Open die forging, also called free forging, is where the billet is placed within dies which do not enclose the metal entirely. The dies used for this process are usually flat in shape although some are specially shaped for more custom projects. The opposite of this process is closed die forging where the workpiece is completely enclosed.
Open die forging is usually used for shapes such as discs, hubs, shafts, blocks, plates and other custom shapes. This type of forging process strengthens the material being used by refining the grain of the metal, this in turn also reduces porosity. Cast parts are usually not as strong or have the wear resistance of similar forged parts.
Closed die forging is the process of compressing a piece of metal in an enclosed die impression. The raw material is approximately the same size as the final forged part which is placed in the bottom die. The top die impacts on the raw material to form it into the required shape. Any sized parts can be made using this process, some of the smaller parts can be forged cold.
Most metals can be forged using this process, this includes carbon steel, alloy steels, stainless steel, tool steel, aluminium, copper and sometimes certain titanium. Magnesium requires a more specialist forging process due to it being a temperature-sensitive material. Titanium is another material which requires specialist equipment and processes for forging.
The advantages of closed die forging over open die forging are the reduce machining time due to the better surface finish, this makes it cost-effective for large production runs. If the part is simple then open die forging is better for short production runs, as there is little or no tooling cost. Machining will be required to achieve the level of accuracy required.
Depending on what you require will determine which type of process you choose. If your design is simple and only a few are needed, then open die forging is for you. On the other hand, if the shape is more complex and a larger production run is needed then closed die forging should be a consideration.