Fatigue Behavior and Failure Mechanisms of Direct Laser Deposited Ti-6Al-4V

Sterling, A. J., Torries, B. A., Shamsaei, N., Thompson, S.M., & Seely, D. W. (2016). Fatigue Behavior and Failure Mechanisms of Direct Laser Deposited Ti-6Al-4V. Materials Science & Engineering A. Elsevier. 655, 100-112. DOI:10.1016/j.msea.2015.12.026.

Abstract

The fatigue behavior of Ti-6Al 4V fabricated using Laser Engineered Net Shaping (LENS), a type of Direct Laser Deposition (DLD) process, is investigated in this study. This additive manufacturing (AM) method is capable of producing different microstructures than that of traditionally-manufactured, wrought Ti-6Al-4V, due to the high and non-uniform cooling rates. As a result, the mechanical properties of LENS-fabricated Ti-6Al-4V can be different than those of wrought Ti-6Al-4V. In order for DLD parts to be more widely adapted, it is important to have their mechanical properties well-characterized. Therefore, fully-reversed strain-controlled fatigue tests were conducted on a series of specimens manufactured out from wrought Ti-6Al-4V, LENS-fabricated Ti-6Al-4V, annealed LENS-fabricated Ti-6Al-4V, and heat treated LENS-fabricated Ti-6Al-4V. Scanning Electron Microscopy (SEM) was used to examine the fracture surfaces of fatigue specimens to determine failure mechanism, crack initiation site, and to observe and quantify visible defects, such as porosity. Results indicate that the fatigue lives of the LENS-fabricated specimens are shorter than those of the as-received wrought specimens. Porosity was found to be a large contributor to the shorter fatigue lives for LENS-fabricated specimens. Annealing the LENS-fabricated specimens had negligible effect on their fatigue resistance. As the porosity seems to be the main contributor to the fatigue behavior of DLD parts, it is important to optimize the manufacturing process and design parameters to ensure fully-dense components.