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Powder metallurgical products

Powder metallurgy is a process technology that USES forming and sintering process to make materials and products of metal powder (or metal powder and non-metallic powder). It is a branch of metallurgy and material science.

Powder metallurgy is a process technology that USES forming and sintering process to make materials and products of metal powder (or metal powder and non-metallic powder). It is a branch of metallurgy and material science. The application range of powder metallurgy products is very wide ranging from general mechanical manufacture to precision instruments. From hardware tools to large machinery

Hard alloy mechanical forming machine ; From electronics to motor manufacturing; From civilian industry to military industry; The process of powder metallurgy can be seen from the general technology to the advanced technology. Development history of powder metallurgy: The method of powder metallurgy originated from around 3000 BC. The first method of making iron is essentially a powder metallurgy method.

There are three important signs in the development of modern powder metallurgy:

1. Overcome the difficulties caused by the melting of refractory metal. In 1909, the manufacture of electric lamp tungsten wire promoted the development of powder metallurgy; In 1923, the appearance of the powder metallurgical hard alloy was hailed as a revolution in mechanical processing.

2. In the 1930s, porous oil bearing was successfully made; Then the development of powder metallurgical iron based mechanical parts has been used to make full play of the advantages of the powder metallurgy cutting and cutting even without cutting.

3. Develop new materials and processes more advanced. In the forties, metal ceramics and dispersion strengthened materials were present. In the late 1960s and early 1970s, the powder high-speed steel and powder high temperature alloy appeared. High strength parts can be made by using the powder metallurgy forging and heat isostatic pressure.

Advantages of powder metallurgy technology:

  • 1. Most refractory metals and their compounds, false alloys and porous materials can only be made by means of powder metallurgy.
  • 2. Since the powder metallurgy method can be compressed into the final size of the blank, without the need or need for the subsequent machining, it can greatly reduce the metal and reduce the cost of the product. When the powder metallurgy method is used to make the product, the loss of the metal is only 1-5%, and the loss of the metal can reach 80% when produced using the normal casting method.
  • 3. due to the powder metallurgical process in the material does not melt in the process of production, are not afraid with the crucible and deoxidizer and other impurities, and sintering in vacuum and reduction atmosphere, not afraid of oxidation, will not give any pollution materials, therefore, likely to preparing high purity materials.
  • 4. The powder metallurgy method can guarantee the correctness and uniformity of material composition ratio.
  • 5. Powder metallurgy is suitable for the production of the same shape and quantity of products, especially for the high processing cost of gear and other processing costs. The production cost can be greatly reduced by using powder metallurgy method.

Disadvantages of the powder metallurgy process: the overall disadvantages:

  • 1) there are always pores in the products;
  • 2) the strength of ordinary powder metallurgical products is lower than corresponding forgings or castings (about 20%~ 30%);
  • 3) due to the fact that the powder is much less liquid than liquid metal in the forming process, there are certain restrictions on the shape of the product.
  • 4) the high pressure required to suppress the forming process and the ability of the products to be suppressed;
  • 5) high mold cost, generally applicable only to batch or mass production. Metal powder: the quality of the final product is difficult to control; Metal powder is expensive; The powder does not obey the laws of hydraulics, but the shape of the product is limited.

Manufacturing equipment and methods: 1) pressure machine: often requires expensive power presses 2) pressure die: it is consumable, the cost is higher 3) sintering furnace 4) the powder is easy to oxidize and the mixing takes a long time 5) the size and shape of the products are limited.

The basic process of powder metallurgy technology is:

1. Preparation of raw material powder. The existing methods of flour milling can be divided into two categories: mechanical and physical chemistry. Mechanical methods can be divided into: mechanical pulverization and atomization. The physical and chemical methods are divided into: electrochemical etching, reduction, legal, reduction-and legal, gas-phase deposition, liquid-phase deposition and electrolysis. The most widely used method is the reduction method, the atomization method and the electrolytic method.

2. The powder form is the blank block of the desired shape. The purpose of molding is to make a certain shape and size of blank, and make it have certain density and strength. The method of forming is basically divided into pressure forming and no pressure forming. The most application of pressure molding is mold forming.

3. Sintering of slab. Sintering is the key process in powder metallurgy. After forming, the pressure billet is sintered to get the final physical mechanical properties required. Sintering is divided into unit sintering and multi - line sintering. The sintering temperature is lower than the melting point of metal and alloy used in the solid phase sintering of the unit and the multivariate system. For the liquid-phase sintering of multivariate systems, the sintering temperature is generally lower than the melting point of the refractory components, and higher than the melting point of the fusible composition. In addition to ordinary sintering, there are special sintering techniques such as loose sintering, melting and soaking, and hot-pressing.

4. Post-order processing of products. After sintering, various methods can be adopted depending on the product requirements. Such as fine finishing, oil immersion, machining, heat treatment and electroplating. In addition, some new processes such as rolling and forging have been applied to sintering of powder metallurgical materials in recent years.

The future development direction of powder metallurgy materials and products:

1. Representative ferro-based alloys will be developed for high-volume precision products and high quality structural components.

2. Manufacturing high performance alloy with uniform microstructure, processing difficulty and complete density.

3. A special alloy consisting of mixed phases is usually made using enhanced densification process.

4. Make non-uniform materials, amorphous, microcrystalline or amorphous alloys.

5. Process unique and non-general form or component parts.

The number and application scope of cemented carbide

GRADE Material performance Recommended use ISO standards
Co% Grain size micronsμm Hardness Bending strength≥N/mm2 density
ZK01F 3 ultrafine 94 1550 15.2 Suitable for carbon steel, high-speed steel finishing, its wear resistance is very good, is suitable for the production of alloy glass blade, steel wire, tungsten, and tantalum hard materials such as high speed control, and surface finish high non-ferrous metal material control (line). K01
ZK05F 5 ultrafine 93.5 1750 15 Applicable to cast iron, harden steel, nonferrous metal, wood and other non-metal materials processing. K05
ZK10F 6 fine 92 2200 14.9 Suitable for cold hard cast iron, nodular cast iron, ordinary cast iron and heat-resisting alloy steel. K10
ZK20F 8 fine 92 2550 14.7 The processing of glass steel, titanium alloy, high hardness hardening steel, yellow steel and other materials is widely used in the processing of PCB milling cutter. K10-K20
ZK30F 10 ultrafine 91.8 3300 14.4 Applicable to the processing of steel, cast iron, stainless steel, heat resistant steel, drilling, high temperature alloy, nickel base and titanium alloy materials, widely used in cutting tool type milling cutter, CNC blade and integral circular saw blade, etc. K20-K30
ZK32F 12 ultrafine 91.4 3500 14.1 Suitable for cold hard cast iron, high temperature alloy, glass fiber material, titanium alloy, alloy steel, large feed amount, drilling, milling and addition. K30
ZK35F 15 ultrafine 90 3850 14.1 Applicable to the plug-in, silicon steel sheet and cold rolled sheet mixing stamping, punching needle, hobbing cutter and fiber knife. K30-K40
ZK10 6 In the 91 2200 15 Applicable to cast iron, non-ferrous metal and alloy, non-metallic materials medium speed semi-finishing and finishing. K10
ZK20 8 In the 90 2450 14.8 Applicable to cast iron, non-ferrous metal and alloy, non - metal materials low - speed precision alloy bars and tensile molds. K20
ZK35 15 In the 87.5 3600 14.2 Cold rolled steel sheet, silicon steel sheet and other materials such as stamping, die plate, gear, toothed drilling teeth, drill teeth. K30-K40
ZK40 20 In the 87 3900 13.7 It is suitable for stamping die materials such as punching watch parts, spring pieces of Musical Instruments, etc., such as die, small size steel ball, screw, nut, etc. K40
ZK45 25 In the 85.5 4100 13 It is mainly used to make straight poles, six pieces of die, bolt, top forging die, upsetting die etc., which have high impact resistance and good thermal fatigue stability. K40
ZP20 8.5 fine 91 1850 11.5 It is suitable for the flat and fine cars of carbon steel and alloy steel during continuous cutting, rough milling and discontinuous cutting. P20
ZP30 9.2 In the 90 2200 13 Applicable to carbon steel and alloy steel (including forgings, stamping parts, cast iron cuticle) discontinuous cutting, rough planer, semi - rough plane. P30
ZG20 8 coarse 88.5 2600 14.7 Is suitable for low exploration and drilling tools, coal mining with light insert, medium and small ball tooth impact drill, rotary exploration drill alloy plate, oil well drill bit, used for drilling f 14 or less in the soft rock, hard rock. G20
ZG30 11 coarse 86.5 3300 14.3 It is suitable for alloy cylindrical teeth, insert and impact drilling tools, drilling hard rock and hard rock formation, for f is greater than 18, and also can be used for other drilling tools alloy and damaged hammer alloy. G20-G30
ZG35 15 coarse 86 3600 14 It can be used in mining, heavy drill and drilling tools, etc., which can be used in hard rock formations and hard rock formations, as well as other cylindrical teeth drilling tools with alloy teeth. G30
ZG40 20 coarse 85 4000 13.5 It is suitable for cold forging, cold stamping and cold pressing mould of standard parts, bearings and tools. G40
ZG45 25 coarse 83 4300 13.1 Applicable to head, nut, wall screw, large standard mould and high carbon steel. G50

What is powder metallurgy

Powder metallurgy is producing metal powder or with metal powder (or the mixture of metal powder and nonmetal powder) as raw material, after forming and sintering, manufacturing metal materials, composite materials and processing technology of all kinds of products. The powder metallurgy method is similar to the production ceramics, which belong to powder sintering technology. Therefore, a series of new powder metallurgy techniques can also be used in the preparation of ceramic materials. Because of the advantages of powder metallurgy, it has become the key to solve new material problems and plays an important role in the development of new materials.

Powder metallurgy includes powder and products. The powder is mainly metallurgical process, and literally. Powder metallurgical products often go far beyond materials and metallurgy, and are often multi-disciplinary (materials and metallurgy, mechanics and mechanics, etc.). Especially modern metal powder 3 d printing [1-2], mechanical engineering, CAD and reverse engineering technology, layered manufacturing technology, numerical control technology, material science, laser technology, the technology of powder metallurgy products become more across disciplines of modern integrated technology.

Powder metallurgy has unique chemical composition and mechanical and physical properties, which are not obtained by traditional casting methods. The application of powder metallurgy can be made directly into porous, semi-dense or full-dense materials and products, such as oil bearing, gear, CAM, guide rod, cutter, etc., which is a kind of less cutting technology.

(1) powder metallurgy technology can minimize the composition of alloy components and eliminate large, uneven casting tissues. In the preparation of high performance rare-earth permanent magnetic materials, rare earth hydrogen storage materials, rare earth luminescent material, rare earth catalyst, high temperature superconducting materials, new type metal materials (such as Al - Li alloy, heat-resistant Al alloy, super alloy, corrosion resistant stainless steel powder, powder high speed steel and high temperature structural materials intermetallic compounds, etc.) plays an important role.

(2) preparation of amorphous and microcrystalline, quasi crystal, nanocrystalline and super saturation solid solution and a series of high-performance nonequilibrium materials, these materials have excellent electrical, magnetic, optical and mechanical properties.

(3) it is easy to realize multiple types of compound and give full play to the characteristics of each group of materials, which is a low cost production of high performance metal base and ceramic composite materials.

(4) can produce ordinary smelting method cannot produce with special structure and properties of materials and products, such as new porous biological material, porous membrane material, high performance structure ceramics abrasive material and functional ceramics, etc.

(5) it can achieve near-net formation and automatic mass production, thus effectively reducing the resources and energy consumption of production.

(6) it can make full use of ore, tailings, steelmaking sludge, rolling steel scales, recycling waste metal as raw materials, and it is a new technology that can effectively carry out material regeneration and comprehensive utilization. We are familiar with machining tools, hardware tools, many of which are made of powder metallurgy.

The preparation of powder:Making powder is the first step in powder metallurgy. Powder metallurgy materials and products are continuously increasing, its quality is constantly improving, and the kinds of powder that are required to provide more and more. For example, from the material range, not only use metal powder, but also alloy powder, metal compound powder and so on; From the appearance of the powder, it is required to use the powder of various shapes, such as when the filter is produced, to form the powder. According to the powder particle size, the powder of all kinds of particle size is required, and the particle size of coarse powder is 500~1000 micron and the particle size is less than 0.5 micron and so on.

In order to meet the various requirements for powder, also will have various production method of these methods is made of metal, alloy powder or metal compounds in solid, liquid or gaseous state into a powder. The various methods of making powder and the powder of the legal system. The methods for converting metal and alloy or metal compounds into powder in solid state include:

(1) mechanical pulverization and electrochemical corrosion of metal and alloy powder from solid metal and alloy:

(2) from solid metal oxides and salts from metal and alloy powder reduction method in metal and non-metallic powder alloy powder, metal oxide and metallic compound powder reduction - legally The conversion of metal and alloy or metal compounds into powder by liquid includes:

(1) the atomization method for the preparation and alloy powder of liquid metal and alloy

(2) the replacement and reduction of metal alloy and coated powder from metal salt solution are replaced by substituting method and solution hydrogen reduction method. Molten salt is used to precipitate metal powder from metal molten salt. Metal bath is used to remove metal compound powder from secondary metal bath.

(3) electrolysis of metal and alloy powder from metal salt solution is used to extract water solution. Molten salt electrolysis of metal and metal compounds from metal molten salt electrolysis.

A method of converting metal or metal compounds into powder in the gaseous state:

  • (1) steam condensing method from metal vapor condensation to metal powder;
  • (2) carbon - based thermal dissociation method for metals, alloys and coated powders from gaseous metal carbon-based dissociation
  • (3) gas phase reduction of metal, alloy powder and metal and alloy coatings from gaseous metal halide gas phase; Chemical vapor deposition of metal compounds and coatings from gaseous metal halide deposition.

However, from the essence of the process, the existing methods of flour milling can be generalized into two major categories: mechanical and physical chemistry. Mechanical method is the process of crushing raw material machinery, while chemical composition basically does not change. Physical and chemical method is using chemical or physical effect, changing the chemical composition of raw material or the state of aggregation and process of powder, powder production methods many from industrial scale, the most widely used Hans reduction method, the atomization method and electrolysis method some methods such as vapor deposition method and liquid phase deposition method in special applications is also very important.

The basic process of powder metallurgy technology is:

  • 1. Preparation of raw material powder. The existing methods of flour milling can be divided into two categories: mechanical and physical chemistry. Mechanical methods can be divided into: mechanical pulverization and atomization. The physical and chemical methods are divided into: electrochemical etching, reduction, legal, reduction-and legal, gas-phase deposition, liquid-phase deposition and electrolysis. The most widely used method is the reduction method, the atomization method and the electrolytic method.
  • 2. The powder form is the blank block of the desired shape. The purpose of molding is to make a certain shape and size of blank, and make it have certain density and strength. The method of forming is basically divided into pressure forming and no pressure forming. The most application of pressure molding is mold forming. In addition, 3D printing technology can be used to make the embryo block.
  • 3. Sintering of slab. Sintering is the key process in powder metallurgy. After forming, the pressure billet is sintered to get the final physical mechanical properties required. Sintering is divided into unit sintering and multi - line sintering. The sintering temperature is lower than the melting point of metal and alloy used in the solid phase sintering of the unit and the multivariate system. For the liquid-phase sintering of multivariate systems, the sintering temperature is generally lower than the melting point of the refractory components, and higher than the melting point of the fusible composition. In addition to ordinary sintering, there are special sintering techniques such as loose sintering, melting and soaking, and hot-pressing.
  • 4. Post-order processing of products. After sintering, various methods can be adopted depending on the product requirements. Such as fine finishing, oil immersion, machining, heat treatment and electroplating. In addition, some new processes such as rolling and forging have been applied to sintering of powder metallurgical materials in recent years.

Property of powder The general term for all performance of the powder. It includes: the geometrical properties of powder (particle size, ratio surface, aperture and shape, etc.); Chemical properties of powder (chemical composition, purity, oxygen content and acid insolubility); Mechanical properties of powder (loose packing density, fluidity, forming, compressibility, accumulation Angle and shear Angle, etc.); Physical properties and surface properties of powders (true density, luster, absorbance, surface activity, ze%26mdash; Ta (% 26 ccedil;) Potential and magnetism, etc. The performance of powder metallurgy products is often determined by powder performance. The basic geometric properties are the particle size and shape of the powder.

(1) the particle size. It affects the processing of powder, the contraction of sintering and the ultimate performance of the product. The performance of some powder metallurgical products is almost directly related to the granularity. For example, the filtering precision of the filter material can be obtained by dividing the average particle size of the original powder particles by 10. The properties of cemented carbide products have a lot to do with the grain size of wc, and to get the hard alloy with fine grain size, only the more granular wc raw materials can be used. The powder used in production practice ranges from hundreds of nanometers to hundreds of microns. The smaller the granularity, the greater the activity, the easier it is to oxidize and absorb water. When small to hundreds of nanometer powder storage and transport is not very easy, but when small to a certain degree of quantum effects began to work, its physical performance will happen great changes, such as ferromagnetic powder will become superparamagnetism powder, with melting point is reduced with the decrease of the size.

(2) particle shape of powder. It depends on the method of making powder, such as the powder produced by electrolysis, the granule is a branch. The particles of iron powder obtained by reduction are spongy. The gas atomization is basically spherical powder. In addition, some powders are egg shape, disc, needle, onion and so on. Powder particle shape affects the liquidity of powder and the apparent density, due to mechanical engagement between particles, irregular powder compact intensity is big, especially the dendritic powder blocking its pressure intensity is the largest. But for porous material, it is best to use spherical powder.

The mechanical properties of mechanical properties are the process properties of powder, which is an important technological parameter in powder metallurgy forming process. The density of the powder is the basis of the volume method. The fluidity of powder determines the filling speed and the production capacity of the press. The compressibility of the powder determines the difficulty of pressing the process and the pressure. The forming of powder determines the strength of the blank.

Chemical properties depend on the chemical purity and powder method of raw materials. The higher oxygen content reduces the mechanical properties of the suppression performance, blank strength and sintering products, so there are certain provisions in the most technical conditions of powder metallurgy. For example, the allowable oxygen content of powder is 0.2% ~ 1.5%, which is equivalent to 1% ~ 10% of oxide content.

Discharge plasma sintering system (SPS) New materials, especially the types and demands of new functional materials, are constantly increasing. New functions of materials call for new preparation techniques. Discharge Plasma Sintering, Spark Plasma Sintering, SPS) preparation of functional materials is a new technology, it has a fast heating, short Sintering time, organization structure, control, energy conservation, environmental protection and other characteristics, can be used for the preparation of metal materials, ceramic materials, composite materials, also can be used for the preparation of nano block material, amorphous block material, gradient materials.

Development and application of SPS at home and abroad SPS technology is used to heat sintering directly through the pulse current between powder particles. Therefore, in some literature, it is also referred to as plasmaactivatedsintering or plasma assistedsintering (plasmaactivatedsintering -PAS or plasma-assistedsintering- PAS) [1, 2]. As early as 1930, American scientists proposed the principle of pulse current sintering, but it was not until 1965 that the pulse current sintering technology was applied to the United States and Japan. The technology of SPS was patented by Japan, but it failed to address the low production efficiency of the technology, so the SPS technology was not widely used.

In 1988, Japan developed the first industrial SPS device and used it in the field of new material research. After 1990, Japan introduced the third generation of SPS, which can be used for industrial production, with sintering pressure and pulse current of between 10 and 100t and 5000 ~ 8000A. Recently, the pressure of 500t has been developed, and the pulse current is 25000A large SPS device. Because of SPS technology has the advantages of fast, low temperature, high efficiency, and abroad in recent years, many universities and research institutions have equipped with SPS sintering system, and the use of SPS for new material research and development [3]. In 1998, Sweden purchased SPS sintering system, and studied more research on carbide, oxides and biological ceramics.

In the last three years, we have also carried out research work on the preparation of new materials with SPS technology [1, 3]. Several SPS sintering systems have been introduced, which are mainly used for sintering nanometer materials and ceramic materials [5 ~ 8]. As a new technology of material preparation, SPS has attracted wide attention both at home and abroad. The sintering principle of SPS

3.1 plasma and plasma processing technology [9, 10] SPS are sintered with discharge plasma. Plasma is a state of matter under high temperature or specific excitation, the fourth state of matter except solid, liquid and gas. Plasma is ionized gas, composed of a large number of positive and negative charged particles and neutral particles, and exhibits a quasi-neutral gas of collective behavior. Plasma is a high temperature conducting gas that is dissociated, which can provide a high reactive state. Plasma temperature 4000 ~ 4000 ℃, its gaseous molecules and atoms in a heightened state of activation, and within the plasma ionization degree is high, these properties make plasma has become a very important material preparation and processing technology. Plasma processing technology has been widely used, such as plasma CVD, low-temperature plasma PBD, plasma and ion beam etching. At present, plasma is used for oxide coating and plasma etching, and there are some applications in the preparation of high purity carbide and nitride powder. Another potential application of plasma is the sintering of ceramic materials [1]. The methods of producing plasma include heating, discharge and light excitation. The plasma produced by discharge includes dc discharge, radiofrequency discharge and microwave discharge plasma. SPS use dc discharge plasma.

The basic principle of SPS device and sintering The SPS device mainly includes the following parts: axial pressure device; Water-cooled head electrode; Vacuum chamber; Atmosphere control system (vacuum, argon); Direct current pulse and cooling water, displacement measurement, temperature measurement, and safety control unit. The basic structure of SPS is shown in figure 1.

The SPS are similar to the heat pressure (HP), but the heating method is completely different, and it is a pressure sintering method that utilizes the direct and electric sintering of the pass-dc impulse current. The main effect of the on-off dc pulse current is to generate the discharge plasma, discharge shock pressure, joule heat and electric field diffusion effect [11]. The pulse current of the SPS is shown in FIG. 2. In the process of SPS sintering, the instantaneous discharge plasma generated by the electrode into the dc pulse current makes the particles in the sintering body uniformly generate the joule heat and activate the surface of the particles. In the same way that the heating reaction synthesis method (SHS) is similar to the microwave sintering method, SPS can effectively use the internal heat of the powder to sintered. The process of SPS sintering can be regarded as the result of particle discharge, conductive heating and pressure synthesis. In addition to the two factors that promote sintering, in SPS technology, the effective discharge between particles can produce local high temperature, which can make the surface of the surface melt and the surface material peel off. The sputtering and discharge impact of high temperature plasma scavenged the surface impurities (such as the surface oxides) and the adsorbent gas. The effect of the electric field is to accelerate the diffusion process [1, 9, 12].

The technological advantages of SPS SPS process has obvious advantages: uniform heating, heating speed, low sintering temperature, sintering time is short, high production efficiency, product fine uniform, can keep the natural state of raw materials and be able to get high density material, can be sintered gradient materials, and complex workpiece [3, 11]. Compared with HP and HIP, SPS devices are easy to operate and do not require specialized skills. Literature [11] reported that the total time of the ZrO2 (3Y)/stainless steel gradient material (FGM) with a diameter of 100mm and thickness of 17mm was 58min, including 28min heating time, 5min of heat preservation time and 25min cooling time. Compared with HP, SPS technology can reduce the sintering temperature of 100 ~ 200 ℃ [13]. The application of SPS in material preparation

At present, there are many researches on the preparation of new materials by SPS, especially in Japan, and some products have been put into production. The kinds of materials that SPS can process are shown in table 1. In addition to the preparation of materials, SPS can also be connected to materials such as MoSi2 and stone mill [14], ZrO2/ Cermet/Ni, etc [15]. In recent years, the research on the preparation of new materials by SPS is mainly focused on: ceramics, metal ceramics, intermetallic compounds, composite materials and functional materials. Among them, the most studied are functional materials, including thermoelectric materials [16], magnetic materials [17], functional gradient materials [18], composite functional materials [19] and nanometer functional materials [20]. The preparation of amorphous alloy, shape memory alloy [21], diamond and so on were also tried. Graded materials

The composition of functional gradient material (FGM) is gradient, and the sintering temperature of each layer is different, and the traditional sintering method is difficult to burn. Using CVD, PVD and other methods to prepare the gradient materials, the cost is high and it is difficult to realize industrialization. Using the step - shaped stone grinding mould, the temperature gradient can be generated due to different current density on the bottom and lower ends of the die. The gradient temperature field generated by SPS in the stone grinding mould can be used for a few minutes to sintering the composition of the components with different gradient materials. Currently, the gradient materials successfully prepared by SPS are: stainless steel/ZrO2; Ni/ZrO2; Al/high polymer; Al/plant fibre; PSZ/T and other gradient materials.

In self-propagating combustion synthesis (SHS), the electric field with larger activation effect and function, especially in the field before activation effect can make composite material can be successful synthesis, enlarged the composition range, and can control the phase composition, but get is porous material, need further processing to improve the density. Using similar to electric field activation of SPS SHS technology, for ceramics, composite materials and gradient materials synthesis and densification simultaneously, can get 65 nm nanocrystals, less than a SHS dense chemical sequence [22]. Using SPS to prepare large-size FGM, the larger FGM system prepared by SPS is ZrO2 (3Y)/stainless steel disc with a size of 100mm x 17mm [23]. With ordinary sintering and hot pressing WC powder, it is necessary to add additives, while SPS makes sintering pure WC possible. The vickers hardness (HV) and fracture toughness of the WC/Mo gradient materials prepared by SPS reached 24Gpa and 6Mpa·m1/2 respectively, greatly reducing the cracking [24] caused by thermal stress caused by the mismatch between WC and Mo thermal expansion.

Thermoelectric materials Due to the high reliability and pollution-free characteristics of hotspot conversion, the recent thermoelectric converter has aroused great interest and studied many thermoelectric conversion materials. It is found that in the study of the preparation functional materials of SPS, there are many researches on thermoelectric materials.

(1) one of the effective ways to improve the hotspot efficiency of thermoelectric materials is to improve the composition of thermoelectric materials. For example, the composition gradient of the beta FeSi2 is a more promising thermoelectric materials, can be used for thermoelectric conversion between 200 ~ 900 ℃. Beta FeSi2 is not toxic and has good antioxidant properties in the air and has high conductivity and thermal power. The higher the quality factor of the hot material (Z = alpha 2/k rho, where Z is the quality factor, alpha is the Seebeck coefficient, k is the thermal conductivity coefficient, and rho is the resistivity of the material), and the thermal power conversion efficiency is higher. The test shows that the beta FeSix(Si content variable) of the composition gradient prepared by SPS is greatly improved than the thermoelectric performance of beta FeSi2 [25]. Examples of this are Cu/Al2O3/Cu[26],MgFeSi2[27], beta Zn4Sb3[28], tungsten silicate []29] etc. (2) the traditional semiconductor materials used for thermoelectric refrigeration are not only poor in strength and durability, but also mainly used in single-phase growth method, which is long and cost high. In recent years, some factories in order to solve this problem, used for calcining semiconductor refrigerating materials, is to improve the mechanical strength and improve the material utilization rate, but the performance of the thermoelectric performance is far less than single crystal semiconductor, now USES the SPS production semiconductor refrigerating materials, preparation of a complete within a few minutes to semiconductor material, and the crystal growth is to over ten hours. The advantage of SPS to prepare semiconductor thermoelectric materials is that they can be directly processed into wafers, which do not require the cutting process of one-way growth method, which saves the materials and improves the production efficiency.

The performance of thermal pressure and cold pressure-sintered semiconductor is lower than that of crystal growth. Presently used in thermoelectric refrigeration semiconductor materials mainly of Bi, Sb, Se and Te, at present the highest Z value of 3.0 x 10 / K, and preparation of SPS thermoelectric semiconductor Z value has reached 2.9 ~ 2.9 x 10 / K, almost equal to the performance of single crystal semiconductor [30]. Table 2 is the comparison between SPS and other methods to produce BiTe materials. Ferroelectric materials SPS sintering ferroelectric PbTiO3 ceramics, under 900 ~ 1000 ℃ sintering 1 ~ 3 min, average grain size after sintering < 1 microns, the relative density of more than 98%. Since the hole in ceramics is less [31], the dielectric constant between 101-106hz is not changed with frequency. When the ferroelectric material Bi4Ti3O12 ceramics was prepared by SPS, the ceramic was rapidly densified while the sintering grain elongation and roughening. It is easy to get a good sample of grain intake with SPS, and there is a strong anisotropy of the electric properties of Bi4Ti3O12 ceramics with grain selection. The ferroelectric Li replacement IIVI semiconductor ZnO ceramics were prepared by SPS, which improved the Tc to 470K, while the previously cold-pressed ceramics had only 330K [34].

Magnetic materials With SPS sintering Nd Fe B magnetic alloy, if under the high temperature sintering, be able to get high density, but the sintering temperature is too high can lead to high temperature will lead to alpha phase and grain growth, magnetic can deteriorate. If sintering at a lower temperature can maintain a good magnetic energy, the powder cannot be compacted completely, so the relationship between density and performance should be studied in detail [35]. In sintering magnetic materials, SPS have the advantages of low sintering temperature and short thermal time. Nd Fe Co V B insulation 5 min under 650 ℃, can burn up close to the fully dense block magnet, found no grain growth [36]. 865 fe6si4al35ni preparation with SPS and MgFe2O4 composites (850 ℃, 850 mpa), with high saturation magnetization Bs = 12 t and high resistivity of rho = 1 x 10 Ω m. [37]. The soft magnetic alloy thin strip, which was prepared by rapid solidification method before, has reached the small grain structure of dozens of nanometers, but can not be prepared into alloy blocks, and the application is limited. Now, the magnetic properties of block magnetic alloy prepared by SPS have reached the soft magnetic properties of amorphous and nanocrystalline materials [3]. Nanometer material

The preparation of compact nanomaterials is becoming more and more important. It is difficult to ensure that the nanometer materials can meet the requirements of nanometer size grain and complete density by using the traditional methods such as hot pressing and sintering. Using SPS technology, the sintering time is short and the grain size can be significantly inhibited due to the fast heating speed. For example, TiN powder with an average particle size of 5 mu m was sintered by SPS (1963K, 196 ~ 382MPa, sintering 5min), and the TiN dense entity with an average grain of 65nm was obtained [3]. In the literature [3], some examples are given to show that the grain growth of SPS is restrained by the maximum limit, and the sintered body has no loose and obvious grain growth.

During SPS sintering, although the pressure is small, but in addition to the effect of pressure leads to the activation capacity Q reduce, due to the effect of discharge, also can make the grain get activation and make the Q value to further reduce, thus can promote grain growth, so from that point of view, with the SPS sintering preparation of nanometer materials has the certain difficulty. However, an example of a TiN dense entity with an average particle size of 65nm has been successfully prepared. In the literature [38], the Fe90Zr7B3 nanomagnetic material of Fe90Zr7B3 was prepared with SPS sintering. In addition, it has been found that grain size is relatively slow in the sintering temperature of SPS, so the mechanism of the preparation of nanomaterials and the effect on grain growth will be further studied.

A commonly used carbide component

(1) tungsten cobalt (WC + Co) cemented carbide (YG)

The main components are tungsten carbide (WC) and binder cobalt (Co). Its brand name is made up of "YG" (" hard, cobalt ") and an average percentage of cobalt. For example, the YG8, which means the average WCo = 8 percent, the rest is tungsten and cobalt hard alloys for tungsten carbide. It is composed of WC and Co, with high flexural strength, good thermal conductivity, but poor heat resistance and abrasion resistance, mainly used in processing of cast iron and nonferrous metals. Fine grain YG hard alloy (e.g. YG3X, YG6X), with the cobalt content, its hardness resistance ratio YG3, YG6, high strength and toughness, suitable for hard cast iron, austenitic stainless steel, heat resistant alloy, hard bronze, etc.

(2) tungsten and cobalt (WC +TiC+ Co) cemented carbide (YT)

The main components are tungsten carbide, titanium carbide (TiC) and cobalt. Its brand name is composed of "YT" (" hard, titanium ") and the average content of carbon titanium. For example, YT15, which means the average WTi = 15%, and the rest of the tungsten and cobalt alloys with tungsten carbide and cobalt content. Due to the hardness of TiC and melting point were higher than the WC, so compared with YG, the hardness, wear resistance and increase red hardness, bonding temperature, oxidation resistance is strong, and will generate TiO 2, at high temperatures may reduce bonding. But the thermal conductivity is poor, the bending strength is low, so it is suitable for processing Ductile materials such as steel.

(3) cobalt tungsten tantalum class (WC + TaC + Co)

On the basis of YG hard alloy, TaC(NbC) is added to improve the temperature, hardness and strength, thermal shock resistance and abrasion resistance Process cast iron and stainless steel. (4) tungsten titanium tantalum cobalt (WC +TiC+TaC+ Co)) hard alloy (YW) The main components are tungsten carbide, titanium carbide, tantalum carbide (or carbonized niobium) and cobalt. This kind of hard alloy is also called universal hard alloy or universal hard alloy Quality, and the alloy. Its brand name is made up of "YW" (" hard "and" ten thousand ") with serial number, such as YW1. Adding TaC(NbC) on the basis of YT - cemented carbide, improved flexural strength, impact toughness, high temperature hardness, anti-oxygen ability and wear resistance. It can process steel and process cast iron and nonferrous metals. Therefore, it is often referred to as universal hard alloy (also known as universal cemented carbide). eye It is mainly used for refractory materials such as heat resistant steel, high manganese steel and stainless steel.

(5) steel bonded hard alloy

Performance is between high speed steel and cemented carbide. It is one or several carbides (such as TiC and WC) For the hardening phase, powder metallurgy material is made of carbon steel or alloy steel (such as high speed steel, chromium molybdenum steel, etc.) as binder, after ingredients, mixing, pressing and sintering. After annealing, it can be machined. After quenching and tempering, it has the high hardness and high wear resistance of hard alloy. It can also be forged and welded, and has the properties of heat resistance, corrosion resistance and oxidation resistance. It is suitable for making all kinds of complicated cutting tools, such as hemp drill, milling cutter, etc., also can make the mold and wear-resisting parts that work at high temperature.