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.

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.

Evacuation systems in steelmaking plants contribute to the security of the operators around the furnace and help to gain the emission levels stated in the environmental regulations, furthermore play a major role in the mass and heat balance of the factory. The aim of the dissertation is to study both primary and secondary emission capture systems of an electric arc furnace steelmaking plant by means of 3D computational thermal fluid dynamics calculations. The overall performance of the post-combustion chamber, and consequently the primary line, is controlled by the size of the gap downstream the fourth hole of an electric are furnace. The impact of the opening coefficient (ratio between the gap area and the total area) on the post-combustion chamber performance has been investigated by means of a comprehensive 3D steady CFD simulation comprising radiative heat exchanges and detailed chemical reactions. It was found that there is not a unique value of the opening coefficient capable of optimizing all the relevant quantities of the evacuation process. A value of the opening coefficient in the range 0.40-0.52 appears advisable. The impact of the (mostly unknown) boundary conditions was also assessed and inefficiencies of the assumed post-combustion geometry have been highlighted. The secondary line's capturing efficiency during the charging phase was simulated with both transient and steady-state solvers with different turbulent models, namely the standard k-e and the Large Eddy Simulation models. The results revealed that steady-state simulations provide sufficient information for designing and optimizing the geometry of the secondary capture system. The simulations also pointed out several geometries, which cause significant pressure drop and, as a result, diminish capturing ability of the canopy hood and the additional evacuation system. The boundary conditions were imposed with the help of experimental measurements in the simulated steelmaking factory.

The anisotropic dimensional change on sintering of a prior cold compacted iron was investigated by dilatometry. Shrinkage is larger along the compaction direction than in the compaction plane. This phenomenon is very pronounced during the heating ramp in alpha phase and in particular below the Curie temperature, while in austenitic field is quite poor. The results of dilatometry tests were elaborated according to the shrinkage kinetics model of classical sintering theory, to calculate an effective diffusion coefficient along the two directions, which resulted higher for direction parallel to the compaction direction than perpendicular to it. In both directions, the effective diffusion coefficient is larger than that reported in the literature for pure iron, corresponding to an equilibrium density of structural defects. It also varies during the isothermal holding time. This discrepancy is attributed to the defectiveness introduced by cold compaction, that increases diffusivity through the activation of dislocation pipe mechanism, which is particularly intense below the Curie. This interpretation may also justify anisotropy of shrinkage due to the inhomogeneous deformation of interparticle contact regions that was measured with ISE method and EBSD analysis. The anisotropic shrinkage was also described through a modified micromechanical model proposed by the continuum mechanics approach, where the porous body is composed by aligned, elongated particles and elliptic pores, whose geometrical parameters were obtained through image analysis of SEM microhgraphs. The dislocation density calculated for different sintering temperatures was comparable to that measured experimentally. The effect of green density on anisotropy of shrinkage was investigated, too. Anisotropy tends to increase with green density, because of the larger plastic deformation introduced in the interparticle regions by the compaction pressure.

The influence of the powder material and the main processing parameters on geometrical features and dilution of the clads is investigated and discussed. Physical and analytical model that allow the explanation of the process and the prediction of the clad geometry and dilution is discussed. Using these models, useful tools for cladding operators and engineers are proposed. The energetic balance of the process is presented. Energetic redistribution in laser cladding process is analysed in detail, and quantification of process efficiency and energy losses is given. The influence of the processing parameters and the chemical/physical properties of the materials is considered throughout the various experiments performed. Some selected properties of the coating produced in different processing conditions are analysed. In specific, the variation of the chemical composition of the clad due to substrate dilution is considered, and its effect on the characteristics of the coatings is discussed.

The standard powder metallurgy process is composed by three main step, the powder production, the compaction and the sintering, and the possible secondary operation that allow to improve the mechanical properties and/or the dimensional and geometrical precision. The present work aims at investigating the influence of processing variables on the dimensional and geometrical precision of parts produced by Powder Metallurgy.