Forging Processes: Open and Closed Die Forging

Definition

Forging is a manufacturing process where metal is shaped using localized compressive forces delivered by hammers, presses, or forging machines. It is one of the oldest and most reliable metal forming techniques, used to produce parts with superior mechanical properties by refining the grain structure of the metal.

Main Content

1. Open Die Forging

Often referred to as "smith forging," this process involves shaping a workpiece between two flat or simple-contoured dies.
The metal is not fully constrained by the dies, allowing the operator to manipulate the workpiece to achieve the desired shape.

      Upper Die (Moving)
        |  |  |  |
      -----------
      |         |
      | Work    |
      |         |
      -----------
        |  |  |  |
      Lower Die (Stationary)

(Diagram: Basic setup of Open Die Forging)

2. Closed Die Forging

Also known as impression die forging, the metal is placed into a die cavity that contains the negative impression of the final part shape.
The metal is squeezed under high pressure until it fills the entire cavity, resulting in high-dimensional accuracy and complex geometries.

      Upper Die (Cavity)
       /  \____/  \
      |            |
      |   Metal    |
      |____________|
       \  /    \  /
      Lower Die (Cavity)

(Diagram: Metal constrained within a Closed Die)

3. Comparison of Flow

In Open Die forging, the material flows laterally, which is known as "barreling."
In Closed Die forging, excess material is pushed out into a small gap between the dies, forming a "flash" which is later trimmed off.

Working / Process

1. Heating the Workpiece

The metal billet is heated to its plastic deformation temperature, which is generally above the recrystallization temperature for hot forging.
Proper heating ensures the metal is ductile enough to flow without cracking during the deformation process.

2. Deformation

For Open Die: The workpiece is compressed and repeatedly struck or pressed while being rotated or moved by the operator.
For Closed Die: The billet is placed in the lower die, and the upper die strikes the metal, forcing it to flow into the intricate contours of the mold.

3. Finishing and Trimming

Post-forging, the part may undergo "flash trimming," where excess material extruded from the die gap is removed.
Finally, the component undergoes heat treatment (like quenching or tempering) to achieve the desired hardness and mechanical strength.

Advantages / Applications

Forging significantly improves the mechanical properties, such as toughness and fatigue resistance, by aligning the grain flow of the metal with the part geometry.
Closed die forging is ideal for mass production of complex, high-strength parts like connecting rods, gears, and engine components.
Open die forging is the preferred method for producing very large, heavy components such as shafts, rings, and pressure vessels that are not feasible to produce in a closed die.

Summary

Forging is a fundamental metal-forming technique that uses compressive force to shape metals into durable, high-strength components. While Open Die forging offers flexibility for large, simple shapes, Closed Die forging provides the precision necessary for complex industrial parts. Important terms to remember include flash, grain flow, plastic deformation, and billet.