Powder bed fusion processing of Ni-base superalloys - Defect formation and its mitigation

• Powder bed fusion of Ni- and Ni-Fe-base superalloys is actively considered a promising manufacturing technology for critical components for the aerospace and industrial gas turbine industries. Such components often operate under harsh conditions, and hence, high demands are placed on both process and feedstock material to meet the strict safety and long-term reliability requirements. The aim of this thesis is to provide knowledge regarding the formation of damage-relevant defects in Ni- and Ni-Fe-base superalloys fabricated by powder bed fusion as well as how they can be mitigated. 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.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Materialteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering (hsv//eng)

Nyckelord

hot isostatic pressing

Ni-base superalloys

non-metallic inclusions

cracking

additive manufacturing

laser powder bed fusion

defect formation

Alloy 718

powder re-use

IN-738LC

Publikations- och innehållstyp

dok (ämneskategori)

vet (ämneskategori)