Investigation of structure, mechanical properties and crystallization of aluminum alloys containing aluminum oxide nanoparticles

A357 and 6082 aluminum alloys strengthened by aluminum nitride nanoparticles were obtained. The process of crystallization of the A357-0.5 wt% Al2O3 and 6082-0.5 wt% Al2O3 alloys was studied under conditions of varying the cooling rate. The A357 and 6082 aluminum alloy structure and hardness were analyzed for the Al2O3 content from 0 to 1 wt%.


Introduction
Aluminum and its alloys are widely used in modern industries [1,2] such as machine building, aerospace, shipbuilding, chemical, electrical engineering, etc. Their wide application is due to the presence of a number of properties, namely high corrosion resistance, small linear shrinkage, crack resistance, low mold shrinkage and good fluidity [3][4][5]. The development of nano-and microtechnologies for producing various powder materials [6][7][8] allows one to apply them in non-ferrous metallurgy and create metal matrix nanocomposites (MMNC) and metal matrix composites (MMC) based on traditional aluminum alloys. In this case, composite material properties will be determined by the type and amount of the particles introduced. The results obtained [9][10][11][12][13][14] allow us to assert that MMNCs based on aluminum and with nanoparticles introduced into them have higher performance characteristics, in particular, ultimate strength, yield strength, plasticity, hardness, heat resistance, etc. [15][16][17][18]. The production of MMNCs based on aluminum is possible in different ways, such as by casting (in a chill mold, sand, centrifugal, etc.), powder metallurgy methods (solid-phase sintering [19], liquid-phase sintering [20], hot pressing [21], shockwave compaction [22]), impregnation [23], laser cultivation, etc. [24][25][26]. However, casting is the most universal way among the listed ones, because it has the following advantages: high productivity, simple equipment, its ability to fabricate large-volume and large-weight billets, its ability to produce complex geometric shapes, low cost of castings, etc.
Using alumina particles is of great interest as this compound remains stable over time, unlike carbides or magnesium oxides which can react with water and decompose.
When nanoparticles are introduced into the melt during casting, there exist a number of problems: agglomeration and floating to the surface because of the poor wettability of the particle surface by the metallic melt. These can be solved in various ways, namely by introducing nanoparticles through master alloys and treating the melt by external means (mechanical mixing, electromagnetic, vibration or ultrasonic treatment). Ultrasonic treatment of the melt seems to be a promising technology for producing composite materials based on light alloys. Ultrasonic treatment contributes to the intensification of various processes in the melt due to cavitation, namely those of degassing, refining the structure (grain refinement), particle dispersion and deagglomeration and their redistribution [27][28]. In addition, when obtaining reinforced aluminum alloys, an important parameter is the cooling rate which can significantly affect the final properties of the casting. The presence of nanoparticles in the melt can significantly affect the crystallization of the melt during the cooling process [29].
The aim of the work was to obtain aluminum alloys containing alumina nanoparticles and study their structure and mechanical characteristics at various cooling rates.

Materials and methods
The initial materials used in this work were aluminum oxide nanopowder (<100 nm) obtained by the electric explosion of wire (EEW) technique [30], aluminum powder ASD6 (<50 μm) and aluminum alloys (6082 and A357).

Preparation of powders
For aluminum oxide nanoparticles to be introduced, a powder mixture of Al-5wt% Al 2 O 3 was prepared. To provide deagglomeration and a uniform distribution of the nanoparticles in the powder mixture, stirring was carried out using stearic acid as a surfactant. 200 ml of petroleum ether and 1.5 wt% of stearic acid were added to the powder mixture. After that, its mechanical stirring was carried out for 20 minutes. Finally, the powder mixture was air-dried and sieved.

Obtaining the master alloy
A master alloy is a metal matrix with particles and is intended to be introduced into the melt. To obtain the master alloy needed based on the previously prepared powder mixture, we applied the shock-wave compacting method. As an explosive, we used the commercial explosive Uglenit whose density is ~ 1.2 g/cm 3 , the detonation speed is 2300 m/s and the detonation pressure is 2 GPa. A detailed scheme of shock-wave compacting of powder mixtures is described in this paper [31].

Preparation of aluminum alloys
The 6082 and A357aluminum alloys were melted in a graphite crucible in air with the total volume of each melt of 500 g. The ultrasonic treatment was carried out using a 5 kW water-cooled magnetostrictive converter with an operating frequency of 17.5 kHz. A conic waveguide with an operating amplitude of ~ 30 μm was made of niobium. Ultrasonic degassing was carried out at a melt temperature of 730°C for 1 minute. Then, the obtained master alloy (Al-5 wt% Al 2 O 3 ) was introduced into the zone of ultrasonic cavitation in the melt directly under the waveguide. After introducing the master alloy, the melt was ultrasonically treated for another 2 minutes at a temperature of 730°C. Then, the melt was cast into a metal mold at a temperature of 710°C. The nominal content of aluminum oxide nanoparticles in the alloys was 0.5 and 1wt%. The initial alloys were obtained using the similar treatment parameters, but without introducing a master alloy.
To investigate the influence of the cooling rate on the structure and properties of the aluminum alloys containing alumina nanoparticles, wedge-shaped castings were made, at the base of which a smaller cooling rate was realized, while a higher cooling rate was at the top.

Research methods
Obtaining the samples of composite materials was followed by the preparation of metallographic sections to study the composite microstructures. The specimens prepared were chemically etched in a solution of picric acid to reveal the grain boundaries of the resulting material. Analysis of the structure of the samples was carried out using an Olympus GX71 optical microscope. To calculate the average grain size, the random secant method was used. Figure 1 shows the optical microstructure images of the 6082-0.5 wt% Al 2 O 3 composite in various sections of the wedge-shaped specimen. It can be seen that with decreasing the size of the casting section (an increase in the cooling rate), the grain size decreases from 200 to 100 μm, that can be directly related to the change in the cooling parameters in different zones of the wedge-shaped casting. At the same time, the alumina nanoparticles are likely to have little effect on the crystallization of the 6082 aluminum alloy. The average dendritic cell size of the 6082 alloy is reduced from 42 to 13 µm. Figure 2 shows the structure and the grain size distribution for the 6082 aluminum alloys before ( Figure 2a) and after introducing nanoparticles (Figures 2b, с).

Microstructure of the 6082-Al 2 O 3 alloy
According to the obtained images the average grain size of the aluminum alloy with 0.5 wt% of the nanoparticles introduced increases from 155 to 240 μm and slightly changes from 240 to 220 μm after increasing the content of alumina (up to 1wt%). The negative effect of nanoparticles can be due to their agglomeration during the production process. Agglomeration and the inability to break such agglomerates with ultrasound lead to a decrease in the efficiency of ultrasonic treatment of the aluminum matrix.  Table 1 shows the results of the microhardness measurements (HV) for the composites obtained. It can be seen that the introduction of aluminum oxide nanoparticles does not change the 6082 alloy hardness. At the same time, the results of the microhardness measurements for the initial alloy have a wide range of values, which decreases with the introduction of nanoparticles. This may be due to hardening of the aluminum substrate, but the hardness of the alloy surface areas containing intermetallics [32] is higher and the total contribution of nanoparticles to hardening is negligible. Figure 3 shows the optical microstructure images of the A357-0.5 wt% Al 2 O 3 composite in various sections of the wedge-shaped specimen. It can be seen that the average size remains practically unchanged, that indicates an insignificant effect of the cooling rate and the presence of nanoparticles on the alloy structure. Meanwhile, the cooling rate significantly affects the average dendritic cell size of the А357 alloy, which is reduced from 152 to 45 µm. Table 2 illustrates some change in the hardness of the resulting alloys.  Figure 4 shows the microstructure and the grain size distribution in the A357-0.5 wt% Al 2 O 3 ( Figure 4a) and A357-1wt% Al 2 O 3 (Figure 4b) aluminum alloys. It can be seen that introducing the nanoparticles does not affect the hardness of the A357 aluminum alloy, that may be due to the fact that the process of obtaining such a composite needs their proper introduction and distribution, so the method applied requires further analysis and optimization.

Conclusion
The cooling rate of the 6082 aluminum alloy has been revealed to directly affect its grain size.
The introduction of more than 0.5 wt% of aluminum oxide nanoparticles has been found to lead to negative consequences for the microstructure of the 6082 alloy (an increase in the average grain size of the alloy was from 155 μm to 220-240 μm).
The change in the cooling rate and the introduction of aluminum oxide nanoparticles into the A357 alloy have been shown to slightly affect the change in the average grain size of the alloy.
Also, it has been found that the introduction of aluminum oxide nanoparticles does not significantly affect the hardness of the aluminum alloys.