The present PhD thesis summarizes the results of experimental activities carried out on the production and characterization of cast aluminum alloys and Al-based nanocomposites for room and high temperature applications. Two quaternary Al-Si-Cu-Mg alloys (A354 and C355) were studied, aiming to investigate the effect of chemical composition, solidification rate and heat treatment condition on the tensile and fatigue behavior at room and high temperature. Heat treatment optimization of A354 alloy was carried out. The overaging behavior of A354 and C355 alloys was compared to that of A356 (Al-Si-Mg) alloy, in order to evaluate the thermal stability of the alloys. As a result, the concurrent presence of Cu and Mg confers, by precipitation hardening, enhanced mechanical properties and higher thermal stability in comparison to the traditional Al-Si-Mg alloy. A preliminary study aimed to evaluate the effect of Molybdenum addition on A354 overaging response was also carried out. Enhanced mechanical properties after long-term overaging were registered in A354-0.3wt.%Mo alloy, in comparison to the base A354. Casting techniques for the production of Al-matrix composites were implemented at the laboratory scale. The stir-casting method assisted with ultrasonic treatment and in situ reactive casting were applied to produce Al2O3-A356 micro/nanocomposites and ZrB2-A356 composites, respectively. Friction Stir Process (FSP) was evaluated as possible solid state processing route to: (i) enhance Al2O3 nanoparticles distribution in a semisolid processed AA2024-based nanocomposite, and (ii) directly distribute Al2O3 nanoparticles into AA7075 alloy at the solid state. Experimental results highlighted difficulties in obtaining an even distribution of nanoparticles, by both liquid and semi-soli state routes, due to the low wettability of nano-sized ceramic reinforcement. The application of FSP led to enhanced nanoparticles distribution, mitigation of casting defects associated to nanoparticles addition (porosity, nanoparticles clusters) and microstructural homogenization, thus allowing to better exploit nanoparticles strengthening effect.

The research activities described in the present thesis have been oriented to the design and development of components and technological processes aimed at optimizing the performance of plasma sources in advanced in material treatments. Consumables components for high definition plasma arc cutting (PAC) torches were studied and developed. Experimental activities have in particular focussed on the modifications of the emissive insert with respect to the standard electrode configuration, which comprises a press fit hafnium insert in a copper body holder, to improve its durability. Based on a deep analysis of both the scientific and patent literature, different solutions were proposed and tested. First, the behaviour of Hf cathodes when operating at high current levels (250A) in oxidizing atmosphere has been experimentally investigated optimizing, with respect to expected service life, the initial shape of the electrode emissive surface. Moreover, the microstructural modifications of the Hf insert in PAC electrodes were experimentally investigated during first cycles, in order to understand those phenomena occurring on and under the Hf emissive surface and involved in the electrode erosion process. Thereafter, the research activity focussed on producing, characterizing and testing prototypes of composite inserts, combining powders of a high thermal conductibility (Cu, Ag) and high thermionic emissivity (Hf, Zr) materials The complexity of the thermal plasma torch environment required and integrated approach also involving physical modelling. Accordingly, a detailed line-by-line method was developed to compute the net emission coefficient of Ar plasmas at temperatures ranging from 3000 K to 25000 K and pressure ranging from 50 kPa to 200 kPa, for optically thin and partially autoabsorbed plasmas. Finally, prototypal electrodes were studied and realized for a newly developed plasma source, based on the plasma needle concept and devoted to the generation of atmospheric pressure non-thermal plasmas for biomedical applications. / L’attività di ricerca svoltasi durante il Dottorato è stata orientata alla progettazione e allo sviluppo di componenti e processi tecnologici innovativi atti ad ottimizzare le prestazioni di sorgenti plasma nel trattamento avanzato di materiali. Sono stati in particolare studiati e sviluppati consumabili di torce al plasma termico per il taglio di materiali metallici (PAC, plasma arc cutting), nell’ambito della cosiddetta alta definizione. L’attività di tipo sperimentale in ambito PAC si è concentrata sulla valutazione e realizzazione di modifiche dell’inserto emettitore, rispetto alla configurazione attuale di elettrodi standard, che prevede un inserto piatto in afnio (Hf) inserito per interferenza in un corpo elettrodo in rame (Cu). Le soluzioni proposte per l’attività di ricerca sono state basate su un’approfondita analisi bibliografica e brevettuale. Il comportamento di inserti in Hf operanti ad alte correnti (250A) in torce PAC è stato sperimentalmente analizzato, ottimizzando la forma iniziale della superficie emittente per incrementarne la vita utile. Sono inoltre state studiate le modificazioni microstrutturali dell’inserto emettitore al fine di comprendere i fenomeni coinvolti nel processo di erosione. Infine, l’attività di ricerca su elettrodi PAC si è concentrata sulla produzione, caratterizzazione e test di inserti compositi prototipali, realizzati unendo polveri ad alta conduttività termica (Cu, Ag) e polvere ad alta emissività termoionica (Hf, Zr). La complessità del sistema torcia ha inoltre richiesto un approccio integrato, che affiancasse attività di simulazione modellistico-computazionale con le valutazioni sperimentali, di natura chimico-fisica e microstrutturale. È stato in questo senso implementato un modello per il calcolo del coefficiente di emissione netto (NEC) del plasma di Argon in funzione di temperatura (1000-25000 K) e pressione (50-200 kPa), per plasmi otticamente sottili o parzialmente auto-assorbiti. Sono infine stati inoltre studiati e realizzati elettrodi prototipali per sorgenti di plasma non termico, finalizzate in particolare ad applicazioni biomedicali, nella configurazione detta plasma needle.

L’attività di ricerca della presente tesi di dottorato ha riguardato sistemi tribologici complessi di interesse industriale per i quali sono stati individuati, mediante failure analysis, i meccanismi di usura dominanti. Per ciascuno di essi sono state studiate soluzioni migliorative sulla base di prove tribologiche di laboratorio. Nella realizzazione di maglie per macchine movimentazione terra sono ampiamente utilizzati i tradizionali acciai da bonifica. La possibilità di utilizzare i nuovi microlegati a medio tenore di carbonio, consentirebbe una notevole semplificazione del ciclo produttivo e benefici in termini di costi. Una parte della tesi ha riguardato lo studio del comportamento tribologico di tali acciai. E’ stato anche affrontato lo studio tribologico di motori idraulici, con l’obiettivo di riuscire a migliorarne la resistenza ad usura e quindi la vita utile. Sono state eseguite prove a banco, per valutare i principali meccanismi di usura, e prove di laboratorio atte a riprodurre le reali condizioni di utilizzo, valutando tecniche di modificazione superficiale che fossero in grado di ridurre l’usura dei componenti. Sono state analizzate diverse tipologie di rivestimenti Thermal Spray in termini di modalità di deposizione (AFS-APS) e di leghe metalliche depositate (Ni,Mo,Cu/Al). Si sono infine caratterizzati contatti tribologici nel settore del packaging, dove l’utilizzo di acciai inox austenitici è in alcuni casi obbligatorio. L’acciaio inossidabile AISI 316L è ampiamente utilizzato in applicazioni in cui siano richieste elevate resistenze alla corrosione, tuttavia la bassa resistenza all’usura, ne limitano l’impiego in campo tribologico. In tale ambito, è stata analizzata una problematica tribologica relativa a macchine automatiche per il dosaggio di polveri farmaceutiche. Sono state studiate soluzioni alternative che hanno previsto sia la completa sostituzione dei materiali della coppia tribologica, sia l’individuazione di tecniche di modificazione superficiale innovative quali la cementazione a bassa temperatura anche seguita dalla deposizione di un rivestimento di carbonio amorfo idrogenato a-C:H / The research focused on complex tribological systems of industrial interest for which the dominant wear mechanisms have been identified, through failure analysis. Therefore alternative solutions were studied on the basis of tribological laboratory test. For the tracks of high power excavators are widely used conventional Q&T steels. The possibility of using the new microalloyed medium carbon steels, allow a considerable reduction of manufacturing costs. This section focused on the study of the tribological behavior of these steels. It was also addressed the tribological study of hydraulic motors, in order to improve the wear resistance and the durability of this motors. Bench tests were carried out to assess the main wear mechanisms, and tribological laboratory test were performed to reproduce the real conditions, evaluating surface modification techniques that were able to reduce the wear of the components. Various types of thermal spray coatings, in terms of deposition mode (AFS -APS ) and metal alloys used (Ni, Mo, Cu/Al), were analyzed. Tribological contacts were finally characterized in the packaging sector, where the use of austenitic stainless steel is required in many cases. The AISI 316L stainless steel is widely used in applications where high corrosion resistance are required, however, the low wear resistance, limit the use of this steel in the tribology field. In this context, a tribological issue has been analyzed related to automatic machines for dosing of pharmaceutical powders. Alternative solutions have been studied: the complete replacement of the materials of the tribological pair, and the identification of innovative surface modification techniques, such as low temperature carburizing followed by the deposition of hydrogenated amorphous carbon a-C:H coating(DLC)

Bimodal bulk nanocristalline (nc)/ultrafine (UF) aluminum was produced after cryomilling and spark plasma sintering consolidation process. The samples obtainedwere plastically deformed in uniaxial compression. We show that there is a significant fraction of plastic strain (11%) that can be recovered after unloading. High-energy synchrotron X-ray diffraction experiments revealed that, there is a correlation between plastic strain recovery and microstructural evolution detected during in-situ loading-unloading experiments. Using a deconvolution approach, the nanostructured volume fraction (grain size below 100 nm) and the UF counterpart (grain size above 100-150 nm)were separated in terms of lattice strain, microstrain, crystallite size and crystallographic texture. During loading-unloading cycles we observe a lattice strain splitting between nc and UF volume fractions, a complete recovery of the peak broadening and a recovery of texture. These intriguing phenomena were explained to be strictly correlated with the lattice strain splitting behavior which act as the driving force for dislocation recombination.

Copper is widely used in many applications demanding high thermal and electrical conductivity, unfortunately its low hardness and wear resistance limit its performance. Work hardening has been proposed as a successful strengthening mechanism for the production of harder copper material, keeping the intrinsic conductivities. In this PhD thesis initially mechanical milling (MM) has been considered as suited strengthening technique due to the severe strain hardening and microstructural refinement induced by severe plastic deformation during the process. Then an enhanced hardening has been obtained by dispersion of a second harder phase in the copper matrix by mechanical alloying (MA), leading to the production of metal matrix composites (MMC). In this PhD thesis strain hardened and dispersion hardened copper materials have been sintered by Spark Plasma Sintering (SPS). Firstly the MM behaviour of Cu as function of milling time has been studied, it consists in three stages: flaking, welding and fracturing process. Since stearic acid has been added as process control agent (PCA), its decomposition has been analysed to limit the residual porosity in sintered samples. Several focused attempts have been made and the best results have been obtained by using a fine particle size, decreasing the heating rate and applying the SPS pressure once the decomposition of PCA was completed. However the presence of copper oxide and microstructure defects induced by the severe strain hardening hinder the densification. The residual porosity is responsible of a decrease of hardness in sintered sample and consequently to a limited wear resistance, to a decrease of thermal conductivity and to a loss of ductility. For the production of MMC a ceramic reinforcement (0.5wt% of TiB2) has been selected. Increasing milling time the dispersion of the hard phase among the matrix becomes more homogeneous and refinement of TiB2 is highlighted. The evolution of particle size and morphology during MA is similar to MM; also the densification mechanism during SPS are the same consisting in powder rearrangement, local and bulk deformation. The final density generally decreases by increasing milling time, by the way an increasing hardness confirms that strain hardening and dispersion hardening abundantly compensate the negative effect of porosity. Has been proved that the hard particles successfully enhanced sliding and abrasion wear meanwhile the copper matrix guarantees high thermal conductivity, satisfying the requirements. Therefore considering the characteristics of the initial copper powder, promising results have been obtained for MMCs showing an increased hardness combined with a high wear resistance and a thermal conductivity comparable to atomized copper and much higher than the commercial Cu-Be alloy. On the other side mechanical milled samples exhibited some limits, but they allowed a deep understanding of the MM process of copper.

Hot work tool steels (HWTSs) are ferrous alloys for tooling application, particularly developed to meet high toughness and good hot hardness. Increasing hardness generally leads to a decrease in toughness, therefore metal matrix composite (MMC) coatings and functionally graded materials have been proposed as a good solution for improving wear resistance. In this PhD thesis powder metallurgy has been applied for the production of particle reinforced HWTSs. Mechanical milling (MM) and mechanical alloying (MA) have been considered as suited techniques for the production of powders showing higher sinterability and finer microstructure. Spark plasma sintering (SPS) has been used for the consolidation. As reinforcement a harder high speed steel (HSS) and different ceramic powders (TiB2, TiC and TiN) have been selected. The production of HWTS/HSS blends has highlighted the negative interaction on densification of the two components due to their different sintering kinetics. This interference can be minimised by selecting powders with smaller particles size. With this respect MM was proved to be a very useful method, which enhances sintering. Fully dense blends with good dispersion of the reinforcing particles can be sintered using small sized powders and setting the particle size ratio (PSR) smaller than 1. For the production of MMCs the formation of aggregates has been overcome by MA which promotes a uniform dispersion of hard particles into the parent steel. Among the reinforcement considered in this work, TiB2 is not suitable because it reacts with steel depleting carbon and producing TiC and brittle Fe2B. HWTS composites with 20%vol of TiC can be fully densified by SPS at 1100 °C for 30 minutes and 60 MPa uniaxial pressure. On the other hand TiN-reinforced MMC shows high resistance to densification and fully dense materials could not be produced.

Cemented carbides are composite materials formed by high amount of WC bonded by a soft phase, usually Co. They are used in many applications, such as drawing dies, cutting tools and hot rolls due to theirs remarkable properties of high hardness and wear resistance. Mechanical properties are strongly related to microstructure, namely the binder amount and the carbide grain size. Increasing the binder content and the carbide grain size, the hardness decreases ad the fracture toughness increases. In this PhD, the correlations between the mechanical properties of WC-Co and the microstructural characteristics, in parts taken from industrial production, were defined. After that, the influence of the residual microporosity on the mechanical properties was evaluated. Considering the production process, another important modification of the final microstructure of WC-Co occurs due to the liquid cobalt migration phenomenon. Based on this, also the liquid cobalt migration that occurs during sintering was investigated. At the end of the thesis, since a few data are available in literature, Thermal Fatigue and oxidation damage in WC-Co were studied. The main results of this PhD thesis show that the hardness and fracture toughness of WC-Co are defined by the mean binder free path and not by the contiguity since the high standard deviations, the microstructural fineness and also the high carbide grain size scatter. Differently, in case of mechanical strength, also the residual microporosity that depends on the dewaxing stage must be defined. Furthermore, the dewaxing stage acts on the liquid cobalt migration that affects the surface properties and also the final microstructure of the WC-Co part in industrial production. At the end, considering the damages that occur during high temperature applications, the TF and oxidation resistance of WC-Co results affected by the Co content: high cobalt content leads to a better condition of TF damage and s higher oxidation resistance.

Rapid Manufacturing (RM) technologies as Electron Beam Melting (EBM) and Selective Laser Melting (SLM) are able to produce fully dense parts from pre-alloyed powders in a layer-wise way. Moreover, they are able to create tailored surfaces with interconnected porosity. Applied to biomedical prostheses, such porosity can favour cell adhesion and osteointegration. The most important intrinsic characteristic of RM techniques is the large undercooling the parts undergo during the process. This undercooling results in peculiar, very fine, metastable microstructures, associated to peculiar mechanical behaviour. Metastable microstructures can change on post-melting operations, making the materials match the standard requirements and gain interesting properties.

The aim of the present PhD work is the study of the production of a nanostructured copper by Spark Plasma Sintering. The nanostructured powder was produced by cryomilling an atomized powder, using a ball-to-powder ratio of 30:1 for 8h; it has a mean grain size of 19±2 nm and shows quite a high thermal stability, as shown by a DSC investigation. The influence of temperature, particle size, pressure on the densification and sintering mechanisms as well as that of heating rate and holding time on the structural evolution has been investigated. Particle rearrangement, local deformation, bulk deformation and sintering are the SPS mechanisms occurring successively during the sintering process of the atomized copper. These mechanisms are enhanced by the peculiar heating mechanism in SPS, and the surface overheating above the melting temperature in the contact regions has been demonstrated. In the cryomilled powder, sintering occurs at much lower temperature than in the atomized powder, due to effect of the high density of structural defects on the mass transport phenomena responsible for neck growth. The increase in heating rate tends to promote a bimodal grain size distribution (both nanomentric and ultrafine grains) while an increase in holding time increases grain size slightly. A promising combination of strength and ductility was measured on tensile specimens produced under selected conditions, and a dimpled fracture morphology was observed.

The contact fatigue and surface damage of prealloyed (Fe-0.85Mo, Fe-1.5Mo) and diffusion bonded (Ni-free, low-Ni, high-Ni) powder metallurgy (PM) steels were investigated. Materials subjected to contact stress fail due to the nucleation of subsurface cracks (contact fatigue cracks), nucleation of brittle surface cracks, and surface plastic deformation. The occurrence of these contact damage mechanisms was predicted using theoretical models, which were developed by assuming that crack nucleation is preceded either by local plastic deformation (contact fatigue and surface plastic deformation) or local brittleness (brittle surface cracks ) of the metallic matrix. With reference to the mean yield strength of the matrix (mean approach) or the yield strength of soft constituents (local approach), the models predict the theoretical resistance of materials to the formation of damage mechanisms. The models were then verified using experimental evidence from lubricated rolling-sliding contact tests. In addition, the effect of compact density and microstructures of materials on the resistance to contact damage mechanisms was investigated. Density and microstructure were modified by varying green density, alloying elements, sintering temperature and time, and applying strengthening treatments: carburizing and shot peening on prealloyed (homogenous microstructure) and carburizing, sinterhardening and through hardening on diffusion bonded (heterogeneous microstructure) steels. The theoretical resistance to subsurface and surface crack nucleation in prealloyed materials was predicted using the mean approach since the microstructure is homogeneous. But the local approach is applied for diffusion bonded materials (Ni-free and low-Ni); exceptionally, the mean approach was applied for some homogeneous microstructure of Ni-free material sintered at a prolonged time. However, the models have a limitation in predicting the contact damage mechanisms in a high-Ni material. This issue may require further investigation to modify the model. Shot peening provides higher resistance to the nucleation of surface cracks. High compact density, high sintering temperature and time, and sinterhardening improve the resistance to contact damage mechanisms for Ni-free and low-Ni materials.