Hybrid powder metallurgy processing for high temperature strength Ni-base superalloy Inconel 718

Jennings, Rachel Elizabeth (2022). Hybrid powder metallurgy processing for high temperature strength Ni-base superalloy Inconel 718. University of Birmingham. Ph.D.

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Abstract

Additive layer manufacturing (ALM) processes such as powder bed fusion (PBF) provide an opportunity for the manufacture of new more complex designs of Ni-base superalloy components, which have the potential to extend the operating temperatures and efficiencies of gas turbine engines. There are however challenges to the industrial implementation of ALM, in particular the slow production times associated with these processes and required post processing. In recent years hybrid additive manufacturing has been proposed as a way of increasing ALM production efficiency. The most popular hybrid concept to date combines additive and subtractive manufacturing in the direct laser deposition process.

In this work an alternative approach to hybrid manufacturing- the in-situ shelling process- is proposed for the production of Ni-base superalloy Inconel 718. This method combines PBF with Powder Metallurgy Hot Isostatic Pressing (PM HIPping). The HIPping canister is manufactured and filled in-situ during PBF, followed by consolidation of the powders inside the canister by HIPping.

Powder characteristics and their interaction with PBF and HIPping processes are examined extensively and the importance of control of powder packing densities and powder chemistry for the success of each processing route is highlighted. Parametric studies are presented for laser and electron PBF, with the aim to optimise conditions to meet the property requirements of HIPping canisters. In-situ shelling components are manufactured and the strength of bonding across the laser PBF-PM HIPped interface is examined by tensile testing. Two mechanisms for improving bond strength are proposed and tested.

Throughout this work scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and energy dispersive and wave dispersive spectroscopy (EDS & WDS) have been implemented to evaluate microstructures, grain morphologies and chemistries.

The in-situ shelling process shows promise as a production route for the manufacture of Inconel 718 when electron PBF is implemented at the shelling stage. The use of laser PBF for shelling resulted in significant oxide formation at the PBF- PM HIPped interface leading to poor bonding and a substantial diminishment in tensile properties. Yet, initial results suggest that through manipulation of the LPBF surface, improved bonding may be achievable. It is suggested that chromia oxide forming Ni-base superalloys may be better candidates for in-situ shelling owing to chromia being less stable than alumina.

The overall novelty of this work lies in the examination of Inconel 718 for the in-situ shelling process, however throughout investigations into different aspects of the in-situ shelling process, several additional novel research outcomes emerged. The diffusion zone in state-of-the-art PM HIPped material was found to result in a diminishment of tensile properties as it created a Nb concentration gradient, reducing Nb available for strengthening phases. Surface optimisation studies in in-situ shelling work showed minimising continuous oxides and increasing point contacts between power particles and the LPBF surface to result in an increase in grain growth across LPBF-PM HIPped interfaces owing to an increase in recrystallisation. Oxide chemistry was suggested to influence whether PPB formation was dictated by the oxygen content of the powder or by powder particle size in state-of-the-art PM HIPping.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):