Polishing Cast Aluminum.

[pauljohn]

I recently had a custom part cast for me by a small company. I am fairly pleased with the result but while polishing it I have noticed a number of small pinholes in the casting that will be detrimental to my planned high quality polished finish. I have tried to sand them out but with no luck. What are my options here?

Am I left with a choice between a decorative chrome finish where the copper plate will (hopefully) fill in the pinholes, tig welding the holes (long, laborious process and possibly opening up more holes) or is there still a way I can acheive a good surface by filling in somehow.

The part is mainly decorative and is made from A356 with some 3003 welded to it. Thanks for any help.

 

[swall]

The pinholes are "microporosity" due to hydrogen in the melt. They will be present through the entire cross section, so you will continue to expose more. This kind of porosity can be mitigated with proper foundry practice (degassing, mainly) and a bit of good luck.I would choose the plating option at this point. I would start with electroless nickel, as it is conformal and will not just bridge the pits. Then follow up with the copper,nickel, decorative chrome.

 

[TVP]

Another option is to impregnate the porosity prior. Loctite makes a number of products for this purpose; Resinol RTC is one of them:

http://www.henkelna.com/cps/rde/xch...tredDotUID=productfinder&redDotUID=0000000HVS

 

[CoryPad]

Another option is burnishing. You may be able to deform the surface to close/eliminate the porosity.

Regards,

Cory

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

 

[pauljohn]

Thank you for replies. A company who chromes and polishes say they can fix it when they chrome it so I will probably going with this option. I like the burnishing idea but I don't think it would work in my case.

 

[unclesyd]

Your plater is probably going to try electropolishing.
I have had a lot of Al parts electropolished but never any 356. I won't hazard a guess on how alloy will polish.

Bright dipping is another possibility but I've had varying results with 356 permanent mold castings. The problems were mainly due to the high Silicon content.

How is your casting made?

http://www.ep-systems.com/aboutme1.htm

http://www.ep-systems.com/favorite.htm

 

[pauljohn]

Unclesyd

If I understood the plater correctly I don't think he is going to electropolish. He is first going to clean and polish, then copper plate, then give back to the polisher who in the act of polishing will force the copper into the holes and build up enough to fill them. If warranted another copper-plate and polish can be done(or a few more if the holes are big). Then chrome. Hope I have that right.

 

[swall]

That is pretty the standard method platers use for filling pits when doing a decorative chrome job.

 

[unclesyd]

As I understand it based on your last post the plater is going to use the copper strike to fill in the holes. This can be good or bad depending on the end use of the product as thick copper doesn't normally offer very good support for Cr plating. We like to keep the copper plate at the minimum to let the base metal support the Cr plate.

I would keep my all my options open for either the mechanical burnishing or the electropolishing. Essentially what is happening is that the copper is being burnished. If I were to use plating to correct the finish I would trend towards the system suggested by swall.