Nathaniel Jay Lies
Advisor: Prof. Aaron P. Stebner

will defend a doctoral thesis entitled,

Suppressing Intergranular Cracking in Blown Powder Laser Beam Directed Energy Deposition Additive Manufacturing of Molybdenum

On

Thursday, July 24th at 01:30 p.m.

GTMI 114

or

On Teams:

Meeting ID: 213 063 553 176 4

Passcode: qV32ZF7L

Committee
            Prof. Aaron P. Stebner – School of Materials Science and Engineering  (advisor)
            Prof. Arun Gokhale –  School of Materials Science and Engineering
            Prof. Josh Kacher – School of Materials Science and Engineering

 Prof. Richard W. Neu – George W. Woodruff School of Mechanical Engineering

         Dr. Daniel Driemeyer – The Boeing Company

 Dr. Ronald D. Noebe – NASA Glenn Research Center

Abstract

    Molybdenum (Mo) alloys have long been limited in their manufacturability by low room temperature ductility and oxidation concerns. Despite these issues, Mo has found valuable application in several industries requiring high temperature structural materials such as aerospace, nuclear power, and metalworking. The geometric freedom and atmospheric control of blown powder laser beam directed energy deposition additive manufacturing (DED-LB-BP) makes it an attractive choice for expanding applications of Mo alloys.

    This thesis work utilizes metallurgical insight to enable DED-LB-BP of Mo by: (1) Designing a fixture to provide repeatable solidification for comparison of alloy properties. (2) Through use of the fixture, quantitatively shows deformation twinning in molybdenum alloys with dilute rhenium additions, explaining a decades-old discussion about the efficacy of these additions as a ductilizing agent. (3) Identifying coherent thermal expansion and contraction as the cause of intergranular cracking and stochastic fluctuations in the density of pure Mo deposits, frustrating statistical determination of a processing zone where >99% density pure Mo deposits were achieved in DED-LB-BP for the first time. (4) Evaluation of three different alloying approaches to reduce intergranular cracking, reduce stochastic density fluctuation, and thereby enable the development of stable, repeatable deposition of Mo-base alloys. Of the evaluated methods, grain refinement and solid solution strengthening are shown to be the most promising candidates.

    Altogether, this work provides valuable insight into Mo alloys themselves as well as directions for development of new Mo alloys to enable DED-LB-BP of high temperature, complex geometry components.