Powder bed fusion processing of Ni-base superalloys - Defect formation and its mitigation
Doctoral thesis, 2020
The first part of the thesis presents the connection between the surface oxidation of Alloy 718 powder for EBM, as a consequence of powder re-use, and the presence of oxide-related defects in the EBM fabricated material. The results indicate a clear connection between powder re-use and surface oxidation of the powder. Surface analysis of the progressively re-used powder by means of SEM, XPS and AES reveals significant growth of Al-rich oxide, which occurs via selective oxidation of Al due to the environment in the build chamber. Furthermore, the increased amount of oxide on the surface of the re-used powder results in an increased amount of oxide inclusions and lack of fusion defects in the EBM fabricated material. The morphology of the defects reveals that they originate from Al-rich oxide particulates on the surface of the re-used powder.
The second part of the thesis presents a study on the cracking of IN-738LC fabricated by means of LPBF. Implementation of custom designed powder grades with varying content of B and Zr indicates that both elements have a strong negative effect on the susceptibility to grain boundary microcracking of the alloy during LPBF. The XPS, AES and APT analyses show the enrichment of B and Zr at the cracked grain boundaries. Moreover, a significant portion of both elements are found to be connected to oxide. Hence, it is suggested that the increased microcracking susceptibility of IN-738LC is connected to the embrittlement of high-angle grain boundaries due to the formation of B- and Zr-containing oxide. In addition, post-LPBF hot isostatic pressing (HIP) is evaluated as a concept for microcrack healing. A HIP strategy that suppresses formation of macrocracks during the HIP treatment is developed by tailoring the temperature and pressure profiles during the heating stage. However, when applying the developed HIP strategy to the material grade with high levels of B and Zr, brittleness-inducing secondary phase particles at the grain boundaries appear after HIP at 1210°C, leading to a significant reduction of the impact toughness. Formation of the secondary phase is suppressed by lowering the HIP temperature to 1120°C. Results from microscopy and Charpy impact testing suggest that significant healing of the microcracks is accomplished when applying the developed HIP strategy.
hot isostatic pressing
laser powder bed fusion
Chalmers, Industrial and Materials Science, Materials and manufacture
Effect of Powder Recycling on Defect Formation in Electron Beam Melted Alloy 718
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science,; Vol. 51(2020)p. 2430-2443
Effect of Powder Recycling in Electron Beam Melting on the Surface Chemistry of Alloy 718 Powder
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science,; Vol. 50(2019)p. 4410-4422
Effect of Powder Recycling on the Fracture Behavior of Electron Beam Melted Alloy 718
Powder Metallurgy Progress,; Vol. 18(2018)p. 40-48
Powder bed fusion (PBF) is a sector within AM that utilizes a laser beam (laser powder bed fusion, LPBF) or an electron beam (electron beam melting, EBM) to build the components by successive melting of thin layers of metal powder in a powder bed. PBF has seen a huge growth the last decade as a consequence of an increasing interest for additive manufacturing of metal components for a variety of different applications, including Ni-base superalloy components for the aerospace and industrial gas turbine industries.
Even though PBF has many advantages compared to many conventional manufacturing processes, there are also some challenges that have to be met. Among these is the need to be able to reach a sufficient quality and repeatability of the fabricated material for its application in aerospace engine and industrial gas turbine components.
More specifically, there is a risk that an increased amount of critical material defects may occur when re-using the metal powder that is not consumed during the PBF process. As the non-consumed powder often constitutes a large portion of the powder bed, powder re-use has a great value from both an environmental as well as an economic perspective. The first part of this thesis describes the effect of re-using Alloy 718 powder in EBM. It is shown that the Alloy 718 powder may suffer from significant surface oxidation due to the high temperature in the EBM process chamber. Furthermore, when re-using the oxidized powder, there is an increased risk of forming brittle non-metallic defects, which may act as failure initiation points during mechanical loading. Hence, this part of the thesis emphasizes the necessity of controlling a good condition of the feedstock powder during powder re-use in EBM.
The second part of this thesis is focused on the formation and mitigation of defects in IN-738LC, fabricated by means of LPBF. IN-738LC is a common material used for high-temperature applications in industrial gas turbines and is traditionally used in its cast form. Due to its complex chemistry, the major challenge for its application in LPBF is the formation of cracks during LPBF as well as during subsequent heat treatments. First, it is shown that the alloying elements boron and zirconium have a negative effect on the cracking susceptibility of the alloy during LPBF. It is suggested that the increased cracking susceptibility is connected to formation of boron- and zirconium-containing oxide at the solidification grain boundaries, which is promoted by a higher level of oxygen in LPBF compared to the casting process. Furthermore, it is shown that hot isostatic pressing (HIP) can be used for extensive healing of the cracks formed in the LPBF process. It is also shown that the problem of crack formation during the HIP treatment can be mitigated by tailoring of the temperature and pressure profiles in the HIP process.
Areas of Advance
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4824
Chalmers University of Technology
Opponent: Prof. Dr. Eric A. Jagle, University of the Bundeswehr, Germany