Control of Microstructure in Laser Additive Manufacturing
Farshidianfar, Mohammad Hossein
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Since its arrival in the late 1980’s, Laser Additive Manufacturing (LAM) has come a long way to establish itself as one of the most advanced versatile manufacturing technologies in the 21st century. LAM implies a novel layer by layer solidification of powder injected materials for the formation of arbitrary configurations. Production of complex shaped functional metallic components, including metals, alloys and metal matrix composites (MMCs), with desired mechanical and metallurgical properties is currently the main focus in the LAM industry. A potential problem in applying the LAM technique however, is the possibility of an inconsistent microstructure throughout a complex component. The emphasis of this thesis is to develop an automated closed-loop system in order to control deposition microstructure of the LAM process in real time. An infrared imaging system is developed to monitor thermal properties of the process as feedback signals. Cooling rate and melt pool temperatures are recorded in real time to provide adequate information of the thermal process. The aim is to provide a consistent microstructure by controlling thermal characteristics involved in the LAM. An experimental analysis is developed to identify cooling rate and melt pool temperature effects on the final microstructure using two combined parameters: the effective energy density and the effective powder deposition density. The analysis provides critical insight of how the microstructure is specifically dependent on the cooling rate and its variations. Further study is conducted to evaluate cooling rate effects on the microstructure properties such as the morphology, grain size and phase transformations. Positive correlation is observed between microstructure evolutions and the cooling rate. On the other hand, cooling rate variations are also studied with respect to the traveling speed, in order to identify a suitable controlling action for the controller. Using the identified correlations between the cooling rate, travelling speed and the clad microstructure, a novel feedback PID controller is established to control the cooling rate. The controller is designed to operate the cooling rate around a desired point by tuning the travelling speed. The performance of the controller is examined on several single-line and multi-line closed-loop claddings in order to achieve desired microstructures with specific properties. Results show that the closed-loop controller is capable of generating a consistent controlled microstructure during the LAM process in real time.