Metal Forging – the process

  • Forging is the oldest metal forming process. It produces parts by the process of metal deformation.
  • Used for the production of simple components like bolt heads, pressure vessels and shafts.
  • Increases the strength, toughness and fatigue life of the material. This is due to realignment of grain structure.
  • Great for high volumes of parts, but post-processing is necessary and part geometry capability of the process is limited.
metal-forging-crankshaft

A forged crankshaft. Image ©IPH Hannover

Metal forging is the oldest metal forming process which produces parts by the process of metal deformation.

The forging process is classified into two different types on the basis of the deformation mechanism: impact forging and compression forging.

In impact forging, an impact force is applied to the work piece using a hammer. In compression forging, a constant pressure is applied to the work piece. Forging processes can also be classified into two more types based on the temperature of the work piece; hot forging is done at the recrystallization temperature of the material, and cold forging done at room temperature.

Regardless of which process type is used, the process is quite simple. The work piece is placed on the bottom die and the top part die applies the impact or compression load to deform the work piece into the required shape.

four connecting rods - spare parts of a disel engine

Forged connecting rods

Applications

  • Used for the production of simple components like bolt heads, pressure vessels and shafts.
  • It can also be used for the production of sensitive components like connecting rods, gears, and cam shafts.
  • Used in the aerospace industry for turbine disks and large rotating components.

Materials for Metal Forging

A material must be ductile in nature in order to be processed by forging. Materials like low alloy steel, high alloy steel, stainless steel, carbon steel, magnesium alloy, aluminum, nickel alloy, copper and titanium alloy can be forged.

Design Considerations

  • Part complexity is limited due to mold design restrictions.
  • Features like inserts and internal cavities are not possible.
  • Machining allowances have to be added which can be from 0.8 to 6 mm depending upon the size of the part.
  • Minimum section size for forging is 3mm.

Process variations

There are six different types of forging operations:

  1. Open Die Forging uses a flat die set to deform the work piece.
  2. Impression Die Forging uses a die with a cavity and applies compression force to give the work piece the shape of the cavity.
  3. Flashless Forging uses a fixed amount of material and compresses it in a closed die.
  4. Hand Forging is used by blacksmiths. The work piece is heated and then shaped using hand tools like hammers.
  5. Upset Forging use two dies to grip the work piece and a third die to shape the head section of work piece.
  6. Roll Forging use rollers to reduce the thickness of a ring or donut shaped work piece.

Economic Considerations

  • Production rates vary a lot depending upon the geometry and size of the product, from a single unit to several hundred units per hour.
  • Lead time to produce a die set can be several weeks.
  • About 25 percent of the material is wasted in a single operation, but it can often be reused.
  • Labor, equipment, and tooling costs are high but finishing costs are moderate.

Quality Considerations

  • Increase in strength, toughness and fatigue life due to realignment of grain structure.
  • Voids, defects and porosity is quite low.
  • Die wear is high and hot material increases this.
  • High residual stresses in parts.

Advantages

  • Very high production rate for simple parts.
  • Better microstructure control compared to some casting processes.

Disadvantages

  • Post processing is required.
  • Very limited part geometries are possible.

One Response to Metal Forging – the process

  1. sherry.chen says:

    yes,that's very good point,we are a factory doing forging parts in China,and it's better microstructure control compared to some casting processes,that's true,love this article.

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