Cold working

Cold forming is a metalworking process in which metal is shaped at a temperature below its recrystallization temperature, usually at the ambient temperature. Cold forming techniques are usually classified into four major groups: squeezing, bending, drawing, and shearing.

Advantages & disadvantages

Advantages:

• No heating required
• Better surface finish
• Superior dimensional control
• Better reproducibility and interchangeability
• Directional properties can be imparted into the metal
• Contamination problems are minimized

The increase in strength due to strain hardening is comparable to that of heat treating. Therefore, it is sometimes more economical to cold work a less costly and weaker metal than to hot work a more expensive metal that can be heat treated, especially if precision or a fine surface finish is required as well. The cold working process also reduces waste as compared to machining, or even eliminates with near net shape methods. The material savings becomes even more significant at larger volumes, and even more so when using expensive materials, such as copper. The saving on raw material as a result of cold forming can be very significant, as is saving machining time. Production cycle times when cold working are very short. On multi-station machinery, production cycle times are even less. This can be very advantageous for large production runs.

During cold working the part undergoes work hardening and the microstructure deforms to follow the contours of the part surface. Unlike hot working, the inclusions and grains distort to follow the contour of the surface, resulting in anisotropic engineering properties.

Disadvantages:

• Greater forces are required
• Heavier and more powerful equipment and stronger tooling are required
• Metal is less ductile
• Metal surfaces must be clean and scale-free
• Intermediate anneals may be required to compensate for loss of ductility that accompanies strain hardening
• The imparted directional properties may be detrimental
• Undesirable residual stress may be produced

Due to the large capital costs required to setup a cold working process the process is usually only suitable for large volume productions.

Intermediate annealings may be required to reach the required to continue cold working a workpiece, otherwise it may fracture if the ultimate tensile strength is exceeded. An anneal may also be used to obtain the proper engineering properties required in the final workpiece. Also, the distorted grain structure that gives the workpiece it's superior strength can lead to residual stresses.

Cold worked items suffer from a phenomenon known as springback, or elastic springback. After the deforming force is removed from the workpiece the springs back slightly. The amount a material springs back is equal to Young's modulus for the material from the final stress.

Processes

• Squeezing
  • Rolling
  • Swaging
  • Extrusion
  • Forging
  • Sizing
  • Riveting
  • Staking
  • Coining
  • Peening
  • Burnishing
  • Hubbing
  • Thread rolling
• Bending
  • Angle bending
  • Roll bending
  • Draw and compression
  • Roll forming
  • Seaming
  • Flanging
  • Straightening
• Shearing
  • Slitting
  • Blanking
  • Piercing
  • Lancing
  • Perforating
  • Notching
  • Nibbling
  • Shaving
  • Trimming
  • Cutoff
  • Dinking
• Drawing
  • Tube drawing
  • Wire drawing
  • Spinning
  • Embossing
  • Stretch forming
  • Sheet metal drawing
  • Ironing
  • Superplastic forming

References

Bibliography

• Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4.