Mechanical properties of Invar 36 alloy additively manufactured by selective laser melting

Invar 36 alloy is widely used in aerospace engineering, owing to its extremely low coefficient of thermal expansion. This work aims to explore the underlying influence of process parameters on the microstructures and mechanical properties of Invar 36 additively manufactured by selective laser melting (SLM) with island scanning strategy. A full factorial design for a wide range of SLM process parameter sequence was established. Density and the mechanical properties including hardness, tensile strength as well as microstructures, fractography were characterized. The results confirm that the increment of exposure time and the reduction of point distance remarkably induce stable melting and achieve superior density and hardness. The laser energy density shows a pronounced effect on the microstructures, as extremely low laser energy density induces considerable lack-of fusion pores, unmelted powder particles and blurry boundaries of islands. While, very high laser energy density induces a few keyhole pores, and makes the laser scanning tracks and island boundaries transfer to be irregular. The optimal laser energy density is recommended to be 99.2 J/mm³, and the as-fabricated Invar 36 shows the relative density of 99.5%, Vickers hardness of 1.8 GPa and ultimate tensile strength of 480 MPa those are very comparable to conventionally fabricated one. The testing temperatures of 200 °C and 600 °C induce the coarsening of γ phase, and result in a significant degeneration of tensile properties. The correlations between the SLM process and mechanical properties provide experimental basis of the additive manufacturing Invar 36 alloy.