Additively Manufactured Metals: Effect of Microstructure and Defects on Multiaxial Plasticity and Fracture Behavior

Open Access
- Author:
- Wilson-Heid, Alexander
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 25, 2021
- Committee Members:
- Allison Michelle Beese, Dissertation Advisor/Co-Advisor
Allison Michelle Beese, Committee Chair/Co-Chair
Todd Palmer, Committee Member
Guhaprasanna Manogharan, Committee Member
Jingjing Li, Outside Member
John C Mauro, Program Head/Chair - Keywords:
- Additive manufacturing
Ti-6Al-4V
Stainless steel 316L
Multiaxial plasticity and fracture
Stress state
Porosity - Abstract:
- Metal additive manufacturing (AM) processes build 3-dimensional (3D) components in a layer-by-layer fashion, which allows for the manufacturing of geometrically complex components that cannot be produced via traditional manufacturing methods, making it an attractive option in many industries. Laser powder bed fusion (L-PBF) AM is a process category of AM that involves using a focused laser to selectively melt and fuse metallic powder to make a component. A common type of defect in AM is lack-of-fusion porosity, where the newly melted powder fails to fully-fuse to the material adjacent or below due to imperfect selection of processing parameters. Understanding the mechanical behavior of material used in AM is important for the safe, reliable, and repeatable application of the technology in industry. This thesis work provides a new understanding of the static uniaxial and multiaxial plasticity and fracture properties of L-PBF Ti-6Al-4V and stainless steel 316L in two material orientations. In particular, a series of experiments over a wide range of stress states that included uniaxial tension, equibiaxial tension, plane strain tension, pure shear, and combined tension/shear were probed to characterize both the plasticity and subsequent failure behavior. Finite element method simulations were used in combination with the experiments to calibrate and validate the stress state dependent, anisotropic plasticity behavior. Fracture behavior of both alloys was found to be stress state dependent in both orientations, and the equivalent plastic strain to failure under the wide range of stress states studied was accurately captured using calibrated ductile fracture criteria, namely the modified Mohr-Coulomb and Hosford-Coulomb models. Hypothesized microstructural driven shear softening in 316L shear dominated experiments was captured in simulations by adopting shear damage criterion in conjunction with the anisotropic plasticity model. Furthermore, this thesis for the first time characterized the effect of penny-shaped defects of varying size on the deformation and failure response of L-PBF 316L under uniaxial tension and three high stress triaxiality stress states. Pores were intentionally introduced using the unique capability of AM to embed enclosed defects at the center of samples. As a result, the 316L material was found to be defect tolerant under uniaxial tension; where the pore did not impact material ductility until the pore was 9% of the sample cross-sectional area. Strain to failure was stress state dependent until a large pore size occupying 4% of the sample cross-sectional area was introduced and then failure became pore size dependent and stress state independent.