Additive Layer Manufacturing, ALM, of metal components has been steadily developed over the past decade. Further work is necessary to understand the metallurgical response of alloys to user defined processing parameters to establish the robustness of individual ALM systems and how the response affects microstructure and mechanical properties. This thesis addresses several areas to this end for the Nickel-Iron superalloy Inconel 718; Identification of key process variables for two types of commercially available additive systems (powder-bed EOS M270 and blown-powder Trumf DMDSOS). Development of a processing theme for Inconel 718 on the EOS M270 to build simple 3D shapes. The melting response to user defined variables. Analytical modelling of the melt pool geometry and the local solidification conditions (i.e. cooling rate, temperature gradient, and isotherm velocity). A microstructural investigation of the as-deposited grain structure. Simple mechanical testing. Statistical Design of Experiments (DOE) in the form of Central Composite Design (CCD) was used extensively to minimise experimental effort throughout the project. For the investigation of the EOS M270, processing maps were produced to identify a processing window where fully-dense, pore-free parts were obtained with the key variables being beam velocity and offset distance between adjacent melted lines. This was necessary as Inconel 718 had no defined processing conditions at the time of the investigation. Test samples were built and their microstructure investigated for a range of processing conditions. The grain structure of all samples was seen to consist of fine, dendritic, columnar grains orientated with a strong < 001 > fibre texture aligned perpendicular to the horizontal layers being melted. The DMD505 investigation considered single track thin walls deposited over a range of laser powers and beam velocities. The grain structures obtained varied across the process window but did not fit into classical 'equiaxed' or 'columnar' morphologies. 'Mixed' microstructures consisting of long grains which are continuous across melted layer boundaries and short discontinuous grains were observed at high laser powers and beam velocities. This corresponded to fluctuations in the top surface of the weld tracks. At lower powers, long continuous grains were observed along the total wall height. As velocity decreases there is a change to elongated grains which are contained within a single melted layer. Melt pool geometry and solidification conditions were modelled using analytical heat transfer equations which showed good agreement (±lO%) with experimental results for geometry and cooling rate for both processes. The shape of the melt pool is shown to influence heavily the resulting grain structure. Other materials and processing issues in ALM are considered such as surface roughness and thermally induced stress. These are discussed in relation to material response to user defined processing parameters and a material's thermal and physical properties which are related by underlying heat transfer equations. Material selection charts are used to compare the properties of different engineering alloys which in turn can be used as a basis for parameter selection during processing.

Porous pseudo-elastic NiTi alloy with a nearly three-dimensionally interconnected pore structure with structural characteristics that show promise for implant applications has been successfully fabricated by the metal injection moulding (MIM) process, followed by transient liquid phase sintering, using a mixture of Ni and Ti elemental powders. Three different powder volume fractions ranging from 65.5 to 69.5 vol% with a nominal Ni-Ti composition of SO.9at%Ni were mixed with a binder system, comprising a mainly water-soluble binder system known as polyethylene-glycol (PEG), in a new technique using a speed mixer, principally incorporating a dual asymmetric centrifuge (DAC). The powder-binder mixture was then analysed using a capillary rheometer at various temperatures and shear rates. It was found that the feedstock exhibited pseudo-plastic behaviour, which is favourable for the MIM process. A temperature range of 120°-130°C was considered as the optimum operating condition for the injection moulding process. The parts were moulded into cylindrical shapes, leached in warm water (60°C for 10 hours), thermally debound in argon and subsequently sintered in a vacuum furnace at different temperatures ranging from 950°C to 1250°C and with different holding times. The physical, thermal and mechanical behaviour of the as-sintered parts, in terms of pore morphologies, phase constituent analysis, phase transformation temperatures and load-unload compression test, were systematically investigated. The binder system used in MIM not only serves as a temporary vehicle to support the metal powder, particularly during mixing, injection and debinding, but also can act as a pore former, particularly during the water leaching and thermal debinding processes which finally facilitate the enlargement of pore channels during the subsequent sintering process. On sintering, the particles bond together first into a network, forming several inter-metallic phases such as NiTiz, NiTi, Ni3 Ti and Ni4 Ti3, with fine scale porosity in that network due to Kirkendall effects. The formation of a transient liquid phase (TLP) close to the eutectic composition leads to rapid bonding. This liquid covers the surface of Ni particles by capillary force and consequently enlarges the pore channels. As the alloy homogenises, this liquid phase disappears, and asymmetric diffusion of the nickel into the titanium particle network results in swelling and macroscopic expansion of the structure. Increasing the sintering temperature and holding time enhances the inter-diffusion between the elemental powders, leading to a major fraction of the desired B2 NiTi phase, whilst minimizing other secondary phases such as Niz Ti, Ni3 Ti and Ni4 Ti3, which are known to be brittle and unresponsive to pseudo-elasticity. For all the processing conditions employed, the variation of porosity, as well as the average pore size, was very small; 3S-4S% and 80-120 ~m, respectively. However, it seems that the use of greater powder loading (69.Svol%) and greater sintering temperature (> 10S0°C) was remarkable in terms of better isotropic dimensional changes after sintering, due to greater inter-particle friction, and low impurity content, due to the smaller amount of binder used and the greater amount of transient liquid, leading to better phase homogenisation. As a result, the ductility of the porous samples was enhanced remarkably (maximum strength and strain are> 400 MPa and> 30 %, respectively), and is slightly higher than with other PM routes such as SHS and HIP techniques using a mixture of elemental Ni and Ti powders. The porous samples also exhibited a quite promising pseudo-elasticity; the elastic deformation for all samples was around 10% strain and> 80% unloading recovery at room temperature for the load-unload compression up to 8% strain, and the recovery was nearly completed (<5% plastic strain) after the samples were heated above Aj temperature. The result is much higher than that of the conventional elastic deformation of ordinary alloys. Further, the average stiffness calculated from the stress-strain curves was around 2-3 GPa during loading and S-8 OPa during unloading, very close to that of cancellous bone(<3 GPa), which makes these alloys attractive as bone implants in biomedical applications.

Un procédé de traitement de surface avant projection plasma d’alumine, développé pour des substrats en acier C40E, a été étendu à des substrats en nickel. Afin de remplacer l’étape de sablage, une pré-oxydation sous CO2 a été réalisée, ayant pour objectif de créer une couche de protoxyde de nickel avec une épaisseur contrôlée. Cette couche d’oxyde permet d’augmenter significativement l’adhérence du dépôt d’alumine sur le substrat nickel. La formation de cette couche de protoxyde a été suivie par une étude de la cinétique d’oxydation. L'observation des interfaces alumine/nickel au MET a mis en évidence que des relations d'hétéroépitaxie entre le monoxyde de nickel et l’alumine sont à l’origine d'une adhésion chimique forte du dépôt. Cette forte adhérence (environ 105 MPa) a pu être quantifiée à l’aide d’un test de traction modifié (ASTM C633-13), dont le développement a été mis en place pour des échantillons en acier C40E pré-oxydés. / A surface treatment process, developed for C40E steel substrates before alumina plasma spraying, has been extended to nickel substrates. As an alternative to sand blasting, nickel substrates were pre-oxidized under CO2, to create a protoxide nickel layer of controlled thickness. This oxide layer significantly increases the alumina coating adherence on nickel substrates. A kinetic study of nickel oxidation under CO2has been realized, in order to better control the nickel preoxidation. TEM investigations of alumina/nickel interfaces showed a heteroepitaxial growth of alumina on nickel monoxide, which justify the strong alumina coatings adhesion. This high adherence (around 105 MPa) has been measured using a modified tensile test (ASTM C633-13), specifically set up for pre-oxidized steel C40E substrates.