Extrusion products may get defect to the considerable deformation associated with extrusion operations; some defects can occur in extruded products.
The defects in extruded products can be classified into three categories:
- Center burst
- Piping and
- Surface cracking
In below illustrated the defects formed in extruded products.
(a) Center burst:
Center burst defect is an internal crack that develops as a result of tensile stresses along the center line of the workpart during the extrusion process. Although tensile stresses may not usually be seen in a compression process such as extrusion process, tensile stresses tend to occur under conditions that cause large deformation in the regions of the work away from the central axis. The significant amount of material movement in these outer regions stretches the material along the center of the product. If the stresses are high, a bursting occurs. Conditions that promote center burst defect are high die angles, low extrusion ratios and impurities in the work metal that serve as starting points for crack defects. It is difficult to identify the center burst defect. Center burst is an internal defect that is usually not noticeable by visual inspection. Other names of the center burst defect include Arrowhead fracture, center cracking and Chevron cracking.
Piping is a defect associated with a direct extrusion. As in the above figure (b), it is the formation of a sinkhole at the end of the billet. The use of a dummy block whose diameter is somewhat less than that of the billet helps to avoid a piping defect in the extrusion process. Other names of the piping defect are tailpipe and fishtailing.
(c) Surface cracking:
Surface cracking defect results from high workpart temperatures that cause cracks to develop at the surface. They often occur when extrusion speed is very high, leading to high strain rates and associated heat generation. Other factors contributing to surface cracking defect in extrusion process are high friction and surface chilling of high-temperature billets in the hot extrusion process.