Search

Metallic biomaterials have an essential portion of uses in biomedical applications. Their properties can be tuned by many factors resulting in their process tuneability. Among metallic biomaterials for biomedical and specifically orthopedic applications, titanium and its alloys exhibit the most suitable characteristics as compared to stainless steels and Co-Cr alloys because of their high biocompatibility, specific strength (strength to density ratio), and corrosion resistance. According to their phase constitution, Ti-alloys are classified into three main groups, namely alpha, beta, and alpha+beta alloys. Depending on the degree of alloying and thermomechanical processing path, it is possible to tune the balance of ? and ? phases, which permits to tailor properties like strength, toughness, and fatigue resistance. (alpha+beta) Ti alloys, especially Ti-6Al-4V, are widely used alloys in biomedical applications; however, they have some drawbacks like the presence of toxic elements i.e., V and relatively high elastic modulus to that of bones. In view of the lower elastic modulus of body center cubic beta phase (50GPa

This thesis summarizes the results obtained during the PhD research period in Industrial Engineering at the Industrial Engineering Doctorate School of University of Padua. The research project was mainly focused on the effect of the electrical current on the plastic deformation and on the microstructure of different metal alloys known as Electroplastic Effect (EPE). The research project was conducted in collaboration with the Metallurgy, Mechanical and Electrical Engineering groups of the Industrial Engineering Department of University of Padua. This thesis is organized in five different chapters. The first chapter deals with the state of the art of the Electroplastic Effect, starting from the first scientific papers and then focusing mainly on the various theories that try to explain the phenomenon. In the second chapter, already performed researches at University of Padua are reported. The third chapter focuses on the experimental apparatus used during the research period. Fourth chapter is focused on the experimental results obtained using continuous and pulsed direct current on four types of high technological interest metal alloys (pure aluminum, titanium grade five, an experimental TWIP steel and four grades of duplex stainless steels). The fifth chapter deals with electropulsed treatment on a super duplex stainless steel. It has been decided to include two appendixes that contributed to the scientific training of the candidate. Appendix A reports the results concerning the influence of small volume fraction and different morphologies of secondary phases on the ductile-to-brittle (DBT) transition of a UNS S32205 duplex stainless steels. Finally, appendix B describes the influence of cold rolling on the laser weldability of UNS S32750 duplex stainless steel. Most of the research activities were carried out in the laboratories of the Metallurgy group, the Electrical Engineering group and the Mechanical Engineering group of the Industrial Engineering Department of University of Padua. Specifically, tensile tests were carried out at the Precision Manufacturing Laboratory under the supervision of Professor Stefania Bruschi and Professor Andrea Ghiotti, electrical measurements were performed at the Laboratory of Electroheating of Padova (LEP) under the supervision of Professor Michele Forzan and Professor Renato Gobbo, finally, the characterization of the samples were performed at the metallurgy laboratories supervised by Professor Irene Calliari and Professor Manuele Dabalà. Some activities were conducted outside the University of Padua in collaboration with other institutions. In particular laser welding and phase quantification by means of electromagnetic measurements were carried out thanks to Professor Istvan Mészáros of the Department of Materials Science and Engineering of the Budapest University of Technology and Economics and finally, impact toughness tests were conducted at the Quality Control Laboratories of Acciaierie Valbruna S.p.A. under the guidance of Engineer Enrico Piva. This research work helped to improve the knowledge on the electroplasticity phenomenon, in particular for a specific class of steels in which two phases with different metallurgical characteristics are present.

The evolution of additive manufacturing (AM) techniques has had such an exponential increase especially in recent years that various and remarkable techniques have been developed for the production of metallic materials. These techniques allow to obtain products with remarkable mechanical characteristics. Therefore, the different AM techniques that employed metallic materials were analysed and their strengths and weaknesses were considered. In particular, investigations were carried out on artefacts made by Direct Metal Laser Sintering (DMLS) technique in two different metal alloys: Inconel-625 and titanium grade 2. In relation to Inconel-625, tomographic analyses were carried out for the detection of ad hoc defects, ultrasound analyses to evaluate anistropy, micrographs and tensile tests to evaluate their mechanical characteristics. The titanium grade 2 products were compared with samples made by the traditional fusion technique to assess their suitability in the dental field. The results show that artefacts made by DMLS technique have overall better features than fusion samples: the defects are less widespread and smaller, the hardness - characteristic of mechanical properties - higher.

Low-cost techniques for the manufacturing of lead and lead-alloys foams have been investigated in this work. In particular the “sintering and dissolution process” (SDP) and “replication process” (RP) have been focused and discussed. The effects of some process parameters have been investigated too: powder compacting pressure, composition of the base metal, granulometry, sintering time and temperature. Spherical urea and NaCl have been employed in the SDP method. In the replication process the melt viscosity has been found the main parameter. Furthermore melt infiltration in the NaCl particles has been found much easier if external pressure is applied. Lead recovery and recycling of exhaust batteries into foam-based electrodes has been examined too with a novel method for the direct conversion of Pb scrap into lead foam.

The demand for ductile cast iron components, with weights ranging from a few kilograms to several tons, has increased significantly in recent years, both for technical and economic reasons. In fact, the lower cost compared to other materials, the good castability, which allows to obtain near-net shape components in as-cast conditions, and the mechanical properties that can be obtained, are just some of the motivations that attract mechanical designers. In the case of large components, the knowledge of mechanical behaviour is however limited and incomplete. What is known is that, by increasing the thicknesses of the castings, the solidification times increase and the cooling rates are greatly reduced. In these critical conditions, solidification can lead to the formation of microstructural defects, sometimes unavoidable, which negatively influence the local mechanical properties of ductile cast iron components. From an initial and in-depth bibliographic analysis, it emerged that, in literature, these issues have been studied in a systematic way only in the recent years. As a result, the number of data available for engineers is limited and it can be noted that there are many lacks, especially with regard to the mechanical characterization and fatigue behaviour of these materials. Specifically, most of the work has been carried out considering traditional ferritic ductile iron used, for example, in the production of wind turbine components. In some studies, traditional cast irons with pearlitic matrix have been characterized, while the data concerning the mechanical and microstructural properties of new-generation cast irons with a solid solution strengthened ferritic matrix are very limited. The First Chapter contains a brief introduction concerning the above mentioned topics, with particular attention to the state of the art and to the recent works published in the literature The Second Chapter describes the experimental campaign conducted during the doctorate, following which an in-depth microstructural and mechanical characterization of different types of spheroidal cast iron characterized by different conditions of cooling and solidification was carried out. During the experimental campaigns, microstructural analyses were performed using an optical microscope and image analysis software on polished and etched samples. In addition, tensile and fatigue tests were performed on specimens obtained from the areas of interest within the castings. The fracture surfaces were then analysed by SEM to identify the causes of fracture. In the Third Chapter the most significant experimental results obtained from the microstructural and mechanical characterization of a traditional ferritic matrix iron are reported. The aim is to expand the literature with further experimental data on this type of material in specific cooling and solidification conditions. In the Fourth Chapter, because of the few works in the literature, cast iron with pearlitic matrix was studied. In particular, through the realization of three experiments, the effect that the post-inoculation process have on the microstructural parameters and on the mechanical properties has been investigated. In the Fifth Chapter, the results obtained on new-generation solution strengthened ferritic ductile irons are reported. In particular, the effect of different amount of silicon and antimony on the properties of castings characterized by long solidification times has been investigated. In the Sixth Chapter, the microstructural, mechanical and fatigue properties of a particular grade of solution strengthened ferritic ductile iron have been evaluated as a function of different section thicknesses and solidification times. In the Seventh Chapter, a model that allows estimating the fatigue strength of as-cast ductile irons containing solidification defects is proposed based on the new experimental data. The Eighth Chapter presents concluding remarks on the work in order to discuss the main results

The optimization of metallic solid surfaces is one of the greatest industrial challenges about surface engineering. Surface hardening treatments are applied to improve the wear resistance of parts without affecting the more tough and soft core of the treated substrates. This doctoral work is focused on the study of gaseous multistage oxynitrocarburizing treatments for low carbon steels and hard-anodizing processes applied on aluminum HPDC components. There is still a lack of knowledge both in literature and in industry about these treatments. The aim of this work is to investigate the microstructural evolution during gaseous multistage oxynitrocarburizing treatments and the impact of the substrate’s characteristics on the hard-anodic layer, respectively. It is important to understand the impact of the process parameters and the substrate’s microstructure on the respective resulting layers; in this way, it is possible to improve the scientific knowledge and therefore to understand the basics for following replications in an industrial context. A wide-ranging view of the whole thermochemical and hard-anodizing processes has been provided by a review of the literature and several treatments replicated in an industrial plant.

Two are the key players of this doctoral thesis: secondary AlSi alloys and the High Pressure Die Casting (HPDC) process. As the levels of CO2 and other greenhouse gasses increase, the industry sector is called to reduce the emissions produced and to contain the energy consumptions. Therefore, secondary Al ingot production shows a 6% annual rate growth, mainly drawn by the transport sector. In particular, automotive industry and, in general, transportation one increasingly needs light components in order to reduce total weights and therefore limit harmful emissions and fuel consumption. To this end, HPDC process is getting more and more crucial. It is, on one hand, versatile and highly productive but, on the other, the elevated amount of defects found in the castings sometimes compromises the characteristics of the final product. The research activities performed during the Ph.D. years focused on the High Pressure Die Casting (HPDC) process and the assessment of the correlations between process parameters, casting quality and mechanical response improvement. The research work comprises three interconnected topics: (i) the effect of the process on the casting quality in terms of defect generation and eutectic Si morphology, (ii) how defects and microstructure influence the mechanical response of the components and (iii) how the mechanical properties can be improved through specific heat treatments. The thesis starts dealing with the work developed within the EU MUSIC project aimed at transforming a production-rate-dominated manufacturing process into a quality/efficiency-driven and integration-oriented one. In the frame of the EU MUSIC project, an investigation procedure was developed which led to the definition of preliminary correlations between process parameters, defect content and static mechanical properties obtained analyzing both reference castings and commercialized industrial demonstrators. A specific focus was always devoted to the possible industrial application in order to narrow down the gap between academic research and industry. The second section deals with the definition of a so-called Penalty Index based on FE simulation model and fractographic investigations. It gave satisfactory predictions of both static strength and fatigue life of castings based on defect locations on the fracture surfaces and their projected areas. The elaborated criterion gives good results even though it does not take into account the real morphology of defects (which can be a very complex 3D morphology in the case of irregular flaws); its main drawback is that it is a "post-mortem" application. However, this limit can be overcome thanks to the wider diffusion of industrial high-resolution Computed Tomography (CT). The third section is mainly dedicated to the study of HPDC microstructure. Two secondary Al-Si alloys, an AlSi12(b) and an AlSi12Cu1(Fe), were used to produce HPDCs. In both cases, there was an evident non-homogeneity of the microstructure, in particular of the eutectic Si morphology which could be correlated, to some extent, to the different as-cast mechanical response of the castings. Heat treatments at 350°C and soaking times ranging from1 to 8 hours were performed Mechanical testing, hardness and metallographic investigations were carried out in order to understand the effect of the heat treatments. Microstructural observations using OM, SEM, TEM and EBSD investigations were also performed. Based on the obtained results, the heat treating procedure was effective since it did not cause surface blistering (a real issue in the heat treatment of die castings) or component distortion. The visible mechanical improvement in terms of both static and fatigue response was determined by Si crystals’ size and morphology. Thanks to heat treatment, the Si particle morphology changed from an interconnected ‘network’ to a more fragmented and smooth one. In terms of mechanical behavior, from a rapid crack propagation which led to a drastic reduction of the load-bearing area during the component use, the heat treatment caused a less instantaneous failure since the crack had to propagate through the ductile Al matrix. The best solution was assessed and proposed to obtain significant mechanical properties and low costs. Preliminary lab tests aimed at verifying the effect on the eutectic Si of common alloying elements such as Cu and Fe were finally conducted. A small mold was designed and manufactured in order to produce castings with solidification velocities comparable with those typical of the HPDC process. The base alloy was an AlSi12(b) ingot which was melt and several elements (i.e. Cu, Fe, Mn) were added to reach the AlSi12Cu1(Fe) alloy ranges. A refining effect of Cu was observed, however further investigations are necessary in order to better understand its joined effect with Fe. This doctoral thesis deals with a complex topic; a high number of sequential actions are involved in the process. This is the reason why the developed work brought interesting and useful results, but it can also be considered a starting point for further investigations. Further analyses can be conducted to understand the causes of the non-uniform eutectic Si morphology and the nature of the Si precipitates within the α-Al phase as well as an enhancement of the elaborated Penalty Index can be obtained.

Solar sails exploit the radiation pressure as propulsion system. Sunlight is used to propel space vehicles by reflecting solar photons from a large and lightweight material, so that no propellant is required for primary propulsion. Kapton seems to be the most suitable material for the sail production and in the space missions till now activated booms as deployment systems have always been used. In this work, an innovative self-deploying system based on NiTi shape memory wires has been designed and manufactured in a small-scale prototype. As kapton has always been employed with a thin Al coating on the surface of the sail, commercial pure Al thin sheets with thin adhesive kapton have been used in order to simulate the sail. The attention has been focused, in the deployment experiments performed in the laboratory, on the effect of different heating methods and different pressure conditions on the activation times. The folded configuration chosen has been deployed in atmospheric condition and in low pressure condition (0.05 bar) inside a oven connected to a rotary pump. For what concerns the heating methods, the attention has been focused on low-pressure oven ISCO NSV 9035 (1.3 kW) and on halogen lamp (1 kW) in order to obtain the self-deployment of the sail. Some comparisons between the two configurations in the different environmental conditions have been performed. In all cases, the full self-activation of the sail has been achieved.

Precursor effects of indium melting have been investigated by means of Mechanical Spectroscopy (MS) and High Temperature X-ray Diffraction (HT-XRD). MS tests evidenced a sharp drop of dynamic modulus in the temperature range between 418 K and 429 K (melting point). At 429 K, HT-XRD showed partial grain re-orientation, peak profile broadening, in particular in the lower part, and peak shift towards lower angles. Experimental results are consistent with density increase of self-interstitials and vacancies in the crystal lattice before melting. Self-interstitials and vacancies play a synergic role in the solid–liquid (S-L) transformation. The increase of self-interstitials over a temperature range of about 10 K before melting has the effect of weakening interatomic bonds (modulus drop) that favors the successive vacancy formation. Finally, the huge increase of vacancy concentration above 428 K leads to the collapse of crystal lattice (melting).

The recovery of different types of metals from wastes has had a spreading interest in the last years. One reason is that wastes, and in particular e-waste, contain metals which are considered strategic. In fact, the availability of these metals is limited and decreases, as natural resources are limited, and their prices fluctuate according to the markets and the management policy of the resource country. Actually, strategic metals are generally defined as those metals that are required for the national defense of a country, but are threatened by supply disruptions due to limited domestic production. However, the definition of strategic metals can be also including those metals that are important not only for national defense, but also for industries that play an important role in the economic development of a country such as energy, aerospace, telecommunication, computer or mobile technology. Thus, it has become quite interesting and urgent to find a strategic way to recover these metals from the wastes. In this work, the recovery of resources from different wastes was studied. In particular, after the introduction and the description of the experimental systems used in this research, the recovery of gold, together with other metals (silver, copper and tin), from the printed circuit boards (PCBs) of end of life (EOL) mobile phones is discussed in Chapter 3. Actually, both the most dangerous and the most precious metals could be founded in PCBs. In particular, it was studied a process which consists in an acid leaching process followed by the gold complexation with thiourea or thiosulphate, two substances that represent an alternative to the more toxic cyanides typically used for the selective gold dissolution. Furthermore, the effect of the ultrasound during the leaching was examined, in order to improve the recovery yield. In fact, in the latest years, ultrasound has been investigated to assist hydrometallurgical metals extraction from ores and minerals but their industrial application is until now limited, although the advantages derived from their application are: a greater metal release in shorter time, lower consumption of reagents and lower process temperature. Therefore, the use of ultrasound could represent an advantage because it increases the recovery of precious metals from the waste. Subsequently, a study was conducted to evaluate a method to recovery silver from the end-of-life solar cells and, more generally, to design a simple and cheap process for the full recovery of the materials constituent the photovoltaic panels. In fact, different methods both hydrometallurgical and pyrometallurgical were tested and the best results were obtained using a combination of a heating treatment and three hydrometallurgical ones. The process parameters were optimized, both for heating and chemical treatments. The results of this study are presented in Chapter 4. Moreover, because the idea seems economically valuable, the process in lab scale was also improve at TRL 5 and the obtained results were used to apply successfully for European funding. In fact, “ReSiELP” project was approved, with the aim to improve the TRL of the technology from 5 to 7. Successively, in Chapter 5 is described the study of recovering Tantalum from EOL capacitors and Neodymium from permanent magnets. In order to separate tantalum from silica, a new treatment with hydrofluoric acid was investigated and tested, whereas the process known in literature for neodymium recovery, was modified introducing a treatment with ammonia which increase the purity of recovered neodymium and sodium double salt. After these studies about metals extraction from e-wastes, a method to increase the added value of recovered material was investigated. In fact, besides the intrinsic value of these metals, a further value could be achieved by recovering these metals in form of nanoparticles, which exhibit very interesting and promising properties in comparison with the corresponding bulk materials. Size-dependent properties include, but are not limited to, the following: optical, magnetic, catalytic, thermodynamic, electrochemical properties and electrical transport. Moreover, nanomaterials may provide solutions to technological and environmental challenges in the areas of solar energy conversion, catalysis, medicine, and water treatment. Different nanoparticles were produced using as raw material the solutions in which electronic scraps were previously leached. In particular, nanoparticles of tin dioxide, silver, copper, cuprous oxide, gold and iron were synthetized and characterized and the results of the study are presented in Chapter 6. All the nanoparticles were synthesized using hydrometallurgical processes and the reagents used were environmentally friendly (like ascorbic acid or glucose syrup) as well as the technologies (ultrasound) used to produce them and to improve the yields in metals extraction from the e-waste. The use of ultrasound in nanoparticles production allows to reduce the sizes of the products thanks to the cavitation effect which produces locally extreme conditions during the nanoparticles nucleation and growth processes. Different processes were developed to produce the six different type of nanoparticles. The materials obtained were analysed by inductive coupled plasma, scanning and transmission electron microscopy, x-ray diffraction, laser diffraction particle size analyser and UV-spectroscopy. The results showed that the developed methods allow to recover the metals with high yield and to produce high purity nanoparticles of tin dioxide, silver, copper, cuprous oxide, gold and iron with sizes between 2 and 200 nm. Finally, some interesting and innovative application for the recovered metals, in particular silver and copper, were also studied and the results are presented in Chapter 7 of this thesis work. The silver and copper powder were introduced into Plasma Electrolytic Oxidation coating. Plasma electrolytic oxidation (PEO), also called ‘Microarc Oxidation’ (MAO) is relatively new surface modification technique that shows an increasing interest in the production of oxide ceramic coatings on light alloys such as aluminium, titanium or magnesium. PEO coatings can enhance the corrosion and wear resistance properties of these metals, or can confers to the light alloys various other functional properties including anti-friction, thermal protection, optical and dielectric features. Furthermore, PEO can be used as a pre-treatment to provide load support for top layers. PEO is a process derived from conventional anodizing which shows many advantages such as higher corrosion and wear resistance performances of the coatings, more environmental friendly electrolytes and the possibility to include into the coatings particles coming from the electrolyte. PEO of metals is a complex process that combines oxide film formation, dissolution and dielectric breakdown: the sample, as anode, is immersed in an electrolyte and it works with high voltages and current densities inside a tank which constitute the cathode of the electrolytic cell. Due to the high voltage that needs to overcome the dielectric breakdown potential of the oxide layer, the formation of a persistent anodic micro-discharges on the surface during the PEO treatment is observed. These short-lived micro-discharges are the key of the process; they move randomly over the surface and produce the growth of an oxide ceramic coating and they also allow to incorporate compounds from the electrolyte into the coating. In the studies presented in this thesis silver and copper powder were introduced into PEO coating by direct addition into the electrolyte to improve respectively the antimicrobial/antifungal and the antifouling properties of the sample surfaces.

High quality scrap is the dream of all the smelters but it is expensive. Therefore low quality scrap has also to be re-melted. Here two strategies have to be taken into account: increasing the scrap quality inside the refinery and removing the non-metal content during the re-melting of the scrap The aim of this doctoral thesis is to support the aluminium recycling industry in both these aspects highlighting the influence of the pre-melting scrap treatments and the melting practices on the effectiveness of the recycling process in terms of metal recovery.

The gold market is one of the major world markets and the Italian companies place the country as one of the major players in this sector. However, the production in these companies is mostly based on the use of artisan skills, without investing in research and studies about the production processes of these materials. In fact, often the process depends on a "craft" way of working, in which the technical and scientific knowledge are dictated more by experience than by a systematic scientific approach. The international bibliography regarding the world of machining of gold alloys is not as developed as in the case of other metal alloys (steels and aluminum alloys), in which the aesthetic appearance is much less important than the functional aspect. The increase in the nationally and internationally competitiveness caused a growing interest to the study of these materials and their metallurgical properties, with the need to create more complex products that are able to maintaining their extremely high quality. Many of the metallurgical concepts of other materials (in particular face-centered cubic structure alloys) can be applied to gold alloys. However, the greatest variety in composition, the well-defined standards in form, the mechanical and aesthetic properties that they have to satisfy, made necessary the knowledge of mode and characteristics, with which the different constituents influencing the machining, heat treatments and the final quality of the semi-finished product. In the first part of the thesis, the main properties of pure gold and gold alloys will be presented, with a particular attention to the main jewelry production technologies and the main usage of gold in the world market. In first chapter the supply, demand and pricing of gold will be generally described to allow an easy collocation of the information contained and the results explained in this thesis, in a larger context such as the world gold market. After, in chapter 2, will be described the main features of gold, which make its the most important metal used for the production of precious artifacts. The attention will be focused mainly on the electronic configuration of gold, and on the physical, optical, and crystallographic properties that result from this. In chapter 3, the aim is to introduce the metallurgical principles of alloying for improve particular properties, such as strengthening, or for adjust other characteristics of gold alloys in order to meet certain requirements. Will be introduced the industrial operations of casting, deformation, heat treatment, and interesting using of innovative approaches to the goldsmith production. Chapter 4 is an overview of the main technologies used in gold jewelry manufacture, the aim is to give a more complete idea of the means of the term “jewelry”, and of the key role of a scientific approach, or the scientific research, focused to improve and develop the industrial production system. The second part of this thesis summarizes the work carried out during the three-year Ph.D in Industrial Engineering. The objectives of the project are the determination of the relationships between the composition and different properties of the gold alloys (the caratage, the color, the mechanical properties,…) used by the goldsmith company in the different stage of the production cycle for the realization of full pipe and cable chain. In particular will be studied the effect of the various constituents and their quantity in the alloys, the mechanical properties and the weldability of these. The investigation will be performed from the melting process, through different plastic deformation and annealing steps, until the realization of the welded semi-finished product. A further objective of the project is the study of the mechanisms of plastic deformation of the different alloys used in the company, in order to optimize the processes of deformation and heat treatments depending on the material used, the various alloying elements and the different production cycles. In chapter 5, will be briefly described the experimental apparatus used for the characterization of the samples (taken from the whole production process in the goldsmith company FilK S.p.A.), in the laboratories of the Department of Industrial Engineering of the Padua University. In chapter 6, will be described the production cycle of the gold welding wires used to solder the hollow gold chain produced. The effect of the microstructure and residual stresses on the corrosion and mechanical properties will be discussed. The study will focus on the control of production parameters in order to improve machinability of the gold wires and to increase their properties. In chapter 7, a statistic analysis of some mechanical and compositional characteristics of gold alloy wires will be reported. The purpose is to individuate an approximation of the possible existing influences and relationships between these and the energy absorbed during deformation. The properties that will be investigated are elongation, Young modulus, ultimate strength and the concentration of gold. In Chapter 8, the production processes of gold alloys plates will be described. The aim of this part of thesis is to optimize the microstructure and the mechanical properties of the products, which, successively, will be welded to an iron sheet to create the base for the final hollow chain products. The parameters of production cycles and the intrinsic properties of the different gold alloys will be studied, with particular attention to the stresses and the microstructures generated from the subsequent deformation and annealing steps. Finally, in chapter 9 the plating process will be analyzed, in order to define the influence of the process parameters on the characteristics of the semi-finished products. Furthermore, different ageing treatments for various gold alloy compositions will be described, the results will allow to optimize time and temperature of the age-hardening treatments for the company’s gold alloys. The whole production tests were performed, in collaboration with Ing. Fabrizio Furlan, in Filk S.p.A., the goldsmith company partner of this collaboration and unique supplier of the material for this research. The main part of the characterization of the different gold samples was carried out at the metallurgy laboratories in the Department of Industrial Engineering, University of Padua, under the supervision of prof. Manuele Dabalà and in collaboration with Ing. Luca Pezzato. Some analyzes, especially the chemical etching of the specimens with different solutions of cyanides, have been performed at other structure: in the laboratories of Progold S.p.A. in collaboration with Ing. Daniele Maggian, Dr. Patrizio Sbornicchia and Dr. Valerio Doppio. The obtained results allowed improving the workability of the gold alloys used by the company with a reduction of the industrial wastes. The optimization of the production parameters, in the different processes, allowed to increase the microstructural quality of the semi-finished products with an improvement on the mechanical and metallurgical characteristics. A suitable control on the production processes and, the optimization of the gold alloy’s compositions, allowed to increase the constancy, the reproducibility of the results and the quality in the production of hollow and cable pipe chains. The obtained results also have a scientific relevance that have allowed their presentation in numerous national and international conferences and publication in scientific journals as reported at the end of the thesis.

The present work investigates the effect of heat treatments in air on surface and structure of titanium hydride (TiH2) and hydrogen desorption. TiH2 has been heated in air at 440 °C and 540 °C for increasing time up to 180 minutes to get samples representative of 12 different oxidation conditions. The samples have been then examined by Temperature Programmed Desorption (TPD), X-Ray Diffraction (XRD) and Photoelectron Spectroscopy (XPS). Experimental results are presented and discussed.

Solar sails are propellantless systems where the propulsive force is given by the momentum exchange of reflecting photons. In this study, a self-deploying system based on NiTi shape memory wires and sheets has been designed and manufactured. A small-scale prototype of solar sail with carbon fibre loom has been developed. Different configurations have been tested to optimize material and structure design of the small-scale solar sail. In particular the attention has been focused on the surface/weight ratio and the deployment of the solar sail. By reducing weight and enlarging the surface, it is possible to obtain high values of characteristic acceleration that is one of the main parameters for a successful use of the solar sail as propulsion system. Thanks to the use of shape memory alloys for self-actuation of the system, complexity of the structure itself decreases. Moreover, sail deployment is simpler.

Recently aluminum foaming has been of much interest due to its characteristics properties of light weight structure. Metallic foams are highly porous materials which present complex structure of three-dimensional open cells. This aspect causes strong limitations in mass transport due to electro-deposition technology. In this work, the electro-deposition of copper on aluminum open-cell foams substrates was developed, in order to enhance the thermal and mechanical properties of these cellular materials. The mechanical and thermal characterization of the produced samples was lead through compression and conductivity tests. On the basis of the experimental results, analytical models are proposed to predict the quantity and the quality characteristics of the coating.

Liquid Pb-Bi eutectic (LBE) alloy is of great interest as coolant and neutron spallation source for the accelerator driven systems (ADS). Scanning Photoemission Microscopy (SPEM) at the ELETTRA synchrotron radiation facility of Trieste was carried out on samples quenched from melt at different temperatures. SPEM analyses show that the two components (Pb and Bi) are not homogeneously distributed in the range of temperature from melting up to 518 °C, in particular they form clusters enriched in Pb and Bi. The cluster size decreases as temperature increases hence diffusion occurs between clusters and matrix leading to the progressive disgregation of clusters that is almost completed at 518 °C. Such micro-chemical evolution is accompanied by a structural change of the liquid evidenced by the results of High-temperature X-ray Diffraction (HT-XRD). In particular, radial distribution function (RDF) curves show a progressive change of the ratio between the shell radii of 1st and 2nd nearest neighbours. © (2014) Trans Tech Publications, Switzerland.

The shape recovery efficiency of Ni-Ti shape memory springs has been investigated upon the application up to 6 X 105 thermo-activation cycles. The hysteretic behaviour of the Martensitic-Austenitic phase transition has been characterized by resistivity measurements and infrared thermography. A loss in the recovery efficiency of the original shape has been observed and has been ascribed to functional fatigue leading to the formation of the R phase upon sample heating. Nevertheless, one way shape memory effect was found to exhibit an asymptotic stable behaviour which makes possible the realization of Ni-Ti actuators able to operate for a relative large number of activation cycles.

Microstructural stability is one of the utmost important requirements for metallic materials in engineering applications, particularly at high temperatures. The paper shows how Mechanical Spectroscopy (MS) (i.e., damping and dynamic modulus measurements) permits the monitoring of the evolution of lattice defects, porosity, and cracks which strongly affect the mechanical behavior of metals and sometimes lead to permanent damage. For this purpose, some applications of the technique to different metals and alloys (AISI 304 stainless steel, PWA 1483 single crystal superalloy, nanostructured FeMo prepared via SPS sintering and tungsten) of engineering interest are presented. These experiments have been carried out in lab conditions using bar-shaped samples at constant or increasing temperatures. The results can be used to orient the interpretation of frequency and damping changes observed through other instruments in components of complex shape during their in-service life.

High pressure die casting (HPDC) is a widely used process for manufacturing components with high production rate and complex geometries. However, high pressure die casting is still considered a “defect generating process”, since an amount of 5-10% of scrap is usually detected in castings. For this reason, the availability of new standards and tools for optimizing the process is one of the prominent needs of foundry field. Both these issues are tackled in this Thesis. First, a new classification of defects and reference dies for estimating the static mechanical properties of aluminium alloys have been developed and then published as CEN Technical Reports, satisfying the need for new standards. Then, a novel and comprehensive methodology has been developed for optimizing HPDC process through the definition of meaningful behavioural models. Some newly defined process parameters have been hence introduced, by representing the most important physical phenomena affecting casting quality and the integral nature of HPDC, and an original approach is outlined. In particular, in order to provide an effective representation of the different process stages, the following parameters have been found to be more influential and effective in representing transmitted forces, mechanical energy exchange and heat removal: the root mean square acceleration in the second stage, the energy associated to the flow forces in the whole cycle, the work of the pressure forces in the third stage and the normalized thermal gradient. The first two parameters embody the plunger motion time-history, the third parameter represents the hydraulic pressure time-history, while the last one concentrates on some thermal aspects. These parameters take advantage from signal processing techniques of the measured position and pressure profiles. Four experimental campaigns (in the whole 210 castings) using different injection machines, different alloys and different geometries of the die have been carried out to validate the novel process parameters and to prove the general validity of the approach. The statistical correlation with the measured static mechanical properties, density and percentage of porosity, as well as the metallographic analysis (percentage of oxides on fracture surfaces, analysis of different kinds of internal defect) prove the soundness of the developed method. Given the significance of the plunger motion profile in explaining the casting quality, analytical models for computing the root mean square acceleration and the energy associated to the flow forces have been developed. These analytical models permit selecting in advance the best plunger motion profile, which guarantees reliability and soundness of castings, thus satisfying the need expressed by foundries for effective optimization tools.

This thesis summarizes the work carried out during the three-year Ph.D in Industrial Engineering and involve the study and characterization of coatings obtained on light alloys with the technique known as Plasma Electrolytic Oxidation (PEO). PEO process is, from the practice point of view, similar to the traditional anodic oxidation process as it's based on the electrochemical growth of a protective oxide layer on a metal surface. Compared with the traditional anodizing, PEO process works at higher currents and higher voltages, thus modifying the characteristics of the obtained layer. In recent years the importance of PEO process is increasing both in the research and in the industrial world. In fact the potentiality of the coatings obtained with this type of process are higher than those of the coatings obtained with the traditional techniques of chemical conversion or anodizing. However, the relatively high cost and some problems related to the process (in particular the need of a post treatment to ensure galvanic corrosion) have now slowed to the widespread use on an industrial scale. So the scientific research on one hand is looking for new solutions to further improve the properties of the coatings, in order to justify the higher costs, on the other is trying to modify the existing process to reduce the above-mentioned costs. The obtained results explained in this thesis have allowed an expansion in the knowledge regarding the PEO coatings and in particular to move towards greater industrial development of the technique. In fact new process parameters that permit to reduce the total time for the obtainment of good PEO coatings maintaining good corrosion resistance were found, especially working with higher current densities if compared with the ones reported in literature. Moreover the addiction of molybdenum and lanthanum salts as additives in the electrolyte used in the PEO process, has permitted to improve the performances of the coating in terms of corrosion resistance. The addiction of graphite nanoparticles and silver particles has permitted to obtain respectively coatings with improved corrosion and wear resistance and coatings with an intrinsic antimicrobial effect. PEO process was also successfully applied on steels.

Al foams are used to produce aluminium foam sandwiches (AFS) panels or other mechanical parts of complex shape, which are of great interest for many industrial applications. AFS panels with a core made of AlSi10 foam and the skins of AISI 316 steel have been prepared by in situ bonding process. To optimise the process conditions, a first set of experiments has been carried out to realise samples of reduced lab scale (20 × 20mm2), then the AFS panels of pre-industrial size (100× 100mm2) havebeenproduced. The steel-foam joints were investigated by scanning electron microscopy, energy dispersion spectroscopy and X-ray photoelectron spectroscopy. The performed examinations showed that the interface formed during the foaming was characterised by a good inter-diffusion of alloying elements, testifying the excellent quality of metallurgical joints. Furthermore, the pre-industrial panels exhibited the same characteristics of lab samples; therefore, it can be concluded that the scale-up process has been implemented successfully.

In this research the shape memory effect of commercial NiTi sheets has been investigated. Four commercial SMA elements have been characterized. Tensile tests (load-unload cycles) have been performed at various constant temperatures (from room temperature up to 60 °C). Stress-strain curves have been analyzed and discussed in order to quantify the shape-recovery which has been found higher at increasing testing temperature. A study on the energy dissipation on each cycle has been carried out too and the energy absorption on the whole load-unload cycle has been found higher as the temperature increases.

Liquid Pb–Bi eutectic (LBE) alloy has been selected as coolant and neutron spallation source for the development of MYRRHA, an accelerator driven system (ADS). The alloy has been characterized in liquid state from melting (125 °C) to 750 °C by mechanical spectroscopy, i.e. internal friction (IF) and dynamic modulus measurements. The experiments have been carried out using hollow reeds of austenitic stainless steel filled with Pb-Bi alloy and sealed at the extremities. Dynamic modulus showed a remarkable drop in the range 350-520 °C. In the same temperature range radial distribution functions (RDFs), determined from X-ray diffraction patterns, evidenced variations of the mean distance between the 1st nearest neighbour atoms. The phenomenon has been explained as a structural re-arrangement of atoms in the liquid metal.

One-way shape memory effect (OWSME) in NiTi springs has been investigated in this work. The main goal is the definition of a guide-line for the design of a linear actuator for high cycles duty. Some SMA and steel springs with various geometrical features have been produced from wires with different diameters. SMA spring's behavior has been analyzed measuring the maximum length (austenitic condition, T > A F – austenite finish) under different applied loads. The measurement of this length has been performed at successive thermomechanical complete working cycles (150, 5350, 43,000 and 600,000) under a constant applied load necessary to full recovery in the martensitic phase. It has been found that the higher the thermomechanical cycles the lower the reached maximum length. In particular the length loss is negligible at higher cycles. Starting from these considerations and the knowledge of the mechanical characteristics of the springs, a linear actuator (SMA spring–harmonic steel spring) for high-cycle duty can be designed. The right working conditions have been verified too.

The mechanical characteristics of dentine have been investigated on local scale by micro-hardness and instrumented indentation tests. FIMEC, an indentation technique employing a cylindrical punch, permitted measurements of elastic modulus, yield stress, stress-relaxation and creep. The punch diameter (Φ = 0.5 mm) is much larger than the tubule size thus data are not so largely scattered as in micro- and nano-indentation tests but, at the same time, is small enough to guarantee a good resolution in mapping the mechanical properties in different tooth positions. The results are in agreement with literature data obtained by means of various experimental techniques. Furthermore, through tests of mechanical spectroscopy carried out on bar-shaped samples (13 mm x 4 mm x 0.8 mm) the dynamic modulus and the damping factor Q-1 have been measured.

A remote and non-destructive method for the characterization of residual stress in metallic components is here proposed. Such a method is based on the application of infrared thermography for the evaluation of thermal diffusivity, which is expected to be dependent on the local dislocation density in the material lattice induced by plastic deformations. Preliminary experimental results obtained on a yielded ASTM 516 grade 65 steel specimen are presented and discussed on the basis of microhardness and optical metallographic investigations carried out on the same sample.

The structural stability of the single-crystal PWA 1483 superalloy has been investigated by internal friction (IF) and dynamic modulus measurements from room temperature to 1073 K. The examined samples were in the solubilized state. The vibrating reed technique with electrostatic excitation and frequency modulation detection of flexural vibrations has been employed. Frequency was ∼350 Hz. IF spectra recorded in successive test runs on the same samples show a Q−1 maximum (M1) above 623 K, whose intensity and position change from one run to another; in correspondence with M1 the modulus undergoes a slow decrease followed by a sudden increase. Sometimes another maximum (M2) has been observed at lower temperature (∼523 K). After each run the values of the modulus and of Q−1 change indicating that a progressive irreversible transformation occurs. Damping phenomena have been attributed to the rearrangement of dislocation structures in the disordered matrix. This rearrangement modifies the density and the average distance of pinning points. This explanation is supported by transmission electron microscopy (TEM) observations.

The solidification of two binary alloys, In-10Sn and Sn-13Pb, has been investigated by X-ray diffractometry (XRD) at high temperatures. The high-temperature X-ray camera, which was mounted on a diffractometer, had a sample holder suitable for molten metals and operated in a controlled Ar atmosphere. Melts were cooled down slowly and XRD spectra were recorded step by step in order to monitor the structural evolution of the liquid prior to and during solidification. Temperature was kept constant while the XRD data were collected. The radial distribution functions (RDFs) were then determined from the spectra for each temperature. Experiments showed that atomic clusters form in the melt immediately before the appearance of the first solid. The experimental problems encountered in real-time monitoring of phase transformations involving liquid metals were looked at. To avoid convective motion in the liquid and to arrive at the best possible experimental conditions, we discussed the possibility of carrying out these studies at the reduced gravity found on the International Space Station.