Characterisation of Nickel-Based Superalloys Manufactued by Electron Beam Melting

Doctoral thesis, 2010

Nickel-based superalloys have been manufactured by electron beam melting (EBM). EBM is an additive manufacturing process for direct production of metal parts. The part is build up by successive melting of metal powder layers in a vacuum chamber using electron beam. In this study, the microstructure, texture and the physical properties of nickel-based superalloys manufactured by EBM were thoroughly characterised by different methods, e.g. XRD, DTA, SEM, EDS, EBSD, TEM and APFIM. The investigated alloys were Inconel alloy 600, Udimet alloy 720 and Inconel alloy 718; they are strengthened by different hardening mechanisms. The EBM materials were highly textured and oriented with the <100> directions in the building direction and in the scanning directions of the electron beam. Furthermore, the grains are elongated in the building direction and are up to several millimetres in length. The mechanical properties of heat treated Inconel alloy 718 manufactured by EBM are to large extent comparable with conventional material. Additionally, the same alloys were manufactured by casting and directional solidification, in order to compare the solidification and precipitation behaviour with the EBM samples. The solidification rate was fastest in the EBM process and slowest in directional solidification. Macrosegregation did not occur in the EBM samples, but in the cast and directionally solidified samples. Enhanced understanding about rapid solidification and local melting has been gained during this study. This should be of value also for similar processes, e.g. additive manufacturing and welding. EBM is a promising technology for direct manufacturing of highly textured nickel-based superalloy parts. A suitable application would be parts currently manufactured by directional solidification. Using EBM, both cost and time can be reduced as no mould is required. Furthermore, it would be possible to tailor both the geometry and the texture of each single part in the EBM process.