Braze-welding defects in cast bronze with O/A

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A friend of mine has started doing some bronze sculpture with a local foundry (college metalworking class's foundry).  My friend has no skills with welding or brazing, so has asked me to help him by using my O/A gear to fill his castings' surface defects.The castings are silicon bronze, so I obtained some silicon-bronze rods of various diameters.  I bought rods of the exact type recommended by the metalwork instructor at the college.I've had general success in filling the defects.  If they are quite small, I use a 1/8" drill to mill a dimple into the casting surface, which makes it easier for the molten brinze to flow in.  Of course, I'm also using brazing flux.With larger defects, there is no need to drill a dimple first.The issue has not been either getting the bronze to flow in nor getting a good bond.  It's that with some fills there has remained a hairline crack (barely visible sometimes) around the fill - sometimes not 360 degrees around, but partially.  In all cases, when a crack is visible it's when the excess bronze fill (above the surface level of the casting) is gound down to the level of the surface.  Yet, in these cases, the plug is solidly bonded and does not come loose with grinding.I've been careful to heat around the void or hole (defect), heating the main metal, not just the rod's tip.  Still, my sculptor friend has become nervous when he watches me do this, because he thinks I may melt the cast metal too much and possibly worsen the surface  defect.  I've carefully explained to him that it would be useless simply to melt the filler rod, and that no decent bond could result.  (And I've asked him not to watch me!)I want to get a better result, free of all surface cracks.  But I'm in a ticklish place of wishing to eliminate any crack around the fill, while finding myself feeling wary about overheating the base metal and possibly actually damaging a casting.  Thanks for pointers and general advice.
Reply:Are these hollow castings and if so, how thick are the walls? If they are thick enough for some peening not to damage them, you might try doing that to the  metal you added above the surface, as it cools. I can't remember off-hand if this alloy has some (relatively low) temperature where it becomes very brittle (hot short) as it cools; if so, this might be a bad idea. Otherwise, it would help to compact the filled-in area and reduce cracking.Someone else will correct me if I'm wrong here.
Reply:How much flux are you putting on the rod???  It's common, most people use far more flux than necessary, just heat the rod a little, dip it, even partially wipe it on the way out of the can.  Even the precoated brazing rods, usually have more flux than I like on them.  The flux, especially going into a hole, may be getting in the way of a good bond at the top of the hole, since it floats.Don't be afraid, of experimenting, with no flux at all, especially for an ornamental or art application like this.  There are several different fluxes available, you might look around, maybe another flux would be more appropriate?Is it possible, at all, to get some scraps or reject pieces, from the foundry, to practice and experiment with?  You can always return them later, so they can thow it back into the metal pile for future castings ...You weren't too clear in your original post, is this a commercial foundry, that the college metalworking class uses, or is it a foundry, that the college owns themselves?  If it is commercial, check with the people there, most foundrys do their own repairs and touching up, they may have some suggestions.
Reply:Oldiron2 asked: “Are these hollow castings and if so, how thick are the walls?”All are hollow to one extent or another.  Some of the smaller pieces that I’ve worked on have had walls 3/16” thick or so.  Some of the larger pieces have walls about 3/8” and sometimes much thicker.Mark 8310 asked:  “You weren't too clear in your original post, is this a commercial foundry, that the college metalworking class uses, or is it a foundry, that the college owns themselves? If it is commercial, check with the people there, most foundrys do their own repairs and touching up, they may have some suggestions.”It’s not a commercial foundry that’s doing the casting – it’s metalworking students under the guidance of an instructor in a college shop/studio.  Because my friend is not a student there, they are just pouring the castings and not spending any extra time repairing the defects.I’ll experiment with fluxes.  I’m not sure if that’s the problem or not, but I’m sure it’s good to bear it in mind.
Reply:Originally Posted by Joel_BC my sculptor friend has become nervous when he watches me do this, because he thinks I may melt the cast metal too much and possibly worsen the surface  defect.  I've carefully explained to him that it would be useless simply to melt the filler rod, and that no decent bond could result.  (And I've asked him not to watch me!)
Reply:Originally Posted by pulserI understand the base metal is silicon bronze and the filler is silicon bronze, so it seems logical as you say to melt the base metal and filler metal to "weld" fill the imperfections, however this may be related to your cracking problem, just guessing.  If you were to "braze" the defects, by not melting the base metal, only melting the filler, you may have better luck, and as will all brazing it is possible to get an excellent bond without melting the base metal.
Reply:Silicon bronze with TIG doesn't need flux, but with OA I'm sure it is needed.  Something to think about, try using a slightly carburizing flame, rather than the perfect neutral cone, or oxidizing.
Reply:Silicon Bronze is easily contaminated with lead, as from leaded brass. Lead Silicate, ie glass, is the result of that. So long as you've only used Silicon Bronze rod, that should not be a problem though.I took a quick look at a few books, but didn't find what I wanted about the alloy. Some non-ferrous alloys go through a brittle stage during cooling, and if shrinkage has occurred, cracking can occur easily. The solution might be to heat a much larger area so it all expands, causing less localized shrinkage when it's going through that stage. Overheating can be just as bad by causing too much stress and possibly getting more metal to be at that brittle stage, or by making the brazed area  stay in the stage longer. I don't even remember if any of this should apply here. but I do like the suggestion of borrowing some larger scrap items to test procedures or practice on. Have you tried the peening yet? Maybe wait 'till you get the scrap pieces.