Mysterious blue particles in A356 aluminum castings

[EngineerDave]

On the metallograph, there are some blue colored particles. The only thing I can guess is that it might be from light diffraction hitting oil or some residue polishing compound, however I have my doubts about this because it is restricted to the particle itself?

Do you have any explanations for the appearance of these blue particles after viewing at 100, 200X, 400X, etc?

 

[Maui]

A picture's worth a thousand words. If you can post a micrograph, we might be able to offer some suggestions.


Maui

 

[etch]

okay what etchant and what addition do you make to the metal,

i would narrow it down to one of three things


silicon based either primary crystals
titanuim or Ti-B
Mg particles - unliklel

I used to have a boo that told you what elements showed up l;ike under etchant, did you use kellers reagent?

 

[EngineerDave]

Believe it or not,this was unetched. I will get a picture. I haven't uploaded any to this before. Is there a special trick to uploading pictures?

 

[CoryPad]

EngineerDave,

Mg₂Si particles are blue.

Regarding pictures - you can't upload them to an Eng-Tips server. Rather, you have to store them on another server, then link to them using the TGML code shown in the Process TGML link (this link is included within Step 2 Options at the bottom of the Your Reply window.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EngineerDave]

Thanks guys. For now I'll hold off on uploading a picture, because quite honestly the blue isn't as evident in the digital photomicrograph as it is when you look through the scope.

 

[TVP]

As CoryPad mentioned, Mg2Si particles are blue. Proper solution treatment should dissolve these, so this may be an indication of incomplete homogenization during the solution treatment.

 

[EngineerDave]

Is it correct to predict that the Mg2Si particles will behave in a brittle fashion?

Also a general forums based question. How long does a thread stay on here after being inactive. My interest is basically what happens after no responses have been made for 6 months or a year. Does it get deleted? I know some forums that go back 2-3 years, just curious about this one, because it'd be great to refer back to these. I think to be safe, I'll just save them from time to time.

 

[NickE]

I thought that Mg2Si was soft?

nick

 

[CoryPad]

Mg₂Si particles are brittle.

Currently, all posts are kept. This policy may change, although I think this is highly unlikely.





Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.