Fig. 2. In situ synchrotron X-ray observations and keyhole/molten pool measurements during laser welding.

a–c In situ high-speed synchrotron X-ray observations of laser welding at the welding speeds of 0.33, 1.00, and 1.42 m/s at times $t_0$, $t_0+0.3$, and $t_0+0.6$ ms, where $t_0$ was arbitrarily selected. See Supplementary Movies 1–5 for in situ observations at five different welding speeds. d Depth of keyhole rear wall. e Molten pool (MP) length. f Keyhole width (measured) and maximum melt velocity ($u_{\max }$, calculated by Eq. 1). The error bars in (d–f) represent the standard deviation, calculated from at least 3 measurements for each sample from the in situ observations.