# which type of casting will give higher cooling rate, casting in thin water-cooled copper mold or casting in thick copper die?

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.

• 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. – Jeffrey J Weimer Dec 11 '18 at 14:49
• 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. – blacksmith37 Dec 11 '18 at 16:16
• 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. – joojaa Dec 12 '18 at 7:18
• 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! – wwarriner Dec 19 '18 at 21:31