Closed Die Forging
Closed die forging, also known as impression die forging, is one of the processes utilized to forge metal. The process of closed die forging involves hammering or pressing preformed metal blanks into a die shaped as the desired product. The piece is fully enclosed by the hammer and anvil, which forces the metal to completely conform to the die. Closed forging differs from open die forging, the latter of which involves the die coming into contact with part of the piece at a time. Configurations made by the closed die forging process can be complex with tight tolerances, and the amount of possible shapes has very few limits.
Closed die forging is the most cost effective forging process for high-volume production. The initial costs for the tooling and the equipment is higher, but can be a wise investment since the recurring cost for the parts are very low. Both symmetrical and asymmetrical parts can be created with closed die forging, and popular materials include titanium, tool steel, nickel, copper, stainless steel, alloy metals, aluminum, carbon steel, naval brass, and high alloy steel. Closed die forgings is used extensively in a multitude of industries, including food processing, hardware, construction, semiconductor, manufacturing, commercial, automotive, aerospace, and electronics.
In a closed die forging machine, two tooling dies are used: a stationary die, or anvil, and a moving die, or hammer. Both the hammer and the anvil feature an impression of the shape of the desired component. The hammer presses down onto the anvil and the metal workpiece, forcing the metal to fill every part of the impression and take its shape. Closed die machines may be enclosed, but they are not sealed tightly, and some of the metal material may escape. Therefore, at the edges of the two dies is a flash gutter, or a small recess that collects any excess metal. The excess metal quickly cools, and serves as a sort of plug that keep the rest of the metal inside the die. Sometimes, the closed die forging process requires more than one impression cavity. One set of impressions form the rough shape of the final product, while the finisher cavity helps in adding the finishing touches.
There are several methods of closed die forging: cold forging, warm forging, and hot forging. In the cold forging process, the metal is formed using high pressure instead of direct heat. The dies are usually circular, lubricated, and at room temperature. Materials that are commonly cold forged are carbon and standard alloy steel forigings. The parts that are cold forged are typically symmetrical and weigh under 25 lbs. Some advantages of cold forging include a high production rate, a safer work environment, a long die life, and improves the mechanical properties of the metal. One disadvantage is that the improvement of the metal’s mechanical properties is not necessary in many forging applications. The advantages of cold forging far outweigh its drawbacks, making it the most widely used of the three types of forgings.
In the warm forging process, alloys such as steel alloys are the recommended material choice. Depending on the alloy, the usual temperature can range anywhere from above room temperature to 1800 degrees Fahrenheit (982.2 degrees Celsius). The ideal temperature range, however, is 1000 to 130 degrees Fahrenheit. With warm forging, manufacturers are able to create complex shapes. Warm forging also helps reduce tooling loads, increase the ductility of steel, eliminates the need to anneal or heat treat the metal prior to forging. Common disadvantages of warm forging includes strictness with temperature control and the requirement of heating equipment.
In the hot forging process, heat is transferred from the workpiece to the die, creating thermal gradients in the workpiece. Titanium alloys, aluminum alloys, nickel alloys and steel are common metals that undergo the hot forging process. Depending on the metal, temperatures can range from 1700 to 2300 degrees Fahrenheit (925 to 1260 degrees Celsius). Hot die forging has both its advantages and disadvantages. The benefits of hot die forging include reduced material costs, close tolerances, less blocking and preforming operations, and the requirement of fewer and smaller machines. Disadvantages, however, include the requirement for materials that are controllable and consistent, a vacuum atmosphere around the die and workpiece to prevent oxidation, and a low production rate in order to allow the die to be properly filled. When hot forging, it is important to consider the surrounding atmosphere. The use of a vacuum or an inert gas is recommended as a protective measure for both the workpiece and the dies in order to prevent oxidation.
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