With such pressing, the processes of forming and sintering the workpiece are technologically combined to obtain the finished part. This method produces parts of hard alloys and special heat resistant materials that are characterized by high strength, density and uniformity of the material. When hot pressing, graphite molds are used. The high temperature of the powder can significantly reduce the required pressure. There are also significant shortcomings: low productivity, low durability of molds (4-7 compacts), the need for processes in the protective gas environment, which limit the use of this method.
This method is used to produce metal-clad billets, to which nc show high requirements for accuracy. The pressed powder 3 (Figure 8.3) is covered in a waterproof elastic rubber or metal shell 2, placed in the chamber 1, and carefully compacted. The air is then pumped out so that it does not separate from the pressed billet and does not lead to the formation of cracks in it. Then hydrostatic pressure is applied through the shell with a fluid 4 (emulsion, oil).
Fig. 8.3. Hydrostatic compaction scheme:
1 - the sealed chamber; 2 - the elastic shell; 3 - powder; 4 - liquid
The fluid pressure p reaches 3000 MPa, which ensures the production of blanks of high strength and density. There is no need to use expensive molds. The overall dimensions of the billets produced depend on the design of the sealed chamber.
Extrusion (isostatic extrusion)
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In this way, rods, pipes and profiles of various cross-sections are manufactured. The extruder scheme is shown in Fig. 8.4.
Fig. 8.4. Scheme of the device for isostatic extrusion:
1 - the receiving cylinder; 2 - press washer; '3 - punch; 4 - the compressing fluid; 5 - powder; 6 - compaction; 7 - the stop plate of the matrix; 8 - product; 9 is the matrix
The process of preparing the blanks consists in extruding the powder 5 through the combined hole of the mold matrix 9. The plasticizer is added to 10-12% by weight, improving the process of joining the particles and reducing the friction against the walls of the press- form. The profile of the part being manufactured depends on the shape of the calibrated hole. Hollow profiles are produced using a dissector, cermet - extruded on hydraulic and mechanical presses.
Rolling. This method of obtaining thin ribbons, strips, sheets of powders is one of the most productive and promising methods for processing metal-ceramic materials. Rolling methods differ in the way the powder is fed (horizontal or vertical feed) and the direction of flow. The basic scheme of rolling with vertical feed is shown in Fig. 8.5.
Fig. 8.5. Scheme of rolling of powders with vertical feed:
a - obtaining a single-layer tape; b - getting a two-layer tape; 1 - the bunker; 2 - powder; 3 - rolls; 4 - homogeneous tape; 5 - bunker partition; 6 - the second powder; 7 - Bimetallic tape
The powder is continuously fed from the hopper 1 into the clearance between the rollers 3. When rotating, the powder 2 is compressed and drawn into a belt or strip 4 of a certain thickness. The process can be combined with sintering and final processing of the resulting blanks. In this case, the tape passes through the sintering furnace, and then proceeds to rolling ensuring a predetermined thickness. Rolled ribbons are obtained from various metal-ceramic materials (porous, carbide, friction, etc.). Due to the use of bins with a partition 5 (Figure 8.5, b ), tapes 7 are made of various materials 2 and 6 (two-layered). In Fig. 8.6 shows the scheme of rolling of belts with horizontal powder feeding.
Fig. 8.6. Scheme for rolling powders with horizontal forced feed:
1 - bunker with powder; 2 - auger; 3 - roll
Rolled metal powders are produced with a thickness of 0.02-3.0 mm and a width of up to 300 mm. The use of rolls of a certain shape makes it possible to obtain bars of various shapes, including wire with a diameter of 0.25 mm to several millimeters.Sintering and finishing of blanks
Sintering is carried out to compact the powder, fill more pores, increase the strength of preforms prepared by pressing or rolling. Due to the temperature mobility of the atoms of the powders, processes such as diffusion, reduction of surface oxides, recrystallization, etc. proceed simultaneously. The sintering temperature is usually 0.6-0.9 of the melting point of the powder of a one-component system or below the melting point of the base material for powders, several components. The process is recommended to be carried out in three stages: I - heating up to 150-200 ° C (moisture removal); II - heating to 0.5 sintering temperature (removal of elastic stresses and active cohesion of particles); III - final heating to the sintering temperature. The holding time after reaching the sintering temperature over the entire section is 30-90 min.
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Increasing the time and temperature of sintering to certain values leads to an increase in strength and density as a result of activation of the formation of contact surfaces. Exceeding these technological parameters can lead to a decrease in strength due to the growth of crystallization grains.
Electric resistance furnaces or induction heating furnaces are used for sintering. To prevent oxidation, the process is carried out in neutral or protective environments. It is possible to re-press and sinter to increase the density and strength of the resulting blanks. The required accuracy is achieved with the help of finishing operations: calibration and machining.
The workpieces are calibrated by additional pressing in special steel molds or by pushing the bar material through a calibrated hole. This increases the accuracy and compacts the surface layer. Machining by cutting (turning, drilling, milling, tapping, etc.) is used in those cases when parts of the specified sizes and shapes can not be pressed by pressing. When machining metal-ceramic billets, their porosity must be taken into account. It is not recommended to use ordinary coolants, which, when absorbed into the pores, cause corrosion. Oil impregnation of porous preforms before processing is also undesirable, because during the cutting process, oil flows out of the pores and, when heated, smokes.
When cutting, use a tool equipped with plates of hard alloy or diamond. To maintain porosity during machining, a well sharpened and finished tool must be used.
The described methods are used in the manufacture of various gears and gear pulleys, bushings and pistons of shock absorbers, parts from antifriction and friction materials for work in various braking and transfer units, various filter elements from metal powders.
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