Most cases are cast aluminum and usually not tempered.
Most cases and pistons use a high pressure die casting process. These castings are done using metal molds where molten aluminum is forced into the molds under very high pressure similar to injection molding of plastic.
Cylinders and heads that use sand casting are usually made from A 356 aluminum and have to be heat treated and tempered before the machine work is done or they will not hold their shape and dimension as the casting ages.