Why do magnetic materials need to test green embryo and sintering density?
Why do magnetic materials need to test green embryo and sintering density? The magnetism of magnetic materials (soft magnetic) is affected by the purity of materials, alloy composition, material density, internal porosity and internal stress. The sintering density directly affects the quality of magnetic induction B, permeability μ, coercive force HC and resistivity ρ. The sintering density of materials is directly affected by the density of green embryo. Because the product is finished by green embryo forming and then sintering, the shrinkage and deformation of the product during sintering are directly affected by the green embryo density, so the test of green embryo density is the key.
1. Material purity: when C, S, O, N and other impurity atoms intrude into the crystal lattice, the permeability (μ) and coercivity (HC) of the material will be adversely affected. Therefore, high purity raw powder should be used as much as possible, and the lubricant should be completely burned out during sintering, and appropriate sintering atmosphere should be selected to reduce the influence of the above factors.
2. Alloy composition: appropriate addition of alloying elements (such as P, Si, Sn, etc.) can improve the magnetic properties of the material, such as BS, μ, he, P, etc. the addition of these elements can use element powder, compound powder or master alloy powder.
3. Material density: with the increase of material density, B and μ will increase, while HC and ρ will decrease. In the low density region, the magnetic properties of the material are very poor due to the large number of pores, which will hinder the movement of the magnetic wall. With the increase of density, the porosity decreases, the B and μ m values gradually increase, and the HC value gradually decreases, which makes the whole magnetic quality better. In order to increase the density, the following methods can be adopted:
- (a) Use iron powder with good compressibility
- (b) Using high temperature sintering
- (c) Addition of phosphorus, boron and other elements can promote sintering
- (d) Re compression and re sintering.
4. Internal pores: for some structurally sensitive magnetic properties such as μ and he, the smaller the number of pores in the internal structure, the better, the rounder the shape and the smaller the size. These are all affected by the particle size of raw powder, sintering temperature, whether the liquid phase is produced or not.
5. Internal stress: the internal stress of material caused by post-processing such as correction, re compression or cutting will cause the bending difference of μ and he values. The original magnetic properties can be restored after stress relief by annealing.
Source: China Permanent Magnet Manufacturer www.rizinia.com