Remote Sensing of Temperature for Laser Metal Deposition of 3D Free Form Structures

"In order to maintain the process stability and avoid local defects in laser metal deposition (LMD), it is required to have a greater control over the process parameters such as laser power and speed. For this reason, among the various detectable indicators of the process, one of the most relevant is the optical emission. A small wavelength portion of the light emitted by the process can be correlated to the process temperature being used as a feedback signal to control process parameters. To this purpose firstly, it is preferable to employ a remote sensing strategy to maintain the high flexibility of the motion system so as not to limit the process head movement. Secondly, a coaxial monitoring approach is more advantageous compared to an off-axis one since it allows observing the process directly without the requirement to be reoriented at any head motion which is key point if the objective is observing the molten pool. This paper describes and exploits the advantage of the coaxial and remote sensing of the temperature based on optical signals acquired through the optical combiner (TOC) of the multimode fiber laser. In the work, the adequate optical emission zone is defined and required optical arrangement is studied. The sensing system is applied to monitor temperature in laser metal deposition of 3D free form during the realization of a thin wall structure. The capabilities of the instrument to detect local process variations of the temperature due to speed variation in correspondence of transient positioner settlements are demonstrated. Signal characteristics of process irregularities are defined and compared with one deriving from a lateral placed thermal camera."