Other names / variants: ring-rolling, open-die forging, closed-die forging, drop forging
Related processes in this database include: metal extrusion
Forging is one of the oldest forms of metalworking. Metal shaping process in which a hot piece of metal is hammered (forged) between the stationary part and the moving (dropping) part of a two-piece hollow metal die (called forging die) in one or several operations.
It produces parts and components that are stronger than those that are machined or cast. As metal is shaped during the forging process, its internal grain structure deforms to follow the general shape of the part or component. As a result, the grain structure is continuous throughout the part or component, leading to superior strength characteristics, such as reliability at high stress points.
Forging is generally defined as the working of metal into a desired shape using compressive forces.
This metal forming can be accomplished by either “hot forging” or “cold forging” processes, both of which have multiple sub-categories. One of the most common processes is a type of hot forging known as impression die forging (also known as closed die forging). This method uses powerful presses and/or hammers and relies on the shape of the dies, presses metal between 2 dies that contain a precut profile of the desired part.
A heated blank is placed between 2 halves of a die
A single compressive stroke squeezes the blank into the die to form the part. In hammer or drop forging this happens by dropping the top of the mould from a height. An alternative is to squeeze the moulds together using hydraulic pressure.
Once the die halves have separated, the part can be ejected immediately using an ejector pin.
The waste material, flash, is removed later.
Materials and shapes
Any metal can be forged, provided the blank is hot enough (( 60% of the melting temperature).
Typical possible sizes for closed dies range from 10g to 10kg, depending on complexity.
The part is left with good surface and mechanical properties, although cold-forging can perform even better.
Complex parts can be formed using a series of forging dies with increasing levels of detail.
Impression die forging has a high initial cost due to the expense of engineering, designing and manufacturing dies, which can be repeatedly used.
However, the benefit is a low recurring cost for each forged part or component produced, thus becoming increasingly economical with volume. And, impression die forgings generally have about a 20% higher strength-to-weight ratio when compared to machined or cast parts of the same material.
Specialized and powerful presses are used in impression die forging.
The two main types of presses are mechanical and hydraulic. Mechanical presses function by transforming the rotational force of a motor into a transitional force that preforms the pressing action. One advantage is that different forces are available at different stroke positions. Another advantage is that mechanical presses are faster than hydraulic ones. Hydraulic presses use fluid pressure and a piston to generate force. The advantages of a hydraulic press over a mechanical press are its flexibility and greater capacity. The disadvantages include a slower, larger, and costlier press to operate.
- The Extreme Machines Magazine has published forging processes video.
- The Forging Page on Wikipedia.
- Forging Industry Association.