Unique thermal expansion behaviors and magnetic properties of Super Invar alloy fabricated by laser powder bed fusion

The highly-anticipated combination of the laser powder bed fusion (LPBF) process and the Fe-Ni Invar alloy material leads to its even-puzzled thermal expansion behaviors. In this study, the unique thermal expansion behaviors and magnetic properties of Super Invar alloy (Fe-32Ni-4Co) samples manufactured via LPBF within the volumetric laser energy density (E_V) range of 59.52–89.29 J/mm³ were thoroughly investigated. Overall, a positive correlation among the average grain size, proportion of low-angle grain boundaries (LAGB), texture intensity, Curie temperature and E_V while a negative one between dislocation density and E_V could be identified within the investigated E_V range despite the weird E_V = 74.40 J/mm³. These samples fabricated at this critical E_V oddly exhibit the highest LAGB proportions, abnormal dislocation density increase, strongest texture intensity and geometry-dependent Curie temperature shift (approximately 8–10 ℃ enhancement of vertical-built sample over horizontal-built one), which crashes with many experiential conclusions in previous researches upon LPBF manufactured Invar 36 alloy (Fe-36Ni). Quite little martensite formed around −120 ℃ within LPBF processed Super Invar alloy contributes to its unexpected dimensional stability in cryogenic scenarios. LPBF processed Super Invar alloy generally shows much lower average coefficient of thermal expansion (CTE) within the temperature range of 30–100 °C (-0.04 ×10⁻⁶∼0.41 ×10⁻⁶ °C⁻¹) compared to LPBF built Invar 36 (∼0.36 ×10⁻⁶ °C⁻¹, 30–100 °C) and standard Fe-32Ni-5Co Super Invar alloy (0.84 ×10⁻⁶ °C⁻¹, 30–100 °C). Mainly attributed to the considerably high residual stresses, all vertical-built samples possess notable negative thermal expansion behaviors, and LPBF fabricated Super Invar alloy anomalously exhibits even larger average CTEs than the standard suggests and LPBF processed Invar 36 alloy within a wider temperature range of 30–300 or 30–400 ℃, and the widely-reported positively-correlated relationship between CTE and saturation magnetization within traditional Fe-Ni Invar alloy could not be expanded to LPBF built Super Invar alloy.