The basic technology of powder metallurgy production process include: preparation, powder mixing, compacting and sintering and subsequent processing, etc. In diagram in figure 7-1. Ceramic products of the production process and there are many similarities of powder metallurgy, the process including powder preparation, forming and densification of three stages.

2.1 powder preparation

2.1.1 powder preparation

The basic materials of powder is sintered parts. Powder preparation methods have a lot of kinds, sum up can be divided into mechanical and physical and chemical method two kinds big.

(1) mechanical method, mechanical method is mechanical crushing method and liquid atomization method.

Mechanical crushing method is the most commonly used in ball mill method. The method in 10 ~ 20 mm diameter steel or hard alloy of metal ball mill, is suitable for the preparation of some brittle metal powder (e.g., iron alloy powder). For soft metal powder, adopt spiral grinding method.

Atomization method is currently used more of a mechanical milling method, especially for making alloy powders, such as low alloy steel powder, stainless steel powder. Will the molten metal liquid slowly down through the holes, with high-pressure gas (such as compressed air) or liquid (water) injection, through the mechanical force and quenching effect to make metal liquid and spray. The results for different metal powder particle size. Figure 7-2 for the gas atomization powder. Atomization method is simple, can be continuous, mass production, is widely used.

(2) the physical and chemical method, the common physical methods have spirit from the liquid phase

Deposition method. Metals such as zinc, lead, gas condensate and low melting point metal powder.

Such as metal carbonyl content Fe (CO) 5, Ni (CO) 4 liquids such as the 180 ~ 250 ℃ heating thermal dissociation method, can obtain high purity superfine iron and nickel powder, known as the carbonyl iron and nickel carbonyl.

Chemical method are mainly electrolytic process and reduction process. Electrolytic process is manufacturing industry

Copper powder's main method, namely using copper sulfate electrolytic aqueous solution, precipitation of high pure copper. Reduction method is the main production of industrial iron powder method, adopt the method of solid carbon reduced iron phosphorus or iron ore powder. Reduction after get the sponge iron, after crushing of iron powder annealing under hydrogen atmosphere, the sieve is made of iron powder you need. Figure 7-2 gas atomized powder

2.1.2 powder performance

Performance of its forming and sintering process, powder and products performance has significant influence, and thus the performance of the powder must be understood. The performance of the powder can be divided into physical properties, chemical properties and process performance. Physical properties of the particle shape, particle size and particle size composition, density, hardness, strain hardening and plastic deformation capacity and microstructure, etc.; Chemical properties have a chemical composition; Process performance is powder, such as apparent density, mobility and repressive. Usually use the following several main performance to evaluate the performance of the powder.

(1) the particle shape, particle size and particle size

A. particle shape, particle shape is the main factor determining powder process performance. Made of different methods for powder shape is different, as shown in table 7-2. The shape of the particles as shown in figure 7-3. Particle shape of powder compacting and sintering. Such as smooth surface of the powder particles, the liquidity is good, is good to improve the compact density. But the complex shape of powder, good for improve compact strength of the product, at the same time can promote the sintering.

Table 7-2 grain shape, apparent density and powder production method

Powder production method

Powder particle shape

Apparent density g/cm3

Powder production method

Powder particle shape

Apparent density g/cm3

Carbonyl iron powder

Atomized iron powder

Reduced iron powder

The spherical powder

Spherical or irregular shape

Irregular spongiform

3.0

2.2

Electrolysis of iron powder

Ball mill grinding iron powder

Spiral grinding iron powder

dendritic

flake

The disk

0.35

2.0 ~ 2.1

1 2 3 4 5 6 7 8

Figure 7-3 powder particle shape

Spherical nearly spherical polygon flake 5 4 3 2 1 dendritic 6 irregular porous spongy 7 of 8 dish

B. The particle size and particle size distribution of powder granularity refers to the size of the particles. For the powder body, particle size refers to the average particle size. Industrial manufacture of powder, particle size range of 0.1 ~ 400 microns, usually use mesh size (inch length is the number of mesh on the screen). Granularity has special measuring method, such as the sieve analysis method and microscope method and settlement method, etc, the most commonly used is the sieve analysis. Particle size directly affects the performance of the products, such as hard alloy, ceramic material, particle size as fine as possible. For commonly used production of powder metallurgy products, not only to determine average particle size of powder body, the more important is to determine the content of different particle size and particle size distribution for short. Powder's particle size distribution on forming and sintering has certain influence. Such as powder size distribution properly, the porosity between powder particles is small, the high density of forming and sintering easier.

(2), apparent density, mobility and repressive

A. apparent density apparent than is apparent density also called, is refers to the unit volume free loose powder quality, usually in g/cm3. The apparent density of the powder is a comprehensive performance, it is powder particle size, particle size, particle shape and intragranular pore. The influence of such factors as Apparent density using the powder flow meter to measure.

B. liquidity powder liquidity refers to the unit mass of powder after free fall into the stream of time, commonly used s / 50 g said. Powder liquidity reflects the powder filling a certain shape container ability, to realize automatic suppression and for the suppression of complicated shape products uniform filling is very important. Powder liquidity is a comprehensive performance, mainly depends on the coefficient of friction between the powder, with powder shape, particle size, particle size and surface water absorption and volume and so on. Liquidity is also using the powder flow meter to measure.

C. repressive powder repressive including compressibility and formability. Powder compressibility is refers to the powder in the process of suppression of compression capability. Are generally used under a certain pressure (400 mpa) to suppress the compact density (g/cm3). It's critical to the strength of the compact and density. Powder compressibility is mainly composed of powder of hardness and plastic deformation ability and the work hardening properties of decision, and in a considerable extent related to the size and shape of the particles. After powder formability is refers to the powder compacting, compact shape stays on. Generally with proper strength to compact (only handling is not broken or not happen deformation intensity) needed to represent the pressure. Is the direct reflection of forming of powder compacting intensity, and therefore available to compact the compressive strength and bending strength of expressed quantitatively. Main of forming of powder and particle shape, particle size and particle size composition and other physical properties.

(3) the chemical composition of the powder chemical composition should include the main content of metal or alloy components and the content of impurities. To meet the requirements of general products manufacturing, metal or alloy powder in the content of alloy components cannot be less than 98 ~ 99%, and in manufacturing magnetic alloy and some special purpose is material when its higher purity requirements. Powder is mainly refers to combine with the main metal impurities of Si, Mn, C, S, P, O, etc. Some of the elements; SiO2, Al2O3, Portland, or refractory metal carbides, such as acid fusion objects; Powder surface adsorption of oxygen, moisture, such as N2, CO2 gas. Powder impurities on subsequent process and final products quality will have a significant impact. So must be strictly controlled. If iron powder acid insolubles under 0.2%, hydrogen reduction reduction is less than 0.2%.

2. 2 powder

Powder mixed refers to two or two or more components of the powder mixing process. Mixed not only affects the quality of the forming process and the quality of compact and will seriously affect the sintering process and final quality of the products.

Mixed mainly divided into mechanical method and chemical method two kinds. The wide application of which is mechanical method. Mechanical method is divided into wet mixing dry mix. Using dry mixing iron-based products production; Cemented carbide mixture preparation often adopts wet mix, such as when mixing a certain proportion to join cemented carbide balls in gasoline to fully wet milling. Chemical mixture is add metal or compound powder and metal salt solution mixing; Or is every all mixed in some form of salt solution, and then by precipitation, drying and reduction by a mixture of uniform distribution, such as used for preparing tungsten - copper - nickel alloy with high density, magnetic iron nickel material, tungsten silver - contactor alloy mixture of raw materials.

In order to improve the powder to suppress performance, reduce product uniform density distribution, die wear and demoulding, often to join a small amount of lubricant, such as ferrous products need to join the right amount of zinc stearate, its technical requirements are shown in table 7-4. Zinc stearate in the sintering process produces decomposition, volatile was left in the corresponding parts of the products needed for the pores and make the product final pore connectivity to each other, will also act as a pore-forming.

Table 7-3 zinc stearate technical requirements

The metal zinc content

The moisture

Free acid

Melting point

Particle size

The appearance

10.2-11.2%

< 0.2%

< 0.5%

> 120 ℃

- 200 mesh

white

The commonly used mixer is used for powder mixed type as shown in figure 7-4. Filling quantity, powder density difference and mixed system, the structure of the mixer and revolutions, mixing time and mixing medium will influence mixture uniformity. Mixing shall ensure that the chemical composition of certain combinations of materials, process performance and mixing evenness and other technical requirements.

V type mixer

Double cone mixer

Rotating cube mixer

Eccentric rotating turret type mixer

Horizontal rotation mixer

Eccentric rotation mixer

Figure 7-4 various contour diagram of mixer

Ceramic powder material for organic compounds, and very fine particles, plasticization and granulation processing is required, can be used for forming. Plasticizing refers to the so-called material adding plasticizer make the material malleable. Plasticizer capable of plastic material is directed block, according to their different role in the ceramic forming can be divided into the adhesives solvent, plasticizer, and three categories. Adhesives (usually with polyvinyl alcohol, polyethylene glycol (peg) and the paraffin wax, etc.) make room temperature powder particles together, make the billet forming properties and has a certain strength, oxidation and decomposition at high temperature and volatilization. Plasticizer (usually with glycerin, oxalic acid, etc.) dissolved in organic binder, form a liquid layer between the powder, improve the plasticity of the billet. The solvent (usually water, anhydrous alcohol, acetone, benzene, ethyl acetate, etc.) can dissolve the binder and plasticizer and can form a plastic material and material liquid. So-called granulation is made larger particles to small particles of powder or granule process, commonly used to improve the filling of fine powder. Commonly used methods of ceramic powder granulation are classified into such three types:

Ordinary granulation method - will be adding suitable amount of binder mixtures was made granule on the roller, disc and sieve machine;

Briquette binder granulation method - will be added the mixture under low pressure preloading into pieces, and then crushing sieving;

Spray granulation method - will join the binder liquid is atomized into a dry powder in dry atmosphere.

7.2. 3 press forming

Press forming refers to a loose powder body compacted into certain shape, size, density and intensity of compacting process. Compacting method are many, such as continuous forming mold forming, isostatic pressing forming, powder, powder injection molding and slip casting forming, etc., and mould pressing forming is the most widely used powder forming technology. Mold forming on the mechanical press or hydraulic press, usually at room temperature and under certain pressure. Powder metallurgy suppressing pressure generally is 140 ~ 840 mpa, for pressing of materials of ceramic pressure 40 ~ 100 mpa. It is will be a certain amount of powder mixture in precision die, die in pressure, under the action of the powder body pressure, pressure, pressure relief, and then again from compact cavity die out process, as shown in figure 7-5.

On the die

Female die

The die

A loose pack

B compression

C mold release

Figure 7-5 pressing process diagram

2.3.1 filling

Generally the volume method is adopted, the powder loading Yin with prescribed volume cavity model. Commonly used filling method has the following three:

7 (1) into (6 a) send powder to die cavity and core rod forming mold, free powder fell into the cavity.

(2) the method of suction under (7-6 b) die in the ejection of compact position, move send powder mold, die under drop rises (or the cavity die mandrel) reset, the powder into a cavity.

(3) too much filling (7-6 - c) die mandrel dropped down position, after powder fell into the Yin model cavity, core rod rises will be redundant powder ejector, and feeding device was blown away. When this method is applied to forming thin-walled parts compacting.

A fall into the law

B inhalation method

C too much filling

Figure 7-6 automatic filling method

Only one, the purpose of all kinds of packing methods mentioned above is to compact powder evenly and accurately into the cavity.

2.3.2 to suppress

(1) the stress distribution

With a simple cube as shown in figure 7-7, as a body force in the molding powder

Schematic diagram.

(F) : a. thrust pressure applied to the die shape powder body force. Suppressing pressure consumption mainly has two parts: the net pressure needed for powder body density (F1) and used to overcome the powder particles and the friction between the die wall (F2) : F = F1 + F2

Figure 7-7 compacting force diagram

B. the lateral pressure (P) : the powder body within the molding pressure, compact will be around to expand, the mould wall will give compact an equal and opposite reaction. Because between powder particles and powder body and friction between the die cavity wall, the influence of such factors as to powder of die profile is always less than the pressure stress. Such as suppression of iron powder, the side of P and F assembly direct proportion relationship between lateral (P = 0.38 ~ 0.41 F total).

C. external friction, powder compression downward movement in the die, as a result of the existence of the lateral pressure, powder creates friction between the die wall, its size is equal to the product of the coefficient of friction and lateral pressure.

(2) motion and deformation of powder

Powder body force within the die, the loosely packed into a dense state, form has a certain shape and intensity of the compact. This is due to the powder particles by the unbalance force after movement (displacement) and the deformation results.

A. powder powder body of the displacement in the die free loose-packed, due to the friction and mechanical bite powder particles, the particle overlap each other, cause many times larger than the particle pore, this phenomenon is called "arch bridge" effect, as shown in figure 7-8. Powder body after stress, powder "arch bridge" in the body is destroyed, the powder particle rearrangement, pore filling each other, increase the contact, the powder body the porosity is reduced greatly. Displacement situation of powder particles, can be used as shown in figure 7-9 model figure more vividly. Powder body, of course, what happened in the state of compression displacement situation is more complicated, several displacement may occur simultaneously, and the displacement is always accompanied by deformation.

B. Several types of figure 7-9 powder displacement

Separation of the powder particles is close to b powder particles c powder particle sliding

Figure 7-8 "arch bridge" effect diagram

D the rotation of the powder particles

E powder particles through the crush of mobile

B. after the mechanical deformation of powder particles, resulting from the mutual contact of particles movement blocked extrusion deformation, powder particles such as all solid material also forced deformation, elastic deformation, plastic deformation and brittle fracture. Force of the powder is belong to the three to the stress state of stress, but the unbalanced force of movement and deformation caused by powder, only along the direction of positive pressure, compact by lateral pressure relatively is much smaller. Friction and powder movement in the opposite direction, impact velocity, powder for powder direction does not work.

Therefore powder movement and deformation of the biggest features are: (1) powder is mainly along the pressing direction as a straight line, a small amount of lateral movement. (2) the pore are compact horizontal get squashed.

(3) the distribution of the density and size of the intensity

A. compact density figure 7-10 said the relationship between pressure and density of different powder. After pressure on, "arch bridge" destruction, particle displacement and fill the gap, and reach the maximum packing density, compact density increased rapidly; When stress continues to increase, the powder surface reveal certain compression resistance, at this stage, with the increase of pressure, the density did not increase; In the stress exceed the critical stress value of the powder materials, yield strength or ultimate strength, deformation of powder particle increased and fracture (such as brittleness powder), due to the displacement and deformation at the same time, the compact density and then increases slowly.

Repression, due to the role of friction, die stress transfer is not uniform on the powder body, thereby compacting density distribution is uneven. As shown in figure 7-11 for one-way suppress nickel powder compacting density distribution of the actual situation.

B. compact intensity refers to the mechanical strength of the powder compact. It depends on the suppression of pressure and the kinds of metal powder, namely, the influencing factors of compact strength are: compact density, powder particles surface roughness, surface area, surface oxidation and pollution, the apparent density of the powder, the amount of additives, etc.

Compact density and strength have a direct impact on the quality of the sintered body size, density and even distribution, high strength, is the quality of the sintered body is high also.

Figure 7-10 density to the pressing force

Figure 7-11 one-way suppress nickel powder compact state of real density (g/cm3) distribution

The unit pressure 630 mpa; Height/diameter = 0.87)

Means of (4) according to the powder body when pressing force, powders of the movement and distribution of the density and intensity of the size of the rule, used for press forming of basically has the following four ways.

A. one-way suppression in the process of pressing, die cavity and the core rod, only die on pressure from one direction. Uneven distribution of the density of the compact (large under small), it is suitable for pressing thickness without the steps class (figure 7-12 a) smaller parts.

B. two-way suppression in the process of suppression, cavity die fixed, die and pressure at the same time from two directions method. Income compact uniform density distribution is one way to neutralize, upper and lower density is higher and the basic equal, among the lowest layer is located in the compact density, it is suitable for crushing thick parts without the steps class (figure 7-12 b).

C. floating suppression in the process of suppression, cavity die for the spring bearing and lower die and fixed. The burst pressure of upper die, as powder is compressed, cavity die wall and the friction between the powder increased gradually to the greater than the spring support, cavity die with die on the decline, is equivalent to die under the rose, like a two-way repression. Compared with two-way pressing, its overall density increased, and the lowest density distribution is longer, the green strength increase (FIG. 7-12 - c).

D. down mould cavity block movement is by press rather than by the friction work, is suitable for the friction is small, some products can not float. Female die down type is similar to the female die forced under pressure, and the cavity die downward movement of the distance (that is, the change of relative movement between the die and cavity die) according to the need to precisely control of each products. To compact low density layer to reduce to a minimum, in the middle of the middle and low density layer can be adjusted to the required position (FIG. 7-12 d).

Figure 7-12 four basic way

A one-way to suppress

B. two-way pressing

C floating down

D by mould

We can see from this, should according to different shapes and compact requirements, choose the right way.

2.3.3 the holding and demoulding

(1) the holding means the powder body under the maximum suppression under pressure to stay for a period of time, so that (1) pressure transmission, thus is advantageous to the compact density of uniformity; (2) between the powder have enough time to escape the air in the pore; (3) for each mesh and deformation of the powder particles in plenty of time. Implementation of compact density and strength increase. For the use of compressibility and poor formability of raw material powder pressing, complex shape or volume larger compact is particularly important.

With 6 t/cm 2, for example, pressure pressing iron powder, no pressure, compact density of 5.65 g/cm 3; After 0.5 minutes holding its density of 5.75 g/cm 3; After 3 minutes the holding its density of 6.14 g/cm 3, namely the compact density increased by 8.7%.

(2) stripper is to suppress forming compact smooth out from the die cavity. Commonly used mold release means has:

Ejection type: die down as compared to the female mold cavity up relative motion, thus will compact ejector cavity. As shown in figure 7-12 - c.

Die down type: the motionless, Yin cavity down relative motion, thus will compact ejector cavity die. As shown in figure 7-12 d.

2.4 the sintering

Sintering is powder or powder compact, under appropriate conditions of temperature and atmosphere heating phenomenon or process. Sintering is the result of the particles by mechanical engagement between into atomic crystal, sinter strength increase, and in most cases, the density is also improved. So what is the force makes the density and strength increased ware? What changes have taken place in the sintering process?

Against 2.4.1 sintering impetus

(1) surface energy due to the highly fragmented, powder and powder particle surface is uneven, so the powder body compared with dense metal and after sintering products, has a large specific surface, thus there is a lot of surface energy.

(2) the distortion energy powder in the manufacturing process, the particles inside lattice distortion, and produce various kinds of defects. In addition, in the process of suppression, the deformation of the powder particles have much lattice distorted seriously, thus powder compact can store a large amount of distortion.

The above-mentioned two aspects of energy makes the atoms within the compact powder particles in a state of flux. Sintering in the unstable state of atom will tend to reduce the energy. From the whole, compact powder particles together must be a spontaneous process in energy reduction.

2.4.2 the basic process of sintering

Under the effect of sintering temperature, the great energy of atoms will cause material migration, main form of migration: diffusion and flow. So that the powder body has experienced changes in figure 7-13 signal process.

Adhesive (1) the early stages of sintering, the powder particles due to the high temperature surface atom diffusion, between two particles in the formation of bonding surface, and with the growing adhesive surface, sintering neck is formed between particles, crystal is formed between particles. At this stage sintering shrinkage a little, density increase.

(2) the stage of sintering neck grew up along with the sintering process continues, the atom to the particle surface mass migration, sintering neck grew up and form a continuous pore network. Grain boundary migration, grain growth, make pore annexed to each other, concentration, total pore volume decreased, sinter contraction, the density increases, the strength is greatly improved.

(3) closed pore spheroidization stage and shrink when the porosity of the sintered body is less than 10%, most of the pore are completely separate, lots of closed pore, spheroidizing and narrowing, even disappear, still slow contraction, sintered body in sintering, the sintering largely discontinued. Sintering, the above three steps are often interrelated and overlapping, cannot be divided strictly.

Figure 7-13 sintering process interface and pore shape change model

A raw contact between particles and the binder

B sintering neck grew up

C closed pore spheroidization and narrow

2.4.3 sintering type

According to the atmospheric pressure sintering and pressure sintering sintering methods can be divided into two broad categories.

Atmospheric pressure sintering is to compact under the atmospheric pressure or under low gas pressure sintering method. Atmospheric pressure sintering, does not produce liquid phase sintering as solid phase sintering. Of liquid phase sintering is also known as liquid phase sintering. Liquid phase sintering process, the liquid will penetrate into pores, at the same time to speed up the shrinking, increase the density of the sintered body.

According to the different methods of pressure, can be divided into the hot pressed sintering and hot isostatic pressing sintering, powder hot forging, etc. Hot-pressing refers to the loose powder or graphite mould of powder compact heating at the same time, the uniaxial pressure sintering process, in order to improve the sintering density, such as powder metallurgy friction plate, double metal antifriction materials; Al2O3, BeO, the SiO, BN, AlN ceramics are available hot-pressing sintering, the sintering temperature can be reduced about 100 ~ 150 ℃. Powder hot forging is usually first to compact sintering, and then under the high temperature of appropriate implementation of the forging. Hot isostatic pressing refers to packed in bag loose powder is heated at the same time exert isotropic and static pressure of the sintering process. Solve the ordinary hot-pressing lack of lateral pressure and the pressure caused by the uneven density inequality problem.

2. 5 subsequent processing

Some sintering parts can be used after sintering. Many parts need to be added, however, the processing, in order to make the shape of the sintering parts with prescribed, size accuracy and use performance. Subsequent processing include: usually finishing (plastic, calibration, after pressure), heat treatment, steam oxidation treatment, cars, milling, grinding, drilling, tapping, rolling light, oiled or leaching resin, electroplating, metal, etc. Almost all suitable for metal forging process is suitable for powder metallurgy products.