Abdul Karem, Waleed
(2009).
Vibration assisted filling of thin section castings.
University of Birmingham.
Ph.D.
Abstract
Understanding of the mechanism of the vibration needed to fill thin section or one with sharp edges in profile shapes and clarifying the dominant control parameters of the vibration in thin wall investment casting is key to producing sound casting (one free of misrun defects). It's also a central issue for study in this thesis. The filling capability in thin wall investment casting method was assessed in relation to metal head. It was found that the effect of the vibration on the metal head is markedly dependent on acceleration. Generally, it was observed that the metal head required to force the metal in thin sections in the casting vibrated at (1g) acceleration is approximately half that used in castings made without vibration. Two potential mechanisms were observed from the experimental result during the filling process in thin wall casting i] discontinuous propagation flow in vibration conditions; and ii] continuous propagation flow without vibration. These mechanisms may be acting to modify the contact angles between liquid metal and a wall of the mould. Experiments also showed that two features of the transition can be observed from the front of the morphology; i] a coherent liquid metal front - this occurs in thin wall investment casting when the acceleration due to vibration is less than (1g); and ii] jetting at the free surface - this occurs in thin wall investment casting when the acceleration due to vibration exceeds 1g. This is present in terms of a unifying concept, using a frequency and amplitude ( f - a ) map. The time of the vibration operation has a moderate effect on the relative filling area when the acceleration is less than 1g. However, it is more effective when the acceleration of the vibration is greater than 1g. The mathematical models comprised one-dimensional heat transfer with phase change and had an established flow field for molten A356 alloys flow in the thin section ceramic channel mould. The work was concerned with the fluidity of A356 alloys in thin wall investment casting with and without vibration in two type of filling (flowability and fillability filling types), combining heat and metal flow in addition to the simultaneous solidification stage. The results of the mathematical model, produced agreement with the experimental test carried out in the foundry and also agreed with other published data. The results on fluidity indicated that the fluidity of the molten metal was affected by mould temperature, pouring temperature, the velocity of the molten metal flow relative to the surface tension and the channel thickness. The data used in the mathematical model of the fluidity in thin section under vibration condition were deduced experimentally; namely, velocity of the molten metal and the heat transfer coefficient between the liquid metal and the chilled surface of the mould. This model was used to estimate the fluidity characteristics in thin wall investment casting with and without vibration. Real-time X-ray observation and computer modelling of the metal head-driven mould filling sequences reveal that no surface turbulence occurred when the liquid metal flowed into the thin section and the advance metal front continued to flow under surface tension control. X-ray was also used to measure the flow time and the velocity of the metal inside the thin channel and confirm the modification on Bernoullis Equation (kinetic energy+ potential energy = constant) to estimate the velocity relative to surface tension in the fluidity mathematical model. Flow-3D software was used to calculate the velocity of the liquid metal in the flowability filling type and the fluidity characteristics. Weibull analysis identifies the acceleration vibration as practical criterion to judge the reliability of casting. A vibration mould with vertical direction in the thin wall investment casting after filling can make the liquid metal flow into the thin section under surface tension control. This technique is used to achieve mould filling free from misrun defects and surface turbulence and this makes vibration casting a promising technique for producing high quality castings. On the basis of these findings, an operation window for the production of reliable castings has for the first time been developed in this research.
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