Hot forging uses repeated hammer blows to shape a heated metal ingot into a desired shape. Dies made of tool steel are used to withstand the process. Open die forging allows for expansion, while die forging uses shaped dies to create the final product. Flashless forging is preferred due to less waste.
Hot forging is a metalworking process that gradually shapes a heated piece of metal, called an ingot. It consists of forming the ingot with repeated blows of a hammer or die which is lifted and then dropped onto the piece, flattening it or forcing it into a mould. Depending on the complexity of the part design, the process sometimes requires the use of several dies in progression. The stamping process usually produces a close approximation of the finished part, but additional machining is usually required to bring it back within tolerance. Hot forging is widely used in the automotive industry in the production of various engine parts, gears and axles.
The dies used for forging are typically made from a high alloy steel, called tool steel. Forging dies are designed to be resistant to impact and wear and can generally withstand thousands of rapid heating and cooling cycles. Forging dies are usually made in two halves. The top half, called the hammer, is attached to the block which is lifted and dropped onto the ingot. The bottom half, called the anvil, is usually a fixed die against which the ingot is forged.
Open die forging is done with dies that do not completely enclose the workpiece. The molds are usually flat, although shaped or cutting molds can also be used. The open design allows the ingot to expand as it is hammered to the desired thickness.
Techniques commonly associated with open die forging include hobbing and beading. Cogging is the process of progressively flattening a bar or ingot lengthwise. This process is used to achieve the desired thickness, after which it can be beaded. Edging is usually done with a concave die. This technique concentrates and shapes the material along the sides and ends of the forged part for uniform edges and adequate width.
Die forging, sometimes referred to as closed die forging, uses dies that are shaped like a die. When the hammer is dropped on the part, hot metal is forced into the die cavities to create the part’s final shape. As the metal is forced to conform to the die, excess material, called flash, is ejected. The flash must be removed after the forging is complete.
Another common type of closed die forging is called flashless forging. This process is also referred to as true closed die forging, as the workpiece is completely enclosed by the die, preventing burrs from forming. Many makers prefer flashless forging because the flash produced by die forging can account for nearly half of the original ingot.
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