It is probably a solid state welding process that is being used , not a fusion process to join Al to steel.
Several options are used including:
Friction Weld
Explosion Welding
Each method has limitations and not at all like typical fusion welding processes.

Hi

I have previously joined Al to Cu by first buttering the Cu with a Silver solder brazing alloy. Following this, I welded the Al to the silver solder using an Al filler. Possibly you could try something simmilar, if this is what you had in mind.

If this is the type of joining you had in mind, let me know and I will scratch out the filler and brazing alloy details.

Regards
Niekie Jooste