This thesis examines the thermal distribution and material behavior in in-space additive manufacturing (ISAM) using metal directed energy deposition (DED), addressing challenges of the orbital environment, including microgravity and vacuum. Experimental studies with a Scandium-modified Al-5183 alloy, conducted via Wire Are Additive Manufacturing, validated heat transfer coefficients and simulated ISAM thermal profiles. A finite element model, developed in Ansys Mechanical and calibrated with experimental data, accurately predicted overall thermal histories despite underestimating melt pool temperatures. Applied to orbital conditions, the model showed environmental healing effects were minimal for small components but significant for larger structures, supporting ISAM’s potential for space infrastructure. Material analysis revealed enhanced mechanical properties due to Scandium addition, with refined grains and Al3Sc precipitates. Integrating experiments, simulations, and characterization, this work advances ISAM thermal modeling, offering insights for future refinements in simulation accuracy and orbital testing.

This thesis describes an investigation in the modeling and manufacturing of Nitinol (NiTi) superelastic metamaterials. To achieve an accurate agreement between model and experiment regarding superelasticity in Ni-rich NiTi metamaterials, the research focuses particularly on the martensitic transformation behavior. Each chapter contributes to a better understanding of the entire additive manufacturing process chain, including constitutive modeling, heat treatments and structural design, to develop NiTi-based metamaterials with tunable superelastic properties.
NiTi shape memory alloys exhibit unique shape memory effect (SME) and superelasticity due to reversible martensitic transformations. These properties make NiTi a suitable material for adaptive structures, biomedical devices, and aerospace components. Though computational models can be used to design NiTi structures and metamaterials with superelasticity and SME, the successful additive manufacturing of these designs remains challenging. Achieving full superelasticity in complex geometries produced via laser powder bed fusion (L-PBF) is particularly difficult. Thus, aiming for model-experiment consistency of superelastic metamaterials, the main research gap lies in establishing clear qualitative and quantitative relationships between material models, properties, mesoscopic structures and the resulting macroscopic responses.

In Chapter 3, the research started with the development of analytical expressions for the effective transformation stress and numerical models to evaluate the superelastic behavior and energy dissipation of truss-based metamaterials. NiTi truss-based metamaterials with body-centered cubic (BCC) and octet structures were selected to represent bending- and stretching-dominated architectures, respectively. A detailed parametric finite element analysis was performed to study the relationship between relative density and effective transformation criteria. Using L-PBF, crack-free BCC and octet samples were successfully fabricated from Ni-rich NiTi powder. However, the as-fabricated samples exhibited only partial superelasticity and premature fracture.
In Chapter 4, the study is focused on inhomogeneity of microstructural and functional properties and the underlying reasons for partial superelasticity. Through both numerical and experimental approaches, the study investigated how geometric factors, such as relative density, affect microstructural inhomogeneity and thermomechanical properties of NiTi in body-centered cubic (BCC) structures. Geometric effects on melt pool behavior lead to different solidification textures and inhomogeneous response to indentation. The numerical simulation shows that inhomogeneous transformation temperatures cause narrow stress hysteresis in the macroscopic response. This chapter reveals the interdependent relation between relative density, microstructure, localized properties of NiTi and the macroscopic response.

The focus in Chapter 5 is on understanding and mitigating premature fracture in Ni-rich NiTi metamaterials produced by L-PBF. To investigate the origins of fracture, a comparative analysis of two unit cell architectures, the a Gyroid network (bending-dominated) and a Diamond shell (stretching-dominated), was conducted. Due to the inherent tension-compression asymmetry of NiTi, the structural stability of these designs was found to be reversed compared to conventional elastic-plastic responses, leading to premature fracture and limited superelasticity in the as-fabricated samples. As large deformation can not be achieved through martensitic transformation or dislocation slip systems, partial superelasticity and low deformation recoverability were observed in the as-fabricated samples. Heat treatments were applied to address these issues and achieve qualitative agreement between experimental data and model predictions.

After the superelasticity was successfully achieved, the transformation stress-temperature relation and energy absorption in Ni-rich NiTi superelastic metamaterials were investigated in Chapter 6. Temperature dependence often restricts the practical use of superelasticity. In metallic metamaterials, energy absorption typically relies on the elastoplasticity of ductile metals; however, achieving energy absorption with recoverable deformation has not been fully explored. To address this, a numerical model of the Diamond shell structure was developed to predict temperature-dependent superelasticity and energy absorption. A heat treatment was applied to ensure agreement between the model and experimental results. The findings show that the transformation stress-temperature coefficient decreases from 9.5 MPa/°C for bulk samples to 0.9 MPa/°C for Diamond samples. Under uniaxial compression, the effective transformation stress can be controlled by relative density, with values of 41.8, 52.1, and 65.3 MPa for relative densities of 0.15, 0.2, and 0.25, respectively. A specific energy absorption of 3.5 J/g was achieved in cyclic compression tests with 15 cycles. The recoverable plateau-like response in the macroscopic stress-strain curves originated in a continuous transformation region forms along the macroscopic [100] direction under uniaxial compression. Post-yielding plasticity in the macroscopic stress-strain curves is related to a plastic shear band formed along the [110] direction.

In summary, this work successfully developed a model-manufacturing strategy for superelastic NiTi metamaterials. By addressing multiscale challenges such as microstructural inhomogeneity and tension-compression asymmetry, this study demonstrates that computation-based design and additive manufacturing can create functional NiTi structures with tunable thermomechanical properties. Multiscale issues often prevent computational designs from being fully realized in experiments. By identifying and controlling variable interdependencies across scales, this research achieves largely tunable superelasticity in experiments. This approach provides a foundation for practical applications of NiTi metamaterials in fields such as biomedical devices, aerospace, and civil engineering.

Printing patterns in additive manufacturing have been extensively studied regarding their effects on the microstructure and mechanical properties of the materials. However, weaving, a welding technique that produces good quality welds in large areas and ensures good weld penetration has not been applied in additive manufacturing. This study focuses on comparing four weaving patterns to two traditional line printing patterns in Wire arc additive manufacturing of austenitic stainless steel 316L. This thesis covers the groundwork of weaving pattern characterization, starting from optimization of printing parameters and printing path, followed by pattern characterization through measurements of voltage, current, thermal history, and thermal profile, and investigation of surface quality of the samples. Microstructure characterization was then conducted by optical microscopy, electron backscatter diffraction, and X-ray diffraction; and mechanical properties were obtained through microhardness and tensile testing. The weaving patterns showed an improved deposition rate and avoided lack-of-fusion defects compared to the line printing patterns. Two out of four weaving patterns achieved superior surface quality over the line printing patterns and are free of macro-defects such as side wall collapse and spattering. The weaving patterns have excessive heat accumulation and lower cooling rates compared to line printing patterns, which led to coarser microstructures and inferior microhardness and tensile properties. The thermal gradient is also more uniformed and aligned to the build direction, increasing the degree of grain alignment and texture of the weaving patterns across the samples.

This thesis investigates designing and verifying a new frame for the HyperScout, a hyperspectral optical instrument used for earth observation. Due to changes in the instrument's size, the research aims to develop a frame that meets the thermal and structural requirements. The study explores six design concepts, focusing on innovative materials like titanium, PEEK, and wood and advanced manufacturing techniques such as 3D printing. Three promising designs were selected for further development and underwent structural, thermal, and optical testing to ensure their suitability for space conditions. Finite Element Modeling (FEM) and Thermal Vacuum Chamber (TVAC) tests were used to simulate the harsh environment of space and validate the designs. The findings highlight the strengths and limitations of each material and design approach, with recommendations for future research in optimizing spacecraft components.

An investigation into the application of additive manufacturing to the production of structural joints was conducted for this thesis. The main goal was to determine all necessary aspects that someone needs to consider when designing a structural part that is intended to be printed with additive manufacturing. As an example, the branched column connection was selected due to its inherent limitations, which can be effectively addressed through the integration of additive manufacturing techniques.
Within the construction realm, wire arc additive manufacturing (WAAM) stands out as particularly advantageous due to its ability to yield high mechanical properties comparable to conventionally manufactured materials, coupled with a high deposition rate. As with any manufacturing process, WAAM operates within certain constraints dictating the types of objects feasible for production.
To ensure compatibility with WAAM while achieving desired structural and aesthetic benchmarks, three laboratory tests were conducted. These tests scrutinized the impact of input process parameters, overhang, and overlapping on the quality of the build. Factors such as travel speed, wire feed speed, voltage, and current were examined for their influence on welding bead dimensions and quality. Additionally, an investigation into how the percentage of overlap affected print quality was done. Among the constraints, the most pivotal one was the overhang limitation, determining the minimum allowed overhang angle in perpendicular and parallel directions depending on the direction of the print.
The primary objective in designing the connection was to reduce the necessary material for the branched column connection’s manufacture. Topology optimization (TO) played a crucial role in achieving this goal. Various models were constructed, each differing in TO input parameters to find the model with the lowest required mass while maintaining adequate structural performance. These models varied based on TO objectives (equivalent von Mises stress, compliance, mass, and volume) and TO constraints (either mass retain percentage or maximum stress), while also integrating manufacturing constraints obtained from lab tests. Ultimately, the model that offered the lowest mass and satisfactory structural performance focused on compliance as its objective, while retaining 15% of its initial mass.
Based on the above explained research a design guideline could be proposed and includes all necessary steps and considerations that someone needs to take into account when designing a connection manufactured with WAAM. The steps of the guideline include: 1. Selection of additive manufacturing process; 2. Material selection; 3. Determination of additive manufacturing process manufacturing limitations; 4. Design phase including TO with the proposed input parameters.
In illustrating the advantages and limitations of WAAM in the construction industry and proposed design guideline, a case study involving a comparative analysis between a steel plate and a WAAM branch column connection was done. This study centered on the real life project, 6 Bevis Marks in London. The findings showcased a notable reduction in the required material. However, the limitations of this approach were apparent in the increased manufacturing time and costs.

Hot cracking is a significant imperfection in laser welding of thin transformation induced plasticity (TRIP) steel sheets. This study aims to evaluate strategies to mitigate the hot cracking susceptibility through beam shaping, by performing free edge welding tests. TRIP steel sheets were welded with a laser beam with a constant laser power for various spot sizes and incident angles at different distances from the free edge. A high-speed camera and a welding camera are applied to film the behaviour of the plasma plume and the development of the molten fluid flow. Pre-attached thermocouples are utilized for in-situ transient temperature measurements. The weld pool dimensions and microstructures of the fusion zones are characterized by optical microscopy and semi-quantitative elemental analysis is conducted by scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS). Experiments showed that, as the heat input increases, the vaporization of material induces a recoil pressure and leads to keyhole formation, and the direction of maximum thermal gradient and the cooling rate determines the microstructure. At a fixed beam spot size and incident angle, an increased travel speed decreases the heat input, and results in a smaller bead width with less solidification shrinkage and thermal contraction. In the case of varying spot sizes with a fixed incident angle and travel speed, a small spot size of 0.2 mm refers to a high energy density, the concentrated beam creates an open keyhole that leads to energy loss, and the absorption rate of thermal energy is low. Such a condition generates a narrow bead width, which results in less shrinkage that mitigates the hot cracking susceptibility significantly. When the spot size increases, a wider bead is created that enhances the contraction and therefore higher tensile stresses, which causes the metal to be more prone to cracking. When the diameter of the spot increases to 1.2 mm, the thermal energy is so dispersed that the heat is not sufficient to evaporate significant amounts of metal and the operation mode switches from a keyhole mode to a conduction mode, which only partially penetrates the steel sheet. In the case of varying the incident angle with a fixed spot width and fixed travel speed, the inclined beam projects a prolonged elliptical impingement area and shifts the location of highest density according to the incident angle. A leading beam configuration, i.e. a small incident angle, acts as post-heating source of the molten liquid. For large incident angles of a trailing configuration, the beam impinges on the front wall of the keyhole and acts as a pre-heating source. A prolonged elliptical heated area increases the time for the back feeding liquid to heal the gaps between the columnar grains growing into the fusion zone and suppresses the hot cracking susceptibility. If the incident angle becomes too small (45°), the consequence is similar to a too dispersed spot size, i.e. partial penetration of the sheet. A trailing configurated beam can reflect on the front keyhole wall and enhances absorption, but it generates turbulent flow. When the angle comes too large (135°), the process is not entirely stable. In conclusion, for producing defect-free welds in industrial applications, it is recommended to utilize a small spot size that corresponds to less solidification shrinkage. Furthermore, a leading configuration is preferred, but the incident angle should not be too low, in order to achieve full penetration.

This thesis describes an investigation of the deposition of Stellite 6 on SS316L stainless steel substrates using a Gas Metal Arc Welding (GMAW) based Wire Arc Additive Manufacturing (WAAM) system. The primary aim of this research was to optimize the deposition process with a focus on reducing heat input and understanding the development of residual stress, a critical factor in the performance of hardfacing materials. The project utilized a zigzag toolpath deposition strategy requested by adaptation from current manual operation, aiming to achieve low heat input while maintaining layer integrity.
The experimental methods involved finite element analysis (FEA) to simulate the deformation of the sample for a better understanding of the material thermo-mechanical responses to the zigzag deposition strategy used. The deformation simulation agrees with the measured deflection. The model, however, computed very large residual stresses. To have a better evaluation of the residual stress resulted from the deposition process, Incremental Central Hole Drilling (ICHD) method was used for residual stress analysis. Residual stresses were introduced in the samples by depositing one or two layers, with different clamping configurations. The measured residual stresses together with the observations noted during the experiments were analysed, compared, and discussed. The results show that double layer deposition can reduce residual stress gradients and provide a more stable stress profile along the thickness of the deposited layer. Furthermore, although single-sided clamping allowed for a higher freedom of thermal expansion and contraction during the deposition process, which leads to a more balanced stress distribution, it also increases deformation. Therefore, the use of uniform clamping during deposition of Stellite 6 should be implemented for actual application case. Additionally, depending on the desired thickness of Stellite 6, a multi-layer deposition strategy can be implemented to minimize residual stress build up.
The research concluded that defect-free Stellite 6 layers can be successfully deposited using the GMAW-based WAAM process. The residual stress measurement showed that preheating and reducing the thermal gradient can effectively reduce the residual stress within the deposited material. The obtained results can be helpful for the further development of automated toolpath generation and the integration of 3D vision control systems for more efficient and reliable WAAM processes.

This study explores the potential of using Profilometry Indentation Plastometry (PIP) to develop a methodology for guiding the process parameters optimisation in large-scale Wire Arc Additive Manufacturing (WAAM) deposits. The reliability of PIP measurements in evaluating the mechanical properties of WAAM deposits is assessed in comparison to traditional mechanical testing methods, while also examining how process parameters influence the microstructure and mechanical properties (material responses). The focus is more on the mechanical properties including the hardness, yield strength and tensile strength as the material (Mn Si alloyed steel solid wire, ER70S-6) investigated is well developed and studied in the literature.
The research focused on depositing sample using two deposition modes for investigating the material responses to the process. The two deposition methods are pulsed welding and Super Active Wire Process (SAWP) using a Gas Metal Arc Welding (GMAW) system from Panasonic. SAWP is a short circuit welding process working together with its mechanical push pull motor to manage heat input. A total of 119 samples were produced from single bead ramping tests, along with three larger blocks (250 mm × 90 mm × 250mm) deposited using heat inputs of low, medium and high levels, respectively. The mechanical behaviour of the ramping samples was measured using a PIP device and then the measurements were analysed through curve fitting to find any potential physical trends. An exponential relation was found between the material response and the heat input. Additional single beads were also deposited on the substrate with different starting temperatures using a selected condition. This to simulate the bead deposition when depositing 3D block. Subsequently, the cross sections of these samples were prepared and measured using the PIP device. The trend curves then plotted with different exponential power index to find the correlation between the PIP measurements and the substrate starting temperature. These curve plotted together with PIP measurements of the samples extracted from the block deposited using different heat inputs, in which a good agreement was found. These results shown that it is promising to develop a systematic procedure that can correlate the single bead depositions with the large scale component to be deposited. The PIP measurements can help to reduce the R&D lead time and to guide the parametric optimisation when using WAAM for 3Dmetallic printing. The power index value may have correlation with the cooling rate, as the resulting material properties are determined by the resulting microstructure, which depending on its chemical composition, grain size and orientation. These are interested aspects to be further investigated but cannot be included in this thesis.
Additionally, the obtained results also shown that the methodology of combining the single bead ramping together with the PIP measurement can effectively guide the selection of processing conditions and inter pass temperature for larger WAAM deposits, which are important conditions to define the 3D printing procedure. Process parameters, particularly heat input, had a significant impact on the resulting mechanical properties of WAAM deposits, with a too high heat inputs, it can lead to reduced hardness. PIP hardness measurements shown a good agreement with the Vickers hardness measurements, and PIP strength measured shown reasonable agreement with the uniaxial tensile test results. It was noticed that yield strength measured using the PIP device shown the values systemically lower than the uniaxial measurements. This is likely due to limitations in the device’s fitting model inputs that has been predefined within the PIP device as a Blackbox towards the normal user. Further researches are expected to enable a better utilization of the PIP device to measure the WAAM deposits. Meanwhile it also expected that the device manufacturer (in collaboration with us in this thesis work) can further improve the device stability, accuracy and user friendliness based on our feedback that has been communicates. Nonetheless, the main research question of using PIP for assisting speed up of the process optimization was successfully answered.

Sintered silver is known for its high thermal and electrical conductivity and is considered a promising die attach material for wide bandgap semiconductors at high power. This study investigates the evolution of microstructure and mechanical properties of sintered silver materials during aging in different atmospheres. The study used a combination of experimental methods, including shear tests, scanning electron microscopy, and EBSD, to evaluate the effects of air and nitrogen atmospheres on the reliability of the material. Advanced simulation methods, such as dynamic Monte Carlo and phase field methods, were combined to predict grain growth and void evolution during aging. The results show that air aging accelerates grain growth and pore aggregation, and oxidation is suppressed in a nitrogen atmosphere. These findings provide insights into optimizing the sintering process to improve the long-term stability and performance of sintered silver in semiconductor packaging applications.

Advanced high strength steels (AHSS) have been widely employed in the automotive industry to meet the requirements of improving crash performance while reducing vehicle weight. The excellent performance of AHSS in strength and ductility comes from a dedicated design of alloy composition and microstructure. However, the addition of some alloying elements may lead to poor weldability. Solidification cracking tends to occur in AHSS containing detrimental elements such as phosphorous or sulfur under unfavorable welding conditions.

Solidification cracking is a complex problem associated with multiple physical phenomena at different length scales. During welding, with a localized heat input, the material is heated to melt, forming a weld pool. Upon cooling, a mushy zone consisting of solid grains and liquid channels forms behind the weld pool. As temperature drops, solid grains in the mushy zone grow, which tends to close liquid channels in the mushy zone. However, during solidification, detrimental elements segregate in the liquid, inhibiting the closure of the liquid channel. Meanwhile, due to solidification shrinkage and thermal contraction, the liquid channels experience a tensile load, which tends to open the liquid channel. If there is insufficient liquid feeding, solidification cracking occurs. Solidification cracking can be avoided by controlling parameters including alloy composition, microstructure and processing conditions like power, welding velocity, laser beam shapes, etc.

Due to the complexity, an accurate prediction of solidification cracking under various welding conditions is challenging. A full-field simulation of a welded component which incorporates all the physical phenomena and is capable of handling various welding conditions is not realistic with current computational power. To achieve an accurate prediction of solidification cracking, approximations and simplifications must be made while major physical mechanisms are still properly captured. Moreover, existing modelling techniques should be improved or adapted to fulfill the requirements of welding simulations.

This research starts from explicitly modelling segregation and solidification during welding. In the literature, cellular automata (CA) models and phase field (PF) models have been widely used to simulate segregation and solidified microstructure. The CA method is more computationally efficient compared to the PF method and thus is mainly adopted in the current research. A cellular automata solidification model is developed, where the growth velocity is determined based on kinetic undercooling at the interface. The state-of-the-art model incorporates a decentered growth algorithm to suppress grid anisotropy and a generalized height function method to calculate curvature accurately. To remove the dependency on the mesh size, a new diffusion term is proposed to handle the diffusion between the interface cells and liquid cells. Moreover, a solute redistribution method has been applied for each interface cell to resolve the mass balance error introduced by the virtual liquid cell assumption. The developed CA model is validated by simulating single-dendritic solidification in an Al-3Cu (wt.\%) alloy. The simulated tip velocities agree with the prediction of the Kurz-Giovanola-Trivedi (KGT) model. With improvements in the aspects of mesh-size independency and mass balance, the developed CA model is suitable for solidification simulation with a high undercooling, as is common in welding. It also provides an easier way to achieve multi-component solidification simulation compared to conventional CA solidification models, which need to solve a system of mass balance equations in interface cells. Despite the improvements made, due to the poor discretization of the solid-liquid interface, the CA method is less accurate compared to the phase field method. Up to now, no CA models can reproduce the dendrite tip velocity predicted by a Green function method in single-dendritic solidification simulations.

For better accuracy, the research was extended with a focus on PF modelling of segregation in the liquid channels in the mushy zone during welding. Following the frozen temperature gradient approximation, the complex thermal conditions during welding are approximated with a directional solidification condition defined by a constant temperature gradient and pulling velocity, which is the moving velocity of the liquidus isotherm. Under directional solidification conditions, columnar dendrite grains form and liquid channels exist in between neighboring grains, where solutes accumulate. The liquid channel structure and segregation is simulated with the PF model, while the solidification cracking susceptibility (SCS) is then quantified by calculating the pressure drop from the dendrite tip to the coalescence point of the liquid channels with an analytical model, the Rappaz-Drezet-Gremaud (RDG) model. A larger pressure drop represents a larger SCS. With the modelling setup, the influence of the temperature gradient and the pulling velocity on SCS have been investigated. Increasing the pulling velocity or decreasing the temperature gradient increases the pressure drop from the dendrite tip to the coalescence point, leading to an increase in SCS. Decreasing the primary dendrite arm spacing (PDAS) decreases the permeability of the liquid channel and the liquid channel length at the same time, resulting in a decrease in the pressure drop and SCS when the PDAS is small. Consideration of the PDAS dependency on the temperature gradient and the pulling velocity influences the value of the pressure drop but does not change the tendency of the SCS. The findings indicate that solidification cracking can be avoided by either decreasing the pulling velocity or increasing the temperature gradient or refining the grain size, as supported by experimental results. However, due to the high computational cost, the size of the simulation domain is limited. Moreover, the influence of process parameters on SCS cannot be considered directly due to the lack of macroscopic modelling.

To include the influence of process parameters, macroscopic thermal-mechanical modelling and microstructure modelling of the whole weld pool are necessary. The former is relatively easy to achieve, while the latter is, nevertheless, not achievable with the aforementioned CA model or PF model, as both models require a fine mesh size to simulate the liquid channel structure and segregation within the liquid channels. Simulating the solidification microstructure for the whole weld pool or the whole mushy zone requires a huge amount of computational resources. As an alternative to the numerical solution, the segregation in the liquid channel can be calculated analytically with the Scheil-Gulliver calculation, while the remaining question is how to achieve the microstructure simulation of the whole weld pool.

Therefore, the research addressed microstructure modelling of the whole weld pool. To this purpose, a special kind of CA model is adopted, which calculates the growth velocity as a function of local undercooling based on analytical models (LGK model or KGT model). In this case, there is no need to solve the concentration profiles numerically, which permits a coarse mesh size and thus a reduction in the requirement of computational resources. These kind of CA models are called CAFE models, as such CA models are always coupled with a thermal finite element (FE) model. In this research, a CAFE model which is two orders of magnitude faster than conventional CAFE models has been developed. The acceleration comes from three different sources. Firstly, by adopting an exact temporal integration and a multi-layer capture algorithm, a large time step can be employed without compromising the simulation results. Secondly, the parallelism of the simulation codes is achieved in a shared-memory environment, enabling a more efficient load balance. Thirdly, a subdomain activation and deactivation method is employed to reduce the computation tasks. The proposed model is validated by simulating the grain morphology and texture of additively manufactured samples. A good agreement is achieved between the simulations and the experiments.

By coupling the CAFE model with a thermo-mechanical FE model and a granular model to calculate liquid pressure, a multi-physics multi-scale modelling framework is developed to predict solidification cracking. The thermo-mechanical FE model calculates the profiles of temperature and strain rate for the welded component during welding; the CAFE model simulates the solidification microstructure in the whole weld pool; and the granular model calculates the pressure drop in the liquid channel network determined based on the simulated microstructure and the Scheil-Gulliver calculations. The developed modelling framework is then validated by simulating welding experiments of a TRIP steel. With a constant ratio between the power and the welding velocity, increasing the welding velocity increases the maximum pressure drop in the mushy zone, indicating an increase in SCS. Grain refinement or decreasing the freezing temperature by changing the alloy composition leads to a decrease in the maximum pressure drop in the mushy zone, representing a decrease in SCS. The predictions from the modelling framework are supported by experimental findings in the literature.

In conclusions, the research starts from microstructure modelling of segregation and liquid channel structure with both the CA method and the PF method and finalizes with a multi-physics multi-scale modelling framework of solidification cracking. In this approach, several approximations and simplifications have been made to reach the final modelling framework for solidification cracking. The developed multi-scale multi-physics modelling framework incorporates major physical mechanisms at different length scales and paves a way to understand and predict solidification cracking under various welding conditions. It provides a theoretical basis to eliminate solidification cracking by tuning parameters including alloy composition, microstructure and processing parameters like power, welding velocity and laser beam shapes, etc. Moreover, with the acceleration in microstructure simulation, the developed CAFE model contributes to the development of a digital twin of additive manufacturing.

A multi-material structure combines diverse materials, allowing for precise tailoring of properties in specific regions. The integration of dissimilar materials leads to superior performance compared to single-material components. Bimetallic structures, a subset of multi-material systems, offer site-specific performance owing to the contrasting properties of the materials involved.

This thesis focused on fabricating a thin, functionally graded bi-metallic wall using Wire Arc Additive Manufacturing (WAAM) with Inconel 625 and HSLA steel. Functional grading of metallic materials, like Inconel 625 and HSLA steel, enables precise customisation of component properties to fulfil distinct functions within a structure. Despite its promise, this combination presents challenges, notably the formation of intermetallic compounds that act as a catalyst for forming solidification cracks in combination with varying material composition and high heat input.

Optimal process parameters, notably reduced heat input, are critical in mitigating these issues. The current study involved process parameters optimisation for Inconel 625 single bead-on-plate welds to control dilution levels at dissimilar interfaces. Extensive characterisation using Light Optical Microscopy (LOM) and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive Spectroscopy (EDS) was conducted to study the microstructural evolution of the as-fabricated sandwich structure.

Results indicated that interface 1 (Inconel 625/HSLA steel) exhibited minimal dilution and zero defects, while interface 2 (HSLA Steel/Inconel 625) exhibited substantial dilution and was prone to solidification cracks. The EDS results affirmed that the variations of elemental composition and heavy dilution at interface 2 lead to inhomogeneous microstructural features, elemental segregations and the formation of brittle intermetallic phases, thereby leading to a solidification crack at this particular interface.

X-ray diffraction (XRD) measurements were conducted to identify phases at the dissimilar interfaces, corroborating the microstructural features observed using SEM and LOM. The identified phases confirm the presence of brittle intermetallics, such as the laves phase, extensively present at interface 2. Vickers hardness testing was performed to assess the mechanical behaviour at both interfaces, revealing a consistent trend of decreasing values with a sudden increase in hardness values at both interfaces due to microstructural transition.

Based on the EDS results, an optimal range of elemental compositions was speculated, focusing primarily on the significant elements Fe, Ni, and Cr. These compositions, with Ni content of approximately 20-25 wt%, Fe content of around 70-75 wt%, and Cr content of approximately 5-10 wt%, are crucial in reducing cracking susceptibility at interface 2. This study elucidates the initiation and propagation of solidification cracks at interface 2, establishing a definitive link between microstructural evolution, mechanical behaviour, and crack formation.

Ultimately, this thesis lays the foundation for future research to delve deeper into these insights, fine-tune process parameters, and fabricate compositionally graded multi-material structures with enhanced susceptibility to solidification cracking at dissimilar interfaces.

Additive manufacturing (AM) refers to a series of techniques in which parts are created by successive deposition of layers. Among the different technologies, Wire Arc Additive Manufacturing (WAAM) employs a welding system in combination with a motion mechanism to deposit layers of molten metal. This technology possesses high deposition rates and an unconstrained build envelope, limited only by the reach of the motion mechanism making it suitable for fabricating large-scale parts. The use of this technique to manufacture technologically interesting materials as Invar 36, a low thermal expansion alloy, could lead to reduced material waste and components with a geometrical complexity only attainable via AM. The high heat input associated with WAAM has been proven to induce columnar grain growth causing anisotropic behavior in the materials, and in the case of Invar 36, it also promotes cracking. In this work, the addition of nucleating agents, known as inoculation, has been implemented during the deposition process to induce grain refinement and mitigate the aforementioned effects. TiC and NbC were selected as possible inoculants based on the results from the implementation of the edge-to-edge matching model as selection criterion. The inoculants, in the form of powders, were mixed with an organic carrier to create suspensions at 50 wt.% and 75 wt.% which then were applied as a coating to each layer during the deposition process. Invar 36 cuboidal specimens with dimensions of 15x15x120 mm were fabricated using GTAW-based WAAM with a heat input of 550 J mm^-1. Microstructural characterization showed that specimens with added inoculants achieved significant grain size reduction, reduced crack formation, and an increase in hardness. Defect-free depositions were attained in the specimens with inoculant-loaded suspensions at 75 wt.%.

Qualification of the mechanical properties of Wire Arc Additive Manufacturing (WAAM) products is an essential step for a successful introduction of this process in industry. To realise this, the relationship between processing conditions, the thermal history and the resulting mechanical properties needs to be established. Hardness is an easy-to-check property that correlates well with such tensile properties. This thesis describes the prediction of the relationship between thermal cycling during deposition of a WAAM produced multilayer block and the resulting hardness. A 3D transient heat model was built to investigate the effect of the thermal behaviour. The weld process was simulated using temperature boundary conditions instead of a direct heat source to calculate the thermal field. The material deposition was simulated using the element birth-death method. The calculated thermal data was used for hardness estimation based on the t8/5 time, i.e. the time to cool from 800 to 500 °C, to determine the hardness distribution throughout the deposited blocks. Temperature was measured during the experiments with thermocouples to validate the thermal model, while hardness was measured to verify the hardness results. The thermal boundary condition based model was found to be in good agreement with experimental results. The modelled results show a correct prediction of the decrease in hardness with increased layer height.

In this research, a thermal analysis of the wire and arc additive manufacturing process is presented based on the F.E. method.

An important quality of the F.E. model that is presented in this research is to describe the temperature field that is experienced by the deposited material in the wire and arc additive manufacturing process. In contrast to the traditional method of monitoring the substrate temperature, the F.E. model allows to describe the temperature field that is experienced by the deposited material constituting both the multilayer weld deposit and the component. Accordingly, the F.E. model is capable to describe the effect of the locally attained temperature field on the microstructure of the deposited material constituting the multilayer weld deposit in terms of the thermal characteristics including the temperature distribution, the locally attained temperature values, the cooling rates and the temperature gradients throughout the component.

Two cooling methods are proposed to control the heat dissipation from the component to the environment based on the application of an interlayer waiting time and immersing the component into a cooling medium. The results show a significant effect of the cooling methods on the temperature field that is experienced by the material constituting both the multilayer weld deposit and the component. Indicating that the cooling methods that are proposed in this research are effective to control the microstructure and the temperature field that is experienced by the component in the wire and arc additive manufacturing process. The resulting microstructure is characterised in terms of the microstructural morphology and the microstructural constituents, using optical microscopy based on the average grain size and the distribution of alloying elements throughout the material constituting the multilayer weld deposit. In addition, the average grain size and the distribution of the alloying elements throughout the material are evaluated in terms of the hardness values.

The current research work investigates the possibility of using machine learning models to deduce the relationship between WAAM (wire arc additive manufacturing) sensor responses and defect presence in the printed part. The work specifically focuses on three materials from the nickel alloy family – Inconel 718, Invar 36 and Inconel 625, and uses three sensor responses (welding voltage, welding current and welding audio) for predictions. A variety of types of prints, including ramp tests, single bead depositions, and walls were explored. Three different machine learning models are used – artificial neural networks (ANNs), K-Means clustering and random forests (RF), and the performances are compared. In addition to separate material analysis, cross-material predictions are conducted using two supervised models to investigate the prediction capabilities of such an approach. The results indicate that models are indeed capable of finding connections between welding parameters and defect formation, and the accuracies range from 60% to 90% and the correlation coefficient is less than 0.5 (indicating weak positive correlation) depending on the model and material. The cross-material predictions are significantly worse, with accuracies ranging from 20% to 27% and very weak correlation coefficients (less than 0.1). Analysis of the results indicates that the importance of audio sensor response depends on the nature of defect, and that additional sensors like spectrometers could give a wider range of information to cover more types of defects, potentially raising the performance of cross-material predictions. Between the models, random forest is found to perform the best overall, with ANNs coming in a close second. The versatility of ANNs indicates that increasing the dataset size and resolving the class imbalance could potentially tip the scales in the favor of ANNs.

For the use of screening potential glassmaking recipes the seeding method has been applied to a Molecular Dynamics simulated CaO-SiO₂ system in order to attain the parameters for the Classical Nucleation Theory and construct a Time-Temperature-Transformation (TTT) diagram. Using this TTT diagram, the non-crystallisation temperature and the critical cooling rate of the material was determined, two quantities important for the prevention of crystallisation during the glassmaking process. The implementation of the seeding method on the CaO-SiO₂ system involved creating a novel local bond order based detection method for distinguishing the crystalline structure of Wollastonite-1A from the glass melt. Regrettably this method had less than desirable precision which resulted in results for the nucleation rate that can only be used qualitatively. In contrast the results for the crystalline growth rate can be used quantitatively, while the resulting TTT diagram again can only be used qualitatively.

Recycled concrete aggregates (RCA) have been recognized as a sustainable solution to satisfy the demand and address the environmental impacts of the concrete industry. However, construction engineers and contractors still have reservations about RCA due to their heterogeneous origin and lower quality compared to natural aggregates. It is essential to know the properties of the parent concrete, in particular its chemical composition, before demolishing concrete structures in order to identify the source of the raw materials. Despite its importance, this has never been done before. This thesis aims to analyze in-situ chemical composition of concrete and to identify the type of cement used in concrete structures by applying the non-destructive characterization technique of a handheld X-ray Fluorescence (hXRF) analyzer. To achieve this, laboratory experiments were performed to assess important factors in measuring cement paste and concrete surface chemical composition using the hXRF. These examined the impact of different water-to-cement (w/c) ratios, curing age, relative humidity, and carbonation on the chemical composition analysis with hXRF, and to gain valuable insights regarding the oxide concentrations of concrete and the most influential factors in hXRF measurements. The results indicated no impact on the chemical composition measurements with the hXRF despite the w/c ratios, the curing age, and carbonation. It was identified that higher relative humidity conditions than 75% affect the measurements with hXRF. Subsequently, three case studies were considered for the in-situ assessment of chemical composition. Conducting tests with hXRF demonstrated the device’s potential for analyzing concrete characteristic oxides (MgO, Al2O3, SiO2, SO3, CaO, Fe2O3). As a result, it facilitated cement-type recognition and provided valuable information regarding its opportunities and challenges. Drawing on insights from the literature review, laboratory experiments, and in-situ measurements, a guideline for in-situ testing of concrete composition was developed.

Wire Arc Additive Manufacturing (WAAM) uses an arc welding process and consumable filler wire to deposit near net shaped 3d structures in a layer by layer manner. The highly flexible nature of the process enables the production of components with materials that are otherwise challenging to process using subtractive manufacturing methods. Invar 36 is a low thermal expansion Fe-Ni alloy deriving its name from the invariability of the thermal expansion to temperature, which is the lowest at a nickel content of 36 wt.%. Invar is challenging to machine due to a low thermal conductivity, high toughness and high susceptibility to work hardening, resulting in low productivity and high machining costs. Consequently, additive manufacturing methods such as WAAM can provide a good alternative to conventional production processes.

The processing parameters required for defect-free deposition of a commercial invar 36 welding wire using GTAW-based WAAM were investigated in this research. Multi-layer structures with a target cross sectional area of 15x15 mm2 were deposited using processing parameters corresponding to heat inputs ranging from 200- 550 J/mm. Microstructural evaluation revealed a coarse columnar grain structure with a mostly cellular mixed mode solidification substructure. Additionally, an increasing heat input resulted in a higher concentration of voids and (micro)cracking along the grain boundaries. Ductility-dip cracking was the most likely cracking mechanism due to the observed locations and fracture surface of the cracks. Defect-free invar weldments were deposited at a heat input of 200 J/mm.

Two 20x120x60 mm3 invar 36 deposits were made at 200 J/mm and 550 J/mm to allow for the machining of tensile testing samples. A higher average strength and elongation was observed in the 200 J/mm heat input samples when compared to the 550 J/mm samples. Samples oriented horizontally in the block displayed a 5.2 % higher UTS and a 8.5 % lower Young's modulus. The highly directional solidification grain structure resulted in anisotropic behaviour due to the Hall-Petch relationship and elastic anisotropy of the austenite crystal. A high concentration of grain boundary defects caused premature failure in 75 % of the 550 J/mm samples.

Multi-wire WAAM, where both invar 36 and nickel welding wires were used, was used to deposit invar alloys with a variable nickel content. A stepwise graded wall was deposited, where the invar composition was increased every three layers ranging from 33-38-42-48-55 wt.% Ni. Energy Dispersive X-ray Spectroscopy (EDS) analysis showed a homogeneous composition in each layer and a gradual increase of the nickel content throughout the wall. A defect-free invar 42 block was deposited at a heat input of 215 J/mm. The nickel composition was found to be homogenous throughout the sample and in accordance with the model created to calculate the weld metal composition as a function of the individual wire feed speeds. X-ray diffraction (XRD) analysis of all invar compositions deposited in this research showed a diffraction pattern matching the fcc diffraction pattern.

Liquid metal embrittlement (LME) is a problem encountered in the resistance spot welding joining process of advanced high-strength steels in the automotive industry. Its occurrence reduces the mechanical performance of welds. The nature of resistance spot welding prevents in-situ characterisation of thermo-mechanical conditions causing LME. Laser-beam welding (LBW) under tension is proposed as an alternative method to analyse LME cracks growing during the welding process of a DP1000 dual-phase steel grade. The influence of global thermo-mechanical parameters on the degree of embrittlement is investigated through Gleeble hot tensile tests. LBW schedules are explored, and material characterisation used to find and prove LME crack occurrence. Finite element analysis with COMSOL is used to connect results from Gleeble hot tensile tests with results from LBW and relevance to RSW is outlined. Results show that a temperature dependent ductility trough is present between 750 and 900°C. The Fe-Zn system is further found to require specific mechanical conditions (stress and strain rate) to become susceptible to LME. The proposed LBW setup is found to be susceptible to LME, but not in a high enough severity to be detectable through SEM and EDS. Changes should be made to the loading setup of the LBW setup to induce LME crack growth to a sufficient degree to allow for in-situ monitoring of local thermo-mechanical conditions surrounding the crack.

Oxide dispersion strengthened (ODS) steels are promising candidates for use as structural materials in the next generation fission and fusion reactors. Compared to conventional ferritic or martensitic steels, ODS steels exhibit improved high-temperature creep properties and irradiation resistance. Favourable properties are mainly attributed to the fine grain features and the high number density of nanosized oxide particles in the steel matrix. These nanoparticles can act as pinning sites for dislocations and stable sinks for irradiation introduced defects, leading to significantly enhanced mechanical properties. In order to be employed in nuclear systems with large, complex structures, the fabrication and welding of ODS steels with reproducible and superior properties are inevitable and essential. However, after 10–20 years of studying since the emergence of ODS steels, these issues remain the major bottlenecks limiting further development. This thesis is concerned with ODS Eurofer steel, which is one of the representatives of ODS steels and has been the research focus in terms of promising nuclear materials within the European Union. With the aim to develop suitable and effective methods for the fabrication and welding of ODS Eurofer, the result of this study should help to extend the use of ODS steels in future nuclear applications.