This type of composite materials includes materials such as SAP (sintered aluminum powder), which are aluminum reinforced with dispersed particles of aluminum oxide. Aluminum powder is obtained by spraying molten metal, followed by grinding in ball mills to a size of about 1 micron in the presence of oxygen. With increasing grinding time, the powder becomes finer and its aluminum oxide content increases. Further technology for the production of products and semi-finished products from SAP includes cold pressing, pre-sintering, hot pressing, rolling or extruding a sintered aluminum billet into a mold finished products, which can be subjected to additional heat treatment.
Alloys of the SAP type are used in aircraft engineering for the manufacture of parts with high specific strength and corrosion resistance, operating at temperatures up to 300 - 500 °C. They are used to make piston rods, compressor blades, shells of fuel elements and heat exchanger pipes.
Reinforcement of aluminum and its alloys with steel wire increases their strength, increases the elastic modulus, fatigue resistance and expands the temperature range of the material's service life.
Reinforcement with short fibers is carried out using powder metallurgy methods, consisting of pressing followed by hydroextrusion or rolling of blanks. When reinforcing sandwich-type compositions consisting of alternating layers of aluminum foil and fibers with continuous fibers, rolling, hot pressing, explosion welding, and diffusion welding are used.
A very promising material is the aluminum-beryllium wire composition, which realizes the high physical and mechanical properties of beryllium reinforcement and, first of all, its low density and high specific rigidity. Compositions with beryllium wire are obtained by diffusion welding of packages of alternating layers of beryllium wire and matrix sheets. Aluminum alloys reinforced with steel and beryllium wires are used to make rocket body parts and fuel tanks.
In the “aluminum - carbon fiber” composition, the combination of low density reinforcement and matrix makes it possible to create composite materials with high specific strength and rigidity. The disadvantage of carbon fibers is their fragility and high reactivity. The aluminum-carbon composition is obtained by impregnating carbon fibers with liquid metal or using powder metallurgy methods. Technologically, the easiest way to do this is to draw bundles of carbon fibers through molten aluminum.
Aluminum-carbon composites are used in the fuel tank structures of modern fighter aircraft. Due to the high specific strength and rigidity of the material, the weight of fuel tanks is reduced by 30%. This material is also used for the manufacture of turbine blades for aircraft gas turbine engines.
Composite materials with non-metallic matrix
Composite materials with a non-metallic matrix have found wide application in industry. Polymer, carbon and ceramic materials are used as non-metallic matrices. The most widely used polymer matrices are epoxy, phenol-formaldehyde, and polyamide. Coal matrices are coked or obtained from synthetic polymers subjected to pyrolysis (decomposition, disintegration). The matrix binds the composition, giving it shape. Strengtheners are fibers: glass, carbon, boron, organic, based on whisker crystals (oxides, carbides, borides, nitrides, etc.), as well as metal (wires), which have high strength and toughness.
The properties of composite materials depend on the composition of the components, their combination, quantitative ratio and strength of the bond between them.
The hardener content in oriented materials is 60 - 80 vol. %, in non-oriented (with discrete fibers and whiskers) - 20 - 30 vol. %. The higher the strength and elastic modulus of the fibers, the higher the strength and stiffness of the composite material. The properties of the matrix determine the shear and compressive strength of the composition and the resistance to fatigue failure.
Based on the type of reinforcement, composite materials are classified into glass fibers, carbon fibers with carbon fibers, boron fibers and organofibers.
In layered materials, fibers, threads, tapes impregnated with a binder are laid parallel to each other in the laying plane. Planar layers are assembled into plates. The properties are anisotropic. For the material to work in a product, it is important to take into account the direction of the acting loads. You can create materials with both isotropic and anisotropic properties. Fibers can be laid at different angles, varying the properties of the composite materials. The flexural and torsional rigidities of the material depend on the order in which the layers are laid across the thickness of the package.
Reinforcers of three, four or more threads are used (Fig. 7). The most widely used structure is a structure of three mutually perpendicular threads. Reinforcers can be located in the axial, radial and circumferential directions.
Three-dimensional materials can be of any thickness in the form of blocks or cylinders. Bulky fabrics increase peel strength and shear strength compared to laminated fabrics. A system of four threads is constructed by placing the reinforcement along the diagonals of the cube. The structure of four threads is equilibrium and has increased shear rigidity in the main planes. However, creating four directional materials is more difficult than creating three directional materials.
Rice. 7. Scheme of reinforcement of composite materials: 1- rectangular, 2- hexagonal, 3- oblique, 4- with curved fibers, 5 – system of n threads
The most effective from the point of view of use in the most severe conditions of dry friction are antifriction materials based on polytetrafluoroethylene (PTFE).
PTFE is characterized by a fairly high static coefficient of friction, however, during sliding friction, a very thin layer of highly oriented polymer is formed on the surface of PTFE, which helps to equalize the static and dynamic coefficients of friction and smooth movement when sliding. When the sliding direction changes, the presence of an oriented surface film causes a temporary increase in the friction coefficient, the value of which again decreases as the surface layer is reoriented. This frictional behavior of PTFE has led to its widespread use in industry, where unfilled PTFE is mainly used for the production of bearings. In many cases, non-lubricated bearings must operate at higher friction rates. At the same time, unfilled PTFE is characterized by high values of the friction coefficient and wear rate. Composite materials, most often based on PTFE, have found widespread use as materials for non-lubricated bearings operating in such conditions.
The simplest way to reduce the relatively high wear rate of PTFE during dry friction is to introduce powdered fillers. At the same time, creep resistance during compression increases and a significant increase in wear resistance during dry friction is observed. The introduction of the optimal amount of filler makes it possible to increase wear resistance up to 10 4 times.
Polymers and composite materials based on them have a unique set of physical and mechanical properties, thanks to which they successfully compete with traditional structural steels and alloys, and in some cases, without the use of polymer materials it is impossible to ensure the required functional characteristics and performance of special products and machines. The high manufacturability and low energy intensity of technologies for processing plastics into products, combined with the above-mentioned advantages of PCM, make them very promising materials for machine parts for various purposes.
Composite materials with a metal matrix. To operate at higher temperatures, metal matrices are used.
Metal CMs have a number of advantages over polymer ones. In addition to a higher operating temperature, they are characterized by better isotropy and greater stability of properties during operation, and higher erosion resistance.
The plasticity of metal matrices imparts the required viscosity to the structure. This contributes to the rapid equalization of local mechanical loads.
An important advantage of metal CMs is the higher manufacturability of the manufacturing process, molding, heat treatment, and the formation of joints and coatings.
The advantage of metal-based composite materials is higher values of characteristics depending on the properties of the matrix. These are, first of all, temporary resistance and tensile modulus of elasticity in the direction perpendicular to the axis of the reinforcing fibers, compressive and bending strength, ductility, and fracture toughness. In addition, composite materials with a metal matrix retain their strength characteristics to higher temperatures than materials with a non-metallic matrix. They are more moisture-resistant, non-flammable, and have electrical conductivity. The high electrical conductivity of metal CMs protects them well from electromagnetic radiation, lightning, and reduces the danger of static electricity. The high thermal conductivity of metal CMs protects against local overheating, which is especially important for products such as rocket tips and wing leading edges.
The most promising materials for matrices of metal composite materials are metals with low density (A1, Mg, Ti) and alloys based on them, as well as nickel, which is currently widely used as the main component of heat-resistant alloys.
Composites are obtained by different methods. These include impregnation of a bundle of fibers with liquid melts of aluminum and magnesium, plasma spraying, and the use of hot pressing methods, sometimes followed by hydroextrusion or rolling of blanks. When reinforcing sandwich-type compositions consisting of alternating layers of aluminum foil and fibers with continuous fibers, rolling, hot pressing, explosion welding, and diffusion welding are used. Casting of rods and pipes reinforced with high-strength fibers is obtained from the liquid metal phase. The bundle of fibers is continuously passed through a melt bath and impregnated under pressure with liquid aluminum or magnesium. When leaving the impregnation bath, the fibers are combined and passed through a spinneret to form a rod or tube. This method ensures maximum filling of the composite with fibers (up to 85%), their uniform distribution in the cross section and continuity of the process.
Aluminum matrix materials. Aluminum matrix materials are mainly reinforced with steel wire (SWI), boron fiber (BFA) and carbon fiber (CF). Both technical aluminum (for example, AD1) and alloys (AMg6, V95, D20, etc.) are used as a matrix.
The use of an alloy (for example, B95) as a matrix, strengthened by heat treatment (hardening and aging), gives an additional effect of strengthening the composition. However, in the direction of the fiber axis it is small, while in the transverse direction, where the properties are determined mainly by the properties of the matrix, it reaches 50%.
The cheapest, most effective and accessible reinforcing material is high-strength steel wire. Thus, reinforcing technical aluminum with VNS9 steel wire with a diameter of 0.15 mm (σ in = 3600 MPa) increases its strength by 10-12 times with a fiber volume content of 25% and by 14-15 times with an increase in content to 40%, after which temporary resistance reaches 1000-1200 and 1450 MPa, respectively. If you use wire of a smaller diameter for reinforcement, i.e., greater strength (σ in = 4200 MPa), the temporary resistance of the composite material will increase to 1750 MPa. Thus, aluminum reinforced with steel wire (25-40%) in its basic properties significantly exceeds even high-strength aluminum alloys and reaches the level of the corresponding properties of titanium alloys. In this case, the density of the compositions is in the range of 3900-4800 kg/m 3 .
Strengthening aluminum and its alloys with more expensive fibers B, C, A1 2 O e increases the cost of composite materials, but at the same time some properties are improved more effectively: for example, when reinforced with boron fibers, the elastic modulus increases 3-4 times, carbon fibers help reduce density. Boron softens little with increasing temperature, so compositions reinforced with boron fibers retain high strength up to 400-500 ° C. A material containing 50 vol.% of continuous high-strength and high-modulus boron fibers (VKA-1) has found industrial application. In terms of elastic modulus and temporary resistance in the temperature range of 20-500°C, it surpasses all standard aluminum alloys, including high-strength (B95), and alloys specially designed for operation at high temperatures (AK4-1), which is clearly presented in Fig. 13.35. The high damping capacity of the material ensures the vibration resistance of structures made from it. The density of the alloy is 2650 kg/m 3, and the specific strength is 45 km. This is significantly higher than high-strength steels and titanium alloys.
Calculations have shown that replacing the B95 alloy with a titanium alloy in the manufacture of an aircraft wing spar with reinforcement elements from VKA-1 increases its rigidity by 45% and provides a weight saving of about 42%.
Aluminum-based carbon fiber-reinforced composites (CFRPs) are cheaper and lighter than boron fiber composites. And although they are inferior to the latter in strength, they have a similar specific strength (42 km). However, the production of composite materials with a carbon strengthener is associated with great technological difficulties due to the interaction of carbon with metal matrices when heated, causing a decrease in the strength of the material. To eliminate this drawback, special carbon fiber coatings are used.
Materials with magnesium matrix. Materials with a magnesium matrix (MCM) are characterized by a lower density (1800-2200 kg/m3) than with aluminum, with approximately the same high strength of 1000-1200 MPa and therefore higher specific strength. Deformable magnesium alloys (MA2 and others), reinforced with boron fiber (50 vol.%), have a specific strength > 50 km. The good compatibility of magnesium and its alloys with boron fiber, on the one hand, makes it possible to manufacture parts using the impregnation method with virtually no subsequent mechanical processing, and on the other hand, it ensures a long service life of parts at elevated temperatures. The specific strength of these materials is increased by the use of light lithium alloyed alloys as a matrix, as well as by the use of lighter carbon fiber. But, as stated earlier, the introduction of carbon fiber complicates the technology of already low-tech alloys. As is known, magnesium and its alloys have low technological plasticity and a tendency to form a loose oxide film.
Titanium-based composite materials. When creating titanium-based composite materials, difficulties arise due to the need to heat to high temperatures. At high temperatures, the titanium matrix becomes very active; it acquires the ability to absorb gases and interact with many strengthening agents: boron, silicon carbide, aluminum oxide, etc. As a result, reaction zones are formed and the strength of both the fibers themselves and composite materials as a whole is reduced. And, in addition, high temperatures lead to recrystallization and softening of many reinforcing materials, which reduces the strengthening effect of reinforcement. Therefore, to strengthen materials with a titanium matrix, wire made of beryllium and ceramic fibers of refractory oxides (Al 2 0 3), carbides (SiC), as well as refractory metals with a high elastic modulus and high recrystallization temperature (Mo, W) are used. Moreover, the purpose of reinforcement is mainly not to increase the already high specific strength, but to increase the elastic modulus and increase operating temperatures. Mechanical properties of titanium alloy VT6 (6% A1, 4% V, the rest A1), reinforced with Mo, Be and SiC fibers, are presented in table. 13.9. As can be seen from. Table, the specific stiffness increases most effectively when reinforced with silicon carbide fibers.
Reinforcement of the VT6 alloy with molybdenum wire helps maintain high elastic modulus values up to 800 "C. Its value at this temperature corresponds to 124 GPa, i.e., decreases by 33%, while the temporary tensile strength decreases to 420 MPa, i.e. more than 3 times.
Nickel-based composite materials. Heat-resistant CMs are made on the basis of nickel and cobalt alloys, strengthened with ceramic (SiC, Si 3 Ni 4, Al 2 O 3) and carbon fibers. The main task in creating nickel-based composite materials (NBC) is to increase operating temperatures above 1000 °C. And one of the best metal reinforcements that can provide good strength at such high temperatures is tungsten wire. The introduction of tungsten wire in an amount from 40 to 70 vol.% into a nickel-chromium alloy provides strength at 1100°C for 100 hours, respectively, 130 and 250 MPa, while the best unreinforced nickel alloy, designed for work in similar conditions, has a strength of 75 MPa. The use of wire made from tungsten alloys with rhenium or hafnium for reinforcement increases this figure by 30-50%.
Composite materials are used in many industries and primarily in aviation, rocket and space technology, where reducing the weight of structures while increasing strength and rigidity is especially important. Due to their high specific strength and rigidity characteristics, they are used in the manufacture, for example, of horizontal stabilizers and flaps of aircraft, rotor blades and containers of helicopters, bodies and combustion chambers of jet engines, etc. The use of composite materials in aircraft structures has reduced their weight by 30-40% , increased the payload without reducing speed and range.
Currently, composite materials are used in power turbine construction (turbine working and nozzle blades), automotive industry (car and refrigerator bodies, engine parts), mechanical engineering (machine bodies and parts), chemical industry (autoclaves, tanks, containers), shipbuilding (hulls of boats, boats, propellers), etc.
The special properties of composite materials make it possible to use them as electrical insulating materials (organic fibers), radio-transparent fairings (fiberglass), plain bearings (carbon fibers) and other parts.
Composite materials with ceramic matrix. For the highest operating temperatures, ceramics are used as a matrix material. Silicate (SiO 2), aluminosilicate (Al 2 O 3 - SiO 2), aluminoborosilicate (Al 2 O 3 - B 2 O 3 - SiO 2) materials, refractory aluminum oxides (Al 2 O 3), zirconium are used as ceramic matrices (ZrO 2), beryllium (BeO), silicon nitride (Si 3 N 4), titanium borides (TiB 2) and zirconium (ZrB 2), silicon carbides (SiC) and titanium (TiC). Composites with a ceramic matrix have high melting points, resistance to oxidation, thermal shock and vibration, and compressive strength. Ceramic CMs based on carbides and oxides with metal powder additives (< 50об. %) называются cermets . In addition to powders, metal wire made of tungsten, molybdenum, niobium, heat-resistant steel, as well as non-metallic fibers (ceramic and carbon) are used to reinforce ceramic CMs. The use of metal wire creates a plastic frame that protects the CM from destruction when the fragile ceramic matrix cracks. The disadvantage of ceramic CMs reinforced with metal fibers is their low heat resistance. CMs with a matrix of refractory oxides (can be used up to 1000°C), borides and nitrides (up to 2000°C), and carbides (over 2000°C) have high heat resistance. When ceramic CMs are reinforced with silicon carbide fibers, a high bond strength between them and the matrix is achieved, combined with resistance to oxidation at high temperatures, which allows them to be used for the manufacture of heavily loaded parts (high-temperature bearings, seals, working blades of gas turbine engines, etc.). The main disadvantage of ceramics - the lack of ductility - is to some extent compensated for by reinforcing fibers that inhibit the propagation of cracks in ceramics.
Carbon-carbon composite . The use of amorphous carbon as a matrix material, and crystalline carbon (graphite) fibers as a reinforcing material, made it possible to create a composite that can withstand heating up to 2500 °C. Such a carbon-carbon composite is promising for astronautics and transatmospheric aviation. The disadvantage of the carbon matrix is possible oxidation and ablation. To prevent these phenomena, the composite is coated with a thin layer of silicon carbide.
Carbon matrix similar to physical and chemical properties carbon fiber, provides heat resistance to CCCM
The most widely used methods for producing carbon-carbon composites are:
1. carbonization of the polymer matrix of a preformed carbon fiber preform by high-temperature heat treatment in a non-oxidizing environment;
2. deposition from the gas phase of pyrolytic carbon, formed during the thermal decomposition of hydrocarbons in the pores of the carbon fiber substrate.
Both of these methods have their advantages and disadvantages. When creating the UKCM they are often combined to give the composite the necessary properties.
Carbonization of the polymer matrix. The carbonization process is the heat treatment of a carbon fiber product to a temperature of 1073 K in a non-oxidizing environment (inert gas, coal bed, etc.). The purpose of heat treatment is to convert the binder into coke. During the carbonization process, thermal destruction of the matrix occurs, accompanied by mass loss, shrinkage, the formation of a large number of pores and, as a result, a decrease in the physical and mechanical properties of the composite.
Carbonization is most often carried out in resistance retort furnaces. A retort made of a heat-resistant alloy protects the product from oxidation by atmospheric oxygen, and the heating elements and insulation from contact with volatile corrosive products of binder pyrolysis and ensures uniform heating of the furnace reaction volume.
The mechanism and kinetics of carbonization are determined by the ratio of the rates of dissociation of chemical bonds and recombination of the resulting radicals. The process is accompanied by the removal of evaporating resinous compounds and gaseous products and the formation of solid coke, enriched with carbon atoms. Therefore, in the carbonization process, the key point is the choice of temperature and time conditions, which should ensure maximum formation of coke residue from the binder, since the mechanical strength of the carbonized composite depends, among other things, on the amount of coke formed.
The larger the dimensions of the product, the longer the carbonization process should be. The rate of temperature rise during carbonization is from several degrees to several tens of degrees per hour, the duration of the carbonization process is 300 hours or more. Carbonization usually ends in the temperature range of 1073-1773 K, corresponding to the temperature range of the transition of carbon to graphite.
The properties of CCCM largely depend on the type of initial binder, which is synthetic organic resins that produce a high coke residue. Most often, phenol-formaldehyde resins are used for this purpose due to their manufacturability, low cost availability, and the coke formed in this process is highly durable.
Phenol-formaldehyde resins have certain disadvantages. Due to the polycondensation nature of their curing and the release of volatile compounds, it is difficult to obtain a homogeneous dense structure. The amount of shrinkage during carbonization of phenol-formaldehyde binders is greater than for other types of binders used in the production of CCCM, which leads to the occurrence of internal stresses in the carbonized composite and a decrease in its physical and mechanical properties.
Furan binders produce denser coke. Their shrinkage during carbonization is less, and the strength of coke is higher than that of phenol-formaldehyde resins. Therefore, despite the more complex curing cycle, binders based on furfural, furfurylidene acetone, and furyl alcohol are also used in the production of CCCM.
Coal and petroleum pitches are very promising for obtaining a carbon matrix due to their high carbon content (up to 92-95%) and high coke number. The advantages of pitches over other binders are availability and low cost, exclusion of solvent from technological process, good graphitability of coke and its high density. The disadvantages of pitches include the formation of significant porosity, deformation of the product, and the presence of carcinogenic compounds in their composition, which requires additional safety measures.
Due to the release of volatile compounds during thermal degradation of the resin, significant porosity appears in carbonized plastic, which reduces the physical and mechanical properties of CCCM. Therefore, the carbonization stage of carbon fiber completes the process of obtaining only porous materials that do not require high strength, for example, low-density CCCM for thermal insulation purposes. Typically, to eliminate porosity and increase density, the carbonized material is again impregnated with a binder and carbonized (this cycle can be repeated several times). Repeated impregnation is carried out in autoclaves in the “vacuum-pressure” mode, i.e., first the workpiece is heated in a vacuum, after which a binder is supplied and an excess pressure of up to 0.6-1.0 MPa is created. During impregnation, solutions and melts of binders are used, and the porosity of the composite decreases with each cycle, so it is necessary to use binders with reduced viscosity. The degree of compaction during re-impregnation depends on the type of binder, coke number, porosity of the product and the degree of pore filling. As the density increases during repeated impregnation, the strength of the material also increases. Using this method, it is possible to obtain CCCM with a density of up to 1800 kg/m 3 and higher. The method of carbonization of carbon fiber is relatively simple, it does not require complex equipment, and ensures good reproducibility of the material properties of the resulting products. However, the need for repeated compaction operations significantly lengthens and increases the cost of obtaining products from CCCM, which is a serious drawback of this method.
Upon receipt of the UKCM by method of deposition of pyrolytic carbon from the gas phase hydrocarbon gas (methane, benzene, acetylene, etc.) or a mixture of hydrocarbon and diluent gas (inert gas or hydrogen) diffuses through the carbon fiber porous frame, where, under the influence of high temperature, decomposition of the hydrocarbon occurs on the heated surface of the fiber. The precipitating pyrocarbon gradually creates connecting bridges between the fibers. The kinetics of deposition and the structure of the resulting pyrolytic carbon depend on many factors: temperature, gas flow rate, pressure, reaction volume, etc. The properties of the resulting composites are also determined by the type and content of the fiber, and the reinforcement scheme.
The deposition process is carried out in vacuum or under pressure in induction furnaces, as well as in resistance furnaces.
Several technological methods for producing pyrolytic carbon matrix have been developed.
With the isothermal method the workpiece is located in a uniformly heated chamber. Uniform heating in an induction furnace is ensured with the help of a fuel-generating element - a susceptor made of graphite. Hydrocarbon gas is supplied through the bottom of the furnace and diffuses through the reaction volume and the workpiece; gaseous reaction products are removed through an outlet in the furnace lid.
The process is usually carried out at a temperature of 1173-1423 K and a pressure of 130-2000 kPa. A decrease in temperature leads to a decrease in the deposition rate and an excessive lengthening of the process duration. An increase in temperature accelerates the deposition of pyrolytic carbon, but the gas does not have time to diffuse into the volume of the workpiece and surface layering of pyrolytic carbon occurs. The process takes hundreds of hours.
The isothermal method is usually used for the manufacture of thin-walled parts, since in this case the pores located near the surface of the product are filled predominantly.
It is used for volumetric saturation of pores and production of thick-walled products. non-isothermal method, which consists in creating a temperature gradient in the workpiece by placing it on a heated mandrel or core or by directly heating it with current. Hydrocarbon gas is supplied from the side having a lower temperature. The pressure in the furnace is usually equal to atmospheric pressure. As a result, the deposition of pyrolytic carbon occurs in the hottest zone. The cooling effect of gas flowing over a surface at high speed is the primary way to achieve a temperature gradient.
An increase in the density and thermal conductivity of the composite leads to a movement of the deposition temperature front, which ultimately ensures volumetric compaction of the material and the production of products with high density (1700-1800 kg/m3).
The isothermal method for producing CCCM with a pyrocarbon matrix is characterized by the following advantages: good reproducibility of properties; simplicity of technical design; high density and good graphitability of the matrix; the ability to process several products simultaneously.
The disadvantages include: low deposition rate; surface deposition of pyrolytic carbon; poor filling of large pores.
The non-isothermal method has the following advantages: high deposition rate; possibility of filling large pores; volumetric seal of the product.
Its disadvantages are as follows: complex hardware design; only one product is processed; insufficient density and graphitability of the matrix; formation of microcracks.
3.4.4. High-temperature heat treatment (graphitization) of CCCM. The structure of carbonized plastics and composites with a pyrocarbon matrix after compaction from the gas phase is imperfect. The interlayer distance d002, which characterizes the degree of ordering of the carbon matrix, is relatively large - over 3.44·10 4 μm, and the crystal sizes are relatively small - usually no more than 5·10 -3 μm, which is typical for two-dimensional ordering of the basic layers of carbon. In addition, during the production process, internal stresses may arise in them, which can lead to deformations and distortions of the structure of the product when these materials are used at temperatures above the temperature of carbonization or deposition of pyrolytic carbon. Therefore, if it is necessary to obtain a more thermally stable material, it is subjected to high-temperature treatment. The final heat treatment temperature is determined by operating conditions, but is limited by the sublimation of the material, which occurs intensively at temperatures above 3273 K. Heat treatment is carried out in induction furnaces or resistance furnaces in a non-oxidizing environment (graphite backfill, vacuum, inert gas). Changing Properties carbon-carbon materials in the process of high-temperature heat treatment is determined by many factors: the type of filler and matrix, the final temperature and duration of heat treatment, the type of medium and its pressure, and other factors. At high temperatures, energy barriers in the carbon material are overcome, preventing the movement of multinuclear compounds, their attachment and mutual reorientation with a greater degree of compaction.
The duration of these processes is short and the degree of conversion is determined mainly by temperature. Therefore, the duration of high-temperature heat treatment processes is much shorter than in the case of carbonization or deposition of pyrocarbon, and usually amounts to several hours. During high-temperature heat treatment of carbonized plastics, irreversible deformations of the product and gradual “healing” of defects occur. For well-graphitized materials based on pitches at temperatures above 2473 K, intensive growth of three-dimensionally ordered carbon crystallites is observed up to the transition to a graphitic structure. At the same time, in carbonized plastics based on poorly graphitized polymer binders, structural defects persist up to 3273 K and the material remains in a non-graphitized structural form.
This type of composite materials includes materials such as SAP (sintered aluminum powder), which are aluminum reinforced with dispersed particles of aluminum oxide. Aluminum powder is obtained by spraying molten metal, followed by grinding in ball mills to a size of about 1 micron in the presence of oxygen. With increasing grinding time, the powder becomes finer and its aluminum oxide content increases. Further technology for the production of products and semi-finished products from SAP includes cold pressing, pre-sintering, hot pressing, rolling or extrusion of a sintered aluminum billet in the form of finished products that can be subjected to additional heat treatment.
Alloys of the SAP type are used in aviation technology for the manufacture of parts with high specific strength and corrosion resistance, operating at temperatures up to 300–500 °C. They are used to make piston rods, compressor blades, shells of fuel elements and heat exchanger pipes.
Reinforcement of aluminum and its alloys with steel wire increases their strength, increases the elastic modulus, fatigue resistance and expands the temperature range of the material's service life.
Reinforcement with short fibers is carried out using powder metallurgy methods, consisting of pressing followed by hydroextrusion or rolling of blanks. When reinforcing sandwich-type compositions consisting of alternating layers of aluminum foil and fibers with continuous fibers, rolling, hot pressing, explosion welding, and diffusion welding are used.
A very promising material is the composition “aluminum - beryllium wire”, which realizes the high physical and mechanical properties of beryllium reinforcement, and first of all, its low density and high specific rigidity. Compositions with beryllium wire are obtained by diffusion welding of packages of alternating layers of beryllium wire and matrix sheets. Aluminum alloys reinforced with steel and beryllium wires are used to make rocket body parts and fuel tanks.
In the aluminum-carbon fiber composition, the combination of low density reinforcement and matrix makes it possible to create composite materials with high specific strength and rigidity. The disadvantage of carbon fibers is their fragility and high reactivity. The aluminum-carbon composition is obtained by impregnating carbon fibers with liquid metal or powder metallurgy methods. Technologically, the easiest way to do this is to draw bundles of carbon fibers through molten aluminum.
The aluminum-carbon composite is used in the construction of fuel tanks of modern fighter aircraft. Due to the high specific strength and rigidity of the material, the weight of fuel tanks is reduced by
thirty %. This material is also used for the manufacture of turbine blades for aircraft gas turbine engines.
GENERAL CHARACTERISTICS AND CLASSIFICATION
Traditionally used metallic and non-metallic materials have largely reached their structural strength limits. At the same time, the development of modern technology requires the creation of materials that work reliably in a complex combination of force and temperature fields, when exposed to aggressive environments, radiation, high vacuum and high pressures. Often, the requirements for materials can be contradictory. This problem can be solved by using composite materials.
Composite material(CM) or composite is a three-dimensional heterogeneous system consisting of mutually insoluble components that differ greatly in properties, the structure of which allows one to take advantage of the advantages of each of them.
Man borrowed the principle of constructing CM from nature. Typical composite materials are tree trunks, plant stems, human and animal bones.
CMs allow you to have a given combination of heterogeneous properties: high specific strength and rigidity, heat resistance, wear resistance, heat-shielding properties, etc. The range of properties of CMs cannot be obtained using conventional materials. Their use makes it possible to create previously inaccessible, fundamentally new designs.
Thanks to CM, a new qualitative leap has become possible in increasing engine power, reducing the weight of machines and structures, and increasing the weight efficiency of vehicles and aerospace vehicles.
Important characteristics of materials operating under these conditions are specific strength σ in /ρ and specific stiffness E/ρ, where σ in is the temporary resistance, E- modulus of normal elasticity, ρ – density of the material.
High-strength alloys, as a rule, have low ductility, high sensitivity to stress concentrators, and relatively low resistance to the development of fatigue cracks. Although composite materials may also have low ductility, they are much less sensitive to stress raisers and are better resistant to fatigue failure. This is explained by different mechanisms of crack formation in high-strength steels and alloys. In high-strength steels, a crack, having reached a critical size, subsequently develops at a progressive rate.
A different mechanism operates in composite materials. A crack, moving in the matrix, encounters an obstacle at the matrix-fiber interface. Fibers inhibit the development of cracks, and their presence in the plastic matrix leads to an increase in fracture toughness.
Thus, the composite system combines two opposing properties necessary for structural materials - high strength due to high-strength fibers and sufficient fracture toughness due to the plastic matrix and the mechanism of dissipation of fracture energy.
CMs consist of a relatively plastic matrix base material and harder and more durable components, which are fillers. The properties of CM depend on the properties of the base, fillers and the strength of the bond between them.
The matrix binds the composition into a monolith, gives it shape and serves to transfer external loads to the filler reinforcement. Depending on the base material, CMs are distinguished with a metal matrix, or metal composite materials (MCM), with polymer - polymer composite materials (PCM) and with ceramic - ceramic composite materials (CCM).
The leading role in strengthening CMs is played by fillers, often called strengtheners. They have high strength, hardness and elastic modulus. Based on the type of strengthening fillers, CMs are divided into dispersion strengthened,fibrous And layered(Fig. 28.2).
Rice. 28.2. Schemes of the structure of composite materials: A) dispersion strengthened; b) fibrous; V) layered
Small, evenly distributed refractory particles of carbides, oxides, nitrides, etc. are artificially introduced into dispersion-strengthened CMs, which do not interact with the matrix and do not dissolve in it up to the melting temperature of the phases. The smaller the filler particles and the smaller the distance between them, the stronger the CM. Unlike fibrous ones, in dispersion-strengthened CMs the main load-bearing element is the matrix. An ensemble of dispersed filler particles strengthens the material by resisting the movement of dislocations under loading, which makes plastic deformation difficult. Effective resistance to the movement of dislocations is created up to the melting temperature of the matrix, due to which dispersion-strengthened CMs are distinguished by high heat resistance and creep resistance.
The reinforcement in fibrous composite materials can be fibers of various shapes: threads, tapes, meshes of different weaves. Reinforcement of fibrous CM can be carried out according to a uniaxial, biaxial and triaxial scheme (Fig. 28.3, A).
The strength and rigidity of such materials is determined by the properties of the reinforcing fibers that bear the main load. Reinforcement gives a greater increase in strength, but dispersion strengthening is technologically easier to implement.
Layered composite materials (Fig. 28.3, b) are composed of alternating layers of filler and matrix material ("sandwich" type). The filler layers in such CMs can have different orientations. It is possible to alternately use layers of filler made of different materials with different mechanical properties. For layered compositions, non-metallic materials are usually used.
Rice. 28.3. Fiber reinforcement schemes ( A) and layered ( b) composite materials
DISPERSE-RESTROENED COMPOSITE MATERIALS
During dispersion strengthening, particles block sliding processes in the matrix. The effectiveness of hardening, subject to minimal interaction with the matrix, depends on the type of particles, their volume concentration, as well as the uniformity of distribution in the matrix. Dispersed particles of refractory phases such as Al 2 O 3, SiO 2, BN, SiC, which have a low density and a high elastic modulus, are used. CMs are usually produced by powder metallurgy, an important advantage of which is the isotropy of properties in different directions.
In industry, dispersion-strengthened CMs are usually used on aluminum and, less commonly, nickel bases. Typical representatives of this type of composite materials are materials such as SAP (sintered aluminum powder), which consist of an aluminum matrix strengthened by dispersed particles of aluminum oxide. Aluminum powder is obtained by spraying molten metal, followed by grinding in ball mills to a size of about 1 micron in the presence of oxygen. With increasing grinding time, the powder becomes finer and its aluminum oxide content increases. Further technology for the production of products and semi-finished products from SAP includes cold pressing, pre-sintering, hot pressing, rolling or extrusion of a sintered aluminum billet in the form of finished products that can be subjected to additional heat treatment.
Alloys of the SAP type are satisfactorily deformed in a hot state, and alloys with 6–9% Al 2 O 3 - even at room temperature. From them, cold drawing can be used to produce foil up to 0.03 mm thick. These materials are easy to cut and have high corrosion resistance.
SAP grades used in Russia contain 6–23% Al 2 O 3 . There are SAP-1 with a content of 6–9, SAP-2 with 9–13, SAP-3 with 13–18% Al 2 O 3. With increasing volume concentration of aluminum oxide, the strength of composite materials increases. At room temperature, the strength characteristics of SAP-1 are as follows: σ in = 280 MPa, σ 0.2 = 220 MPa; SAP-3 are as follows: σ in = 420 MPa, σ 0.2 = 340 MPa.
Materials such as SAP have high heat resistance and are superior to all wrought aluminum alloys. Even at a temperature of 500 °C their σ is at least 60–110 MPa. The heat resistance is explained by the inhibitory effect of dispersed particles on the recrystallization process. The strength characteristics of SAP type alloys are very stable. Long-term strength tests of SAP-3 type alloys for 2 years had virtually no effect on the level of properties both at room temperature and when heated to 500 °C. At 400 °C, the strength of SAP is 5 times higher than the strength of aging aluminum alloys.
Alloys of the SAP type are used in aviation technology for the manufacture of parts with high specific strength and corrosion resistance, operating at temperatures up to 300–500 °C. Piston rods, compressor blades, shells of fuel elements and heat exchanger pipes are made from them.
CM is produced using powder metallurgy using dispersed particles of silicon carbide SiC. The chemical compound SiC has a number of positive properties: high melting point (more than 2650 °C), high strength (about 2000 MPa) and elastic modulus (> 450 GPa), low density (3200 kg/m3) and good corrosion resistance. The production of abrasive silicon powders has been mastered by industry.
Aluminum alloy and SiC powders are mixed, pre-compacted under low pressure, then hot-pressed in steel containers in a vacuum at the melting temperature of the matrix alloy, i.e. in a solid-liquid state. The resulting workpiece is subjected to secondary deformation in order to obtain semi-finished products of the required shape and size: sheets, rods, profiles, etc.
38.1. Classification
Composite materials are materials reinforced with fillers arranged in a certain way in a matrix. Fillers are most often substances with high energy of interatomic bonds, high strength and high modulus, however, highly plastic fillers can also be used in combination with brittle matrices
Binding components, or matrices, in composite materials can be different - polymer, ceramic, metal or mixed. In the latter case, we talk about polymatrix composite materials.
According to the morphology of the reinforcing phases, composite materials are divided into:
zero-dimensional (designation: 0,), or strengthened by particles of varying dispersion, randomly distributed in the matrix;
one-dimensional fibrous (symbol: 1), or reinforced with unidirectional continuous or discrete fibers;
two-dimensional layered (designation: 2), or containing identically oriented reinforcing lamellas or layers (Fig. 38.1).
The anisotropy of composite materials, “designed” in advance with the aim of using it in appropriate structures, is called structural.
Based on the size of the reinforcing phases or the size of the reinforcement cell, composite materials are divided as follows:
submicrocomposites (reinforcement cell size, fiber or particle diameter<С 1 мкм), например, дисперсноупрочненные сплавы или волокнистые композиционные материалы с очень тонкими волокнами:
microcomposites (reinforcement cell size, diameter of fibers, particles or layer thickness ^1 μm), for example materials reinforced with particles, carbon fibers, silicon carbide, boron, etc., unidirectional eutectic alloys;
macrocomposites (diameter or thickness of reinforcing components -100 microns), for example parts made of copper or aluminum alloys reinforced with tungsten or steel wire or foil. Macrocomposites are most often used to increase the wear resistance of friction parts in technological equipment.
38.2. Interfacial interaction in composite materials
38.2.1. Physicochemical and thermomechanical compatibility of components
The combination in one material of substances that differ significantly in chemical composition and physical properties brings to the fore in the development, manufacture and connection of composite materials the problem of thermodynamic and kinetic compatibility of the components. Under pressure
Dynamic compatibility is understood as the ability of the matrix and reinforcing fillers to be in a state of thermodynamic equilibrium for an unlimited time at production and operating temperatures. Almost all artificially created composite materials are thermodynamically incompatible. The only exceptions are a few metal systems (Cu-W, Cu-Mo, Ag-W), where there is no chemical and diffusion interaction between the phases for an unlimited time of their contact.
Kinetic compatibility - the ability of components of composite materials to maintain metastable equilibrium in certain temperature-time intervals. The problem of kinetic compatibility has two aspects: 1) physical and chemical - ensuring a strong bond between the components and limiting the processes of dissolution, hetero- and reaction diffusion at the interfaces, which lead to the formation of brittle interaction products and degradation of the strength of the reinforcing phases and the composite material as a whole; 2) thermomechanical - achieving a favorable distribution of internal stresses of thermal and mechanical origin and reducing their level; ensuring a rational relationship between the strain hardening of the matrix and its ability to relax stress, preventing overload and premature destruction of the strengthening phases.
There are the following possibilities for improving the physical and chemical compatibility of metal matrices with reinforcing fillers:
I. Development of new types of reinforcing fillers that are resistant to contact with metal matrices at high temperatures, for example, ceramic fibers, whiskers and dispersed particles of silicon carbides, titanium, zirconium, boron, aluminum oxides, zirconium, silicon nitrides, boron, etc.
II Application of barrier coatings on reinforcing fillers, for example coatings of refractory metals, titanium carbides, hafnium, boron, titanium nitrides, boron, yttrium oxides on carbon fibers, boron, silicon carbide. Some barrier coatings on fibers, mainly metal ones, serve as a means of improving the wetting of fibers by matrix melts, which is especially important when producing composite materials by liquid-phase methods. Such coatings are often called technological
No less important is the plasticization effect discovered during the application of technological coatings, which manifests itself in the stabilization and even increase in the strength of the fibers (for example, when aluminizing boron fibers by drawing them through a melt bath or when nickeling carbon fibers with subsequent heat treatment).
III. The use in composite materials of metal matrices doped with elements with a greater affinity for the reinforcing filler than the matrix metal, or with surfactant additives. The resulting change in the chemical composition of the interfaces should prevent the development of interfacial interaction. Alloying matrix alloys with surface-active or carbide-forming additives, as well as applying technological coatings to fibers, can help improve the wettability of the reinforcing filler with metal melts.
IV. Alloying the matrix with elements that increase the chemical potential of the reinforcing filler in the matrix alloy, or with additives of the reinforcing filler material to saturation concentrations at the temperatures of production and operation of the composite material. Such alloying prevents the dissolution of the reinforcing phase, i.e. increases the thermal stability of the composition.
V. Creation of “artificial” composite materials similar to “natural” eutectic compositions by choosing the appropriate composition of components.
VI. Selection of optimal durations of contact of components during a particular process for producing composite materials or under the conditions of their service, i.e., taking into account temperature and force factors. The duration of contact, on the one hand, must be sufficient for the formation of strong adhesive bonds between the components; on the other hand, do not lead to intense chemical interaction, the formation of brittle intermediate phases and a decrease in the strength of the composite material.
Thermo-mechanical compatibility of components in composite materials is ensured by:
selection of matrix alloys and fillers with minimal differences in elastic moduli, Poisson's ratios, and thermal expansion coefficients;
the use of intermediate layers and coatings in reinforcing phases, reducing differences in the physical properties of the matrix and phases;
transition from reinforcement with a component of one type to poly-reinforcement, i.e. a combination in one composite material of reinforcing fibers, particles or layers that differ in composition and physical properties;
changing the geometry of parts, the pattern and scale of reinforcement; morphology, size and volume fraction of reinforcing phases; replacing a continuous filler with a discrete one;
the choice of methods and modes for the production of a composite material that ensure a given level of bond strength of its components.
38.2.2. Reinforcing fillers
To reinforce metal matrices, high-strength, high-modulus fillers are used - continuous and discrete metal, non-metallic and ceramic fibers, short fibers and particles, whiskers (Table 38.1).
Carbon fibers are one of the most advanced and advanced reinforcing materials in production. An important advantage of carbon fibers is their low specific gravity, thermal conductivity close to metals (R = 83.7 W/(m-K)), and relatively low cost.
Fibers are supplied in the form of straight or twisted myogofilament strands, fabrics or ribbons made from them. Depending on the type of feedstock, the diameter of the filaments varies from 2 to 10 microns, the number of filameites in the bundle - from hundreds to tens of thousands of pieces.
Carbon fibers have high chemical resistance to atmospheric conditions and mineral acids. The heat resistance of the fibers is low: the temperature of long-term operation in air does not exceed 300-400 °C. To increase chemical resistance in contact with metals, barrier coatings of titanium and zirconium borides, titanium carbides, zirconium, silicon, and refractory metals are applied to the surface of the fibers.
Boron fibers are produced by deposition of boron from a gas mixture of hydrogen and boron trichloride onto tungsten wire or carbon monofilaments heated to a temperature of 1100-1200 °C. When heated in air, boron fibers begin to oxidize at temperatures of 300-350 °C, and at 600-800 °C they completely lose their strength. Active interaction with most metals (Al, Mg, Ti, Fe, Ni) begins at temperatures of 400-600 °C. To increase the heat resistance of boron fibers, thin layers (2-6 µm) of silicon carbide (SiC/B/W), boron carbide (B4C/B/W), boron nitride (BN/B/W) are applied in the gas phase.
Silicon carbide fibers with a diameter of 100-200 microns are produced by deposition at 1300 °C from a vapor-gas mixture of silicon tetrachloride and methane, diluted with hydrogen in a ratio of 1: 2: 10, on tungsten wire
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Carbon fibers
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TABLE 38.2 ALLOYS USED AS MATRIX IN COMPOSITE MATERIALS
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or pitch carbon fibers. The best fiber samples have a strength of 3000-4000 MPa at 1100 °C
Coreless silicon carbide fibers in the form of multifilamentite bundles, obtained from liquid organosilanes by drawing and pyrolysis, consist of ultrafine f)-SiC crystals.
Metal fibers are produced in the form of wire with a diameter of 0.13; 0.25 and 0.5 mm. Fibers made from high-strength steels and beryllium alloys are intended mainly for reinforcing matrices made of light alloys and titanium. Fibers from refractory metals alloyed with rhenium, titanium, oxide and carbide phases are used to strengthen heat-resistant nickel-chromium, titanium and other alloys.
Whiskers used for reinforcement can be metal or ceramic. The structure of such crystals is monocrystalline, the diameter is usually up to 10 microns with a length to diameter ratio of 20-100. Whiskers are obtained by various methods: growth from coatings, electrolytic deposition, deposition from a vapor-gas environment, crystallization from the gas phase through the liquid phase. by the vapor-liquid-crystal mechanism, pyrolysis, crystallization from saturated solutions, visceration
38.2.3. Matrix alloys
In metal composite materials, mainly matrices are used from light wrought and cast alloys of aluminum and magnesium, as well as alloys of copper, nickel, cobalt, zinc, tin, lead, and silver; heat-resistant nickel-chromium, titanium, zirconium, vanadium alloys; alloys of refractory metals chromium and niobium (Table 38 2).
38.2.4. Types of bonds and interface structures in composite materials
Depending on the filler material and matrices, methods and modes of obtaining composite materials across interfaces, six types of bonds are implemented (Table 38.3). The strongest bond between components in compositions with metal matrices is provided by chemical interaction. A common type of bond is mixed, represented by solid solutions and intermetallic phases (for example, the composition “aluminium-boron fibers” obtained by continuous casting) or solid solutions, intermetallic and oxide phases (the same composition obtained by pressing plasma semi-finished products), etc.
38.3. Methods for producing composite materials
The technology for the production of metal composite materials is determined by the design of the products, especially if they have a complex shape and require preparation of joints by welding, soldering, gluing or riveting, and, as a rule, is multi-transition.
The elemental basis for the production of parts or semi-finished products (sheets, pipes, profiles) from composite materials most often are so-called prepregs, or tapes with one layer of reinforcing filler, impregnated or coated with matrix alloys; metal impregnated fiber tows or individual fibers coated with matrix alloys.
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TYPES OF BONDING ALONG INTERFACE SURFACES IN COMPOSITE MATERIALS
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Parts and semi-finished products are obtained by combining (compacting) the original prepregs using the methods of impregnation, hot pressing, rolling or drawing prepreg packages. Sometimes both prepregs and products made from composite materials are manufactured using the same methods, for example, using powder or casting technology, but under different modes and at different technological stages.
Methods for producing prepregs, semi-finished products and products from composite materials with metal matrices can be divided into five main groups: 1) vapor-gas phase; 2) chemical and electrochemical; 3) liquid phase; 4) solid phase; 5) solid-liquid phase.
38.4. Properties of metal matrix composite materials
Composite materials with metal matrices have a number of undeniable advantages over other structural materials intended for use in extreme conditions. These advantages include: high strength and... rigidity combined with high fracture toughness; high specific strength and rigidity (the ratio of the tensile strength and elastic modulus to the specific gravity of a/y and E/y); high fatigue limit; high heat resistance; low sensitivity to thermal shock, to surface defects, high damping properties, electrical and thermal conductivity, manufacturability in design, processing and connection (Table 38 4).
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COMPOSITE MATERIALS WITH METAL MATRICES COMPARED TO THE BEST METAL STRUCTURAL MATERIALS |
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TABLE 385 |
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MECHANICAL PROPERTIES OF COMPOSITE MATERIALS WITH METAL MATRIXES
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In the absence of special requirements for materials regarding thermal conductivity, electrical conductivity, cold resistance and other properties, the temperature ranges of operation of composite materials are determined as follows:<250 °С - для материалов с полимерными матрицами; >1000 °C - for materials with ceramic matrices; Composite materials with metal matrices cover these limits
The strength characteristics of some composite materials are given in Table 38 5.
The main types of connections of composite materials today are bolted, riveted, adhesive, connections by soldering and welding, and combined. Connections by soldering and welding are especially promising, since they open up the opportunity to most fully realize the unique properties of a composite material in a structure, but their implementation represents a complex scientific and technical task and in many cases it has not yet left the experimental stage
38.5. Problems of weldability of composite materials
If by weldability we mean the ability of a material to form welded joints that are not inferior to it in their properties, then composite materials with metal matrices, especially fibrous ones, should be classified as difficult-to-weld materials. There are several reasons for this.
I. Welding and soldering methods involve joining composite materials over a metal matrix. The reinforcing filler in a welded or brazed seam is either completely absent (for example, in butt welds located transverse to the direction of reinforcement in fibrous or layered composite materials), or present in a reduced volume fraction (when welding dispersion-strengthened materials with wires containing a discrete reinforcing phase), or there is a violation of the continuity and direction of the reinforcement (for example, during diffusion welding of fibrous compositions across the direction of the reinforcement). Consequently, a welded or soldered seam is a weakened area of a composite material structure, which requires consideration when designing and preparing the joint for welding. In the literature, there are proposals for autonomous welding of composition components to maintain continuity of reinforcement (for example, pressure welding of tungsten fibers in a tungsten-copper composition), however, autonomous butt welding of fibrous composite materials requires special preparation of the edges, strict adherence to the reinforcement pitch and is only suitable for materials reinforced metal fibers. Another proposal is to prepare butt joints with overlapping fibers beyond the critical length, but this poses difficulties in filling the joint with matrix material and ensuring a strong bond at the fiber-matrix interface.
II. It is convenient to consider the influence of welding heating on the development of physicochemical interaction in a composite material using the example of a connection formed when an arc melts a fibrous material across the direction of reinforcement (Fig. 38.2). If the matrix metal does not have polymorphism (for example, Al, Mg, Cu, Ni, etc.), then 4 main zones can be distinguished in the joint: 1 - zone heated to the matrix return temperature (by analogy with welding of homogeneous materials, we will call this zone the main material); 2 - zone limited by the temperatures of return and recrystallization of the matrix metal (return zone); 3-zone,
limited by the temperatures of recrystallization and melting of the matrix (recrystallization zone); 4 - heating zone above the melting temperature of the matrix (let's call this zone the weld). If the matrix in the composite material is alloys of Ti, Zr, Fe and other metals that have polymorphic transformations, then subzones with complete or partial phase recrystallization of the matrix will appear in zone 3, but for this consideration this point is not significant.
Changes in the properties of the composite material begin in zone 2. Here, recovery processes remove the strain hardening of the matrix achieved during solid-phase compaction of the composite material (in compositions obtained by liquid-phase methods, softening is not observed in this zone).
In zone 3, recrystallization and growth of matrix metal grains occurs. Due to the diffusion mobility of matrix atoms, the further development of interphase interaction, which began in the processes of production of the composite material, becomes possible; the thickness of the brittle layers increases and the properties of the composite material as a whole deteriorate. When fusion welding material
When obtained by methods of solid-phase compaction of powders or prepregs with a powder or sprayed matrix, porosity is possible along the fusion boundary and adjacent interphase boundaries, deteriorating not only the strength properties, but also the tightness of the welded joint.
In zone 4 (weld seam), 3 sections can be distinguished:
Section 4", adjacent to the axis of the weld, where due to strong overheating under the arc of the metal matrix melt and the longest duration of the metal being in the molten state, complete dissolution of the reinforcing phase occurs;
Section 4", characterized by a lower heating temperature of the melt and a shorter duration of contact of the reinforcing phase with the melt. Here this phase is only partially dissolved in the melt (for example, the diameter of the fibers decreases, cavities appear on their surface; the unidirectionality of the reinforcement is disrupted);
Section 4"", where there is no noticeable change in the dimensions of the reinforcing phase, but intense interaction with the melt develops, layers or islands of brittle interaction products are formed, and the strength of the reinforcing phase decreases. As a result, zone 4 becomes the zone of maximum damage to the composite material during welding.
III. Due to differences in the thermal expansion of the matrix material and the reinforcing phase in welded joints of composite materials, additional thermoelastic stresses arise, causing the formation of various defects: cracking, destruction of brittle reinforcing phases in the most heated zone 4 of the joint, delamination along interphase boundaries in zone 3.
To ensure high properties of welded joints of composite materials, the following is recommended.
Firstly, among the known joining methods, preference should be given to solid phase welding methods, in which, due to the lower energy input, minimal degradation of the properties of components in the joint zone can be achieved.
Secondly, pressure welding modes must be selected so as to prevent displacement or crushing of the reinforcing component.
Thirdly, when fusion welding composite materials, methods and modes should be selected that ensure minimal heat input into the joint zone.
Fourth, fusion welding should be recommended for joining composite materials with thermodynamically compatible components, such as copper-tungsten, copper-molybdenum, silver-tungsten, or reinforced with heat-resistant fillers, such as silicon carbide fibers, or fillers with barrier coatings, such as fibers boron with boron carbide or silicon carbide coatings.
Fifthly, the electrode or filler material or the material of intermediate gaskets for fusion welding or soldering must contain alloying additives that limit the dissolution of the reinforcing component and the formation of brittle products of interfacial interaction during the welding process and during subsequent operation of the welded units.
38.5.1. Welding of composite materials
Fibrous and laminated composite materials are most often joined in a lapped manner. The ratio of the length of the floor to the thickness of the material usually exceeds 20. Such connections can be further strengthened with riveted or bolted connections. Along with lap joints, it is possible to make butt and corner welded joints in the direction of reinforcement and, less commonly, across the direction of reinforcement. In the first case, with the correct choice of methods and modes of welding or soldering, it is possible to achieve equal strength of the connection; in the second case, the strength of the connection usually does not exceed the strength of the matrix material.
Composite materials reinforced with particles, short fibers, and whiskers are welded using the same techniques as precipitation-hardening alloys or powder materials. In this case, equal strength of welded joints to the base material can be achieved provided that the composite material is manufactured using liquid-phase technology, reinforced with heat-resistant fillers and when selecting appropriate welding modes and welding materials. In some cases, the electrode or filler material may be similar or similar in composition to the base material.
38.5.2. Gas shielded arc welding
The method is used for fusion welding of composite materials with a matrix of chemically active metals and alloys (aluminum, magnesium, titanium, nickel, chromium). Welding is carried out with a non-consumable electrode in an atmosphere of argon or a mixture with helium. To regulate the thermal effect of welding on materials, it is advisable to use a pulsed arc, compressed arc or three-phase arc.
To increase the strength of joints, it is recommended to make seams using composite electrodes or filler wires with a volumetric content of the reinforcing phase of 15-20%. Short fibers of boron, sapphire, nitride or silicon carbide are used as reinforcing phases.
38.5.3. Electron beam welding
The advantages of the method are the absence of oxidation of the molten metal and the reinforcing filler, vacuum degassing of the metal in the welding zone, high energy concentration in the beam, which makes it possible to obtain joints with a minimum width of the melting zone and heat-affected zone. The latter advantage is especially important when making connections of fiber composite materials in the direction of reinforcement. With special preparation of joints, welding using filler spacers is possible.
38.5.4. Resistance spot welding
The presence of a reinforcing phase in a composite material reduces its thermal and electrical conductivity compared to the matrix material and prevents the formation of a cast core. Satisfactory results were obtained when spot welding thin-sheet composite materials with cladding layers. When welding sheets of various thicknesses or composite sheets with homogeneous metal sheets, in order to bring the core of the weld point into the plane of contact of the sheets and balance the difference in the electrical conductivity of the material, select electrodes with different conductivity, compressing the peripheral zone, change the diameter and radius of curvature of the electrodes, and the thickness cladding layer, use additional gaskets.
The average strength of the weld point when welding monoaxial boron-reinforced aluminum plates with a thickness of 0.5 mm (with a volume fraction of fibers of 50%) is 90% of the strength of boron-aluminum of an equivalent section. The joint strength of bora-aluminum sheets with cross-reinforcement is higher than that of sheets with uniaxial reinforcement.
38.5.5. Diffusion welding
The process is carried out at high pressure without the use of solder. Thus, boron aluminum parts to be joined are heated in a sealed retort to a temperature of 480 °C at a pressure of up to 20 MPa and maintained under these conditions for 30-90 minutes. The technological process of diffusion resistance spot welding of bora-aluminum with titanium is almost no different from fusion spot welding. The difference is that the welding mode and the shape of the electrodes are selected so that the heating temperature of the aluminum matrix is close to the melting temperature, but below it. As a result, a diffusion zone with a thickness of 0.13 to 0.25 microns is formed at the point of contact.
Specimens lap-welded by diffusion spot welding, when tested in tension in the temperature range 20-120 °C, are destroyed along the base material with tear-out along the fibers. At a temperature of 315 °C, samples are destroyed by shear at the joint.
38.5.6. Wedge-press welding
To connect ends made of conventional structural alloys with pipes or bodies made of composite materials, a method has been developed for welding dissimilar metals that differ sharply in hardness, which can be called micro-wedge press. The pressing pressure is obtained due to thermal stresses that arise when heating the mandrel and holder of the thermocompression welding device, made of materials with different thermal expansion coefficients (TE). The ending elements, on the contact surface of which a wedge thread is applied, are assembled with a pipe made of a composite material, as well as with a mandrel and holder. The assembled device is heated in a protective environment to a temperature of 0.7-0.9 from the melting point of the most fusible metal. The fixture mandrel has a higher CTE than the holder. During the heating process, the distance between the working surfaces of the mandrel and the holder is reduced, and the protrusions (“wedges”) of the thread on the tip are pressed into the cladding layers of the pipe. The strength of the solid-phase connection is not lower than the strength of the matrix or cladding metal.
38.5.7. Explosion welding
Explosion welding is used to join sheets, profiles and pipes made of metal composite materials reinforced with metal fibers or layers having sufficiently high plastic properties to avoid crushing of the reinforcing phase, as well as for joining composite materials with bracings made of various metals and alloys. The strength of the joints is usually equal to or even higher (due to strain hardening) than the strength of the least strong matrix material used in the parts being joined. To increase the strength of joints, intermediate gaskets made of other materials are used.
There are usually no pores or cracks in the joints. Melted areas in the transition zone, especially during the explosion of dissimilar metals, are mixtures of eutectic-type phases.
38.6. Soldering of composite materials
Soldering processes are very promising for joining composite materials, since they can be carried out at temperatures that do not affect the reinforcing filler and do not cause the development of interfacial interaction.
Soldering is carried out using conventional techniques, i.e. immersion in solder or in an oven. The question of the quality of surface preparation for soldering is very important. Joints made with brazed solders using fluxes are susceptible to corrosion, so the flux must be completely removed from the joint area.
Soldering with hard and soft solders
Several options for soldering boron aluminum have been developed. Solders for low-temperature soldering have been tested. Solders of the composition 55% Cd -45% Ag, 95% Cd -5% Ag, 82.5% Cd-17.5% Zn are recommended for parts operating at temperatures not exceeding 90 °C; solder composition 95% Zn - 5% Al - for operating temperatures up to 315 °C. To improve wetting and spreading of solder, a layer of nickel 50 microns thick is applied to the surfaces to be joined. High-temperature soldering is carried out using eutectic solders of the aluminum - silicon system at temperatures of the order of 575-615 ° C. Soldering time should be kept to a minimum due to the risk of degradation of the strength of boron fibers.
The main difficulties in soldering carbon-aluminum compositions both with each other and with aluminum alloys are associated with the poor wettability of carbon-aluminum compositions with solders. The best solders are alloy 718 (A1-12% Si) or alternating layers of foil from alloy 6061. Soldering is carried out in an oven in an argon atmosphere at a temperature of 590 ° C for 5-10 minutes. To connect bora-aluminum and carbon-aluminum with titanium, solders of the aluminum-silicon-magnesium system can be used. To increase the strength of the connection, it is recommended to apply a layer of nickel to the titanium surface.
Eutectic diffusion soldering. The method consists of applying a thin layer of a second metal to the surface of the parts being welded, forming a eutectic with the matrix metal. For matrices made of aluminum alloys, layers of Ag, Cu, Mg, Ge, Zn are used, the eutectic temperature of which with aluminum is 566, 547, 438, 424 and 382 °C, respectively. As a result of the diffusion process, the concentration of the second element in the contact zone gradually decreases, and the melting temperature of the compound increases, approaching the melting temperature of the matrix. Thus, solder joints can operate at temperatures higher than punkka temperature.
When diffusion soldering of boron aluminum, the surfaces of the parts to be joined are coated with silver and copper, then compressed and maintained under pressure of up to 7 MPa at a temperature of 510-565 ° C in a steel retort in a vacuum or inert atmosphere.