Numerical Simulation of the Temperature Distribution and Microstructure Evolution in the LENS Process

Transcription

1 The Seventeenth Solid Freeform Fabrication Symposium Aug 14-16, 2006, Austin, Texas Numerical Simulation of the Temperature Distribution and Microstructure Evolution in the LENS Process L. Wang 1, S. Felicelli 2, Y. Gooroochurn 3, P.T. Wang 1, M.F. Horstemeyer 1 1. Center for Advanced Vehicular Systems, Mississippi State University 2. Mechanical Engineering, Mississippi State University 3. ESI Group, Bloomfield Hills, MI

2 Outline Introduction Objectives Finite Element Modeling Results and Discussions Conclusions Future Work

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 available in the literature. Optimize the process parameter (laser power) in order to achieve a pre-defined molten pool size for each pass. Investigate the effect of the thermal cycles on the phase transformation and consequent hardness.

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 7.62 mm/s Substrate Moving time of the laser beam for each pass 1.3 s Idle time of consecutive layers deposition 0.7 s 5 Time to finish one layer 2 s 20 Total time to finish the part 20 s Weld direction: Same direction for each pass. Material properties of the deposited part and the substrate are the same.

8 Thermal Properties (SS410) Thermal properties depend on the temperature, and the phase proportions.

9 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.

10 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.

11 Element Activation Technique Dummy material method is applied to the element activation: M1: Deposited layers + substrate Material with actual thermal properties and phase transformation M2: Layer being deposited Material with actual thermal properties and starting with dummy phase Dummy phase Austenite phase (T>T aus ) M3: Layers to be deposited Material with dummy low thermal properties and without phase transformation Fixed mesh is used for the plate and substrate. M3 M2 M1 V

12 Heat Source h z h r r r r i e e o ) )( ( = ) ( ) ( t v y y x x r o o + = = exp 1 2 r r h z h r P Q r π 3D Conical Gaussian Function r Q P - Input energy density (W/mm 3 ) - Absorbed laser power (W) Part of energy generated by the laser beam is lost before being absorbed by the part. Absorbed laser power is used in the calculation. The nominal laser power is calibrated by matching the predicted temperature profile with measured data. Q r

13 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

14 Model Calibration Temperature distribution (2-D View) 4 mm Distance (mm) The calibration calculation is performed only for the deposition of the top layer (the 10 th layer). T 0 = 600 C, P abs = 100W, P l = 275W, E = 36.4% (30-50%) (Unocic and DuPont, 2004) Temperature ( C) Cooling Rate ( C/s) Modeling Measured (Hofmeister et al., 1999)

15 Molten Pool Size mm Nominal Laser Power (W) Pass Number The molten pool size is determined by melting temperature (1450 C for SS410) One and a half layers are melted for each pass About 5% decrease in laser power is needed from one layer to the next subsequent layer in order to keep a fairly constant pool size

16 Temperature Distribution Laser beam is at the center of the 5 th pass Laser beam is at the center of the 10 th pass 3D temperature distribution for 10-pass LENS process Similar molten pool size and temperature distribution surrounding the molten pool size are obtained by both cases.

17 Thermal Cycles ( C) 1 st layer 3 rd layer 5 th layer 10 th layer Cross-section micrograph of H13 tool steel thin wall* Ms (s) Thermal cycles for the mid-points of layers 1, 3, 5, and 10 of the built plate. * Griffith et al., Thermal Behavior in the LENS process, J. Mater. Des. 20 (1999) Hardness versus distance from top of wall*

18 Cooling Rates ( C/s) Max. cooling rate for each layer Max. cooling rate for 1 st layer (s) Cooling rates for the mid-points of layers 1, 3, 5, and 10 of the built plate.

19 Temperature Contour Movie

20 Conclusions A 3-D model has been developed to predict the thermal cycles and cooling rates during the 10-pass LENS process of a SS410 plate with SYSWELD. The model predicts temperature profiles and cooling rates that agree qualitatively and quantitatively well with measured data. About 5% decrease in laser power for each pass is required in order to keep the molten pool size in the predefined range. The tempered martensite is transformed at the lower layers due to the thermal cycles, which will cause the hardness of the upper part to be higher than that of the lower part.

21 Future Work Experiments will be performed to measure the thermal profiles and temperature gradients for SS410 plate to calibrate the current model. Using the calculated thermal profiles, the phase proportions and hardness of the LENS material will be predicted with SYSWELD. Measurements in hardness and microstructures will be performed to calibrate the model.

22 Acknowledgements Dr. John Berry (ME, Mississippi State University) Jim Bullen (Optomec Co.) Benton Gady (National Automative Center) The project is sponsored by U.S. Army TACOM.