Enhancement of Low Cycle Fatigue Performance From Tailored Microstructures Enabled by Electron Beam Melting Additive Manufacturing Technology

Abstract

Electron beam melting (EBM) additive manufacturing (AM) technology has allowed the layerwise fabrication of parts from metal powder precursor materials that are selectively melted using an electron beam. An advantage of EBM technology over conventional manufacturing processes has been the capability to change processing variables (e.g., beam current, beam speed, and beam focus) throughout part fabrication, enabling the processing of a wide variety of materials. In this research, additional scans were implemented in an attempt to promote grain coarsening through the added thermal energy. It is hypothesized that the additional energy caused coarsening of Ti6Al4V microstructure that has been shown to increase mechanical properties of asfabricated parts as well as improve surface characteristics (e.g., reduced porosity). Fatigue testing was performed on an Lbracket using a loading configuration designed to cause failure at the corner (i.e., intersection of the two members) of the bracket. Results showed 22% fatigue life improvement from Lbrackets with asfabricated conditions to Lbrackets with a graded microstructure resulting from the selective addition of thermal energy in the expected failure region. Three Lbrackets were fabricated and exposed to a triple melt cycle (compared to the standard single melt cycle) during fabrication, machined to specific dimensions, and tested. Results for fatigue performance were within ∼1% of wrought Lbrackets. The work from this research shows that new design procedures can be implemented for AM technologies that involve evaluation of stress concentration sites using finite element analysis and implementation of scanning strategies during fabrication that help improve performance by spatially adjusting thermal energy at potential failure sites or high stress regions.