Alternative to polishing aluminum castings that we machine and polish

[surfit]

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?

[Wes Bucey]

Originally Posted by surfit

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?

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:

1. tumble the parts in polishing medium
2. sandblast each piece in a sandblast box
3. 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.

[apestate]

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.

[Caster]

Originally Posted by surfit

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.

[surfit]

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.

[Crusader]

Originally Posted by surfit

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

[Dave.C]

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.

[Crusader]

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.