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Aviation History
1962
1962 - 1957.PDF
FLIGHT International, 6 September 19 •> SOME TYPICAL TEST APPLICATIONS In an effort to improve the grain structure of castings, BISRA are using electro-magnetic vibrators to find out how agitation of the melt affects the mode of crystallization. The stirring of the melt just before pouring a casting, with the object of improving the quality of a casting, has been practised for a long time. Like so many empirical methods, it works well, although the fundamental mechanism underlying it is far from understood, and results are by no means consistent. It is now accepted that stirring does encourage finer and more even grain development, but to be able to take full advantage of the possibilities, the metallurgist must know exactly how and why this comes about. In attempts to elucidate the precise behaviour of metals on freezing, and the effect of agitation on the mode of nucleation, the British Iron and Steel Research Association, at its Battersea Laboratories, is engaged on a programme of original research, to simulate conditions in the molten metal as it solidifies. For obvious reasons, it is difficult to carry out detailed continuous observations on the internal behaviour of a mass of molten metal, so it was decided to use a trans parent and easily purified liquid to simulate conditions in a typical melt. In the BISRA experiments benzene has been found to meet these requirements. It is known that on freezing, nucleation, or the initial orientation of atoms into the lattices of crystal unit cells, may be either homogeneous or heterogenous. In the first case, crystallization occurs spontaneously throughout the body of the material; in the second, nucleation is 'triggered' by particles of some impurity, round which the atoms begin to re-form in a manner broadly analogous to the condensation of a gas on particles of dust. Benzene has a melting point of 5-48°C and the commercial product can readily be supercooled by about 10°C without the necessity of any purification. If the liquid is supercooled, nucleation, when it does occur, is followed by rapid crystallization. The use of a supercooled liquid therefore has the advantage that the success of any attempt to initiate nucleation is immediately apparent. Elaborate equipment is not required. This is an important advantage from the point of view of the BISRA experiments, because at this early stage the work is very much in the nature of an exploration of possible lines of approach, and the use of expensive apparatus is not justified until results indicate the need for further research along a particular line. The simple set-up used in the experiments is illustrated in Fig. 1. The insulated cylinder in the lower part of the picture forms an outer container which is packed with a salt-ice mixture to provide the necessary low temperature. The ice mixture is in fact too cold, having a temperature anything down to about —20°C. The required working temperature is therefore obtained by adding an electric heating element, the connections to which can be seen at the top of the cylinder. The heat input is adjustable to Fig.l The tube in the foreground, filled with benzene, is normally placet! m the cylindrical cooling vessel. The vibrators at the top of the rig appl]' and measure vibration applied to the supercooled liquid. maintain the required temperature of—5°C. as indicated on a thermometer. This arrangement means that the required conditions can be maintained for a useful period, until the temperature of the ice-salt mixture rises above —5°C. The heating element is in an inner cylindrical c filled with liquid paraffin, which is kept in c motion by stirring. The driving pulleys for the powered by a small electric motor, are seen oi side of the electrical connections to the heate paraffin remains liquid at the working temperaU acts as a heat transfer medium between the ice c and the benzene. The latter is contained in the gl; which, in the photograph, has been withdrawn f >mtlie inner vessel and hangs in front of the cylinder. Submerged in the benzene is the end of a clos i &&^ : amber instant tirrers. either The •e, and ; amber ss tube
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