Researchers have manufactured Super-invar Fe64-Ni32-Co4 (wt %) alloy through wire-arc additive manufacturing(WAAM), revealing the association between the micro-structure and the value of the G (the temperature gradient)/R (the solidification rate) during the deposition process, and achieving the coefficient of thermal expansion (CTE) of 0.265×10-6K-1 from 20℃ to 100℃. Published in Advanced manufacturing, the findings provide a reference for the fast fabrication of super-invar alloy components through WAAM, which promotes the applications of super-invar alloy in aerospace.
Super-invar alloy, distinguished by their extremely low coefficient of thermal expansion (CTE) below the Curie point, have promising applications in fields requiring high dimensional stability, such as electrical communication, aerospace, precision measuring devices, and railway transport. However, the machining of super-invar alloy component is challenging due to their low heat conductivity, high ductility, and noticeable work hardening. Addressing this challenge, researchers from Beijing Institute of Technology and Beijing Institute of Aerospace Systems Engineering, have manufactured a thin-wall rectangular sample by the hot-wire technique based on wire-arc additive manufacturing (WAAM).
As for WAAM, the deposited process is important because it can affect the surface quality and the mechanical properties of the deposited parts. In order to ensure the stability of the manufacturing process, an industrial high-speed camera is used to observe the manufacturing process. Researchers analyzed the transfer mode of liquid droplets during the manufacturing process and found that the main transfer mode was liquid bridge transition, which minimizes the impact on the molten pool and guarantees a better surface of the deposited parts.
The unique micro-structure was found in super-invar alloy specimens, it was columnar cellular micro-structure. Researchers established a numerical simulation model and obtained the temperature gradient and the solidification rate. In order to verify the accuracy of the temperature simulation, the infrared thermal imager was applied to measure the temperature distribution during the actual manufacturing process. Finally they found that the formation of the columnar cellular micro-structure is associated with the value of G/R.
The uniformity of the hardness distribution along the deposition direction of the sample reflects the uniformity of the material deposition and the stability of the manufacturing process. Researchers found that the hardness of the sample is uniformly distributed along the deposition direction, with a 152HV0.2 average hardness.The anisotropy of the mechanical properties was found in super-invar alloy specimens, the longitudinal specimens have better mechanical properties. In addition, both longitudinal and transverse specimens have ductile fractures.
As a low-expansion material, the coefficient of thermal expansion of super-invar alloy is the most interesting parameter for all researchers. In this study, super-invar alloy samples have an extremely low coefficient of thermal expansion, which was measured to be 0.265×10-6 K-1 from 20 ℃ to 100 ℃.
While the team acknowledges the need for further refinement in manufacturing process of super-invar alloy, this research offers some insights into the super-invar alloy components manufactured by WAAM. By verifying the mechanical properties and the coefficient of thermal expansion, this study lays the foundation for better applications in some areas of precision engineering.
This paper "Microstructure and mechanical properties of super-invar alloy fabricated by wire-arc additive manufacturing" was published in Advanced manufacturing.
Ye S, Xu L, Guo Y, Di X, Han Q, et al. Microstructure and mechanical properties of super-invar alloy fabricated by wire-arc additive manufacturing. Adv. Manuf. 2025(1):0005, https://doi.org/10.55092/am20250005.
