Cyclic-hardening and Fatigue Behavior of LENS Processed Metal

Pascu, A., Xue, Y. "., Smelser, N., Wang, L., Wang, P., & Horstemeyer, M. (2008). Cyclic-hardening and Fatigue Behavior of LENS Processed Metal. ASME 2008 International Mechanical Congress and Exposition (IMECE). Sheraton Boston Hotel, Boston, Massachusetts, United States: ASME.

Abstract

Laser Engineered Net Shaping (LENS™), a rapid, flexible fabrication process of metallic objects by direct metal deposition, has potential for complex prototype fabrication, small lot production, precision repair or feature addition, and tooling. The real promise of the technology relays on the process parameter optimization to manipulate material fabrication and properties by precision deposition of the material using thermal behavior controlled, layered or graded deposition of multi-materials. Cylindrical bar stocks of 316L steel, with the radius and length at 0.375"x6.0" and 0.600"x4.0", respectively, were fabricated using LENS process at two sets of processing parameters, which produced two distinct microstructures. The microstructure features were first examined using X-ray detection, optical and scanning electron microscopes. Some layered periodic microstructural features were identified. The constant amplitude, strain-controlled, strain-life tests were conducted on the specimens machined down from the stock. The fracture surfaces were further examined using SEM. The large pore was identified as the initiation site for fatigue damage incubation. Micromechanical simulations were conducted on a unit-cell (UC) that represents the typical microstructures in LENS process 316L steel. The nonlocal microplasticity and its distribution were evaluated on the UC using a internal state variable plasticity model. The microplasticity was used to evaluate the fatigue damage incubation life. The small crack growth rate was characterized as proportional to the crack tip displacement using the first-order estimation of the MSF model developed by Xue, et al. Finally, the multistage fatigue model was developed. The microstructure-fatigue property relations were quantified for the LENS processed 316L steel obtained with two processing parameters.