Alloy 718: Laser powder bed additive manufacturing for turbine applications

Abstract

Alloy 718 has been utilized successfully in both static and rotating turbo machinery applications for four decades. The combination of high strength, fatigue capability, rupture strength, corrosion and creep resistance at temperatures through 650 °C are key attributes of this alloy. Conventional manufacturing routes include cast, wrought, sheet, joining and fabrication by welding and brazing, powder metallurgical processing and metal injection molding. Recent investigation of aerospace materials like Alloy 718 produced by additive manufacturing technology has provided an opportunity for disruptive component manufacturing methods, geometries, and component capabilities which expand design space for complex applications. At GE Aviation (GEA), development of laser powder bed direct metal laser melting (subsequently referred to as DMLM) Alloy 718 was a natural choice following the successful commercial application of DMLM CoCrMo in GE Aviation and Safran’s LEAP platform fuel tip component and the GE90 T25 sensor part. GEA’s DMLM 718 development started with demonstrator military applications, and now has expanded to include multiple commercial engine applications across the size range of the GEA product line. The additive (laser) process development for Alloy 718 involved a combination of laser processing parameter investigation and heat treatment development to produce both acceptable build geometry and metallurgical microstructures. Initial developments started with 20 μm build layer thickness, and expanded to 50 μm thicknesses for improved build productivity. From the onset, the materials engineering focus was on isotropic, fine grain, pore free and fully developed microstructures for Alloy 718 by heat treat design. Mechanical characterization included consideration of build direction, machine type, machine to machine variation and processing gas effects. This paper will discuss various metallurgical challenges and related, microstructure & mechanical characterization of DMLM Alloy 718.