Why do powder metallurgy products crack?
Feb 06, 2023
Why do powder metallurgy products crack?
The reason why powder metallurgy products crack is the knowledge we share today. Let's follow zhongwei precision knitting to see.
1. Crack formation
Generally, a solid is separated or split into two or more parts under the action of force, which is called fracture. The cause of fracture can be crack and crack growth under load. For the formation of powder metallurgy forming body, the crack can be mainly classified into one of the following reasons: inter-particle interlocking fracture; No inter-particle interlocking is formed The main reason for inter-particle interlocking fracture is the blocking or stretching of powder particles. Interlock fracture may be caused by the action of applied tension or transverse shear force or the combination of the two forces. The fracture area may be deeper than the shallow surface, and may also be irregular and discontinuous. On the other hand, if the stress concentration is caused by the external stress or internal stress reaching the micro-stress intensity level, it will lead to crack. Therefore, When the conventional stress component of the transverse or horizontal powder particle crack surface reaches the cracking stress, cracks caused by external or internal stress will appear.
2. Solution to cracking of powder metallurgy products
(1) Wrong material integration
For various reasons, metal powder uses additives. For example, adding appropriate lubricants for mixing will improve the compressibility and reduce the release force. However, adding too much lubricant to the mixed iron powder will inhibit the formation of inter-particle bond. Adhesives, impurities and even residual air will have a negative effect on the formation of bond
(2) Interparticle displacement
The inter-particle bond is initially formed by plastic deformation and powder block movement. Under ideal conditions, the densification process is bidirectional, symmetrical and synchronous, and the inter-particle edge displacement will not occur. The displacement movement after the densification process will prevent the formation of inter-particle bond, and can destroy the bond formed in the early stage of forming.
(3) In the process of abnormal plastic strain separation forming
Irrecoverable plastic deformation will occur to particles. In addition, recoverable plastic deformation will also occur. After the final forming stage, the relevant pressure will be reduced and finally reduced to zero during the demoulding process. At the moment of the release of the forming pressure, the compression stress will be released, and the forming embryo will suddenly change from plastic to elastic stage. If the internal stress is greater than the strength limit of the forming body, cracks will occur.
(4) High tension, shear force
In the powder metallurgy embryo state, if the tension and shear force of the formed body caused by external or internal causes are higher than the embryo strength of the formed body itself, then cracks will occur.
(5) Cracks due to material integration
When air remains in the powder material forming process, the amount of powder decreases at a ratio of about 2:1. The air filled in the pores between the loose particles must be squeezed out during the forming process, and must be completed in a relatively short time. The air must be squeezed out of the powder matrix and the mold components with very small design fit gap tolerance.
The above is the reason why powder metallurgy products split. I hope this article will help you. If you need to know more about it, please pay attention to the zhongwei precision official website.
