3D Laser Shock Peening – A new method for the 3D control of residual stresses in Selective Laser Melting

This paper describes a hybrid additive manufacturing process – 3D Laser Shock Peening (3D LSP), based on the integration of Laser Shock Peening (LSP) with selective laser melting (SLM). The well-known tensile residual stresses (TRS) in the as – built (AB) state of SLM parts in the subsurface region have a detrimental effect on their fatigue life. LSP is a relatively expensive surface post treatment method, known to generate deep CRS into the subsurface of the part, and used for high end applications (e.g. aerospace, nuclear) where fatigue life is crucial. The novel proposed 3D LSP process takes advantage of the possibility to repeatedly interrupt the part manufacturing, with cycles of a few SLM layers. This approach leads to higher and deeper CRS in the subsurface of the produced part, with expected improved fatigue properties. In this paper, 316L stainless steel samples were 3D LSP processed using a decoupled approach, i.e. by moving back and forth the baseplate from an SLM machine to an LSP station. A clear and significant increase in the magnitude and depth of CRS was observed, for all investigated process parameters, when compared to the AB SLM parts, or those traditionally LSP (surface) treated.

3D laser shock peening - A new method for improving fatigue properties of selective laser melted parts

Nikola Kalentics, Christian Leinenbach, Roland Logé, Manuel Ortega Varela De Seijas

Although Selective Laser Melting (SLM) currently revolutionizes the way parts are being manufactured, certain process inherent limitations such as accumulation of residual stresses and increased porosity content can lead to a decrease in fatigue life of produced parts. 3D Laser Shock Peening is a new hybrid additive manufacturing process whereby a periodic Laser Shock Peening (LSP) treatment is added to the standard SLM process. LSP can be applied selectively in the 3D volume of metallic parts, and is shown to influence the bulk internal stress state and hardness, as well as the distribution of porosities in the material. Here, a decoupled 3D LSP process was used, i.e. LSP treatments were applied during the SLM process, by removing the sample from the SLM chamber whenever needed. We report fatigue lives of 316 L stainless steel multiplied by more than 15 times compared to standard SLM parts, and more than 57 times compared to conventional manufacturing, with a significant improvement over other state-of-the art methods. The effect of the 3D LSP treatment on SLM parts and the consequence on crack initiation and propagation is investigated by measuring the near surface residual stresses, imaging the fractured surface, measuring the porosity content and analyzing the microstructure and microhardness.

ELSEVIER2020

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

Nikola Kalentics, Roland Logé, Markus Strobl

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.

ELSEVIER2020

Geometrical and material characterisation of quenched and self-tempered steel reinforcement bars

Eugen Brühwiler, Alain Nussbaumer, Marina Rocha Pinto Portela Nunes

Quenched and self-tempered (QST) steel reinforcement bars (rebars) are manufactured by Thermex or Tempcore processes. Characterization studies of QST rebars are mostly limited to reveal the hardened outer layer and some improved mechanical properties compared to hot-rolled (HR) and cold-worked (CW) rebars. However, investigations on residual stresses and imperfections originating from the manufacturing process as well as stress concentrations arising from the ribbed profile are rarely found in the literature. Surface residual stress may be beneficial or detrimental to the fatigue performance of rebars although they have been studied only on the subsurface of QST rebars. Surface imperfections are zones of stress concentration from where fatigue cracks may initiate. Imperfections are usually identified in the fractured cross section resultant from fatigue tests of rebars. Stress concentrations can also arise from the rib geometry. Although the rib geometric parameters affect the stress concentration factor Kt on rebars, stress concentration analysis are restricted to two-dimensional (2D) finite-element models (FEMs). The study presented herein is part of a further detailed investigation on the fatigue behavior of HR-CW and QST rebars in the very high cycle domain. In the present work, characterization analyses of QST rebars include (1) experimental investigation of surface and subsurface residual stresses on QST rebars by cut compliance and X-ray diffraction techniques (residual stresses from both techniques are discussed); (2) identification of surface imperfections by scanning electron microscopy (SEM) analysis; and (3) three-dimensional (3D) finite-element analysis of stress concentrations on the ribbed profile. The influence of the rib geometry such as radius, width, height, and inclination as well as the rebar diameter on Kt values are analyzed. The critical zones are determined along the ribs. (C) 2016 American Society of Civil Engineers.

Asce-Amer Soc Civil Engineers2015

Geometrical and material characterisation of quenched and self-tempered steel reinforcement bars

Eugen Brühwiler, Alain Nussbaumer, Marina Rocha Pinto Portela Nunes

Asce-Amer Soc Civil Engineers2015