There are several different ways to bond aluminum to steel without significant heating and associated diffusion.

A thin layer of aluminum (up to 5 microns) on steel can be obtained by vacuum vapor deposition (PVD and CVD methods).

Thicker layers of aluminum on steel are obtained by high velocity spraying (coold spaying) of aluminum powder. In this way, a coating with a thickness of tens of micrometers to several millimeters can be obtained.

Another method used for joining sheet materials is explosion cladding.

However, the simplest way to obtain a thin layer of aluminum on steel without heating is to mechanically rub the surface of the steel with a piece of aluminum (a type of friction welding, but at the micro level).

Dear Himanshu Rai , in addition to Vadim Verlotski answer, another way to obtain a thin film of aluminum over steel would be by means of DC- sputtering. Alternatively pulsed laser deposition (PLD) could be used as well, but in many cases the substrate must be heated.

Your question suggest you want to join together two pieces (one of steel and another of Al). If this is the case, you could polish a face of each piece to achieve a close contact.

Hope this helps. Good luck with your research.

Thanks, Vadim Verlotski Manuel Gómez Mohammad Pourgharibshahi