after several helpful replies on the topic of Magnesium,
my questions have shifted a bit, and am reposting under a new
topic.

recent experience:
pair of brake calipers showed up from a gokart... really light,
i figured they were cast aluminum. i've welded cast alum before,
so i fired up the MIG and had at them. the mig wire blew RIGHT
THROUGH the calipers. it did a better job that my drill press!

fast forward a few weeks:
bought a Lincoln Invertec TIG welder (205T AC/DC)
i'm having trouble selecting filler metals for TIG welding aluminum.
-- because i dont know what type of aluminum i'm trying to weld.

i do this as a hobby.. but enjoy it enough to want to learn more.

most of what shows up here at home are castings...
brake calipers, guides (fences?) for woodworking equip,
oil pans, pulleys, etc..

any good way of identifying what type of aluminum these are
made of (if its aluminum at all)? and if they are weldable?

the books i have are very broad on aluminum classification
(ie, all castings are the same species of aluminum).. either that,
or they presume outright that i have a spec sheet telling me
what type of aluminum it is.

are there general purpose aluminum filler rods that i can use
blindly? an old standby? or is aluminum too sensitive?

thanks again,
-tony