We produce some small parts of CDA 798-2 Nickel-Silver that require laser stenciling. The stenciling is done by an outside vendor. The customer is complaining of dark spots on the surface next to the marking. The laser people said that it would not be from the laser operation but from the residue on the surface prior to stenciling. They said it could be a film or even dust that gets heated up by the laser. What would be the recommended method of precleaning the parts? R.S.
To best remove the contaminant, it would be helpful if you could first identify where it was coming from and what it might be. Your shop performing the laser stenciling could be correct. An organic residue of some sort could turn dark after experiencing the heat of the laser. This organic residue would not necessarily be confined to the stencil, but also the area immediately surrounding it, since some of the organic contaminant may volatilize to some extent and then redeposit adjacent to the laser marking.
Before setting up a large cleaning operation to combat this, though, I would start off conservatively with a couple cleaning trials to see if the situation improves. Not knowing the size of these parts, I am not sure what the best process would be, but it may be as simple as starting off with a simple hand wiping with a solvent to remove any gross contaminants. From there, you may also want to try aqueous cleaning to remove the potential contaminant.
The reason I am suggesting you approach this conservatively is that particular alloy that you are having laser stenciled may also be causing you problems. I am not familiar with the CDA designation 798-2. The standard CDA (Copper Development Association) nomenclature uses either a three-digit in older references or five-digit number in more recent literature to designate the alloy. The 700 series of alloys are copper-nickels used primarily for corrosion protection in salt or marine environments. The C735 to 799 series are copper-nickel-zinc alloys, also referred to as nickel silvers. The C798 specifically is a copper-nickel-zinc that also contains lead and manganese (CDA lists three versions, 79800, 79830 and 79860 with varying levels of alloying elements).
The reason I bring this up is that I am wondering if one or more of the alloying elements could be volatilized from the energy of the laser and then redeposited back onto the surface. At atmospheric pressure, zinc can turn to a gas at about 1,650°F, while lead is somewhat higher. This is speculative, but it may be possible that the zinc is sublimating from a solid to a gas locally at the area where the laser contacts the metal. If this is the case, no amount of cleaning will help you, only a change to a different alloy. Here again, this would be easy enough to evaluate by trying an alternate copper alloy that did not contain these elements.
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