Impact of compositional gradients on selectivity of dissolvable support structures for directed energy deposited metals

Functionally gradient materials (FGM) have many important applications due to their ability to possess vastly different material properties across the gradient. Recent work on dissolvable supports in stainless steel components fabricated using directed energy deposition (DED) show the utility of exploiting differences in the corrosion suseptibility in FGM metals. In order to better control the feature resolution of DED dissolvable supports, it is first necessary to understand how dilution and mixing within the gradient impact the local corrosion susceptibility and etch rates. In this work, FGMs with varying numbers of tracks and layers were fabricated onto a 304 stainless steel build plate; first one to three layers of 91 carbon steel followed by one to ten layers of 431 stainless steel. Metallography, potentiodynamic polarization plots, energy dispersive x-ray spectroscopy, and 3D contact profilometry data were collected to show that mixing within the gradient is very inhomogeneous. Incomplete mixing is observed throughout individual tracks along with widely varying composition and material properties from track-to-track, even within a single track. This paper demonstrates that the impact of incomplete mixing and composition gradients within a layer must be considered for DED-fabricated FGM dissolvable supports.