Laser Directed Energy Deposition of Inconel 625 Alloy for Repair & Feature Addition Applications: An Experimental and Numerical Investigation

Abstract

Engineering components after the prolonged hours of service in harsh environments often go through several defects such as wear, cracks and deformation. These defects alter the actual part geometry thereby affect its performance. In general, these parts are changed with new one during the overhauling process. However, replacing a part with new one is uneconomical specially if the part has high intrinsic value. For high value engineering parts, repairing the damages surfaces is more cost effective than replacing the entire part with new one. Welding has been the primary method for repair practices. However, this method has limitations, such as high heat affected zone and inability to repair complex shape cracks. With the advancement in metal Additive Manufacturing (AM), it is anticipated that this technology can be a game changer for repair industries. To this end, the present thesis is focused to explore the repair capabilities of Laser Directed Energy Deposition (LDED) process which is a class of metal AM process where material is deposited by feeding the stock material (usually metal powder) through a nozzle into the melt pool formed on a substrate by a laser source. The work presented in this thesis starts with the fundamental understanding of LDED process to its applications in the sector of repair and feature addition for Inconel 625 (IN625). First, a laser surface melting study is presented where the focus is to understand how laser material interaction occurs. Effect of the process parameters, such as laser power and scan speed on the melt pool geometry and microstructure were studied. In addition, a Finite Volume Method based numerical model was established to understand the effect of considering fluid dynamics on the predicted melt pool geometry, cooling rates and thermal gradients. For the validation of numerical model, the results of cooling rates and thermal gradients were compared with the microstructure. Next, a single-track study was conducted to identify the process parameter window for sound deposition and to understand effect of process parameters on the deposited track height and width. A finite element based numerical model was established to predict the track height and width. Subsequentially, the optimized process parameters were chosen and a total of six thin walls were built to mimic feature addition applications on a IN625 substrate. Effect of process parameters on the thin wall build geometry, surface roughness, imicrostructure and mechanical properties were investigated. A FE based numerical model was established to understand the variation in melt pool geometry, cooling rates and thermal gradients with the change in process parameters and over the layers. Further, a study was carried out to understand the repair capabilities of LDED process, where the samples were extracted from a wrought plate of IN625 and were subjected to a fatigue load to mimic a component in service for repairing. Further, deposition was carried out on the surfaces (i.e., Top, Top & bottom, One side and Both sides) of these fatigued tensile sample. The samples were also solution-treated at 1200°C for 90 mins. Microstructure and mechanical properties were evaluated and then compared between the different deposition strategies and sample heat-treatment conditions. Tensile properties were compared for all the three sample conditions viz., wrought alloy, as repaired and solution treated. Results indicate sound deposition with minimal porosity in all the four deposition strategies using the LDED process. IN625 being a choice of material for high temperature applications, it is important to understand the thermal stability of the parts repaired using LDED process. To this end, the repaired samples were also tested for high temperature oxidation. Using the LDED process, an IN625 block was fabricated. The LDED IN625 samples were also subjected to solution treatment. In order to compare the performance of both LDED IN625 and solution treated LDED IN625, test coupons were also extracted from the wrought plate. Oxidation study was carried out for as-built, solution treated and wrought alloy at 800° and 1000°C for up to 100 hours in air. In the conclusions of this thesis, LDED is found to be a promising technique to repair and add features on existing component with least porosity and high mechanical properties. However, the results of this study do indicate that the selection of an optimum process parameter can be useful to achieve consistent build quality during the thin wall deposition. Also, a suitable post-processing technique such as solution treatment is required for the achieving a homogenized microstructure, consistent mechanical properties and high thermal stability in the repair components.