A method of forming an assembly is provided. The method includes forming an insert, coating the insert with a bond material and placing the insert within a casting mold or die. The casting mold or die is purged with an inert gas and the casting mold or die is filled with molten metal to encapsulate the insert. The encapsulated insert is diffusion bonded to the molten metal to form a diffusion bonded insert, which is placed within a cavity of a secondary casting mold or die. The secondary casting mold or die is filled with molten metal to form a composite casting assembly. After or during the casting, the composite casting assembly is heat treated. In another method of the present disclosure, the insert includes ferrous alloys, nickel-based alloys, super alloys, and nonferrous alloys.