Shot Peening and Roller-Burnishing to Improve Fatigue Resistance of the (a+B) Titanium Alloy Ti-6A1-4V

Author:  Kocan, Ostertag and Wagner
Source:  Conf Proc: ICSP-8 Sept. 16-20, 2002 Garmisch-Partenkirchen, Germany
Doc ID:  2002060
Year of Publication:  2002
Authors Marcin Kocan 1, Alfred Ostertag 2 and Lothar Wagner 1 1 Chair of Physical Metallurgy and Materials Technology, Technical University of Brandenburg at Cottbus, Cottbus, Germany 2 Ecoroll AG, Celle, Germany Introduction It has long been recognized that mechnical surface treatments such as shot peening or roller-burnishing can significantly increase the fatigue performance of structural components. Regarding the application of light-weight alloys, it is known that titanium and magnesium alloys as opposed to aluminum alloys can respond quite critically to a shot peening treatment. For example, a very marked over-peening effect was observed on the high-strength magnesium alloy AZ80 [1,2], i.e., the fatigue life as a function of Almen intensity first dramatically increased compared to an electropolished reference followed by a drastic drop as the intensity increased. This sensitivity was attributed to the limited deformability by slip of the hexagonal magnesium crystal structure. The response of titanium alloys to shot peening is reported to strongly depend on many factors, as alloy class (a, (a+B) and metastable B) and its cyclic deformation behavior which in turn determines the cyclic stability of the process-induced residual compressive stresses. For example, metastable B alloys exhibited only slight improvements of the fatigue performance while a alloys responded much more beneficially. Further, previous work [3] has shown that the response of the (a+B) titanium alloy Ti-6A1-7Nb to shot peening and roller-burnishing was clearly related to the mean stress sensitivity of the particular microstructure and crystallographic texture. Conditions with an anomalous mean stress sensitivity [4,5] showed little improvement in fatigue performance as opposed to conditions with a normal mean stress sensitivity. The present investigation was performed on the well known (a+B) titanium alloy Ti-6A1-4V having a typical commercially available mill annealed microstructure. In order to establish optimum conditions with regard to fatigue performance, shot peening and roller-burnishing were performed using a wide variation in Almen intensity and rolling force, respectively. Additional polishing treatments were performed to reduce process-induced roughnesses and microcracks in order to find out if the fatigue behavior can be further improved.

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