Dicast you have 3 options, magnesium, zinc - zamack or aluminum. Some of the alloys contain all 3 to varying degrees, but so long as its not zamack its weldable, zamack feels noticeably heavier than a similar sized lump of aluminum, nearly on a par with steel. If you can even approximate its volume working out its density should narrow it down into likely to be weldable or not.
Lots of the casting grades of aluminum generally weld very nicely as a rule of thumb, if you consider what makes good casting properties wise of going from liquid to solid, casting grades of metal generally makes for nice welding properties, (exception of iron) its the extruded grades that are normally the bigger issues, though most can be welded pretty reliably these days too.
This would probably be a bad idea if your going to spin the chuck fast, but my understanding, your going to be in the low hundred rpm range, so even if it fails, its not going to be seriously dangerous. If you do want to go fast, then it probably would be better to look at a solid steel part - back plate. Aluminum has a very definite fatigue life, especially when your playing - welding with uncertain alloys.