Investigation of the effect of porosity on fatigue behavior for DMLS built parts using 3D X-ray Computed Tomography

Following recent developments in manufacturing technology, Additive Manufacturing (AM) has become an important figure in the production of complex geometries in which conventional subtractive methods are typically incapable of producing or are excessively high costs. However, AM techniques have their capabilities and limitations. One limitation that affects mechanical properties, particularly fatigue strength under cyclic loads, is unwanted material porosity causing fatigue failure. Fatigue failure, due to its nature, is random, and a reason for this stochastic nature is the uncertainty of the porosity dispersion in the AM-built part. A commonly used AM process is Laser Powder Bed Fusion (LPBF), which can print near full-density (99.5+%) parts if process parameters are set correctly. Therefore, EOSINT M 280 printer based on Direct Metal Laser Sintering (DMLS) technology, which is also an LPBF technique, was used to manufacture fatigue test specimens using 1.2709 metal powder with a chemical composition 18Ni9Co5Mo1Ti. Porosity in twenty-five fatigue specimens was analyzed using 3D X-ray Computed Tomography (XCT). Bruker SkyScan 1275 micro-CT was used to scan the samples. Fatigue tests were performed with a custom-made torsion fatigue test machine. Twenty tests have been conducted, five tests each for four different torsion angles at 5 Hz frequency and R = -1. Fatigue test results were compared with the porosity values measured by XCT in order to build a stochastic material model for predicting the fatigue lives of larger-sized objects in which scanning for porosity is not applicable.