Video Monitoring and Control of the LENS Process

Abstract

The LENS (Laser Engineered Net Shaping) process has significant potential impact to the manufacturing community in producing near-net shape rapid prototypes, tooling and customized small lot parts. LEINS has its roots in stereolithography and weld surfacing. Parts are built up in layers by delivering powder carried in an inert gas stream directed via nozzles to a laser-produced molten pool. A robust implementation of this technology requires a thorough understanding of how the thermal history during part fabrication influences the dimensions, microstructure and properties of the part. This understanding, in combination with effective closed loop feedback control of the process, and modeling of the part to be formed, is required to ensure routine fabrication of components with appropriate properties Thermal behavior at high temperatures (above 800 C) can be readily monitored by visible light radiation pyrometry. In this work a high speed digital camera with a narrow bandpass optical filter was used to obtain thermal images of the LENS process zone. The thermal imaging system was incorporated into the optical path of the laser so that the melt pool and adjacent areas of the part could be monitored without intrusive hardware add-ens at the lens/powder nozzle/process zone vicinity. The output of the digital camera was collected by a fiarne grabber card in a personal computer (PC). Characteristics of the melt pool were evaluated and then used as inputs to a Proportional-Integral-Derivative (PID) control algorithm also running on the PC. The output of the PID algorithm was then used to control the laser power. Running the closed loop control resulted in significant stabilization of the melt pool size during simulated fabrication experiments. We will describe the equipment, algorithms, experiments and results obtained from LENS-formed simple shapes of 316 Stainless Steel.