Process Induced Preheating in Laser Powder Bed Fusion Monitored by Thermography and Its Influence on the Microstructure of 316L Stainless Steel Parts
Abstract
:1. Introduction
2. Materials and Methods
2.1. Material and Specimen Manufacturing by L-PBF
2.2. In-Situ Thermographic Monitoring and Temperature Analysis
2.3. Analysis of Microstructure Using Electron Back Scatter Diffraction (EBSD)
2.4. Analysis of Cellular Substructures by Scanning Electron Microscopy (SEM)
2.5. Analysis of Chemical Composition
3. Results
3.1. Surface Temperatures
3.2. Grain Size Analysis
3.3. Cellular Substructure
3.4. Chemical Composition
4. Discussion
4.1. Surface Temperatures
4.2. Grain Size Analysis
4.3. Cellular Substructure
4.4. Chemical Composition
4.5. Subsumption of the Results with Regard to Hardness
5. Conclusions
- Preheating temperature: An increase of VED can rise the preheating temperature as the heat input into the material is increased. The rise of preheating temperature by a decrease of ILT is much more significant. This is related to the reduced time for heat dissipation. Temperature measurements have revealed a change in preheating temperature by more than a factor of 2 in the upper part of the specimens for short ILT (18 s) compared to longer ILT (65 s and 116 s). Intrinsic preheating temperatures of up to about 600 °C were revealed. In turn, this resulted in heterogeneity of the microstructure and differences in material properties within the same specimen, as specified below.
- Grain sizes: A significant increase of grain sizes and sub-grain sizes was identified in sections of specimens with increased preheating temperature. Differences in grain size of more than a factor of 2.5 were found within the same specimen, which was attributed to the variations in build height and parameter combination.
- Spacing of cellular substructure: The measurement of cell spacing is handicapped by significant measurement uncertainty due to the high degree of local changes within very small areas, i.e., within the size of individual melt pools. Despite this scatter, a trend to increasing cell sizes was observed and was related to differences in solidification rate and thermal gradients induced by differences in scanning velocity and preheating temperature. The average cell size within this investigation was between 0.52 µm and 0.76 µm, depending on the parameter combination.
- Hardness: Eventually, the examined and discussed differences in grain sizes and cell sizes were related to differences in hardness examined in a previous study [5]. A general trend of decreasing hardness (from 221 HV1 to 176 HV1) with increasing microstructural feature size was revealed.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Specification | C | Si | Mn | P | S | Cr | Mo | Ni | N | Fe |
---|---|---|---|---|---|---|---|---|---|---|
Min. | - | - | - | - | - | 16.5 | 2.0 | 10.0 | - | bal. |
Max. | 0.03 | 1.0 | 2.0 | 0.045 | 0.03 | 18.5 | 2.5 | 13.0 | 0.1 | bal. |
Powder | 0.017 | 0.6 | 0.92 | 0.012 | 0.004 | 17.7 | 2.35 | 12.6 | 0.1 | bal. |
Processing Parameters | Level | |
---|---|---|
Layer thickness | 0.05 mm | |
Laser power | 275 W | |
Hatch distance | 0.12 mm | |
Platform preheating temperature | 100 °C | |
Inter layer time | Short: 18 s | |
Intermediate: 65 s | ||
Long: 116 s | ||
Volumetric energy density | Low: 49.12 J∙mm−3 | vs = 933 mm∙s−1 (75% of basis VED) |
Basis: 65.48 J∙mm−3 | vs = 700 mm∙s−1 | |
High: 81.85 J∙mm−3 | vs = 560 mm∙s−1 (125% of basis VED) |
Level of Inter Layer Time | Level of Volumetric Energy Density | Method of Analysis | ||||
---|---|---|---|---|---|---|
In-Situ Thermographic Measurment | EBSD Grain Size Measurement | Cell Structure Analysis with SEM | Chemical Analysis | Analysis of Oxygen Content | ||
Short ILT | High VED | x | x | x | x | x |
Basis VED | x | x | x | x | - | |
Low VED | x | x | x | - | - | |
Intermediate ILT | High VED | x | x | - | - | - |
Basis VED | x | x | x | x | x | |
Low VED | x | x | - | - | - | |
Long ILT | High VED | x | x | - | - | - |
Basis VED | x | x | x | - | - | |
Low VED | x | x | - | x | - |
Measurement Technique | Measuring Device | Chemical Element |
---|---|---|
Combustion/IR-detection | Elementrac CS-i (Eltra GmbH, Haan, Germany) | C |
S | ||
Carrier gas hot extraction | G8 Galileo (Bruker Corporation, Billerica, MA, USA) | N |
O | ||
X-ray fluorescence spectrometry | NITON XL3t (Thermo Fisher Scientific Inc., Waltham, MA, USA) | Mn |
Cr | ||
Mo | ||
Ni | ||
Inductively coupled plasma optical emission spectrometry | Spectro Arcos (SPECTRO Analytical Instruments GmbH, Kleve, Germany) | Si |
P |
ILT | VED | sub-Grain Size in µm² Lower Part | Sub-Grain Size in µm² Upper Part | ||
---|---|---|---|---|---|
Mean Value | Standard Deviation | Mean Value | Standard Deviation | ||
Short | High | 462.4 | 3.6 | 1386.4 | 5.1 |
Basis | 454.6 | 4.0 | 992.9 | 5.3 | |
Low | 291.5 | 3.2 | 457.7 | 4.5 | |
Intermediate | High | 456.8 | 4.2 | 511.4 | 4.4 |
Basis | 389.4 | 3.7 | 466.5 | 4.0 | |
Low | 262.5 | 3.1 | 308.8 | 3.3 | |
Long | High | 435.2 | 4.0 | 484.5 | 4.2 |
Basis | 347.6 | 3.6 | 532.3 | 4.2 | |
Low | 289.4 | 3.2 | 403.4 | 3.8 |
ILT | VED | Grain Size in µm² Lower Part | Grain Size in µm² Upper Part | ||
---|---|---|---|---|---|
Mean Value | Standard Deviation | Mean Value | Standard Deviation | ||
Short | High | 689.9 | 4.1 | 1830.6 | 5.9 |
Basis | 592.6 | 4.3 | 1222.5 | 5.8 | |
Low | 370.9 | 3.5 | 628.6 | 5.0 | |
Intermediate | High | 674.2 | 4.6 | 709.9 | 4.8 |
Basis | 520.0 | 4.0 | 596.2 | 4.3 | |
Low | 335.2 | 3.4 | 396.4 | 3.7 | |
Long | High | 610.4 | 4.3 | 682.0 | 4.6 |
Basis | 478.6 | 4.0 | 532.3 | 4.2 | |
Low | 351.2 | 3.4 | 403.4 | 3.8 |
Element | Uncertainty | Short ILT High VED | Short ILT Basis VED | Intermediate ILT Basis VED | Long ILT Low VED | ||||
---|---|---|---|---|---|---|---|---|---|
Lower Part | Upper Part | Lower Part | Upper Part | Lower Part | Upper Part | Lower Part | Upper Part | ||
C | 0.0020 | 0.0149 | 0.0142 | 0.0161 | 0.0152 | 0.0156 | 0.0159 | 0.0168 | 0.0171 |
Si | 0.05 | 0.59 | 0.59 | 0.61 | 0.52 | 0.55 | 0.52 | 0.62 | 0.56 |
Mn | 0.03 | 0.89 | 0.92 | 0.91 | 0.94 | 0.87 | 0.90 | 0.91 | 0.94 |
Cr | 0.4 | 17.9 | 17.9 | 18.0 | 17.9 | 17.9 | 18.0 | 17.9 | 17.9 |
Mo | 0.04 | 2.38 | 2.40 | 2.42 | 2.39 | 2.37 | 2.42 | 2.39 | 2.41 |
Ni | 0.3 | 12.8 | 12.8 | 12.9 | 12.8 | 12.7 | 12.9 | 12.8 | 12.9 |
N | 0.005 | 0.075 | 0.077 | 0.078 | 0.077 | 0.079 | 0.078 | 0.082 | 0.083 |
P | 0.002 | 0.008 | 0.009 | 0.010 | 0.009 | 0.008 | 0.009 | 0.009 | 0.009 |
S | 0.0004 | 0.0043 | 0.0041 | 0.0042 | 0.0043 | 0.0042 | 0.0042 | 0.0042 | 0.0042 |
Fe | - | bal. | bal. | bal. | bal. | bal. | bal. | bal. | bal. |
Measurement Uncertainty | Short ILT High VED | Intermediate ILT Basis VED | ||
---|---|---|---|---|
Lower Part | Upper Part | Lower Part | Upper Part | |
0.004 | 0.036 | 0.034 | 0.031 | 0.031 |
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Mohr, G.; Sommer, K.; Knobloch, T.; Altenburg, S.J.; Recknagel, S.; Bettge, D.; Hilgenberg, K. Process Induced Preheating in Laser Powder Bed Fusion Monitored by Thermography and Its Influence on the Microstructure of 316L Stainless Steel Parts. Metals 2021, 11, 1063. https://doi.org/10.3390/met11071063
Mohr G, Sommer K, Knobloch T, Altenburg SJ, Recknagel S, Bettge D, Hilgenberg K. Process Induced Preheating in Laser Powder Bed Fusion Monitored by Thermography and Its Influence on the Microstructure of 316L Stainless Steel Parts. Metals. 2021; 11(7):1063. https://doi.org/10.3390/met11071063
Chicago/Turabian StyleMohr, Gunther, Konstantin Sommer, Tim Knobloch, Simon J. Altenburg, Sebastian Recknagel, Dirk Bettge, and Kai Hilgenberg. 2021. "Process Induced Preheating in Laser Powder Bed Fusion Monitored by Thermography and Its Influence on the Microstructure of 316L Stainless Steel Parts" Metals 11, no. 7: 1063. https://doi.org/10.3390/met11071063