Welding Tips and Tricks

I've been given a part from a vintage stationary engine to repair. It's a casting which has broken due to being badly fited but I'm not entirely sure what metal it is and what process to repair it with. It is dark grey and very 'grainy' which along with its age leads me to believe its some sort of zinc alloy.
Should I TIG it? I have AC TIG and have welded aluminium (I'm English!) and cast iron with good results. I also have oxy/acetylene, should I braze it?
Any help with identifying the metal and filler choice would be great. Cheers.

You may be looking at cast magnesium. Hot work is very unfriendly to this alloy unless you do it in a purge box so none of the metal sees oxygen.

The test is easy; Break off a small sample and expose it to oxy-fuel heat way at the end of the flame so it sees air. Magnesium will burn with a nearly blinding white light and a lot of white smoke.

Mg can be tig'd but you'll need to do the work in an entirely O2 free environment. It has a low melting point, and welding on anything cast is usually difficult. A preheat may be needed. This might be a question to ask Jody directly.

Steve

I would say that you have either aluminum or magnesium. And for something like that, TIG would be the best option.
An easy way to find out if the metal you have is magnesium (or not) is to put a drop of stay-clean soldering flux on the metal. If it turns black (instantly) you have a high level of magnesium in your metal. If the drop stays clear, like water, then you have something else.
If you have magnesium and are going to weld it, cleanliness of both base metal and filler metal is extremely important. Chemical or mechanical cleaning of the joint are and filler metal, to remove surface oxidation, should be done before welding. The totally oxygen-free environment is not necessary, this will be practically identical to welding aluminum. With this being an engine casting, it is going to be very dirty. You may have to make several passes over the same joint to completely burn out the impurities and get good weld metal in there. It is an extremely tedious process. You will probably be using either AZ61A or AZ92A filler metal with AZ92A being the most versatile.

Remember, burning magnesium is classified as a class "D" fire and cannot be extinguished by class A,B, or C agent.
Good luck.

Thanks for the information, I'll try these tests to rule it out but I forgot to mention that this stuff is heavy. I'm an aircraft engineer so although I've not welded mag I deal with mag and alu castings all the time. It's dark grey in colour but polishes up real bright, (nickel?) It's not like any other metal I've dealt with before.
BTW I also use a Dynasty 200DX with an SSC pedal, brilliant.

If it's heavy enough to rule out Mg and Al, you can probably rule out Zn too. You may be looking at a cast steel with a high Cr content, similar to modern stainless steels. I've seen cast 304 stainless, and it has a grey color and grainy structure when broken. If this is the case, a welded repair (done with TIG, I suggest) will require a pre-heat, post-heat and gradual cooling (normalizing). I'd have to do some research to recommend a filler rod; I'm not sure 308 will have the same shrinkage as a stainless casting.

If money's not a serious obstacle, you can send a small sample to a metalurgist and get a clear answer.

Good luck,

Steve

If it is "heavy" as you say, I would say there may be two possibilities. 1. The casting has a high zinc content. or 2. As Steve said, it may not be magnesium at all. It's hard to say because your perception of "heavy" may be different than mine.

Hi there,

Two tests that I can think of. 1 Put a magnet on it, to rule out any form of ferrous metal.
2. The spark test- Grind a bit of it to see what the sparks look like, ie : colour, shape and length. You can find an explanation here-

http://en.wikipedia.org/wiki/Spark_testing Copy and paste into address bar.

Also being very old is it likely to be an exotic metal? Might just be a form of cast iron? There are lots of types.

Hope it helps.

Kind regards
Mick

for a reference, the most common thing you'll see cast in zinc is a carburetor. now, I've never broken a carburetor, so I don't know what a cracked edge looks like, but the zinc castings get a lot darker than Al or Mg as they oxidize, similar to tin. I've never tried, but it was my understanding that you can't weld tin and brazing it is difficult.


as for your stationary engine, unless it was made by a super low budget firm (not enough money for a cast iron furnace) there's no reason for it to have been made out of anything but iron. stationary=weight insensitive. pictures would help ID it, after you do a spark test. cast iron is pretty easy to spot(the sparks look like little fireworks), anything non-ferrous won't make sparks.

If it's a zinc alloy, it will have a lot less ductility than an aluminum alloy. So if it's fractured, you can see if the metal yielded much along the fracture line before completely fracturing.

Another clue, although not always conclusive, is the manufacturing method. Zinc alloy is often selectedd because it melts at a low temperature, so can be poured into a metal (IE: steel) casting mold and out pops a shiny, die-cast part. The undamaged steel mold is ready to re-cast the next part, so many copies can be made inexpensively. Parts made this way generally have fine casting detail and smooth surfaces from the metal mold surfaces being usually smooth and able to have fine detail. It's not always conclusive though, because sometimes you will find aluminum alloys cast in the metal molds (die-cast).

If the part is cast but does not have smooth surfaces, it indicates it is sand-cast (where the mold is made out of sand particles that is generally used to make one part only,) and in that case there will not generally be a zinc alloy, but is likely to be an aluminum alloy.

Zinc alloys have a whitish sheen to them that's kind of hard to describe but if you've seen enough examples can help indicate if a piece is such an alloy as well.

Finally, you can always clean it and try TIG welding it. If it's zinc alloy, shortly after melting, it will smoke and burn and leave powdery "smut" (tiny, solidified particles of zinc oxide) around the weld area. You can TIG weld it to some degree like this, but not without some porosity inside the weld bead. If you can get a layer of 4043 or something buttered on the surface, you can TIG weld to that nicely. Be sure to let it cool often when welding it, and don't get it too hot or the low-melting temp zinc alloy part might suddenly collapse on you.