Real-time monitoring of laser powder bed fusion process using high-speed X-ray imaging and diffraction

We employ the high-speed synchrotron hard X-ray imaging and diffraction techniques to monitor the laser powder bed fusion (LPBF) process of Ti-6Al-4V in situ and in real time. We demonstrate that many scientifically and technologically significant phenomena in LPBF, including melt pool dynamics, powder ejection, rapid solidification, and phase transformation, can be probed with unprecedented spatial and temporal resolutions. In particular, the keyhole pore formation is experimentally revealed with high spatial and temporal resolutions. The solidification rate is quantitatively measured, and the slowly decrease in solidification rate during the relatively steady state could be a manifestation of the recalescence phenomenon. The high-speed diffraction enables a reasonable estimation of the cooling rate and phase transformation rate, and the diffusionless transformation from β to α ’ phase is evident. The data present here will facilitate the understanding of dynamics and kinetics in metal LPBF process, and the experiment platform established will undoubtedly become a new paradigm for future research and development of metal additive manufacturing.


Determination of melt pool profiles
For the melt pool, as presented in Fig. 2, it can be recognized easily above the metal base; however, below the base surface, it becomes fairly difficult to detect directly through human eyes. In order to show the melt pool dynamics clearly, particularly in the region of interest below the base surface, we developed a set of algorithm codes based on the intensity profile of each horizontal line on the X-ray images. First, as illustrated in Supplementary Fig. 3b, the X-ray image at the time of t was where Di is the length between the two intersection points of the C-L or the L-S interface over the ith horizontal line of an X-ray image, Δh is the height interval between the neighboring horizontal lines, and n is corresponding to the maximum penetration depth (Fig. 3b, d = n•Δh). The calculation results of nominal areas are summarized in Fig. 3d. In addition, the maximum melt pool widths (w) were measured, and the ratios of d/w are shown in Fig. 3c, as a function of the frame time, t.

Particle tracking
The fast compressive tracking technique proposed by the Yang group provides an efficient solution for tracking a moving object in real-time 1 . In this study, we modified and improved the technique to accommodate our applications of tracking the particle motions accurately through a set of Xray images. First, from the original images (e.g. Fig. 4a), the locations of the targeted particles (e.g. P1-P5) were marked out by the Matlab built-in function of imfindcircles and were confirmed by human eyes. Second, the X-ray images were enhanced through the Local Equalization function built in the Image-Pro Plus software (Media Cybernetics Manufacturing, Warrendale, PA) to distinguish the targeted particles from their backgrounds 2 ; the enhanced local backgrounds also provided better references for detecting the targeted particles. Third, the algorithm codes were run to track the trajectory of each targeted particle (e.g. P1-P5 in Fig. 4b), and the results were confirmed by human eyes. As an example, the particle tracking process was illustrated in Supplementary Video 7. Among all the particles outside the powder bed, the ejection speeds and angles were statistically analyzed, and the results are shown in Figs. 4c and 4d, respectively.

Determination of solidification rate
After the laser fusion process, the melt pool started to cool down, and columnar grains grew along the radial direction. In Fig. 5a and Supplementary Video 8, we chose five points of interest (P1-P5) on the L-S interface, and tracked their motions. The solidification rate, ς, is estimated as where Δt is the time unit (50 µs), and Δl is the corresponding moving distance of the targeted point on the L-S interface. Fig. 5b shows typical history of the solidification rate of a targeted point (e.g. P3). Over the time spanning the plateau in Fig. 5b, the solidification rate was averaged, and the results are shown in Fig. 5c. Figure S1. Energy spectrum of X-rays generated by the undulator with a period of 1.8 cm at the 32-ID beamline of the Advanced Photon Source. The gap of the undulator is set to 12 mm. The first harmonic energy is 24.4 keV.

Supplementary Videos
Video S1. High-speed X-ray imaging of laser powder bed fusion process of Ti-6Al-4V with 340 W laser power.
Video S2. High-speed X-ray imaging of laser powder bed fusion process of Ti-6Al-4V with 520 W laser power.
Video S3. Melt pool dynamics in the Ti-6Al-4V metal base with 340 W laser power.
Video S4. Melt pool dynamics in the Ti-6Al-4V metal base with 520 W laser power.
Video S5. Powder motion tracking with 340 W laser power.
Video S6. Powder motion tracking with 520 W laser power.
Video S7. Particle tracking using modified fast compressive technique.
Video S8. High-speed X-ray imaging of the rapid solidification process of Ti-6Al-4V.
Video S9. High-speed X-ray diffraction of laser powder bed fusion process of Ti-6Al-4V.