A parametric neutron Bragg edge imaging study of additively manufactured samples treated by laser shock peening

Matteo Busi,Takenao Shinohara,Manuel Morgano,Roland Logé,Christian Leinenbach,Markus Strobl,Nikola Kalentics,Seth Griffiths,Anton S Tremsin

doi:10.1038/s41598-021-94455-3

Abstract

Laser powder bed fusion is an additive manufacturing technique extensively used for the production of metallic components. Despite this process has reached a status at which parts are produced with mechanical properties comparable to those from conventional production, it is still prone to introduce detrimental tensile residual stresses towards the surfaces along the building direction, implying negative consequences on fatigue life and resistance to crack formations. Laser shock peening (LSP) is a promising method adopted to compensate tensile residual stresses and to introduce beneficial compressive residual stress on the treated surfaces. Using neutron Bragg edge imaging, we perform a parametric study of LSP applied to 316L steel samples produced by laser powder bed fusion additive manufacturing. We include in the study the novel 3D-LSP technique, where samples are LSP treated also during the building process, at intermediate build layers. The LSP energy and spot overlap were set to either 1.0 or 1.5 J and 40% or 80% respectively. The results support the use of 3D-LSP treatment with the higher LSP laser energy and overlap applied, which showed a relative increase of surface compressive residual stress (CRS) and CRS depth by 54% and 104% respectively, compared to the conventional LSP treatment.

Highlights

The as built condition (AB) and B samples were built with parallel laser scanning strategy whereas the laser shock peening (LSP) and 3D-LSP specimens were built with chess strategy, and they were all built with high speed and no support structures
The sample treated with buried LSP shows a similar behavior that differs from the as built mainly in a confined zone of CRS between approximately 500 and 900 μ m depth under the surface, which is introduced by the LSP treatment at the corresponding depth under the final surface
These results indicate and confirm that the heat induced by the Laser powder bed fusion (LPBF) processing of the successive layers, is insufficient to fully relax the CRS introduced by the LSP treatment in the “buried layer”

Summary

Introduction

Neutron diffraction and the X-ray diffraction (XRD) are widely used XRD is in general limited to surface analyses due to the limited penetration depth in many metals and neutron diffraction techniques are typically limited in spatial resolution. To overcome these limitations, neutron imaging methods based on diffraction c ontrast[14] have been introduced. We have applied Bragg edge imaging to assess the residual stress introduced in additively manufactured steel samples by post processing treatments such as laser shock peening (LSP)[24]. The influence of the printing and 3D-LSP treatment conditions on the residual stress in the samples was analyzed with a specific focus on the quantification of the magnitude and depth of the CRS introduced by the LSP treatments

Methods

Results

Conclusion