in Fig. 3 is Al2 Cu intermetallic phase. In Figs. 6-8, the SEM images of nanocomposite samples are given. The reinforcement particles with the fine size below 1 μm are clearly shown in these microstructures. As shown in these microstructures, some of Al2 O3 particles are agglomerated, which is attributed to the fine grain size. Fig. 7 presents the EDX spectra of two particular points. The presence of Al and Cu peaks in point A as well as Al and O peaks in point B indicates the existence of Al2 Cu and Al2 O3 phases in the microstructure.

3.3. Porosity measurements Fig. 9 shows the amounts of porosity. The arrestment of environmental gases in particle masses causes porosity formation during casting [25]. According to Fig. 9, in sample NA (no particles entering into the melt), a significant amount of gas porosity may be due to the trapped gasses and suction of environmental gasses into the melt during the mixing process, leading to ∼2% porosity. While a lower amount of porosity is observed in sample NB, meaning that the milling process of nano alumina-aluminum powders highly affects the amounts of porosity. On the other hand, clustering can be encouraged with an increase in volume fraction of reinforcement particles, leading to porosity formation between particles, which is known as the interparticle porosities. Gas porosity is also increased with the increasing amounts of reinforcement. High viscosity of the melt prohibits the gasses to exit, leading to the increment of trapped gases.

3.4. Compression test The main purpose of metal matrix composite fabrication is to improve the mechanical properties. To evaluate the effect of alumina nanoparticles on the compressive properties of the composite, compression test was carried out on the standard samples. Fig. 10 shows the yield stress (YS) values of the composites. As expected, milling


of ceramic particles with aluminum powders leads to a considerable increase in the mechanical properties of the composites due to incorporation of ceramic particles with a suitable distribution. The principal strengthening mechanisms for the composites may include the load transfer mechanism, dislocation density increment, and interaction of dislocation and particles, such as Orowan strengthening, refining grain size, and increasing plastic constraint [26-27]. As shown in Fig. 10, the flow stress of the nanocomposite increases with the increasing amount of reinforcement up to 3vol%, while the further presence of reinforcement causes a decrease in flow stress. The amount of defects

(particle agglomeration, porosity, etc.) will be increased with the increase of alumina nanoparticles, leading to the reduction in mechanical properties. It should mentioned that the yield stress of sample NA is lower than sample M, which is attributed to high porosity levels in as-cast samples due to entrance of air bubbles into the melt, caused by vortex formation.

3.5. Microhardness measurements Hardness of composite samples is higher than the matrix alloy, as shown in Fig. 11. This can be attributed to the presence of ceramic particles in the matrix, and the alumina particles have a higher hardness than the matrix

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Fig. 3.

SEM image of Al-4.5wt% Cu alloy (sample M).

Fig. 4.

XRD pattern of Al-4.5wt% Cu alloy (sample

M).

Fig. 7. Fig. 5.

EDX spectrum of the white phase in Fig. 3.

Fig. 6.

SEM image of 3vol% nanocomposite (sample

NC).

SEM image (a) and EDX spectra of points A

(b) and B (c) in Fig. 7(a).

Fig. 8. (NB).

SEM image of the 1.5vol% nanocomposite


Fig. 9.

Variation of porosity with Al2 O3 particle con-

tents.

Fig. 10.

hardness measurements of the matrix alloy and composite samples at different aging times are shown in Fig. 12. It is reported [24-25, 27] that due to the different coefficient of thermal expansion (CTE) between the matrix and reinforcement during solidification, a large number of dislocations are generated around the reinforcement, leading to the formation of heterogeneous nucleation sites for precipitates. That is the reason why the nano composite samples have higher microhardness values after a lower aging time than the monolithic samples.

Variation of yield stress for nanocomposite

as-cast samples.

Fig. 12.

Age hardening behavior of casting samples at

different aging times.

Fig. 11.

Vickers microhardness values for casting sam-

ples.

alloy. Alumina particles act as obstacles to plastic deformation and cause a reduction in grain size of the matrix, which is one reason of hardness increase. However, as can be seen, by increasing the volume fraction of reinforcement, an ascendant trend is observed, while the last sample (ND), which contains 5vol% alumina nanoparticles, has a lower value of hardness in respect to sample NC. Porosity formation could lead to crack initiation during local plastic deformation, leading to a reduction in hardness. Aging of Al-4.5wt% Cu alloy leads to an appropriate distribution of Al2 Cu phase in the matrix. The precipitate particles (Al2 Cu phase) act as obstacles to dislocation motion and thereby strengthen the heat-treated alloy [27]. As previous authors [24-25, 27] reported, the presence of ceramic reinforcement could lead to the acceleration in ageing kinetic, comparing to the unreinforced alloy. Micro-

It is observed that the solutionized hardness is increased with the increase of nanoparticles. Before reaching the peak, the hardness of the composite is higher than the matrix for all durations. Based on other investigators’ reports [26-27], an approximate time required in Al-4.5wt% Cu alloy to achieve a maximum hardness (Hv 110) may be about 55 h. It should be mentioned that by increasing the amount of nanoparticles, no change can be observed for the steps of age hardening process. However, the existence of ceramic particles can accelerate and increase the production of intermediate phases as well as their volume fractions. Thus, the composites samples reach faster to the peak of hardness (comparing sample M with NB). The microhardness increases with the aging time to a peak value, and then decreases with the prolonged ageing time.

4. Conclusions (1) Production of aluminum alloy nanocomposites is possible by the stir casting method. (2) The milling process highly affects incorporation of nanoparticles into molten aluminum. (3) Porosity amounts are increased with the higher amount of reinforcement in samples. (4) Compressive yield strength of the composite is considerably higher than that of the matrix alloy. (5) The hardness of the nanocomposite is higher than that of the matrix alloy. (6) Under the heat treatment conditions, the micro-

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hardness increases with the aging time to a peak value, and then decreases after a prolonged aging time. Addition of nanoparticles to Al-4.5wt% Cu alloy considerably accelerates the aging kinetic, and also leads to an increase in hardness, compared with the monolithic alloy. Increasing the volume fraction of reinforcement causes a decrease in microhardness.

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