Investigation of the effect of Laser Shock Peening in Additively Manufactured samples through Bragg Edge Neutron Imaging

Abstract

Additive manufacturing is a promising and rapidly rising technology in metal processing. However, besides a number of key advantages the constitution of a part through a complex thermo-mechanical process implies also some severe issues with the potential of impacting the quality of products. In laser powder bed fusion (LPBF), the most applied metal additive manufacturing process, the repetitive heating and cooling cycles induce severe strains in the built material, which can have a number of adverse consequences such as deformation, cracking and decreased fatigue life that might lead to severe failure even already during processing. It has been reported recently that the application of laser shock peening (LSP) can counteract efficiently the named issues of LPBF through the introduction of beneficial compressive residual stresses in the surface regions mostly affected by tensile stresses from the manufacturing process. Here we demonstrate how lattice strains implied by LPBF and LSP can efficiently be characterized through diffraction contrast neutron imaging. Despite the spatial resolution need with regards to the significant gradients of the stress distribution and the specific microstructure, which prevent the application of more conventional methods, Bragg edge imaging succeeds to provide essential two-dimensionally spatial resolved strain maps in full field single exposure measurements.

Introduction

Laser powder bed fusion is an additive manufacturing process in which a component is built through layer-wise addition of material [1]. It is a powder bed technique, where a laser selectively melts the powder in a scanning procedure covering the cross section of the component in consecutive layers. After constituting a layer through this procedure, the base is lowered and a new powder layer is deposited and the process repeats. This way the material goes through complex local thermo-mechanical cycles and the solidification of a new layer on an already existing solid layer implies the build-up of detrimental tensile residual stresses due to shrinkage during cooling of the top layers [2,3]. Despite competitive material properties achieved nowadays in many cases through process optimization [[4], [5], [6], [7], [8], [9], [10]], these complex stress states can cause distortions, cracking and even delamination and complete process failures during the build process [2,3,11].

A variety of in-situ and post-processing techniques have been developed and applied to circumvent the issues. Post-processing cannot prevent process failure and most techniques have severe limitations and drawbacks. For example, pre-heating and laser re-melting can reduce tensile residual stresses and improve geometrical accuracy, but they are limited in introducing compressive stresses and improving fatigue life, hardness, microstructure and crack density [12]. Similar is true for post process heat and hot isostatic pressing (HIP) treatments, which, while improving density through crack closure, can even negatively affect the microstructure and hence properties such as yield strength through promoting significant grain growth [[13], [14], [15]].

Recently, a novel and very promising approach has been introduced. Laser shock peening (LSP) has previously been applied successfully to conventionally produced components to introduce beneficial compressive residual stresses in the surface region and thus increase fatigue life. LSP is a well-known surface treatment. It is used for the purpose of introducing plastic deformation and compressive residual stresses (CRS) into the subsurface region of the treated material. A high energy laser with a pulse duration in the nanosecond range is directed at the surface of the sample. The high energy laser pulse ablates a shallow layer of the surface thus creating a high pressure shockwave directed towards the bulk of the sample. This shockwave plastically deforms the material thus introducing CRS in the subsurface region [16]. Now it could be shown, that LSP applied to LPBF parts has the ability to convert the tensile residual stresses produced by LPBF into beneficial compressive stresses in the processed surface region [17]. In addition to that, it could be proven that LSP can be applied in a hybrid LPBF process referred to as 3D-LSP [18], which allows to apply LSP at any chosen layer of the build process, where it does not only enable stress fields but local microstructure to be tailored to specific requirements [19,20]. The implied compressive stresses also support crack healing, geometrical integrity and increase in fatigue life [21,22]