Committee:
     PRESIDENT: GARCÍA CANO, IRENE
     SECRETARI: CALVO MUÑOZ, JESICA
     VOCAL: SORGENTE, DONATO
Thesis abstract: Shear spinning is an incremental sheet metal forming process where a flat metal blank or pre-form is usually converted into an axisymmetric hollow part. Several ductile metals and alloys can be shear formed either at cold and hot conditions, and the most common include steel, aluminum, copper, and nickel alloys. The shape of the shear-formed parts may be conical, concave, convex, or a combination, with wide-ranging applications across industries like aerospace, automotive, and more.The main focus of this research was the study of the forming limit at room temperature in the shear spinning process through experimental trials and finite element simulations. Traditionally, a successful part produced by shear forming is highly dependent on the operator's experience, and usually, the right conditions are found after several trials.The forming limit of any metal forming process is essential data that helps to predict defects appearance in advance, guaranteeing product quality. Accordingly, processing maps to failure were derived for a mild steel DC04, stainless steel AISI 420, precipitation hardening aluminum alloys AA2024 and AA7075, in which the failure limits were found as a function of the thickness reduction, roller attack angle, spindle speed, and feed rate. The experimental findings revealed a convincing relationship between the maximum allowable thickness reduction and the feed rate ratio with the roller nose radius. As the feed rate increased, the maximum allowable thickness reduction exhibited an exponential decrease, described by a parametric equation. Also, it was found that the maximum allowable thickness reduction was 84% for the DC04 and 80% for the rest of the materials.Furthermore, numerical simulations based on the finite element method have been demonstrated as an efficient tool to accurately predict the mechanical response and appearance of defects in metal forming operations. The prediction capabilities of the numerical simulation are based on the evolution of field variables through the operation, i.e., displacements, strain, strain rate, stresses, and ductile damage. Different models of shear spinning were built with the ABAQUS commercial software. First, the efficiency of the different kinematic strategies and solving methods used to model the shear spinning were compared. An efficient segment model was proposed that reduces the computing time up to 300 times compared to conventional models.Further, different uncoupled ductile damage models were implemented via the user materials subroutine (VUMAT). It was found that the ductile damage models can accurately predict failure conditions using a non-linear damage evolution formulation.This research enhances the understanding of material deformation in the shear spinning process and provides a framework for improving operational efficiency in incremental forming processes.

Committee:
     PRESIDENT: FRANCO URQUIZA, EDGAR ADRIÁN
     SECRETARI: DE SOUSA PAIS ANTUNES, MARCELO
     VOCAL: YANG, YUNXIAN
Thesis abstract: Acrylonitrile-butadiene-styrene (ABS) is one of the most widely used and versatile thermoplastic due to its toughness, chemical resistance and processability. These characteristics make it suitable for various applications as in electronic and automotive fields. However, its inherent flammability could limit its use. To counteract it, the incorporation of phosphorus-based flame retardants (PFRs) in ABS, as a more environmentally friendly alternative to the halogenated ones, has been used in the past years. However, the use of PFRs has an adverse impact on ABS mechanical properties, limiting its use where both fire safety and mechanical performance are required. In order to solve this problem, new strategies to enhance mechanical behavior of PFR-ABS composites were proposed in the present thesis. Also, due to the current challenge of promoting more ecological solutions, new research regarding the use of bio-wastes of forestry industry, as potential bio-based flame retardants in ABS was also made.The first part of the thesis is devoted to the improvement of mechanical properties of an ABS composite, containing ammonium polyphosphate (APP) and aluminum diethyl phosphinate (AlPi), by combining an ultrahigh molecular weight silicone rubber (UHMW-SR) and ethylene methyl acrylate copolymers (EMA) with different wt.% of methyl acrylate (MA). Their effect on the microstructure, mechanical and fire behavior of the ABS composite was discussed. Improvement in interfacial interaction between the flame retardant particles and ABS matrix, as well as, in the molecular mobility in the presence of EMA and/or UHMW-SR was noticed. As a result, remarkable synergies were observed in impact strength. Particularly, a 212% increase was achieved when added 5 wt.% of EMA, with 29 wt.% of MA, and 2 wt.% of UHMW-SR in flame retarded ABS, maintaining its self-extinguished behavior.To address the development of more friendly flame retardant systems for ABS, cork powder (C), a bio-waste of cork industry with no economic value, was proposed to be used as a potential bio-flame retardant (bio-FR). First, to better understand the flammability behavior of C, the effect of cork components on its pyrolysis decomposition was analyzed. It was observed that suberin plays a major role in enhancing the thermal stability of cork; however, it did not show a significant charring ability, contrarily to cork lignocellulosic part. Furthermore, the effect of combining APP with C on ABS microstructure, thermal stability and fire performance was discussed. By only replacing 3 wt.% of APP by cork, an optimum improvement in fire retardancy was achieved with the highest efficiency in reducing the heat release rate (HRR). It was also revealed that replacing 20 wt.% of APP by cork led to a similar fire performance to the ABS with 30 wt.% APP.Moreover, to increase C efficiency as bio-FR, phosphorus moieties were grafted to its surface by chemical modification. Phosphorylation of C with glyphosate (Cg), triethyl phosphate (Ct) and dimethyl-3-triethoxysilanepropylphosphoramidate (Cd) resulted in a registered phosphorus content of 0.9, 0.3 and 2.4 wt.%, respectively. Compared to natural C, phosphorylated cork contributed to an enhancement of flame retardancy in ABS composites. The highest improvement was observed for ABS with 30 wt.% Cd, by showing a higher combustible gasses dilution effect on the gas phase and a more cohesive char layer formation on the condensed phase that led to the reduction of heat and mass transfer phenomena.

Committee:
     PRESIDENT: MARI, DANIELE
     SECRETARI: FARGAS RIBAS, GEMMA
     VOCAL: BOTERO VEGA, CARLOS ALBERTO
Thesis abstract: Polycrystalline cubic boron nitride (PcBN) is a composite competitive cutting tool material that excels performance when machining difficult-to-cut materials (e.g. hardened steels, superalloys, etc) due to its exceptional mechanical properties. In practice, PcBNs are coated with a ceramic film to prevent and prolong the onset of tribo-oxidation and abrasive wear. Although there is not a unique consolidated opinion regarding the benefits of coatings in PcBN, most of the bibliography agrees on their use when involving hard turning operations. Most of the research address the tool performance and wear mechanisms, whereas information involving materials science aspects of PcBN and coated PcBN tools, on the basis of understanding microstructure – mechanical properties correlations, is quite limited. In this regard, this thesis focusses on studying hardness, fracture toughness and wear resistance as they are key mechanical properties controlling the mechanical integrity and reliability of the tool, which are related with the contact response, fracture resistance (e.g. premature chipping) and effective tool life, respectively. Four distinct substrates of the coated PcBN grades are studied and first characterized, including the development of characterization and testing protocols. In doing so, focus ion beam tomography and three-dimension (3D) image reconstruction was implemented to study the bulk microstructural characteristics of a PcBN grade with high cBN content and metallic binder. It was found to be a powerful and useful method to gain in-depth knowledge and understanding the microstructural characteristics of PcBN composite materials, additionally to those gathered by conventionally 2D method. The study then focuses on the assessment of the micromechanical properties of PcBN composite materials, as they are known to be key for optimizing their performance through microstructural design. High-speed nanoindentation is successfully implemented to characterize and correlate microstructure with local mechanical properties of such hard and stiff composite materials; where two different methodologies, 1D and 2D Gaussian, are used for statistically deconvoluting the data. It is found that the harder PcBN grade is clearly related with the high cBN content. A scaled-up method was employed to evaluate the contact response of uncoated PcBN grades. The higher cBN content and metallic binder grade, exhibits higher resistance to crack nucleation and a more gradual transition through different damage scenarios due to the concomitant increase of hardness and fracture toughness of this grade. Regarding fracture toughening mechanisms, crack path changes from propagation across the ceramic binder to transgranular fracture through cBN particles, as the cBN content increases. Very interesting, fracture toughness is enhanced by crack interaction with intrinsic sub-grained or twin boundaries within the individual cBN particles as well as by crack deviation through nano cBN particles dispersed in the binder. Afterwards, research was aimed to characterize coated PcBN (with different chemical nature and bias voltage), mainly in terms of coating adhesion strength and mechanical integrity of bulk coated systems as a function of the PcBN substrate microstructural assemblage. Mechanical response of the coated system, assessed by using Rockwell C indentation technique and scratch testing, is strongly dependent on the underneath substrate microstructural assemblage; and therefore, its different intrinsic hardness-toughness correlation. Finally, TiAlN-coated PcBN inserts were used to mill a hardened cold work tool steel. It was proposed as an exploratory study of milling performance of coated PcBN systems to assess the onset of coating failure without involving the emergence of other wear phenomena (e.g. thermally-driven ones).

Committee:
     PRESIDENT: MONTERO MARTÍN, JAVIER
     SECRETARI: DÍEZ ESCUDERO, ANNA
     VOCAL: OLDE DAMINK, LEON
Thesis abstract: Bone defects pose a major clinical and socio-economic burden and there is a clear need for new bone grafting strategies that take into account the physical-chemical properties of the native bone, to help treat bone defects in a time- and cost-effective way.Autografts are considered the gold standard due to their biological performance, however, additional surgical procedures for bone harvesting poses drawbacks. For this reason, the industrial- and scientific communities have focused on the development of synthetic bone grafting solutions. The present PhD thesis advances in the development of biomimetic personalised bone grafting solutions, including the clinical validation and the development of new material formulations with improved mechanical performance. Chapter 1: Gives an insight into the general context of bone regeneration and the materials used as bone substitutes, emphasising the need for innovative personalised synthetic bone grafts. This chapter offers an overview of additive manufacturing techniques and frequently employed ceramics in bone tissue engineering and their consolidation strategies. Further, a global presentation is given on their clinical translation, especially emphasising calcium phosphates and MimetikOss® 3D. Chapter 2: Focuses on evaluating the clinical performance of the 3D-printed bone graft MimetikOss® 3D in a horizontal vestibular augmentation. The ridge in the anterior maxilla is reconstructed with a synthetic patient-specific bone graft with a staged approach for dental implant placement. 3D-printed bone grafts permit a perfect fit in the surgical site without any additional shaping, which reduces surgery time compared with other bone augmentation techniques (e.g., guided bone regeneration, standard blocks). The bone graft is completely osseointegrated and its macropores colonised by newly formed bone at 10-months post-surgery without signs of encapsulation. A stable bone gain is achieved, resulting in a fully restored bone width. Dental implants were placed without the need for regrafting and stayed stable at 1- year post-loading, demonstrating the clinical relevance of these bone grafts in vestibular bone augmentations. Chapter 3: Encompasses two routes for incorporating PLGA, as a binder or as a coating, to 3D-printed self-setting scaffolds, taking advantage of their low-temperature hardening, to enhance their mechanical performance. The addition of PLGA increases the capacity for plastic deformation, which significantly improves their toughness (by a 2.6-fold and 4.2-fold change in flexion for PLGA as a binder and as a coating, respectively; and by an 8-fold and 1.6-fold change in compression, respectively), while preserving the in vitro cell viability of MimetikOss® 3D (with MG-63 and hMSC cells). The configuration with PLGA as binder is the better option regarding the enhancement in mechanical performance and osteogenic differentiation (2-fold and 1.5-fold change increase for ALPL and RUNX2 expressions, respectively). Screwability tests demonstrate that the enhanced mechanical properties increase the fixability of the scaffolds in a complex fixation indication in the jaw. Chapter 4: Discusses the impact, limitations, and challenges of 3D-printed biomimetic bone grafts and emphasises the steps remaining before transferring the new technology to the market. It is shown that the developments made in this PhD thesis can be beneficial for the patients and have a positive impact on society. Composite patient-specific bone grafts have an impact on cost, time and performance of the future bone grafting solutions, and they strive towards added value in personalised medicine. This will help treat complex and large bone defects in a way that benefits both the clinician and the patient and encourages sustainable healthcare based on synthetic biomaterials. This technology was protected by a filed patent application, however, there is still work left before it can be translated to the market.

Committee:
     PRESIDENT: OLIVER SOLÀ, JORDI
     SECRETARI: GONZÁLEZ COLOMINAS, MARTA
     VOCAL: GASOL MARTÍNEZ, CARLES
Thesis abstract: This thesis aims to deepen in the understanding of the circular economy of plastics through a scientific and design approach. It has focused on packaging waste as it is the sector with the highest priority and contribution to the European legislation for the transition to a circular economy of plastics. This legislation states that at least 50% of its plastic waste must be available to be recycled by 2025. The waste production and management system was first characterized using MFA (material flow analysis) and LCA (life cycle assessment). The second part of the research is focused on understanding the mixed fraction of waste and the behaviour of the materials obtained from its open loop recycling. That was conducted characterizing the materials and their application. Using the principles of “cascade recovery”, a suitability indicator has been modelled, useful for design decisions and the definition of circularity policies.The main results indicate that the effective recycling rate in Spain for 2017 (the last year for which figures are available) is 11%, far below the 50% required by European regulations and even further away from the 70% declared by the official organism in charge of its management and monitoring. The figures are also difficult to improve, even if selective collection were to increase, due to the high presence of around 50% of film plastics, with very low recyclability. In order to achieve the EU’s objectives, an alternative solution based on technically and economically viable solutions has been proposed. Even though it would greatly improve the circularity of the system, it would also lead to an increase in GHG emissions and a pressure on forest resources that is not sutainable.The second part of the thesis focused on the characterisation of the mixed fraction of plastics, as it is the material stream that will play the most important role in a scenario of massive selective collection. It is also the least known fraction. The recycled materials obtained have been characterised in terms of composition, shaping processes, mechanical properties, environmental impact and aesthetic features. On this basis, an investigation has been conducted into the range of applications that have been manufactured using that material over the last 20 years. An environmental modelling of open loop recycling has been developed by evaluating the substitution of different types of materials and comparing it to a hypothetical close loop recycling of plastics. According to the results, the environmental performance of open loop recycling is superior to that of close loop recycling in almost all scenarios and is therefore a preferable alternative in terms of CO2 emissions.In the third block of the investigation, the potential for implementation in the mixed plastic market is defined. The suitability of the different applications taking into account both the ecological credentials, as well as the aesthetics and the industrial operation has been determined. The results indicate that applications such as road markings and reusable industrial packaging are highly suitable applications for mixed recycled plastics with a large potential market. However, an important problem has been detected in terms of aesthetics and smell for use in domestic products. Once the most suitable applications and their respective market shares have been determined, the consequences at a national scale of the massive introduction of a new flow of materials such as mixed recycled plastic has been evaluated. The results obtained indicate that open loop recycled plastic has a greater potential for improving the circularity of plastics than close loop recycled plastic, with a greater potential for reducing CO2 emissions and with a greater potential market.

Committee:
     PRESIDENT: TORRALBA CASTELLÓ, JOSE MANUEL
     SECRETARI: LLORCA ISERN, NURIA
     VOCAL: CARREÑO GOROSTIAGA, FERNANDO
Thesis abstract: Refractory high-entropy alloys (RHEAs) prepared by mechanical alloying followed by spark plasma sintering usually outweigh the high-temperature mechanical behavior of as-cast counterpart alloys. Despite that, few studies have been conducted in order to understand their deformation mechanisms associated with these alloys and their particular microstructures (ultrafine grain size and considerable presence of secondary phases). The present work reports the synthesis, the microstructural characterization, and the constitutive modeling the AlCrxFeMoNbTiV2 (x = 0.15, 0.4, 0.8) RHEAs prepared by powder metallurgy, aiming to contribute to the comprehension of the deformation mechanisms behind the high-temperature mechanical behavior of RHEAs obtained with this fabrication route, as well as of the composition-microstructure-properties of RHEAs.In order to do so, the study was divided into three parts. In the first one, the effect of composition and milling time over themicrostructural and particle evolution of the powder was investigated. In the second part, the effect of Cr content and of a heat treatment over phase formation, microstructure, and grain size was investigated. In the third part, the samples were subjected to compression testing between 950 °C and 1100 °C and between 0.0005 s-1 and 0.01 s-1 to obtain the constitutive equations of the peak stress. Additionally, the microstructure of some of the deformed samples was characterized to find microstructural hints associated with the different potential softening and deformation mechanisms. During the milling study, it was observed that most of the changes occurred during the firsts 50 h, resulting in an average particle size of 7 µm and a nanostructured bcc+hcp microstructure. Except for Mo, none of the constituent elements considerably affected the microstructure or the particle size of the milled powders. The as-sintered samples, fabricated using powder milled for 50 h, successfully presented an ultrafine-grained and multiphase microstructure, constituted by a V,Mo-rich bcc matrix, accompanied by Fe,Nb-rich Laves phases, Al2O3 particles, and Ti,Nb-rich carbides. For reference, a hardness of 1124 HV0.3 was obtained in the as-sintered samples. The average grain size of the matrix phase decreased from 0.40 µm to 0.21 µm with further Cr content; however, the high-temperature mechanical properties were not affected. On the other hand, after the heat treatment, the average grain size increased up to 1.5 µm without affecting the phase equilibrium though. In opposition to the Cr content, the heat treatment enhanced the yield strength in a considerable manner: a specific yield strength of 98 MPa·g-1·cm3 at 1000 °C was obtained in these samples, three times that of the as-sintered sample. Regarding the constitutive modeling, the power law excellently fitted the experimental data, resulting in an exponent of 2.45, indicating that grain boundary sliding governed the high-temperature deformation of the studied alloy, just as it does in ultrafine-grained size traditional alloys. Additionally, an elevated activation energy of 527 kJ·mol-1 was obtained, associated either with a high softening resistance as well as with the considerable presence of secondary phases. In the case of the heat-treated samples, dislocation climbing and glide seemed to govern the deformation (at least at 1000 °C), explaining the enhanced strength, attributed to the hindered grain boundary mobility due to the larger grain size.