Production of Aluminum Matrix Composites via Hot Pressing

Summary

In situ metal matrix composites (MMCs) are multiphase materials in which reinforcement phases are synthesized through chemical reactions within the metal matrix during production. Particle-reinforced MMCs are widely used in weight-sensitive aerospace applications, industrial sectors and transportation. Al2O3-reinforced MMCs are used in nozzles, cylinder liners and rotors, while TiB2-reinforced MMCs are used in cutting tools and armor materials. The aim of this study is to investigate the effects of different sintering temperature and furnace dwell time parameters on the production of Al2O3 and TiB2-reinforced MMCs manufactured by in situ reaction hot pressing (HP) method. Al-TiO2 and B2O3 powders were used as starting materials. Powder mixtures were sintered at temperatures of 900-1000°C and furnace dwell times varying between 15-60 minutes. The microstructures of the obtained composites were examined using XRD and SEM techniques. Hardness values of samples subjected to pin-on-disk wear testing were measured using the Brinell hardness method. According to the results obtained; the number of particles synthesized within the composite increased proportionally with increases in both manufacturing temperature and furnace dwell time. These applied parameters resulted in increased hardness and wear resistance of the composites.

1. Introduction

Aluminum matrix composites are among the material groups frequently preferred especially in aerospace and automotive industries due to their low densities. Reinforcement elements applied to the matrix externally or formed within the matrix (Sharifi et al., 2011) modify the composite's properties including hardness, corrosion and wear resistance. Reinforcement elements are generally in the form of particles, whiskers and fibers (Tjong et al., 1999). Through the mixing of these hard reinforcements in the matrix, the existing properties of aluminum-based materials are improved (Jun et al., 2004). Hard ceramic particles such as SiC, TiC, B4C, Al2O3, Si3N4, TiB2 (Balcı et al., 2014) are among the most commonly added reinforcements to the matrix or synthesized within the matrix. High melting points, hardness, electrical conductivity, superior wear resistance and chemical stability are among the common properties of these compounds. In situ TiB2-reinforced Al MMCs can be produced using Ti and B powders through displacement reactions. In the same reaction system, the Al2O3 compound also serves as a second reinforcement in the matrix due to the combination of 'Al' and 'O2' elements in the matrix material. The use of oxide forms of starting materials contributes to reducing production costs (Zhu et al., 2008). In powder systems where Al and Ti elements are used together, Al3Ti intermetallic transition phases, which have a brittle structure and are located in the microstructure as long rods (>100 μm), can typically form (Tjong et al., 2004). It is possible to eliminate this intermetallic phase through the application of sintering temperature, furnace dwell time and heating rates (Onal and Gavgali, 2014).

2. Experimental Procedure

For aluminum metal matrix composite production, Al powder (99% purity; 1-5 μm), TiO2 (94% purity; 0.3-1 μm) and B2O3 powders (98.5% purity; 1-2 μm) were used as starting components in sequence. The powders were mixed for 45 minutes in a three-axis turbula mixer. Following the mixing operation, drying was carried out in an oven at 70°C for one hour to remove moisture. Cold pressing in a mold was applied at 70 bar pressure for two hours to increase powder densities. Sintering was performed using the conventional hot pressing method. Sintering was conducted in an argon gas atmosphere at temperatures of 900-1000°C at intervals of 15-60 minutes. To minimize porosity in the structure, composite samples were hot pressed at 10 bar pressure in the semi-solid/semi-liquid temperature range. The samples were then removed from the furnace and cooled to room temperature in air. Microstructural characterization studies were performed using XRD and SEM techniques (SEM, Zeiss Evo LS-10). The produced composites were subjected to pin-on-disk wear testing under a 20 N load, 2000 m sliding distance and 2 m/s sliding speed. Composite hardness was determined using the Brinell method - with 31.25 kgf and a 2.5 mm diameter ball for 10 seconds, taking at least 5 measurements from each sample (Innovatest 422D).

3. Results and Discussion

The XRD analyses of composites obtained after equal amounts of powder mixtures were sintered at 900-1000°C at intervals of 15-30-45 and 60 minutes in the furnace are given in Figure 1. Figure 1 shows that in the internal structure of the composites, the amount of aluminum, which is the matrix component, decreases with rising temperature. From this, it can be concluded that as aluminum quantity decreases, there is an increase in the number of Al2O3 particles synthesized in the composite body, and therefore the amount of aluminum reacting increases with rising temperature. In their 2001 study, Lü and colleagues observed that with increasing volume content of TiB2 reinforcement elements in the composite, the Al, Ti and B elements initially subjected to reaction began to decrease, resulting in an increase in Al2O3 and TiB2 content. The SEM images of the microstructures of composites produced at the same temperature (900°C) after being held in the furnace for 15 and 60 minutes are shown in Figure 2.   Figure 2.b shows that as furnace dwell time increases, there is also an increase in the number of particles formed. In their study on the synthesis of Al2O3 particles in pure aluminum, Dikici and Gavgalı state that as sintering time increases, there is a rapid increase in the formation rate and number of submicron α-Al2O3 particles in the microstructure (Dikici and Gavgalı, 2013). With increasing furnace dwell time, coarsening of the particles formed in the composite is observed. German M.R. notes the existence of similar information regarding particle coarsening in studies on sintering. From this, it can be concluded that increased sintering time affects the number and size of reinforcement particles. The graph showing changes in composite hardness values with temperature and furnace dwell time is as shown in Figure 3. At both temperatures, hardness values are seen to increase with increasing dwell times. Balcı et al., in their study to obtain TiB2 in an aluminum matrix using the mechanical alloying method, state that as the amounts of reinforcement elements increase, the hardness values of the composites also increase (Balcı et al., 2014). The graph showing the hardness values of composites sintered at different temperatures and dwell times is presented in Figure 3. The graph showing the change in weight loss with time resulting from measurement of initial weights and weights after wear of the produced composites under 20 N load, 2000 m sliding distance is as shown in Figure 4. Wear amount was calculated in 'mg/m' units.   The decrease in wear amounts observed (Figure 4), which confirms the increase in hardness values presented in Figure 3, explains that hardness and wear are inversely proportional as they increase. With increasing numbers of hard ceramic particles formed in the composite, the hardness values of the composites also increase, and consequently their wear resistance also increases. Kök in 2005 notes that the increase in the number and size of Al2O3 particles obtained by the vortex method in a 2024 aluminum alloy matrix increases the wear resistance of the composites (Kök, 2005).

4. Conclusion

In this study in which in situ Al2O3 and TiB2 particles were successfully synthesized by the hot pressing method in an aluminum matrix; the aim was to reduce production costs through the use of oxide forms of powders in composite production. It was understood that sintering temperature increased the amount of starting components undergoing reaction, and with increasing furnace dwell time, the number of particles formed increased and larger forms were obtained. It is understood that these applied parameters had a positive effect on increasing the hardness and wear resistance of the composites. Instructor Metin Onal / Department of Electrical and Energy / Yüzüncü Yıl University Prof. Dr. Mehmet Gavgalı / Department of Mechanical Engineering / Atatürk University Disclaimer This study was presented as a full text paper at the International Professional Sciences Symposium held between 18-20 October 2017 and was financially supported by TÜBİTAK (Project No: 114M913).

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