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Metal Casting Technologies : March 2011
36 www.metals.rala.com.au TECHNICAL FEATURE Experimental procedure A hollow aluminium tube is used as the consumable electrode. The bottom end of the tube is made flat and sealed off by hammering. Aluminium chips produced during lathe machining were collected and rammed inside the hollow tube. The chips were rammed manually by a steel rod. The top end of the tube was made flat by hammering. This top end was attached to the electrode holder using nut bolt arrangement. The slag contained 30% cryolite (Na3AlF6), 30% potassium chloride (KCl) and 40 wt % sodium chloride (NaCl) and had a melting point of around 6000C. Slag was made in an induction furnace in a graphite crucible. The molten slag was poured in a clay graphite crucible and allowed to solidify. The slag was later crushed and sieved into fine powder. The slag was stored in a desiccator. The mould is kept on the bottom plate and clamped tightly. The process is started by starting an arc between electrode tip and the bottom plate. Preheated slag was added immediately in the arc zone. It was observed that the slag melted partially and there was continuous arcing between electrode and base plate. The rest of the slag was added continuously in the arc zone. The slag didn't melt and the ingot produced was highly non uniform and unsatisfactory. Power was varied during arcing to make the slag molten but there was not much difference. This solid start arcing is a problem in aluminium ESR as the slag melting temperature is approximately near to the metal melting point. The slag may melt partially and remaining may not melt. There will be continuous arcing between electrode and bottom plate starting block. ESR melt of aluminium proved difficult to initiate by arcing due to the incomplete slag fusion. As the solid slag with arcing failed molten slag addition was tried into the mould directly. This will give a quick smooth start of the process. The slag was kept ready in a molten condition in another furnace and added quickly in the mould through a transferring crucible ladle arrangement. The electrode end was kept about half inch above the base plate. As soon as the slag was poured the open circuit voltage reduced and the process started. The parameters were adjusted accordingly and the voltage was kept around 25 V. The process was stable throughout. At the end of the process hot topping was tried by reducing the voltage and current to give maximum yield. The ESR ingot was stripped out and analyzed. Characterisation The ingot was cut to for determining mechanical properties. The other cut portion was macroetched using Tucker's reagent to reveal the macrostructure. Samples were made from the bottom and the centre of ingot for checking the reproducibility of the results. All the tests were carried on transverse and longitudinal specimens. The chemistry of the ingot was checked in an optical emission spectrometer. Tensile samples were made according to ASTM E 8 specifications. The gauge length is kept at 20 mm and gauge diameter is 4 mm. Charpy impact samples with a V Notch (10 x 10 x 55mm) were made according to ASTM E 23. Vickers hardness values were taken at 100 gms load. Four hardness readings were taken for each sample and averaged. Two samples were made from transverse and longitudinal directions. A part of the aluminium ESR ingot was cut for cold rolling. Cold rolling was done in a 75 tons capacity rolling mill. The starting thickness was 21 mm and it was rolled to 10.2 mm giving 51.42% reduction. Tensile and charpy impact samples were made from Figure 2. Aluminium ingot prepared by ESR process using arcing method THE PROCESS IS STARTED BY STARTING AN ARC BETWEEN ELECTRODE TIP AND THE BOTTOM PLATE. PREHEATED SLAG WAS ADDED IMMEDIATELY IN THE ARC ZONE. IT WAS OBSERVED THAT THE SLAG MELTED PARTIALLY AND THERE WAS CONTINUOUS ARCING BETWEEN ELECTRODE AND BASE PLATE.