Systematic Discussion on Three Typical Materials and Application Challenges of Aerospace Additive Manufacturing

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Aerospace is one of the key directions for the development of the world's scientific and technological powers. Its development is inseparable from metal components with the characteristics of lightweight, difficult processing, and high performance. Laser additive manufacturing has opened up a new process for the design and manufacture of high-performance metal components, which can solve the new challenges of materials, structure, process, performance and application in the development of aerospace and other fields.

Professor Gu Dongdong, editorial board member of China Laser and School of Materials Science and Technology of Nanjing University of Aeronautics and Astronautics, wrote a long review paper on Laser Additive Manufacturing of Aerospace High Performance Metal Components, which systematically discussed the laser additive manufacturing of 3 types of typical materials and 4 types of typical structures in the aerospace field and the progress of aerospace applications, the direction of material-structure-process-performance integration of laser additive manufacturing technology is summarized and prospected. In this issue, 3D printing technology reference to the summary of the content of the material part of the key content of the introduction, belonging to the aerospace topic one.

Laser Additive Manufacturing of Aluminum Alloy and Aluminum Matrix Composites

Aluminum alloys and aluminum matrix composites are typical difficult materials for laser additive manufacturing, which is determined by their special physical properties (low density, low laser absorption rate, high thermal conductivity and easy oxidation, etc.). From the point of view of the additive manufacturing process, the density of aluminum alloy is small, the powder fluidity is relatively poor, the uniformity of laying on the SLM forming powder bed is poor or the continuity of powder transport in the LMD process is poor, so the precision and accuracy of the powder/powder delivery system in the laser additive manufacturing equipment are required to be high.

Compared with the wide applicability of titanium-based, nickel-based, etc. to SLM and LMD processes, the research work and application verification of laser additive manufacturing of aluminum-based materials are more focused on the SLM process. At present, there are more than 10 kinds of aluminum alloy and aluminum matrix composite materials based on SLM forming, and most of them are Al-Si series. Due to the material nature of cast aluminum alloy, the tensile strength of such alloys is difficult to exceed 400MPa even if they are prepared by optimized process, thus limiting their use in aerospace bearing components with high service performance requirements.

Mechanical properties of laser additive manufacturing aluminum alloy and its composites

In order to obtain higher mechanical properties, Al-Cu, Al-Mg, Al-Zn and other systems have also been used as SLM forming materials in recent years. However, the high alloy element content and wide cooling and solidification temperature range in this kind of aluminum alloy make the precipitation strengthened alloy easy to form cracks and even crack during laser additive manufacturing. Compared with aluminum, magnesium and lithium are more likely to vaporize under the high temperature action of high-energy laser, thus, the composition stability and mechanical properties of the formed parts are affected. Therefore, the design and regulation of composition, physical parameters and phase change are particularly important for laser additive manufacturing of high-strength aluminum alloys. In recent years, people have designed a Al-Mg-Sc-Zr alloy powder modified and reinforced by rare earth element scandium specially for laser additive manufacturing. After additive manufacturing and supplemented by appropriate heat treatment process, its comprehensive mechanical properties can be significantly improved (tensile strength is higher than 500MPa, elongation rate is more than 10%).

The preparation of aluminum matrix composites is an important way to strengthen and toughen aluminum alloys. Aluminum matrix composites have the excellent characteristics of light alloy, ceramic, fiber and other reinforcements, high specific strength, specific modulus and volume stability, and excellent performance such as high temperature resistance, wear resistance and oxidation resistance, as well as material designability. Laser additive manufacturing aluminum-based composite materials highlight the "multi-phase material designability" in the material selection, emphasize "high controllability" in the additive manufacturing process, and highlight "high performance/multi-function" in the use effect, which also represents an important development direction of additive manufacturing technology. Nano-ceramic reinforcement and in-situ ceramic reinforcement can effectively improve the wettability and bonding of the ceramic/metal interface, inhibit the microscopic pores and cracks on the interface, and improve the mechanical properties of laser forming parts.

Laser Additive Manufacturing of Titanium Alloy and Titanium Matrix Composites

Titanium-based materials are widely used in aerospace, biomedical, food and chemical industries due to their excellent specific strength, corrosion resistance and biocompatibility, and are often used in the field of additive manufacturing. The current challenges in laser additive manufacturing of titanium-based alloys are:

1) Laser additive manufacturing forming fully dense complex structure titanium-based components are still difficult, forming process components are prone to produce pores, cracks and surface spheroidization and other processing defects, these processing defects often become adiabatic shear band and crack sprouting source, reduce the mechanical properties and service performance of the forming parts.

2) The extremely large cooling speed and temperature gradient in the laser additive manufacturing process will induce martensitic transformation, causing large residual stress inside the component; With the increase of the number of processing layers, the residual stress gradually increases, resulting in the formation of hot cracks and the forming parts are prone to warping. When the processing defects accumulate to a certain extent, the forming parts will crack and seriously reduce the plasticity and toughness of the parts.

3) In the process of laser processing, the heat flow is mainly conducted along the direction parallel to the additive manufacturing, which is easy to form a coarse columnar crystal tissue, resulting in a strong anisotropy of the microstructure and mechanical properties of the component.

Mechanical Properties of Titanium and Titanium Alloys by Laser Additive Manufacturing

Titanium-based materials show strong applicability to both SLM and LMD laser additive manufacturing processes. At present, titanium alloys used in laser additive manufacturing are mainly concentrated on industrial pure titanium (CP-Ti) and traditional titanium-based materials such as TC4. The microtissue regulation of laser additive manufacturing components is the basis of its mechanical performance improvement, and the tissue evolution is controlled by the process, so the high-performance component laser additive manufacturing needs to establish the integrated regulation theory and method of material-tissue-process-performance.

Due to the fast cooling rate of the molten pool in the laser additive manufacturing process and the large temperature gradient along the additive manufacturing direction, the solidification tissue of titanium alloy is often columnar crystal structure, resulting in the anisotropy of the mechanical properties of the forming parts. In order to improve the anisotropy and mechanical properties of titanium alloy during laser additive manufacturing, it can be improved from the aspects of material design (such as alloying) and process optimization (such as applying composite energy field).

In addition to the idea of alloying to develop laser additive manufacturing of new titanium alloys, the preparation of ceramic-reinforced titanium-based composites is also an important means to improve the mechanical properties of titanium-based components. Titanium has strong chemical activity, and titanium components are easy to react with other components in situ in the process of laser additive manufacturing, which significantly increases the difficulty of regulating the phase and tissue of laser forming materials, so the selection of ceramic reinforcement phase of titanium-based composite materials should be careful.

Laser Additive Manufacturing of Nickel-based Superalloys and Their Composites

Nickel-based superalloy itself contains more alloying elements, which generally have problems such as strong crack sensitivity, serious element segregation, significant anisotropy of microstructure, and poor controllability of mechanical properties in the laser additive manufacturing process. On the one hand, chromium and aluminum elements with strong oxygen affinity in nickel-based alloys are easy to interact with oxygen elements in the forming atmosphere under high temperature to form fine oxide slag, but the wettability between them and the matrix interface is poor, resulting in cracks and reducing mechanical properties; on the other hand, carbon, niobium, molybdenum and other elements are easy to aggregate at grain boundaries, significantly increasing the content of eutectic phase with low melting point, the formation of hot cracks in the heat affected zone is aggravated. In addition, various types of grain boundary precipitates consume the strengthening phase-forming elements in the nickel matrix, which significantly reduces the mechanical properties of laser additive manufacturing nickel-based components.

At present, the laser additive manufacturing of nickel-based high-temperature alloys is mainly focused on the Inconel series of alloys, in which the precipitation-enhanced Inconel718 and solid-soluble Inconel625 are weldable, and are also suitable for laser additive manufacturing processes based on powder melting/solidification metallurgical processes. The microstructure regulation of laser additive manufacturing nickel-based high-temperature alloy is mainly achieved by optimizing the process parameters and then changing the temperature gradient, solidification speed and cooling rate of the molten pool, and then combining the subsequent heat treatment process to achieve the regulation of grain shape, size and precipitation phase morphology, content and distribution. In addition, the optimized laser scanning strategy can also change the grain growth texture to obtain high strength and toughness nickel-based alloy materials.

Mechanical Properties of Ni-based Superalloy and Its Composites by Laser Additive Manufacturing

Heat treatment can realize the strengthening of laser additive manufacturing nickel-based high-temperature alloy, but will sacrifice the toughness of the material to a certain extent, at the same time, the post-treatment needs to reasonably control the heating temperature, insulation time, cooling medium and hot and other static pressure and other parameters, the cost is higher, the process is more complex, the probability of defect formation is also greater. The hot isostatic pressing (HIP) technology based on high temperature and high pressure treatment can eliminate the residual pores in the laser additive manufacturing nickel-based high-temperature alloy components, inhibit the initiation and propagation of cracks, and then improve the mechanical properties of the formed parts.

The preparation of ceramic reinforced nickel-based composites is another important way to improve the mechanical properties of nickel-based superalloys, which can make the composites have higher specific strength, specific stiffness and heat resistance without reducing the toughness.

Summary

In general, lightweight high-strength alloys represented by aluminum and titanium alloys, as well as load-bearing heat-resistant alloys represented by Ni-based high-temperature alloys, are one of the key materials developed in new material research and development plans in various countries, and are also important application materials in laser additive manufacturing. The characteristics of the research and development of additive manufacturing materials can be summarized in three points:

The development of new high-performance materials is the basic guarantee for the improvement of the mechanical properties and application level of laser additive manufacturing components.

Nanocomposite, in-situ reinforcement and gradient interface design are effective ways to enhance the strengthening and toughening of traditional metal laser additive manufacturing.

Laser additive manufacturing process regulation and technological innovation are the fundamental means to improve the microstructure and performance of metal components.