Effect of building-height-dependent heat accumulation on microstructure and properties of Super Invar alloy fabricated by laser powder bed fusion

The poor thermal conductivity of Super Invar alloy (Fe-32Ni-4Co) further enhances the intrinsic building-height-dependent heat accumulation during its laser powder bed fusion (LPBF) process, resulting in its non-negligible microstructural inhomogeneity and significant property variation. In this paper, a detailed study for the effect of heat accumulation on microstructures, and thermal, mechanical and magnetic properties within a 10 × 10 × 100 mm³ vertically-built Super Invar alloy bar sample was conducted. Overall, LPBF fabricated Super Invar alloy exhibits considerably high degree of deformation and suffers very similar building-height-dependent thermal histories to the typical annealing process. The Curie temperature and Vickers hardness show a negative correlation with the lattice constant but a positive correlation with compressive residual stresses within this LPBF-manufactured Super Invar alloy bar. The vertically-measured coercivity is negatively correlated with the columnar grain size on the side surfaces, notwithstanding the pore-defect-induced coercivity enhancement within first-built layers. The segment of the bar located at a height of 50–75 mm exhibits relatively high bending degrees of magnetic domains, resulting in an extremely low coefficient of thermal expansion (CTE). In contrast, the 75–100 mm segment shows slightly higher CTE values compared to the 0–25 mm and 25–50 mm segments. The first-built segment of the Super Invar alloy bar sample contains coarse grains and significant small pores due to poor melting. This issue gradually diminishes as sufficient heat accumulates. As the building height increases further, a noticeable decline in dislocation density is observed without any change in grain size, indicating the occurrence of recovery at the building height of 30–40 mm. This recovery process causes a decrease in the Curie temperature and a reduction in hardness. Fine columnar grains can be achieved through heat accumulation processing, suggesting a critical recrystallization temperature range of approximately 316–408 °C at the building height of 40–50 mm. This recrystallization process leads to an increase in dislocation density and lattice contraction, which enhances the Curie temperature. When the building height reaches 50 mm, the microstructure, mechanical properties, and magnetic properties generally stabilize. However, when the surface temperature exceeds the recrystallization temperature (especially above 500 °C), grain growth, a decline in dislocation density, and an enhancement in saturation magnetization are observed in the last-built segment of the Super Invar alloy.