Try Durafix instead. It’s an alloy that melts a bit below the temperature at which aluminium melts, and far below the melting point of steel and iron. The company has demo videos on youtube; you’ll need a blowtorch (of the cheap variety, at least - I use a Würth torch that cost around 40$) to heat the point that needs fixing.
It is a lot stronger than aluminium, but not as strong as steel. Occasionally it is used to fix cracked engine blocks, though I wouldn’t use it for that. But an exhaust pipe with a leak? No problem. It’s important to scrub away oxidation, even as you heat the exhaust pipe, then you’ll get a perfect bonding with the alloy.
Google durafix, they have local resellers everywhere.