High resolution digital image correlation for microstructural strain analysis of a stainless steel repaired by Directed Energy Deposition

doi:10.1016/j.matlet.2020.127632

Materials Letters

Volume 270, 1 July 2020, 127632

https://doi.org/10.1016/j.matlet.2020.127632 Get rights and content

Highlights

•
Fatigue life of thin (re) manufactured specimen assessed by a self-heating testing method.
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Quantitative and qualitative analysis of the different microstructures.
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Investigation of the effect of the process parameters on the properties (static and cyclic).
•
Study of the orientation effect during self-heating tests.

Abstract

Deformations within a microstructural gradient zone of stainless steel repaired specimens are investigated. The repair, added material by Directed Energy Deposition over a hot rolled sheet substrate, was tested in monotonic tensile experiments. In situ tests, scanning electron microscope images combined with high resolution digital image correlation and electron backscatter diffraction maps, permitted to monitor the local strain distribution. The strain distribution is homogeneous in the substrate and exhibits a heterogeneous pattern in the printed part with localization correlating spatially with the position of interlayers. The vicinity of the interface has smaller strains and exhibits larger hardness.

Graphical abstract

Introduction

Additive manufacturing allows to repair components by the direct addition of material over the damaged zone. In particular, Directed Energy Deposition (DED) [1] is a leader in this field. In DED, the powder/wire feedstock is directly injected into a moving heat-source (laser or electron-beam) which melts the material. After deposition, the molten material undergoes complex melt-pool dynamics and a rapid cooling in the solid state. Addition of further layers subjects the material to heating–cooling thermal cycles at varying temperature rates. These phenomena are driven by process parameters and generate a hierarchical microstructure [2], [3] that is different from conventionally processed materials. It further implies that the original part and the repaired material exhibit very different microstructures. Therefore, under mechanical loading, these differences generate particular load distribution and strain localization, significantly unlike a part with a homogeneous microstructure.

This article proposes an analysis of the gradient of microstructural properties and the associated deformation mechanisms around the jointing interface for repair configurations. For the latter, deformation patterns were obtained through High Resolution Digital Image Correlation (HR-DIC) during an in situ tensile test inside a Scanning Electron Microscope (SEM). The local strain distribution was monitored by associating the HR-DIC data with electron backscatter diffraction (EBSD) maps.

Section snippets

Materials and methods

A single-track thickness wall with the dimension $50 \times 30 \times 0.7 {mm}^{3}$ was deposited by a DED machine on a thin 316L stainless steel (SS316L) hot rolled sheet substrate with the dimension $55 \times 40 \times 0.7 {mm}^{3}$ . A back and forth printing strategy was employed with the following DED process parameters: laser power of $225 W$ , deposition speed of 2000 mm/min, powder flow of 6.5 g/min, pause time between successive deposited layers of 1 s and a vertical spacing between successive layer of 0.12 mm. The SS316L powder

Microstructure

The microstructure of the RS is illustrated by polar figures of EBSD maps in Fig. 1. Elongated grains along the building direction can be observed for the cross section, i.e. the normal plan, and the printing plan. The magnifying box of the cross section exhibits epitaxial growth of several grains at the interface which insures a solid metallurgical bond [8]. Next, a quantitative analysis and the HR-DIC will be performed on the printing plan.

The grains are described by: (i) the equivalent grain

Conclusion

The microstructural gradient of properties and the associated deformation mechanisms at the repair interface were studied for a 316L stainless steel hot rolled sheet substrate repaired by DED. We observed an indisputable difference of the microstructures: small equiaxed grains for the substrate and larger and elongated grains for the printed part. Additionally, during in situ tensile test in an SEM combined with HR-DIC and EBSD maps, the observed strain was homogeneous in the substrate part

CRediT authorship contribution statement

Yanis Balit: Investigation, Data curation, Visualization, Writing - original draft. Camille Guévenoux: Data curation, Visualization. Alexandre Tanguy: Investigation, Resources. Manas V. Upadhyay: Investigation. Eric Charkaluk: Investigation, Methodology, Supervision, Funding acquisition. Andrei Constantinescu: Investigation, Methodology, Conceptualization, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors would like to thank the Direction Générale de l’Armement (DGA), for the funding of the experimental printing facility and to acknowledge the help of Simon Allais for the SEM measurements and test.

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