High-temperature fatigue investigation of SS316L fabricated via laser wire-directed energy deposition

Additive manufacturing (AM) techniques offer opportunities to fabricate structural components with complex geometries in automobile, maritime, and other industries, where the components may undergo dynamic loadings at elevated temperatures. Stainless steel 316L (SS316L) is a popular material in these industries because of its high toughness and ductility. It possesses excellent weldability, making it suitable for AM fabrication techniques such as laser-powder bed fusion (L-PBF) and directed energy deposition (DED). DED provides numerous advantages over L-PBF like low material waste, less fabrication time, and the presence of commercial-ready feedstock. Although low cycle fatigue (LCF) investigations of L-PBF SS316L specimens at elevated temperatures have been reported, there is a lack of understanding of the high-temperature LCF behavior of SS316L fabricated via laser wire-directed energy deposition (LW-DED). This work addresses this research gap by performing LCF investigations of LW-DED SS316L specimens at elevated temperatures. The specimens are fabricated in a MELTIO M450 system with system-specific parameters. Computed tomography (CT) is then employed to characterize the fabricated specimens, showing that the maximum porosity area is 1.3 %, and lack-of-fusion pores are dominant over gas pores in the specimens. The fatigue investigation includes four different temperatures and two strain ranges at reversed loading conditions. Besides total fatigue life, initiation lives are obtained by compliance evaluation from the load-displacement data. It is observed that the fatigue life decreases with the increase in temperature and strain range, and the initiation life is proportional to the fatigue life. The fractography analysis presents unconventional fractured surfaces where the initiation zone is found in the interior of the specimens. CT and electron back-scattered diffraction analysis further confirm that these unconventional fractured surfaces are due to lack-of-fusion pores. In summary, compared to wrought specimens, coarse columnar microstructures and lack-of-fusion pores originating from the printing process are found to deteriorate the fatigue life of LW-DED SS316L specimens.