Skip to content

Characterizing Additively Manufactured 316L Stainless Steel with the Velocity Pro EBSD Camera

Introduction

The Velocity™ Pro is the latest addition to the Velocity series of Electron Backscatter Diffraction (EBSD) detectors. It can collect EBSD data at rates up to 2,000 indexed points per second while requiring less than a 10 nA beam current. This versatile detector can characterize the microstructure of materials with high indexing success rates and orientation precision performance at these analytical conditions. The Velocity Pro was used with the APEX™ Software for EBSD to characterize additively manufactured (AM) 316L stainless steel samples in this application note.

Results and Discussion

AM materials are of significant research interest from both manufacturing and materials properties perspectives [1]. In this work, two different AM samples were analyzed using the Velocity Pro with an acquisition speed of ≈ 2,000 indexed points per second, operating at 20 kV and ≈10 nA incident beam current. The AM samples were 316L stainless steel alloys fabricated using laser powder-bed-fusion. The two samples had the same general physical shape. Still, the build direction was varied between the two, with the surface analyzed with EBSD aligned with the AM build direction for one sample (termed the normal direction). The AM build direction is aligned with the horizontal X-axis of the image (termed the transverse build direction) for the other. EBSD was used to identify the differences in the observed microstructures relative to the different build directions across a range of length scales.

Initially, the Montage feature of the APEX Software was used to characterize the entire sample surface to give an overview of the microstructures. Montage Large Area Mapping uses stage control to collect an array of EBSD maps at different positions across the sample surface and then stitches the data together for subsequent analysis. This functionality allows for large area characterization of samples and can be collected efficiently with the high collection rates available with the Velocity cameras. Figure 1 shows combined image quality (IQ) and inverse pole figure (IPF) orientation maps from both the normal and transverse build direction samples. The IPF map colors each pixel according to the crystallographic orientation aligned with the normal direction of the sample analysis surface, while the IQ is a greyscale shading where the brightness increases with increasing EBSD pattern quality. The differences in orientation and grain structure between the two build directions are easily visible. The significant green coloring in the normal build direction corresponds to a <110> crystal direction, aligning with the build direction. Additional analysis of the orientation development can be done using pole figures.

Montage Large Area IQ + IPF orientation maps of the normal (top) and transverse (bottom) build directions.
Figure 1. Montage Large Area IQ + IPF orientation maps of the normal (top) and transverse (bottom) build directions.

Pole figures depict the distribution of selected crystal orientations relative to the sample reference frame. Figure 2 shows (001), (110), and (111) pole figures for both build directions. The intensity of the pole figures is determined using a harmonic series expansion approach with the intensities displayed in multiples of times random. The A1 and A2 directions aligned with the vertical and horizontal directions shown in the EBSD maps, while the center of the pole figures (A3 direction) corresponds to the sample normal direction.

Pole figures for the a) normal and b) transverse build directions.
Figure 2. Pole figures for the a) normal and b) transverse build directions.

For the normal build direction, the center peak in the (110) pole figure again corresponds with the alignment of the normal direction with the build direction. The alignment of the peaks in the (001) pole figure with the A1 and A2 sample directions correspond to crystallographic alignment with the deposition laser’s x and y scan direction. For the transverse build direction, the alignment of the (110) intensity with the A2 sample direction shows a 90° change in the build direction.

The microstructural differences between the two build directions can be seen in the IQ + IPF maps in Figure 3. For both directions, the microstructure is complicated. Samples fabricated with laser powder-bed-fusion can exhibit non-equilibrium microstructures, and differences in grain morphology show that grain shape varies relative to the build direction. The patchwork appearance of the normal build direction is a result of the 90° rotation of the laser rastering after each AM deposition layer. The transverse direction map shows that the grain structure evolves across the build layers, as the material is repeatedly melted and rapidly solidified during the deposition process. These structures were also analyzed with 3D EBSD to better understand how these structures develop [2].

IQ + IPF orientations maps from the normal (left) and transverse (right) build directions.
Figure 3. IQ + IPF orientations maps from the normal (left) and transverse (right) build directions.

The alloys fabricated in this manner have high residual stresses that can be investigated with EBSD and the Velocity Pro. Figure 4 shows a PRIAS™ (center ROI) map from both build directions. PRIAS is an innovative imaging approach that uses regions of interest (ROIs) defined within the EBSD detector image as virtual electron detectors. With these images, an ROI was selected within the center of the EBSD detector. As the electron beam is rastered across the sample surface, the intensity within this ROI is recorded and then used to create the PRIAS image. With the center ROI, the primary contrast observed is orientation (channeling) contrast. These images show significant intensity variation within the primary grains, corresponding to a local deformation structure. Grain boundary overlays can be added to these images, where different misorientations and special grain boundary types can be colored for differentiation. Other PRIAS ROIs are useful to show phase/atomic number and topographic contrasts.

PRIAS (center ROI) maps from the normal (left), and transverse (right) build directions.
Figure 4. PRIAS (center ROI) maps from the normal (left), and transverse (right) build directions.

The misorientations within the microstructure can be visualized in a variety of ways [3]. Figure 5 shows Local Orientation Spread (LOS) maps for both build directions. For each pixel in a LOS map, the misorientation for each pixel is calculated relative to a kernel of adjacent pixels, with a third nearest-neighbor kernel is used for these images, and the calculated LOS values are colored according to the scale shown. The grain structure and internal subgrain structure can be visualized with this approach. Velocity detectors are useful to achieve misorientation precisions less than 0.1°. This microstructure influences the material structure-property relationships, so the ability to characterize this residual deformation is important. Geometrically necessary dislocations can also be measured with improved sensitivity, achieved using cross-correlation-based high-angular resolution EBSD analysis.

Third nearest-neighbor LOS maps from the normal (left) and transverse (right) build directions.
Figure 5. Third nearest-neighbor LOS maps from the normal (left) and transverse (right) build directions.

Conclusion

These results show how the Velocity Pro is useful to rapidly and accurately characterize the microstructures of additively manufactured materials. Information regarding the orientation, grain boundary structure, grain morphology, and local deformation can be measured and displayed to provide an understanding of how the AM processing variables influence the final microstructure and resultant material properties.

Acknowledgments

EDAX would like to thank Thomas Voisin at Lawrence Livermore National Laboratory for providing the AM samples.

References

1. T. Voisin et al., New insights on cellular structures strengthening mechanisms and thermal stability of an austenitic stainless steel fabricated by laser powder-bed-fusion. Acta Materialia 203 (2021) 116476.

2. D. J. Rowenhorst et al., Characterization of Microstructure in Additively Manufactured 316L using Automated Serial Sectioning. Current Opinion in Solid State and Materials Science 24 (2020) 100819

3. S.I. Wright et al., A Review of Strain Analysis Using Electron Backscatter Diffraction. Microsc. Microanal. 17 (2011) 316-329.