Recrystallization in non-conventional microstructures of 316L stainless steel produced via laser powder-bed fusion: effect of particle coarsening kinetics

Abstract

Alloys processed by laser powder-bed fusion show distinct microstructures composed of dislocation cells, dispersed nanoparticles, and columnar grains. Upon post-build annealing, such alloys show sluggish recrystallization kinetics compared to the conventionally processed counterpart. To understand this behavior, AISI 316L stainless steel samples were constructed using the island scan strategy. Rhodonite-like (MnSiO3) nanoparticles and dislocation cells are found within weakly-textured grains in the as-built condition. Upon isothermal annealing at 1150 °C (up to 2880 min), the nucleation of recrystallization occurs along the center of the melt pool, where nuclei sites, high stored elastic energy, and local large misorientation are found in the as-built condition. The low value of the Avrami coefficient (n = 1.16) can be explained based on the non-random distribution of nucleation sites. The local interaction of the recrystallization front with nanoparticles speeds up their coarsening causing the decrease of the Zener-Smith pinning force. This allows the progression of recrystallization in LPBF alloys, although sluggish. These results allow us to understand the progress of recrystallization in LPBF 316L stainless steel, shedding light on the nucleation mechanisms and on the competition between driving and dragging pressures in non-conventional microstructures. They also help to understand the most relevant microstructural aspects applicable for tuning microstructures and designing new LPBF alloys. Graphical abstract

Texture and Grain Boundary Network Evolution of Laser Powder Bed Fusion Processed Pure Ni During Post-printing Annealing

“…It appeared SRX nucleation takes place inhomogeneously within the as-built microstructure (Figures 5(c) and (d)), preferentially on laser beam tracks due to higher local stored energy (dislocation density) imposed by the L-PBE process, as reported elsewhere. [6] The SRX grains largely have equiaxed morphology, free of dislocations and promote the formation of low energy CSL boundaries (i.e., P 3 and P 9) through multiple twinning (Figures 5(c) to (f)). Therefore, the partial recrystallization of the microstructure at 1000 °C resulted in a significant change in the misorientation angle distribution, revealing a relatively strong peak at ~60 deg and a small peak at ~39 deg (Figure 4 3 boundaries; this is known as twin multiplication.…”

“…The current AM process is known to create materials with a high dislocation density (i.e., ~10 14 m À2 ), [5,6] appearing as dislocation cells. The dislocation substructure originates from thermal distortions during printing, primarily initiated by constraints surrounding the melt pool, very rapid cooling and subsequent alternating heating/cooling cycles.…”

“…[1,2] The directional thermal gradients inherent in this process cause microstructural complexities such as anisotropic texture and the formation of dislocation networks within the grain interiors. [3][4][5][6] Due to good metallurgical, mechanical, and high oxidation resistance properties of Ni-based superalloys, L-PBF processing of these alloys is technologically viable in the energy, transportation, and semiconductor industries. The microstructure of L-PBF processed Ni-based alloys with the FCC (Face Centered Cubic) crystal structure typically displays elongated/coarse grains along the build direction, promoting specific crystallographic texture (e.g., 100 h i and/or 110 h i).…”

“…[10] However, the introduction of strong texture leads to an anisotropy in the material performance, including yield strength, corrosion, and electrochemical behaviour. [9,[11][12][13][14][15][16] This also influences the grain boundary network, for example, restricting the formation of coincidence site lattice (CSL) boundaries (such as annealing twins) in FCC materials, [6,17,18] which control the material toughness [19] and corrosion behaviour. [20] The grain morphology and size can be altered by changing the laser scanning strategy, which appears to improve the material performance.…”

“…More recently, post-built annealing treatment were performed on AM metals to improve the material performance through the modification in microstructure and texture. [6,27,28] However, they largely worked on recrystallization in highly alloyed materials (e.g., AISI 316L and IN718 alloys), where the presence of a second phase and composition inhomogeneity, to large extent, retards the grain boundary mobility (i.e., kinetics of recrystallization) and subsequently affects the extent of microstructure and texture evolution. [6,27,28] In addition, the prior studies provide limited information on the effect of post-built annealing treatment on the evolution of grain boundary network.…”