College of Engineering
Materials Engineering Department
BS in Materials Engineering
Inconel 718 (IN718) is a popular wrought superalloy, and is currently being investigated for additive manufacturing (AM) applications in the aerospace industry. However, overaging and the presence of microcracks have caused a significant reduction in properties. The purpose of this study is to meet or exceed the mechanical properties of wrought IN718 by varying the composition and build direction of the AM alloy. Alternative compositions were selected with Oerlilon Metco’s Rapid Alloy Development (RAD) software, and differ in niobium content, which increases the fraction of the primary strengthening phase (γʺ). Direct metal deposition (DMD) was used to fabricate the samples, which then underwent a heat treatment to precipitate γʺ. Tensile testing, metallography, and scanning electron microscopy (SEM) were performed on the samples. Tensile testing found that the AM samples could exceed wrought strength with the appropriate composition and build direction. The horizontal build containing the highest niobium percentage achieved an average yield strength of 1400 MPa, higher than the 1218 MPa for the wrought. Samples in the horizontal build direction were consistently stronger than vertical alternatives due to their anisotropic grain morphology. Despite comparable strength, AM samples of all compositions had significantly reduced ductility with an average range of 1-3% elongation compared to an average of 21% for the wrought samples. Microstructural analysis revealed dendritic structures and cracks between print layers in the AM samples, which contributed to this reduction in ductility.
Available for download on Tuesday, June 11, 2019