Effect of substrate yield strength and grain size on the residual stress of direct energy deposition additive manufacturing measured by neutron diffraction

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Highlights

  • Residual stress distribution inside parts built by direct energy deposition(DED) was successfully measured by the neutron diffraction.

  • It was found that the characteristic of substrates can affect the residual stress inside DED parts.

  • Substrates with low yield strength can reduce the residual stress as thermal shrinkage of deposited metal can be more compensated by plastic deformation of the substrates.

  • Substrates with larger grain sizes can reduce the residual stress as the grain growth inside the deposited metal is promoted, providing an additional stress-reducing effect.

Abstract

Among the several metal additive manufacturing technologies, direct energy deposition (DED) is an innovative way to fabricate near-net-shaped parts and is also applicable to repairing damaged parts due to the simplicity of the machine. However, materials processed with this process have complex residual stress distribution due to the complex thermal history. Excessive residual stress inside the parts can induce shape distortion and cracks that degrade the performance of the parts. In this study, residual stress generated during the DED process was measured with the neutron diffraction method, and the result was compared with the FEM simulation result. Furthermore, to study the effect of the substrate characteristics on the residual stresses developed during DED, the DED samples were produced on two different substrates having different grain sizes and yield strengths. The neutron diffraction and the FEM simulation results fit well and show that the use of a softer substrate would reduce the residual stresses in the DED parts. Moreover, the substrate characteristics do not affect the tensile properties of the DED part.

Introduction

Metal additive manufacturing (MAM) is a technology that creates three-dimensionally designed parts by adding metallic material feedstock [[1], [2], [3], [4], [5], [6], [7], [8]]. Among them, direct energy deposition (DED) is a method that uses metal powder as the feedstock and simultaneously melts the deposited metal powder by exposing a focused laser source [9,10]. Melting, deposition, and solidification cycles are repeated layer by layer until the final part is built. Various advantages of the DED process have been studied for industrial purposes [11]. Among them, the simplicity of DED machines compared to other MAM processes allows direct application of the DED process to existing parts, making them a strong candidate for parts repairing and feature addition processes [12].

However, repeated and rapid temperature variation during the DED process causes severe thermal stress inside the parts [13]. Mercelis et al. [14] proposed mechanisms of thermal stress generation during the MAM process. The underlying material restricts the expansion of the heated top layer, inducing compressive strains. The molten top layer shrinks with a solidification during the cooling state, inhibited by the underlying material, introducing tensile stress in the top layer. The thermal stress exceeding the yield strength causes permanent deformation of the material, and the elastic components remain, inducing deflection during the detaching part from the base plate. This internal elastic stress, called residual stress, can distort the shape or create undesirable cracks inside the parts, and it is a very important issue for the reliability of the process [15]. In addition, it is well known that tensile residual stress distribution on the surface region results in fatigue life deterioration [16]. But measuring the residual stress inside MAM processed parts is quite complicated and remains a challenging area.

X-ray diffraction, hole drilling, and contour method for measuring residual stress have been widely used in the past. X-ray can't penetrate deeper into the metallic material, and its analysis is limited only to the sub-surface. Hole drilling and contour methods are destructive methods, so such ways are not free from the deviation in results originating from destructive measurement procedures [17]. Meanwhile, neutron diffraction is a novel method for measuring internal stress, and it can overcome the disadvantages of the stress measurement methods previously described [18]. Neutron diffraction is a non-destructive analysis method, and the neutron can penetrate deeply into the material owing to its zero electric charges. X-rays interact with the electrons, whereas neutrons only interact with the atomic core [19]. Incident neutron beam penetrates the object, and diffraction occurs due to the repeating lattice structure of the material, which act as silt. Since the interplanar distance of a crystal lattice can be calculated by measuring the diffraction angle, the strain of lattice structure can be known by comparing the change in the interplanar distance. Various attempts exist to use neutron diffraction to measure complex residual stress distributions inside parts manufactured by MAM technology. Woo et al. [20] studied the residual stress in the DEDed part in different scan strategies. In each scan strategy, three orthogonal stress components were obtained by neutron diffraction, and it was confirmed that the 90° rotation or island strategy could reduce the residual stress. Em et al. [21] observed the residual stress distribution according to the location of the wall structure fabricated by DED and showed the possibility that sidewall fracture could occur depending on the ductility of the deposited material.

Meanwhile, various methods are suggested to reduce the thermal deformation of AM processed parts. Stress relaxation heat treatment is the most common way of the post process. It is quite effective for removing residual stress inside fabricated parts, but this method can't prevent deformation during the process as it is a post-processing step. Then, the base plate preheating is suggested to reduce the thermal gradient during the process. Buchbinder et al. [22] demonstrated reduced distortion of the base plate with preheating during laser powder-bed fusion processing of aluminum. Baek et al. [23] and Li et al. [24] also report the similar effect of preheating in M4 high-speed steel parts and Ti–Al alloy, respectively.

However, the above analysis was focused on the deposited part only, neglecting the effect of the base plate. The initial state of the base plate affects the residual stress of the DED process. Also, from the point of view of repair using DED, it is not always possible to preheat because there may be various sizes and shapes of base plates. In-depth consideration of the effect of the base plate's initial state is required, especially for the part repairing process with DED in which existing parts become the base material.

In this paper, residual stress generated during the DED process is measured with neutron diffraction. The effect of the substrate yield strength and grain size on the residual stress generation is discussed. Two different substrates were used; (1) as-receive (non-heat treated) from the material supplier and (2) heat-treated. The residual stress evolution is observed experimentally and compared with FEM simulation. The microstructural analysis is also conducted to explain the experimental results.

Section snippets

Heat treatment of the 316L stainless steel substrate

316L stainless steel plate of 10 mm thickness was cut to 30 × 30 × 10 mm3 and used as substrates. The substrates were heat-treated to make a difference in grain size and mechanical strength. Substrate without heat treatment is denoted as NH (non-heat treated), and substrate with heat treatment is denoted as HT (heat-treated) for brevity. Heat treatment was conducted at 1050 °C for 30 min, then quenched in water which is commonly accepted in the industry [25].

Direct energy deposition of 316L stainless steel

15 × 15 × 15 mm3 cubic samples were

HT and NH substrates microstructures and properties

The EBSD-IPF maps of the NH and HT substrates are shown in Fig. 5a and b, respectively. From the IPF maps, the difference in grain size could be seen. The average grain size of the substrate is ∼26 μm for NH substrate and ∼33 μm for HT substrate. Grain size distribution is compared in Fig. 5c, and the HT substrate had a larger grain size. In the texture analysis, both substrates showed no preferred orientation (Figs. 5a–1 and 5b-1). It can be considered that the heat treatment of the substrate

Conclusions

In this study, the residual stresses of DED processed 316L stainless steel parts were measured using neutron diffraction. Simplified FEM simulations and microstructural analysis were performed to analyze the mechanisms of residual stress generation during the DED processing. Further, the effect of the substrate yield strength on the residual stresses of the DED parts was studied. The FEM and neutron diffraction results confirmed that the yield strength and grain size of the substrate affect the

CRediT authorship contribution statement

Sang Guk Jeong: Writing – original draft, Conceptualization, Investigation. Soung Yeoul Ahn: Writing – original draft, Software. Eun Seong Kim: Resources, Validation. Gangaraju Manogna Karthik: Writing – review & editing. Youl Baik: Data curation, Formal analysis. Daehee Seong: Resources, Investigation. You Sub Kim: Resources. Wanchuck Woo: Supervision, Resources. Hyoung Seop Kim: Writing – review & editing, Supervision, 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.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government, the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (2017M2A2A6A05017653, 2021R1A2C3006662, 20201510100030, and 2022R1A5A1030054). Dr. GMK is supported by the Brain Pool Program through the NRF of Korea, funded by the MSIT (2019H1D3A1A01102866).

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