Model Learning and Predictive Control of Laser Powder Bed Fusion
Open Access
- Author:
- Ren, Yong
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 14, 2023
- Committee Members:
- Guhaprasanna Manogharan, Major Field Member
Qian Wang, Chair & Dissertation Advisor
Xiang Yang, Major Field Member
Robert Kunz, Professor in Charge/Director of Graduate Studies
Edward Reutzel, Outside Unit & Field Member - Keywords:
- Laser Powder Bed Fusion
Machine Learning
Optimal Control
Finite Difference Method
Melt-Pool Geometry
Temperature Evolution - Abstract:
- Additive manufacturing (AM) provides a transformative approach for industrial applications, enabling the fabrication of near-net-shape components directly from computer-aided design files. As a subcategory of metal AM processes, Laser Powder Bed Fusion (L-PBF) utilizes a high-speed, fine-diameter laser heat source to melt layers of powder that have been evenly distributed by a recoater. While L-PBF has emerged as the most widely-used commercial metal AM technology, numerous critical challenges still persist in process modeling and control of this approach. Addressing these issues is crucial for enhancing the geometric accuracy and overall quality of additive manufactured components. The objective of this research is to employ machine learning and numerical techniques for the development of comprehensive multiscale models, facilitating prediction and control in the L-PBF process across various process conditions. Using machine learning algorithms allows for constructing robust computational models based on training data, offering accurate predictions and informed decision-making for a wide range of physical systems. In this research, a variety of machine learning techniques were primarily used for fine-scale modeling and control of L-PBF processes. This was demonstrated through single-layer multi-track cases as a proof-of-concept study. In order to accurately model the relationship between process parameters and melt-pool sizes, a physics-informed method was adopted to identify critical input features for machine learning models. A two-level architecture was implemented for both model training and validation. Notably, the initial temperature at the melting point was recognized as a crucial variable in characterizing the thermal history for precise melt-pool size predictions. To achieve consistent melt-pool distribution during multi-track laser processing, a physics-informed optimal control method was devised to adjust laser power based on Gaussian process regression. The study's findings demonstrate that nonlinear regression analysis techniques, such as Gaussian process regression, are effective in predicting melt-pool geometry. When these techniques are further combined with optimal control, they can regulate the melt-pool size to a desired reference value. Regarding the evolution of temperature at the part-scale level, a novel finite-difference model was introduced, providing fast predictions of interlayer temperature and facilitating model-based thermal control. Interlayer temperature, defined as the layer temperature after powder spreading but before scanning a new layer, serves as the initial condition for the subsequent scan and thus plays an important role in the melt-pool morphology and the final build quality. The effectiveness of the proposed modeling method was evaluated through thermal analysis of a square-canonical geometry made from Inconel 718. Based on the part-scale thermal model, an optimal control utilizing layer-wise laser power adjustments was further developed to regulate the interlayer temperature below a preset threshold, thereby mitigating excessive heat buildup during the build process. The optimized laser power profiles, initially obtained by solving a convex program based on the finite-difference model, were then programmed on the EOS M280 system for a feedforward control to build the square-canonical parts. In-situ, real-time measurements of interlayer temperature were collected using infrared (IR) thermal imaging during the build process to validate the model and control. Post-process optical micrographs were also captured to compare the melt-pool morphology under optimized laser power profiles with that obtained under the default constant laser power. The control performance was evaluated through numerical simulations and experimental studies. Research findings confirm the efficacy of the proposed optimal thermal control in reducing overheating during the L-PBF build process.