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Role of Superficial Defects and Machining Depthin Tensile Properties of Electron Beam Melting (EBM)Made Inconel 718
KTH, School of Industrial Engineering and Management (ITM), Production Engineering.ORCID iD: 0000-0002-2582-9910
KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Manufacturing and Metrology Systems.ORCID iD: 0000-0002-5960-2159
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.ORCID iD: 0000-0002-6339-4612
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2021 (English)In: Journal of materials engineering and performance (Print), ISSN 1059-9495, E-ISSN 1544-1024Article in journal (Refereed) Published
Abstract [en]

Since there is no report on the influence of machining depth on electron beam melting (EBM) parts, this paper investigated the role of superficial defects and machining depth in the performance of EBM made Inconel 718 (IN718) samples. Therefore, as-built EBM samples were analyzed against the shallow-machined (i.e., only removal of outer surfaces) and deep-machined (i.e., deep surface removal into the material) parts. It was shown that both as-built and shallow-machined samples had a drastically lower yield strength (970 ± 50 MPa), ultimate tensile stress (1200 ± 40 MPa), and ductility (28 ± 2%) compared to the deep-machined samples. This was since premature failure occurred due to various superficial defects. The superficial defects appeared in two levels, as (1) notches and pores on the surface and (2) irregular pores and cracks within the subsurface. Since the latter occurred down to 2 mm underneath the surface, shallow machining only exposed the subsurface defects to outer surfaces. Thus, the shallow-machined parts achieved only 68% and 8% of UTS and elongation of the deep-machined parts, respectively. This low performance occurred to be comparable to the as-built parts, which failed prematurely due to the high fraction surface voids and notches as well as the subsurface defects.

Place, publisher, year, edition, pages
2021.
Keywords [en]
additive manufacturing, electron beam melting, failureanalysis, Inconel 718, near-net shaped manufacturing
National Category
Manufacturing, Surface and Joining Technology Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-290946OAI: oai:DiVA.org:kth-290946DiVA, id: diva2:1531818
Note

QC 20210824

Available from: 2021-02-26 Created: 2021-02-26 Last updated: 2024-01-17Bibliographically approved
In thesis
1. Additive manufacturing of Ni-based Superalloys- an analysis of parameter and strategy driven properties in Electron Beam Melting Process
Open this publication in new window or tab >>Additive manufacturing of Ni-based Superalloys- an analysis of parameter and strategy driven properties in Electron Beam Melting Process
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Metal AM processes produce significantly rough surfaces as compared to wrought or machined components. Currently, the as-EBM surfaces are extremely rough and consequently, the as-built components could not be directly used as commercial products. The state-of-the-art research has focused primarily on deeply machined (DM) samples. The mechanical properties of the EBM components as reported thus are not real representative of the process. This can largely dispute the main merits of the EBM process: print on demand and low buy-to-fly ratio. Hence, the research here was designed to bridge this research gap and systematically analyze the properties of net-shaped (NS) and near-net-shaped (NNS) components.

This thesis is divided into three levels, including the fundamental, validation and optimization level. The fundamental research work is performed on the commercialized processing parameters from Arcam, for Inconel 718 (IN718). The NNS samples made with and without the commercial multi-spot contour are tested for their mechanical properties. These samples achieved smoother surfaces and better tensile behavior. Thus, they were later compared with the NS and DM samples which were made with the same parameter settings but varied machining depth. The purpose of this work was to test the NS and NNS samples and analyze the cause of the failure. It was found that the premature failure of these samples was linked to the porous surface and subsurface region. Therefore, a machining protocol was set up according to the depth of the porous region which can theoretically improve the tensile strength of the machined samples to a level comparable to wrought material. The findings in case of IN718 were also validated for another superalloy, Inconel 625(IN625). The results again showed that the machining requirement of the as-EBM samples with contour is more than 2 mm. Therefore, a novel contouring strategy was later developed, optimized and tested in order to eliminate the machining requirement. To do so, the continuous contour with different energy, overlapping ratio and sequence to the hatching region was tested. The high-energy continuous contour applied after hatching was found to be the optimal strategy, which was able to maintain a comparable surface condition as the multi-spot contour whilst it could generate a denser subsurface region. Accordingly, the NNS samples with such contour achieved comparable tensile properties as DM samples.

It should be noted that all these research works were focused on the vertical samples (along the building direction) to avoid high thermal stresses in horizontal direction that seriously limit deformation free manufacturing of the samples. To deal with the challenge of thermal stresses and deformation, the last part of this research was designed to assess the thermal stresses and strains of EBM manufactured IN625 samples. The effects of different processing parameters and scanning strategies were tested to provide a set of guidelines about how to produce the horizontal and longer components. The results showed that one could effectively minimize the thermal stress/strain and deformation of the parts using a bidirectional scanning pattern with proper layer rotations angles to deliver shorter scans in each layer.

Abstract [sv]

Additiva tillverkningsprocesser i metall ger avsevärt grövre ytor jämfört med smidda eller maskinbearbetade komponenter. Ytor tillverkade med as-EBM blir, med dagens teknik, extremt grova och följaktligen kan dessa komponenter inte användas direkt som kommersiella produkter. Den senaste forskningen har primärt fokuserat på djupt bearbetade (DM) prover. De mekaniska egenskaperna som rapporterats för dessa EBM-komponenter är således inte representativa för processen. Detta kan till stor del ifrågasätta de viktigaste fördelarna med EBM-processen: beställtryck och låg volymkvot mellan materialbit och komponent. Således var denna forskning utformad för att överbrygga detta forskningsgap samt systematiskt analysera egenskaperna hos slutformade (NS) och nära slutformade (NNS) komponenter.

Denna avhandling är indelad i tre nivåer bestående av grundläggande, validering och optimering. Det grundläggande forskningsarbetet har utförts på kommersialiserade processparametrar från Arcam och materialet Inconel 718 (IN718). NNS-proverna gjorda med och utan den kommersiella smältningsstrategin med flerpunktskontur har testats för utvärdering av mekaniska egenskaper. Dessa prover uppnådde jämnare ytor och bättre dragegenskaper. Således jämfördes de senare med NS- och DM-proverna vilka gjordes med samma parameterinställningar men olika bearbetningsdjup. Syftet med detta arbete var att testa NS- och NNS-proverna och analysera orsaken till brott. Det konstaterades för dessa provbitar att tidiga brott var kopplat till den porösa ytan och regionen nära ytan. Därför implementerades en bearbetningsprocedur upp i enlighet med djupet av den porösa regionen vilket teoretiskt kan förbättra draghållfastheten för de bearbetade proverna. Detta till en nivå som är jämförbar med smidda material. Utfallet med IN718 kunde även valideras för en annan superlegering, Inconel 625 (IN625). Resultaten visade återigen att bearbetningskravet för as-EBM-proverna med strategin för flerpunktskontur är mer än 2 mm. Därför utvecklades, optimerades och testades en ny kontureringsstrategi med syfte att eliminera bearbetningskravet. För att göra detta testades strategier med kontinuerlig kontur och olika energi, överlappningsförhållande och sekvens till regionen med fyllnadsläggning. Den kontinuerliga högenergi-konturen som applicerades efter fyllnadsläggning visade sig vara den optimala strategin som kunde bibehålla jämförbar kondition på ytan som strategin med multipunktskonturen, samtidigt som den generera en tätare struktur i regionen under ytan. Följaktligen uppnådde NNS-proverna med sådan kontur jämförbara dragegenskaper som DM-prover.

Det bör noteras att forskningen var fokuserad på vertikala prover (längs byggriktning) för att undvika höga termiska spänningar i horisontell riktning som kraftigt begränsar deformationsfri tillverkning av provbitar. För att hantera utmaningen med termiska spänningar och deformation utformades den sista delen, inom denna avhandling, för att bedöma termiska spänningar och töjningar av EBM-tillverkade provbitar i IN625. Effekten av olika processparametrar och skanningsstrategier utvärderades för att ta fram en uppsättning riktlinjer gällande tillverkning av horisontella och längre komponenter. Resultaten visade att termisk spänning, töjning och deformation kund minimeras genom att använda ett dubbelriktat skanningsmönster med rätt lagerrotationsvinklar för att ge kortare skanningar i varje lager.

Place, publisher, year, edition, pages
Stockholm Sweden: KTH Royal Institute of Technology, 2022. p. 133
Series
TRITA-ITM-AVL ; 2022:16
Keywords
Additive manufacturing, Electron beam melting, Super-alloys, process optimization, surface defects, subsurface defects, mechanical properties, thermal stresses, thermal deformation
National Category
Production Engineering, Human Work Science and Ergonomics
Research subject
Production Engineering
Identifiers
urn:nbn:se:kth:diva-311557 (URN)978-91-8040-259-0 (ISBN)
Public defence
2022-06-10, M311 / https://kth-se.zoom.us/j/63824172963, Brinellvägen 68, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2022-05-19 Created: 2022-05-18 Last updated: 2022-06-25Bibliographically approved

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Zhao, XiaoyuRashid, AmirHulme-Smith, ChristopherDadbakhsh, Sasan

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