We manufacture aftermarket automotive products. Some of them are aluminium castings that we machine and polish. Porosity is a problem that we cannot detect untill after the part has been machined then polished.:mad: Is there some type of coating or plating that will give the same look as a polished process?

We manufacture aftermarket automotive products. Some of them are aluminium castings that we machine and polish. Porosity is a problem that we cannot detect untill after the part has been machined then polished.:mad: Is there some type of coating or plating that will give the same look as a polished process?

Depending on the equipment you have available and the chemical composition of the aluminum casting, as well as the quantity of parts and the amount of machining (machining ALL surfaces or only a couple of mating surfaces and threaded holes?), there are several options, BUT any coating you put on will only be as smooth as the underlying substrate.

In terms of simplicity, from simple to complex, here are some options:

tumble the parts in polishing medium
sandblast each piece in a sandblast box
convince customer to switch to bar stock and machine part complete from cold drawn bar stock or plate, using progressive tooling which allows a finish nearly equivalent to polishing (depending on quantity, some "boutique" mills provide special heats of bar stock comparable to the casting which are often less expensive than net cost of casting tooling and casting process with less opportunity for spoiled castings due to porosity.) Scrap can be segregated and resold to mill.Everything is dependent on customer's final use and function of the part. There may be other options and it is important to note "Control Plans" are best determined when the supplier has as much knowledge as possible about product and its final function.

I agree with Wes that anodized and hard coated surfaces are not much smoother than the underlying material. I can't learn from your post if you are polishing the parts for appearance or if you are polishing only to inspect for porosity. What are the requirements?

I would also wonder what features need to be polished and whether or not the tolerances on that feature are tight.

Red rouge buffing compounds make aluminum look like jewelry. You can get aluminum to best shine within 20 seconds on a lathe, with red rouge. It will also round the corners and scrub off a surprising amount of material.

Incidentally, WD-40 is an excellent fluid for machining aluminum. It eliminates chip welding almost completely.

We manufacture aftermarket automotive products. Some of them are aluminium castings that we machine and polish. Porosity is a problem that we cannot detect untill after the part has been machined then polished. Is there some type of coating or plating that will give the same look as a polished process?

Surfit

Can your casting supplier not help eliminate this porosity? Do not let them tell you otherwise...it can be done, it's just difficult. It is a very good time to look for foundries, we are all hungry for work. But tell us your expectations up front. We can't prevent porosity if we don't know.

You could try a chrome plate process with a first layer of copper. The copper fills in minor casting defects and is buffed, then the chrome plate process goes ahead from the perfect copper layer....expensive but effective.

Thanks for the input.
We manufacture superchargers and the polishing is only for cosmetic purpouses.
The parts that get polished would not be cost effective to machine out of billet.
The casting vendor is aware of the problem, I have RTV'd many parts after putting the cost of machining and polishing into the part.
The problem will go away then return, usually on the last day of the month.
I have heard of tumbling with polishing media have any of you tried this? If so do you have a contact?
I was leaning towards the plating option, does anyone have a contact for that process?
The parts are sand castings 356-T6.
Thanks for the input,
Jeff.

The parts are sand castings 356-T6.

Jeff,
you could try the Edelbrock Foundry in San Jacinto, CA. Have you inquired? They cast 808 and 356. They have a diverse customer base - not just automotive.

Lee

We had similar problems with cast aluminium radiator headers and charge air coolers we used in radiator manufacture for CAT. The root cause is gas porosity from the casting process, due to the fact that Hydrogen is very soluble in Aluminium. This is exacterbated by the polishing as you can open up leak paths by polishing off the cast surface.

I suspect you are using a small jobbing foundry for small runs? How do they degas their melt - addition of salts or gas injection lances?
If your supplier cannot eliminate the problem, change supplier to one that will!

Another option would be to send defective castings away for ion implantation. This is a well known method of "curing" porosity in castings.

Proper degassing is critical to remove porosity in cast aluminum. Companys such as Pyrotek, have a unit to introduce Nitrogen into the metal bath. The most important step in the degassing process is to perform either a reduced atmospheric test or a density test before tapping the furnace for production. Keep in mind that some gas is important for flowability so don't over-degass.

We manufacture superchargers and the polishing is only for cosmetic purpouses.
.....
The problem will go away then return, usually on the last day of the month.

I have heard of tumbling with polishing media have any of you tried this? If so do you have a contact?....Jeff.

Jeff

Years ago I had some castings vibratory finished by Roto-Finish. They looked fantastic, smooth and shiny. Here are the people who did it for us. Really top level people to deal with, they knew their business.

http://www.roto-finish.com/apps/apps2000/apps-2717.htm

I laugh when I read you get the bad ones at month end...likely all the months returns coming back to you for another try.

As others have said, this is likely gas porosity. It sounds like this scrap is very costly to both you and the foundry. Can you offer them a few cents more to invest to improve their process. Then they win and you win?

Chrome will be very expensive indeed. And you will have another supplier to deal with. You may end up with scrap chromed parts due to porosity. At some point you need to fix the root cause problem.

There are foundries who know how to give you a good part to polish with no porosity. Can you change? Or use the threat of a change to improve your current supplier?

Are you in Northern North America? If so I could point you to a good chrome shop.

Good luck with it, please let us know how it works out.

[quote=Dave.C]We had similar problems with cast aluminium radiator headers and charge air coolers we used in radiator manufacture for CAT. quote]

Dave.C

I used to work in a foundry that specialized in CAC tank castings. They are a tricky bit of casting business to get right. We made parts much like shown on your website...brings back lots of memories, some good, some bad.

Now I make Aluminum wheels, they are another thing all together. We make a wheel to the right chemistry, x-ray it to prove it is sound, heat treat it so it is strong, machine it to insane dimensional tolerances, carry out fatigue tests, balance and runout tests, then coat it and after all that it gets rejected for a small fleck of dirt I can't even see, that will get covered in disk brake dust that the customer never washes off his wheels anyway.....poor, poor, pitiful me. We do amazing castings, machined on awesome equipment, and almost ALL our problems are cosmetic.

You are dead on with your comment that a foundry can eliminate porosity...they just need to know it is a problem and get the equipment and process controls set up.

"Tanks" for the memories!