Chem-film Finishes on Die-Cast Aluminum?
Greetings to all! I'd like some advice regarding chem-film finishes for die-cast aluminum parts. Our company's products often use small aluminum housings that are machined from 6061-T6 alloy and chem-filmed per Mil-C-5541 [link by ed. to spec at TechStreet]. We're now investigating the feasibility of producing some of these parts via die casting.
Recently we received some sample parts that were die cast from alloy A380 aluminum and were chem filmed per MIL-C-5541, Class 1A. Unlike the machined parts (which have colors that are uniform and range from pale to golden yellow), some of the cast parts possess a mottled green appearance that can be easily rubbed off. This is unacceptable.
Also a few other samples I received have a brownish color, and I noticed that heating them to about 125 F-150 F causes their color to darken to a deep bronze. The texture of the finish also becomes flatter, almost chalky.
Some individuals have indicated that this result is typical of chem-filmed die cast aluminum parts, especially when using the A360 or A380 alloys. However, I'd like to hear additional, detailed opinions on this topic.
Also, if the chem-film finish is inadequate for this application, what is an economical alternative? Keep in mind that the electrical conductivity that the chem-film finish offers is a very desirable feature.Keith B. Olasin
- Whippany, New Jersey, USA
360 and 380 both have about 9% silicon in the alloy which makes life quite difficult vs 6061. It will require an acidic deox/desmut that contains a goodly amount of fluoride. Tri acid-sulfuric/nitric/HF being a common one that is extremely strong. If that is the only alloy being run in that chem-film tank, then it could be tweaked with the help of the vendor for optimum performance on the cast alloy. Color changes, minor ones,are moderately common. It should not have major ones and certainly should not be chalky or rub off.
140 F is the maximum that a chromate coating should be heated to , in most cases.Alternatives would be return to the wrought alloy, change the casting alloy to one that was more amenable and find better vendors for the casting and chem-film. You might have to bring the chem-film "in house" . This may cause more problems than it solves. There may be vapor deposition processes that would give satisfactory tradeoffs, but The price would very probably be higher.
It is worth looking into from some of the folks that advertise here.James Watts
- Navarre, Florida
You're pretty much right on the button with your thoughts and what others have been telling you. Die casted chromated parts are nowhere near as uniform as a machined finish because of the porous structure of the casting, you can limit this by polishing down the casting to a smooth surface, but you eliminate the cost savings of going with a casted instead of a machined surface by doing this.
The finish probably appeared rather patchy in areas, but as far as rubbing off, it shouldn't do that no matter what class of chromate you use Class 1/3 whatever. It won't last as long corrosion wise on the casting material as it will on a machined surface. I want to say off the top of my head that 336 is of course the machined surface salt spray requirement in hours and casting material (dependent on the type, I think 380 is the called out material in the ASTM spec) its around 58 hours or something incredibly low like that. Which in my opinion is proof enough to signify the difference! s, the governing bodies already
realized and noted that you will not have good salt spray resistance, and if not mistaken, your electrical requirements also drop down as far as the conductivity goes in regards to the casting material. Are there other options along these lines, maybe, but I don't know of one myself. Good Luck to you, if you have any further questions, feel free to let them fly, I don't have an reference material in front of me to consult and all of this is coming off the top of my head so it might not be 100% accurate, but it does give you a solid A to B starting point to acknowledge.Matthew Stiltner
- Toledo, Ohio
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