Is there a way to break the bond between aluminum and steel besides brute force?

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You don't need a weatherman to know which way the wind blows - Robert A. Zimmerman

try some acids: lemon / lime juice, vinegar, toilet boil cleaner.. they get stronger.. but do you really want to mess with that?

if its in threads somewhere: numerous high heat and cooling cycles will help break the bonds.

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Is this related to an alloy wheel that won't come off the hub?

Caustic eats aluminum. I worked in an aluminum plant for 20 years and we used a 50% caustic solution to leach the alumina out of the ore. Get a bottle of caustic drain cleaner and watch the bubbles start. Caustic won't eat the steel so let it soak until the Al. is gone.