Hmm, I dont wanna be a killjoy but I dont think thats gonna work, induction heaters rely on the heated material being magnetic, ally isnt so probably wouldnt heat up, or maybe just a tiny amount due to the impurities.
I have a chinese wsm200 tig welder that I got off ebay for 150 quid, it will produce pulsed dc and welds ally really well, if your ally is really thin or oxidised or contains impurities then you'll need a true ac tig set which are pricey.
I have a nifty circuit for making an induction heater around 200 watts just using 4 or 5 electronic components, heres the link, part 2 is the one that I tried, part 3 on gets complicated,
**broken link removed**
Using an audio amp isnt a good idea, you need higher frequencies than most amps will produce, and there are some designs that will but most will not work into a low impedance load without going unstable and blowing to bits.