Enabling multi-material gradient structure in laser powder bed fusion

Abstract

The layer-by-layer building in the laser powder bed fusion (LPBF) process can be exploited to achieve graded alloy compositions along the build direction. The local control over alloying element composition would allow for tailored material properties. This work demonstrates the use of LPBF to achieve gradient structures by mixing two austenitic steels, namely AISI 316L and Fe35Mn, by varying the relative deposition amounts of two alloy feedstocks. For this, a custom-built LPBF system equipped with a double-hopper and a mixing chamber was used. The system allowed the powders to be mixed on demand before deposition of each layer. The process parameters were studied to produce graded specimens starting from AISI 316L, and gradually changing to Fe35Mn, along the build direction. Characterization of the elemental composition verified good mixing of the elements, both due to the preparation of the mixed powder within the machine, and homogeneous melting of each new layer into the underlying layer of the build. Element-wise chemical composition control was therefore achieved by gradually substituting Ni, Cr, and Mo (in AISI 316L) with Mn (in Fe35Mn). The specimens were characterized for their mechanical properties at different chemical compositions along the build direction. The microhardness and the ultimate tensile strength could be varied from 240 HV to 150 HV and from 750 MPa to 600 MPa over 6 mm distance along the build direction. The results confirmed the functional gradient and the possibility to use this technique to design complex components with locally specified mechanical properties and geometry.