Concept proposed
As shown in the figure below, a certain amount of stress is suddenly added to a polycrystalline metal sample. The elastic strain immediately produced by the sample is only what the stress should cause. A part (OC) of the total strain (OH), and the rest of the strain (CH) are gradually generated while keeping the stress magnitude unchanged. This phenomenon is called positive elastic aftereffect, or elastic creep or Cold creep. When the external force is suddenly removed, the elastic strain disappears, but not all the strains disappear at the same time, but only a part (DH) disappears first, and the rest (OD) also gradually disappears. This phenomenon is called rebound aftereffect.
Schematic diagram of elastic after-effect
The elastic after-effect usually referred to in engineering refers to this kind of rebound after-effect. In short, this behavior of continuous development of strain over time under the action of stress and the gradual recovery of strain after stress is removed can be collectively referred to as elastic aftereffects.
The elastic aftereffect phenomenon is extremely important in the instrument and precision machinery manufacturing industries. For example, for force-measuring spring materials, film materials, etc. that bear long-term loads, the issue of positive elastic aftereffects should be considered. For example, the force measuring spring of an oil pressure gauge (or air pressure gauge) is not allowed to have elastic aftereffects, otherwise the measurement will be distorted and even unusable. Usually, a straightened workpiece will bend after being placed for a period of time. This is the result of the rebound after-effect, or it may be the result of the positive elastic after-effect caused by the Type I residual internal stress in the workpiece. . The former can be achieved by reasonably selecting the tempering temperature (300~450℃ for steel and 150~200℃ for copper alloy) after straightening, and try to maximize the rebound aftereffect during the tempering process, thereby avoiding the workpiece being damaged in the future. Deformation occurs during use.
Elastic expansion phenomenon
During the paste forming process, the particles not only undergo plastic deformation, but also elastic deformation. When the pressure is removed from the press block or demolded, due to the relaxation of elastic stress, the press block will elastically expand and increase in volume. This phenomenon is called elastic aftereffect. The size of the elastic after-effect is expressed by the percentage of volume expansion of the compact. The result of the elastic after-effect is that the internal stress between the paste particles is reduced, and the contact area between the particles is also reduced. This results in fractures between the particles and the formation of larger Cracks, resulting in cracked waste products. This phenomenon sometimes occurs immediately when demoulding, and sometimes occurs after it has been left for a period of time. Therefore, in order to prevent the raw product from cracking before roasting, it should be loaded into the furnace for roasting as soon as possible. Experiments have shown that the elastic expansion of a molded product in the height direction is greater than its expansion in the diameter direction. This is because the pressure on the molded product in the height direction is greater than the lateral pressure it experiences in the diameter direction, so that the elastic expansion in the height direction Caused by more concentrated stress.
The main reason for the elastic expansion phenomenon is that after the powder body is subjected to pressure during the pressing process, the powder particles undergo plastic deformation, thus accumulating a large internal stress inside the compacted body, the direction of which is consistent with the force exerted on the particles. The direction of the external force is opposite, trying to prevent the particle from deforming. When the pressing pressure is eliminated, the elastic internal stress will be relaxed and released, thereby changing the shape of the particles and the contact state between particles, which causes the powder compact to expand. As mentioned before, the force in each direction of the pressed blank is different, so the elastic internal stress is also different. Therefore, the elasticity of the pressed blank has anisotropic characteristics. Since the axial pressure is greater than the lateral pressure, the elastic aftereffect along the height direction of the pressed blank is larger than that in the transverse direction. The dimensional change of the pressed blank in the pressing direction can reach 5~6, while the dimensional change perpendicular to the pressing direction is 1~3.
Influencing factors
The result of the elastic aftereffect is to reduce the internal stress between the paste particles, and also reduce the contact area, causing the particles to contact and break into larger cracks , causing the generation of waste products. This phenomenon sometimes occurs during demoulding or immediately after being pressed out of the mold nozzle. If the molding pressure is very high, it will crack into layers upon demoulding (layer cracking). Sometimes it can also occur after being left for a period of time. , Therefore, in order to prevent the briquettes from cracking before roasting, it is beneficial to load the furnace for roasting in time or as early as possible.
The elastic aftereffect has a great influence on molded products, followed by isostatic forming, extrusion forming and vibration forming. Therefore, special attention should be paid to molding production.
There are many factors that affect and change the elasticity effect, the main ones are as follows.
1. Plasticity of powder
Metal materials should be annealed to improve the plasticity of pressed powder. The kneading temperature of carbon materials should not be too high. The amount of binder should be increased or decreased according to its particle size. Provide a sufficient thickness of binder layer on the surface of each particle. During forming, the compaction temperature should not be too low. For example, for compressed powder using coal pitch with a softening point within 80°C as a binder, the temperature of the compressed powder during molding should not be lower than 15°C. When more graphite powder is added to metal pressed powder, the plasticity will be reduced due to the different plastic deformations of graphite and metal. Therefore, metal pressed powder containing more than 20% graphite powder must add appropriate binders to improve its plasticity.
2. The particle size composition of the product and the properties of the particles
There is a great relationship between the size of the elasticity of the embryo and the particle size composition. There are many fine particles, and their specific surface area is large. There are fewer large particles when they bite each other, and the internal stress is easy to release. At the same time, due to the large specific surface area and the friction surface between particles, in order to obtain a compact with the same density as the coarse particles, the pressing pressure must be increased, so the internal stress stored in the compact is large, which manifests as a large elastic aftereffect. The finer the powder particle size, the greater the elasticity aftereffect. The mechanical interlocking and interweaving effects between particles with smooth surface and regular shape are smaller than those of particles with rough surface and irregular shape, so the elastic aftereffect is greater.
3. Plasticity of the paste
The paste has an appropriate amount of binder, uniform composition, good kneading quality, good plasticity of the paste, and the expansion force of the elastic aftereffect of the briquette is less than If the adhesive force is insufficient, the elasticity of the product will be smaller. If the paste contains too little binder, the kneading is uneven or the pressing temperature is too low, the plasticity becomes poor, and the expansion force of the elastic after-effect is greater than the adhesive force of the paste, and the product will crack due to the increase in elastic after-effect.
4. Forming pressure
The elastic aftereffect increases with the increase of forming pressure, and is more significant for pastes with poor plasticity. For pastes with good plasticity and rough particle surfaces in the paste, when the pressure increases, the contact surface of the particles increases accordingly. Therefore, the pressure has less impact on the elastic aftereffect. For the molded press block, the elastic expansion in the height direction is greater than the lateral expansion. This is because the positive pressure in the height direction is greater than the lateral pressure on the side of the same section. And because the side pressure gradually decreases from top to bottom along the direction of the pressing block. Therefore, its lateral elastic aftereffect also gradually decreases from top to bottom along the direction of the pressing block.
5. Compression ratio and compaction density
When pressing, the greater the compression ratio of the powder, the greater the elastic aftereffect, which means the greater the relative density of the compaction. , the greater its elasticity will be. vice versa.
6. The material and structure of the stamper
The material and structure of the stamper have an impact on the elastic aftereffect. If the die wall has high hardness and the structure is simple, the elastic aftereffect will be small. Therefore, when designing the mold and die nozzle, due to the uneven elastic after-effect inside the blank, cracks will appear in the weak parts or stress-concentrated parts during demoulding. The expansion of this elastic after-effect of the compact should be taken into consideration value.
Mitigation measures
In order to reduce the elasticity after-effect, the following methods can be used:
1. The mixing temperature should not be too high, and the mixing time should not be too long. Master it well. The amount of binder added. When molding, the temperature of the paste should not be too low, which can improve the plasticity of the paste.
2. Prolonging the action time of pressure can make the particles in closer contact. Because the displacement, deformation and gas discharge of particles all require a certain amount of time, when pressing large products or thick briquettes, maintain the highest pressure for a few seconds to 2 to 3 minutes, or divide the pressure into 2 to 3 minutes from low to high. The 3-stage pressurization can make the particles move fully and combine more closely. The density and strength of the briquettes increase, which helps to reduce the elastic aftereffects, and the density and strength are slightly increased. When pressing small products, the pressure maintenance time has no significant impact on the strength of the briquettes.
3. Applying pressure at a slower speed will help to uniform the density and strength of the briquettes, and can also reduce the elastic aftereffects. Iron is especially suitable for finer pressed powders because their ability to transmit force is poor.
Thick products should be pressed more slowly, whereas thinner products such as carbon resistors can be pressed faster.
4. During the forming process, due to the friction between the powder and the mold wall and the friction between the powder particles, the uneven pressure distribution causes uneven density, which is the reason for the fluctuation of product quality. One of the important reasons. Using the method of additional vibration can overcome this uneven phenomenon to a large extent, thereby reducing the elastic aftereffect. This is because vibration can promote the displacement of powder particles, arrange the particles reasonably, and eliminate the bridging phenomenon of particles. Therefore, higher density briquettes can be obtained under lower pressure.
5. Two-way molding is also helpful in reducing elastic aftereffects.