High-speed directed energy deposition (HS-DED) fundamentally alters laser–material interaction dynamics due to coexisting fast plasma-related and delayed thermal responses, complicating process stability in transition regimes. This work investigates the dynamic behavior of AISI 316 L processed on a commercial HS-DED system. It is shown that conventional grey-box models based on spatially integrated photometric signals fail to describe the process response at critical operating points. Using multispectral photometric measurements, the emission signal is temporally separated into plasma-related and thermal components. Based on this, a time-domain metric termed spectral lag is introduced, providing an emissivity-independent indicator that reliably captures regime transitions.
Gajek et al. (Wed,) studied this question.