Abstract
Relationships between microstructures and hardening nature of laser powder bed fused (L-PBF) 316 L stainless steel have been studied. Using integrated experimental efforts and calculations, the evolution of microstructure entities such as dislocation density, organization, cellular structure and recrystallization behaviors were characterized as a function of heat treatments. Furthermore, the evolution of dislocation-type, namely the geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs), and their impacts on the hardness variation during annealing treatments for L-PBF alloy were experimentally investigated. The GND and SSD densities were statistically measured utilizing the Hough-based EBSD method and Taylor's hardening model. With the progress of recovery, the GNDs migrate from cellular walls to more energetically-favourable regions, resulting in the higher concentration of GNDs along subgrain boundaries. The SSD density decreases faster than the GND density during heat treatments, because the SSD density is more sensitive to the release of thermal distortions formed in printing. In all annealing conditions, the dislocations contribute to more than 50% of the hardness, and over 85.8% of the total dislocations are GNDs, while changes of other strengthening mechanism contributions are negligible, which draws a conclusion that the hardness of the present L-PBF alloy is governed predominantly by GNDs.
Highlights
Laser powder bed fusion is one of the most promising additive manufacturing (AM) technologies that has been successfully applied to produce structural components with complex geometries and outstanding performance through a layer-by-layer method [1,2]
When the temperature increases to 900 °C only vague traces are left, and they completely disappear after annealing above 1050 °C, demonstrating that atomic diffusion becomes more significant from 900 to 1200 °C
It means that these three microstructural configurations are associated with element segregation, the segregation on grain boundaries (GBs) may be higher in comparison to the cellular structures
Summary
Laser powder bed fusion is one of the most promising additive manufacturing (AM) technologies that has been successfully applied to produce structural components with complex geometries and outstanding performance through a layer-by-layer method [1,2]. Apart from cellular structures, the microstructural features include melt pool boundaries (MPBs), columnar grains, highly serrated grain boundaries (GBs) and nanoparticles [5,6], which in turn significantly affect the mechanical properties [4,5,6]. Porosity is inevitably formed in the as-built L-PBF alloy as a result of entrapped gases, lack of fusion and solidification shrinkage [8]. Even though these microstructural defects always lead to premature failure [9] under the mechanical loading, the mechanical properties of L-PBF alloys are still better than the conventionally processed ones [4]. Because of the huge potential of the LPBF technology, many studies related to the manufacturing of metal parts via L-PBF, such as nickel-based superalloy [10], high entropy alloy [11] and titanium alloy [12], stainless steel [13] have been carried out
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