One, crushing process: powder forming technology is the basic forming of powder metallurgy production method; In touch pressure filling powder, and then under the press pressure, demoulding after get the required shape and size of compact products, and roughly three stages:
1, press block density increases with the increase of pressure; Porosity decreased dramatically.
2, the briquette density increase slowly, because the pore eliminate a lot of in phase 1, continue to pressure just let grain yield elastic deformation.
3, the increase of pressure may achieve powder material yield limit and ultimate strength of powder particles in the plastic deformation or brittle fracture under pressure. Because of the brittle fracture of the particle debris fill in the pore space, briquette density increase.
Two, pressing pressure, pressing pressure is divided into two parts; It is under the condition of no friction, make the powder pressure actually realized to some extent the pressure needed for the static pressure (P1); 2 it is to overcome the friction between powder particles and die pressure to the lateral pressure (P2).
The lateral pressure, the greater the ejection pressure is greater, the poisson coefficient of cemented carbide powder generally between 0.2 0.25.
Three, pressure distribution in the pressing process: the main cause of pressure distribution is uneven between powder particles as well as the friction between the powder and the mold wall. Briquetting elevation, pressure distribution is uneven. A two-way pressure or increasing the pressure diameter, can reduce the nonuniformity of the distribution of pressure.
Four, briquette density distribution: the more complex the briquetting, density distribution is uneven; In addition to uneven pressure distribution (pressure drop), the way of filling is not correct, the degree of compression at different positions of the briquetting inconsistencies, can cause uneven briquette density.
1, fill factor: refers to the density of the briquetting pressure and the apparent density of grain to Y Y the ratio of pine; Compression ratio: refers to the powder material grain filling height h and the ratio of the pressure of the press block height h; In the numerical filling coefficient and the compression ratio is equal.
2, in order to reduce the non-uniformity of briquette density distribution:
(1) improve the surface finish of mold;
(2) reduce friction resistance;
(3) improve the liquidity of material grain;
(4) use reasonable means;
3, the effect of powder size on suppressing;
(1) the bigger the powder dispersion (the smaller the apparent), the greater the pressure. The smaller the briquette density; Have the strength of the larger value, good formability.
(2) the material is relatively coarse grain, pressure piece of easy to achieve high density, but its density distribution is not uniform; In general, briquetting strength along with the addition amount of forming agent.
Five, elastic aftereffect of briquetting:
1, elastic stress: powder particles and the internal contact between particles on the surface, due to the gravity and suction of the interaction between the atoms, will generate a force in the opposite direction and particles, and strive to prevent deformation of particles, in order to achieve and repression called elastic stresses in the pressure balance of forces.
2, elastic aftereffect: after removing the thrust pressure and the pressure piece of demoulding, due to the internal stress relaxation effect caused by the pressure piece of volume expansion phenomenon is called springback.
3, in many cases, elastic expansion in the aftermath of the press block demoulding was the cause of press block crack and delamination.
4, powder particle size, particle roughness decreased, bonding strength between particles to reduce (forming or inadequate water content), and the oxide and springback increase will increase the content of impurities. Grain drying is too dry and harden, can increase elastic aftereffect.
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