CC BY
28iSHS 2019
Moscow, Russia
FABRICATION OF Al-AlN NANOCOMPOSITE BY POWDER METALLURGY METHOD USING AlN NANOPOWDER OF
SHS-AZ BRAND
A. A. Kuzina", A. P. Amosov*fi, D. A. Zakharovc, and Yu. V. TitovaA
aSamara National Research University, Samara, 443086 Russia bSamara State Technical University, Samara, 443100 Russia Joint-stock company Volgaburmash, Samara, 443004 Russia
*e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10077
Dispersion-strengthened composites based on aluminum alloys, reinforced by ceramic particles of aluminum oxide and silicon carbide of the micron sizes from 1 to 50 ^m, are widely used for production of modern articles of the space and aviation industry. Currently, much attention is paid to the use of nanoscale (less than 0.1 |im) reinforcing particles, the introduction of which into the composites provides high strength characteristics of the composites with a low content of the dispersed phase, that allows to keep in such composite good ductility and workability of the aluminum matrix. Such nanocomposites are made by solid-phase methods of powder metallurgy and liquid-phase methods of casting. The liquid-phase methods differ in simplicity and profitability due to the possibility of using the existing low-cost foundry equipment and fabrication of castings of relatively large size and complex shape. Therefore, numerous attempts are being made to introduce nanoparticles of various refractory compounds (AhO3, SiC, TiC, Si3N4, AlN, etc.) into the aluminum melt. However, nanoparticles are very active, oxidize at rather low temperatures, stick together in strong agglomerates, poorly wetted by liquid aluminum, therefore, their direct incorporation and uniform distribution in the aluminum melt are difficult. Variety of methods are used for introduction of such nanoparticles, for example, the introduction of particles by a jet of inert gas, the use of the semi-solid state of aluminum, the use of special methods of physical action on the melt, the use of the pressed briquettes of master nanopowder composites [1-8]. However, all these liquid-phase methods make it possible to introduce and distribute a relatively small amount of nanosized reinforcing phase in the matrix phase. The solid-phase methods, in contrast to liquid-phase methods, make it possible to introduce large volumes of the reinforcing phase, which can be poorly wetted by aluminum melt, to achieve a uniform distribution of the reinforcing particles over the matrix. The solid-phase methods of manufacturing the dispersion-hardened aluminum matrix composites include methods of powder metallurgy, mechanical alloying, friction with mixing, diffusion welding, etc. [2-4]. However, the use of these methods is limited by the high cost of the equipment and energy consumption of the process of manufacturing the components from the dispersion-hardened aluminum-matrix composites.
Currently, Al-AlN composites, reinforced by AlN nanoparticles, are promising for use in aerospace engineering at high temperatures up to 400-550°C [9]. But these composites have been studied less than Al-AhO3 and Al-SiC composites, and, unlike the latter, they have not yet mastered industrial production technologies [4, 9]. Therefore, it is important to study and develop technologies for manufacturing the Al-AlN nanocomposites that are economically promising for industrial production. A review of the methods and properties of aluminum-matrix composite materials, discretely reinforced with AlN aluminum nitride nanoparticles, is presented in [10]. It follows from the review that the study of the possibility of using the achievements of a simple energy-saving powder technology based on the process of self-
propagating high-temperature synthesis (SHS) is of great interest for manufacturing the Al-AlN nanocomposites. The cost of nitrides nanopowders, obtained by the azide SHS technology, for their subsequent use in manufacturing the nanocomposites can be an order of magnitude less than the cost of similar nanopowders, produced by the method of plasma chemical synthesis [11]. As a result of our previous studies on the liquid-phase fabrication of Al-AlN nanocomposites using various methods of introducing a nanopowder of the SHS-Az brand of composition (AlN-35% Na3AlF6) into the melt of aluminum (or its alloys), it was possible to obtain cast Al-AlN nanocomposites with a nanoscale reinforcing phase AlN content of up to 7.7% [10-12].
This paper presents the results of studies of the possibility of fabrication of Al-AlN nanocomposite with a significantly higher content (up to 75% by weight) of AlN nanoparticles by a solid-phase powder metallurgy method using PA-4 aluminum powder and SHS-Az nanopowder (AlN-5% Na3AlF6).
The technological scheme for the fabrication of such aluminum-matrix composite hardened with AlN nanoparticles included the following powder metallurgy operations: mixing of the initial powders, one-side cold pressing of the powder mixture, and sintering in vacuum without a load. The following powders were used as the initial powders: aluminum powder PA-4 with average particle size of 70-80 p,m and SHS-Az nanopowder (AlN-5% Na3AlF6) in the form of AlN nanofibers with diameter of 100-300 nm and length of 3 p,m with an impurity 5% Na3AlF6 halogen salt. This nanopowder was synthesized beforehand using the azide technology of SHS by burning a mixture of 20Al + (NH4)3AlF6 + 6NaN3 powders [13].
The preparation of the powder mixes Al-AlN was carried out by mechanical activation of the initial powders with the addition of 1% paraffin (as binding) and without the addition of paraffin in the planetary ball mill Fritsch Pulverisette 5 within 1 minute with speed of rotation of 250 rpm of hard-alloy grinding balls by diameter of 10 mm. The powder mixtures Al-AlN with the content of the disperse phase AlN of 5, 10, 15, 20, 25, 30, 50, and 75% were prepared. According to random samples taken from the prepared mixtures, the powder components are almost evenly distributed throughout the volume of the obtained powders. The maximum decrease in the average size of powder particles is observed in mixtures of Al-50% AlN and Al-75% AlN. The smallest average size of powder particles of 15-50 p,m was obtained in mixture of Al-75% AlN. The maximum values of the specific surface and bulk density of the powders were obtained from mixtures of Al-50% AlN and Al-75% AlN, and were respectively 10.57 m2/g and 0.993 g/cm3; 13.73 m2/g and 0.995 g/cm3. The highest values of the mass fraction of the denser AlN phase in these powder mixtures explains the maximum values of the bulk mass and specific surface of the powder mixture Al-75% AlN. However, all the obtained Al-AlN powder mixtures did not have flowability, since the initial powders PA-4 and AlN-5% Na3AlF6 are not flowable.
The compaction of the powder mixtures Al-AlN into briquettes was carried out with the PSU-50 press by uniaxial cold pressing with pressure of 200 and 300 MPa in the cylindrical die with the internal diameter of 16,5 mm. The height of briquettes was up to 4 mm, the weight was up to 2.5 g. The highest value of compact density was obtained for Al-50% AlN and Al-75% AlN mixtures, 2.86 g/cm3 and 2.95 g/cm3, respectively, what is explained by the highest values of the mass fraction of the denser AlN phase in these powder mixtures.
The sintering of the obtained briquettes was carried out in the vacuum furnace of VKPGr model at temperatures of 580 and 650°C, the vacuum was 210-1 mm of a mercury column, isothermal exposure time was 40 min.
At compaction of the powder mixtures Al-AlN without paraffin, the press tool (the punch and the die) previously was greased with Vaseline (thickness of lubricant did not exceed 1 mm). The use of the lubricant is explained by the need to reduce the energy and power parameters of pressing, as well as the prevention of possible welding and adhesion of aluminum to the press tool during the process of deforming powders, which can lead to the scratches on the surface of
ISHS 2019 Moscow, Russia
the compacted samples. On the surface of the Al-AlN compacts without the addition of paraffin, the scratches were observed, that was a defect of processing. With increase in the content of the reinforcing AlN phase in samples prepared without the addition of paraffin, the number of scratches increased, that led to the distortion of the samples.
In Al-AlN samples prepared with the addition of paraffin, the minimum porosity was observed under a compaction pressure of 300 MPa. The maximum porosity in the compacted samples Al-AlN was observed at the AlN content of 50 and 75% and was 20 and 25%, respectively, that resulted in the increased brittleness of these samples and complicated their sintering.
After sintering in vacuum at temperature 650°C, almost all Al-AlN samples exhibited the shape distortion (warping of samples). The maximum porosity in Al-AlN samples was observed after sintering at 650°C with aluminum nitride content of 50 and 75% and was 18 and 23%, respectively, and after sintering at 580°C, it was 15 and 20%, respectively. Since the sintered Al-AlN samples containing 50 and 75% aluminum nitride were quite porous, further studies were carried out with sintered Al-AlN samples with aluminum nitride content up to 30%.
At sintering at temperature of 650°C, strong oxidation and exudation of aluminum from the studied samples were observed, that led to strong distortion of samples at this temperature. After sintering at smaller temperature of 580°C the samples Al-AlN with addition of paraffin, which were compacted under a pressure of 300 MPa, the external defects were not observed.
In Al-AlN samples (sintered at 580°C), the oxygen content was noticeably less than in samples sintered at 650°C, the element nitrogen was found in small mass fractions. This result can be explained by the fact that the liquid phase of aluminum does not appear at 580°C, the processes of oxidation of aluminum and aluminum nitride proceed, but not as intensively as at 650°C, and in the absence of the liquid phase, there is no Al exudation from the sample.
Minimal porosity after sintering was achieved in the sample Al-5% AlN and amounted to 4.5%; the maximum porosity was in the sample Al-75% AlN and amounted to 20%. Sintered Al-AlN samples with aluminum nitride content of 50 and 75% were obtained rather porous and brittle, therefore, the actual incorporation of AlN nanoparticles in this amount into aluminum by the solid-phase powder metallurgy method in the conditions under consideration is not possible. Figure 1 shows the microstructures of samples Al-5% AlN and Al-30% AlN obtained by sintering at temperature of 580°C.
a b
Fig. 1. Microstructure of Al-AlN samples after sintering at temperature of 580°C: (a) Al-5% AlN; (b) Al-30% AlN.
According to Fig. 1, the pores are observed in the fabricated samples, the presence of which is confirmed by the calculated values of the porosity of the sintered samples, studied by the hydrostatic weighing method.
The X-ray phase analysis of sintered Al-AlN samples containing aluminum nitride from 5
to 30% showed the actual phase composition of the composite materials under investigation, according to which three phases were found: free aluminum, aluminum oxide and aluminum nitride.
Thus, the use of solid-phase powder metallurgy technology made it possible to incorporate up to 30% (by weight) AlN nanoparticles, obtained by SHS azide technology, into aluminum-matrix composite.
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