| 1. |
- Grolig, Jan Gustav, 1986
(författare)
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Coated Ferritic Stainless Steels as Interconnects in Solid Oxide Fuel Cells - Material Development and Electrical Properties
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Solid oxide fuel cells (SOFCs) are attracting increasing interest as devices with potentialuses in decentralized and clean electricity and heat production. Several challengeswith respect to materials have to be overcome to achieve efficiencies and life-spansthat are sufficient for long-term applications.An important element of an SOFC stack is the interconnect component, which connectstwo adjacent fuel cell elements. Interconnects, which are commonly composedof ferritic stainless steels, have to be corrosion-resistant, mechanically stable and costoptimized.This work aimed to investigate economic solutions for interconnect materials and tounderstand the underlying mechanisms of degradation and electrical conduction ofthese materials. Mainly two substrates, a commercially available steel (AISI 441) anda ferritic stainless steel that was optimized for an SOFC application (Sandvik SanergyHT) were combined with different barrier coatings and exposed to a cathode-sideatmosphere. A method was developed that allows for the electrical characterizationof promising material systems and model alloys, thereby facilitating a fundamentalunderstanding of the dominant electrical conduction processes linked to the oxidescales that grow on interconnects. The AISI 441 steel coated with reactive elementsand cobalt showed good corrosion and chromium evaporation profiles, while AISI 441coated with cerium and cobalt also had promising electrical properties. The SanergyHT steel was examined with coatings of copper and iron and copper and manganese,respectively. The corrosion and chromium evaporation profiles of Sanergy HT wereimproved by coating with copper and iron. The copper and iron-coated Sanergy HTshowed lower area specific resistance values than cobalt-coated Sanergy HT. Chromia,which is the main constituent of oxide scales, was synthesized using differentmethods. The electrical properties of chromia were found to be sensitive to not onlyimpurities, but also heat treatment. Finally the electrical properties of cobalt- andcobalt cerium-coated Sanergy HT steels were investigated. It was revealed that theaddition of cerium improved the conductivity of the interconnect by both slowingdown chromia growth and preventing the outward diffusion of iron into the spinel.
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| 2. |
- Lozinko, Adrianna, 1992
(författare)
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Microstructure development and mechanical properties of cast and thermo-mechanically treated eutectic high-entropy alloys
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The recently emerging high-entropy alloys (HEAs) present a novel alloying strategy, significantly expanding the scope of metal alloy design. Single-phase HEAs, nevertheless, suffer from the strength-ductility trade off as seen in conventional metallic materials. A possible solution is found in the recently developed eutectic HEAs (EHEAs), which borrow the concept of using lamellar structures as in-situ composites to balance mechanical properties. The first such alloy, AlCoCrFeNi2.1, with an FCC(L12) + BCC(B2) lamellar microstructure, remains the most studied EHEA. Despite much work put into its characterization, much remains to be understood. For example, more efforts have been given to optimize the mechanical properties while less are given to quantitatively describe the microstructure. Various thermo-mechanical treatments have been used to modify the mechanical properties of the AlCoCrFeNi2.1 alloy, however, previous studies mainly focused on the fully recrystallized materials, while a clear understanding of the recrystallization process is still missing, and the potential of partial recrystallization remains to be explored. The first part of this thesis work focuses on the as-cast microstructures of the eutectic and near-eutectic compositions of the AlCoCrFeNi2.1 system. Quantification of the phase volume and lamellar spacing is performed as a function of the Ni content. Orientation relationship and misorientation angle-axis changes in the five investigated alloys are also studied, with the previously unknown dependency of misorientation angle on the Ni content revealed. Some attention is also given to irregular microstructures in eutectic and near-eutectic compositions, which have not been discussed in previous studies. In the second part of this thesis work, a systematic study of the recrystallization process and the correlation between microstructure and mechanical properties in the thermo-mechanically treated AlCoCrFeNi2.1 alloy is undertaken. Distinctive behavior of the constituent phases during recrystallization, with varying rates of recrystallization and grain growth are observed, providing new insights to the recrystallization process in this alloy. Furthermore, it is shown that by controlling the annealing temperature and time, hetero-deformation induced hardening could lead to abnormal hardening in the as-rolled alloy, providing a new strategy to achieve high-strength with acceptable ductility in EHEAs.
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| 3. |
- Hosseini, Seyedehsan, 1994
(författare)
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Additive-Driven Improvements in Interfacial Properties and Processing of TMP-Polymer Composites
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Efforts to address environmental concerns have resulted in new regulations designed to plan the reduction of plastic and synthetic polymer usage, necessitating the search for sustainable natural alternatives with comparable cost-effectiveness and mechanical performance. Thermomechanical pulp (TMP) fibres are one of the most affordable natural fibres that have no chemical refining in production, production have a high yield of 90-98% and TMP fibres have been demonstrated to improve the mechanical characteristics (strength, stiffness and toughness) of wood-polymer composites (WPCs) compared to the pure polymer. The integration of TMP fibres with non-polar synthetic polymers remains a challenge due to surface polarity differences. This PhD thesis aims to ease the processing of TMP fibre composites through the incorporation of additives. The hypothesis posits that incorporating magnesium stearate (MgSt), molybdenum disulfide (MoS2) and alkyl ketene dimer (AKD) as additives in TMP composites will enhance interfacial properties, resulting in improved processability and flow behaviour at high temperatures. MoS2 is known for its interaction with lignin, which exists in TMP and MgSt is recognised for its ability to improve flow in pharmaceutical processing when combined with cellulose, also a component of TMP. AKD modifies the hydrophilic properties of lignocellulosic surfaces. The experimental work explores the effect of these additives on the properties of TMP composites of ethylene acrylic acid copolymer (EAA) and polypropylene (PP) matrices. The dynamic mechanical analysis (DMA) and mechanical analysis results reveal that MoS2 exhibits superior interaction with TMP fibres, yielding enhanced interfacial properties compared to MgSt in between EAA and TMP fibres. Rheological studies elucidate the transition from a fluid-like state to a network-like structure upon the incorporation of TMP into the PP matrix. The incorporation of AKD with C18 reduces the viscosity of TMP-PP composites and PP itself, and, as determined through theoretical Hansen solubility parameter (HSP) calculations, increases compatibility between cellulose in TMP fibres and PP. The addition of AKD influences both the colour (lighter) and shape (smoother surface) of the extrudate filaments in the TMP-PP composites, indicative of improved processing. In addition, frictional analysis demonstrates the reduction of the coefficient of friction (COF) between metal and TMP fibre by MgSt and AKD treatments.
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| 4. |
- Edgren, Aina, 1995
(författare)
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Microstructure and high temperature properties of Mo(Si,Al)2 - The effect of particle strengthening and alloying
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High temperature heating processes within the steel industries result in significant emissions of CO2, primarily due to the combustion of fossil fuels. Electrification of these processes, such as through the implementation of resistive heating elements, holds great promise for reducing emissions. However, a bottleneck in the transition to a more environmentally friendly industry is related to the materials used for these heating elements. Mo(Si,Al)2 is a ceramic material commonly used for heating elements in various high temperature furnaces and is being considered for large-scale industrial-scale applications. While its oxidation properties have been extensively studied, its mechanical properties, which are crucial when increasing the size of the heating elements, have received limited attention. In this thesis, the high temperature deformation behaviour of Mo(Si,Al)2-based materials, and potential routes for their improvement, have been investigated. This work has shown that diffusion-driven grain boundary sliding is the main deformation mechanism in polycrystalline Mo(Si,Al)2, particularly in fine-grained materials. In coarse-grained materials, the slip of dislocations also contributes to deformation. Moreover, coarse-grained Mo(Si,Al)2 relaxes through the formation of low-angle grain boundaries and dynamic recrystallization. The addition of Al2O3 particles, to achieve particle strengthening, results in a competition between a negative effect from grain refinement at low fractions (up to 15 wt.%), and a positive effect from inhibition of grain boundary sliding at higher fractions. Also alloying with W, Nb, Ta, and V has been studied, among which W was the most promising alternative. The solid solubility of W in Mo(Si,Al)2 was high, and it also led to a slight improvement in high temperature strength. The solubility of the alloying elements Nb, Ta, and V was found to be low in Mo(Si,Al)2. Instead, these elements were enriched in secondary phases. Additionally, Y alloying has been explored to investigate its effect on oxidation behaviour. However, the oxide adhesion was adversely affected.
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| 5. |
- Chen, Zhe
(författare)
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Surface Integrity and Fatigue Performance of Nickel-based Superalloys
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Due to global warming, the demand for more efficient gas turbines has increased. A way to achieve this, is by increasing the operating temperature of the gas turbine. Therefore, nickel-based superalloys have been developed to withstand these extreme temperatures and loads, especially in the hot sections. Today, the way of operating land-based gas turbines is changing. Instead of running for long periods of time, the operation is becoming more flexible, with ever-increasing cyclic loads and number of start and stop cycles. To handle the increased stress and cycles, component resistance to fatigue failures need to be improved.Surface integrity is critical to fatigue performance, since fatigue cracks normally initiate at a surface. Machining changes the surface integrity which can result in worse fatigue resistance. The work presented in this Ph.D. thesis was conducted in collaboration with Siemens Industrial Turbomachinery AB in Finspång, Sweden. Surface integrity changes which are induced during machining and their effects on fatigue performance have been studied on the alloy Inconel 718. Inconel 718 is a widely-used nickel-based superalloy for high temperature applications in modern gas turbines.Broaching, milling, and wire electrical discharge machining, related to component manufacturing in turbo machinery industries, were included in this study. Machining induced surface defects provide preferential sites for fatigue crack initiation which influence the fatigue performance of the alloy. If compressive residual stresses are induced during machining, they benefit the fatigue life by retarding fatigue crack initiation away from surface regions. Shot peening was performed on machined Inconel 718, by which high compressive residual stresses are deliberately induced. It results in enhanced fatigue performance.The high temperatures in gas turbines generally deteriorate the surface integrity. Recrystallization often occurs in the highly deformed surface layer. Microstructural degradation, in the form of α-Cr precipitates, have also been frequently observed in the deformed surface and sub-surface microstructure. Oxidation at elevated temperatures degrades the surface integrity and thereby also the fatigue performance. Fatigue cracks are preferably initiated at oxidized surface carbides, if thermal exposure has been made prior to the test. It is even worse when high temperatures relax the beneficial surface compression induced by shot-peening and thereby lowering the fatigue resistance.Machinability of a newly developed nickel-based superalloy, AD 730TM, and the surface integrity induced during turning have also been studied in this thesis project. AD 730TM is a candidate for turbine disc applications with an operating temperature above 650 °C. At such high temperatures, Inconel 718 is no longer stable and its mechanical properties start to degrade.To summarize, the results from this thesis work show the importance of understanding surface integrity effects for fatigue applications, especially in harsh environments. Moreover, the knowledge gained through this work could be used for surface enhancement of turbine components which are subjected to a high risk of fatigue failure. These will contribute to more efficient and flexible power generation by gas turbines.
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| 6. |
- Hanning, Fabian, 1988
(författare)
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Weld Cracking of Precipitation Hardening Ni-based Superalloys - Investigation of repair welding characteristics and susceptibility towards strain age cracking
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High temperature resistance and strength requirements make nickel-based superalloys the material of choice for the hot section of aero engines. Fabrication in terms of combining wrought and cast parts in the manufacturing of hot structural components enables component optimisation via the use of wrought high-strength parts, where geometrical constraints allow, and cast parts to produce complex geometries. Such an approach requires that the materials involved are weldable. Due to the complex microstructure of precipitation hardening nickel-based superalloys, welding comes with the risk of weld cracking, more specifically solidification cracking, heat affected zone (HAZ) liquation cracking and strain age cracking (SAC). While the first two types require a liquid phase to be present, SAC occurs during heating to post-weld heat treatment, in which age-hardening reactions coincide with the relaxation of weld residual stresses. Increasing engine operating temperatures as well as the intermittent cycling of land-based gas and steam turbines motivates research on the weldability of highly temperature-stable alloys. Hence, the main objective of this work has been the investigation and analysis of microstructural changes and their effect on weldability in terms of susceptibility towards weld cracking of the nickel-based superalloys Haynes® 282® and ATI 718Plus®. This has been addressed by the means of repair-welding studies and a simulative test approach using a Gleeble system. Microstructural changes were found to significantly affect HAZ cracking in cast ATI 718Plus®, where high amounts of Laves phase showed an increased resistance towards cracking. Haynes® 282® shows good weld-cracking resistance, as no HAZ cracks were present after multi-pass weld operations and subsequent post weld heat treatments. A simulative Gleeble test was developed to provide more data on ductility in the SAC temperature range and its dependence on ongoing microstructural changes during thermal exposure. Comparison with Waspaloy showed that the high resistance of Haynes® 282® towards SAC is correlated with the moderate age-hardening kinetics of the alloy and the rapid formation of a grain boundary strengthening carbide network. Furthermore, grain size was found to be a major factor affecting ductility and hence SAC susceptibility.
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| 7. |
- Joseph, Ceena, 1982
(författare)
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Microstructure Evolution and Mechanical Properties of Haynes 282
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Precipitation-hardened nickel-based superalloys find wide applications in aero engines and land-based gas turbines due to a combination of properties such as high temperature strength, resistance to oxidation and corrosion, fabricability, and creep strength. Structural engine components are traditionally cast to achieve higher degree of geometrical design freedom. However, the latest fabrication strategy to achieve low cost and light weight structural components is by joining materials based on temperature needs. The challenge in this strategy is to tailor the heat treatment to suit the multi-material structures and still be able to meet the desired property requirements. This requires a profound understanding of the process-structure-property relationships in these complex alloys. The newly introduced Ni-base superalloy Haynes 282 has been attracting interest due to its high-temperature properties and excellent weldability. These properties are achieved due to the precipitation of strengthening phase (γʹ, Ni3 (Al,Ti)) and grain boundary carbides (mainly M23C6 and M6C) during heat treatment. As Haynes 282 has showed sensitivity to heat treatment temperatures within the typical tolerance limits around the conventional heat treatment, the main objective of this research was to understand the microstructural evolution and mechanical properties with changes in heat treatment conditions. The effect of heat treatment variations on microstructure and mechanical properties has been systematically studied. Its influence on microstructure and tensile properties between room temperature and 730 °C are presented. The results show that γ׳ does not precipitate during rapid cooling but it precipitates as fine spherical particles during air cooling from the carbide stabilization temperature, and it changes to bimodal distribution with square and spherical morphology during slow cooling. During ageing, γ׳ is seen to precipitate intergranularly, as well as along the grain boundaries. The solvus temperature for this phase was above 1010 °C (higher than previously suggested), and depending on the combination of temperatures and times of the heat treatments, the γ׳ morphology changes from spherical to bi-modal to cuboidal. The grain boundary carbide morphology depends strongly on heat treatment temperature and is seen to change from continuous film to brick wall structure and finally to discrete particles. These microstructural changes strongly affect both strength and ductility of the material. Furthermore, Haynes 282 forgings show ductility variations in short transverse direction. The lower limit of ductility in this direction is close to the design tolerance and thus creates a need to understand the underlying cause. In this part, the study is focused to understand ductility variation by microscopic investigations. Carbide segregation and banding is seen to influence the ductility when oriented perpendicular to the tensile axis. This influence is also qualitatively captured through micromechanical modelling. Haynes 282, gamma prime, carbides, isothermal transformation, anisotropy ductility, heat treatment, microstructure, solution treatment, carbide stabilization treatment
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| 8. |
- Tolvanen, Sakari, 1984
(författare)
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Welding of Ti-6Al-4V: Influence of welding process and alloy composition on microstructure and properties
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Titanium alloys are widely used for components in the fan and compressor sections of aeroengines mainly because of their superior strength-to-weight ratio. Large static compressor components can be manufactured by welding together smaller subcomponents, which has potential to provide benefits such as higher buy-to-fly ratio and improved performance of the components. This is the background for why welding and the mechanical properties of welds have been investigated in this project. The aim of the work was to study what kind of microstructures and defects are formed in welding of Ti-6Al-4V with different welding processes and how these aspects affect the mechanical properties of the welds. Furthermore, the influence of chemical composition of the alloy on the formation of microstructures and defects was studied. The welding processes compared were electron beam welding, laser beam welding, plasma arc welding and TIG welding. High energy beam welding processes rendered a finer weld microstructure in comparison to the coarser microstructure produced by arc welding processes. The finer weld microstructure was found to be beneficial for tensile ductility and low cycle fatigue performance. Porosity was observed in welds produced by all the processes. Large pores and pores located close to the specimen surface the most detrimental to the fatigue strength. Fatigue life in the welds produced by arc weld processes was more sensitive to porosity than in the high energy beam welds. The finer microstructure has a higher resistance to micro crack initiation and growth which contributed to the better fatigue performance of welds produced by electron beam welding and laser beam welding. The alloy composition had a significant influence on the microstructure of the welds and the formation of defects. A small boron addition induced significant grain refinement in weld in boron alloyed material. Narrow columnar prior-β grains were formed in the fusion zones of the boron alloyed welds. The α colonies and α plates were also refined, as compared to the standard Ti-6Al-4V welds. In the cast base material, the TiB particles were located along the prior-β grain boundaries restricting the grain growth in the heat affected zone. In the fusion zone of welds, TiB particles had decreased in size and formed networks of stripes along the interdendritic regions. EBSD combined with prior-β grain reconstruction was an effective method to reveal the prior-β grain structure in the different weld zones. A significant batch to batch variation in amount of porosity was observed in laser welding of Ti-6Al-4V. The most significant factors affecting formation of porosity were the material batch, pulse length and welding speed. The material batches that were most susceptible to formation of porosity had increased amount of carbon and oxygen. The formation of porosity could be minimized in all material batches by optimizing the welding parameters.
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| 9. |
- Hearn, William, 1992
(författare)
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Development of Structural Steels for Powder Bed Fusion - Laser Beam
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Over the past decade, powder bed fusion – laser beam (PBF-LB) has attracted noticeable attention from both academia and industry. However, there remains a scarcity of approved material for the process, as fewer than 40 alloys are commercially available. Although structural steels are some of the most commonly used materials in traditional manufacturing, they have yet to be developed for PBF-LB as their high carbon content makes them susceptible to cracking. The objective of this thesis was to develop structural steels for PBF-LB by determining the impact of various process parameters on part quality, microstructure and mechanical properties. This involved the production and analysis of various carbon (0.06 to 1.1 wt.% C) and low-alloy steels (AISI 4130, 4140, 4340 and 8620). In terms of part quality, specimen density was related to the volumetric energy density (VED) and the carbon content of the alloy. Regarding the VED, specimens produced at low VED formed lack of fusion porosity, while specimens produced at high VED formed keyhole porosity. As for the carbon content, increasing the carbon content would reduce lack of fusion porosity at low VED, while lowering the required VED to form keyhole porosity. As for cold cracking, this occurred in structural steels with ≥ 0.38 wt.% C as elevated carbon contents would increase specimen hardness. However, cracking could be mitigated by increasing the VED, laser power or build plate preheating temperature, as each enhanced the level of in situ tempering during PBF-LB. From these findings, process windows were established for each structural steel that produced defect-free and high-density specimens (> 99.8%). In terms of the microstructure, the as-built specimens were primarily composed of tempered martensite, with retained austenite also observed in alloys with ≥ 0.75 wt.% C. During PBF-LB, martensite formed during layer melting and was initially in a quenched-like state, with carbon atoms segregating to dislocations and martensite lath boundaries. Subsequent tempering of this martensite was due to micro-tempering within the heat affected zone and macro-tempering within the previously solidified material. Although both influenced martensite tempering, micro-tempering had the most significant effect as it reduced martensite hardness by up to ~380 HV. This noticeable reduction in hardness was due to the precipitation of nano-sized carbides at the previously carbon enriched regions of martensite. Lastly, mechanical testing found that structural steels produced by PBF-LB achieved a high ultimate tensile strength (4140: ∼1400 MPa, 4340: ∼1500 MPa, 8620: ∼1100 MPa), impact toughness (4140:∼90–100 J, 4340:∼60–70 J, 8620:∼150–175 J) and elongation (4140:∼14%, 4340:∼14%, 8620:∼14–15%) that met or exceeded the ASTM standards. Additionally, these specimens displayed limited directional anisotropy due to small grains with weak crystallographic texture, a homogenous microstructure and low levels of internal defects. These findings are meant to highlight that these alloys are not only suitable but actively take advantage of PBF-LB to achieve properties that meet or exceed those of conventionally produced alloys.
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| 10. |
- Mehta, Bharat, 1993
(författare)
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Development of high performance aluminium alloys tailored for powder bed fusion-laser beam
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The powder bed fusion-laser beam (PBF-LB) process has lately been regarded as a top choice for creating complicated structures which are not possible via conventional manufacturing. Nevertheless, the pace of alloys for PBF-LB has been slower. Commercially available alloys are derived from cast/ wrought counterparts with limited knowledge of their suitability to PBF-LB. To fully exploit the inherent advantages of PBF-LB process, there is thus a growing need to develop alloy compositions with help from computational tools. This research work focused on the development of aluminium alloy systems tailored for the PBF-LB process. Leveraging the possibilities and limitations of PBF-LB process and with the help of CALPHAD tools, two types of alloying approaches were investigated namely in-situ alloying and ex-situ mixing. The key alloy design objectives were to avoid solidification cracking while attaining higher solid solubilities combined with a refined microstructure. The mechanical property objective was >450 MPa strength and high-temperature strength up to 573 K combined with general corrosion resistance. Al-Mn-Cr-Zr based alloy system resulting from this thesis study include several variants with different amounts of alloying elements. Gas-atomised powder was used, and fully dense samples were processed using optimised PBF-LB process. This was followed by post-processing heat treatments to optimise mechanical properties. This created an alloy system with mechanical properties including yield strengths 250-500 MPa, elongation to failure 5-25% and bending fatigue 140-200 MPa. In as-printed state, strengthening was caused by a combination of solid solution strengthening and grain size effect. The strengthening from precipitates was observed after direct ageing heat treatments. The microstructure was characterised by SEM, TEM and in-situ synchrotron measurements. Long-term isothermal testing at 623 K for >1000 h showed a superior performance (-17 HV or 12% drop). High-temperature tensile testing at 573 K showed yield strengths >150 MPa, surpassing most commercially available Al-alloys. These novel high performance alloys expand the available material performance envelope and create an edge over currently available systems while completely avoiding critical or rare earth elements. Such tailored alloy systems are shown to better utilise PBF-LB processing conditions to enhance material properties thus increasing the potential applications.
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