Microstructures and unique low thermal expansion of Invar 36 alloy fabricated by selective laser melting

Invar 36 alloy, which presented extremely low coefficient of thermal expansion (CTE), was fabricated by selective laser melting with island scanning strategy. The microstructures and CTE were systematically characterized. The as-SLMed Invar 36 is mainly composed of fcc γ phase and sparse bcc α precipitates, which are consistent with the phases of the wrought one. The grains grow along the maximum temperature gradient directions, as the microstructure on the side surface is dominated by large columnar grains. The cross-section shows a large number of small columnar grains in the melt pool. The low laser energy density results in the lack-of fusion pores and corresponding high level porosity up to 15%. When the laser energy density is 99.2 J/mm³ and 198.4 J/mm³, excellent quality is obtained, as the porosity is only within 0.2–0.4%. Due to the porosity and nickel element evaporation, the CTE is 1.72–1.96 × 10^–6 °C⁻¹ that is lower than that of wrought one. Besides, lower than 99.2 J/mm³, the decreasing laser energy density leads to the reduction of CTE, since the pores provide extra space for internal thermal expansion. Higher than 99.2 J/mm³, the metal vaporization rate and vapor pressure are enhanced with the increasing laser energy density, and then the loss of nickel element leads to the reduction of CTE.