Prediction of Residual Stress Evolution for End-To-End Process Chain of Laser Powder Bed Fusion Process and Determination of Fatigue S-N Curves

Abstract

This paper presents prediction of residual stress evolution for end-to-end process chain of laser powder bed fusion (L-PBF) for an aero-casing component made of IN718. The end-to-end process chain includes the simulation of the L-PBF build process, removal of support structures, heat treatment cycle for stress relief, and application of surface-hardening processes such as shot peening and laser shock peening. The simulation of the end-to-end process chain was performed using validated process models for IN718. Validation of the L-PBF process model was carried out for the aero-casing component, where predicted and measured distortions were found to be in good agreement. The predicted residual stresses after each process of the chain were used to develop theoretical fatigue S-N curves using the endurance limit approach. This approach requires knowledge for the surface roughness, as well as the ultimate strength and relative density of the material (defect level). Understanding the evolution of stresses in manufacturing process chains as well as prediction of material properties for functional performance is essential to reduce iterations during the process and product development of L-PBF parts. The advantage of this approach is that S-N curves can be determined very rapidly at every location of the aero-casing or any other component without conducting any fatigue tests physically, which saves considerable cost and time. The S-N curves after the L-PBF build process, heat treatment, and surface hardening were determined and discussed. It was concluded that the proposed S-N curve predictive methodology can be employed in design workflows for estimation of high cycle fatigue in L-PBF process chains very early in the design stage. This would enable designers to mitigate fatigue problems by designing parts more proactively for L-PBF process constraints such as surface roughness, ultimate strength, residual stresses, and relative density or defect level in the given material.

Metal powder bed fusion process chains: an overview of modelling techniques

“…Similar approach for LPBF manufacturing process chain of an aero-casing was used and demonstrated in [134]. In both research works [109,136], validated process models have been applied to demonstrate the simulation of a manufacturing process chain, but the validation on the process chain has been highlighted as a future research.…”

Maturity Assessment of Laser Powder Bed Fusion Process Chain Modelling and Simulation

“…A simulation of the manufacturing process chain of an aero-casing component made of Inconel 718 was presented by Afazov et al [3]. They simulated the L-PBF using the inherent strain approach.…”

“…Yaghi et al [27] predicted the overall distortion and residual stress field of L-PBF followed by post-milling of an industrial impeller made of stainless steel 316 L. They validated the predicted distortions after L-PBF and postmilling. The L-PBF has been modelled using the inherent strain method, while the post-milling has been modelled by deactivating elements from the model and mapping surface-induced stresses from milling using the mapping techniques presented in [3]. They used the predicted distortion to pre-distort the original STL file in the negative direction and produced a final STL file to compensate for the accumulated distortion in the process chain.…”

“…Distortion may be compensated for at the design stage through modelling and simulation. Residual stresses can be mitigated or manipulated through post-process thermal stress relaxation or mechanical surface finish such as laser shock peening (LSP) or shot peening (SP) [3].…”

Investigation of the right first-time distortion compensation approach in laser powder bed fusion of a thin manifold structure made of Inconel 718

“…The inherent strain approach has shown accurate predictions of distortion of Inconel 718 components, e.g. an aero-casing [33] and impeller [34] where the inherent strains have been directly prescribed without the use of finite element thermal transient analysis.…”