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Influence of Heat Treatments on Heat Affected Zone Cracking of Gas Tungsten Arc Welded Additive Manufactured Alloy 718
University West, Department of Engineering Science, Division of Welding Technology.ORCID iD: 0000-0002-4598-4790
University West, Department of Engineering Science, Division of Welding Technology.ORCID iD: 0000-0002-0234-3168
University of Manitoba, Department of Mechanical Engineering, Winnipeg, MB R3T 5V6, Canada.
University West, Department of Engineering Science, Division of Welding Technology.ORCID iD: 0000-0001-9065-0741
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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. Vol. 9, no 8, article id 881
Keywords [en]
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: urn:nbn:se:hv:diva-14595DOI: 10.3390/met9080881ISI: 000484510000071Scopus ID: 2-s2.0-85073280907OAI: oai:DiVA.org:hv-14595DiVA, id: diva2:1361525
Available from: 2019-10-16 Created: 2019-10-16 Last updated: 2020-03-06
In thesis
1. Process Understanding and Weldability of Laser-Powder Bed Fusion Manufactured Alloy 718
Open this publication in new window or tab >>Process Understanding and Weldability of Laser-Powder Bed Fusion Manufactured Alloy 718
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
Laser-Powder Bed Fusion; Alloy 718; Process Parameters; Heat Treatments; Weldability; Hot Cracking; Varestraint Testing, 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:nbn:se:hv:diva-15047 (URN)978-91-88847-53-9 (ISBN)978-91-88847-54-6 (ISBN)
Public defence
2020-04-02, Albertssalen, 10:00 (English)
Opponent
Supervisors
Available from: 2020-03-10 Created: 2020-03-06

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Raza, TahiraHurtig, KjellAndersson, JoelSvensson, Lars-Erik

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