Influence of Build Direction on Interface Regions in Additive Manufacturing of Multi-Material Refractory Alloys

In the pursuit of optimum performance, materials engineering seeks to design the microstructure and thus the properties of a material through the control of the material composition and processing. Functionally graded materials (FGM) are designed to incorporate location-specific material properties through compositional changes within the part. Moving toward location-specific design of material properties requires the ability to produce material gradients in three dimensions which can be accomplished through the use of additive manufacturing (AM). This research examines the composition-process-structure-property relationship of early iteration titanium (Ti) and tantalum (Ta) graded alloys built in a novel laser powder bed fusion (LPBF) process through the characterization of vertical and horizontal graded orientations. Ultimate tensile strength, fractography, and Vicker’s microhardness (HV) are used to evaluate the mechanical properties and materials characterization includes X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD). The binary Ti-Ta alloy system is of great interest to many fields of engineering including biomedical and aerospace because of the unique thermal and mechanical properties it possesses. This work discovered that the concentration of Ta required to promote full beta phase stabilization in Ti is 25% greater than what has been previously reported when used in LPBF. Understanding of AM processing effects includes influences from both the thermal behavior and machine processing strategy which impacts composition location control and influences phase evolution. Elemental segregation occurs due to incomplete mixing in the melt pool and remelting at the horizontal interfaces. It is also established that the interfaces are structurally sound, but phase transformations and resulting microstructures differed between the vertically and horizontally graded specimen from a combination of the residual stresses ensuing from the thermal effects and machine variables in the LPBF process. The ability to grade material composition in multiple directions in LPBF AM provides the capability to fabricate complex geometries with spatially varying functional and structural properties tailored to the desired application; however, as the results of this study demonstrate, the complexity of LPBF AM process results in compounded spatial variations in the material structure and resulting properties