Accurate determination of laser spot position during laser powder bed fusion process thermography Ivan Zhirnov ⇑ , Sergey Mekhontsev, Brandon Lane, Steven Grantham, Nikola Bura National Institute of Standards and Technology, United States article info Article history: Received 16 April 2019 Received in revised form 11 October 2019 Accepted 7 December 2019 Available online 10 December 2019 Keywords: Laser powder bed fusion Additive manufacturing Thermography abstract High-speed thermography is useful tool for researching the laser powder bed fusion process by providing thermal information in heat affected zone. However, it is not directly possible to ascertain the position of the laser spot with respect to the melt pool, which could provide key information regarding how laser energy is distributed and absorbed. In this paper, we demonstrate a procedure for registering the laser spot position with the melt pool using a bright illumination source co-axially aligned with the laser to project a sharp spot on the build plane. This spot is fixed to the laser position and used as a reference frame for registering the laser spot with the melt pool radiance temperature distribution. Measurement results demonstrate the effect of varying process parameters (laser power and scan speed) on the melt pool thermal field and respective position of the laser spot. Published by Elsevier Ltd on behalf of Society of Manufacturing Engineers (SME). 1. Introduction The physics of the LPBF process exhibit a wide range of temper- atures (up to or beyond 3000 °C), large temperature gradients (10 5 °C/m to 10 7 °C/m), high surface heating and cooling rates (10 3 °C/s to 10 6 °C/s), and small melt pool scale (<1 mm) [1]. Opti- mization of the material melting, as well as layer-by-layer fabrica- tion of parts, is a nontrivial task. For this purpose, multi-physics simulations are used to numerically replicate the process [2,3], and precise measurements are required to inform and validate these simulations. One important factor is the exact position of energy input relative to the melt pool. Measurement of this factor can guide development of energy input models [4], keyhole or vapor depression models [3,5], or melt pool fluid convection [6]. Ultimately, simulations of melt pool phenomena can guide estima- tion of the resultant solidified microstructure [7–9]. However, the spatial relationship between laser input and melt pool shape or temperature field is seldom observed or highlighted. Few studies have attempted to correlate the laser spot position to melt pool phenomena. For instance, Hooper [10] indicated the position of the laser beam coincides with the hottest area in the molten pool. In contrast, Gusarov et al. [11] simulated the effect of scanning speed on the temperature distribution and melt pool size using a conduction based model, which always resulted in the peak temperature located behind the laser profile. Khairallah et al. [2] showed a more complex effect of recoil pressure and Mar- angoni convection in the melt pool flow which creates complex variations in molten pool surface temperature. Leung et al. showed the position of the laser with respect to vapor depression (without temperature measurement) using high speed X-ray imaging [12]. This paper describes a method for measuring the spatial posi- tion of a laser beam relative to the radiance temperature distribu- tion in the melt pool. This enables direct measurement of the laser energy distribution with respect to a surface temperature map of the heat-affected zone (HAZ). This technique allows key physical relationships between the laser energy input and resulting thermal field to be explored. 2. Measurement methods Measurements were performed on a custom LPBF system called the Additive Manufacturing Metrology Testbed (AMMT). A high- speed, high magnification staring imaging system (static field of view position directly in working zone) was constructed with a long working distance (54 mm) microscope lens, 520 nm bandpass filter, laser cut-off filter at 1000 nm, and attached mirror that allows close-range observation of the melt pool without obstruct- ing the laser (Fig. 1). The camera resolution was 3.07 lm/pixel and frame rate set to 20 000 frames/s. A green laser (532 nm, 0.25 mW) is installed on the same optical path as the ‘hot’ laser (1070 nm, up to 500 W). The green laser is passed through a field stop to project a sharp spot on working surface inside the AMMT chamber. Both green and hot laser focus points are observed in the same camera https://doi.org/10.1016/j.mfglet.2019.12.002 2213-8463/Published by Elsevier Ltd on behalf of Society of Manufacturing Engineers (SME). ⇑ Corresponding author. E-mail address: i.zhirnov.m@gmail.com (I. Zhirnov). Manufacturing Letters 23 (2020) 49–52 Contents lists available at ScienceDirect Manufacturing Letters journal homepage: www.elsevier.com/locate/mfglet