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[Sep 2005]
Piston Dilatometry Confirms Theoretical Predictions for Aluminium Alloys
Recent piston dilatometry tests on a series of aluminium-silicon alloys have produced results for liquid metal density and volumetric expansion coefficient that closely match theoretical predictions produced by NPL's Virtual Measurement System.
In piston dilatometry, a cylindrical sample of the test alloy is placed between ceramic pistons in a ceramic cylinder and is subjected to a small axial compressive force in a conventional mechanical dilatometer normally used for linear thermal expansion measurement of solids. Once the sample melts sufficiently to completely fill the cavity between the pistons, a direct measure of the liquid density as a function of temperature can be made through much of the so-called solid/liquid 'mushy zone' and into the purely liquid condition as the metal expands against the ceramic pistons.
On cooling, it may be possible to follow the process of liquid metal contraction and the first stages of mush solidification. The principal limitation with many metals is the reaction between the metal and the ceramic parts, which can cause the pistons to jam.
A combination of pure aluminium run in a hot-pressed boron nitride cell in an argon atmosphere shows minimal wetting and the process can be followed on both heating and cooling. Pure aluminium shows no melting range, so a very sharp change in volume is seen at the melting point.
Most recently, tests on a series of aluminium-silicon alloys have produced results for liquid metal density and volumetric expansion coefficient that match well with theoretical predictions from NPL's Virtual Measurement System (VMS).
Piston dilatometry has also been successful in acquiring data on cast irons, copper alloys and nickel alloys using an alumina ceramic cell. Success seems to rely on there being sufficient oxide or reaction product being produced on the sample surface to restrict leakage of molten alloy along the pistons. It is the very low melting alloys, such as those based on lead, tin and antimony that cannot be measured reliably.
For more information on the technique, contact NPL's Roger Morrell at: roger.morrell@npl.co.uk
ENDS