Surface Integrity of Machined Advanced Nickel Alloys Produced by an Additive Manufacturing Route

Abstract

Inconel 718 is a widely used Nickel-based alloy in the aerospace sector. However,
its exceptional mechanical properties make it a very difficult material for machining operations. Additive manufacturing presents itself as a promising alternative, as it would reduce the need for post-printing material removal by machining. However, not enough research has been done about the behaviour of this material in machining process. In order to introduce this technology into high-demanding applications, much more is to be studied about the mechanical integrity of machined surfaces of additively manufactured superalloys.

The present report studies the effect of machining parameters such as feed, cutting speed and tool wear on the appearance of the white layer and the depth of subsurface damage due to plastic deformation. This is done by means of electron microscopy, EBSD, nanoindentation and x-ray diffraction for residual stresses and pole figure acquisition.

The different machining parameters studied did not substantially affect the width of the white layer, with values between 0.6 and 1 µm. The swept region depth was observed to be affected mainly by the presence of wear on the cutting tool, effectively doubling the depth of subsurface damage for the highest value studied. No clear correlation was found between this plastic deformation and the values of speed and feed.

Both cutting directions, parallel and perpendicular to the build direction, showed different behaviours against the same machining conditions. Cutting along the build direction showed slightly decreased depth of the plastically deformed region and residual stresses on the machined surface.

Popular Abstract

Inconel 718 is a popular Nickel-based superalloy used in high-temperature applications in fields like aerospace or nuclear energy. Additive Manufacturing, commonly known as 3D printing, arises as an alternative manufacturing route to overcome challenges imposed by conventional processes. Processes like Laser Beam Powder Bed Fusion enable the manufacturing of more complex and near-net shapes than forging or casting. However, for most cases, some post-processing in the form of heat treatments or machining is needed to meet very demanding requirements of the produced components. Due to the extreme plastic deformation involved in the machining process, the appearance of features like a white layer and subsurface damage in the form of strain hardening can compromise the mechanical performance and fatigue lifetime of machined components.

This report investigates the effect of three cutting parameters (feed, cutting speed and tool wear) on the surface integrity of an additively manufactured Inconel 718 workpiece subjected to an orthogonal cutting operation. We also study differences when cutting in two different directions: parallel and perpendicular to the build direction in the printing process.

Surface integrity is studied by making use of a number of different techniques. First, Electron Channelling Contrast Imaging (ECCI) is used to quickly observe the white layer and the depth of the deformation region. Further analysis is done by using Kernel Average Misorientation Maps (KAM) obtained through Electron-Backscattered Diffraction (EBSD), and hardness profiles obtained by nanoindentation. Additional experiments are done involving residual stresses and pole figure acquisition from X-Ray Diffraction (XRD) on the machined surface.

• author: Carrero Robles, Jesus
• supervisor: Rachid Msaoubi; Filip Lenrick
• organization: Production and Materials Engineering
• course: MMTM05 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: superalloy, Inconel 718, additive manufacturing, surface integrity, white layer
• report number: LUTMDN/(TMMV-5373)/1-58/2025
• language: English
• id: 9185860
• date added to LUP: 2025-03-01 13:31:16
• date last changed: 2025-03-01 13:31:16