The research group of Prof. Zhu Qiang in made the latest progress in the field of performance improvement for additive manufacturing composites
Recently, Ph.D. candidate of Professor Zhu Qiang's research team from Department of Mechanical and Energy Engineering of Southern University of Science and Technology, Guo Chuan, has published the latest research results in the leading journal of material corrosion: Corrosion Science, and proposed a new technology to significantly improve the corrosion resistance of a nickel-based superalloy manufactured by the laser powder bed fusion additive manufacturing technology, using oxide nano-particles.
Additive manufacturing, commonly known as 3D printing, is a new component manufacturing technology to manufacture or repair complex components in a layer by layer manner. 3D printing is considered to be a novel manufacturing technology leading “3rd manufacturing revolution”. Laser powder bed fusion is a metallic additive manufacturing process with powders as feedstock. Nickel-based superalloys are widely used in the aerospace industry, such as turbine disks, blades, fuel nozzle and high-temperature strength and oxidation resistance are the key properties for their high-performance applications. Laser powder bed fusion provides great flexibility to directly introduce nanoparticles onto the original powders for manufacturing high-performance composites.
Oxide (Y2O3) nanoparticles with different weight contents were introduced onto the precipitate-strengthening nickel-based superalloy (IN738LC) powder and block samples were printed using the laser powder bed fusion machine. High-temperature oxidation experiments under 1095 oC were performed and the mass-gain of samples was used to evaluate the oxidation resistance of alloys. Fig. 1 shows the oxidation kinetic and oxidation rate curves under different contents of the Y2O3 addition. It can be seen that the oxidation resistance of the alloy with 0.05 wt% addition was significantly improved, while excessive addition (0.2 wt%, 0.6 wt%) would increase the oxidation rate of the alloys.
Fig. 1. The curve of the (a) oxidation kinetic and (b) oxidation rate curves under different contents of Y2O3 addition.
The surfaces of oxide scales were observed using a laser confocal microscope, as shown in Fig. 2. It is apparent that the nodular morphology and the spallation of oxide scales were the key factors affecting the surface quality. Compared with the alloys without and with 0.05 wt% Y2O3, the excessive addition aggravated the spallation of oxide scales, resulting in relatively high surface roughness.
Fig. 2. Reconstruction images of the surfaces of oxide scales with different addition of Y2O3, (a)0 wt%，(b)0.05 wt%，(c)0.2 wt% and (d) 0.6 wt%.
It is found that the oxide scale of the alloy without Y2O3 was mainly composed of a relatively thin chromium-rich surface layer and an alumina sublayer (Al2O3) (Fig. 3a). The addition of 0.05 wt% nanoparticles had no significant effect on the element distribution of the oxide scale, except that the chromium-rich layer was thicker and more continuous (Fig. 3b). However, the addition of 0.2 wt% and 0.6 wt% induced the chromium-rich layer to occupy almost the whole oxide scale, while the formation of alumina was restricted in the matrix and distributed in a dendritic and discontinuous manner (Fig. 3c and d).
Fig. 3. SEM images of the sections of oxide scales with different addition of Y2O3, (a)0 wt%，(b)0.05 wt%，(c)0.2 wt% and (d) 0.6 wt%.
Through the observation and analysis of the samples, the oxide scales were divided into two types, i.e., aluminum type (0 wt%, 0.05 wt%) and chromium type (0.2 wt%, 0.6 wt%). The formation mechanism is shown in Fig. 4. In the initial stage of oxidation, chromium oxide was first formed and reacted with subsequent nickel ions to form nickel chromate (NiCr2O4) with a spinel structure, which could prevent further reaction between the material and oxygen. Nano-Y2O3 oxide particles can promote the nucleation of chromium oxide, thus improving the oxidation resistance of the alloy (0.05 wt%). However, CET and PBR models were used to calculate the stress in the oxidation process and it is found that the stress in chromium oxide was higher than that in alumina, which made the chromium oxide scale be easier to peel off and had an adverse effect on the oxidation resistance of the alloys (0.2 wt%, 0.6 wt%).
This study solved the problem that the added particles could not be uniformly introduced in the traditional process and explained the influence of these particles on the microstructure and properties of the alloy. It provides a new technical approach for improving the corrosion resistance for nickel-based superalloy using laser powder bed fusion and a new idea for improving the performance of superalloy for aircraft engines and ground gas turbines. In the future, the research results will also have a broad development prospect in the field of key parts of aircraft and corrosion-resistant materials in the harsh environment.
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