REAL-TIME OPTIMIZATION OF PRINTING SEQUENCE TO MITIGATE RESIDUAL STRESS AND THERMAL DISTORTION IN METAL POWDER BED FUSION PROCESS
Creators: name: Pour, Ehsan Maleki
Publisher: name: Purdue University Graduate School
Year: 2024
Subjects: classificationCode: 401401
subject: Additive manufacturing

description: The Powder Bed Fusion (PBF) process is increasingly employed by industry to fabricate complex parts with stringent standard criteria. However, fabricating parts free of defects using this process is still a major challenge. As reported in the literature, thermally induced abnormalities form the majority of generated defects and are largely attributed to thermal evolution. Various methodologies have been introduced in the literature to eliminate or mitigate such abnormalities. However, most of these methodologies are post-process in nature, lacking adaptability and customization to accommodate different geometries or materials. Consequently, they fall short of adequately addressing these challenges. Monitoring and controlling temperature, along with its distribution throughout each layer during fabrication, is an effective and efficient proxy to control the thermal evolution of the process. This, in turn, provides a real-time solution to effectively overcome such challenges. 
The objective of this dissertation is to introduce a novel online thermography and closedloop hybrid-control (NOTCH)©, an ultra-fast and practical control approach, to modify the scan strategy in metal PBF in real time. This methodology employs different mathematical-thermophysical concept-based or thermophysical-based models combined with optimization algorithms designed to optimize the printing sequence of islands/stripes/zones in order to avoid or mitigate residual stress and distortion. This methodology is adaptable to different geometries, dimensions, and materials, and is capable of being used with machines having varying ranges of specifications. 
NOTCH’s objective is to achieve a uniform temperature distribution throughout an entire layer and through the printed part (between layers) to mitigate residual stress and thermally related distortion. To attain this objective, this study explores modifying or optimizing the printing sequence of islands/stripes in an island or the strip scanning strategy. This dissertation presents three key contributions: 
First, this work introduces two potential models: the Genetic Algorithm Maximum Path (GAMP) strategy and Generalized Advanced Graph Theory. Preliminary results for a printed/simulated prototype are presented. These models, along with the Tessellation algorithm (developed in my M.Sc. thesis), were employed within NOTCH.
Second, I developed two optimization algorithms based on the greedy and evolutionary approaches. Both algorithms are direct-derivative-free methods. The greedy optimization provides a definitive solution at each printing step, selecting the island/stripe that ensures the highest temperature uniformity. Conversely, the evolutionary algorithm seeks to obtain the final optimal solution at the end of the printing process, i.e., the printing sequence with the highest uniformity in the last printing step. This approach is inspired by the concept of Random Search algorithms, offering a non-definitive solution to find an optimal solution. 
Last, this work presents the NOTCH methodology, enabling real-time modification of printing sequences through the integration of a novel thermography methodology (developed in my M.Sc. thesis), developed models, and optimization algorithms.
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