Development and Characterization of Functionally Graded Al-Si Alloy System and Al-Si/SiCP Composites using Centrifuge Casting

Abstract

FGM is a material that shows change in magnitude of property values from one end of a specimen or component to the other end. FGM has an intermediate layer whose structure, composition and morphology vary smoothly from one end of the specimen to the other end. Fabrication of FGM and their components with gradient microstructures and properties are challenging. Most of the investigations which focus on material behavior of FGMs are limited to analytical or numerical studies. One of the major bottlenecks with experimental studies is the preparation of FGMs having large property gradation. This necessitates the development of a suitable technique to produce such FGMs with reproducibility of structure and properties. The present work aims at developing a manufacturing technique for the FGMs in order to meet the wide range of and also suitable mechanical and tribological properties for specific thermal and mechanical engineering applications. Among the processing techniques available the most commonly used is horizontal centrifugal method, but, this method is used to produce mainly hallow cylinders. In this work a centrifuge setup is fabricated and FGMs have been successfully developed to produce solid castings. The major advantage of this machine when compared to the conventional machine is that the pouring is done while the mold is stationary and machine operates about a vertical axis. The principal advantage of this is good mold filling combined with microstructural control, which usually results in improved mechanical properties. In this process, when the melt is subjected to high G forces the lighter particles segregate towards the axis of rotation, while the denser particles move away from the axis of rotation depending on the density difference between melt and the reinforcement. This segregation depends on several process parameters such as mold rotational speed (G Factor), pouring temperature, mold temperature etc. The use of aluminum, its alloys and aluminum based composites in the present day has shown many advantages through its unique combination of physical and mechanical properties. The light weight, strength, formability, corrosion resistance, ofIV aluminum, its alloys give it the potential to meet a wide range of design challenges. Taking into consideration the advantages such as high wear resistance, controlled thermal-expansion coefficient, good corrosion resistance, and improved mechanical properties over a range of temperatures that Al alloys and its composites can provide, in this work manufacturing and characterization Al-Si FG alloys and Al-Si-SiCP FG composites have been taken up. Two Al-Si alloys eutectic (12%Si) and hypereutectic (17%Si) were used for producing FG alloys. Further effect of 3 mold rotational speed 200, 300, 400rpm, 2 pouring temperatures 800oC , 900oC and 2 mold temperatures ambient and preheating the mold at 180oC temperature were studied. Similarly FG composites were also produced using Al-17%Si and Al-12% Si as matrix with SiCP as reinforcement. Three different volume fractions of SiCP were used to produce FG composites. The FG composites were produced using 900oC pouring temperature with preheating the mold at 180oC under 200, 300, 400rpm mold rotational speed. The structure and properties of the FG alloys and Composites are studied to understand the effect of different process parameters. The Al-Si FGM specimens are studied for distribution of Si along the length of the specimen (from bottom to top) using optical microscope. The hardness's is measured along the length of the specimen using Brinell hardness tester. Sliding wear tests at room temperature are conducted at normal loads of 40, 60, and 80N and at 1.466m/s sliding speed for a constant sliding distance 879.6m in order to measure the wear resistance and friction characteristics. Similar tests were carried out for FG composites. Diametral compressive strength were conducted to know the strength of the specimen along the length at bottom, middle and top regions. It is found that the FG alloy and Composites are produced successfully using centrifuge technique. In both alloy and composite the gradation occurs at higher rpm, teeming temperature and mold temperature. The experimental findings of hardness and the wear tests provide adequate proof on the gradation characterization (% volume fraction of primary Si, % volume fraction of SiCP and rim thickness) done using microstructural studies.