Need Hard Coating for Cast Aluminum with integrated Stainless Steel

Question

Need Hard Coating for Cast Aluminum with integrated Stainless Steel

Albigger (Aerospace)

(OP)

9 Mar 07 11:35

Hello Everyone -

What I need is some type of coating for aluminum parts.  The aluminum is T355 Cast Aluminum, with integrated stainless steel tubing (the tubing is covered everywhere except the two places it protrudes from the aluminum part).

The coating needs to give hardness (and lubricity, if possible).  My choice would be a hard-coat anodizing but this causes problems with the dissimilar metals.

The coating does NOT need to coat the stainless steel, and I don't even care if it coats the whole aluminum part, just the surfaces that will be used.  I want to cut a contour, polish the surface, and then coat it, so the coating should apply in a uniform thickness.

The current castings vary in size, but are at least 10x12x38 inches.  The process would need to be applicable for larger pieces as well (say up to 120 inches long).

Please, any suggestions would be an extreme help!  Thanks.
--Jay

swall (Materials) · Answer 1 · 2007-03-09 14:24:02

Am I correct in assuming that the stainless tubing is cast in place? I am thinking that hardcoat anodizing might work for you if the exposed stainless could be masked with some sort of wax or plastisol prior to anodizing. Could you clarify the geometry of the area of interest?

Albigger (Aerospace) · Answer 2 · 2007-03-09 14:45:01

Yes, the tubing is cast in place.

In general, the geometry will vary from part to part.  I have attached two images of what one detail might look like.  Of course this would be fully machined, with holes etc... before anodizing, but this might give you an idea of the shape.

In the image, the yellow would be of most importance (cavity), with the blue surfaces being secondary importance, and the green surfaces I really don't care about.  The two lighter green tubes sticking out the back is the stainless tubing.

I spoke with our local anodizing house, and they didn't sound too confident.  They said it would have to be masked (I'm not sure what they use to mask) and then just hope the masking holds up and seals well.  They said if it doesn't, it will just eat right thru the steel, and these details (once machined) are too expensive just to "hope" it seals well.

http://img180.imageshack.us/img180/1867/det1backsidelp6.jpg

http://img214.imageshack.us/img214/4972/det1cavitysidevl8.jpg

swall (Materials) · Answer 3 · 2007-03-09 16:05:01

I think I would get some test pieces made up out of 355 casting with two drilled holes and pressed-in pieces of ss tubing. Then you could use these test pieces to try out various masking schemes. Another possibility, if you haven't made the castings yet, would be to coat the stainless with IVD aluminum prior to casting. The third possibilty would be to use a flame sprayed coating on the wear surfaces instead of anodizing.

kenvlach (Materials) · Answer 4 · 2007-03-09 16:05:43

There shouldn't be a problem -- just mask the stainless tubes with adhesive (platers') masking tape, then brush on an additional layer of liquid maskant (cures to a peelable plastic). It's only difficult if you want a perfect knife-edge line at the aluminum/stainless edge, since hard anodizing slightly undercuts masking. But, if this area's not critical, mask slightly onto the aluminum, then use a gray silicone sealant to mask the small gap afterwards.

Why 355? A356 (lower Cu) is somewhat better for anodizing & corrosion resistance. Both hard anodize dark grayish.

Albigger (Aerospace) · Answer 5 · 2007-03-09 16:34:03

We are proceeding with the castings for now, and we are getting a sample pc to try this process on, but it isn't ready yet.

First I've heard of IVD alum so I'll have to research it.  This would effectively seal the tubing such that it could be anodized at a later date?

I'll also have to research flame sprayed coatings - is there a particular process you can suggest?  can this be applied evenly?

kenvlach - all places I talked to said you could try masking, but none would gurantee it would work (i.e. it would be at our risk and if it fails then oh well).  I don't care about a perfect edge line, so this does sound reasonable, maybe I'll just have to look harder for a reputable place to have this done.

In regards to the aluminum, I'm not sure.  We need this to hold vacuum, and our casting house recommended 355.  We have used 356 for vacuum chucks in the past with success (but those did not have the cast-in oil lines) and we have used the 355 with oil lines in the more recent future (but we did not do any coating to the aluminum).

I will make sure to bring this up to the casting house, as the 355 was their recommendation.  And if it is only marginally better in an anodization process, then I'm sure we can live with it for now.

Thanks everyone for the info so far.  Please keep it coming.

kenvlach (Materials) · Answer 6 · 2007-03-09 17:13:02

IVD aluminum probably isn't suitable for this application: Too expensive since the part is large & a very thick deposit would be needed in order to hard anodize, and there is a serious problem getting enough anodize current to flow through the stainless substrate (about 1/10 as conductive as aluminum).

Masking is really Mickey Mouse since you don't need the sharply detailed interface. Since leaks are critical, spend $1 on masking materials instead of $0.50! Plug the tube ends, tape the whole tube, then apply the liquid maskant over both plug & tape and slightly onto the adjoining aluminum. Let dry, inspect, then apply another coat as security blanket.

Do the masking yourself if the local anodizers are incompetent. Tape, plugs & brush-on masking:
http://specialmasking.com/smcoatingmask.html
http://www.shercon.com/

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