Self-propagating high-temperature synthesis (SHS) is a method for producing both inorganic and organic compounds by combustion-like exothermic reactions in solids of different nature.
A variant of this method is known as solid state metathesis (SSM). Since the process occurs at high temperatures, the method is ideally suited for the production of refractory materials with unusual properties, for example: powders, metallic alloys, or ceramics with high purity, corrosion–resistance at high–temperature or super-hardnessity.
In its usual format, SHS is conducted starting from finely powdered reactants that are intimately mixed. In some cases, the reagents are finely powdered whereas in other cases, they are sintered to minimize their surface area and prevent uninitiated exothermic reactions, which can be dangerous. In other cases, the particles are mechanically activated through techniques such as ball milling, which results in nanocomposite particles that contain both reactants within individual chemical cells. After reactant preparation, synthesis is initiated by point-heating of a small part (usually the top) of the sample. Once started, a wave of exothermic reaction sweeps through the remaining material. SHS has also been conducted with thin films, liquids, gases, powder–liquid systems, gas suspensions, layered systems, gas-gas systems, and others. Reactions have been conducted in a vacuum and under both inert or reactive gases. The temperature of the reaction can be moderated by the addition of inert salt that absorbs heat in the process of melting or evaporation, such as sodium chloride.
That extra energy (such as electric heating or laser) trigger the igniter combustion, causing the reaction material (solid phase and gas phase a solid – solid, liquid, and with equal) spontaneous reaction, heat release temminick high-speed, reaction by local automatically in the form of combustion wave spread the system, finally obtain a new technology of synthetic material.Powder directly stick each other reactants can be element or elements of powder and gas, also can use a mixture of metal oxides and reductant and non-metallic powder reaction generates heat to maintain ongoing, reaction products must be stable compounds. This technology has high product purity, simple equipment, low energy consumption, ‘the advantages of flexible production, fault is a strict process parameters are controlled.Hundreds of compounds have been prepared by this method, such as nitrides, carbides, borates, silicides, oxides, hydrides and so on of various metals.S} -is technology has developed into SH5 powder making technology, SHS sintering technology, 5H5 compaction technology, 5HS melting casting technology, 5H5 welding technology, SHS coating technology, etc.
Any combustion of a chemical substance as long as the result is the formation of a product or material of a practical condensed matter can be called a SHS process.In the SHS process, the substance involved in the reaction may be in a solid, liquid or gaseous state, but the final product is usually a solid state.
Basic requirements for combustion synthesis
1) the use of chemical reaction to exothermic, completely or partially without external heat source;
(2) the products of the required components and structures are obtained through the self-sustaining reaction of rapid automatic wave combustion;
(3) control the speed, temperature, conversion rate and composition and structure of products by changing the release and transmission speed of heat.
The products prepared by SHS technology have high purity, low energy consumption and simple process. The non-equilibrium, non-stoichiometric ratio and functional gradient materials can be prepared by SHS technology.
Its features are
- Gamma is a fast synthesis process;
- Carbon dioxide has energy-saving effect.
- Torque can improve the purity of synthetic materials.
- Waste products tend to form porous tissue;
- The microstructure of char combustion products has greater dispersion.
Therefore, it is the key to ensure the industrialization of SHS to explore the combustion and synthesis rules of various SHS systems and obtain uniform tissue.
SHS powder synthesis involves a process similar to powder metallurgy.But there are differences between the two, the main difference in the synthesis process.
The process flow of HS powder synthesis technology is shown as follows
What is centrifugal casting
Centrifugal casting is the technology and method of injecting liquid metal into the high-speed rotating casting mold and filling the mold and forming casting with metal liquid under the action of centrifugal force.Centrifugal force enables the liquid metal to fill the mold well in the radial direction and form the free surface of the casting.Cylinder inner hole can be obtained without core.Helps to remove gases and inclusions from liquid metals.It affects the crystallization process of metal, so as to improve the mechanical and physical properties of casting.According to the spatial position of the mold axis, the common centrifugal casting can be divided into horizontal centrifugal casting and vertical centrifugal casting.Mould rotation axis in the level of state or with small Angle (4 °) horizontal centrifugal casting is called horizontal centrifugal casting.Vertical centrifugal casting is called vertical centrifugal casting when the rotating axis of the casting is in a vertical state.Centrifugal casting with large Angle between the axis of rotation and horizontal and vertical lines is called inclined axis centrifugal casting, but it is seldom used.
Centrifugal casting was first used in the production of cast pipes, and the process was developed rapidly.Centrifugal casting technology is adopted in metallurgy, mining, transportation, drainage and irrigation machinery, aviation, national defense, automobile and other industries at home and abroad to produce steel, iron and non-iron carbon alloy castings.Among them, the production of centrifugal cast iron pipe, cylinder sleeve and shaft sleeve of internal combustion engine is the most common.For some forming tool and gear castings, centrifugal pouring can also be applied to the molten model shell, which can not only improve the precision of casting, but also improve the mechanical properties of casting.
Advantages of centrifugal casting
1) there is almost no metal consumption in the pouring system and the riser system, so as to improve the production rate of the process;
2) the production of hollow castings without core, when long tubular castings in production can greatly improve the metal filling ability, reduce casting wall thickness of the ratio of length and diameter, simplify the production process of the sleeve and the pipe casting parts;
3) high casting density, less defects such as porosity and slag inclusion, and high mechanical performance;
4) it is convenient to manufacture composite metal castings of tube and sleeve, such as copper sleeve of steel back and double metal roll.When casting is formed, the filling capacity of metal can be improved by centrifugal force, so thin-walled casting can be produced.
Disadvantages of centrifugal casting
There are some limitations in the production of heteromorphic castings.
Inner hole diameter is not accurate, the surface of inner hole is rough, the quality is poor, and the machining allowance is large.
As a result, it is not suitable for alloy castings (such as lead bronze) which are prone to producing specific gravity segregation. In particular, it is not suitable for alloys whose impurity proportion is greater than that of liquid metal.
The combustion temperature of many high exothermic SHS systems exceeds the melting point of combustion products.This SHS system is combined with the conventional metallurgical method to produce SHS technology, which USES SHS method to obtain the melt, and USES conventional metallurgical method to treat the melt [14,15].SHS metallurgy includes SHS casting and SHS- centrifugal technology.Aluminum thermal reaction is widely used in SHS metallurgy because of its high exothermic temperature.
The chemical reaction formula is:
In many oil chemical, electric power and metallurgy industry, the service life of steel pipe become one of the most concern of the people, however, because of the large steel pipe diameter is small, the length, with other anticorrosive processing methods can be difficult to achieve, and using this technology can be easily solved.It is a high exothermic chemical reaction between aluminum, magnesium, silicon, zirconium and other powder and metal oxides.The whole system is molten because the reaction temperature exceeds the melting point of ceramics and metals.Under the action of centrifugal force, the melt is stratified according to the density, and the high-density components are combined with the steel tube matrix. The low-density ceramic components are coated on the inner wall of the steel tube to form the ceramic coating.At present, the production technology of coating lining steel pipe has been quite mature.
More straight steel tube using centrifugation is possible, if is bend or other irregular shaped steel tube is still using centrifugation is obviously not feasible, through the efforts of the engineering technical researchers, using the principle of gravity makes the melt in the process of SHS coating to the steel tube wall.High temperature because of the thermite reaction makes the reactants in the molten state, in the steel tube on the reaction of material formed by metal Fe and ceramic two phase composition of the melt pool, due to the density of Fe is greater than the density of the coating phase, under the effect of gravity, the separation of two melts, Fe deposit in the bottom of the pool, the molten coating float to the top of the molten pool.As the self-propagating reaction progresses, the liquid level drops gradually, causing the liquid phase and ceramic liquid phase of Fe to attach to the inner wall of the steel tube and crystallize and solidify successively, thus forming a continuous and uniform coating on the inner wall of the steel tube.
The principle is shown in the figure
Centrifugal casting process
Metal filtration: in some alloy fluids, there are more residuals that are difficult to be removed. A variety of filters can be placed in the casting system to remove residuals, such as foam ceramic filters and glass wire filters.
Pouring temperature: most of centrifugal casting for tubular, set, ring parts, liquid metal filling encountered resistance is small, and the centrifugal pressure or centrifugal force to strengthen the liquid metal filling ability, so the centrifugal casting when pouring temperature is gravity casting low 5 ~ 10 ° C.