The prevalence of additive manufacturing (AM), or 3D printing, has grown in recent decades as a method of manufacturing complex geometries, prototype parts, or high tuned microstructures. One method of additive manufacturing is selective laser melting (SLM), a powder bed fusion technique which utilizes a laser to melt a metal alloy powder layer by layer to form the final geometry. Inconel 718 is a nickel-based superalloy commonly used in high temperature applications due to its superior mechanical and corrosion resistance properties. Owing to a difficult in machining, additive manufacturing has become particularly advantageous method for the production of Inconel 718. The SLM process is associated with a greater degree of process defects as compared to a traditional production method, in particular a poor surface condition, porosity and an anisotropic microstructure. These surface defects pose an issue in response to cyclic loading, as they serve as stress concentrators and can become crack initiation sites. As a result, post-processing treatments are typically required to improve the quality and mechanical properties. This master work focused on two primary aims. First, the characterization of SLM Inconel 718 processed with different layer thicknesses, build directions and post-processing surface treatments and second, the development of a prediction model for fatigue life based on process defects. Smaller layer thickness was found to result in lower surface roughness, which in turn resulting in fatigue performance as compared to larger thicknesses. Differences in surface quality and properties were identified on samples built at an incline. The plane opposite the build direction (the ‘downskin’ side) exhibited higher surface roughness and increased near surface hardness. Mechanical polishing of the surface led to a significant improvement in fatigue performance associated with dramatic reduction in surface roughness and near-surface voiding. Electropolishing as a post-processing treatment did not have a significant impact on surface quality or on fatigue performance. Surface defects were identified as the location of fatigue crack initiation. Removal of these defects by post-processing would therefore provide the greatest improvement in the fatigue performance of SLM Inconel 718. The Murakami-Endo method of fatigue life prediction showed promising results in predicting fatigue life of a sample population based on defects, with good agreement to experimental data.

The exposure of certain carbon steels to sour environments can result in severe hydrogen induced cracking (HIC) damage in the oil and gas industry. Current mitigation techniques in this field have low reliability or are not able to provide long-term protection against such damage. Recent advancements in thermal spray technology have resulted in a promising and cost-effective solution. Improvements in particle velocity and deposition efficiency have enabled coatings to achieve higher density and uniformity. High-velocity air fuel (HVAF) thermal sprayed NiCrMoW coatings are particularly interesting due to their outstanding corrosion resistance and mechanical properties. To ensure that equipment is sufficiently protected against the harsh environment of this industry, a thick coating is desired. However, as coating thickness increases, the performance of thermal sprayed coatings is frequently affected by residual stresses and unfavourable microstructural features. To identify this effect, three NiCrMoW coatings with thicknesses of 250, 375, and 500 extmu m were applied with HVAF thermal spray technology on S235JR carbon steel. Samples were analyzed in order to evaluate differences in terms of microstructure, mechanical behaviour, HIC resistance, and corrosion resistance. An AK07 HVAF instrument in a controlled setting at the IOT research centre of the University of Aachen was used to ensure consistency among the coatings during the spraying process. Experiments to evaluate HIC resistance and corrosion resistance involved prolonged immersion in a sour environment, cathodic charging, open circuit potential measurements, and potentiodynamic polarization tests. Microstructural variation was examined with the use of SEM-EDS and optical microscopy. Additionally, subsurface microhardness measurements of the coating and underlying substrate were used to evaluate hardness and give an indication of the presence of residual stresses. Findings indicate that the coatings exhibit excellent corrosion resistance. A small but noticeable decrease in resistance was however observed with increasing coating thickness. This decline can be attributed to two factors: an increase in the degree of oxidation and accumulation of residual stresses within the thicker coatings. Additionally, it is noteworthy that while the degree of oxidation and residual stresses increased with coating thickness, the porosity fraction decreased. Microstructural features in the coatings varied as a result of differences in thermal input, cooling passes and the influence of shot peening effects. Resistance to HIC of carbon steel in a sour environment was significantly improved by the application of the coatings in comparison with uncoated samples. This can be attributed to the excellent corrosion resistance, uniformity and absence of through-coating porosity in the coatings, the thickness did not have an influence. Furthermore, it was found that the galvanic interaction between the NiCrMoW coating and the S235JR carbon steel significantly accelerates the corrosion of the underlying substrate. Thicker coatings might be able to provide a greater physical defect-free barrier which can resist breaking, damage and erosion to prevent this galvanic effect.