Sanchez, Salomé
(2022)
Creep behaviour of Laser Powder Bed Fused alloy 718.
PhD thesis, University of Nottingham.
Abstract
Laser Powder Bed Fusion (LPBF) is characterised by high design flexibility and no tooling requirement. This makes LPBF attractive to many modern manufacturing sectors (e.g. aerospace, defence, energy and automotive). Nickel-based superalloys are crucial materials in modern engineering and underpin the performance of many advanced mechanical systems. Their physical properties (high mechanical integrity at high temperature) make them difficult to process via traditional techniques. Consequently, manufacture of nickel-based superalloys using LPBF has attracted significant attention.
However, components manufactured by LPBF are currently limited by their performance for use in critical applications. LPBF materials have microstructural defects, such as suboptimal grain size and morphology, and macroscale anomalies, such as lack of fusion. This results in LPBF materials performing below their wrought counterparts for various mechanical properties, such as creep, which has seldom been researched. Consequently, heat treatment of products post additive manufacture is now considered hugely important and the development of appropriate heat treatment is required to ensure material performance. Furthermore, the build time of LPBF can be slower than traditional manufacturing processes, especially for higher volumes of parts. Multi-laser machines, which have the potential to significantly reduce process time do exist, but there is currently little understanding of how interlaced scan strategies impact mechanical properties.
Therefore, to permit a wider application, a deeper understanding of the mechanical behaviour, particularly of creep properties, of LPBF nickel-based superalloys needs to be achieved. Hence, the aim of this work is to establish process-structure-property relationships for the creep properties of LPBF nickel-based superalloy, to benchmark it against wrought equivalents and to provide insights on how to improve the creep performance.
To do this, LPBF alloy 718 parts were fabricated using various scan strategies, build orientations and both single and multi-laser strategies, before being heat treated and creep tested. Results confirmed the necessity for heat treatment, which increased the creep life by a factor of 5. The build orientation, and its effect on the grain orientation as well as the stress state of the material were shown to be determining factors in the creep failure mechanisms. The meander scanning strategy resulted in a 58% increase in creep life compared to the stripe strategy, due to the detrimental effects of the numerous laser overlapping regions in the stripe strategy. There are numerous parameters, such as the fraction area of solidified layer, the fraction of powder underneath the layer and the interlayer rotation of the scan strategy that vary for each layer and means the layers themselves as well as the samples were heterogenous. Despite this, the creep life for any given sample was within 73h (i.e.17%) of its repeats, giving confidence in the results.
The results also showed that multi-laser scan strategies have no adverse effects on the creep properties of LPBF alloy 718 at different build orientations, demonstrating the potential of using multi-laser strategies for faster build rates without compromising the mechanical properties. Indeed, it is shown that for samples built vertically (i.e. where the build direction is parallel to the loading direction), multi-laser samples outperformed their single-laser counterparts and had a similar creep life and secondary creep rate to wrought alloy (1% difference). The presence of numerous large globular carbides in the wrought alloy 718 were also identified as the reason for the material’s curtailed creep life, compared to its LPBF counterpart, despite having a similar creep rate. Finally, for a given strategy, a 24% increase in creep life compared to wrought alloy 718 was observed. This specimen was explored under thermomechanical and thermal exposure conditions for the purpose of illustrating textural and microstructural evolution and inform a potential heat treatment to improve creep performance.
The results showed the instability of the LPBF microstructure in terms of grain size, precipitate density and crystallographic orientation during creep and thermal exposure, proof of the need for an appropriate heat treatment. The texture increased throughout creep testing for the wrought and LPBF alloy 718, reaching a maximum at the time of fracture. This contrasted with the thermal exposure only, where the instability of the LPBF alloy 718 microstructure was evident as the texture increased with time before decreasing and almost disappearing at the time of fracture. This also highlighted the different roles of the build and loading directions on the texture creation and evolution during creep. Finally, an ideal microstructure for improved creep performance was identified and recommendations on how to heat treat LPBF alloy 718 to reach this microstructure were given.
Overall, this thesis shows that LPBF components can become more performant than wrought and conventional equivalents by developing an appropriate heat treatment and provides an insight into process-structure-property relationships for the creep properties of LPBF alloy 718. The results are promising, despite future work being required and this work demonstrates the applicability of using LPBF for critical high temperature applications.
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