Fretting fatigue of laser shock peened Ti–6Al–4V
Introduction
Laser shock peening is a surface treatment method in which a high power, pulsed laser beam is used to impart compressive residual stresses on the surface. For application on metals, an absorbent coating followed by a transparent overlay (transparent to laser energy) is applied to the surface to be treated. Upon irradiating the surface with a pulsed laser, the absorbent coating rapidly vaporizes to a plasma layer and then expands, resulting in a high amplitude short duration pressure pulse; a portion of the energy propagates as a shock wave into the metal, and the use of transparent overlay significantly increase the shock wave intensity. When the pressure of the shock wave exceeds the dynamic yield strength of the metal plastic deformation occurs, imparting residual stresses to the surface. Detailed description of the LSP process [1], [2] and its effect on metal properties and microstructure [1] is available in the literature. The following describes experiments aimed at quantifying the effect of LSP on fretting fatigue behavior of Ti–6Al–4V subjected to contact loading that generates stresses similar to those occurring in gas turbine blade/disk contacts.
Section snippets
Experimental details
The specimen and pads are machined from a forged plate of alpha plus beta () processed Ti–6Al–4V alloy with 60 primary alpha and a 40 lamellar transformed beta microstructure. Fig. 1 shows the microstructure of Ti–6Al–4V alloy identifying the alpha and beta grains [3]; material properties of the alloy are listed in Table 1 [4].
The fretting contact consisted of a nominally flat specimen in contact with a flat indenter with rounded edges. Both the flat length and radius of the rounded edges
Results and discussion
Load conditions for the experiments were chosen based on available test data for untreated Ti–6Al–4V [7] that could be used as a baseline to estimate the improvement in fretting fatigue lives due to LSP. The contact and fracture surfaces were examined using optical microscopy and SEM to understand the fretting behavior of LSP treated specimens. These observations are discussed in the context of mechanics based life prediction.
Conclusions
Fretting fatigue experiments were conducted using LSP treated Ti–6Al–4V. At the stress levels studied 5-, 10- and 25-fold increase in lives for LSP treated specimens were observed when compared to untreated specimens. While LSP treatment increases fatigue life it does not mitigate the formation of fretting cracks. Metallography of the LSP treated pad surface revealed the existence of an array of contact spots and the fretting damage was distributed throughout the contact surface. Contact
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2022, Optics and Laser TechnologyCitation Excerpt :The result showed that LSP treatment provides much greater fretting fatigue resistance than traditional shot peening (SP) alone due to the much deeper compressive zone. S. Srinivasan [22] also found that compared with untreated specimens, the LSP-treated specimens could increase fatigue life obviously. However, Kevin K. Liu's [23] research results showed that the fretting fatigue life improvements for LSP were less than SP, especially when the applied stress is low.