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《Acta materialia》: a new discovery! Strengthening mechanism of additive manufacturing 316L austenitic stainless steel!

In 316L austenitic stainless steel produced by laser powder bed fusion (l-pbf), the cellular structure formed by rapid solidification plays an important role in achieving high strength and high ductility. However, the understanding of their intrinsic properties (such as crystal orientation, dislocation, precipitation, element segregation) and their effects on the strength and thermal stability of materials is still unclear. It is found that the cell wall follows a specific crystallographic direction. The high density of entangled dislocations in the cell wall has a higher tendency to dissociate and form a wider stacking fault, while the oxide precipitation is confined in the cell wall. These features act as barriers to moving dislocations during plastic deformation and contribute to high strength. Compared with traditional materials, l-pbf 316L SS exhibited higher thermal stability and excellent properties at high temperature.

It is well known that metals and alloys prepared by laser powder bed fusion (l-pbf) have high residual stress and non-equilibrium microstructure. Therefore, in order to eliminate the residual stress and adjust the structure performance relationship, heat treatment is needed. Taking l-pbf 316L stainless steel as an example, the finished material contains a large number of rapidly solidified subgrain structures, such as cellular / dendritic structures, which are composed of dislocations, elements, precipitates and low angle grain boundaries. These solidified structures are very important for mechanical properties and corrosion resistance. Some studies suggest that the strength of l-pbf 316 SS is proportional to the size of dislocation cells, while others have found that the strength is not related to the size of dislocation cells. Dislocation density is considered to be the main factor of strengthening. In addition, precipitates are sometimes considered to increase strength significantly. Therefore, it is urgent to clarify the basic strengthening mechanism of solidification subgrain structure in l-pbf 316L SS aluminum alloy and other l-pbf Al based and Co based alloys.
Due to its unique microstructure, the microstructure changes induced by heat treatment in l-pbf materials are different from those of forged or cold-rolled materials, and the thermal stability of l-pbf 316L SS is different from that of conventional samples. These studies further suggest that the key characteristics of solidification cellular structure in the influence of annealing behavior. The stability of cellular structure in the range of 600-950 ° C has been reported, and different mechanical properties have been obtained under similar annealing conditions. In addition, the underlying mechanism leading to the instability of dislocation cell walls at high temperatures is still poorly understood. For practical purposes, 316L SS is often subjected to high temperature applications, such as nuclear pressurized water reactors. Therefore, it is very important to study its thermal stability and its influence on mechanical properties.
In order to elucidate these strengthening mechanisms (especially the dislocation structures and precipitates in the dislocation walls), the team of Professor Thomas Voisin of Lawrence Livermore National Laboratory conducted several special TEM studies to capture the correlation of these key microstructure characteristics. In order to study the subgrain structure and thermal stability of grain boundaries and their corresponding effects on tensile properties, l-pbf 316L SS was annealed at 200 ℃ from 400 ° C to 1200 ° C for 1 hour. Based on the experimental observation, dislocation dynamics (DD) and CALPHAD modeling were further carried out to clarify the mechanism of precipitation strengthening and thermal stability. It is found that the cell wall follows a specific crystallographic direction. The high density of entangled dislocations in the cell wall has a higher tendency to dissociate and form a wider stacking fault, while the oxide precipitation is confined in the cell wall. These features act as barriers to moving dislocations during plastic deformation and contribute to high strength. The related research results were published in Acta materialia, a top metal journal, with the title of “new insights on cellular structures strengthening mechanisms and thermal stability of an authentic stainless steel fabricated by laser powder bed fusion”.
Paper links: https://www.sciencedirect.com/science/article/abs/pii/S1359645420308934
20201123095636 56683 - 《Acta materialia》: a new discovery! Strengthening mechanism of additive manufacturing 316L austenitic stainless steel!

After annealing at 600 ℃, the microstructure remains stable. The annealing between 600-1000 ° C activated the element diffusion, which led to the gradual disappearance of cell wall and the sharp decrease of yield strength. The low angle grain boundary is stable up to 1000 ° C. Annealing above 1100 ° C eliminates all the l-pbf microstructural footprints and exhibits a similar microstructure to that of the conventional one. Compared with traditional materials, l-pbf 316L SS exhibited higher thermal stability and excellent properties at high temperature.

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Fig.1 Schematic diagram of printing strategy. The island scan was rotated 90 ° and moved forward X and y, adding 200 meters per layer

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Fig.2 microstructure of l-pbf 316L material

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Fig.3 low angle grain boundaries (LAGBs) in l-pbf 316L material. a. EBSD · IPF shows high angle grain boundary (black line) and low angle grain boundary (red line). B. the orientation difference along the black arrow shown in A. C. four TEM images taken in the same region (upper left) and dark field (the other three) show LAGB (red arrow) running along the cell wall in different regions with higher resolution.

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Fig.4 sediment in cellular structure. The illustration shows the sediment size distribution calculated from the images in.

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Fig.5 dislocation structure of solidification cell.

20201123234707 27150 - 《Acta materialia》: a new discovery! Strengthening mechanism of additive manufacturing 316L austenitic stainless steel!20201123234947 35561 - 《Acta materialia》: a new discovery! Strengthening mechanism of additive manufacturing 316L austenitic stainless steel!20201123235010 71592 - 《Acta materialia》: a new discovery! Strengthening mechanism of additive manufacturing 316L austenitic stainless steel!
Fig.6 weak beam dark field of grain structure.

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Fig.7 Effect of annealing temperature on microstructure. a. EBSD, IPF and PF at different annealing temperatures. The construction direction is not in the plane. B grain size and LAGB fraction change with annealing temperature. The grain diameter and orientation difference angle of C and D are respectively distributed at different annealing temperatures. The sxrd diffraction at various annealing temperatures is used for phase recognition. The changes of F and G, lattice parameters and FWHM are functions of annealing temperature, respectively.

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Fig.8 cellular structure at different annealing temperatures. A-D stem / HAADF was taken at three times magnification in samples annealed at 600, 800, 1000 and 1200 ° C, respectively. E. stem / EDS line analysis was performed by the precipitates at different annealing temperatures.

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Fig.9 tensile properties after annealing at different temperatures and high temperatures for 1 hour. The engineering and true tensile strain / stress curves of a and B samples annealed at different temperatures for 1 hour were obtained. c. Ys0.2, UTS and UE are the functions of annealing temperature. D and e normalized work hardening and instantaneous work hardening exponents as a function of annealing temperature. f. The minimum and maximum instantaneous work hardening indices are taken as functions of the corresponding engineering and true yield and ultimate strength under different annealing conditions. G engineering yield and ultimate strength of as built materials tested at room temperature and 300 ° C compared to conventional materials tested at similar temperatures. h. The yield strength is a function of annealing temperature.

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Fig.10 the effect of precipitates is studied by 3D dislocation dynamic simulation.

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Fig.11 dislocation cell structure. a. The diagram represents a single cell up. B diagram, showing the projection results of a group of cells C and D with the same crystal orientation cut by {001} and {111} planes.

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Fig.12 CALPHAD simulation of trapped element diffusion during annealing. a. Stem / EDS element diagram of subgrain structure. b. Energy spectrum analysis a (HAADF) was performed along the white dotted line in. c. The variation of Cr and Mo contents between cell wall and cell wall as a function of annealing temperature was calculated by D.

Source: Network Arrangement – China Metal Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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20201123151944 64588 - 《Acta materialia》: a new discovery! Strengthening mechanism of additive manufacturing 316L austenitic stainless steel!
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《Acta materialia》: a new discovery! Strengthening mechanism of additive manufacturing 316L austenitic stainless steel!
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In 316L austenitic stainless steel produced by laser powder bed fusion (l-pbf), the cellular structure formed by rapid solidification plays an important role in achieving high strength and high ductility.
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