EDM Cracking_ columnar vs equiaxed structure

[MagBen]

We have an EDM cracking issue to solve. The material was a chill casted alloy with a columnar structure perpendicular to the cutting plane. The alloy is very brittle in nature, we have tried different casting methods to make it cracking free before EDM to cut into slices, using copper mold for a directional solidification. I was wondering if a ceramic mold, or even a sand mold for an equiaxed (but coarser)grain structure could be a better option.

 

[EdStainless]

If you don't need the columnar structure why are you casting on a chill?
We used to cast and slice A9 all of the time.
We cast onto steel chills with exothermic sand molds.
We had to use less S in the material that we sliced than in block material.

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P.E. Metallurgy, consulting work welcomed

 

[MagBen]

Thanks Ed! Chill was my former colleague idea, I do not like this idea very much because 1. chill caused big thermal stress which led to cracking even on casting during solidification. 2. columnar structure might be the reason material cracked at EDM slicing (for that I am not sure). Mechanical strength and magnetics are not my concerns as long as it does not crack. By the way this is not AlNiCo9, and so anisotropy is not something needed.

 

[MagBen]

Perhaps this is a general question: EDM cut along the grain growth direction is better than cutting across in terms of preventing cracking?!

 

[EdStainless]

I an guessing that this was done because they wanted to cast larger blocks and try to avoid internal shrinkage voids.
I would use standard sand molds.
Are these a simple shape? like round or rectangular
You could line the mold cavities with some ceramic paper/felt to slow cooling, and hot top them.
This will require larger risers though (lower yield).
This should help prevent shrinkage but avoid directional solidification.
We occasionally saw issues cutting large blocks of A9 in either direction.
This was true with both wire EDM and diamond saw.
We tried to not wire EDM, we either plunge EDM cut or simply ground

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P.E. Metallurgy, consulting work welcomed