Re: contact test : Note:
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: I am Ph.D student at Manchester Materials Science Centre. I am
: investigating the metal-mould contact area during casting solidification
: for heat transfer modelling.
: My experimental set-up is schematically shown above. Al-4.5wt % Cu liquid alloy is poured onto a water-cooled copper chill. The solidification is initiated at the metal -chill interface by the asperity contact between the liquid metal and the Cu block (chill). Then, the solidification proceeds upwardsly against the heat flux one diametionally. As my project involves modelling of heat transfer during the casting solidification with various mould surface roughness, I am trying to obtain a quantitative knowledge about the actual liquid-asperity contact occurring at the interface. (immediately after the liquid metal strikes onto cold Cu chill a surface skin solidifies and it obtains its own surface roughness as shown enlarged section and it turns out to be solid-solid contact).
: I thought that I can trace the contact area with an ultrasonic pulse-echo flaw dedector as shown above despite I found no example of similar measurements in the literature so far.
: First, a backwall echo, from the far end of the Cu block, is seen on the CRT. I usually adjust it to 80-90% of total screen height with the gain adjustment. Then, I pour the liquid metal into the insulating tube (onto the Cu block). With the pouring of the metal the original backwall echo diminishes 10-15% of its height followed by a slight increase within a few sec.
: Although, I am using various roughness of the chill surface (preparing with different grade of emery papers) there are little changes in the decrease of original backwall echo.
: Now it difficult for me to interpret the results for a quantitative conclusions. Could you help me commenting on the following questions.
: 1. whether the decrease in the original backwall echo with the pouring is because of the temperature induced attenuation in the Cu block or is it because some of the sound waves are being transmitted across the interface through the casting.
: 2. I have data from the gain adjustment (gain to compensate the reductions in the BWE to its original position) (dB) . Using this data can I calculate the actual area of contact. How can I take into account the difference in the acoustic properties of two different metals, namely Cu and Al-4.5% Cu.
: 3. What else I should also consider before going calculation.
: I would be grateful if you have a chance to look at these and let me know what you think.
: With regards,
Your contact surface is rough. The soundwave will be scattered at the roughness of this surface.
I think, you should try to measure not the reflected, but the scaterred wave. It could be made very simple if you use the transducer that has a different inclination to the liquid metal - Cu chill interface.
This angle depends on the roughness of this surface. The signal amplitude may be smaller, but it could be possible, that you can find a strong changes, depending on the roughness, temperature and so on.
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