Onset of periodic oscillations as a precursor of a transition to pore-generating turbulence in laser melting

Abstract

• Narrowband oscillations of laser absorption precede multimodal oscillations. • Melt pool hydrodynamics drive oscillations when laser is stationary or scanned. • Simulations and X-ray imaging shows pore formation during multimodal oscillations. Laser melting technologies in welding and additive manufacturing, such as laser powder bed fusion, promises to revolutionize manufacturing. However, a challenge remains in preventing pore defects produced by melt pool instabilities, which degrade part quality. Using X-ray imaging synchronized with ultrahigh-speed (40 ns) absorption radiometry and coupled with high-fidelity simulations, we discovered that periodic oscillations in the melt pool's morphology existed prior to a transition to chaotic and pore-generating turbulence. Over-extension by recoil waves followed by surface-tension driven recession created temporary pores that persisted longer in a multimodal regime. This non-linear hydrodynamics exists in both shallow and deep melt pools, but most strongly affects the latter. Early prevention of this transition will help improve the quality of manufactured parts. This work demonstrates that remote sensing of these oscillations is possible by very high time resolution total light scattering measurements, which is more readily achievable in an industrial setting than 2-dimensional melt pool imaging.