Formation of phase change induced compressive residual stresses in additively manufactured M789 tool steel

Metallic parts produced by Laser Powder Bed Fusion (LPBF) typically suffer from tensile residual stresses at their surface [1], resulting from shrinkage during solidification and cooling. Such residual tensile stresses near the surface can negatively affect the mechanical performance of additively manufactured parts, especially when they are loaded in fatigue [2].

The current study, however, presents experimental evidence for the existence of compressive residual surface stresses in a martensitic maraging steel (M789) fabricated by LBPF. To investigate the origin of these compressive stresses, tower samples were produced with an increasing number of layers. It was found that towers with a low number of print layers exhibit compressive stresses in their top surface, whereas towers with a high number of layers exhibit tensile stresses. It is hypothesised that compressive stresses are introduced by the austenite-to-martensite phase transformation occurring during cooling. To elucidate this mechanism, a part-scale thermal model was implemented, allowing to calculate the part background temperature. This is combined with a meso-scale thermo-mechanical model that allows to simulate the residual stress, taking into account the volume expansion due to the martensite phase transformation. From the combined experimental and modelling results, it can be understood that, when the part background temperature is below the martensite start temperature Ms, the last printed layer immediately transforms to martensite, and its volume expansion is constrained by the material underneath, leading to compressive stresses. However, when the background temperature is above Ms, the entire part transforms when production is completed; and the volumetric expansion induced by the phase transformation is not constrained, leading to the expected tensile stresses.

The talk will focus on outlining the experimental work and elucidating the proposed mechanism for the development of residual stresses in the top surface of the manufactured towers. The modelling results will be used to support the experimental findings, without giving a detailed description of the model.

Abstract

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Dr.-Ing. Jeroen Tacq