US-South Korea Collaborative Research: Additive Manufacturing of Fatigue Resistant Materials

Auburn University, Auburn AL

Investigators

Abstract

Additive manufacturing, or 3D printing, offers the ability to fabricate customized, complex metallic parts traditionally unobtainable for a variety of applications. A paradigm shift in engineering design and product realization is thus occurring, and many industries, such as biomedical and aerospace, are poised to benefit. Some examples include (1) on-site, rapid fabrication of metallic bone implants with patient and injury-specific designs, and (2) fabrication of replacement parts in remote locations (e.g. outer space). Nonetheless, metallic parts made by current additive manufacturing methods tend to have porosity and anisotropy, features typically detrimental to part strength and fatigue resistance. Such parts cannot be used with confidence in load-bearing applications. This award supports scientific investigation that can potentially enable production of fatigue resistant metallic parts by additive manufacturing. The objectives of this research are (1) to establish relationships between microstructure properties (grain orientation, and morphology), porosity distribution (size, shape, and location), and process parameters (laser power, scanning speed, hatch spacing, and layer orientation); and (2) to understand effects of microstructure properties (grain orientation) and porosity of additively-manufactured materials on their multi-axial fatigue resistance. To achieve the first objective, continuum-scale thermophysical models will be developed and used to relate microstructure properties and porosity distribution to process parameters. These models will be experimentally validated. Ti-6Al-4V specimens will be fabricated using laser-based additive manufacturing under various process parameter combinations. The layer orientation will be altered between 0º-90º, while scanning speed, hatch spacing, and laser power will be varied within ranges prescribed from existing knowledge (e.g. published experimental data). Microstructure properties and porosity distribution of fabricated specimens will be measured using X-ray tomography (size, shape, and location of porosity), as well as optical and scanning electron microscopy (grain orientation, and morphology). To achieve the second objective, multi-axial microstructure-sensitive fatigue models based on critical plane approaches will be developed and validated by experiments. Multi-axial fatigue experiments will be conducted on fabricated specimens, using in-phase and out-of-phase discriminating load paths to exercise different critical loading planes. Fractography on the fracture surface of specimens will be performed to determine location, size, and shape of the pore(s) responsible for initiating cracks. Crack replication techniques will be employed to find the orientation of fatigue micro-cracks with respect to critical loading plane and to determine effects of anisotropic microstructure on fatigue behavior. Through the collaboration with the Korea Institute of Industrial Technology, the generated models will be tested on other additive manufacturing methods and materials.