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Repairing Pinholes in Cast Silicon Bronze
IBIndomitus -
Tuesday, October 28, 2008 - 9:29am
Casting | Welding pinholes | silicon bronze | welding defects Hello. I have looked through the other blogs in search of a solution to my problem, but I have not found anything on it( although i have found some great tips and info). I apologize if the question has been answered and I missed it ;-) I have been working with cast silicon bronze for many years now, but there is one problem that I have not been able to find a reliable solution for...maybe someone can help. Occasionally, and it seems to occur mostly on very porous castings, I have pinholes which re-occur continually after welding then chasing. i use a miller syncrowave 250 TIG and silicon bronze weld rod. The most reliable solution I have found is to peen the welds soon after welding the holes up. I think that that helps...it seems to work more often then just a random percentage of the time. However, it isn't a sure thing. I always grind the pinholes out before welding them in order to eliminate variables and work with clean metal. Sometimes it seems like the hole comes back after chasing exactly as they existed before i welded them, but sometimes they will re-emerge as two or more smaller pinholes. I have tried reducing amperage, and very consciously controlling heat...that does seem to work some of the time, but definitely isn't fool proof. The problem is very rare, and it really seems to be limited to castings which have a less then an ideal amount of porosity, but it doesn't seem to matter how much of the original metal I replace...short of cutting out a section and replacing it with new metal (a plate or better cast piece), in some cases, no amount of welding in more silicon rod seems to help. Am I missing an easy fix? Thanks in advance for any solutions or advice!  Pinhole Dilemna
IBIndomitus -
Thursday, October 30, 2008 - 2:28pm
Hey Rich...thanks for the post and the welcome. We do use a degassing compound, although we've only recently started using it. We have definitely reduced the incidence of pinholes and porosity, but occasionally we wind up with a casting that is not quite flawed enough to throw back, but turns up with some pinhole problems. I was curious if anyone had any kind of viable solution for the problem after the fact, but the more I look into it, the more I'm finding that the problem has to be dealt with at the source. Thanks for your input... for now, I think we'll be living with some of those pinholes.  degassing compound with SIB
GeorgeMummert -
Wednesday, November 26, 2008 - 10:26pm
what type of compound? I use only Everdur 873, do not really get pinholes, but we do have a furnace that seems to generate alot of turbulence due to the interior shape. What compound and how to use it? Geo |
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ArtMetal
Welcome to ArtMetal!There
Welcome to ArtMetal!
There is silicon bronze and there is silicon bronze, and not all are created equal. While the general specification for Si bronze calls for 97% copper and 3% silicon (for high silicon bronze), it is acceptable for it to have some tramp elements such as manganese, etc. present in amounts less than one percent. Sometimes, these tramp elements can cause problems such as pinholing and porosity when casting.
Do you use a de-gassing compound when melting your bronze? This can be a big help in overcoming outgassing of lower-melting tramp elements and other causes of porosity.
If your porosity/pinholing is, in fact, caused by tramp elements and/or contamination, those contaminants are present all through the area and will cause the same problems when TIG welding the repairs. The filler rod, being more silicon bronze, may have tramp elements, too. When you come up to the fluidus temperature, the problems start.
If it was one tiny area where this was a problem, I'd suggest just drilling, plugging and peening for a fix, rather than welding. Such is probably not the case however, so you're going to be fighting it all along. I know of no way to get true fusion-welded repairs without subjecting the metal to further pinholing if there are contaminants present. Maybe working in a vacuum would do it, I have no way to test that theory.