I'll say  this about that:   If the wheel is for a vehicle (whether aluminum or mag) I don't provide advice.  Except to approach the OEM of the of the part and get written repair procedures.   I know the AWS is constantly showing pictures of guys in shops welding on "Mag" and aluminum wheels...  I've even done it myself several decades ago...   Not anymore.

Now if this wheel is for static display or  some benign part of some machinery, it will still be important to do some research if possible to identify the magnesium alloy.... (all the above text on Mag identification is good).

My experience, especially with complex shapes, is that unlike aluminum, there can be some benefit in pre-heating the base metal prior to welding in order to have a more even cooling rate of the part (blankets etc, or back into the oven for cooling under control)

100% route of cracks followed by dye penetrant check.... You CANNOT weld over any residual cracking in mag and get away with it.

There is a lot of blather about helium mixes... This is only necessary if your power supply does not have the power to start a weld puddle at once.  Hot and fast.   Note:  Unlike aluminum GTAW... Arc terminations should be slammed shut; Meaning after the last dab of filler take your foot off the pedal all at once... Don't tail off slowly.

The article below provides some data on base metal/filler selections (AZ101 being the most versatile)  Filler rod angle/placement is more important with Mag than aluminum, so pay attention to the instructions given here.  Some solid guidance that will save me a lot of typing.

http://www.totalmateria.com/Article35.htm

For GTAW crack repair I agree with AC... Balance not exceeding 60% EN (so about 6-7 on the Synchrowave balance dial)   60-100 Hz.  Nothing special.   For buildup of lugs and flat surfaces DCEP and a giant tungsten can be used with helium gas. But your scenario does not fit that description.

If you can score us a pic of the part and the defect, I can say more about the dark art of preheat/postheat and defect routing.

One nice thing about Mag is that it does not load up carbide burrs like aluminum does :)

Tim:

My apologies for assuming that you were talking about repairing an auto or truck wheel.

The reason for my knee-jerk reaction is that I have always been puzzled by the fact that there are many companies, in Canada and the US, offering cheap repairs to automotive aluminum wheels, and there seems to be very little information on how the repairs are done while preserving original structural strength, or approximating it. In fact, some operations are franchises, and operational requirements are minimal. Most of these advertise "certified weldors" and "guaranteed safety", but, it is difficult to find out who did the certification and how quality is controlled.
One exception is the province next door, British Columbia, which has a clear set of defined standards for wheel-repair shops, even defining the type of equipment needed, as well as certification required.

http://www.bclaws.ca/civix/document/id/complete/statreg/5_97

In Alberta, though we have many repair shops,  and the rest of Canada, I have been unable to find anything like this.