For high strength and ductility, aluminium casting alloys require controlled additions of a dopant (known as 'modifier' in the foundry trade), usually sodium or strontium. Precise control of casting quality requires a convenient means not only of adding the modifier to the melt, but also of measuring its concentration. For sodium modification Ionotec produces components for a novel doping system known as COSMA, and also a sodium sensor, both of which are based on Ionotec's ion-conductive ceramic membranes. A sensor for strontium is also being developed.
COSMACOntrolled Sodium Modification of Aluminium | ||
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The COSMA unit delivers the additive to the aluminium melt on demand by electrolysis of sodium hydroxide (NaOH), controlled via a feedback loop from the the sodium sensor
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Sodium sensorshown on the right, is used for continuous monitoring of the concentration of the additive in the aluminium melt. In addition to its use for controlling COSMA, the sensor can also be used with conventional means of adding sodium modifier, such as pellets and canisters. |
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COSMAThe COSMA unit is an electrolytic reactor using an inexpensive sodium salt as feedstock to generate pure liquid sodium metal. Feedback between the sensor and the COSMA unit provides continuous automatic control of the modification process, which avoids the need for handling sodium metal and provides precise control of the modifying process. |
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The sodium sensor gives a voltage output that depends on concentration of the sodium modifier:
In continuous control mode the voltage signal from the sensor instructs COSMA to deliver the modifier automatically so that the concentration remains correct. The following graph shows the sensor readout under automatic control pre-set to 75 ppm. During the period labelled OFF the COSMA unit was switched off and the modifier level falls rapidly due to evaporation. Following subsequent switch-on there is rapid recovery to the correct level.
