Date on Master's Thesis/Doctoral Dissertation

5-2020

Document Type

Master's Thesis

Degree Name

M.S.

Department

Mechanical Engineering

Degree Program

Mechanical Engineering, MS

Committee Chair

Berfield, Thomas

Committee Co-Chair (if applicable)

Starr, Thomas

Committee Member

Atre, Sundar

Author's Keywords

Additive manufacturing; selective laser melting; fatigue; stainless steel; thermal treatment

Abstract

Fatigue failure is the leading source of loss in industry. In order for new means of manufacturing to move towards mainstream use a complete understanding of material and mechanical behavior must be gained. This endeavor seeks to aide in that task by observing the fatigue behavior of selective laser melting (SLM) additive manufacturing (AM) specimens and the effect of differing thermal treatment conditions for an optimized AM process. Stainless steel 17-4 PH specimens were fabricated using SLM AM and thermally treated to three conditions: as-built, solutionized and hardened, and direct hardened. These specimens were characterized for material (powder quality, density, and microstructure) and mechanical properties (tensile, hardness, and fatigue). The fatigue specimens were tested in a tension-tension (R=0.1) to observe the effect of the variable thermal treatments on that behavior. The internal and surface defects present within the AM specimens were found to be the first order limiting factor in the fatigue life for a given stress. Ultimately this resulted in scatter and a conclusion that intrinsic limitations of additive manufacturing contributed more to the fatigue performance than other typically used measurements and predictors of mechanical performance such as yield stress. Following this the thermal treatment performed on the specimens compared to as-built material led to a slight increase in fatigue life for a given stress. Currently there is no available literature discussing the effects of variable thermal treatments on fatigue behavior of SLM 17-4 PH stainless steel. There are also limited sources discussing the fatigue behavior of other AM materials and manufacturing methods. This will provide an understanding of the fatigue behavior of 17-4 PH and the intrinsic fatigue behavior of AM.

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