Selective laser melting (SLM) for 3D metal object production is a rapidly developing field of science and technology. 3D printing of aluminum alloys and aluminum matrix composites (AMC) is pretending to become the leading technology for the production of complex shape details for aerospace and automotive engineering [1
The SLM process parameters (laser power, scanning speed, powder feeding rate, etc.) have a decisive influence on the 3D object characteristics [8
]. The quality of the initial powder is quite significant as well [12
]. The properties of aluminum-based alloys are well suited to produce complex shape objects of high strength and density by SLM processes [14
There are many problems in structure formation (for example, porosity) of 3D sintered objects formed by using the powders with a broad particle size distribution. The powders with spherical particles having a narrow size distribution is the best initial material to obtain 3D objects with high quality by SLM. This provides a compact packaging of particles in a melted layer and a stable feeding rate of the powder. For example, aluminum powders with a particle size from 30 to 100 μm are typically used for 3D printing by the SLM process [15
Disadvantages of aluminum powders for additive technologies are their poor flowability, low emissivity factor, high conductivity, and low mechanical properties of synthesized 3D objects [17
]. On the opposite side, aluminum powders reinforced with refractory additives allow to obtain sintered objects with excellent properties, such as good wear resistance, high hardness, and tensile strength [19
]. In such powders, aluminum is the matrix phase and it forms a percolating network during sintering, and the refractory additive is the reinforcement and crystallization center [20
]. SiC, TiC, TiB2
, or Al2
can be used as a refractory additive [21
]. However, the use of Al2
eliminates the possibility of formation of side phases during sintering and interaction of the reinforcement with the aluminum matrix. aluminum-alumina composites are considered as raw materials for the synthesis of potential lightweight and high-strength alloys for aircraft and automotive industry [19
Aluminum particles covered with an alumina shell (Al core and Al2
shell) is an interesting raw material for 3D printing. The Al core and Al2
shell structure of the particles has higher stability, emissivity, thermal resistance, and aging properties in comparison with non-oxidized aluminum particle [22
]. The flowability of such powder is also much higher due to lower surface energy and, as a result, lesser cohesion forces. Despite some advantages, high aluminum oxide content in the obtained object could decrease its strength properties. That is why alumina content in composites should be strictly controlled.
There are different ways of obtaining aluminum-alumina powder composites from aluminum powder. The most obvious is the oxidation of aluminum by air or oxygen [23
]. However, the oxidation of small particles causes the formation of hollow alumina spheres [23
]. As the initial material for the SLM process, hollow alumina has unacceptable characteristics (see Figure 1
). Oxidation of aluminum powder in water is one of the possible solutions [26
]. The aluminum particle oxidation process can be stopped at any oxidation degree by reagent separation (water and aluminum) at a certain time. The oxidized powder can be deleted from the water by filtering or sedimentation. This method is perfectly suitable for producing Al core and Al2
shell powdery composites.
The oxidation of nano- and micron-sized aluminum powders by water is widely studied [22
]. The majority of publications [22
] dealt with the different points of obtaining hydrogen by the reaction of aluminum with water. The primary purpose of such works is an increase in the Al + H2
O reaction rate because aluminum is passivated by oxide layer [26
]. Many works on the aluminum powder oxidation by water and steam are devoted to the theoretical aspects of the aluminum oxidation and hydrogen obtaining [28
]. Some studies have shown that high oxidation temperatures lead to the following problems: Poorly ordered crystal structure of oxidation products, agglomeration of particles, low specific surface area and micropores in oxidation products [31
]. The effect of mechanochemical activation on the reaction kinetics in Al–H2
O system was studied, and the effect of alkali metals was investigated [22
]. The oxidation of the micron-sized aluminum powder by water for the synthesis of pore-free aluminum-alumina composites was also proposed [35
]. This oxidation leads to the formation of aluminum particles with an oxide surface layer (Al core and Al2
The aim of this work is to study the process of oxidation of aluminum with water for the manufacture of Al core and Al2O3 shell powder composites and the obtained powder’s comprehensive characterization.
The method of obtaining aluminum-alumina powdery composites for their subsequent sintering by selective laser melting has been studied. Initial micron-sized aluminum powders were oxidized in water at 120–200 °C and 0.15–1.8 MPa pressure. Oxidation products were dried at 120 °C and calcined at 600 °C. Four high-pressure high-temperature oxidation modes were considered. The alumina content, particle morphology, and particle size distribution for the obtained aluminum-alumina powdery compositions were studied by XRD, SEM, laser diffraction, and volumetric methods.
The temperature of the oxidation process affected the alumina content in the composites. The alumina content was 10.0 wt. % for 120 °C and it was increased to 20.0 wt. % at 200 °C. The beginning of the oxidation reaction of aluminum for all modes was observed at 68 °C.
The particles of the initial aluminum powder had a spherical shape and did not change significantly after processing. The average particle size and size distribution did not differ significantly from the initial aluminum powder indicating low porosity of the formed oxide layer. According to the obtained characteristics of aluminum-alumina powdery composites, they are suitable for further sintering. However, for the removal of agglomerates and additional sieving with 100 µm mesh size is necessary.
Synthesis of 3D objects from aluminum-alumina powdery composites by the SLM process will be considered in the future.