As advanced high strength steel, maraging steel is mainly used in the fields of aviation, atomic energy, and high-end tooling. The traditional methods for the preparation of maraging steel including cold rolling, homogenizing treatment, solution treatment and aging are complex, time-consuming and costly. Additive manufacturing (AM) has a wide application in the forming of complex shaped parts, which mainly depends on its flexibility, parameter controllability, and high cooling rate. AM can provide an effective way to promote martensite transformation, grain refinement, and forming of complex internal cavity structural molds.
The combination of AM technology and maraging steel has great advantages in realizing personalized customization and reducing subsequent processing. The AM of maraging steel opens a new direction for the preparation of high-end die steel. However, it has complicated solidification metallurgy and solid phase transformation behaviors, although maraging steel fabricated by AM has excellent strength, toughness, hardness, corrosion resistance, and wear resistance. Therefore, it is of great significance to deeply understand the correlation among process, composition, and microstructure of maraging steel fabricated by AM.
To improve the mechanical properties of maraging steel fabricated by AM, the post-treatment methods mainly including heat treatment and hot isostatic pressing (HIP) are used, which can effectively decrease the internal defects of forming parts and reduce the internal stress, the porosity, and the un-melted powder particles. However, the maraging steel fabricated by AM has anisotropic mechanical properties along the building direction and the scanning direction. Therefore, the process and properties of maraging steel fabricated by AM need to be further studied.
With the maturity of laser AM technology and the improvement of mechanical properties of building parts, the AM of maraging steel is more widely used in industry and daily life. Laser cladding AM technology has also been applied to metal surface coating, but the research of laser cladding AM technology on coating is relatively less. Above all, the review and prospect of the AM of maraging steel are important in guiding the development of maraging steel with high-performance by AM and regulating of microstructures.
The advances in the improvement of mechanical properties, the post-treatment technology, and the microstructural control of maraging steel are reviewed. Table 1 summarizes the typical mechanical properties of maraging steel fabricated by AM in recent years. There are many researches on additive manufacturing of maraging steel. Bai’s research showed that with the increase of solid solution temperature, the hardness and strength of maraging steel were significantly improved with the formed Ni3Mo, Fe2Mo and Ni3Ti precipitation. Tan et al. eliminated the residual pores of parts by HIP. Compared with that of the as-fabricated parts, the porosity of the maraging steel treated by HIP is obviously reduced [Figs. 2(d-e)]. Yin et al. studied the microstructure of LPBF-18Ni300 maraging steel and found that the very fine cellular structure could improve the hardnesses and other properties of parts [Fig. 2(c)]. According to the transfer mode of powder and the thermal history effect of AM, the heterostructural maraging steel can be obtained. Kurnsteiner successfully prepared Damascus-like maraging steel with a composition of Fe-19Ni-5Ti through direct energy deposition (DED), and the ultimate tensile strength of heterostructural steel was 200 MPa, higher than that of conventional homostructural steel [Figs. 3 and 4(a)]. Tan’s research proved that the heterostructural steel could also be obtained by using the alternating layer printing method [Fig. 4(b)]. AM can achieve a variety of comprehensive properties by combining two metal materials together, i.e., functionally graded structural materials [Figs. 4(c) and 5(b)]. The results show that the thermal diffusion interface between two different materials has good interface bonding and mechanical property stability. Particle-reinforced maraging steel composites with high strength and wear resistance can be prepared by adding ceramic powder in maraging steel. It shows that the WC particles can significantly reinforce the maraging steel composites and the excellent comprehensive mechanical properties are obtained.
The AM of maraging steel breaks through the limitation of a traditional process and shows a great prospect in the manufacturing of molds with complex structures. However, further researches on the AM of maraging steel are needed. In terms of composition design, the development of low alloy elements, the reduction of preparation cost, and the development of special powder for the AM of maraging steel become new research contents. On the optimization of process parameters, reducing laser power and improving scanning speed are worth considering for improve the efficiency and reduce the cost. The microstructure of maraging steel fabricated by AM is dominated in columnar grains, resulting in the mechanical anisotropy between the building direction and the scanning direction, which is also a vital problem to be solved. These defects or undesired microstructures mentioned above can be avoided by controlling the scanning mode of AM in the future, i.e., controlling the energy output and properly preheating of the substrates. An artificial neural network plays an important role in the field of materials science. In the future, machine learning can also be used to assist the composition design of maraging steel, the optimization of process parameters, and the prediction of initial temperature for martensite transformation (Ms).
