Application of Surface Severe Plastic Deformation to α+β and β Titanium Alloys for Microstructure Modification

David A Brice, Purdue University

Abstract

Grain refinement of metallic materials has become a popular topic for investigation because decreasing grain size improves mechanical properties; however, the production of large pieces/parts with refined grain structures is problematic. Early attempts to produce fine grained materials relied on gas condensation, or milling processes that produced nanocrystalline materials. These processes produced small specimens that had nanocrystalline structures, but exhibited decreased ductility. The use of severe plastic deformation processes, namely ECAP, have produced ultra fine grained materials; however, the uniform grain sizes still decrease overall ductility of the piece produced. Recently, surface deformation processes have produced localized refinement of the surface in metallic materials, and reported an increase in strength along with ductility. This study focuses on the use of shot peening(SP) as a severe plastic deformation and its application to Ti based alloys, and has varied goals. Firstly, this project has used SP as a deformation process that coupled with a post SP heat-treatment resulted in clear and consistent grain refinement in a variety of titanium alloys. The alloys used for this investigation were selected to represent classes of titanium alloys of interest to several industries, and more importantly to contrast the capabilities of lesser known alloys with Ti-64. Application of SP produced clear grain refinement of surface layers that measured several microns from the surface. Significant hardening of the surface was measured as a result of the layers produced. Other effects from the processing, such as precipitation of α-Ti in β-alloys, were examined and shown to be influenced by elastic residual stresses. Also, the corrosion resistance of the layers was measured through cyclic potentiondynamic polarization under conditions that mimic biological environment. Proof of concept for self-peening is reported in this study. Self-peening is the practice of performing SP with shot media composed of the same (or similar) material as the part that is being treated. Production of shot media was successfully performed through the exploitation of titanium oxidation. Evidence is provided that clearly shows decreased surface contamination when using the Ti-based shot media to shot peen Ti alloys. Additionally the surface hardening and residual stress imparted by the different shot media , while being used with identical shot peening conditions were compared.

Degree

Ph.D.

Advisors

Trumble, Purdue University.

Subject Area

Aerospace engineering|Materials science|Mechanics|Surgery

COinS