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Process Understanding and Weldability of Laser-Powder Bed Fusion Manufactured Alloy 718
University West, Department of Engineering Science, Division of Welding Technology. (PTW)ORCID iD: 0000-0002-4598-4790
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Laser-powder bed fusion (L-PBF) is an additive manufacturing (AM) process that involves building components by fusing fine metal powders using laser. There is no universal set of process parameters that can yield optimum results for all the different materials, geometries, and L-PBF machines. The research performed at hand regarding the process parameters showed that laser power, scanning speed, laser exposure time, and laser point distance are the most influential process parameters to decrease the amount of lack of fusion. In contrast, gas porosities are unavoidable in the L-PBF material because they can occur either because of powder particles containing inherent gas pores from the powder atomisation process or entrapped shielding gas during the L-PBF process. 

To fully utilise the L-PBF technique as a commercial production process, joining of small parts to build large-sized or complex shaped components, such as structural components for jet engines, can be a solution. The as-built microstructure of L-PBF-manufactured superalloy Alloy 718, which is the material in focus in the present research, has grains mostly oriented in the building direction of the part with a very fine cellular-dendritic structure within them. The microstructure of the alloy also contains NbC, TiN and low melting Laves phase in the interdendritic regions and along the grain boundaries. The specimens in this study were subjected to different heat treatments, such as hot isostatic pressing (HIP), solution heat treatment, and solution and ageing heat treatment, prior to welding, to study the effect of these heat treatments on the microstructure of the L-PBF-718 with regard to the susceptibility towards heat-affected zone (HAZ) liquation cracking during welding. Results showed that L-PBF-718 was susceptible to HAZ cracking during welding in all material conditions. L-PBF-718 subjected to HIP was more prone to HAZ cracking while welding and revealed a lower ductility behaviour in comparison to L-PBF-718 in the as-builtixcondition and wrought Alloy 718. The welding direction with respect to the graingrowth direction in L-PBF-718 was also found to have a significant influence onhot cracking susceptibility. The extent of HAZ cracking was observed to besmaller in samples welded parallel to the elongated grain orientation than insamples where the welding was performed perpendicular to it.

Abstract [sv]

Den laserbaserade pulverbäddmetoden (L-PBF) är en av de vanligaste additiv tillverkningsmetod, där komponenter byggs upp genom att smälta metallpulver med hjälp av laser. Det finns ingen universell uppsättning av parametrarna som kan användas för alla olika material och geometrier. Forskningen som här utförts beträffande processparametrar visade att lasereffekten, skanningshastighet, exponeringstid för laser och dess avstånd mellan två laser punkter var väsentliga för att minska antalet bindfel i materialet. Gasporositet var dessvärre oundvikligt då det primärt härstammar från antingen pulverframställningen eller gasinneslutning under L-PBF processen.

För att kunna utnyttja L-PBF-tekniken fullt ut som en kommersiell produktionsprocess kan sammanfogning av små L-PBF-delar för att bygga större eller mer komplexa komponenter, till exempel komponenter för jetmotorer, vara en lösning. Mikrostrukturen hos L-PBF tillverkad superlegering 718, som undersökts i detta projekt, har kristallkorn som mestadels är utsträckta i komponentens byggriktning. Kristallkornen i den tillverkade komponenten har en mycket fin cellulär till dendritisk struktur. Legeringens mikrostruktur innehåller dessutom utskiljningar som niobkarbid, titannitrid och den lågsmältande lavesfasen i de interdendritiska områdena samt längs korngränserna. Det genomfördes olika värmebehandlingar för att studera dess inverkan påvarmsprickor. Varm Isostatisk Pressning (HIP), upplösningsbehandling ochåldring utfördes på L-PBF-tillverkat material före svetsning för att jämföra medL-PBF-718-material utan värmebehandling samt även för att jämföra med 718 i plåtform. Resultaten visade att L-PBF-718 var känsligt för sprickbildning i HAZunder svetsning efter alla de typer av värmebehandlingar som undersöktes. Orsaken till varmsprickorna var relaterat till återuppsmältning av sekundära faser.

L-PBF-718 som utsatts för HIP-behandling visade sig vara mer benägen försprickbildning och var dessutom mindre duktilt. Svetsriktningen i förhållande till riktningen för korntillväxt visade sig också ha en betydande inverkan på känsligheten för varmsprickor. Sprickor i HAZ observerades dessutom vara markant mindre i de prover som var svetsade parallellt med tillväxtriktningen äni prover där svetsningen utfördes vinkelrätt mot den.

Place, publisher, year, edition, pages
Trollhättan: University West , 2020. , p. 88
Series
PhD Thesis: University West ; 35
Keywords [en]
Laser-Powder Bed Fusion; Alloy 718; Process Parameters; Heat Treatments; Weldability; Hot Cracking; Varestraint Testing
Keywords [sv]
Laser-pulverbäddmetoden; Legering 718; Processparametrar; Värmebehandlingar; Svetsbarhet; Varmsprickor; Varestraintprovning
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
URN: urn:nbn:se:hv:diva-15047ISBN: 978-91-88847-53-9 (electronic)ISBN: 978-91-88847-54-6 (print)OAI: oai:DiVA.org:hv-15047DiVA, id: diva2:1412680
Public defence
2020-04-02, Albertssalen, 10:00 (English)
Opponent
Supervisors
Available from: 2020-03-10 Created: 2020-03-06
List of papers
1. Influence of Heat Treatments on Heat Affected Zone Cracking of Gas Tungsten Arc Welded Additive Manufactured Alloy 718
Open this publication in new window or tab >>Influence of Heat Treatments on Heat Affected Zone Cracking of Gas Tungsten Arc Welded Additive Manufactured Alloy 718
Show others...
2019 (English)In: Metals, E-ISSN 2075-4701, Vol. 9, no 8, article id 881Article in journal (Refereed) Published
Abstract [en]

The weldability of additive manufactured Alloy 718 was investigated in various heat-treated conditions. The microstructure of the base metal was examined in detail in order to understand the effect of different pre-weld heat treatments; i.e., solution, solution and aging, and hot isostatic pressing. After welding, the variation in total crack lengths, maximum crack length and the total number of cracks in the heat affected zone (HAZ) were used as criteria for the cracking susceptibility of each material condition where wrought Alloy 718 was used as the reference material. Selective laser melting (SLM) manufactured Alloy 718 was susceptible to HAZ cracking in all material conditions. Total crack lengths in HAZ were highest in the SLM as-built condition and lowest in the SLM hot isostatic pressed condition. The cracks that were found in the HAZ of the welded materials consisted of liquation cracks, with eutectic product surrounding the cracks, as well as cracks from which liquation products were absent.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
selective laser melting; Alloy 718; heat treatments; gas tungsten arc welding; heat affected zone cracking
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-14595 (URN)10.3390/met9080881 (DOI)000484510000071 ()2-s2.0-85073280907 (Scopus ID)
Available from: 2019-10-16 Created: 2019-10-16 Last updated: 2020-03-06
2. Varestraint testing of selective laser additive manufactured alloy 718: influence of grain orientation
Open this publication in new window or tab >>Varestraint testing of selective laser additive manufactured alloy 718: influence of grain orientation
2019 (English)In: Metals, E-ISSN 2075-4701, Vol. 9, no 10, article id 1113Article in journal (Refereed) Published
Abstract [en]

The effect of grain orientation on hot cracking susceptibility of selective laser additive manufactured Alloy 718 was investigated by Varestraint testing. Electron backscattered diffraction showed that cracks in heat affected zone (HAZ) of the welded samples occurred in high angle grain boundaries. The extent of HAZ cracking was smaller in samples tested parallel to the elongated grain orientation and larger in samples transverse to the elongated grain orientation. However, for solidification cracking in the weld metal, no significant difference with respect to grain orientation in the base metal was found. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.

National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-14708 (URN)10.3390/met9101113 (DOI)000498219400092 ()2-s2.0-85074226227 (Scopus ID)
Funder
Vinnova
Available from: 2019-11-14 Created: 2019-11-14 Last updated: 2020-03-06
3. Microstructure of Selective Laser Melted Alloy 718 in As-Manufactured and Post Heat Treated Condition
Open this publication in new window or tab >>Microstructure of Selective Laser Melted Alloy 718 in As-Manufactured and Post Heat Treated Condition
2018 (English)In: Procedia Manufacturing, E-ISSN 2351-9789, Vol. 25, p. 450-458Article in journal (Refereed) Published
Abstract [en]

Manufacturing of Alloy 718 with selective laser melting (SLM) process faces several challenges. One of the challenges is the process-induced porosities in as-manufactured SLM parts. Another challenge is the microstructure of Alloy 718 that contains a high amount of segregating elements, such as MC-type carbides and γ/Laves phase eutectics in interdendritic regions. The microstructural heterogeneity in the as-manufactured SLM part unavoidably leads to mechanical heterogeneity and hence, post-processing heat treatments become necessary to achieve a homogeneous microstructure. Therefore, by investigating various post heat treatment options the knowledge on how to decrease/eliminate these segregations will be developed. © 2018 Elsevier B.V. All rights reserved.

Keywords
Selective laser melting, Alloy 718, Process parameters, Post heat treatments
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13921 (URN)10.1016/j.promfg.2018.06.100 (DOI)2-s2.0-85065659917 (Scopus ID)
Conference
8th Swedish Production Symposium, SPS 2018; Waterfront Convention CentreStockholm; Sweden; 16 May 2018 through 18 May 2018; Code 147722
Funder
VinnovaSwedish National Space Board
Available from: 2019-06-05 Created: 2019-06-05 Last updated: 2020-03-06Bibliographically approved
4. Varestraint weldability testing of additive manufactured alloy 718
Open this publication in new window or tab >>Varestraint weldability testing of additive manufactured alloy 718
2018 (English)In: Science and technology of welding and joining, ISSN 1362-1718, E-ISSN 1743-2936, Vol. 23, no 7, p. 606-611Article in journal (Refereed) Published
Abstract [en]

The weldability in terms of susceptibility towards hot cracking of selective laser melted (SLM) Alloy 718 was investigated and wrought Alloy 718 was used as reference material. Varestraint testing was carried out by means of investigating the weldability in three conditions; (1) SLM as-built, (2) hot isostatic pressing at 1160°C at 105MPa for 3h and (3) wrought Alloy 718 in the mill-annealed condition. The material exhibited intergranular cracking in all three conditions; however,theSLMHIPedconditionhadanincreasedmagnitudeofcracksusceptibilityatallstrain levels.TheincreasedsensitivityoftheHIPedmaterialwasduetothesignificantlylargergrainsize incomparison.TheSLMas-builtconditionshowedthesamedegreeofcrackingasthewrought material.

Keywords
Weldability; Selective laser melting; Alloy718
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-12077 (URN)10.1080/13621718.2018.1437338 (DOI)000440719100008 ()2-s2.0-85041916665 (Scopus ID)
Funder
VINNOVASwedish National Space Board
Note

Published online: 12 Feb 2018

Available from: 2018-02-13 Created: 2018-02-13 Last updated: 2020-03-06Bibliographically approved
5. A review of selective laser melting: Process parameters and its influence on microstructure, defects and strength in superalloy Alloy 718.
Open this publication in new window or tab >>A review of selective laser melting: Process parameters and its influence on microstructure, defects and strength in superalloy Alloy 718.
2016 (English)Conference paper, Oral presentation only (Other academic)
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-10582 (URN)
Conference
Aerospace Technology Congress 2016, Stockholm, Sweden, October 11-12, 2016
Available from: 2017-01-13 Created: 2017-01-13 Last updated: 2020-03-06Bibliographically approved

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