Real-Time Closed-Loop Control of Microstructure and Geometry in Laser Materials Processing

Abstract

Laser Materials Processing (LMP) is currently one of the fastest growing technologies of the 21st century. Different categories of this technology such as Laser Additive Manufacturing (LAM) and Laser Heat Treatment (LHT) have now paved the way for more versatile methods of manufacturing that were not possible through conventional manufacturing methods. The localized laser heat source provides advantages such as minimal dilution, minimal distortion, small heat affected zones, and improved localized geometry and quality. However, these advantages come at a price, which is the number of inputs, outputs and process parameters involved that make the LMP a complex process for mainstream manufacturing. Current industrial LMP platforms require an extensive amount of manual tuning and process knowledge in order to achieve high quality production. Nonetheless, because of process sensitivity and lack of automation in LMP machines, the material and mechanical properties of LMP-manufactured products are highly inconsistent. Therefore, to take advantage of the technology’s benefits and to establish LMP into the mainstream manufacturing technology, it is highly essential to develop a fully automated closed-loop LMP process that can intelligently control important output characteristics in real-time. In this research, an automated real-time closed-loop process will be studied and developed to simultaneously control two of the most important LMP output properties: (1) microstructure and (2) geometry. A multi-objective thermal-geometry monitoring and control module is developed to enable closed-loop control of microstructure and geometry properties of the LMP process. Geometry features such as clad height of the LAM process are directly monitored through a CCD camera. Geometry control is achieved by direct control of absolute geometrical values in real-time. An infrared thermal image acquisition system is integrated with the CCD-based imaging system to monitor real-time thermal dynamics. Thermal dynamics of the process such as the cooling rate, melt pool temperature, and heating rate are recorded directly in real-time through a specific set of thermal image analyses algorithms. Microstructure control is defined as control of consistency and stability of a desired set of microstructures for specific materials correlated with a set of perceived thermal dynamics and thermal signatures offline. Therefore, by directly controlling the desired set of correlated thermal dynamics in real-time, a consistent controlled microstructure is guaranteed during the process. A complete closed-loop control process is developed by integrating the monitoring system, LMP system and a multi-input-multi-output controller system. LHT and LAM experiments are conducted with thermal monitoring to understand and predict microstructue, hardness and geometry characteristics in real-time. Microstructure features such as martensitic formation and phase transformations are correlated with real-time thermal cooling/heating rates and melt pool temperatures to develop a microstructure prediction method. Important geometry properties such as hardened depth are also correlated with the thermal dynamics to identify a suitable feedback signal for closed-loop control of the depth, which cannot be monitored by a CCD camera. Thermal patterns are identified for online control of the hardness during single-track and multi-track LHT and LAM processes. Furthermore, an accurate and computationally efficient thermal dynamics model is developed and validated for the LHT and LAM processes for real-time estimation of the thermal dynamics of the process with limited information of the thermal boundaries. The dynamic model is integrated into a state observer feedback control system to provide model-based closed-loop control of the thermal dynamics. The intelligent closed-loop process is evaluated for different case studies of single-track and multi-track laser heat treatment and laser additive manufacturing. The real-time control of microstructure and hardness is achieved in the LHT process through a closed-loop control of the peak temperature. State observer feedback control of the peak temperature is also evaluated for the LHT process. Single-input-single-output control of the clad height and cooling rate are also incorporated for individual real-time control of the microstructure and geometry. Finally, an integrated microstructure and geometry control of the LAM process is constructed and tested for single-track and multi-track LAM depositions, to provide consistent material properties with controlled clad height. As a result of the closed-loop multi-input-multi-output control, the consistency and quality of the LMP manufacturing processes have increased significantly. The controller is capable of eliminating the effect of process and environmental disturbances such as irregular workpiece geometries or undesired heat accumulations. As a result, the developed closed-loop system significantly reduces the extensive amount of time and effort required for manual tuning of LMP setups, and automatically adjusts the process inputs to achieve the desired material and geometry properties. In addition, it also provides an essential tool for obtaining in-process knowledge of the LMP manufacturing process.