High-speed Laser Directed Energy Deposition of Stellite 6 Coatings on Steel Substrates

High-speed laser directed energy deposition (HS-L-DED) process, also known as the high-speed laser cladding (HSLC), and extreme high-speed laser application (EHLA), is gaining traction in coating applications and repair, driven by its high production efficiency and minimal disruption to substrate materials. Historical studies on single-track depositions, use of fibre optics with solid state lasers, and the development of dual-laser HS-L-DED system laid the groundwork for HS-L-DED. This leads to recent advancements in single-laser HS-L-DED system, showcasing deposition speed from 5 to 150 m/min. The combination of high process speed and thin material deposition results in refined microstructures, enhancing wear and corrosion resistance of targeted coating materials. 300M steel is a low-alloy ultra-high-strength steel, commonly used for manufacturing of aircraft landing gears. The component surface requires wear resisting coating which has been traditionally applied by electrolytic hard-chrome (EHC) plating. However, the increasing environmental concerns of electroplating processes are stimulating search for alternative coating and application process to reduce its environmental impact. Stellite® 6 is an established cobalt alloy, with a long history of successful use in hard-facing component surface through L-DED deposition. A further study is required to explore the key contributors to its deposit formation and microstructural evolution in HS-L-DED deposition of Stellite® 6 when deposition speed exceeds 10 m/min. The research emphasis in this thesis is on the effects of laser power, deposition speed, laser spot diameter, powder stream offset distance and track overlap on the deposition of Stellite® 6 powder on steel substrates. Process optimisation and the establishment of a process window are key objectives, considering both productivity and substrate integrity. The recommendations for depositing Stellite® 6 on 300M steel substrates through HS-L-DED include an optimal energy density level of 5 J/mm2, consisting of a 3 kW laser power, a 2.4 mm laser spot, a deposition speed of 15 m/min, with a linear powder mass delivery of 1.5 g/m, a 90% track overlap and a 0.8 mm powder stream offset distance from the substrate surface. The resulted thickness of a single-layer, multi-track deposit was measured at 495 ± 46 μm. The microstructural evolution of as-deposited Stellite® 6 exhibits a mixture of columnar and equiaxed grains alternating across the overlaid track boundaries. This is attributed to the incorporations of semi-molten powder particles, as well as an accelerated columnar-equiaxed transition during solidification of the deposit. The increasing proportion of equiaxed grains, as a result of increasing deposition speed, directly correlates with the increase in microhardness, and the wear performance of deposited layers. The wear performance of the optimised HS-L-DED Stellite® 6 coating was evaluated against the conventional L-DED Stellite® 6 coating and EHC plating. The results highlight the potential of HS-L-DED for depositing high-quality Stellite® 6 coatings on steel substrates with improved deposition efficiency. The interfacial bond strength between HS-L-DED deposited Stellite® 6 and 300M steel surpasses the minimum shear strength requirement for EHC coating, even in the presence of pre-existing defects. It signals a potential improvement in wear resistance with a good metallurgical bond to the substrate. In-situ monitoring with monochrome high-speed and welding camera was used to study the high-speed deposit formations in HS-L-DED. A novel observation, melt pool lag (MPL), is introduced. It is considered a result and representation of gradual heat accumulation within the deposition region. While the MPL was clearly observed in laser surface melting experiments, the irradiating pre-heated powder particles posed a challenge to directly observing the MPL phenomenon during HS-L-DED. The MPL development demonstrates a strong correlation with HS-L-DED deposited Stellite® 6 characteristics, which suggests a potential in using the MPL measurement to regulate HS-L-DED depositions. Based on MPL measurements, an innovative approach was examined to align the powder stream spot with the delayed melt pool formation, improving the powder catchment efficiency during HS-L-DED. In summary, this thesis contributes to the evolving field of HS-L-DED, offering insights into deposit formation, process optimization, and the potential for superior Stellite® 6 coatings with enhanced wear resistance and bond strength to steel substrates. The research findings deepen the understanding of microstructural evolution of Stellite® 6 alloy during rapid solidification. Further research is recommended to enhance the understanding of high-speed melt pool formation and its implications on deposit quality.