Closed die forging, the most prevalent method, is accomplished by hammering or squeezing the aluminum between two steel dies, one fixed to the hammer or press ram and the other to the anvil. Forging hammers, mechanical presses, and hydraulic presses can be used for the closed die forging of aluminum alloys. The heated stock is placed in the lower die and, by one or more blows of the ram, forced to take the shape of the die set. In closed die forging, aluminum is shaped entirely within the cavity created by these two dies. The die set comes together to completely enclose the forging, giving lateral restraint to the flow of the metal.
The process of open die forging is similar to that described above but in this method the shape of the forging is determined by manually turning the stock and regulating the blows of the hammer or strokes of the press. Open die forging requires a great deal of skill and only simple, roughly shaped forgings can be produced. Its use is usually restricted to items produced in small quantities and to development work where the cost of making closed-type dies is prohibitive.
The process of rolled ring forging is used in the manufacture of seamless rings. A hollow cylindrical billet is rotated between a mandrel and pressure roll to reduce its thickness and increase its diameter.
Proper lubrication of the dies is essential in forging aluminum alloys. Colloidal graphite in either water or an oil medium is usually sprayed onto the dies for this purpose.
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