Additive manufacturing (AM), also known as 3D-printing, is a manufacturing method in which the parts are built by the addition of raw material layer by layer according to the geometry of a computer model. AM enables production of complex shaped components from a variety of different materials, including polymers, ceramics and metals, without the need for moulds and tooling or extensive machining. At the current state, AM is especially suitable for rapid prototyping as well as for smaller production series of customized consumer goods, spare parts and complex shaped, high-end components for different industrial sectors.
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 during 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. One of these challenges 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.