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Investigation of Mechanical Behavior of Stir Casted Al Based Composites Reinforced With B4C Nanoparticles

Abstract. In this study, aluminum alloy (Al-2wt. % Cu) matrix composites reinforced with 1, 2 and 4 wt. % boron carbide nanoparticles with average size of 80 nm were fabricated via stir casting method at 850°C. The microstructures of composites were studied by scanning electron microscope (SEM). Density measurement, tensile and compressive tests were carried out to identify the mechanical properties of composites and effect of B4C nanoparticles amount. In all fabricated composites, severe agglomeration was observed in the micrographs. With increasing the amount of B4C nanoparticles up to 2 wt. %, yield and tensile strength increased but with more increasing B4C content they were decreased. Also, the compressive strength of samples was increased with increasing weight percentage of B4C nanoparticles.

Introduction

Composites containing discontinuous reinforcements especially particulate metal matrix composites have found commercial applications [1, 2] because they can be fabricated economically by conventional techniques. Al–alloy based composites have attracted attentions due to their processing flexibility, low density, high wear resistance, heat treatment capability and improved elastic modulus and strength [3]. Ultra fine particles such as nanoparticles used as reinforcement noticeably reduce interparticle spacing resulting in increased mechanical properties. On the other hand, nanoparticles have a high tendency to form agglomerates. Thus, for each technique and matrix, it is important to find out the optimum size, reinforcement content and parameters of fabrication to minimize agglomeration [4].

Factors such as different particle sizes, density, geometries, flow or the development of an electrical charge during mixing may lead to agglomeration [5]. In this process, mixing of matrix and reinforcement is a critical step to obtain a homogenous distribution of reinforcing particles in matrix. Since by reducing ceramic particle size the stress concentration level on each particle is decreased and makes it difficult to be fractured, nanoscale ceramic particles have attracted attentions in academia and industry [6, 7].

Recently, B4C reinforced composites have been manufactured via various techniques [8-11]. B4C is the third hardest material after diamond and CBN, Furthermore B4C has a lower specific gravity (2.51 g/cm3 which is less than Al with 2.7 g/cm3), high wear and impact resistance, high melting point, good resistance to chemical agents and high capacity for neutron absorption. B4C is a proper candidate as reinforcement in Al matrix composites [12].

Generally, wettability of the reinforcement ceramic particles by a liquid metal is very poor. Good wetting between ceramic particles and liquid metals leads to a proper bonding between these two during and after casting [13].Various techniques like pretreatment of particles [14], adding surface active agents into the matrix [15], coating or oxidizing the ceramic particles [16], cleaning the particle surface by ultrasonication and different etching methods [17] have been tried to improve wettability.

Among various techniques to fabricate metal matrix composites reinforced with ceramic particles, stir casting is one of acceptable routes for commercial production. However, this method needs delicate optimization of parameters like casting temperature, stirring velocity, reinforcement content, etc [18, 19]. In order to improve ductility of as-cast composites application of plastic deforming processes such as hot extrusion is necessary [20,21].

In this research, three composites with different B4C content as reinforcement were fabricated via stir casting. B4C nanoparticles were wrapped in aluminum foil to facilitate addition to the molten aluminum. The casting temperature was fixed and simultaneous stirring of molten aluminum at constant stirring velocity was carried out. The role of the reinforcement content on the strength and ductility of the produced composites were investigated.

Materials and Methods

Al-2%wt. Cu was used as the matrix and nano-sized B4C with the average paricle size of 80 nm was employed as the reinforcement in fabrication of samples.

According to Canakci et al., the nanoparticles were pretreated in the following procedure; Chemical pre-treatment holding in acid mixture (50 vol. % HF + 50 vol. % H2SO4) for 3 min, mixture was diluted with ethanol then ultrasonic cleaning in ethanol and air drying at room temperature for 6 h and then oven drying at 150°C for 24 h and finally calcination at 400°C for 3 hrs [22].

The samples were prepared using a resistance furnace equipped with a stirring system. After smelting of aluminum ingots, stirring was carried out for 4 minutes before adding the particles at constant rate of 420 rpm and continued stirring for 13 minutes after adding the particles. The casting temperature was 850°C and the melt poured in a steel mold to obtain ingots of 35 mm diameter and 70 mm height. Finally, the as-cast samples were prepared for next microstructural and mechanical analyses.

Bulk density measurement was carried by Archimedes method. Theoretical density was calculated by using simple rule of mixtures. Microstructural studies of fabricated as-cast composites were carried out by scanning electron microscope (SEM-Philips XL 30).

The tension tests were carried out in air at room temperature (Instron Universal Testing Machine-1195 machine) according to ASTM-B557. At least 3 specimens were used for each composite sample. Brinell method was used to measure the hardness of samples after grinding and polishing them down to 1µm.At least 5 indentations on two polished specimens were done to obtain data of hardness.

Results and Discussion

Microstructural Studies of Fabricated As-Cast Composites The microstructural examination of fabricated composites generally revealed that B4C particles were not distributed uniformly in the matrix and regional clusters of particles exist. Typical microstructures of the composites are shown in Fig 1. Since the wettability of particles by molten matrix is poor a uniform distribution of particles cannot be observed in the composites fabricated by stir casting. In addition, other factors like stirring speed, pouring conditions, solidification rate, etc. have noticeable influence on the distribution of particles [23].

2730Manufacturing Science and Technology, ICMST2011

Fig. 1 SEM images of as-cast sample containing (a) 1 wt. % B4Cp (b) 2 wt. % B4Cp (c) 4 wt.% B4Cp

Density and porosity measurements The measured densities of composites vs. B4C nanoparticles content are shown in Fig. 2(a).It is clear that by increasing the reinforcement content density was decreased. The results of measured densities of fabricated as-cast samples demonstrate that by increasing B4C nanoparticles content density was decreased because of higher possibility of agglomeration at higher percentages of nanoparticles. Agglomeration, in turn, leads to porosity formation. In short, by increasing nanoscaled reinforcements, porosity content increased. This result is confirmed by porosity content vs. amount of B4C nanoparticles in Fig. 2(b).

Fig. 2 (a) Relative density of fabricated as-cast samples vs. weigh percent of reinforcement (b) Volume percent of porosity in fabricated as-cast samples vs. weigh percent of reinforcement of fabricated samples

Tensile behavior The results of tensile tests are presented in Fig. 3. It is clear from Fig. 3 that by increasing B4C particle content up to 2 wt.%, the yield and the ultimate tensile strength (UTS) was increased but at 4 wt.% they were decreased. The beneficial effect of B4Cp addition, up to an optimal volume fraction, on the strength could be explained by the reduction of mean free path with increasing B4Cp volume fraction, and also with the increased density of dislocations generated as a result of the difference in thermal expansion coefficients of the matrix and reinforcement. In the sample containing 4 wt. % B4Cp, the mentioned beneficial effect of B4Cp was weakened by the noticeable porosity content which leads to decreased values of the yield strength and UTS.

Also, it can be understood that elongation to fracture, which is a measure of ductility, in fabricated as-cast composites was low. This low level of ductility in as-cast state may be ascribed to the high porosity content; early void formation at low strains during tensile elongation and heterogeneous particle distribution, therefore, ductility is expected to decrease with increasing reinforcement content [24].