Investigation of induction-assisted laser metal deposition of crack susceptible coatings

Abstract

Protecting components against severe wear and corrosion in high temperature environments is an ongoing challenge that is often resolved by applying an advanced functional coating. Environments like steam turbines present ever-evolving demands with industrial push to utilise higher operating temperatures that in turn increase power generation efficiencies. Tribaloy T800 is a superior coating composition that presents challenges in processability due to its low toughness and crack susceptible nature. Weld overlay/ cladding processes based on fusion technology such as Laser Metal Deposition (LMD) (AKA Laser Cladding) offer significant benefits for the application of coating materials including a high strength metallurgical bond and a dense, defect free coating. LMD generates more rapid melting and solidification of the alloy compared to arc cladding, which produces a fine microstructure, but can lead to solidification cracking due to induced thermal stresses during the process and a lack of toughness in the coating material. A small number of papers and published research projects have looked into LMD coatings of Tribaloy T800 with the ultimate aim to prove the feasibility of protecting components in high-temperature (600-1000°C) gas turbines, jet engines, shafts and pumps. Many of these studies conclude the findings of process optimisation, material characteristics and integrity, based on small-scale deposits. Small area deposits can present a lack of representation compared to full-scale coatings, including thermal control/accuracy, residual stresses, and even effects on dilution. In all circumstances, preheating was found to reduce or eliminate cracking by minimising the thermal shock and cooling rate of the alloy upon solidification. In this study, part of the objectives was to review and ensure the relevance of the experiments and findings for transferability to full-scale components. Using Design of Experiments (DoE), experiments were constructed that evaluated the relationship between dilution, hardness and cracking response of the LMD deposited T800 against critical process variables including particularly induction heating temperature. One substrate alloy was investigated, 316L stainless steel, (although there was an intention to transfer developments to In718 nickel superalloy at a later stage). Through this process, it was determined that cracks form during solidification of the meltpool. Another objective was to evaluate two methods of improving coatings speeds for the application of T800, 1) by implementing equipment adaptions to increase melt pool size and material feed rate and 2) to make initial assessment of a novel LMD technique known as Extreme High-speed Laser Application (EHLA). Cracking was only eliminated in the large-scale undiluted coatings of T800 when using high levels of substrate heating (800-950°C) prior and during the coating operation by induction-assisted LMD. The resultant material behaviour of the deposited T800 showed promising results in terms of material integrity, and tribological and protective properties. Improvements in coating speeds were made when using laser defocus methods and increased deposition rate from 300 grams/hour to 600- 1000grams/hour, though increased cracking was observed. Trials using EHLA demonstrated unique process characteristics of high-speed thin coatings with promising cracking behaviour at lower preheat temperatures than required for LMD (200-500°C), ultra-low dilution and good material properties.