Functionally graded materials (FGM) are a class of materials in which the chemical composition or microstructure varies as a function of position, offering unique material properties. In this study, a functionally graded structure was manufactured using Gas Tungsten Arc Welding and a double wire feed device. The wire feed rate was changed step by step of the austenitic AISI 316L and ferritic AISI 430L stainless steel wire, creating a chemically graded duplex stainless steel structure. The structure, approximately 30 mm in height, was investigated using Optical Microscopy, X-Ray Diffraction, X-Ray Fluorescence and Energy Dispersive X-Ray Spectroscopy. The transverse section is composed of large elongated grains. The chemical analysis revealed a relatively smooth change in Nickel and Molybdenum composition over the section, due to remelting of previously deposited layers. The graded material showed a gradual transition in phase fractions from mainly austenite and some ferrite to mainly ferrite and some austenite, to fully ferric structure. There were no brittle phases detected in the structure.

Present stroke rehabilitation devices for the arm are often difficult to use by the patient himself and cannot be used at home. A compliant shell mechanism could overcome the shortcomings of the current available devices. By the use of monolithic shell mechanisms a simple to use device can be designed, which can be made wearable. It would make it easier for stroke rehabilitation patients to do repetitive rehabilitation exercises at home. The focus lies on balancing the gravity during the lifting of the upper-arm. To achieve that a negative stiffness in the compliant shell mechanism is necessary. Negative stiffness arises when tape springs are bent and buckle for a short range of motion. Tape springs are thin-walled beams with a curved cross section. The short range of the negative stiffness limits the use for static balancing over a longer range of motion. In this project an analysis is presented on how the range of the negative stiffness can be increased by changing the geometry. The addition of longitudinal curvature to the tape spring results in a more gradual negative stiffness for a longer range of motion. It is shown why the addition of longitudinal curvature results in a more gradual and longer range of negative stiffness during the bending of shell.

Wire arc additive manufacturing (WAAM) is attributed to higher material deposition rates and medium to large build volumes. Integration of multi-axis (more than three) material deposition kinematics in WAAM can bring a paradigm shift in the metal additive manufacturing industry. The material build-up mechanism employing directed energy deposition results in localized melting and solidification of feeding wire making the fabricated structure inherently prone to deposition defects such as dimensional distortion, residual stresses, solidification cracking, and porosity. Optimization of welding process parameters can improve cracking and porosity behavior. However, the problem of distortion is inevitable, and current mitigation strategies rely on post-processing or symmetric material deposition; while the former decreases the effectiveness of WAAM and increases costs, the latter could only be implemented for specific parts. Tackling this problem with the help of computational design tools is a recent approach, and Fabrication Sequence Optimization dictating material deposition is numerically predicted to limit the extent of the distortion effectively compared to conventional planar horizontal deposition; however, this still lacks experimental validation. This research presents a series of fabrication experiments using Al5356 Aluminum-magnesium alloy wire in a 6-axis robotic WAAM setup in two parts: (1) Process parameters identification for optimal bead characteristics for Al5356 wire, (2) Fabrication of single-bead multi-layered thin-walled shell structures using planar and optimized material deposition. A digital image correlation-based non-contact in-situ distortion measurement setup is constructed to analyze strain development during fabrication process and 3D scanning of fabricated structures is performed to estimate dimensional deviation. The experimental findings revealed an improvement in geometrical formation and a qualitative indication of distortion minimization with an optimized fabrication sequence.

Additive manufacturing (AM) has revolutionized part manufacturing, offering unprecedented design freedom and the ability to fabricate intricate structures. Computational design methods like topology optimization (TO) effectively capitalize on AM's design freedom. However, manufacturability constraints must be considered to ensure reliable manufacturing. A significant constraint is local overheating, prevalent in metal additive manufacturing, leading to defects, part distortion, and diminished mechanical properties. While overhangs are commonly associated with overheating, they are not the only geometric features prone to overheating. This thesis addresses the overheating issue in topology optimization by a novel geometric approach. Specifically, we propose to estimate the local conductivity around each element by evaluating the material distribution in its vicinity. This is then used to generate a pseudo temperature field (i.e., hotspot map) to assess overheating risks. Formulated as a constraint in topology optimization, our approach creates optimized structural layouts free of local overheating during additive manufacturing. Additionally, the approach is implemented in space-time topology optimization, limiting overheating risks by simultaneous optimization of structural layout and fabrication sequence. Compared with existing overheating prevention methods, the geometry-based constraint demonstrates significant computational advantages. Transient thermal AM simulations were conducted on the final designs obtained using the physics-based method from the literature and the novel geometry-based overheating prevention method. The numerical results have shown that the proposed geometric approach is efficient and effective in controlling local overheating in topology optimization for metal additive manufacturing.

Additive manufacturing (AM) is quickly becoming one of the more popular methods to manufacture components made of Ti-6Al-4V in the aerospace and automobile industry due to its flexibility in producing complex geometries and reducing tooling costs. As the world of additive manufacturing is still relatively young, there is no globally accepted method for the certification of AM parts, allowing freedom to develop procedures to utilize the advantages of AM. To greatly reduce material wastage, the use of smaller specimens has been explored to characterize the mechanical properties of the material. This study aims to analyze the influence of test specimen size on the near threshold fatigue properties of Ti-6Al-4V manufactured by conventional processes and Laser powder bed fusion (LPBF) . Two different specimen sizes were utilized, the standard size and the sub-sized specimens. The effect of two heat treatments, stress relief (SR) and annealing (AN) on the LPBF material on the fatigue properties was also studied. In addition, the effect of anisotropy on the fatigue threshold value was also touched upon in this study. A fatigue test setup for the sub-sized samples was designed and a constant Kmax test was employed to characterize the threshold properties. The direct current potential drop (DCPD) method was employed to measure the crack length during the test. The fatigue threshold of the SR microstructure did not show any effect due to sample size. This was attributed to the partly intergranular crack propagation observed playing a prominent role in the fatigue threshold. The conventional and annealed samples witnessed a drop in threshold values of about 0.5 MPa√m when sub-sized samples were used. It was observed that the sub-sized samples had a greater crack depth at threshold leading to increased constraint at the crack tip and hence smaller degree of crack closure explaining the drop in threshold values. The current study has thus established successfully the feasibility of using sub-sized specimens to characterize the near threshold fatigue properties of conventionally and additively manufactured Ti-6Al-4V.

Adolescent Idiopathic Scoliosis (AIS) is a condition of the spine, often characterized by a three- dimensional spinal deformity. Treatment usually involves interventions like exercise, bracing, or if necessary, surgery. Often bracing is prescribed to stop curve progression so that surgery can be avoided. Traditional scoliosis braces are usually rigid devices, which displace the spine to the desired corrective position. With efficacies over 90%, these braces can be quite effective when worn enough. Unfortunately, the activities of daily living (ADL) for patients are reduced drastically when wearing a brace, and as a consequence compliance towards the braces is low. Since AIS develops in around 3% of all adolescents, of which approximately 10% has progressive curves that require treatment of some sort, the need for effective, comfortable bracing is high. To increase the ADL of patients, the current focus has been shifted towards designing a compliant scoliosis brace that can provide needed corrective forces and allow motion. This thesis focusses on the evaluation of spinal motions through the design of a motion capture experiment. The goal of this work is to provide general knowledge about these spinal motions for clinicians, researchers and mechanism designers, such that they can make use of the provided analysis for the design of a new, compliant scoliosis brace. Parts of this analysis are implemented in a brace design quantification strategy, which can be used to facilitate such a brace design project. The key contribution of this master thesis is the characterization of spinal motions for specific vertebrae, to provide substantial kinematic data for the design of a compliant scoliosis brace.

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.

The outstanding mechanical properties of graphene have made it a suitable candidate for awide range of sensor and actuator applications in modern technology. However, before the full potential of future applications can be achieved, a proper characterisation of the fundamental properties of graphene is crucial. The aim of this project was to contribute to the understanding of the mechanics of graphene membranes in presence of surface imperfections. To this end two configurations are investigated: ribbons and cantilevers, respectively. Wrinkled graphene nanoribbons are used to investigate the mechanical behaviour during the transition from the wrinkled state to the flat state. A molecular dynamics model has been developed of a single layer graphene ribbon to describe both the formation of wrinkles as well as the transition from the wrinkled state to the flat state. Also, a continuum model was developed to investigate the formation of wrinkles in graphene nanoribbons. Different constitutive laws have been investigated to describe the mechanical response of wrinkled membranes during the transition from the wrinkled state to the flat state. It was concluded that an exponential version of Hooke’s law fails to describe this transition correctly. The transition is however well described by the first order compressible Ogden’s law. Ogden’s law provided further insights into different mechanical properties of the wrinkled layer. Ogden’s law predicted that wrinkledmembranes exhibit a negative Poisson’s ratio at small strains, which is in agreement with previous research. Also, Ogden’s law predicted a decreasing shearmodulus and an increasing Poisson’s ratio after flattening of the membrane. Single layer graphene cantilevers show great potential, however, due to the difficult manufacturing of these fragile structures they remain virtually unstudied. Herein, a molecular dynamics model has been developed to investigate if nanocantilevers could be stabilised by implying a curvature. We found that, depending on the aspect ratio of the membrane and the applied rate of curvature, single layer graphene cantilevers could be (partly) stabilised by implying a curvature. In conclusion, with this research we provided new insights for designing and investigating the next generation of graphene nanoelectromechanical devices.

Fused filament fabrication (FFF) is the most used additive manufacturing technique that uses a heated nozzle to melt a polymer and a feeder to extrude it on a buildplate. The dependencies of temperature, shear-rate, viscosity and pressure of the melt create complex dynamics within the nozzle, which causes inconsistent extrusion. To improve extrusion control, a better understanding of the dynamics within the nozzle is required. The greatest knowledge gap comes from a lack of experimental data on the pressure inside the nozzle, due to the challenging environment for sensors. This study presents a novel way of monitoring the pressure inside the nozzle of an FFF 3D printer. A pin that is in direct contact with the melt transfers the force applied by the melt through a hole in the nozzle to an externally mounted load cell. The set-up has proven to provide reliable, repeatable pressure data in steady-state, static extrusion. The experimental data on different nozzle geometries and materials, with different flows and temperatures, has been compared to theoretical pressure calculations to identify non-linearities that influence the pressure such as entrance effects, temperature non-uniformity and viscoelastic behaviour of the melt. The proposed design can be used to gain more knowledge on the extrusion process to further develop extrusion control.

The urge for more sustainable and cost-efficient repair methods for damaged parts is growing every day. On the other hand, laser metal deposition (LMD) is gaining momentum. This flexible technique, where material is added in a layer-by-layer fashion with a minimal heat input offers solutions for a wide range of production and repair purposes. Especially restoration or replacement of large and complex high-performance parts with long production times could profit from these developments. Industrial gears are certainly such type of parts. Occasionally, unforeseen tooth breakage of gears occurs, often leading to the replacement of the parts. No reliable repair techniques have emerged yet, due to the desired material properties and the uneven distributed hardness seen in carburized gears. This study was initiated to examine the feasibility of LMD for the repair of industrial gears with the goal to reduce both material waste and down time while retaining or exceeding the required mechanical performance. The initial part of the research is focused on finding the best suitable material and deposition parameters to ensure the desired mechanical properties of the core and a good bonding of the structure to the gear. Multiple deposition parameters are varied followed by destructive and non-destructive testing. It was found that the mechanical properties of deposited structures Inconel 718 and its bonding to a carburized substrate can be adjusted by accurately selecting the deposition parameters. Large differences in outcome were obtained by changing the deposition direction between each layer and tuning of the shielding gas flow rate. In further investigation the effects on the mechanical behavior of a deposited tooth structure were studied by altering the thermal cycle during and after deposition. The tests are performed on a specially developed static load testing setup. The results show a negative correlation between the interpass temperature and both the yield and ultimate tensile strength. This effect is attributed to a lower cooling rate during solidification at higher interpass temperatures. Ultimately, the results of a deposited tooth with hard flanks and built on a case hardened base were compared to a case hardened gear tooth that was manufactured according to the regular production method. It is found that the deposited structure exhibits significantly lower yield strength. Nonetheless, the superior ductility of the newly deposited tooth structure in the presented work is considered as a promising method for future gear repair.

Fused deposition modeling (FDM) is one of the fastest-growing additive manufacturing processes in recent years. Nevertheless, the mechanical properties cannot be matched to those of traditional manufacturing methods such as injection molding.The research presented in this report aims to demonstrate the limitations of fusion deposition models and proposes automated fiber placement (AFP) as a way to mitigate them by improving transversal tensile strength and elastic modulus. The background and main characteristics of the stated processes, as well as the state of the art, are discussed in this paper. It is explained how both methods were integrated for a production line built specifically for the execution of this research. The key parameters are listed, along with their impact on the end part. Mechanical parameters such as tape-to-print bonding strength, tape-to-tape bonding strength, molding unidirectional fabric tensile strength, and 3D printed part tensile strength were all compared in the experiments. The major challenges to be addressed are surface roughness, print-to-tape contact area, and heat transfer when taping, according to the results. Yet, there is still a large potential for these methods to achieve better performance.

Wire Arc Additive Manufacturing (WAAM) is a manufacturing technique with the ability to produce large metal parts with relatively complex geometrical shapes. One obstacle limiting full exploitation of WAAM in the industry is uncertainty on the mechanical properties of manufactured parts. Mechanical properties are strongly influenced by the microstructure of the material which is a product of the thermal history of the material. Thermal history of parts manufactured with WAAM is complex which leads to uncertainty on mechanical properties and microstructure. Therefore a methodology to predict thermal history, microstructure and hardness of metal-based additive manufactured parts is presented, validated and applied in this study. The methodology is applied to high strength low alloy s690 steel parts produced with WAAM. First a part level thermal process model is utilised to predict the thermal history. Subsequently, a model is developed to predict the microstructure by modelling microstructure phase transformations. Based on thermal history and microstructure phase fractions, Vickers hardness is predicted. Predictions are validated with experimental measurements and observations obtained from a collaborating master thesis. Part temperatures of the thermal model agreed well with measurements. Although the predicted microstructure phase fractions show a lower ferrite content than experimentally observed, the trend of the microstructure phase fractions along the part height is the same in the prediction and the experiment. The predicted hardness is 49 HV higher than the measured hardness. However, hardness predictions and measurements show the same trend along the part height. The methodology to predict the thermal history, microstructure and hardness was applied on a study in which the height of a wall was varied and on a study on a geometry in which two walls cross each other at the center. For both studies the martensite content decreases over the part height, whilst the bainite content increases over the part height. This is caused by the decrease of cooling rate over the part height. Hardness also decreases over the part height. For both the change in microstructure phases and the hardness over the part height, the largest change was observed at the bottom of the part. For the study with the crossing geometry, no significant difference was found for the microstructure and hardness between the center and side of the crossing.