Behavior of A356 and A319 Aluminium Alloy castings solidified under oscillatory conditions

Abstract

The investigation on the mechanical and metallurgical properties of A356 and A319 aluminum alloy castings solidified under oscillatory conditions have been carried out experimentally to find out variation in mechanical and metallurgical properties of the casting so prepared as compared to stationary prepared casting. The work pieces were casted in the frequency and amplitude ranges of 0-400Hz and 0-15μm respectively. The present dissertation consists of five chapters. Chapter 1 provides a brief introduction with background of the work and objective of the present work. This chapter contains the introduction regarding the casting of aluminum alloys, techniques of grain refining and improvement in properties (mechanical and metallurgical properties) by mold oscillation during solidification of casting. Mold Oscillation is one of the most popular methods used for degassing of molten metal. The mold vibration as grain refining method has been briefly described in this chapter, which is also followed by the enhancement in the property of castings, described by the overcoming of surface tension of the molten metal and as a result of promoting the flow of the melt and filling the forms. A study of the structure of metal castings, cast and solidified in a vibrating form showed that the structure is much finer and denser as compared with the structure of an alloy, obtained by casting solidified under stationary conditions. In this way, vibration can eliminate or considerably reduce the primary defects in the structure of steel, cast iron, and non-ferrous metal castings. The mechanical and metallurgical behaviors of aluminum alloys components can reveal large variation even when the identical alloy is used. Best ways to get maximum mechanical and metallurgical properties included molten metal quality, metal processing (modification, grain refining), suitable gating methods, filtration, directional solidification, high cooling rates and competent risering. Chapter 2 is literature survey which deals with information pertaining to castability issues of aluminum alloys including characteristics, casting metallurgy, casting process parameters, and effect of frequency and amplitude variation on mechanical and metallurgical properties of casts. Little work has been done on the effect of oscillation on the mechanical properties and its correlation with the microstructure of casting. Therefore, the investigations on the mechanical properties have been carried out on the widely used aluminum alloy casts of A356 and A319 respectively under oscillatory conditions. The microstructure plays the primary role in providing the properties desired for each application. Knowledge about microstructure is thus paramount in component design and alloy development. Vibration can modify the solidified microstructure by promoting nucleation and thus reduce the grain size and lead to a more homogenous microstructure. After application of oscillations during the solidification, both acicular silicon and large grains are fractured into fine grains. From metallurgical point of view the vibrations during solidification give a more uniform grain size. Chapter 3 clearly defines and explains the design and fabrication of the experimental set-up, materials, methodology and testing used in the current research work. For casting under stationary and oscillatory conditions, a set-up was designed and fabricated. Set-up consists of a base plate, oscillatory table, two mild steel shafts and device for coupling the exciter to the oscillatory table rigidly. The oscillatory table rests on two shafts which were mounted on four bearings fixed over the base plate. The oscillatory table has a hole in the vertical plate for coupling the vibration exciter rigidly. The casting mold was clamped on the oscillatory table with the help of C-clamps and angle iron pieces. During casting the work pieces were oscillated at different frequencies and amplitudes of vibration with the help of power amplifier and vibration exciter. Tension test of the cast specimens under stationary and oscillatory conditions on a Instron machine. Yield strength, ultimate tensile strength, percentage elongation were recorded. Impact test on specimens was carried out on Charpy impact testing machine. Microhardness test on the test specimens were conducted on a Vickers hardness testing machine. Metallurgical properties of the castings were carried out on test specimens prepared for microstructure examination and fracture behavior with the help of Scanning Electron Microscope and Optical Microscope. Chapter 4 is enriched with the results of various experiments and in depth discussions on the results obtained, explaining the possible beneficial effects of mold oscillation during the solidification of casting. The main objective of the present investigation is to understand the effect of different frequencies and amplitudes of oscillation during the solidification on mechanical and metallurgical properties of A356 and A319 aluminum alloy casting. The chapter presents the experimental results obtained during the investigation. The frequency varies from 0 Hz to 400Hz and amplitude of oscillation ranged from 0μm to 15μm.The castings prepared under oscillatory conditions have different value of ultimate tensile strength, yield strength, percentage elongation, impact strength and hardness for different frequencies and amplitudes of oscillation during casting. The microstructure of the casting has also been found to vary with the variation in frequency and amplitude of oscillation. Chapter 5 in general provides the concluding remarks of the present research work with recommendations for future work. The salient conclusions from the present investigation are summarized as follows: 1. Both strength and elongation improved in oscillatory prepared casting as compared to that of stationary casting. 2. In general, the strength of the alloy is improved by changing chemical composition, but in the present study, the strength is improved without changing chemical composition. 3. The hardness of casting surface improved because of high cooling rate (due to mold oscillation) as compared to stationary casting. 4. After application of oscillations during the solidification, both acicular silicon and large grains are fractured into fine grains. 5. Average area and average length of silicon particle in alloy under vibratory condition decrease with the increase in frequency of vibration of mold. Aspect ratio and roundness of silicon particle increases with increase in frequency of vibration 6. The SEM image of a tensile fractured surface of vibratory casting reveals that the fracture mode is mixed. It shows dimples with bright cleavage facets in the aluminum matrix. The presence of dimples and facets shows quasi-cleavage fracture. 7. Mechanical and Metallurgical properties of casting prepared under oscillatory conditions improves as compared to that of stationary prepared casting. The maximum improvement was found at 400Hz-5μm for both A319 and A356 aluminum alloys.