CC BY
52APPLICATION OF SHS FOR FABRICATION OF ALUMINUM-MATRIX
NANOCOMPOSITES (REVIEW)
A. P. Amosov*", E. I. Latukhin", A. R. Luts", Yu. V. Titova", A. A. Kuzina",
and D. A. Maidan"
aSamara State Technical University, Samara, 443100 Russia
e-mail: egundor@yandex.ru
DOI: 10.24411/9999-0014A-2019-10008
The first place in the production of composites with a metal matrix is occupied by aluminum-matrix composites (AMCs), due to their low weight, high specific strength, wear resistance, corrosion resistance, a fairly wide temperature range of operation, good technological properties [1-3]. The greatest volume of their application falls on road transport, where they are used for engine parts and brakes. Because of the scarcity of fibers, their high cost and complex manufacturing technology, the AMCs, discretely strengthened with ceramic particles AhO3, SiC, TiC, TiB2, Si3N4, AlN, etc., are more affordable and widely used. The presence of such refractory, high-strength and high-modulus ceramic particles in the matrix of aluminum alloys, which do not dissolve in the matrix, causes a significant improvement in mechanical properties including at elevated temperatures while maintaining a small specific gravity and other properties of aluminum.
However, the wider use of AMCs is constrained by a number of unresolved issues. It is necessary to increase the level of strength properties of AMCs as it is often insufficient especially at elevated temperatures. But the increase in strength due to the increase in the volume fraction of ceramic particles of micrometer size from 0.5 to 50 |im along with positive effects leads to such disadvantages as low crack resistance, low hardness and strength at elevated temperatures, poor mechanical machinability [4]. Much more promising is another direction of increasing the strength of AMCs - reducing the size of the reinforcing particles, the transition to the use of nanoparticles (from 1 to 100 nm) instead of micrometer-sized particles. With such a decrease in the size, other strengthening mechanisms begin to work, and a significant change in the properties of aluminum-matrix composites is achieved with a significantly lower content of the reinforcing phase, and, as a consequence, maintaining high ductility, which is very important to overcome such lacks of AMCs as low ductility and crack resistance with increased strength [5, 6]. In dispersion-strengthened nanocomposites, the matrix bears the main part of the external load, and the effective resistance to the displacement of dislocations in the body of the metal matrix is created by ceramic nanoparticles (Orowan mechanism), grain boundaries (Hall-Petch mechanism), the mismatch of elastic modules (EM) and coefficients of thermal expansion (CTE) of the matrix and nanoparticles. The greater the resistance, the higher the degree of strengthening of the material. Calculations show that the most significant contributions to strengthening are created by the mismatch of CTE, as well as by Orowan mechanism, especially when the particle diameter does not exceed 50 nm with the content of nanoparticles up to 15 vol %.
However, it should be borne in mind that the results of calculations are valid for a perfectly uniform distribution of reinforcing particles over the matrix body with a perfectly continuous contact and a strong adhesion bond between the particles and the matrix. Therefore, in order to realize the large potential of strengthening the AMCs due to the reinforcement by nanoparticles, it is necessary that the technologies for manufacturing such AMCs provide a uniform distribution of nanoparticles over the matrix and a strong adhesive interfacial bond. The
fulfillment of these requirements is a big technological problem since usually nanoparticles are poorly wetted by the matrix material and are prone to the formation of agglomerates from nanoparticles due to interparticle adhesive forces, the value of which increases sharply with decreasing particle size [4-6]. For uniform distribution of nanoparticles, it is necessary to overcome the forces of their adhesion and destroy agglomerates, as well as to ensure wettability of nanoparticles by the matrix material.
Numerous solid-phase and liquid-phase methods of manufacturing the nanoparticle-reinforced AMCs, which are divided into ex-situ and in-situ methods, have been developed. In the case of ex-situ methods, previously prepared reinforcing nanoparticles in powder form are introduced into AMCs and mixed with the matrix metal in solid or liquid state, and in the case of in-situ methods, the reinforcing particles are synthesized directly during the manufacture of AMCs by chemical reactions in the body of solid or liquid matrix metal.
Solid-phase methods (powder metallurgy, mechanical alloying, friction with mixing) in comparison with liquid-phase methods allow to use relatively large volumes of the reinforcing phase, which can also be poorly wetted with aluminum melt, to avoid the formation of undesirable brittle phases in the reaction of the filler with the melt, to achieve a uniform distribution of reinforcing nanoparticles over the matrix, but these methods have a noticeable residual porosity of composites and low adhesion of the matrix with nanoparticles. The use of solid-phase methods is currently limited both by the high cost associated mainly with the multistage and energy consumption of the process of manufacturing blanks from dispersed-hardened AMCs, and by the simple shapes of these blanks, which must be further machined to make the component of desired shape and size [4-6]. But for the sake of justice, it should be emphasized that among the AMCs reinforced with nanoparticles, only the nanocomposites of the SAP brand (Al-AhO3) and Al-AUC3 with the content of the reinforcing nanoparticles of AhO3 and AUC3 up to 22% and with the unique mechanical properties up to 500oC made by solid-phase methods have so far found industrial production and application.
Liquid-phase methods are more cost-effective for industrial production due to the possibility of using the available low-cost equipment for foundry production and fabrication of castings and components of complex shape, however, with a limited amount of reinforcing phase in composites, since the melt loses its fluidity at a high content of the reinforcing phase. However, such a simple and economical ex-situ method as mechanical stirring the reinforcing nanopowders in bulk with the melt of aluminum or its alloys does not lead to success, since nanopowders do not sink in the melt, are not absorbed by the melt. In this regard, to ensure the introduction of nanopowders, it is necessary to use special ex-situ methods for the introduction of nanopowders: mixing in the semi-solid state of the matrix metal, spraying or spinning the melt, physical effects on the melt, primarily by ultrasound and others [4-6]. Technologies based on these methods are complex, require special equipment, inefficient, energy consumption. More simple technology based on the use of a variety of nanopowdery pseudo-master alloys (mixtures of nanopowders with metal powders-carriers, often in the form of pressed briquettes), is limited by the possibility of dissolution of the briquettes in the aluminum melt. The content of the reinforcing ceramic powder in the pseudo-master alloy usually does not exceed several percent, and therefore the content of the nanoscale reinforcing phase in the thus obtained nanostructured cast AMCs is usually small and does not exceed 0.1%.
In the case of in-situ methods, when carrying out chemical synthesis reactions of reinforcing particles directly in the melt, the increased content of the reinforcing phase is provided at a lower cost compared to the expensive separately synthesized nanopowders, as well as a high thermodynamic stability, a more intimate contact and a good bond (adhesion) between the phases of the composite, as the reinforcing phase is not introduced from the outside with the particle surface usually contaminated by oxides and adsorbed gases and moisture, but these particles are formed directly in the body of the melt, are not in contact with the atmosphere, do not contain moisture and have fresh clean surfaces [7]. However, in-situ methods have such
disadvantages as noticeable residual porosity of AMCs, uneven distribution of synthesized reinforcing particles over the matrix, which usually form clusters and agglomerates along the grain boundaries of the matrix alloy. To overcome the last drawback, such methods as ultrasonic treatment of the melt in the synthesis of AMCs or subsequent plastic deformation of the resulting composite ingot (forging or rolling, especially at different speeds of rotation of the rolls) are used [8, 9].
To date, the technological problems of manufacturing the AMCs by liquid-phase methods with a high content (up to 15 vol % ) and the uniform distribution of nanoparticles over the matrix are unresolved, what is necessary for the implementation of large potential opportunities for strengthening the AMCs due to reinforcement by nanoparticles, and there are no economically viable technologies for the industrial production of cast aluminum-matrix composites reinforced with nanoparticles.
A significant contribution to the solution of the above problems can be made by using the achievements of a simple energy-saving powder technology of self-propagating high-temperature synthesis (SHS) of solid chemical compounds (carbides, borides, nitrides, oxides, etc.) and materials based on them [10]. SHS process can be used in solid-phase and liquid-phase methods of manufacturing the dispersion-strengthened AMCs [11]. In the case of development of cast aluminum-matrix composites, discontinuously reinforced with nano-sized ceramic particles, the application of SHS process is possible in three directions: (1) synthesis of inexpensive ceramic nanopowders for their subsequent introduction into the matrix melt (ex-situ); (2) introduction of the previously prepared ceramic nanoparticles into the matrix melt (ex-situ) with the use of SHS process, creating a large gradient of temperature and chemical potential, what promotes wetting and uniform distribution of nanoparticles; and (3) synthesis of inexpensive reinforcing ceramic nanoparticles directly in the aluminum melt (in-situ) with ensuring their good adhesion to the matrix [12].
On the market abroad and in Russia, there are mainly ceramic nanopowders of plasma chemical synthesis, the prices of which (nitrides, carbides and borides) are from 1000 to 3000 euros per 1 kg [13]. Such high prices of nanopowders largely prevent their use for reinforcement of AMCs and the organization of economically valid industrial production of AMCs, dispersion-strengthened by nanoparticles. The analysis shows that the price of SHS nanopowders, in particular, nitrides of azide technology of SHS, can be an order of magnitude less, so the use of SHS nanopowders for reinforcing the aluminum alloys in all three of the above directions is promising [12, 14-19]. The first direction is represented by the results of the successful application of nanopowder products of azide SHS technology for reinforcing the aluminum alloys with a reinforcing phase content of up to 4%, using various ex-situ methods of introducing the nanopowders into the melt of aluminum (or its alloys): in the form of nanopowdery pseudo-master alloys (pressed briquettes of mixtures of nanopowders with metal powders-carriers), in the form of composite master alloys obtained by fusion of flux carnallite KCl-MgCh with nanopowders, by mixing nanopowders into an aluminum alloy in a semi-solid state [14, 15]. In the second direction, ex-situ injection of 7.7% aluminum nitride nanopowder, previously obtained by the azide SHS technology, into the melt of aluminum was carried out in a mixture with a charge Ti + C, leading to the implementation of SHS process of reinforcing phase TiC in the aluminum melt (in-situ) [16]. The third direction of the synthesis of reinforcing ceramic nanoparticles directly in the aluminum melt (in-situ) was carried out in the fabrication of aluminum-matrix composite Al-10% TiC by introducing a charge Ti + C into the melt [12, 17]. Dilution of the SHS charge with inert additives, replacement of the initial powders of pure elements with their precursors, i.e. chemical compounds, the use of fluxes made it possible to achieve the nanolevel of the dispersed reinforcing phase TiC in this composite. In the synthesis of the reinforcing phase by SHS method, the alloying of the aluminum matrix by various elements can also play an important role in increasing the mechanical properties of AMCs [18, 19]. For example, nanocomposite Al-5%Cu-0.5%TiC, obtained by dissolving in
the Al-5%Cu melt of the Al-TiC master nanocomposite, previously synthesized by burning a mixture of Al and Ti powders with carbon nanotubes in vacuum, showed along with increased strength of 540 MPa unique ductility 5 = 19%, which was almost 3 times higher than the original matrix alloy Al-5% Cu with 485 MPa and 6.6%, respectively.
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