1. Full build thermal modeling of the additive manufacturing process
Tom Stockman, Judy Schneider
University of Alabama in Huntsville
Bryant Walker
Keystone Synergistic Enterprises, Inc.
Approved for public release; distribution is unlimited

2. Objective
Develop a model to capture full build cycle thermal history during free form additive manufacturing
Model should be capable of integration into a production environment
Center for Advanced Vehicular Systems, Mississippi State University

3. Additive manufacturing is governed by heat transfer
Effective thermal management can minimize several concerns:
Slumping
Residual stress
Microstructural management
Void distribution
Geometrical tolerancing
High heat input build
Low heat input build

4. Modeling the full build is imperative
Thermal history of the build
Substrate temperature distribution
Melt pool
Modeling the full build can also offer insight into microstructural evolution

5. Requirements for incorporation into a production environment
Quick turnaround
Predictive capability
Integration with CNC software (G code)
Compatibility with current processes:
Powder bed processes: laser, electron beam
Freeform processes
Blown powder: laser, electron beam
Wire fed: laser, electron beam, pulsed-arc

6. Capabilities with pulsed arc, wire fed, free form additive manufacturing process
Avoids safety issues related to fine powders
Not confined to typical dimensions of a powder bed system
Applicable to deposition of multiple materials
Selection of heat source controls deposition rates

7. Free form wire fed with-pulsed arc heat source provides a production platform for model verification
Thermal measurement sensor on robotic head
Multi-axis robotic platform
Arc length control
Z axis build height sensor

8. Thermocouples and infrared (IR) thermal measurements provide thermal data to verify model
Materials used in builds: Inconel 718, Inconel 625, Invar 36 steel, Maraging steel, 4340 steel

9. Complex shapes yield a complex thermal profile
14.5 cm diameter (bottom)
10 cm long x 10 cm high

10. Most thermal modeling of AM process focus on localized temperatures
Models start from melt pool temperature, shape, and size
Thermal history tied to residual stress
Many models based on commercial software (ABAQUS)
Time intensive
Currently not targeting full build
Chin, Beuth, Amon, J. Mfgt Sci. & Engr., Vol. 123, 2000.
Cheng, Price, Lydon, Cooper, Chou, J. Mfgt Sci & Engr, Vol 136, 2014.

11. A customized model offers the ability to rapidly import varying geometries and model the full build
Custom built in free open source programming language (Python 2.7)
Non-linear, transient, mass added numerical model
“G” code compatible for ease of use in production environment
Maintains thermal history of entire build
Temperature predictions can be used to program robotic controls for desired build temp

12. Boundary conditions are convection and radiation, applied differently depending on direction
Free convection in 3 directions
Quasi-static gaseous environment
Cartesian mesh
Black body radiation

13. Temperature evolution governed by heat flux
Heat flux induced by mass addition at melt temperature
Flux verified by comparison to Rosenthal equations and by estimates from The Welding Institute
Incropera, F., 2007, Fundamentals of Heat and Mass Transfer 6th Edition
The Welding Insitute,

14. Each step of the deposition follows a time discretized process flow

15. Example of 2D model output
2D model migrated to 3D

16. Summary
Instructions built with times relative to the start of each pass allowing top layer of build to cool to given interpass temperature before next layer begins
Modeling allows for optimization of interpass temperature
Hold times used for temperature control based on geometrical and thermal profile
Model is being developed to allow future integration with other tools to optimize process parameters and predict microstructural evolution and residual stresses

17. Acknowledgements
Keystone Synergistic Enterprises, Inc., "Extension of Physics Based Laser MDDM Process Mapping,” Internal IR&D.
NASA STTR Phase I with Keystone Synergistic Enterprises, Inc., “Advancing Metal Direct Digital Manufacturing (MDDM) Processes for Reduced Cost Fabrication of Bi-Metallic Cooled Rocket Engines.”