Microstructure and mechanical properties of difficult to weld Rene 77 superalloy produced by laser powder bed fusion

Fabrication of gamma' precipitation strengthened nickel-based superalloys via laser powder bed fusion still remains a challenge. In this study, Rene 77, a high gamma' containing superalloy that is considered as difficult to weld, was processed by laser powder bed fusion. Crack-free parts with high density were fabricated without any compositional modifications or preheating of the built plate. This defect-free structure was maintained upon solutionizing and aging heat treatment. The microstructure of the samples has been characterized in detail following the fabrication and after the heat treatment. Scanning electron microscopy analysis revealed that the as-built microstructure consists of columnar grains mainly aligned in the <100> direction along with extremely fine gamma' precipitates and spherical cell boundary carbides. The grain structure and texture were unaffected by the applied heat treatment due to the pinning effect exerted by the carbide particles. Development of a bimodal gamma' distribution including cuboidal primary and spherical secondary precipitates was observed in the heat-treated sample. Additional carbide formation as a discontinuous grain boundary film was also seen. Tensile deformation behavior for both conditions was also tested at mom temperature and 810 degrees C. Measured strength values for all test conditions were higher compared to a wrought and heat-treated alloy tested at room temperature. The asbuilt sample showed hardening and loss of ductility during elevated temperature testing due to gamma' precipitation at the test temperature. The microstructure of the heat-treated sample was not altered during testing at 810 degrees C. However, improved elongation behavior and transition in fracture mode from cleavage to ductile fracture were observed due to micmtwin formation at elevated temperatures.

Automated summary

The study successfully fabricated crack-free parts of Rene 77 high gamma' containing superalloy through laser powder bed fusion, maintaining a defect-free structure post solutionizing and aging heat treatment. The microstructure analysis revealed columnar grains, fine gamma' precipitates, and a bimodal gamma' distribution after heat treatment. Tensile deformation behavior testing showed higher strength values for all conditions compared to a wrought and heat-treated alloy tested at room temperature.