Drop forging is a metalworking process that gradually shapes a heated piece of metal, called an ingot. It involves forming the ingot with repeated blows from a hammer or die that is raised and then dropped onto the part, flattening it or forcing it into a mold. Depending on the complexity of the part design, the process sometimes requires using several dies in progression. The drop forging process usually produces a close approximation of the finished piece, but additional machining typically is required to bring it within tolerance. Drop forging is widely used throughout the automotive industry in producing various engine parts, gears and axles.
The dies used for drop forging typically are made of a high alloy steel, called tool steel. Forging dies are designed to be impact resistant and wear resistant, and they generally can withstand thousands of rapid heating and cooling cycles. Drop forging dies are usually made in two halves. The upper half, called the hammer, is attached to the block that is raised and dropped onto the ingot. The lower half, called the anvil, usually is a stationary die against which the ingot is forged.
Open-die drop forging is done with dies that do not completely enclose the workpiece. The dies usually are flat, although contoured dies or cutting dies can be used as well. The open design allows room for the ingot to expand as it is hammered to the desired thickness.
Techniques commonly associated with open-die drop forging include cogging and edging. Cogging is the process of progressively flattening a bar or ingot length-wise. This process is used to achieve the desired thickness, after which it can be edged. Edging is usually done with a concave die. This technique concentrates and shapes the material along the sides and ends of the forged part in order to achieve uniform edges and proper width.
Impression-die forging, sometimes referred to as closed-die drop forging, uses mold-shaped dies. When the hammer is dropped onto the workpiece, the hot metal is forced into the die cavities to create the final shape of the part. As the metal is forced to conform to the die, excess material, called flash, is squeezed out. The flash must be removed after the forging is completed.
Another common type of closed-die forging is called flashless forging. This process is also referred to as true closed-die forging, because the workpiece is completely enclosed by the die, preventing flash from forming. Many manufacturers prefer flashless forging because the flash produced by impression-die forging can account for almost half of the original ingot.