Laser shock peening (LSP) has been successfully applied to improve fatigue performance of metallic components. The beneficial effects of LSP are primarily due to near-surface compressive residual stresses which retard fatigue-crack initiation, and near-surface work-hardening which retards fatigue-crack initiation by limiting plastic deformation.

Shot peening at higher temperature increases the fatigue strength. Warm shot peening has been developed to improve the stability of the residual stress and fatigue strength. In addition, warm deep rolling has shown better fatigue performance than room temperature deep rolling both in steel and aluminum alloys.

Compared with shot peening and deep rolling, LSP can produce deeper and larger compressive residual stress, and stronger work hardened surface. The treated surface of the component in LSP has lower surface roughness, which reduces crack initiation at the surface. Moreover, LSP is a flexible manufacturing process, and not limited to sample geometric restriction.

During LSP, a laser pulse penetrates through the confining media and irradiates on the ablative coating material. The ablative coating material is instantly ionized to plasma, the expansion of which leads to the shock wave propagation into the target material and the confining media. When the shock wave pressure exceed the Hugoniot elastic limit (HEL) of the material, plastic deformation occurs, resulting in work hardening and compressive residual stress near the surface. The LSP process at elevated temperatures – so-called warm laser shock peening (WLSP) – results in even better performance.

Dynamic aging (DA) takes places in WLSP because thermal energy is supplied during the high strain rate forming of aluminum alloy to overcome the energy barrier to form nanoscale precipitates. The ultrahigh strain rate results in high density dislocation, which provides a large number of nucleation sites for precipitation. These nucleated nanoscale precipitates in turn interact with dislocations. This interaction results in high density dislocation arrangement and nano-precipitates during the peening process.

LSP leads to an average hardness increase of 27%. Whereas WLSP shows an average increase of 59%. While both LSP and WLSP benefits from strengthening by formation of dislocation and a submicron/ nano-grain structure after deformation, only WLSP can generate high density nanoscale precipitates through DA. Therefore, this significant increase of surface strength after WLSP is mainly due to the formation of nanoscale precipitation as a result of DA.

The surface roughness increases with laser intensity for both LSP and WLSP but WLSP results in lower surface roughness and also leads to higher stability of mechanical properties than LSP. The increased residual stress stability after WLSP contributes to the increase in fatigue life. Similar stability increase has been found in another warm peening process.

In the near future, this innovative material processing might be used in many age-hardenable materials for improvement in fatigue resistance, and reliable mechanical properties at high temperature.

Chang Ye, Yiliang Liao and Gary J. Cheng ; DOI: 10.1002/adem.200900290