Forging is basically involves plastic deformation of material between two dies to achieve desired configuration. Depending upon complexity of the part forging is carried out as open die forging and closed die forging.
In open die forging process, the metal is compressed by repeated blows by a mechanical hammer and shape is manipulated manually.
In closed die forging, the desired configuration is obtained by squeezing the workpiece between two shaped and closed dies.
In the forging process the forces are applied on the raw material such that the stresses induced are greater than yield and less than ultimate so that material is experiencing plastic (or) permanent deformation to get the required shape. But in forging operation force applied can be either continuous or intermittent impact loads.
1 Based on the method of force application
- Hand forging (Drop hammer type)
- Machine forging (Mechanical or hydrostatic forging)
1.1 Hand forging:
Hand forging always uses drop hammer type. Because of the continuous force by the human hand, it is not sufficient to produce the deformation in the workpiece.
1.2 Machine forging:
In machine forging, because the required force is obtained from the machine it is possible to use either continuous force application or intermittent impact load application.
2. Based on the method of the shape obtained
- Open die forging
- Closed die forging
- Semi die forging
2.1 Open die forging
- In open die forging operation only drop hammer type of force application will be used.
- Press forging is not used since there is no time for change the position of the component.
Features of open die forging:
- Repeated impact blows are given on the work.
- Less dimensional accuracy.
- Suitable only for simple shapes of work.
- Requires more skill of the operator.
- Usually used for work before subjecting it to closed die forging (to give approximate shape.
- Dies are simple and less expensive.
- It can be analyzed much easily.
- it is the simplest of all forging operations.
2.2 Closed die forging
- Closed die forging is also called impression die forging because the shape of the dies is impressed on the component.
- In this the type of forging force application may be either press forging type or drop hammer type will be used.
Features of closed die forging:
- Closed die forging involves two or more steps:
- Blocking dies: Work is rough forged, close to final shape.
- Finishing dies: Work is forged to final shape and dimension.
- Both blocking die and finishing die are machined into the same die block.
- More number of dies are required depending on the complexity of the job.
- Two die halves close in and work is deformed under high pressure.
- High dimensional accuracy/close control on tolerances.
- Suitable for complex shapes.
- Dies are complex and more expensive.
- Large production rates are necessary to justify high costs.
2.3 Semi-closed die forging operation
- Only drop hammer type of force application is possible.
- In the case of open die and closed die forging operations, the volume of raw material required is remaining the same as that of the volume of the final finished component. But in the case of closed die forging operation, the volume of the raw material is about 10% to 20% more than the final component to be obtained.
- To accommodate the excess volume of material it is required to provide some open space dies also called the gutter.
- Because of the provision of gutter, the flash is produced in the forged component.
- This flash is unwanted material that has to be removed by trimming operation.
- Flash and gutter are used only in case of closed die operation but not in open die or semi-closed die forging operation.
Some common advantages of choosing forging operation are listed below:
- Forged parts possess high ductility and offer great resistance to impact and fatigue loads.
- Forging refines the structure of the metal.
- It results in considerable saving in time, labor and material as compared to the production process of a similar item by cutting from a solid workpiece and then shaping it.
- Forging distorts the previously created unidirectional fiber as created by rolling and increases the strength by setting the direction of grains.
- The forged parts can be welded easily.
- Because of intense working, flaws are rarely found, so have high reliability.
- High accuracy may be obtained in forging operation.
A few disadvantages of choosing forging operation are listed below:
- Rapid oxidation in forging of a metal surface at high-temperature results in scaling which wears the dies.
- It is difficult to maintain close tolerances in forging operations.
- Forging is limited to simple shapes and has limitations for parts having undercuts etc.
- Some materials are not readily worked by forging.
- The initial cost of forging dies and the cost of their maintenance is high.
- The metals get cracked or distorted if worked below a specified temperature limit.
- The maintenance cost of forging dies is also very high.