In situ monitoring of surface depressions in metal laser additive manufacturing and its interlayer transfer mechanism

• Synchrotron X-ray in situ imaging reveals the dynamic evolution of surface depressions across multiple layers in LPBF, linking them to interlayer defect formation. • Insufficient energy input in the first layer causes balling and fracture of melt tracks, which serve as precursors for subsequent surface depressions. • Increasing energy input in the second layer induces local overheating at gaps between melt tracks, creating surface depressions; reducing energy in later layers hinders melt backflow and enlarges depression regions. • The study provides critical experimental evidence for closed-loop interlayer process control, enabling real-time defect mitigation in laser additive manufacturing.