Hot Extrusion Process Effect on Mechanical Behavior of Stir Cast Al Based Composites Reinforced with Mechanically Milled B 4 C Nanoparticles

Transcription

1 J. Mater. Sci. Technol., 2011, 27(12), Hot Extrusion Process Effect on Mechanical Behavior of Stir ast Al Based omposites Reinforced with Mechanically Milled Nanoparticles A. Alizadeh 1), E. Taheri-Nassaj 1) and M. Hajizamani 2) 1) Department of Materials Science and Engineering, Tarbiat Modares University, P.O. Box , Tehran, Iran 2) Faculty of Materials and Manufacturing Processes, MUT University, P.O. Box , Tehran, Iran [Manuscript received April 21, 2011, in revised form July 26, 2011] In this study, aluminum alloy (Al 2 wt% u) matrix composites reinforced with 1, 2 and 4 wt% boron carbide nanoparticles fabricated through mechanical milling with average size of 100 nm were fabricated via stir casting method at 850. ast ingots of the matrix alloy and the composites were extruded at 500 at an extrusion ratio of 10:1 to investigate the effects of hot extrusion on the mechanical properties of the composites. The microstructures of the as-cast and the extruded composites were investigated by scanning electron microscopy (SEM). Density measurement, hardness and tensile tests were carried out to identify the mechanical properties of the composites. The extruded samples revealed a more uniform distribution of nanoparticles. Also, the extruded samples had strength and ductility values superior to those of the as-cast counterparts. In the as-cast and the extruded samples, with increasing amount of nanoparticles, yield strength and tensile strength increased but elongation to fracture decreased. KEY WORDS: Stir casting; Hot extrusion; Al matrix composite; nanoparticles; Mechanical milling 1. Introduction omposites containing discontinuous reinforcements especially particulate metal matrix composites have found commercial applications [1 3] because they can be fabricated economically by conventional techniques. Al-alloy based matrix composites (AMs) have attracted attentions due to their processing flexibility, low density, high wear resistance, heat treatment capability and improved elastic modulus and strength [4]. AMs are fabricated by incorporating ceramic particles like Si,, Al 2 O 3 with particle size of micron or nano-scale into Al alloy matrix [5]. Ultra fine particles such as nanoparticles noticeably reduce interparticle spacing resulting in the increase of mechanical properties. On the other hand, they have a high tendency to form agglo- orresponding author. Prof., Ph.D.; address: taheri@modares.ac.ir (E. Taheri-Nassaj). nanoparticles merates. Thus, for each technique and matrix, it is important to find out the optimum size, reinforcement content and parameters of fabrication to minimize agglomeration [6]. Factors such as different particle sizes, density, geometries, flow or the development of an electrical charge during mixing may lead to agglomeration [7]. 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 decreases and makes it difficult to be fractured, nanoscale ceramic particles have attracted attentions in academia and industry [8,9]. Recently, reinforced composites have been manufactured via various techniques [10 14]. is the third hardest material after diamond and cubic boron nitride (BN). Furthermore, has a lower specific gravity (2.51 g/cm 3, which is less than that

2 1114 A. Alizadeh et al.: J. Mater. Sci. Technol., 2011, 27(12), Table 1 haracteristics of Average particle size/nm Hardness/(kg/mm 2 ) Density/(g/cm 3 ) Melting point/ of Al with 2.7 g/cm 3 ), high wear and impact resistance, high melting point, good resistance to chemical agents and high capacity for neutron absorption. is a proper candidate as reinforcement in Al matrix composites [15,16]. It can be considered as an alternative to Si reinforced composites, where a high stiffness or a good wear resistance is required [17]. Various mechanical methods like high energy processes such as planetary, attrition and jet milling are available to prepare ultra fine particles [18]. Attrition mill has been widely used for fine grinding of different materials. The main advantages of attrition mills are relatively high energy utilization, fast and efficient fine grinding and simple operation [19]. 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 them during and after casting [20]. Various techniques like pretreatment of particles [21], adding elements such as magnesium and lithium into the matrix as surface active agents [22,23], coating or oxidizing the ceramic particles [24,25], cleaning the particle surface by ultrasonication and different etching methods [26,27] have been tried to improve the wettability. Among various techniques to fabricate metal matrix composites (MMs) 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 [28,29]. In as-cast AMs, inferior ductility limits their performance and applications [30]. Parameters like matrix microstructure, distribution of reinforcements, porosity content affect the ductility. In order to improve ductility application of plastic deforming, processes such as hot extrusion are necessary [31 33]. Secondary processing of the discontinuously reinforced MMs can result in breaking up of particle agglomerates, reduction or elimination of porosity, and improvement of bonding, all of which lead to improvment of mechanical properties [34]. onventional deformation processes including rolling, extrusion and forging can be used to form MMs, among which extrusion is the most common secondary processing due to its excellent preferential axial alignment of discontinuous fibers as well as large compressive hydrostatic state of stress [35]. However, it is necessary to note that the presence of brittle and almost nondeformable reinforcements in MMs may cause undesirable phenomena like reinforcement fracture, debonding of interface, or surface cracking in the extruded product [36]. In this research, three composites with different content as reinforcement were fabricated via stir casting. 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. Then the as-cast composites were extruded. The role of the reinforcement content and hot extrusion process on the strength and ductility of the produced composites were investigated. 2. Experimental Al 2 wt% u was used as the matrix and nanosized was employed as the reinforcement in fabrication of the samples. nanoparticles were synthesized by milling original powders with the mean size of 0.8 µm in an attrition mill (union process, model 1-S) using a hardened stainless steel vial and hardened stainless steel balls with 6 mm in diameter. The milling time was 140 h. The ball to powder ratio and rotational speed were 15:1 and 400 r/min, respectively. Isopropyl alcohol was used as the milling media and the grinding media occupied 80% of the chamber volume. The final mean size of the particles was 100 nm. Some characteristics of powder are listed in Table 1. According to anakci et al. [37], the nanoparticles were pretreated in the following procedure: firstly holding in an acid mixture (50 vol.% HF + 50 vol.% H 2 SO 4 ) for 3 min, the 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 for 24 h and finally calcination at 400 for 3 h. The samples were prepared using a resistance furnace equipped with a stirring system. After smelting of aluminum ingots, stirring was carried out for 4 min before adding the particles at a constant rate of 420 r/min and continued stirring for 13 min after adding the particles. This stirring rate was employed based on the previous studies [38]. The casting temperature was 850 and the melt was poured in a steel mold to obtain ingots of 35 mm in diameter and 70 mm in height. The cylindrical ingots were machined down to 30 mm in diameter, and then extruded to give rods of 10 mm in diameter and about 600 mm in length. The forming process was performed at 500 to yield an extrusion ratio of 10:1. Finally, the as-cast and the extruded samples were prepared for next microstructural and mechanical analyses. Bulk density measurement was carried out by Archimedes method. Theoretical density was calculated by using the simple rule of mixtures. Microstructural studies of the as-cast and the extruded

3 A. Alizadeh et al.: J. Mater. Sci. Technol., 2011, 27(12), Intensity / % (b) Unmilled 50 h 60 h 70 h 80 h 100 h 140 h Size / m (d) Intensity / a.u / deg. Fig. 1 (a) SEM micrographs of the as-received particles, (b) particle size distribution of the ball milled boron carbide, (c) TEM micrograph of particles after 140 h milling, (d) XRD patterns of the ball milled boron carbide particles samples were carried out by scanning electron microscopy (SEM, Philips XL 30). Micrograph of particles after milling was recorded through transmission electron microscopy (TEM, Hitachi H 800). The size of as-received and milled powders was quantified by a laser particle size analyzer (Horiba LB 550). The tensile tests were carried out in air at room temperature (Instron Universal Testing Machine-1195 machine) according to ASTM-B557. At least three specimens were used for each composite sample. Vickers method was used to measure the hardness of samples after grinding and polishing them down to 1 µm. At least five indentations on two polished specimens were done to obtain the data of hardness. 3. Results and Discussion 3.1 Morphology and particle size distribution of nanoparticles As-received powder with the mean particle size of about 0.8 µm was used as the starting material. SEM micrographs of as-received powder revealed that the powder had a wide size distribution and irregular shape as shown in Fig. 1(a). Figure 1(b) shows the variation of particle size distribution and median size (D 50 ) as a function of milling time measured by particle size analyzer. By increasing the milling times, particle size decreased and a narrow size distribution could be seen. During milling, a combination of fracture mechanisms occurs, which includes abrasion, compression and impact mechanism. The particle size distribution of a material after comminution is determined by the combination of all mechanisms. The predominant mechanism is determined according to the material properties and operating conditions. Since hard materials are difficult to be abraded on their surfaces, the impact mechanism plays a dominant role, and thus, the distribution curve showed a monomodal distribution. This result is consistent with that reported by Shinohara et al. [19] regarding grinding of diamond. Figure 1(c) presents the TEM image of the nanoparticles. The nanoparticles were spherical and had a uniform distribution. The XRD pattern, shown in Fig. 1(d), reveals that the nanoparticle of has rombohedral structure. 3.2 Microstructural studies of prepared composites The microstructural examination of the as-cast composites generally revealed that particles were not distributed uniformly in the matrix and regional clusters of particles existed. In the extruded samples, a more even distribution of p could be observed.

4 1116 A. Alizadeh et al.: J. Mater. Sci. Technol., 2011, 27(12), Fig. 2 SEM micrographs: (a) as-cast sample containing 1 wt% p, (b) as-cast sample containing 4 wt% p, (c) extruded sample containing 1 wt% p, (d) extruded sample containing 4 wt% p Relative density (a) Porosity / vol.% (b) Fig. 3 (a) Relative density of samples vs weight percent of reinforcement, (b) volume percent of porosity in samples vs weight percent of reinforcement Although clusters of p still existed in the extruded samples, their sizes were much smaller than those observed in the as-cast state. Typical microstructures of the composites in the as-cast and extruded state are shown in Fig. 2. Since the wettability of particles by molten matrix was poor, a uniform distribution of particles could not be observed in the composites fabricated by stir casting. In addition, other factors like stirring speed, pouring conditions, solidification rate, etc. had noticeable influence on the distribution of particles [39]. 3.3 Density and porosity measurements The measured densities of the as-cast and the extruded samples vs nanoparticles content are shown in Fig. 3(a). It is clear that with increasing reinforcement content, the density decreased in both the as-cast and the extruded samples. Also, the densities of the as-cast samples are lower than those of the extruded ones with the same content of reinforcement. It may be mainly because the volume percent of porosity in the as-cast composites were more than that of the extruded composites. This fact is shown in Fig. 3(b), which shows the porosity content of both kinds of samples. The higher amount of porosity in the as-cast samples can be ascribed to air bubbles entering the melt either independently or as an air envelope to the reinforcing particles [27]. The results of the measured densities of the as-cast samples demon-

5 A. Alizadeh et al.: J. Mater. Sci. Technol., 2011, 27(12), Table 2 Results of tensile tests applied to the as-cast and the extruded samples Sample Yield strength/mpa Ultimate tensile strength/mpa Elongation to fracture ±0.5/% Matrix Al 1 wt% Al 2 wt% Al 4 wt% True stress / MPa matrix 2 Al 1 wt% 3 Al 4wt% 4 Al 2 wt% (a) True stress / MPa (b) matrix 2 Al 1 wt% 3 Al 2 wt% 4 Al 4 wt% True strain / % True strain / % Fig. 4 True stress vs true strain curves: (a) as-cast samples, (b) extruded samples strated that with increasing nanoparticles content, the density decreased because of higher possibility of agglomeration at higher percentages of nanoparticles. Agglomeration, in turn, leads to porosity formation. In short, with increasing nanoscaled reinforcements, porosity content increased. This result is confirmed by porosity content vs amount of nanoparticles in Fig. 3(b). 3.4 Tensile behavior The results of tensile tests for the as-cast and the extruded samples are presented in Fig. 4(a) and (b), respectively. In order to make a comparison, these results are given in Table 2. In the as-cast state, it is clear from Table 2 that with increasing particle content up to 2 wt%, the yield strength and the ultimate tensile strength (UTS) increase but at 4 wt% they decrease. However, in the extruded samples, the values of yield strength and UTS continuously increase with increasing p content. In the as-cast composite samples, beneficial effect of p addition, up to an optimal volume fraction, on the strength could be explained by the reduction of mean free path with increasing p volume fraction, and also with increasing density of dislocations generated as a result of the difference in thermal expansion coefficients of the matrix and reinforcement. With the application of hot extrusion process, a moderate improvement in yield strength, and especially in the high reinforcement containing samples, a substantial increase in tensile strength were observed [40]. In the as-cast sample containing 4 wt% p, the mentioned beneficial effect of p was weakened by the noticeable porosity content, which resulted in the decrease of the yield strength and UTS. On the other hand, in the extruded composites, since the content of porosity was affected by the hot extrusion process and also the distribution of reinforcing p was more uniform (See Fig. 2), the nanoparticles retained their advantageous effect up to 4 wt% p. Thus, the yield strength and UTS increased with increasing reinforcement content. From the tensile test results presented in Table 2, it could be understood that elongation to fracture, which is a measure of ductility, in the as-cast composites was much lower than that of the extruded counterparts. This low level of ductility in the as-cast state could be ascribed to the high porosity content, early void formation at low strains during tensile elongation and heterogeneous particle distribution, and therefore, ductility was expected to decrease with increasing reinforcement content [41]. However, application of hot extrusion process improved the ductility of the composites significantly, leading to ductility levels about 3 to 10 times higher than those of the as-cast samples. This improvement of ductility after hot extrusion was mainly due to the decrease in amount of porosity in the extruded state, more homogenous particle distribution, improvement of particle-matrix bond and refinement of the matrix structure [40]. 3.5 ompressive behavior The result of compressive tests is shown in Fig. 5. It could be concluded from these results that with increasing p content, the compressive strength increases continuously both in the as-cast and the ex-

6 1118 A. Alizadeh et al.: J. Mater. Sci. Technol., 2011, 27(12), ompressive strength / MPa to resistance offered by particles [6]. This is the reason of the increase in compressive fracture strength with adding p content. Also, the compressive strength of the extruded samples was slightly more than that of the as-cast ones with the same p content. It could be primarily ascribed to more uniform distribution of reinforcements in the extruded state. 3.6 Hardness measurements Fig. 5 ompressive strength of as-cast and extruded samples vs reinforcement content Hardness / VHN Fig. 6 Measured values of hardness (VHN) vs weight percent of p content for as-cast and extruded samples truded samples. Although the porosity content of samples increases with increasing p content in both kinds of samples (see Fig. 3(b)), the compressive strength increases. This demonstrated that porosity content had no disadvantageous effect on the compressive strength and the content of reinforcement played a major role, i.e. the compressive strength increased with increasing p content. In the case of the composites, the plastic flow of matrix was constrained due to the presence of these rigid and very strong particles. The matrix could flow only with the movement of particle or over the particles during plastic deformation. While p content was significantly higher, the matrix became constrained considerably to the plastic deformation because of smaller interparticle distance and thus resulted in higher degree of improvement in flow stress. It has been understood that the plastic flow of the composite is due to the plastic flow of the matrix [42]. The strain-hardening of the composite is primarily due to the hardening of the matrix during its plastic flow. The strain-hardening of matrix is expected to be influenced by the following factors: (1) dislocation density and dislocation to dislocation interaction, (2) constraint of plastic flow due The hardness of the samples vs p content is presented in Fig. 6. It is clear that the hardness of all composites is higher than that of the matrix in the as-cast and the extruded state. This is because of the presence of hard nanoparticles. In the as-cast state, with increasing reinforcement content up to 2 wt% p the hardness increases but the hardness of the sample containing 4 wt% p decreases. This is due to heterogeneous distribution of nanoparticles and high porosity content. However, after applying hot extrusion process the hardness of all samples increases and the hardness increases continuously with increasing p content. This behavior is explained by reduced amount of porosity in the extruded samples and more homogenous distribution of nanoparticles in the extruded samples. 4. onclusions Al alloy based composites reinforced with nanoparticles were fabricated by stir casting at 850. The composites were extruded at an extrusion ratio of 10:1 and at 500. To investigate the effect of hot extrusion process, microstructural and mechanical behavior were investigated and the following results were concluded: (1) With increasing reinforcement content, densities decreased in both the as-cast and the extruded samples and the densities of the as-cast samples were lower than those of extruded ones with the same content of reinforcement due to higher level of porosity in the as-cast state. (2) The results of tensile tests revealed that for as-cast samples, both the yield strength and UTS increased with increasing particle content up to 2 wt% but in the extruded state, the values of yield strength and UTS continuously increased with increasing p content. The higher yield strength and UTS in extruded state could be due to less porosity content and more uniform distribution of p in the matrix. (3) Ductility of the as-cast composites was much lower than that of extruded ones because of higher porosity content, early void formation at low strains during tensile elongation and heterogeneous particle distribution. (4) The compressive strength of both the as-cast and the extruded composites increased with increas-

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