Forging is the process of shaping metal through the application of force. Open-die forging is also known smith forging. In open-die forging, a hammer strikes and deforms the workpiece, which is placed on a stationary anvil. Open-die forging gets its name from the fact that the dies (the surfaces that are in contact with the workpiece) do not enclose the workpiece, allowing it to flow except where contacted by the dies. Therefore the operator needs to orient and position the workpiece to get the desired shape. The dies are usually flat in shape, but some have a specially shaped surface for specialized operations. For example, a die may have a round, concave, or convex surface or be a tool to form holes or be a cut-off tool. It is different from closed die forging in that the workpiece is not enclosed by the dies and the dies themselves are more like tools with simple shapes and profiles rather than resembling enclosed molds.
Open die forging is an important technique for many types of manufacturing. It allows rough and finishing shaping of metal, most commonly steel and steel alloys. It requires a die that is open on the sides, allowing the workpiece to move freely, in a lateral direction, when struck. This type of design also allows for forging of very large workpieces, in some cases, weighing many tons and having great length and width. Some open die forges can accommodate pieces weighing as much as 150 American tons (136 metric tons) and 80 or more feet (24.4 m) in length.
Open die forgings can be worked into shapes which include discs, hubs, blocks, shafts (including step shafts or with flanges), sleeves, cylinders, flats, hexes, rounds, plate, and some custom shapes.
Open-die forging lends itself to short runs and is appropriate for art smithing and custom work. In some cases, open-die forging may be employed to rough-shape ingots to prepare them for subsequent operations. Open-die forging may also orient the grain to increase strength in the required direction.
The process of open die forging serves many purposes, besides simply shaping the metal. Forging of the metal aligns and refines the grain of the metal, which increases strength as well as reducing porosity, which is the presence of any air bubbles, even those too small to be seen with the naked eye. It also improves the ability of the metal to respond to machining. Forged metal parts have improved wear resistance and other mechanical properties over similar machined or cast parts.
Advantages of Open-Die Forging
- Reduced chance of voids
- Better fatigue resistance
- Improved microstructure
- Continuous grain flow
- Finer grain size
- Greater strength
Cogging is successive deformation of a bar along its length using an open-die drop forge. It is commonly used to work a piece of raw material to the proper thickness. Once the proper thickness is achieved the proper width is achieved via edging.Edging is the process of concentrating material using a concave shaped open die. The process is called edging because it is usually carried out on the ends of the workpiece. Fulleringis a similar process that thins out sections of the forging using a convex shaped die. These processes prepare the workpieces for further forging processes.
Advanced open die forging techniques are capable of producing complex shapes even in large parts and products in addition to simpler shapes like bars, ingots, and rounds. While steel and related alloys are the most common subjects for open die forging, other metals can be shaped this way as well, including copper, nickel, and titanium alloys. The main drawback of open die forging is that the workpiece must be constantly moved as it is worked, requiring complex mechanical controls or skilled human operators, whereas closed die forging can be entirely automated and is a much simpler process.