Determining the laser absorptivity of Ti-6Al-4V during laser powder bed fusion by calibrated melt pool simulation

Additive manufacturing processes such as laser powder bed fusion (LPBF) are now routinely used for advanced medical and aerospace components. However, moving from these bespoke applications to high-volume com-mercial manufacturing requires higher levels of predictability and control of part properties. Detailed simulations of the LPBF process can assist this transition by providing understanding of defect mechanisms and guiding paths to improvement. An accurate value of laser absorptivity for the material is critical for LPBF simulation but the published literature contains surprisingly little absorptivity data applicable to actual LPBF operating conditions. Here we determine the in-situ laser absorptivity of the alloy Ti-6Al-4V during LPBF to be 0.27 +/-0.03, for a laser wavelength of 1.07 mu m. Our technique involves calibrating melt pool CFD simulations against single-track experiments conducted over a range of energy densities and can be extended to other materials. The simulations incorporate multiple laser reflections and cover the transition from conduction to keyhole mode. We also discuss physical mechanisms that may be responsible for changes in the absorption behaviour at high laser energy density which are observed in this and other work.