Hot workability and microstructure evolution of the nickel-based superalloy Inconel 718 produced by laser metal deposition

The manufacturing of parts from nickel-based superalloys used to be dominated by conventional processes such as forging, which use expensive dies and require cost-intensive machining operations to produce the final part shape, due to the limited ability of the forging process to produce near-net shape parts. The ongoing efforts in developing additive manufacturing (AM) processes could promote an increasing competition of AM with traditional processes. AM offers various benefits such as near neat shaping, integration of functions into parts and the possibility to locally engineer components. However, manufacturing costs and process time in additive manufacturing rise rapidly with part size. For many applications, the disadvantages of AM and forging operations could be levered by mating both processes to new process chains. Adapted pre-forms for forging operations could be created by printing onto existing stock material, thus allowing to reduce the number of forging steps and to avoid high material waste. However, up to the present day, only very limited knowledge is available on the forming behavior of additively manufactured materials. In this paper, the hot deformation behavior and the associated microstructural changes of samples made from the nickel-based superalloy Inconel (R) 718 using laser metal deposition (LMD) have been studied using isothermal hot-compression tests. The as-built state is shown not to be suitable for hot working due to the presence of the Laves phase, which has to be removed by heat treatment. Wrought samples and heat-treated LMD samples are compared with respect to their hot working behavior. Pronounced shifts of characteristic points of the flow curves between both types of material are observed. The results show that heat-treated LMD samples are well hot workable, preferentially at higher values of the Zener-Hollomon parameter where incipient yield and initial work hardening occur at lower stress levels. (C) 2018 Elsevier B.V. All rights reserved.