Thermal Modeling and Experimental Validation in the LENS Process

Transcription

1 The Eighteenth Solid Freeform Fabrication Symposium August 6-8, , Austin, Texas Thermal Modeling and Experimental Validation in the LENS Process Liang Wang 1, Sergio D. Felicelli 2, James E. Craig 3 1. Center for Advanced Vehicular Systems, Mississippi State University 2. Mechanical Engineering, Mississippi State University 3. Stratonics, Inc., Laguna Hills, CA 92653

2 Outline Introduction Objectives Finite Element Modeling Thermal Measurement Results and Discussions Conclusions

3 Introduction Mirror or other beam guiding means Laser Laser beam and powder delivery nozzle Lens Shroud gas inlet Material deposition head Powder material supply Z-axis positioning of focusing lens and powder delivery nozzle assembly X-Y positioning stages Carrier gas Laser Engineered Net Shaping (LENS TM ) Schematic Temperature distribution in molten pool (Hofmeister et al. 1999)

4 Introduction A variety of materials can be used: Stainless Steel (SS410, SS316) Ti-based alloy (Ti-6Al-4V) Inconel, copper, aluminum, etc. Application: Aerospace repair & overhaul Rapid prototyping and 3D structure fabrication Product development for aerospace, defense, and medical markets, etc. Advantages: Low cost & time saving Enhanced design flexibility and automation Highly localized heat-affected zone (HAZ) Superior material properties (strength and ductility) Processing Blade Processing Bar

5 Introduction The mechanical properties are dependent on the microstructure of the material, which in turn is a function of the thermal history of solidification. An understanding of the thermal behavior of the fabricated part during the LENS process is of special interest. Numerical simulation methods have the potential to provide detail information of the thermal behavior.

6 Objectives Develop a 3-D model to simulate 10-pass single build plate LENS deposition of 410 stainless steel (SS410) powder with SYSWELD finite element code. Predict the temperature distribution and cooling rate surrounding the molten pool and compared with experimental data Thermal images in the molten pool and surrounding area were recorded using a two-wavelength imaging pyrometer system The results were analyzed using ThermaViz software to obtain the temperature distribution The dynamic shape of the molten pool was investigated

7 Geometry & Process Parameters Process parameters Values (Unit: mm) Width of the part Thickness for each layer 1.0 mm 0.5 mm 10 pass single build part V 10 Laser beam travel velocity 2.5 mm/s Substrate Moving time of the laser beam for each pass 4 s Idle time of consecutive layers deposition 1 s 5 Time to finish one layer 5 s 20 Total time to finish the part 50 s Weld direction: Same direction for each pass. Material properties of the deposited part and the substrate are the same.

8 Mesh Structure Number of nodes: 104,535 Number of elements: 132,400 Element size in the part: 0.1 X 0.1 X 0.1 mm 3 A dense mesh was used for the plate and the contact area with the substrate, where higher thermal gradients are expected.

9 Mathematical Model Modified heat conduction equation: L ij P T t - phase proportion - temperature -time i, j - phase indexes Q - heat source (T ) A ij ρ - mass density C - specific heat λ - thermal conductivity - latent heat of i j transformation - Proportion of phase i transformed to j in time unit Thermal properties depend on the temperature, and the phase proportions. The latent heat effects due to phase changes are modeled with the specific heat variation.

10 Initial and Boundary Conditions Initial condition T ( x, y, z, t = 0) = T Boundary condition on the bottom of the substrate T ( x, y, z = 0) = T for t > 0 Boundary conditions for all other surface k r 0 As new layers are activated, the surfaces are increased and the boundary conditions are updated. 0 ( ) ( 4 4 T n ) Ω = h( T Ta ) Ω + εσ T Te Ω Qr ΩLaser

11 Experimental Set-up CCD Camera Laser beam & nozzles assembly LENS Glovebox Telephoto lens Digital CCD Camera Specimen Viewport with thin film Substrate Computer ThermaViz software Schematic of thermal imaging experimental setup The thermal camera focuses on the sample through a viewport on the side of the LENS glovebox. Transparent thin film is used in the viewport

12 Calibration Curve Intensity ratio (L/S) Model with film Calibration with film Model w/o film Calibration w/o film Temperature ( C)

13 Process Parameters in the Experiments Specimen No. Laser power (W) No. Layers Laser speed (mm/s) Length of part (mm) Powder flow rate (g/min)

14 Photograph of Thin Wall Plate Samples P=300W, V=2.5mm/s P=600W, V=2.5mm/s

15 Thermal Image & Molten Pool Shape Short wave-length intensity image Thermal image and molten pool size Long wave-length intensity image P = 600W, V = 2.5 mm/s. Intensity is in counts and temperature in C.

16 Temperature & Gradient [600W, 2.5 mm/s] Temperature ( C) x (mm) Temperature gradient ( C/mm) x (mm) Temperature ( C) y (mm) Temperature gradient ( C/mm) y (mm)

17 Effect of Laser Power Temperature ( C) W 450W 600W Temperature ( C) W 450W 600W Distance from center of pool along depth direction (mm) Distance from center of pool along opposite travel direction (mm)

18 Max. Temperature & Max. Cooling Rate Temperature ( C) mm/s 4.2 mm/s 8.5 mm/s Maximum cooling rate along travel direction ( C/s) W 450W 600W Laser Power (W) Laser travel velocity (mm/s)

19 Molten Pool Size Molten pool size along travel direction (mm) W 450W 600W Layer Number Molten pool size along depth direction (mm) mm/s 4.2 mm/s 8.5 mm/s Laser power (W) Modeling results [P=600W, V=2.5mm/s]

20 Thermal Cycles Calculated thermal cycles at the center of each layer at P=600W, V=2.5mm/s

21 Results Comparison Temperature ( C) Experiment Modeling Temperature ( C) Experiment Modeling Distance from center of pool along depth direction (mm) Distance from center of pool along opposite travel direction (mm) Results Comparison between modeling and experimental results for thermal profiles in the molten pool along (a) depth and (b) opposite travel directions at P=600W, V=2.5mm/s

22 Conclusions Thermal behavior of the LENS process for SS410 single wall build was characterized by using a two-wavelength imaging pyrometer Experiments were performed in a LENS 850 machine with a 3kW IPG laser for different process parameters Temperature distribution in the molten pool and the molten pool size were investigated. It was found that the maximum temperature in the molten pool is approximately 1600 C. The molten pool size is approximately 1.0 mm, and the maximum cooling rate in the liquid-solid interface is in the order of 103 C/s.

23 Conclusions (Cont.) The molten pool size and cooling rate significantly depend on the travel velocity and the laser power. The thermal measurements give interesting and useful information when averaging is applied; however, there is a significant fluctuation in temperature readings at the pixel level. More accurate pyrometer performance would be desirable in order to confirm the present measurements.

24 Acknowledgements Dr. John Berry (ME, Mississippi State University) Dave Baker (CAVS, Mississippi State University) Jim Bullen (Optomec Co.)