As the main component of the entire precision optical systems for aerospace, the dimensional stability of the lens tube has an undeniable impact on the overall accuracy and performance of space optical systems. However, the complex thermal environment in which the satellite operates for a long time, such as direct solar radiation or the Earth's infrared radiation, can significantly affect the detection accuracy of the optical system. Therefore, Invar alloy 4J36, with excellent dimensional stability, can be selected to manufacture structural components of lens tubes. Due to its unique "Invar effect", the Invar alloy 4J36 exhibits an extremely low coefficient of thermal expansion at its Curie temperature (230 ℃) , and it can be effectively used in the manufacturing of space optical devices. However, owing to the high hardness and poor machining performance of 4J36, traditional manufacturing methods, such as turning and milling, require long processing time periods and result in serious material waste. Selective laser melting (SLM) is an additive manufacturing technology that has the advantages of high design freedom, a short production cycle, and wide applicability. Furthermore, it can be used to manufacture structurally complex products at a faster rate when compared to traditional manufacturing methods. To date, the SLM additive manufacturing process for Invar alloy lens tubes is not mature. Through a series of adjustments to the powder quality, process parameters, 3D models, and residual stress, we hope to obtain high-quality structural components of lens tubes that satisfy usage requirements. This can aid in further exploration of the universe.

SLM was used to shape an Invar alloy lens tube, and the Invar alloy powder material was selected according to the standard set at the beginning of this study to ensure that the microstructure of the finished product exhibits no obvious defects. The essence of SLM technology is the direct interaction between laser and powder. Hence, to explore the optimal laser selective melting process parameters and optimize the quality of Invar alloy tube products, single factor experiments were conducted with laser scanning spacing and scanning speed ranging from 0.08 mm to 0.11 mm and 900 mm/s to 1200 mm/s, respectively. The goal was to obtain different laser energy densities. Subsequently, the surface of the test block was cleaned and polished, its surface microstructure was observed via a metallographic microscope, and its mechanical properties were characterized using an electronic universal testing machine. Additionally, we optimized the structure of the three-dimensional model of the lens tube, calculated its overall stress and strain through simulations, and compared the simulation results with those of the original model to analyze the optimization. Subsequently, heat treatment was performed on the lens tube to relieve its residual stress, and the printing accuracy and residual stresses at specific points were characterized via X-ray scanning. Finally, the thermal expansion coefficient of the lens tube after heat treatment (530 ℃±10 ℃, 1 h) was tested using a thermal expansion instrument to evaluate its structural stability.

After standardized screening, the loose density of the Invar alloy powder can reach 4.7 g/cm³, and the powder sphericity can reach up to 0.89. Additionally, the smooth flowability of the powder surface is significantly improved to 14.7 s/50 g, and no powder accumulation occurs during the additive manufacturing process using this powder material. The surface of the product was smooth and crack-free (Fig. 3). Experimental results show that the optimal scanning spacing for SLM additive manufacturing is 0.09 mm, and the surface microstructure is smooth without obvious defects such as keyholes and lack of fusion (Fig. 4); the optimal scanning speed is 900 mm/s. Simultaneously, the surface microstructure is complete and smooth, without cracks (Fig. 5), with a tensile strength of 482 MPa and yield strength of 388 MPa. They exhibit excellent mechanical properties (Table 5). After topology optimization, the service strain of the lens tube structure at the same point under the same specifications is only 0.09 mm(Fig. 6). Furthermore, when compared to traditional models, it can save materials and improve efficiency. After the stress-relief heat treatment, there is no evident defects inside the lens tube(Fig. 10), and the maximum residual stress is only 13% of its yield stress. The thermal expansion coefficient of the lens tube is 1.9×10^-6 K^-1 (Table 6), which satisfies the requirements of high dimensional stability.

This study successfully realizes high-quality manufacturing of Invar alloy lens tube using SLM additive manufacturing technology. First, by establishing physical and chemical specifications for the powder materials, macroscopic defects, such as cracks and inclusions, in the lens tube are avoided in the initial stages of the experiment. The SLM process parameters are optimized. The optimal process is determined and corresponds to a scanning spacing of 0.09 mm and scanning speed of 900 mm/s. Simultaneously, the surface microstructure is observed as smooth and free of defects such as cracks and lack of fusion. The best mechanical properties are obtained using this process. The best mechanical properties correspond to a tensile strength of 482 MPa, a yield strength of 388 MPa, an elongation of 29%, and a shrinkage rate of 73%. Topological optimization is performed using the original 3D model of the lens tube. After optimizing the structure, the overall stress concentration of the product under service conditions is significantly reduced, and the maximum deformation degree of the product is only 0.09 mm. Additionally, the structure adopts self-supporting formation, which effectively saves powder materials. Finally, the lens tube is subjected to post-treatment to eliminate residual stresses. The maximum residual stress inside the product after the heat treatment is 60 MPa, which is only 13% of its yield stress. Simultaneously, the lens tube exhibits an extremely low coefficient of thermal expansion (1.9×10^-6 K^-1), which satisfies the requirement of high structural stability of space optical lenses in complex thermal environments.