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Forging is the process of shaping metal through the application of force. It can be done while the metal is cold or hot but not when it is molten. Open die forging is done by impacting a die onto a workpiece that is resting on a stable base called an anvil. Since the process is analogous to the way metal has been forged for centuries by blacksmiths using a hammer and anvil, it is also called smith forging. 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.
The dies used in open die forging are simple in shape and open on the sides to allow the workpiece to move and extend beyond the edges of the die and the anvil beneath. The die is powered by a machine that causes it to descend upon the workpiece that rests upon an anvil or another open die. Open dies are typically flat but may also be convex or concave. Concave dies are used for edging, which shapes the edges of a workpiece, creating defined edge shapes. Convex dies are used to shape the workpiece by making it thinner at the point of impact.
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.
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.