Bird caging is a defect that can occur in drawn wire, particularly copper wire. It is characterized by transverse cracks that occur at regular intervals along the length of the wire, giving it a cage-like appearance.
What causes bird caging?
Bird caging is caused by stresses that develop in the wire during the drawing process. As the wire is pulled through a die, the reduction in diameter produces axial tension. This tension is usually not perfectly uniform across the cross-section of the wire. The outer layers of the wire tend to be more highly stressed than the inner core. These stress gradients cause localized plastic deformation to occur in the surface layers. As drawing continues through multiple dies, these deformations can accumulate and lead to the formation of transverse cracks or fissures at regular intervals, perpendicular to the drawing direction.
When does bird caging occur?
Bird caging tends to occur when wires are subjected to excessive drawing stresses. Some of the key factors that can contribute to bird caging include:
- High reduction in area – Wires drawn through dies with a high reduction in cross-sectional area are more likely to bird cage.
- Improper lubrication – Insufficient lubrication during drawing can increase friction and drawing stresses.
- Inclusion particles – Hard inclusion particles in the wire can create stress concentrations that initiate cracks.
- Surface defects – Pre-existing surface defects, scratches or gouges can also act as stress raisers.
- Residual stresses – Residual stresses from previous cold working operations can add to the stresses of subsequent drawing.
What are the effects of bird caging?
The cage-like cracking caused by bird caging can significantly degrade the quality and performance of drawn wire. Some of the potential effects include:
- Reduced ductility and fracture strength – The transverse cracks are stress concentrators that reduce the fracture resistance of the wire.
- Increased electrical resistance – Cracks disrupt the continuity of the metal, increasing resistivity.
- Corrosion susceptibility – Cracks can allow corrosion to initiate more readily in the wire.
- Surface irregularities – Cracking often causes surface roughness and ridges to form on the wire.
- Failure in service – Wires with extensive bird caging may fail prematurely in service due to fracture at one of the crack locations.
How can bird caging be prevented?
There are several strategies that can help avoid bird caging defects during wire drawing:
- Reduce the percentage of reduction in area per draw – Multiple passes with smaller reductions in area can minimize drawing stresses.
- Maintain proper draw bench set-up – Ensure proper alignment and minimal friction through the die.
- Use adequate lubrication – Drawing compounds should be continually applied to reduce friction.
- Select clean wire rod – Wire rod should be free of seams, scales, and inclusion particles.
- Annealing between draws – Annealing relieves residual stresses allowing further cold reduction.
- Evaluate die wear – Draw dies should be periodically inspected and reworked to maintain proper geometry.
How is bird caging detected?
There are a few methods that can be used to detect bird caging in drawn wire:
- Visual inspection – Carefully examining the wire surface can reveal transverse cracks characteristic of bird caging.
- Microscopy – Laboratory techniques like stereo microscopy or scanning electron microscopy can aid in detecting fine cracks.
- Electrical testing – Checking electrical resistivity down the wire length can identify increased resistance at crack locations.
- Destructive testing – Pull testing fractured wire specimens reveals cracking patterns typical of bird caging.
- Non-destructive testing – Ultrasonic or eddy current techniques can detect subsurface discontinuities associated with bird caging cracks.
What are the industry standards for bird caging?
Some key standards that provide specifications related to bird caging defects include:
- ASTM B947 – Standard Test Method for Length-Dependent Wire Drawing Cracking of Copper Wire
- AMS-WD-1002 – Wire, Drawing Quality Copper, Uncoated, Annealed
- AMS-QQ-W-461 – Wire, Copper, Coated or Uncoated, Silver Bearing, Annealed
- SAE HS-1086 – Steel Wire Specifications
These specifications restrict the amount of transverse cracking allowable on wire surfaces. For example, ASTM B947 sets limits on the number, length, and depth of cracks present per unit length of wire.
How is bird caging repaired?
There are limited repair options once extensive bird caging has occurred in a wire. Some potential corrective actions include:
- Annealing – Heating the wire to soften and recrystallize the metal may help relieve stresses and close some cracks.
- Surface conditioning – Processes like abrasive blasting or grinding can dress cracks and produce a smoother wire surface.
- Coatings – Applying metallic coatings can help seal cracks and inhibit corrosion.
- Scrap and re-draw – Severely caged wire may need to be scrapped and the material re-drawn from rod.
However, repaired wire will still contain flaws and not achieve the quality of properly produced wire. Prevention remains the best remedy to avoid bird caging issues.
Conclusion
Bird caging is a defect characterized by transverse cracking that occurs during wire drawing operations. It is caused by non-uniform stress gradients that develop as the wire is pulled through dies. Bird caging can significantly degrade wire performance and lead to premature failures. By understanding the root causes, implementing quality control testing, following industry specifications, and properly setting up drawing processes, manufacturers can prevent bird caging defects.
| Cause of Bird Caging | Prevention Method |
|---|---|
| High reduction in area | Reduce % reduction per pass |
| Improper lubrication | Use adequate drawing compounds |
| Inclusion particles | Select clean wire rod |
| Surface defects | Start with quality wire rod |
| Residual stresses | Annealing between draws |
| Die wear | Inspect and rework dies |
References
[1] ASM Handbook, Vol 14B – Metalworking: Sheet Forming. ASM International, 2006.
[2] G.E. Dieter, Mechanical Metallurgy. McGraw-Hill, 1988.
[3] ASTM B947-21, Standard Test Method for Length-Dependent Wire Drawing Cracking of Copper Wire. ASTM International, 2021.
[4] SAE HS-1086-2015, Steel Wire Specifications. SAE International, 2015.
[5] AMS-QQ-W-461G, Wire, Copper, Coated or Uncoated, Silver Bearing, Annealed. SAE International, 2012.
