Keyhole Formation by Laser Drilling in Laser Powder Bed Fusion of Ti6Al4V Biomedical Alloy: Mesoscopic Computational Fluid Dynamics Simulation versus Mathematical Modelling Using Empirical Validation
Abstract
:1. Introduction
2. Modelling
2.1. Analytical Modelling
2.2. Numerical Modelling
3. Materials and Methods
4. Results and Discussions
5. Conclusions
- The temperature below the laser irradiation point rises rapidly in LPBF, whereas the powder layer around it remains at a localized temperature. Due to air between the powder particles and the relatively low particle–particle contact areas, laser transverse heat waves have a slower speed because of the high thermal resistance.
- Two types of laser keyholes can be generated during the LPBF process: shallow and deep keyholes. The mode type can be controlled and defined to an extent by the energy density. Increasing the energy density leads to an increase in the amount of energy delivered to a given region, and optical rays encounter numerous reflections due to voids available in the deposited powder layer. In return, this increases the laser beam absorption coefficient. As a result, the shallow keyhole converts into a deep keyhole.
- The deep keyhole usually generates numerous laser beam reflections and stream traces. A deep keyhole experiences a larger energy density compared with a shallow keyhole, pushing the material towards vaporization. The porosity usually occurs if the solid front hits quickly before they escape from the melt pool. Due to an elevated temperature distribution in the deep keyhole, the probability of pores forming is much higher than in a shallow keyhole, as the liquid material is close to the vaporization zone. However, both shallow and deep keyholes exhibited a clockwise direction.
- Due to the specific heat and fusion latent heat, the results demonstrated that, as the temperature increases, the material density decreases rapidly, elevating the fluid volume. This, in return, reduces the surface tension (ST), and is also known as Benard–Marangoni convection or thermocapillary convection. The ST difference largely determines the size of the melt pool. When a liquid’s ST difference widens, a significant pull arises from the high ST end toward the low ST end.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Parameters | Values (Units) |
---|---|
Solidus temperature | 1878 (K) |
Liquidus temperature | 1928 (K) |
Boiling temperature | 3533 (K) |
Latent heat of fusion | 286 (kJ/Kg) |
Latent heat of evaporation | 9830 (kJ/Kg) |
Viscosity | 0.005 (Kg/ms |
Surface tension | 1.68 (N/m) |
Surface tension gradient | −0.00026 (N/mK) |
Parameter | Value | Parameter | Value |
---|---|---|---|
Substrate length × width × depth | 2500 µm × 200 µm × 400 µm | Average size of cell for CFD simulations | 3.3 µm |
Powder layer thickness | 70 µm | Ti6Al4V particle size distribution | D10 = 19 µm, D50 = 30 µm and D90 = 46 µm |
Laser power | 195 W | Laser beam entire absorption coefficient | 0.25 |
Laser scanning speed | 400 mm/s, 1000 mm/s | Number of cells generated for CFD simulation | 6,288,368 |
Effective laser beam radius where the heat flux is 1/e2 of its maximum value | 25 µm |
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Ur Rehman, A.; Mahmood, M.A.; Pitir, F.; Salamci, M.U.; Popescu, A.C.; Mihailescu, I.N. Keyhole Formation by Laser Drilling in Laser Powder Bed Fusion of Ti6Al4V Biomedical Alloy: Mesoscopic Computational Fluid Dynamics Simulation versus Mathematical Modelling Using Empirical Validation. Nanomaterials 2021, 11, 3284. https://doi.org/10.3390/nano11123284
Ur Rehman A, Mahmood MA, Pitir F, Salamci MU, Popescu AC, Mihailescu IN. Keyhole Formation by Laser Drilling in Laser Powder Bed Fusion of Ti6Al4V Biomedical Alloy: Mesoscopic Computational Fluid Dynamics Simulation versus Mathematical Modelling Using Empirical Validation. Nanomaterials. 2021; 11(12):3284. https://doi.org/10.3390/nano11123284
Chicago/Turabian StyleUr Rehman, Asif, Muhammad Arif Mahmood, Fatih Pitir, Metin Uymaz Salamci, Andrei C. Popescu, and Ion N. Mihailescu. 2021. "Keyhole Formation by Laser Drilling in Laser Powder Bed Fusion of Ti6Al4V Biomedical Alloy: Mesoscopic Computational Fluid Dynamics Simulation versus Mathematical Modelling Using Empirical Validation" Nanomaterials 11, no. 12: 3284. https://doi.org/10.3390/nano11123284