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Numerical Simulation and Experimental Investigation of Powder Stream and Laser Attenuation in Fabrication of Functionally Graded Materials by Means of Laser Directed Energy Deposition Process

Sobhanieh, Niloufar | 2022

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 55456 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Akbari, Javad; Moradi, Mahmoud
  7. Abstract:
  8. Functionally Graded Material (FGM)is attaining more and more interests recently due to its outstanding feature of possessing benefits of physical and mechanical properties of two materials in one part. One of the common processes to fabricate FGM is laser directed energy deposition of pre-mixed powder of two materials. The powder stream and laser attenuation in LDED of FGM is studied in this dissertation by the aid of numerical simulation and experimental investigations. Due to numerical simulation results, which is done in FLUENT software, powder particle size and axial shielding gas flow rate has significant effect on composition of powder mixture falling into the molten pool. Considering constant density for each of materials’ powder, the size of powder particles should be chosen in a way that they have equal acceleration in carrier gas flow. In order to obtain the same acceleration for particles of the two different materials, the product of density and diameter square should be equal for both of them. Another parameter that may cause deviation from desired composition is axial shielding gas flow rate. It blows particles off the axial axis and whatever the particles are lighter they are affected more. Increasing axial shielding gas flow rate may cause the lighter particles fall out of meltpool and consequently the presence of lighter material in deposited layer would be less than expected. Experimental investigations were done to verify the results of simulations. The pre-mixed powders used was the combination of WC with SS316L and 309L. The deposited layers were cut in different locations along deposition direction. Composition in each section was surveyed by the aid of SEM photos and ImageJ software. The result of experiments showed that the desired and consistent composition of two materials, is reachable if the ratio of powder size of two materials and axial shielding gas flow rate is chosen correctly. Furthermore, a new method is proposed for calculating laser attenuation without simplification applied in previous works. Despite other studies that consider a predefined powder distribution, the result of simulation of the powder stream is utilized for defining the position of particles. The divergence and spatial distribution of the laser beam are considered by dividing the laser beam in a radial direction. A GUI has been developed in MATLAB to take CFD model output as input for calculating laser attenuation with Beer-Lambert law and plotting the laser intensity on the workpiece surface after being attenuated. The influence of powder mass flow rate, powder size, and workpiece position on laser attenuation and its intensity distribution is investigated. It is shown that increase in powder flow rate would increase laser attenuation almost linearly. It has also indicated that smaller particles would attenuate more energy than larger ones while the powder mass flow rate is kept constant. More specifically, decreasing powder size from 100 μ to 20 μ, increases attenuation from 18% up to 55%. The size of powder particles affects powder stream distribution and consequently, this affects the laser intensity profile on the workpiece surface. The result of investigating the workpiece position shows that the position of the workpiece influences the laser attenuation, powder catchment, and maximum intensity of laser energy
  9. Keywords:
  10. Tungsten Carbide ; Laser Clading ; Functionally Graded Materials (FGM) ; Laser Attenuation ; Laser Energy Distribution ; Laser Directed Energy Deposition (LDED)

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