EXPERIMENTAL CHARACTERIZATION AND MODELING OF MULTIAXIAL PLASTICITY BEHAVIOR OF 304L AUSTENITIC STAINLESS STEEL 304L PRODUCED BY ADDITIVE MANUFACTURING

Open Access
Author:
Wang, Zhuqing
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 02, 2018
Committee Members:
  • Allison Michelle Beese, Dissertation Advisor
  • Allison Michelle Beese, Committee Chair
  • Tarasankar Debroy, Committee Member
  • Hojong Kim, Committee Member
  • Robert Carl Voigt, Outside Member
Keywords:
  • Additive manufacturing;
  • Austenitic stainless steel
  • Martensitic transformation
  • Multiaxial stress state
  • Constitutive modeling
Abstract:
In additive manufacturing of metallic alloys, near-net shape 3D components are built in a layer-by-layer fashion. Austenitic stainless steels have high strength and ductility, as they tend to undergo a strain-induced martensitic phase transformation with plastic deformation. The thesis focuses on quantifying process-microstructure-multiaxial mechanical property relationships in additively manufactured 304L austenitic stainless steel (SS304L) and developing a physically-based plasticity model for this material that relates microstructural phase transformation to macroscopic mechanical properties. The effect of processing parameters on microstructure and mechanical properties was studied using pure SS304L walls. A grain growth model was used to describe austenite grain size as a function of processing parameters and location. A Hall-Petch relationship was used to explain the effect of austenite grain size and morphology on yield strength. The effects of chemistry, stress state, and texture on martensitic phase transformation were investigated using walls made using a mixture of SS304L powder and iron powder. As the concentration of elements that increase the stacking fault energy of austenite decreased, the austenite stability decreased, and the propensity for martensitic transformation increased. Multiaxial mechanical tests, including uniaxial tension, uniaxial compression, pure shear, and combined tension and shear, were performed on the material. As the primary texture resulted in a higher driving force for martensitic transformation under uniaxial compression than uniaxial tension, the rate of phase transformation was higher under uniaxial compression, which contradicted the trend in texture-free materials. A macroscopic plasticity model is proposed to describe the multiaxial plasticity behavior for the material. This model makes use of a chemistry-, stress state-, and texture-dependent martensitic transformation kinetics equation to incorporate the effect of martensitic transformation on mechanical properties. The plasticity model was implemented into a finite element code, and calibrated and validated using experimental data. The good agreement between simulation and experimental results under the stress states studied indicates the model is able to describe and predict the multiaxial mechanical behavior of additively manufactured SS304L. The results in this thesis work enable the use of additively manufactured stainless steels in structural applications, as it provides quantitative links among processing, structure, and mechanical behavior.