356 ALLOY -SILICON SEGREGATION/PULL OUT 1

[Nadimuthu]

We are supplying a lot of A356 alloy castings in T6 condition.Our machining partner complains of shrinkage and blowhole and rejects the castings on many occasions.
Whereas when we do a rough machining of these bores and conduct a Dye Penetrant test, it does not reveal any porosity.
With joint discussions and analysis, we could come to some conclusion that this occurs when the machining process engineer uses PCD(Poly Crystallind Diamond) inserts and goes for very high speed and feed this problem occurs.
We feel this could be due to pull out silicon from the bores.
The same phenomenon is not occuring on the facing sides but only when the cutting direction is perpendicular as in bore machining operations.
Would very much appreciate inputs to go into this phenomenon deeper.I will also provide cutting speed and feeds in subsequent mails.

 

[metengr]

Typically, A356 is an aluminium alloy that has excellent casting characteristics - increased resistance to shrinkage cracks and hot cracking, high fluidity, and increased ductility in comparison to 356.

I presume these are sand castings? What exactly is the part, and approximate dimensions?

I have been involved with situations where we have ordered material and one party claims it is the others fault. Based on your post, I would suggest having an independent evaluation performed on one or two typical castings to assess the extent of internal shrinkage and inclusion content. This can be done by a metallurgical lab at nominal cost.

It will result in scraping one or more castings, but you can use this information to make any necessary adjustments in final machining (switching to carbide tools and lower feed speeds) or if need be, go back to the foundry.

 

[Nadimuthu]

Dear Metengr.
Thank you for the immediate response and suggestion.Yes.It is a sand casting and we have already done microstructure study for the casting of this product which is a housing into which gears are assembled and we have also conducted a microstructure study for another part of the same A356 alloy.
I reproduce below the report of microstructure study done on an earlier occasion during the proving out exercise for the sample of casting under question.
"The sample was cut and ground with a series of emery papers down to 600 grit size and polished with a diamond paste of 1-2 micorns size.It was etched with 0.5% HF.
Microstructure consists of dendrites of Aluminum solid solutions with fine modified particles of eutectic silicon at the interdendritic regions.Fien needle shpaed intermetallic particles are also seen.A fine cavity is seen at the sub surface region.No other defects such as segregation or pinholes were seen"

The fine cavity mentioned is the reason for conducting this study .It was a microporous shrinkage cavity which was eliminated by altering the feeding of the casting.
After the correction, the DPT reveals no defect.
.
This issue has now cropped up after the machining team has gone in for cycle time reduction using PCD tools instead of carbide inserts.

Does this info give you any further insights to our problem.

Look forward to hearing from you.

Regards

 

[metengr]

I would recommend going with a lower feed speed and using carbide tooling. I would suspect the surface indications are inclusions being pulled out of the casting.

 

[Nadimuthu]

Dear Metengr,
Thanks for the suggestions.I will conduct a microstructure study to rule out inclusions first.This is a more serious problem for the foundry and then get along with your suggestion of going in for low feed and speed with carbide tooling.