Casting conundrum

Findhan

Hi, I was wondering does anyone know the difference in single crystal casting and standard polycrystalline casting from a manufacturing persepective? Not specific to any product but just the general rules? Many thanks

tak

Hi,

First, apart from the semiconductor industry, there is little or no direct casting of single crystal components.
It goes without saying that a lot of control over such things as local temperature gradients, diffusion ahead of the solid phase, orientation of the "seed" crystal, etc is needed for this process.
It is highly expensive in terms of plant and skilled staff.
But the financial returns from this performance - which is the basis of our modern electronic age - are obvious.

What they do for say single-crystal gas-turbine blades is to re-crystallise an existing polycrystalline blade with a "root" that has a constriction in its cross section just ahead of the blade proper.
On controlled re-crystallisation through the root the solidification front can be confined to the growth of just one crystal. Careful control of the directional resolidification process is needed so as to keep just this one crystal growing upwards into the main section of the blade.

This sort of process is no cakewalk either by any means. It is justified economically by the increased performance, esp. in resistance of creep deformation, which is the bane of components like turbine blades that are working close to their melting point.

In contrast the usual foundry casting is relatively easy. Sure one has to have regard to the likely crystal size in the final casting - for structural components moreover - as well as avoiding common defects in castings like tears, cold shuts, etc. But that is a much easier proposotion than trying to achieve a single crystal. As well as that for the last 20 years the use of electromagnetic pumps has removed a lot of the turbulence-related problems in conventional casting into sand/shell moulds.

All in all the difference is difficulty.
But when the final material has a lucrative application (like with single crystal Si or single crystal turbine blades) these problems will be taken on !

tak.

P.S Why do you ask ?