Welding Magnesium Aluminum alloy

First, contact the equipment manufacturer's technical support. (Although, they may try to discourage you from modifying their product due to safety and/or legal concerns.) 

Try one of these welding forums:
http://www.hobartwelders.com/weldtalk/
http://weldingweb.com/
http://www.millerwelds.com/resources/communities/mboard/

First try and contact the manufacturer and tell them that you are trying to repair their deck and not trying to modify it. They will feel allot more obligated to help if they think their product has failed. Don't necessary ask them what rod to use but what the alloy is so your welding supplier can recommend a rod. If this doesn't work I would go to the nearest welding supply and tell them what you plan on doing and could they suggest a rod. Also look over the deck and determine if it is cast or a fabricated part. Do you see any welds on it? This would tell you right away that it can be welded. Sometimes things are cast from certain alloys because they are not weldable. I have seen allot of these decks made from what I call pot metal and they are not weldable with any type rod. If you try to weld them they either crack or the metal just doesn't seem to fuse together. If I was going to try to weld without knowing exactly what to use for filler metal I would start with pure tungsten, AC current with high freq., and a rod used for cast aluminium. I forget the number on this rod but most any welding supply has them. Be sure to clean the item super good because any aluminum alloy can't stand contaminants in the weld zone. When you fire the TIG rig up on the metal you will know right away if it is pot metal and you stand a good chance to know if it is weldable by the way the puddle acts.

pipewelder