At what point does a short freezing range alloy become a long freezing range alloy? 1

[dangoo]

Hello All,

I have a large number of alloys with a number of base materials (aluminium, copper and nickel mostly) in a table listing the Liquidus, Solidus, Latent Heat, Feeding Effectivity and Solidification Morphology, stating if the alloy has a short or long freezing range. At first, I thought that if the solidification band was low (5 - 50 degree C range) then it makes sense that the freezing range was "short" however in this table it is listing some alloys with relatively large solidification bands (~120 degrees C or more) as a short freezing range alloy whilst others in the same solidification band region are listed as long freezing range. It may be possible that the data I have in front of me is incorrect but it did raise the question, What is the tipping point between an alloy have a short or long freezing range? and how do I work out if an alloy is one or the other?

Good ol' R.Wlodawer tells us:

"The width of the solidification band is influenced by the following circumstances:
- The phase diagram of the metal indicates the interval which solid and liquid metal exist together (the freezing range). The wider this interval, the longer the time available for the crystals to grow, and the more favourable the solidification pattern becomes.
-The higher the solidification temperature, the more rapidly the temperature falls (hotter bodies radiate heat much more that cooler objects). With a high solidification temperature, therefore, the crystals have little time to increase in length, as the molten metal in the interstices will also solidify rapidly; the freezing range is small, and hence favourable to the production of sound castings
-The lower the thermal conductivity of the metal, the more the dendrites use up their store of heat during growth. with a low thermal conductivity heat is only slowly replenished from the residual melt, growth is hindered, the crystal length is short, and the freezing range is short, and therefore favourable."

I would love to hear if you have any insight on this topic at all.

Even though this sounds like a student type of question I can assure you I am not, I am just trying to satisfy my own curiosity.

Cheers,

Daniel

 

[arunmrao]

Short freezing range alloys have a band of < 50C and Long freezing range alloys have a band of > 110c . I remember from my school days. These can be modified by using principles of directional solidification. You have the best resource material , book by Wlodawer with you.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[EdStainless]

The three points that you mention are the basis of it. If you plot the alloys in those three dimensions you will see where various alloys fall. Most molten metals have very high, and very similar thermal conductivity (compared to the solids). One part of this that is difficult is finding reliable TC and heat capacity data in the melting region.
One way to look at this is that higher melting temp, higher thermal conductivity, and lower heat capacity all tend to speed the rate of solidification. Alloys with these characteristics would need a very large L-S gap to look like long freezing alloys.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

Arunmrao has it correct. I would consider him a resident expert in casting.

 

[arunmrao]

Metengr, thanks, you are too kind. I am blushing at this age!!

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.