TEM observations of variation of dislocation cell structures along the building direction in SLM-316L stainless steel

Ultra-fast cooling and cyclic thermal heating associated with the additive manufacturing by selective laser melting (SLM) often lead to the formation of multi-scale microstructures in manufactured metallic materials. It has been frequently reported that a dislocation cell structure develops within the grains in SLM-316L stainless steels (ss), which has important effects on their mechanical properties and thermal stability. However, the formation mechanism of the dislocation cell structure is still under debate. In this study, we used transmission electron microscopy (TEM) techniques to characterize the variation of dislocation structures along the building direction in a SLM-316L ss sample. It exhibits various dislocation structures in the very surface melt pools. The distribution of dislocations at shallower locations is relatively discrete. While the dislocation cell structures are completely formed at deeper locations, which are similar to the bulk interior of the sample. The thermal cycle is not the main contribution to the formation of dislocation cell structures, but plays the role in enhancing the annihilation of dislocations inside the cells and sharpening the cell boundaries.


Introduction
Dislocation generation and multiplication are usually associated with straining [1].However, a high density of dislocations is prevalent in additively manufactured (AM) virgin metallic materials without plastic deformation.This is due to the complex thermal history caused by the unique melting and solidification behavior of the additive manufacturing process.In common laser additive manufacturing processes, metal powders are scanned and melted with high-energy laser beams, enabling ultra-high cooling rates and solidification temperature gradients during printing.In addition, the build-up matrix is repeatedly subjected to thermal cycle, which could alter the microstructures [2][3].Under such conditions, the microstructures in AM metals often appear different morphologies at multi-scales especially with chemical segregation and high density of dislocations [4].
Dislocations in many AM metals exist in the form of cellular substructures [5][6][7][8].Interestingly, this dislocation substructure has been shown to increase yield strength and provide stable work-hardening properties in some AM alloys.For example, the yield strength of SLM-316L ss is significantly higher (2-3 times) than that of 316L ss produced using conventional techniques [4].In order to better control the microstructures of AM metals to obtain improved mechanical properties, it is necessary to understand the formation mechanism of dislocation cell structure in AM metals.
Currently, the proposed formation mechanisms of dislocation cell structures in AM metals are still controversial.An earlier study suggested that the formation of unique precipitation-dislocation array was promoted by reprecipitation or rearrangement of particles during printing [9].After that, since the cell structures were found to have the characteristics of dendritic growth, it had been generally believed that the subsequent solute segregation was the direct cause of its formation [10].Liu et al. believed that the basic reason is the inherent misorientation between adjacent dendrites during solidification [5].However, new theories proposed in recent years emphasize the role of thermal cycle and suggest that the stress induced by repeated compressive-tensile deformation during printing is the main contribution [11,12].
Thermal cycle is always inevitable in the AM process.But there is no comprehensive study addressing the effect of the extent of thermal cycle on the evolution of cell structures.If the thermal cycle is considered to be the primary factor in the development of cell structures in AM materials, then the cell structures at various depths along the building direction ought to differ apparently.Based on this consideration, we investigate the variation in the development of cell structures along the building direction of an SLM-316L ss sample.By comparing the dislocation characteristics at various depths from the surface, we demonstrated the correlation between the development of dislocation cell structures and thermal cycle.

Experimental Procedure
The 316L ss spherical powder manufactured by gas-atomization was provided from SLM Solutions Group AG (Lübeck, Germany).The particle sizes ranged from 10 to 45 μm.Table 1 shows the chemical composition of the 316L ss precursor powders.The SLM samples in this study were printed as cuboids of 45×90×18 mm 3 (x, y, z) by SLM-280 machine from SLM Solutions.In order to avoid large influence of processing parameters on microstructure, a set of commonly used parameters was selected, including laser power of 200 W, scaning speed of 1000 mm/s, laser spot diameter of 80 μm, hatching space of 120 μm and layer thickness of 30 μm.The building direction (BD) was along with the Z-axis from the base plate to top-surface, while the scanning direction (SD) followed the direction of the laser beam travel.The scanning strategy adopted the continuous scanning method, and the two successive layers were rotated by 67°.Since the SD is always in the xoy plane during printing, the microstructures observed from the two planes, yoz, parallel to BD and, xoy, perpendicular to BD are different.Samples for characterization were therefore sectioned parallel to the xoy and yoz planes, respectively.Scanning electron microscopy (SEM) samples were ground to 3000 grit SiC papers and mechanically polished with 0.5 μm diamond spray polishing compounds.In order to better reveal the boundaries of the melt pools, the samples were electrolytically etched using a 10% oxalic acid solution for 60 s at about 10 V at room temperature.JEOL JSM-7800F was used SEM observations.TEM characterizations were carried out with a JEOL JEM-2100 electron microscope at 200kV.The TEM thin films were first mechanically ground to about 40 μm and then twin-jet electropolished in a solution of 5% perchloric acid and 95% alcohol at 45 V and -20 °C.The local crystallographic orientation of adjacent regions was measured using a semi-automatic Kikuchi analysis method installed in the JEOL JEM-2100 electron microscope [13].

Results and discussion
Figure 1 shows the SEM images covering an area from the upper surface toward the bulk interior of the sample.The side view is shown in it where the melt pool boundaries are clearly visible.The melt pools seen at the very surface layer are freshly formed during the last printing step.These melt pools have average width and depth of 160 μm and 145 μm, respectively, which are more than twice of the correspond values of 70 μm and 65 μm observed at the interior part of the sample.As the powder layer thickness is only 30 μm, these observations of large pool depths indicate that each new melting process not only melts the metal powder layer, but also re-melts more than one layer of last solidified pools.The comparison of dislocation structures at various locations are shown in figure 2. Figures 2a-c were all taken on the [100] zone axis.Figure 2a shows the TEM images at about 40 μm depth from the upper surface.At this location, there is no obvious dislocation cell structure in the most of the regions.Some dislocation entanglements can be observed, and the distribution of dislocations are relatively discrete.Interestingly, complete dislocation cell structure was observed in the most of the regions at about 100 μm depth, as shown in figure 2b.The dislocation density is significantly higher than that at 40 μm depth.A large number of dislocation tangles can be clearly observed at the cell boundaries.From figure 1, the location at about 100 μm depth is located in the very surface melt pools, which has not undergone the effect of thermal cycle.The result indicated that thermal cycle is not the main contribution to the formation of the dislocation cell structures.
At the interior part, the dislocation cell structures were observed as well, as shown in figure 2c.Comparing the distribution of dislocation in figures 2b and c, the only difference between the 100 μm depth and the interior part is whether there are many dislocations inside the cell.Essentially, it is whether they experience the effects of repeated thermal cycle.The original cell in the very surface melt pool with more internal dislocations gradually evolves into a clean cell with few internal dislocations after thermal cycle.In conclusion, the role of thermal cycle is to promote the movement of dislocations form the center to the cell wall and sharpen the cell boundaries.Besides, the misorientation between the cell boundaries were also investigated.Figure 2d shows the statistics results of the misorientation between the cell boundaries.There are clear interfaces between dislocation cells at about 100 μm depth and the interior part, while there is no regular cell structure and complete interface at about 40 μm depth.So, only a few sets of measurements at about 40 μm depth were marked in figure 2a.According to the statistics, the average misorientation angle at about 100 μm depth and the interior part are 0.7° and 0.6°, respective, which are relatively similar.
In this paper, a brief set of observations were made on the characteristics of dislocation structures at various locations.The relationship between the formation of dislocation cell structure and internal stress, solute segregation, etc. will be discussed in future.In addition, the dislocation cells can maintain stability under the effect of thermal cycle, indicating its good thermal stability.The relevant results will also be analyzed in detail in subsequent studies.

Conclusion
In this study, the distribution of dislocations in an SLM-316L bulk sample were observed at various locations using TEM.The effect of thermal cycle on the evolution of the dislocation structures was further demonstrated.The conclusions are the following: 1. Dislocations exhibit different distribution characteristics at various locations in the very surface melt pools.At lower part, the dislocations are relatively discrete.While at deeper part, the dislocation cell structures are completely formed.
2. Thermal cycle is not to dominate the formation of dislocation cell structures, but plays the role in enhancing the annihilation of dislocations inside the cells and sharpening the cell boundaries.

Figure 1 .
Figure 1.SEM images showing the morphology change of melt pools.

Figure 2 .
Figure 2. Bright-field images of the dislocation structures taken at (a) 40 μm depth, (b) 100 μm depth and (c) interior part of the sample using TEM.(d) Statistics of misorientation between the dislocation cell boundaries at 100 μm depth and the interior.

Table 1 .
Chemical Composition of the SLM-316L ss in this study.