What are the Reasons for Cracking in Powder Metallurgy Products?

What are the reasons for cracking in powder metallurgy products?

1. Particle displacement: The bond between particles is initially formed by plastic deformation and movements of the powder body. Under ideal conditions, densification is bidirectional, symmetrical, and synchronized, and there is no displacement of particle edges. However, the displacement movement after densification will prevent the formation of bond between particles and destroy the bond already formed in the early stage of forming.

2. High tension and shear force: If the tension and shear force generated by external or internal factors on the formed body in the embryonic state exceed the strength of the embryonic body itself, cracks will appear.

3. Wrong material integrity: For various reasons, metal powder is used with additives. For example, adding suitable lubricants for mixing can improve compressibility and reduce demolding force. However, excessive addition of lubricant to mixed iron powder will cause bond formation between particles and adhesives, impurities, and even residual air will have a negative effect on bond formation.

4. Abnormal plastic strain separation during forming: Particles undergo irreversible plastic deformation, and reversible plastic deformation also occurs. After the final forming stage, the associated pressure decreases and drops to zero in the demolding process. At the moment when the forming pressure is released, the compressive stress is released, and the embryonic body suddenly changes from the plastic stage to the elastic stage. If the internal stress is greater than the strength of the formed body, cracks will appear.