Investigation of laser shock peening effects on residual stress state and fatigue performance of titanium alloys

Abstract

Laser shock peening can potentially enhance fatigue life of titanium components by inducing compressive residual stresses in surface layers much deeper than caused by traditional shot peening (SP). In the present study, the high cycle fatigue (HCF) performance of α Ti-alloy Ti-2.5Cu, (α + β) Ti-alloy TIMETAL 54M and the metastable β Ti-alloy TIMETAL LCB was investigated after laser shock peening without coating (LPwC). The fatigue results were interpreted by examining the changes of surface morphology, microhardness and residual stress generated in the surface layer. Furthermore, thermal stability of residual stresses in aged Ti-2.5Cu, as an example, was evaluated after annealing LPwC-treated material at various elevated temperatures and exposure times by applying a Zener-Wert-Avrami-approach. The depth profiles of residual stresses were obtained by means of synchrotron X-ray diffraction or by incremental hole drilling method. Results revealed that the HCF performance of Ti-2.5Cu and TIMETAL LCB was markedly improved after LPwC, while it was deteriorated in TIMETAL 54M. Compared to LPwC, better 10 7 fatigue strength of Ti-2.5Cu was obtained after ball-burnishing (BB). Moreover, LPwC-induced residual stresses are thermally more stable than shot peening-induced ones.

Laser shock peening without absorbent coating (LSPwC) effect on 3D surface topography and mechanical properties of 6082-T651 Al alloy

“…Similar RS results were obtained in our previous publications [11,18,29] and are consistent results after LSP in other publication [7,14,42].…”

“…In this case high energy laser pulses (E<100J) and pulse duration up to 100 ns are applied. The coating is usually formed with a black paint or Al foil prior to laser irradiation, and the remaining coating is removed after the treatment [7]. The other called laser shock peening without coating (LSPwC) was discover in 1995 [8].…”

“…1. The principle of compressive residual stress generation by LSPwC was described by Sano et al [7]; when metallic material without protective coating is irradiated underwater by laser pulses of specific characteristics the surface instantaneously evaporates through the ablative interaction. Water confines the evaporating material, which is immediately ionized and forms plasma by inverse braking radiation (bremsstrahlung).…”

Determination of polycrystal diffraction elastic constants of Ti–2.5Cu by using in situ tensile loading and synchrotron radiation

“…This alloy is used in the annealed condition as sheets, forgings and extrusions for fabricating component such as bypass ducts of gas turbine engines [1]. In order to enhance the fatigue performance of this alloy, mechanical surface treatments such as shot peening, laser shock peening or ball-burnishing can be applied [2]. This beneficial influence is often explained by two main contributions, namely surface strengthening by the process-induced high dislocation densities hindering crack nucleation at the surface and compressive residual stresses which retard microcracks growth from the surface.…”

“…Some earlier works [2][3]11] dealt with residual stress evaluation in Ti-2.5Cu alloy by using single crystal elastic constants for pure α-titanium obtained from literature values [12] which could be different from the realistic ones. Moreover, there is poor information about the elastic properties of the lattice planes (hk.l) in Ti-2.5Cu.…”