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It is will known that increasing cooling of a metal casting will lead to finer micro-structure which mean higher mechanical properties. If I have 2 molds one is thick copper die ,& second is just thin-walled copper cup or mold cooled by water. which one of them excepted to cool the molten metal i.e. aluminium faster ?

the calculated heat diffusivity of copper is : thermal conductivity x specific heat x density = 385 x 0.385 x 8960 = 1328096 m²/s

while for water it is just : 0.6 x 4.2 x 1000 = 2520 m²/s only !!

so, the copper can transfere heat 527 times faster than water.

by calculations it seems that copper die have higher cooling power as it has higher thermal diffusivity , but in real life it seems water have more cooling power. I am so confused.

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    $\begingroup$ I suspect you mean $k/\rho \tilde{C}_p$ not $k \rho \tilde{C}_p$. Check your calculations and UNITS. Using water includes convection. For flowing water, convection is higher than conduction. $\endgroup$ – Jeffrey J Weimer Dec 11 '18 at 14:49
  • $\begingroup$ Your basic assumption is incorrect ; Fast freezing and cooling of aluminum will produce lower strength although with finer grain size. It would be solution annealing the casting, more or less. If you want to heat-treat something , it would be best to do it as a separate operation. $\endgroup$ – blacksmith37 Dec 11 '18 at 16:16
  • $\begingroup$ Water can be replaced with new water, copper not so much. Which is less fancier to say convection is the primary tranfer method of water. $\endgroup$ – joojaa Dec 12 '18 at 7:18
  • $\begingroup$ A couple notes: aluminum is highly soluble in copper, so expect the mold properties to change with successive castings. It is also possible for intermetallics to form, which could scale off. Contained or sealed water has extremely high vapor pressures (multiple GPa) at liquid Al temperatures, so be wary of inadvertently creating a bomb with your system. Sufficient convection, and not directly exposing water to liquid metal, should prevent this issue. Stay safe! $\endgroup$ – wwarriner Dec 19 '18 at 21:31
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A company I worked at used tool steel molds and channels in the mold with flowing water to increase cooling. Heed the advice of others, never let ANY water come into contact with molten aluminum. I have seen it hit a 20' ceiling (would have gone farther) and as far as 15' in circumference.

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It depends on what you're casting. For a smaller castings with thin sections, the amount of heat that needs to be absorbed is low and the surface area and casting to mold volume ratio is high. So a chunky solid copper die may absorb the heat quickly enough.

But the heat transfer slows as the die heats up (temperature difference decreases). If the casting is a larger block, a lot more heat has to diffuse through a smaller surface area and as the surface of the die heats up, the cooling rate reduces. In this case having water that can take the heat and move it away from the die surface (by pump or convection), allows the heat to be distributed throughout the cooling system more evenly, keeping the temperature difference high.

You can also use chillers or cool the mould to increase the temperature difference and speed up the cooling, which will give you more nucleation points when solidifying and a smaller grains. But an increased cooling rate often actually decreases the strength due to increased porosity and other casting defects and may not even fill the die properly (I have seen people pre-heat moulds to slow down the cooling rate and get a better casting). Increasing the strength is really best done by alloying with other elements.

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