Study and characterisation of Ni-based superalloys produced by laser additive manufacturing

The laser additive manufacturing techniques (LAM) are of growing interest in different industries due to their capacity to produce near net shape components in a single step. Particularly in the case of the Ni-based superalloys, LAM processes can produce highly complex shape components more cheaply with respect to the traditional technologies. However, nowadays, there is still a lack of knowledge on the study of the process parameters on the microstructure and densification levels and the study of tailored heat treatments to meet standard qualification as well as precise industrial requirements. This thesis presents the research performed on two Ni-based superalloys produced by LAM processes. Inconel 625 (IN625) fabricated by laser powder bed fusion (LPBF) and directed energy deposition (DED) and Hastelloy X (HX) built by LPBF. For LPBF IN625 alloy was studied the effect of different process parameters on the hardness and densification level, obtaining dense samples (relative density over 99.8 %). Afterward, it was studied in detail the microstructure, mechanical and thermomechanical properties of as-built IN625. The results showed that the tensile behaviours of as-built IN625 state are higher than minimum values requirements for wrought IN625 alloys, due to very fine dendritic structures (mainly less than 1.5 micron) coupled with high dislocation density. However, the characterisation of as-built IN625 samples revealed that heat treatments are necessary to decrease the residual stresses, reduce elements segregation, and produce mechanical properties suitable for industrial applications. For this reason, the microstructural evolution of LPBF IN625 under different heat treatments was investigated. According to the performed heat treatment, the mechanical properties of heat-treated IN625 were influenced by phases precipitation (mainly γʺ phases and carbides) and/or recrystallisation, grain growth and dissolution of dendritic structures. For DED IN625 alloy, the aim of this work was to determine the impact of different process parameters on the densification level (relative density over 99.7 %), hardness and microstructure in order to select the appropriate parameters for industrial production. Finally, LPBF HX alloy was studied in a research collaboration with GE AVIO s.r.l. The target of this work was to investigate the microstructure of as-built and post-processed LPBF HX alloy. The as-built HX samples showed different microcracks caused by the LPBF process, so some samples were hot isostatically pressed (HIPed) to close the microcracks. The microstructure, grain size, phases and hardness of the as-built, heat-treated and HIPed HX samples were investigated. The results revealed that in case microcracks appear during production, they can be removed after specific post processing, generating LPBF HX parts with grain size similar to standard solution heat-treated (SHT) wrought HX alloy

Tailoring of the Microstructure of Laser Powder Bed Fused Inconel 718 Using Solution Annealing and Aging Treatments

Inconel 718 (IN718) is a nickel-based superalloy with high weldability and is thus ideal for being processed via laser powder bed fusion (LPBF). Unlike traditional casting, LPBF IN718 develops a complex microstructure due to the rapid solidification that characterizes this manufacturing process. As a result, LPBF microstructures are different from those expected in equilibrium conditions, and for this reason, specific heat treatments should be designed. This paper, using differential scanning calorimetry (DSC), thermal mechanical analysis (TMA), and a field emission scanning electron microscope (FESEM), aims to develop a complete heat treatment that maximizes the material strength, thereby enhancing its microstructure. The paper shows that high-temperature annealing followed by two aging steps is the most suitable way to achieve the abovementioned task. More specifically, a complete dissolution of the δ phase via solution annealing at 1080 °C is the key factor in gaining an even and intense precipitation of γ′ and γ″ during the subsequent aging treatments. The microstructural analyses showed the elimination of needle-like δ particles and detrimental Laves phases. At the same time, intense precipitation of spherical and of discoidal reinforcing particles was achieved by performing the aging treatments at 720 and 630 °C, respectively

Pastas dentífricas branqueadoras: a evidência científica - revisão bibliográfica

A estética do sorriso é muito valorizada pela sociedade, sendo a alteração da cor dentária uma das queixas comummente relatadas por pacientes que procuram tratamento estético. Os fabricantes de produtos para higiene oral estão cientes da insatisfação do consumidor com a cor dentária e desenvolveram uma vasta seleção de pastas dentífricas, visando solucionar o problema. Pretende-se com esta Dissertação, reunir e analisar a informação científica relacionada com os métodos de ação das pastas branqueadoras atualmente disponíveis no mercado, assim como relatar os principais efeitos e consequências associados. Para isso, foi realizada uma revisão de literatura nas bases de dados PubMed/Medline e Science Direct. Apesar da abundância de estudos disponíveis, destaca-se a necessidade da realização de novas pesquisas que visem reduzir o impacto das desvantagens associadas ao uso não controlado destes dentífricos, relacionadas nomeadamente com a determinação de adequados níveis de abrasividade e de concentrações mais eficazes e seguras.Smile aesthetics is highly valued by society and change in tooth color is one of the complaints commonly reported by patients seeking aesthetic treatment. Oral hygiene products’ manufacturers are aware of consumer dissatisfaction with tooth color and have developed a wide selection of toothpaste to solve the issue. The aim of this Dissertation is to gather and analyze scientific information related to the action methods of whitening toothpastes currently available on the market, as well as to report the main effects and associated consequences. To achieve this, a literature review was carried out in the PubMed / Medline and Science Direct databases. Despite the abundance of available studies, there is a need for further research to reduce the impact of disadvantages associated with the uncontrolled use of these toothpastes, namely related to the determination of adequate levels of abrasiveness and more effective and safe concentrations

CO2 from direct air capture as carbon feedstock for Fischer-Tropsch chemicals and fuels: Energy and economic analysis

The investigated plant concept integrates the direct air capture technology with the Fischer-Tropsch synthesis. 250 kt/h of air, with a CO2 concentration of 400 ppm, are used as feedstock to produce the synthetic hydrocarbons. The direct air capture is modelled as a high-temperature calcium recovery loop process. An alkaline electrolyser and a reverse water-gas shift reactor produce the required syngas. The Fischer-Tropsch products distribution is described by a carbide model developed for a Co-Pt/ɣAl2O3 catalyst for alkanes and alkenes of carbon number C1-C70. Five integration scenarios are analysed. In the base case, the energy demand of the direct air capture process is supplied with natural gas from the distribution grid. In improved configurations, the effect of Fischer-Tropsch off-gas recirculation to the reverse water-gas shift and/or the direct air capture units is explored, excluding the need of fossil fuel. An electrified direct air capture solution is also included. In the analysed scenarios, the highest system efficiency corresponds to 36.3 %, while the maximum carbon dioxide conversion is of 68.3 %. The maximum waxes production corresponds to 8.7 t/h. Lastly, capital and operating plant costs are allocated in an economic investigation, considering different market electricity costs and financial risk values. In a medium financial risk scenario (interest rate: 7.5 %), the minimum Fischer-Tropsch waxes production cost corresponds to 6.3 €/kgwax, reaching 5.05 €/kgwax at an interest rate of 0%. Lastly, the effect of learning curves over the production cost at the year 2030 and 2050 is included

Effect of Cold Rolling on Microstructural and Mechanical Properties of a Dual-Phase Steel for Automotive Field

A new advanced dual-phase (DP) steel characterized by ferrite and bainite presence in equal fractions has been studied within this paper. The anisotropy change of this steel was assessed as a progressively more severe cold rolling process was introduced. Specifically, tensile tests were used to build a strain-hardening curve, which describes the evolution of this DP steel's mechanical properties as the thinning level increases from 20 to 70% with 10% step increments. As expected, the cold rolling process increases mechanical properties, profoundly altering the material's microstructure, which was assessed in depth using Electron Backscatter Diffraction (EBSD) analysis coupled with the Kernel Average Misorientation (KAM) maps. At the same time, the process strongly modifies the material planar anisotropy. Microstructural and mechanical assessment and the Kocks-Mecking model applied to this steel evidenced that a 50% strain hardening makes the DP steel isotropic. The material retains or resumes anisotropic behavior for a lower or higher degree of deformation. Furthermore, the paper evaluated the forming limit of this DP steel and introduced geometric limitations to testing the thin steel plates' mechanical properties

Application of Directed Energy Deposition-Based Additive Manufacturing in Repair

In the circular economy, products, components, and materials are aimed to be kept at the utility and value all the lifetime. For this purpose, repair and remanufacturing are highly considered as proper techniques to return the value of the product during its life. Directed Energy Deposition (DED) is a very flexible type of additive manufacturing (AM), and among the AM techniques, it is most suitable for repairing and remanufacturing automotive and aerospace components. Its application allows damaged component to be repaired, and material lost in service to be replaced to restore the part to its original shape. In the past, tungsten inert gas welding was used as the main repair method. However, its heat affected zone is larger, and the quality is inferior. In comparison with the conventional welding processes, repair via DED has more advantages, including lower heat input, warpage and distortion, higher cooling rate, lower dilution rate, excellent metallurgical bonding between the deposited layers, high precision, and suitability for full automation. Hence, the proposed repairing method based on DED appears to be a capable method of repairing. Therefore, the focus of this study was to present an overview of the DED process and its role in the repairing of metallic components. The outcomes of this study confirm the significant capability of DED process as a repair and remanufacturing technolog

Study of the Microstructure and Cracking Mechanisms of Hastelloy X Produced by Laser Powder Bed Fusion

Hastelloy X (HX) is a Ni-based superalloy which suffers from high crack susceptibility during the laser powder bed fusion (LPBF) process. In this work, the microstructure of as-built HX samples was rigorously investigated to understand the main mechanisms leading to crack formation. The microstructural features of as-built HX samples consisted of very fine dendrite architectures with dimensions typically less than 1 µm, coupled with the formation of sub-micrometric carbides, the largest ones were mainly distributed along the interdendritic regions and grain boundaries. From the microstructural analyses, it appeared that the formation of intergranular carbides provided weaker zones, which combined with high thermal residual stresses resulted in hot cracks formation along the grain boundaries. The carbides were extracted from the austenitic matrix and characterized by combining different techniques, showing the formation of various types of Mo-rich carbides, classified as M6C, M12C and MnCm type. The first two types of carbides are typically found in HX alloy, whereas the last one is a metastable carbide probably generated by the very high cooling rates of the process

Development and Characterisation of Aluminium Matrix Nanocomposites AlSi10Mg/MgAl2O4 by Laser Powder Bed Fusion

Recently, additive manufacturing techniques have been gaining attention for the fabrication of parts from aluminium alloys to composites. In this work, the processing of an AlSi10Mg based composite reinforced with 0.5% in weight of MgAl2O4 nanoparticles through laser powder bed fusion (LPBF) process is presented. After an initial investigation about the effect of process parameters on the densification levels, the LPBF materials were analysed in terms of microstructure, thermo-mechanical and mechanical properties. The presence of MgAl2O4 nanoparticles involves an increment of the volumetric energy density delivered to the materials, in order to fabricate samples with high densification levels similar to the AlSi10Mg samples. However, the application of different building parameters results in modifying the size of the cellular structures influencing the mechanical properties and therefore, limiting the strengthening effect of the reinforcement