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.

The growing demand for manufactured products with complex geometries requiring advanced fusion-based manufacturing techniques emphasises the importance of process development and optimisation to reduce the risk of adverse outcomes, which is currently impeded with traditional approaches (trial and error experiments). Development, optimisation and qualification of such procedures are often expensive and time-consuming, particularly when new materials or new material combinations are involved. Process stability is intrinsically linked to the stability of the molten metal melt-pool, which ideally should solidify in a smooth and continuous manner to produce a consistent product, free of undesirable geometric and metallurgical defects. The influence of material properties and process conditions on melt-pool stability are generally difficult to derive from experimental observations; hence process optimisation is often reliant on a trial-and-error approach, mitigated to a large extent by a considerable body of industrial experience.
The challenge addressed in this research is to develop a simulation-based approach to assess the stability of oscillating melt-pools in fusion welding and additive manufacturing, to minimise the number of trial-and-error experiments required for process development and optimisation, which ultimately will lead to shortening the time between design and production. The computational model developed in the present work has a generic construction with specific process influences addressed through appropriate boundary conditions, avoiding the necessity to integrate melt pool and detailed process descriptions in a single simulation. The model is therefore capable of representing a wide range of welding and additive manufacturing technologies through selection of appropriate material properties and boundary conditions. The robustness of the present computational model in predicting the melt-pool behaviour is demonstrated by comparing the numerical predictions with experimental, analytical and numerical data.
Focusing on numerical simulations of solidification and melting using the enthalpy-porosity method, the influence of the permeability coefficient (also known as the mushy-zone constant) on the numerical predictions, which is employed to dampen fluid velocities in the mushy zone and suppress them in solid regions, is systematically analysed for both isothermal and non-isothermal phase-change problems. For isothermal phase-change problems, reducing the cell size diminishes the influence of the mushy-zone constant on the results and the solution becomes independent of the mushy-zone constant for fine enough meshes. Numerical predictions of non-isothermal phase-change problems are inherently dependent on the mushy-zone constant. A method is proposed, based on a Péclet number, to predict and evaluate the influence of the permeability coefficient on numerical predictions of solidification and melting problems.
In many numerical studies in the literature, the transport coefficients of the material, specifically thermal conductivity and viscosity, are artificially increased by a so-called `enhancement factor' to achieve agreement between experiments and numerically predicted melt-pool sizes and solidification rates. However, the use of an enhancement factor has little physical meaning, does not represent the physics of complex transport phenomena and can significantly affect the numerical predictions. The effects of using enhancement factors on the numerical predictions of melt-pool behaviour in fusion welding and additive manufacturing are studied in detail. Moreover, the effects of employing temperature-dependent material properties on the numerical predictions are discussed in the present thesis.
Melt pools in fusion welding and additive manufacturing exhibit highly non-linear responses to variations of process parameters and are very sensitive to imposed boundary conditions. Temporal and spatial variations in the energy-flux distribution, which are often neglected in numerical simulations, are taken into account in the present work. It is shown how deformations of the melt-pool surface, due to fluid motion as well as changes in the system orientation, affect the numerical predictions of thermal and fluid flow fields. The effects of joint shape design on melt-pool behaviour during fusion welding is also studied in the present work.
Changes in power-density and force distributions affect the thermal and fluid flow fields on the melt-pool surface, which in turn can influence the pool shape. Oscillations strongly relate to shape and size of the melt-pool and the surface tension distribution on the molten material surface. Using the simulation-based approach developed in the present work, the frequency and amplitude of melt-pool oscillations and changes in the oscillation modes are predicted, which are not accessible using published analytical models and are generally difficult to measure experimentally. Additionally, using the proposed simulation-based approach, the need for triggering of the melt-pool oscillations is obviated, since even small surface displacements are detectable, which are not sensible to the current measurement devices employed in experiments.
The dynamic features of the oscillation signals cannot easily be derived employing conventional Fourier transform (FT) analysis since the oscillation signals are assumed to be stationary (i.e. the behaviour of the system is linear and time-invariant), which is often not the case in fusion welding and additive manufacturing. The continuous wavelet transform (CWT) has been employed in the present work to overcome the shortcomings of the conventional fast Fourier transform (FFT) analysis in characterising the non-stationary features of the surface oscillation signals received from the melt pool. Employing the continuous wavelet transform, the time-resolved melt-pool surface oscillation signals obtained from the numerical simulations can be decomposed into time and frequency spaces simultaneously.
The simulation-based approach developed in the present work addresses some of the significant challenges involved in assessing the melt-pool stability for process development and optimisation. The numerical predictions of the present computational model enhances the current understanding of the process behaviour, which is often very challenging to achieve from experiments alone. Moreover, the present simulation-based approach can be employed to explore the design space and reduce the costs associated with process development and optimisation.

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.

For in-­situ resource utilisation start-up Maana Electric, an investigation was undertaken to determine whether a cover glass for solar panels can be produced using only desert sand as the raw material. During this investigation, the composition of desert sand, melt formation, processing temperatures, and mechanical and optical properties were considered. The composition of 18 desert sands was analysed by means of X­ray fluorescence and estimations of mineralogical composition were made, after which an attempt was made to melt the unmodified sand samples in a microwave furnace built for the purpose. Melt formation was further observed by melting binary combinations of store bought minerals that were found in the desert sands. The composition data and modelling of temperature-viscosity curves were employed to explore lowering the practical melting of the sand point by modification of the composition through benificiation. `Synthetic benificiated desert sand' was produced and melted based on the results. Glass samples produced were characterized using X­ray fluorescence, visual inspection, optical spectrometry, and fracture mirror analysis. It was found that about half of the desert sand samples assessed contain over 90 wt% silica, making it less feasible for use as raw material for glass due to high melting temperatures and/or large waste streams from benificiation, while sands containing larger fractions of carbonates and/or feldspars will form a melt at less than 1650 degrees Celsius if the SiO2 content is less than 55 wt%. Transmission of 85 % of ~550 nm wavelength light was shown to be possible for desert sand glass of 3 mm thickness if Fe2O3 content is lower than 0.1 wt%, while for the same transmission in the complete effective spectrum of silicon based solar cells the iron content needs to be lowered further. Known absorbing species such as Cr2O3, NiO and CuO were detected in desert sand in trace amounts, but were not present in the synthetic mixtures, the influence of these contaminants on transmission requires further research. Mechanical analysis was inconclusive due to a limited number and low quality of the samples produced, but a review of the literature implies that a Young's modulus of >70 GPa and flexural strength of >45 MPa are attainable in a glass produced from desert sand components.

Additive manufacturing (AM) is currently making a transition from mainly being used for prototyping to an alternative for conventional production methods. AM provides more freedom in design geometry combined with the possibility to produce (complex) products in low quantities. As a result, the need arises for printed parts with consistent mechanical properties that rival those of conventionally produced parts. Currently, the layer-by-layer process used by AM introduces weaknesses at the interlayer (weld) bonds. Polymer science suggests that raising weld bonds to temperatures above the glass transition temperature (Tg) can improve their mechanical properties up to bulk strength. This research aims to verify if fused filament fabrication (FFF) printing at elevated print room temperatures (Tenv) raises interlayer temperatures above Tg and as a result enhances mechanical properties. In order to measure the increase in interlayer properties double cantilevered beam (DCB) testing is used. Additionally, tensile testing is used to evaluate the effect of elevating Tenv on general mechanical properties. DCB and tensile testing of samples printed at elevated Tenv values has shown an increase of 109 % in interlayer energy release rate (G1c), up to 50 % in ultimate tensile strength and 106 % in tensile toughness. This has been determined by comparing samples printed at elevated Tenv with samples printed at room temperature (23 ◦C). To further understand the mechanisms behind the enhanced properties, the temperature history during printing has been determined. This has been done by running simulations and using IR imaging. The temperature history of printed parts has been related to data obtained from mechanical testing. This showed that mechanical properties increased for samples in which the interlayer bonds resided above Tg for prolonged periods of time. In addition, optical microscopy and micro CT has been used to monitor the meso-structure of printed samples. This showed that voids in the sample caused by printing defects contributed to an increased spread in measured values. Mechanical properties corrected for effective surface areas showed a roughly 10 % increase for average values. While significant this is not in the same order of magnitude as the increase in mechanical values reported for printing at higher Tenv values. Combining these results it has been found that elevating Tenv does not significantly affect the meso-structure, but it does cause bonds to spend an increased amount of time above Tg. Therefore, it has been concluded that the enhanced mechanical properties are caused by weld bond healing due to the part residing above Tg.

Preventing solidification cracking is an essential prerequisite for the safety of a welded structure. An undetected solidification crack has the potential to cause premature failure during service. Two conditions generated by a weld thermal cycle are responsible for the initiation of solidification cracks. The first is the presence of excessive stresses/strains imposed on the solidifying weld metal and the second is the existence of a weak solidifying microstructure. For more than five decades, weld solidification cracking has been a subject of considerable interest. Cracking has been observed in various alloys, used in a wide range of engineering applications. Despite achieving a better understanding over this period, an accurate prediction of the occurrence of solidification cracking under a specific set of conditions remains difficult. An alloy with a high susceptibility to solidification cracking can still exhibit good weldability upon selection of appropriate welding conditions. Conversely, an alloy with supposedly high resistance to cracking, can still fail when subjected to inappropriate welding conditions.

The objective of the research work reported in this dissertation is to study and elucidate the solidification cracking phenomenon in two popular and commercially available automotive sheet steels, namely transformation-induced plasticity (TRIP) and dual phase (DP) steels. In particular, the effect of restraint (strain imposed), shape of the weld pool, solidification morphology, segregation, solidification temperature range, dendrite coherency and interdendritic liquid feeding on susceptibility to solidification cracking is considered.

This research focused on the question whether it is possible to predict the density of tropical hardwoods with acceptable accuracy based on drill resistance measurements. To assess the feasibility, the influence of the characteristics of the drill resistance measurement tool; the Resistograph® and the wood species have been investigated. With respect to the measurement tool, drill sharpness, feed speed and rotation rate were examined. For the wood samples, the influence of extractives, moisture content, drill direction and density were investigated. It was found that the influence of drill sharpness cannot be ignored and a calibration procedure was developed. No significant difference in drill resistance was observed between drilling in radial and tangential directions; however, drilling in the longitudinal direction resulted in a significant higher drill resistance. Moisture content influences the drill resistance; however, the effect is small and not statistically significant, it can usually be ignored. The influence of extractives on the drill resistance most likely depends not only on the mass fraction of extractives but also on the sort and the composition of the cell wall. Meranti has a typical density within the range 350 to 860 kg/m3. With an appropriate calibration procedure, it is possible to predict the density of meranti with 95% confidence to an accuracy of ± 70 kg/m3