Thermal monitoring of laser metal deposition strategies using infrared thermography

The thermal monitoring of additive manufacturing techniques implementing high-power energy sources is essential to ensure process quality. Although the number of research works on thermal process monitoring has grown in recent years, there are still few works that focus on using thermal camera monitoring on Laser Metal Deposition (LMD), to analyze thermal effects caused by changes in deposition strategy. This work aimed to monitor the entire thermal field produced by the LMD process during the multitrack deposition of a Nickel-based superalloy. The temperature field monitoring was carried out using an IR thermal camera capable of detecting temperatures up to 2117 °C. Experimental tests were carried out on six deposition strategies, showing that for unidirectional hatch strategies maximum temperatures of 1620 °C were reached, while contouring strategies exceeded 2100 °C. The thermographic map assessed the spatial distribution of the temperature generated by each strategy. Moreover, some key points are chosen to analyze the thermal cycles and relative cooling rates generated during the process. Additionally, an ad-hoc algorithm was elaborated to calculate the thermal gradient of every frame acquired by the thermal camera, and a routine was elaborated to create the thermal gradient map. The thermal gradient profiles were evaluated, showing mean values around 550 °C/mm and marked variations (between 200 and 700 °C/mm) only for contouring strategies. The thermal characteristics are then correlated with the metallographic and microhardness analyses. A high number of cracks was found in areas with higher temperature and thermal gradient variations. Finally, microhardness tests were carried out, and mean values and standard deviations were correlated with the thermal gradient distribution.