DEVELOPMENT OF TiB2 / Y2O3 REINFORCED AA2024 MATRIX COMPOSITES WITH IMPROVED MECHANICAL AND WEAR PROPERTIES THROUGH SEMI-SOLID METAL CASTING

Abstract

Metal matrix composites are developed by combing two or more materials through various processing techniques to attain resultant properties of the constituent phases. In the past few decades, alloys and monolithic materials have been replaced by particle reinforced aluminum composites. However, the applications of these composites in various areas have not been effectively widespread due to higher processing costs, lower mechanical and wear characteristics, etc. Therefore, the purpose of the current research is to develop AA2024 matrix composite with enhanced mechanical and wear properties. To achieve the objectives, two composites were developed through semi-solid metal casting, where the slurry was processed in the semi-solid temperature region of the alloy. The slurry was processed at a semi-solid temperature of 640 oC, which corresponds to a 40% solid fraction. Then, the reinforcements were added to the slurry at series of experiments and stirred thoroughly to distribute them in the melt. In order to determine the appropriate process parameters that would yield the desired semi-solid cast composites, several trial experiments have been conducted by varying the processing temperature. The initial values were determined based on a critical literature survey. During the development of the first composite, three different composite samples with varying amounts of TiB2 particles ranging from 2 wt% to 6 wt% were developed. Additionally, two samples (unreinforced liquid cast sample, and unreinforced stir rheocast sample) were also developed to be used as benchmark. The formation of the reinforcing particles, their distribution, and the formation of other intermetallic phases were analyzed by Optical Microscopy (OM), Field Emission Scanning Electron Microscopy (FE-SEM), Energy Dispersive X-ray (EDX), X-ray Diffraction (XRD) techniques and Transmission Electron Microscopy (TEM). The mechanical and physical properties such as wear, hardness, ultimate tensile strength, yield strength, ductility, fracture toughness, density and percent porosity were evaluated. The results revealed that the mechanical properties were significantly improved after the reinforcement of TiB2 particles in the matrix. Among the developed composite samples, AA2024 reinforced with 2 wt% TiB2 has shown maximum hardness, yield strength, ultimate tensile strength, and elongation of 116 HV, 120 MPa, 285 MPa, and 12.9%, respectively. Dry sliding wear properties of AA2024 composites reinforced with 0wt%, 2wt% and 4wt% of TiB2 particles developed through stir rheocasting process was also studied. The test was conducted on a pin on disc apparatus. The experiment was conducted according to full factorial iii design of experimental technique with three factors and three levels. Wear and friction coefficient were considered as an output response and applied load, reinforcement ratio, and sliding distance were the process parameters varied, and their influence was investigated using analysis of variance. Additionally, the correlation between the input and the output parameters were determined using multiple linear regression equation. From the optimized test results, the most influencing factor of wear was load followed by sliding distance and reinforcement ratio. Similarly, for coefficient of friction, applied load followed by the reinforcement ratio and sliding distance were the influencing factors. Finally, the developed AA2024-TiB2 composites were subjected to secondary processing, such as heat treatment and cryo rolling, to enhance their properties. The developed composite samples were solution treated at 490 °C for 1, 2.5 and 5 hours, and aged at 190 °C for duration of 1-12 hours. After peak aging, an improved Vickers hardness of 147 VH, yield strength of 230 MPa, ultimate tensile strength of 342 MPa and ductility of 6.2% were achieved for AA2024/2wt% TiB2 composite, amongst others. The optimum solution treatment time that contributed to peak hardness and fastest aging kinetics was 2.5 hours. Rolling at cryogenic temperature was also performed to enhance further the mechanical properties of the composites. The high dislocation density induced by severe plastic deformation refined the grains of the samples, which led to significant improvement in mechanical properties but at the expense of ductility. The highest hardness, YS, UTS, and ductility attained were 163 HV, 312 MPa, 415 MPa, and 3.3%, respectively, for the cryo rolled AA2024 reinforced with 2 wt% TiB2. During the development of the second composite, TiB2 was fixed at 2 wt%, and Y2O3 was varied from 0.5 wt% to 1.5 wt% to develop AA2024/ TiB2 + Y2O3 hybrid composites. The liquid cast sample, the unreinforced stir rheocast sample, and AA2024 reinforced with 2 wt% TiB2 were considered as a benchmark. The highest hardness, ultimate tensile strength, and yield strength achieved were 122 HV for the AA2024/ 2 wt% TiB2 + 1.5 wt% Y2O3 composite, 310 MPa for the AA2024/ 2 wt% TiB2 + 0.5 wt% Y2O3 composite, and 230 MPa, for the AA2024/ 2 wt% TiB2 + 1 wt% Y2O3 composite respectively. With the reinforcement of Y2O3 particles in the matrix, the porosity in the samples increased, resulting in lower elongation. The developed hybrid composites were also subjected to aging heat treatment and wear test. In this phase also, applied load, reinforcement ratio, and sliding distance were the varied iv parameters, and volumetric wear loss and friction coefficient were considered as the responses. The dry sliding wear test results revealed that the wear resistance of the hybrid composites increased with an increase in the amount of the Y2O3 particles. Additionally, the optimum solution treatment time that contributed to peak hardness and fastest aging kinetics was 2.5 hours. After peak aging, the maximum hardness, ultimate tensile strength, yield strength, and elongation achieved were 148 HV, 380 MPa, 296 MPa, and 4.5 %EL, respectively, for the AA2024/ 2 wt% TiB2 + 0.5 wt% Y2O3 hybrid composite. From the various investigations carried out in the present work, 2 wt% TiB2 reinforcements seem to create a favorable condition in enhancing the properties of the AA2024/ TiB2 composite. Similarly, the addition of 1.5 wt% Y2O3 particles along with a fixed 2 wt% TiB2 created a favorable condition in enhancing the mechanical properties of the developed hybrid composite.