Extrusion defects

Extrusion defects- 

There are many defects occurs during material Extrusion due to many reasons, For example, poor material mixing, no proper material selection, overheating, etc.

Some major defects are given below

1. Center burst/internal cracks
2. Surface cracking
3. piping/Extrusion defects

Center burst/internal cracks-

• Center burst defect is an internal crack that develops as a result of tensile stresses along the center line of the work part during the extrusion process.
• If the stresses are high, a bursting occurs. also, occurs due to the difference between velocity and tensile strain flow at the center.
• 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.

prevention-

• Die angle
• take the friction into consideration
• Control the Ratio of extrusion

Surface cracking-

• Cracks appear on the surface
• Easy to inspect, they are not regular
• At lower temperatures, the stick-slip phenomenon may cause cracks especially in hydrostatic extrusion where pressures are very high.

Cause-

• High temperature
• high speed of extrusion
• High Friction

Prevention

• By controlling the temperature of the material extrusion
• by controlling the speed of extrusion
• By decreasing and increasing the friction as required.

Piping/Extrusion defects-

• Extrusion defect is basically due to inhomogeneous deformation.
• It mostly occurs in Hot and direct extrusion process.
• The material at the center of the billet comes across least resistance compared to the material near the die wall.
• As a result, rapid flow happens at the center. 
• After one-third of the billet is extruded, the material from the periphery gets entrained towards the center and flows rapidly. 
• Oxides present in peripheral layers are also entrained. Oxides form internal stringers near the center

Prevention-

• Extrusion material should be mixed properly.
• Control the flow of material.