Measurement of mass loss, absorbed energy, and time-resolved reflected power for laser powder bed fusion

Laser powder bed fusion processes are driven by scanned, focused laser beams. Along with selectively melting the metal powder, laser energy may be converted and transferred through physical mechanisms such as reflection from the metal surface, heat absorption into the substrate, vaporization, spatter, ejection of heated particles, and heating of the metal vapor/condensate plume that is generated by the laser-metal interaction. Reliable data on energy transfer can provide input for process modeling, as well as help to validate computational models. Additionally, some related process signatures can serve better process monitoring and optimization. Previous studies have shown that the proportion of the transfer mechanisms depend on laser power, spot size, and scan speed. In the current investigation, the energy conservation principle was used to validate our measurement of reflected energy, absorbed energy, and energy transfer by vaporization on bare plates of Nickel Alloy 625 (IN625). Reflected energy was measured using an optical integrating hemisphere, and heat absorbed into the substrate was measured by calorimetry. Transfer from vaporized mass loss was measured with a precision balance and used to establish an upper bound on energy transfer by mass transfer. In addition to measurement of total reflected energy, the reflected laser power was time-resolved at 50 kHz in the integrating hemisphere, which provided insight into the process dynamics of conduction, transition, and keyhole modes.