Depends on the kind of piston.
For a car or other engine, you normally find the piston head is made of aluminium with 2 0-ring seals along the top edge made of steel. Please remember that only the piston head is made of aluminium- the crank shaft and the rest of the cam assembly are made of high-tensile strength steel.
For firearms, the piston is normally made of stainless steel or chromoly steel which allows soot to be easily wiped off.
For pumps and compressors the pistons, depending on the application, might be made of plastic, although in older designs iron or steel is much more common
For a car or other engine, you normally find the piston head is made of aluminium with 2 0-ring seals along the top edge made of steel. Please remember that only the piston head is made of aluminium- the crank shaft and the rest of the cam assembly are made of high-tensile strength steel.
For firearms, the piston is normally made of stainless steel or chromoly steel which allows soot to be easily wiped off.
For pumps and compressors the pistons, depending on the application, might be made of plastic, although in older designs iron or steel is much more common
