American Welding Society Forum
Can anyone help with some information regarding TIG welding repairs to magnesium castings?
We have a number of differential castings used by sprintcar racers here in Australia which have developed cracks in the diff centre casting. We are wondering if anyone could post a magnesium TIG welding procedure which may help us.
ie- how much pre-heat and post-heat if required? Is peening during welding required? What sort of electrode to use and polarity of machine?
Are gas lenses of any benefit?
Any other relevant information would be much appreciated.
Our machines are 300 amp square wave and pulse TIG machines.
You can call just about any steel foundry, the one that made that casting would probably be the best one to let you know the chemistry of the part to know what electrode would be best to use on it. They would also have the weld procedure to make repairs to that particular casting. All foundries that make steel casting usually have a rpair procedure for each casting.
Lets start with safety. Magnesium (Mag) is a flammable metal, in this case flammable metal means a metal that produces its own oxygen when burning. Breaking the Fuel/Heat/Oxygen cycle is nearly impossible once the stuff gets going. Metal-X fire extinguishers and buckets of sand are necessary to cover burning Mag. Magnesium will literally burn under water. Do NOT use your regular shop ventilation to remove mag grinding particles and dust. Some shops use modified paint booths, which pull the dust into a water medium. Most people cannot or will not invest in the necessary safety measures, however please do use extreme caution if you're doing a lot of grinding of mag.
There is a lot to cover as far as weld repairs of Mag castings go. The more complex the casting geometry the more strictly guidelines should be followed. Firstly you need to identify the type of magnesium alloy you are dealing with. Now with you in Australia and me in the US we may have to work a bit to define terms. None the less, Preheat, inter pass temperatures and postheat as well as filler wire selection are all going to be governed by the specific type of casting your dealing with. Here in the US. We deal with several predominate types, here is a listing of some that I have worked with personally.
AMS4418 …………………Trade name QE22A (Alloyed with Rare Earth and Silver),
AMS4434 …………………Trade named AZ63A (Aluminum Zinc)
AMS 4434 and 4484 ………Trade named AZ92 and AZ92A (Aluminum Zinc)
AMS 4442 …………………Trade named EZ33A (Rare Earth and Zinc)
Now rather than write an entire lecture I'm going to wait to see if you can identify the type of metal you're proposing to work with. Once that's established we could talk about the very important Preheat techniques, Manual TIG repair, Electrode Selection, the effect of hotwork on Temper/ Designations, Shielding Gasses or mixtures, InterPass temperatures, Distortion restraint, grinding and surface prep, and a whole lot more.
The best suggestion of all would be for you to gain access to the AWS Welding Handbook Volume 3 Eighth Edition which gives detailed guidelines in over 40 pages dedicated specifically to Magnesium.
The Ball is in your court.
Lawrence, please forgive my tardiness in not getting back to you sooner.
I had sent an email to the casting manufacturer hoping to get some info with regards to their casting composition but unfortunately they seem to have no interest in replying to me.
At a guess, I would think the casting would likely be a Mag/Alum. alloy but that would only be a guess.
We have some Mag filler wire at work, I'll find out the alloy composition and post it here if that's all right?
it's not a guess, you're absolutely right in saying that the casting you have is actually an aluminum/magnesium casting. Magnesium is a highly chemical active metal, which means that it can not be stored in open air because it inmediately starts transforming into magnesium oxide. Aluminum lowers this activity of magnesium, making the alloy stable to oxidation conditions.
The aluminum/magnesium alloy used for car wheels and other pieces is widely known by the commercial name of Duraluminium.
Giovanni S. Crisi
Sao Paulo - Brazil
Were you able to acquire AWS Welding Handbook Vol. 3 Materials and applications?
I'm not familiar with auto castings; my Mag. experience is strictly with aircraft. None the less, differential castings sound pretty critical. So to give advice without knowing what we deal with makes me nervous.
Here is the tricky part. Whilst Professor Crisi correctly acknowledges that Magnesium is most susceptible to corrosion and in obedience to the second law of thermodynamics, really likes to return to its oxide form. And while *some* magnesium castings contain as much as 10% aluminum we need to be equally clear that as many as 50% of the types of magnesium castings to be found today are NOT alloyed with aluminum. Zinc and or Zirconium being added instead. The composition of the casting, as stated in my last post, has an effect on Pre/interpass/post temperatures and is critical to filler metal selection. While guessing sometimes works, the wrong filler wire will necessitate the removal of all deposited metal and the diluted parent material as well, only then can a sound weld be made with the correct filler if you guess correctly the second time.
This all sounds complicated only because we lack data. Once you know what your working with the actual welding is pretty routine. If the casting is a good one, Mag. Can be welded with relative ease.
The tables in the AWS handbook provide an excellent source of data and if you are going to have to experiment on scrap material, (due to lack of response from your foundry) it may be wise to acquire a copy before you begin. If you must make general assumptions it will be invaluable.
If you cannot do these things, Here are a few rules of thumb, which apply to nearly all Mag. Castings.
1) No old filler wire. If the wire has been exposed to air for more than a month or so chuck it. Unless fresh out of a new sealed package, clean with fresh scotch brite until a *bright* surface is achieved than wipe down with Acetone or Alcohol.
2) Route out entire flaw. Cracks hold impurities, which will foul any attempt to cover them over. Argon backup on any part which may melt thru or has been routed completely thru.
3) Preheat the part in an *oven* <pick a day your wife is gone> with at least a 60 minute soak time. No rosebuds!
4) Cover entire part with an insulation blanket during welding operations leaving only the weld area exposed. Return to oven as necessary to keep heat uniform.
5) If multiple passes are required use tempstick to stay within interpass temperatures.
Keep us updated and good luck
Let me clear up one thing. I didn't want to mean that Magnesium/Aluminum is the ONLY magnesium alloy, but just it's a widely used one.
Giovanni S. Crisi
Lawrence, unfortunately I was unable to access the AWS handbook you recommended. I had sent another email to the casting manufacturer, again, to no avail it seems.
We have some Mag filler wire here and it has the following designation.
Alloy-Magnesium. RTAZ92A. Heat. NJ13011ALA-D3.
We also have some diff castings here which are beyond repair but which will provide me with some practice before attempting the real thing.
Do you think you could provide preheat, interpass and postheat temperature conditions which may apply when using this filler wire?
Any information you can provide is appreciated.
Thanks also to G.S. Crisi, the information provide by you and Lawrence and others make this site so worthwhile.
I'm gonna try to email to you the tables for preheat postheat and a few pages of technical data which I took home and scanned. Send an email and I'll reply with the data attached
Now keep in mind that even though you may have identified the filler wire you have in the shop (AZ92) You still don't have a clue as to the compositon of your parent material.
Are those scrap castings the same componant type as the one you need repaired? if yes thats good! The AZ92 is applicable to a wide variety of Aluminum/zinc casting compositions.
Back to the Original post. Gas lenses are Ok if you can afford to replace them quite often, as the screens will get chewed up during AC welding operations. They certainly are not essential
Go with Zirconium alloyed tungsten electrodes. With your AC set slightly towards max penetration to start and modify as necessary.
Keep the electrode very close to the work, Mag makes this easy because unlike aluminum its tendency is to stay away from the electrode rather than to jump on it and make a mess.
Argon is just fine for shielding and backup. Helium works but is more expen$ive and requires greater volumes. Some folks prefer a mixture if 50/50 on very thick castings in order to make best advantage helium heat transfer and argon coverage but its not a necessity. (I just like playing with my gas mixer)
Just as with AC aluminum operations, its best to employ a liquid cooled Tig torch with as long an electrode as you can get. (Not a long stickout) but a long cap on the back to drink as much heat as possible and keep the tip shape from distorting.
Remove All paint, anodizing or alodine from hot work areas. And reseal with corrosion preventative After stress relief and post weld inspection. Xray is nice but FPI is better for chasing down cracks In castings both in routing original defects and repairing flaws.
When practicing on your scrap. Practice post weld inspection too. Blend a little off the top of each weld bead, Mag can be quite tricky and what appears to be perfect on the surface might well look like swiss cheese just a little below.
Keep us posted
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