This study uses Additive Manufacturing (AM) processes to fabricate 316L stainless steel beams with pockets of unfused powder for increased damping. Modal testing was completed to compare damping factors of beams with varying pocket geometries as well as number of pockets and pocket location. For the first three bending modes that were tested, an initial damping increase was observed when pocket height is greater than powder diameter. Following the initial increase there is a height threshold, which is mode dependent, that is required to achieve a statistically significant increase in damping.

Keen interest in additive manufacturing (AM) of refractory metals such as tungsten has been motivated by the demand for materials capable of enduring extreme temperatures in aerospace and nuclear applications. The aims of this work were to develop alloy compositions for high-temperature applications in the space propulsion and nuclear fusion sectors, and to establish processing windows for these compositions fabricated using laser powder bed fusion additive manufacturing (LPBF-AM). Tungsten (W)-based alloys are well-suited for high working temperatures because of their high melting points, excellent thermal conductivity, low corrosion resistance, and low coefficient of thermal expansion. The integrated computational materials engineering (ICME) approach was implemented to establish the connections among composition-printability-microstructure-properties-performance framework. ThermoCalc-CALPHAD software was used for Scheil-Gulliver solidification simulation (SGSS) of W-based compositions with various alloying element additions. Chromium, vanadium, and niobium were down-selected as suitable alloying elements based on SGSS results. Further, addition of carbon enhanced printability due to eutectic solidification by the formation of various carbides towards the end of solidification leading to crack-free microstructure as well as being vital for control of oxygen. This work demonstrates the successful manufacturing of multiple crack-free W-based alloy components using LPBF-AM, which had a wide range of working temperatures and enhanced mechanical properties.

Fatigue performance is one of the important mechanical properties which is hindering some of the additive manufacturing build structures. The main objective of this study is to investigate the fatigue life dependence of the 21-6-9 austenitic stainless-steel build using the laser powder bed fusion (LPBF) technique. The study is mainly focused on the fracture surface based on defects and the micro constituents, how they are related to the fatigue performance of the samples. 21 samples were build using optimized process parameters in which 9 of the samples were built-in vertical direction and the remaining 12 were built in the horizontal direction. Low cycle fatigue (LCF) testing was carried out with the samples with 2 conditions i.e., room temperature 24⁰C and 750 ⁰C. Before the specimens were subjected to testing the samples were stress relived at 600⁰C for 2 hours. Fractography was carried out on the fracture surface to pinpoint the initiation sites (concluded by analyzing the striations) and the cause of failure. Microstructural characterization was carried out in both the vertical and the horizontal direction of the build. The fatigue crack growth initiated from the surface where defects were present, when the cyclic loads were applied the stress concentration at these regions caused the crack initiation to take place. In some cases, the defect size was smaller at the initiation site, and that correlated to increased fatigue life. To optimize the fatigue life of LPBF built 21-6-9 is important to minimize the grain size. However, the absence of defects is crucial as well, as those typically are the weakest link. Therefore, minimizing those during the manufacturing is essential.

GKN Aerospace AB, Sweden (GAS) is one of the leading companies taking up the charge in manufacturing components using Additive Manufacturing(ed) (AM) techniques in the aerospace sector. They are a hub of engineering and they are a supplier of engine and engine components to the world’s leading aero-engine manufacturers, and airframes to civil and military aircraft manufacturers. A phenomenon that is of interest to designers at GAS is the effects of dwell times on high temperature fatigue, especially how this phenomenon affects the fatigue properties of Laser Powder Bed Fusion (LPBF) Inconel 718 (IN718). IN718 is a versatile alloy that can be used at relatively high temperatures and has excellent weldability and is one of the newer materials replacing expensive materials such as Titanium (and its alloys) in the aerospace industry. The aerospace industry has been pushing for an increase in parts manufactured using AM processes because of the advantage the AM process grants to the production process, however a new manufacturing process for an industry needs to be studied and researched from a failure perspective, i.e. the prominent mode of failure for components manufactured using AM and the underlying factors influencing the failure mechanism must be studied. This thesis explores a solution to predict the life of components based on experimental crack propagation tests wherein the test specimens were subjected to the phenomenon mentioned above. A literature survey was conducted researching ways to model this phenomenon and the factors affecting it. The methods found in the literature survey were far too complex to model for the purposes of this thesis, additionally the methods described in the literature were empirical methods describing the phenomenon, rather than a fundamental study of factors causing the phenomenon and ways to model their influence on the life of the component. Hence, a simple method based on the Palmgren-Miner linear damage summation rule which was coded in the form of a FORTRAN code was utilized to compute the life of the components. Software runs predicting life of physical experiments were conducted and inferences about the predictive method were drawn. The limitations of this method were understood and possible solutions were explored, based on which conclusions were drawn regarding the method’s efficacy in predicting the life of the specimens that underwent dwell loading during fatigue cycling. Finally, the method was applied to a case study to understand the effectiveness of the method. / GKN Aerospace AB, Sverige (GAS) är ett av de ledande företagen som tar upp kampen vid tillverkning av komponenter med hjälp av additiv tillverkning (AM) inom flyg- och rymdsektorn. De är ett nav för ingenjörskonst och de är en leverantör av motorer och motorkomponenter till världens ledande tillverkare av flygmotorer, och civila och militära flygplanstillverkare. Ett fenomen som är av intresse för designers på GAS är effekterna av uppehållstider på högtemperaturutmattning, särskilt hur detta fenomen påverkar utmattningen egenskaper hos Laser Powder Bed Fusion (LPBF) Inconel 718 (IN718). IN718 är en mångsidig legering som kan användas vid relativt höga temperaturer och har utmärkt svetsbarhet och är ett av de nyare materialen som ersätter dyra material såsom titan (och dess legeringar) inom flygindustrin. Flygindustrin har drivit på för en ökning av delar som tillverkas med additiva tillverkningsprocesser på grund av den fördel som tillverkningsprocessen ger en ny tillverkningsprocess för en industri behöver dock studeras och forskat ur ett misslyckandeperspektiv, dvs. det framträdande sättet att misslyckas för komponenter som tillverkats med hjälp av additiv tillverkning och de bakomliggande faktorer som mekanismen måste studeras. Denna avhandling utforskar en lösning för att förutsäga livslängden för komponenter baserat på experimentella sprickutbredningstester där provexemplaren utsattes för fenomenet som nämns ovan. En litteraturstudie genomfördes för att undersöka olika sätt att modellera detta fenomenet och de faktorer som påverkar det. Metoderna som framkom i litteraturstudien var alldeles för komplexa för att modellera för denna avhandling, dessutom är metoderna som beskrivits i litteraturen var empiriska metoder som beskriver fenomenet, snarare än en grundläggande studie av de faktorer som orsakar fenomenet och sätt att modellera deras inverkan på komponentens livslängd. Därav en enkel metod baserad på Palmgren-Miners linjära skadesummeringsregel som kodades i form av en FORTRAN-kod användes för att beräkna livslängden för komponenterna. Programvarukörningar som förutspådde livslängden för fysiska experiment genomfördes och slutsatser om den prediktiva metoden drogs. Begränsningarna med denna metod förstods och möjliga lösningar utforskades. Som låg till grund för de slutsatser som drogs om metodens effektivitet när det gäller att förutsäga livslängden för de prover som genomgick uppehållsbelastning underutmattningscykling. Slutligen tillämpades metoden på en fallstudie för att förstå effektiviteten avmetod.

Additive manufacturing (AM) has the possibility of producing complex-shaped components which can not be produced by conventional manufacturing methods. This gives the opportunity for designers to freely think outside the design spectra which is otherwise limited by conventional manufacturing methods. AM of metal has rapidly been developed for the last three decades, and they now are reaching industrial acceptance levels, metal feedstock for use in AM is also rapidly growing. AM of metals is especially of interest for the tooling industry. The design freedom which AM offers the tooling manufacturer can design complex cooling channels within moulds, which could reduce cycle time and enhance the quality of components produced with the moulds. Maraging steels have been proven to both be able to be processed with AM but also have comparable performance to traditionally carbon-based used tool steels. Laser Powder Bed Fusion (LPBF) is one of the most promising AM systems today, by using powder as a feedstock it can produce high-resolution parts without needing to process them after they have been produced. However, there is a need to better understand processing within the LPBF system. This master thesis is aimed to process a newly developed maraging steel from Uddeholm, and conduct process parameter experiment and study their correlation to be able to produce samples with as few defects possible. It is crucial to conform to a good methodology for how to find those correlations and see how they influence the printed material. LPBF process has a multi-complex variable system, and by narrowing down the complexity by focus on the most influencing parameters as has been proven by many researchers. Even with a reduced focus, it is still a multi-variable problem. In this study a methodology of finding process parameters relations, a Design Of Experiment software was used, namely, MODDE. By screening of process parameter ranges, within the software, a statistical evaluation of operational process window can be found with fewer conducted experiment. Development of process parameter can traditionally be time-consuming and result in an unnecessary large number of experiments to find the operational window. The experiment showed that laser power and point distance had the most influencing effect on relative density, followed by exposure time and hatch distance. The experiment was firstly conducted with a layer thickness of 50 µm, the achieved relative density resulted in over 99.8 percent. However, a large lack of fusion defects was observed inside the specimens. Even though a high relative density was measured, a pore analysis has to be conducted to fully understand the size and shape of defects since they can have a severe impact on mechanical properties. It was believed that the layer thickness was too high and that the defects could be reduced by printing a set with same process parameters but with a lower layer thickness of 40 µm instead. The reduction of layer thickness did result in a significant decrease of the defects observed. However, future work after this thesis must be continued to further optimize and to increase the solidity of printed material to reach closer to its conventional produced relatives / Additiv tillverkning har möjligheten att producera komplext konstruerade komponenter som inte kan produceras med konventionella tillverkningsmetoder. Detta ger konstruktörer möjligheten att fritt tänka utanför designspektra som annars begränsas av konventionella tillverkningsmetoder. Additiv tillverkning av metall har snabbt utvecklats under de senaste tre decennierna och har nu nått industriella acceptansnivåer. Metallråvara för användning i additiv tillverkning växer snabbt. Additiv tillverkning av metaller är särskilt intressant för verktygsindustrin, designfriheten som additiv tillverkning kan erbjuda verktygstillverkaren för att kunna utforma komplexa kylkanaler inuti formar. Det kan således reducera cykeltiden och förbättra kvaliteten på komponenter som produceras med formarna. Maråldringsststål har visat sig att både kunna processas i additiv tillverkning och har jämförbara egenskaper med traditionellt kolbaserade verktygsstål. Pulverbäddsystemet är ett av de mest lovande systemen idag, genom att använda pulver som råmaterial kan systemet producera komponenter med hög noggranhet utan att behöva bearbeta dem efter att processen är klar. Det finns emellertid ett behov av att bättre förstå själva processen inom pulverbädds teknologin. Den här masteruppsatsen syftar till att additivt tillverka ett nyutvecklat maråldringsstål från Uddeholm. Samt att genomföra processparameterexperiment och studera deras korrelation för att kunna producera prover med så få defekter som möjligt. Det är avgörande att hitta en metod för hur man hittar korrelationerna och se hur de påverkar det tillverkade materialet. Pulverbäddsystemet har ett multikomplext variabelsystem. För att minska komplexiteten kan fokus läggas på de mest inflytelserika processparametrarna, vilket har bevisats av många forskare. Även med ett reducerat fokus är det fortfarande ett flervariabelsproblem. I denna studie användes en metod för att hitta relationer mellan processparametrar och en Design Of Experiment-programvara, nämligen MODDE. Genom screening av processparametrar, inom programvaran, kan en statistisk utvärdering av operativt processfönster hittas med färre genomförda experiment. Utvecklingen av processparametrar kan traditionellt vara tidskrävande och resultera i ett onödigt stort antal experiment för att hitta det operativa fönstret av processparametrar. Experimentet visade att lasereffekt och punktavstånd påverkande den relativa densiteten mest, följt av exponeringstiden och spåravståndet. Experimentet genomfördes först med en lagertjocklek av 50 mikrometer, lagertjockleken resulterade i en relativ densitet på över 99,8 procent. Emellertid observerades stora fusionsdefekter inuti proverna. Även om en hög relativ densitet mättes, måste en poranalys genomföras för att fullt ut förstå storleken och formen på defekter eftersom de kan ha en avgörande inverkan på mekaniska egenskaperna. Det misstänktes att lagertjockleken var för hög och att defekterna kunde minskas genom att tillverka en ytterligare uppsättning av samma processparametrar men med en lägre lagertjocklek på 40 mikrometer istället. Minskningen av lagertjockleken resulterade i en signifikant minskning av de observerade defekterna. Framgent efter den här avhandlingen måste dock arbetet fortsätta att ytterligare optimera och öka soliditeten i det additivt tillverkade materialet. Det för att uppnå bättre prover och komma ännu närmre det konventionellt tillverkade materialets egenskaper.

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Laser Powder Bed Fusion (LPBF) is an Additive Manufacturing (AM) method whereby complex parts can be built to near net geometry in an automated environment. Parts formed by LPBF have excellent properties and require little post-processing. While LPBF allows for creative designs to reduce weight and part count by integrating sub-components of assemblies, designers are limited to one material within any LPBF print. Although various materials can be used in LPBF, metals are of special interest to many industrial customers due to their high strength and toughness. The metals commonly used for LPBF include iron, aluminum, and titanium alloys. These alloys are generally designed to fulfill specific needs and even small variations in composition are detrimental to the functionality. However, some alloys are intentionally modified to achieve specific results. For example, Yttria is dispersed in stainless steel to enhance its resistance to radiation damage. The sensitivity of metal alloys to small changes in composition can be exploited to change the properties of LPBF material within a single print. An en-situ doping technique, which is under development, allows for the introduction of small quantities of liquid-suspended additives to any part of the powder bed. The liquid is then evaporated, and these additives integrate with the solid material upon laser fusion to change the properties of the base material. In this thesis, steel-insoluble (zirconia) and soluble (carbon) dopants are introduced into multi-layer parts formed by LPBF. Zirconia significantly increased the porosity of the steel with continuous pores which disrupt the columnar grain structure. The majority of the added zirconia segregated to the outer surface and porous surfaces within the bulk. Although hardness did not increase as expected, the porosity can aid in osseointegration when used for implants, or as a reduced-conductivity thermal barrier in heat sensitive applications. Carbon-doped samples, on the other hand, had nearly 30% increased hardness and more homogeneous microstructure than unmodified material. Hardened surfaces may be a valuable tool for designers who require wear resistance. Although porosity increased from ~0% to over 10% in the worst case, modified parameters resulted in only 1% porosity. The data indicate that changing the processing conditions affects porosity, so the amount of porosity could be adjusted. Finally, carbon was shown to create preferential etching which enables easy removal of support structures. Supports doped with carbon to promote sensitization and etched in an electrolyte bath either broke free without tension, or using no more than 20% of the force required to remove unmodified supports. This is a valuable step for reducing the post-processing required of many LPBF designs.