Dr. Starr is Professor of Chemical Engineering and Assistant Director of the Additive Manufacturing Institute of Science and Technology (AMIST) in the J.B. Speed School of Engineering. He joined the University of Louisville in August 1998 and has served as Chair of the Chemical Engineering Department (2000-2004) and as Associate Dean for Research (2004-2015). Prior to joining UofL, Dr. Starr spent eighteen years at the Georgia Institute of Technology directing and managing research programs in the School of Materials Science and Engineering and in the Georgia Tech Research Institute. Dr. Starr earned a Ph.D. in Physical Chemistry from the University of Louisville and a B.S. in Chemistry from the University of Detroit.
- Ph.D. in Physical Chemistry, University of Louisville, 1976
- B.S. in Chemistry, University of Detroit, 1970
Selective laser melting (SLM) processed stainless steel usually exhibits an inhomogeneous microstructure in the as-built condition. The effect of powder chemical composition on the microstructural evolution of SLM processed 17-4 PH in the as-built condition was studied. A path to achieve a fully martensitic 17-4 PH component in the as-built condition by fine-tuning the alloy composition without any post-built heat treatments was demonstrated. The as-built 17-4 PH phase transformation from δ ferrite to austenite (γ) and subsequently to martensite (α’) was governed by the concentrations of ferrite and austenite stabilizing elements as represented by a chromium to nickel equivalent (Creq/Nieq) value. Electron backscatter diffraction (EBSD) analysis revealed that increase in the WRC-1992 equations based Creq/Nieq value to ≥ 2.65 resulted in coarse δ ferrite grains with a <100> preferential crystal orientation along the build direction. Epitaxial growth of semi-circular and columnar δ ferrite grains accompanied by a marginal volume fraction of retained austenite and transformed martensitic phases was observed. Retained austenite and transformed martensitic phases exhibited a fine grain structure preferentially along the coarse ferrite grain boundaries. Decreasing the Creq/Nieq value to 2.36 induced δ ferrite grain refinement with a significant amount of transformed martensite in the as-built condition. EBSD phase composition analysis along with thermodynamic equilibrium modeling implies that a lower Creq/Nieq value promotes martensite formation resulting in a less retained δ ferrite in the as-built condition.