Microstructures, heat treatments and mechanical properties of AerMet100 steel fabricated by hybrid directed energy deposition

Abstract

As a kind of directed energy deposition (DED) method, wire and arc additive manufacturing (WAAM) is a solution for the short-process manufacturing of medium complex and integral parts. Ultrahigh-strength steel AerMet100 has been widely applied in large-scale aerospace structural parts like landing gear and the increasing demand of integrated design promotes the application of WAAM. The microstructure and mechanical properties of as-deposited AerMet100 steel are greatly affected by post-heat treatments, and thus researches on heat treatments are essential to the wider application of AerMet100 in the aerospace industry. In this paper, an AerMet100 steel sample was fabricated by WAAM with in situ microrolling (a hybrid DED method). Two heat treatment designs, based on standard and homogenization, were applied to the sample. The microstructures of the as-deposited and heat-treated specimens were examined with optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD), and the mechanical properties were examined using an electronic universal testing machine. The results indicate that austenitization is the main grain refinement mechanism of the as-deposited specimens, and a smaller layer height and an appropriately large heat input are beneficial for the refinement. Cellular segregation formed during solidification has severe heredity and strong effects on the microstructures of the nonhomogenized heat-treated group, causing lower mechanical properties and anisotropy. A special fracture mechanism related to segregation causes fracture morphology featuring parallel structures in both the non-homogenized X-direction tensile specimens and X-Y direction KIc specimens. Compared with the nonhomogenized heat-treated group, homogenization heat treatment basically eliminated cellular segregation and achieved a better combination of the tensile and KIc properties.