Effect of annealing treatment on microstructure evolution and deformation behavior of 304 L stainless steel made by laser powder bed fusion

Highlights

• Microstructural evolution of LPBF 304 L SS at all length scales during annealing was comprehensively investigated.

• As-built samples contained the square lattice distortion networks composed of orthogonal strain ripples using HRTEM.

• High annealing temperature resulted in the unidirectional dislocation migration, which destroyed the as-built square lattice distortion networks.

• TKD results disclosed that the local misorientation ranges of cellular interior, cellular walls, and newly formed subgrain boundaries were <0.2°, 0.2°−0.5°, and 0.5°−2°, respectively.

• Deformation mechanisms and work hardening behaviors of as-built and annealed LPBF 304 L SS were revealed.

Abstract

This paper focuses on the microstructural evolution of 304 L austenitic stainless steel (SS) manufactured by laser powder bed fusion (LPBF) after stress-relieving annealing (650 °C) and solution annealing (1050 °C). Multiple advanced characterizations were adopted to disclose the microstructural characteristics and investigate the annealing-driven dislocation migration process. At 650 °C, the dislocation density of cellular walls decreased slightly, associated with a slight decrease of strength. At 1050 °C, the dislocations of cellular walls migrated to more energetically favorable regions, forming subgrain boundaries with higher dislocation density and resulting in a strength-ductility trade-off. The temperature of 1050 °C could slightly increase the recrystallization volume fraction and induce the coalescence of multi-oriented fine-grained tribes into single-oriented grains. The nano-scale characterization indicated that the as-built samples and annealed samples at 650 °C contained the square lattice distortion networks composed of orthogonal strain ripples. However, after annealing at 1050 °C, only unidirectional strain ripples in square lattice distortion networks were retained due to unidirectional dislocation migration. Direct experimental results were provided that the local misorientation ranges of cellular interior, cellular walls, and newly formed subgrain boundaries were <0.2°, 0.2°-0.5°, and 0.5°-2°, respectively. After tensile deformation, interacting deformation twins occurred in <111> and 〈101〉 oriented grains of as-built and annealed specimens at 650 °C, while the twins occurred in all oriented grains of annealed specimens at 1050 °C due to the disappearance of cellular substructure and the increase of tensile elongation. This work yields new insights into misorientation across the cellular walls, dislocation migration process during annealing, strengthening mechanisms, and work hardening behaviors, which can be used to design and optimize future annealing routines for LPBF materials.

Introduction

Laser power bed fusion (LPBF) is one of metal additive manufacturing (AM) processes and has attracted great attention in the engineering fields and research community because LPBF provides unparalleled capabilities to achieve sophisticated and near-net-shape components that cannot be manufactured by conventional fabrication methods (Liu et al., 2020b; Park et al., 2021; Wang et al., 2019a; Zanini et al., 2021). However, the microstructural characteristics of LPBF materials are remarkably different from conventional fabricated materials owing to the layer-by-layer printing (Seede et al., 2021). The high thermal gradient during solidification leads to the columnar grains in the as-built state, thus resulting in the strong anisotropy of microstructure and mechanical properties (Liu et al., 2019; Zhang et al., 2021b). The mechanical properties of LPBF metallic materials are affected by the microstructural features in terms of molten pool boundaries (Paul et al., 2021), low angle grain boundaries (LAGBs) (Pegues et al., 2020), nano-inclusions (Deng et al., 2020a; Li et al., 2019), phase compositions (Zhu et al., 2020), and residual stress (Pokharel et al., 2019). Additionally, the prevalence of defects such as keyholes, gas pores, and lack-of-fusion (LoF) defects can affect the ductility and toughness of AM materials (Kumar et al., 2021). Although the microstructure heterogeneity and intrinsic defects can accelerate the failure of materials, the mechanical properties of AM materials are still better than their traditional counterparts because of the regulation of typical solidification cellular substructures (Li et al., 2021, 2020d; Wang et al., 2018).

Austenitic stainless steel (SS) is commonly used in energy industries such as nuclear implants because of its excellent combination of corrosion resistance and mechanical properties (Cruz et al., 2020). The microstructure homogeneity and the mechanical properties of LPBF austenitic SS are improved based on optimization of process parameters and post-processing heat treatment. However, the optimization of process parameters can usually decrease the porosity but cannot effectively eliminate the microstructure anisotropy (Ghayoor et al., 2020; Liu et al., 2020a, 2019; Zhao et al., 2021). The heat treatment can homogenize microstructure to improve mechanical properties of LPBF austenitic SS. The heat treatment can also adjust the residual stress state induced by the high thermal gradient and high cooling rate (Bartlett and Li, 2019). Therefore, the effect of heat treatment on the microstructure and residual stress is essential to tailor the mechanical properties of LPBF austenitic steel.

Considering the highly heterogeneous microstructure in LPBF austenitic SS, extensive efforts have been made to investigate the microstructural evolution and thermal stability of LPBF austenitic SS at different annealing temperatures and different times. When the annealing temperature is lower than 600 °C, there was no visible microstructure change (Salman et al., 2019; Voisin et al., 2021). When the annealing temperature exceeds 800 °C, the chemical elements were redistributed and the cellular substructures vanished (Brown et al., 2019; Salman et al., 2019; Voisin et al., 2021; Yin et al., 2021). When the annealing temperature exceeds 1100 °C, the grains are coarsened and recrystallized (Brown et al., 2019; Kong et al., 2019; Voisin et al., 2021). Existing experimental results prove that increasing annealing temperature can result in a decline of strength and an increase of ductility (Kong et al., 2019; Salman et al., 2019; Voisin et al., 2021). In addition, Salman et al. (2019) claimed that cell size increases with increasing annealing temperature until the cellular substructure vanishes. Similarly, Yin et al. (2021) found that cellular substructures were coarsened at 700 °C for 100 h and stated that smaller cell sizes led to higher strength. On the contrary, Li et al. (2020c) reported that the strength of LPBF austenitic steel was determined by the dislocation density rather than cell size. Therefore, Cui et al. (2021) statistically demonstrated that the geometrically necessary dislocations (GNDs) density decreased with increasing the annealing temperature by using electron backscattered diffraction (EBSD) technology. Pokharel et al. (2019) found a reduction of dislocation density and no significant change of texture in annealed samples through the in-situ measurement of heat treatment process. They suggested the thermally stimulated diffusional climb-assisted recovery mechanism annealed the process-induced dislocations during heat treatment. Brown et al. (2019) further demonstrated the reduction in dislocation density associated with increasing the annealing temperature using in-situ neutron diffraction. Furthermore, they found that the temperature above 980 °C greatly lowered the ferrite fraction, but the elimination of ferrite only had a modest effect on the macroscopic strength. Particularly, it has been found that the temperature of 400 °C could increase the yield strength because of the additional precipitation of nano-sized silicates along the cellular walls (Chen et al., 2019a). Yan et al. (2018) claimed that the oxide inclusions are the main contributor associated with retarding grain growth and enhancing strength. They also concluded that the MnSiO₃ Rhodonite phase in as-built samples was converted to the equilibrium MnCr₂O₄ Spinel phase at 1200 °C. Deng et al. (2020b) reported that the thermal stability of cellular substructures of austenitic SS is closely related to the alloy elements Mo and Al, which are the main chemical compositions of inclusions in as-built samples (Chao et al., 2017). These results emphasize the crucial role of nano-inclusions in trapping dislocations. The redistribution of elements and the phase transformation during annealing may be the key reasons that affect the thermal stability and dislocation migration of useful cellular walls. With the aid of in situ high-energy X-ray powder diffraction measurement, Ferreri et al. (2020b) reported that the rate of martensitic transformation during deformation decreased steadily for the annealed samples at temperatures up to 1100 °C due to crystallographic recovery, while the rate increased for the annealed samples at higher temperature due to the static recrystallization. However, due to the complexity of microstructure and the difficulty of experimental testing, a deep understanding of the dislocation migration process of LPBF austenitic SS involved in annealing remains missing, which hinders the optimization of the annealing process that can obtain high-performance LPBF austenitic SS.

In this paper, the microstructure evolution and thermal stability of LPBF 304 L austenitic SS were comprehensively investigated by using the typical stress-relieving annealing temperature (650 °C) and solution annealing temperature (1050 °C). The microstructural characteristics of annealed and as-built samples, such as grain morphologies, crystallographic texture, recrystallization behavior, cellular substructures, dislocation distribution, phase compositions, and residual stress were characterized through scanning electron microscopy (SEM), X-ray diffraction (XRD), EBSD, transmission electron microscopy (TEM), elemental mappings, and X-ray residual stress analyzer. The high-resolution TEM (HRTEM) was used to investigate the dislocation cores, strain fields, and dislocation migration after annealing. The local misorientations (GND density) of cellular interior, cellular walls, and newly formed subgrain boundaries were statistically distinguished with the aid of high-resolution transmission Kikuchi diffraction (TKD/t-EBSD). Finally, the tensile tests of all sets of specimens were performed with the in-situ monitoring of acoustic emission (AE) technology, and the various strengthening mechanisms and work hardening behaviors were substantially discussed by analyzing annealed and deformed microstructures.