TIG welding anything cast is a bit of a nightmare.
http://weightweenies.starbike.com/forum/viewtopic.php?f=3&t=49794
Essentially you can treat these frames just like alluminium frames. i.e. you can use a grinder on them (how I removed the shifter hump), use a blow torch on it etc. Heck you can even weld it if you really want to.
@Ruimteaapje started
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http://weightweenies.starbike.com/forum/viewtopic.php?f=3&t=49794