Ripple formations determine the heterogeneous microstructure of directed energy deposition (DED)-printed 316L components

This paper presents an experimental study to characterize the ripples and other morphological formations as well as their influence on the heterogeneous microstructure on the surfaces of the directed energy deposition (DED)-printed stainless steel (316L) components. While ripple formations in the welding literature have been studied extensively, they have not received much attention in the DED process. They are often wrongly conflated with the melt pool geometry. The experiments consisted of printing 10 mm cubic components on a hybrid machine tool (MTS 500 from Optomec) under nine different combinations of laser power, scan speed and dwell time. The ripple formations and the microstructure on the scan surface were subsequently observed and statistically characterized at four different magnifications on an optical microscope. The study establishes that the ripple traces are a main determinant of the dendritic structures, as well as the heterogeneous spatial distribution of the microstructure formations in DED-printed 316 components. The statistical characterization also highlights the limitations of the ripple waveform descriptors employed commonly in the welding literature and introduces spectral quantifiers to adequately capture the complex waveform patterns of ripples in DED.