Fatigue Enhancement with Thermal Stability

Compared to shot peening, laser peened parts retain deeper compressive residual stress profiles in high-temperature environments. Laser peening’s superior heat resistance results from the lower percentage of cold work involved, producing deep compressive residual stresses that remain stable at elevated temperatures.
Surface enhancement techniques like shot peening and laser peening rely upon plastic strain to produce beneficial compressive residual stresses within a material. Laser peening is a mechanical process that utilizes a high-energy, pulsed laser to generate a compressive stress wave at the surface of a part. The compressive stress wave induces plastic deformation as it propagates into the material, causing dislocations in the microstructure that enhance the strength and fatigue resistance of the part. Compressive residual stresses inhibit the initiation and propagation of fatigue cracks.

Thermal Stability of Laser Peening

The graph below illustrates the thermal stability of compressive residual stress profiles in laser peened titanium alloy:

The samples were exposed to different operating temperatures (450° F and 750° F) for a duration of four hours. After these heat treatments, the residual stress profiles were measured and compared to untreated specimens. The graph shows the annealed specimens retained similar residual stress profiles to a measured depth of 1 mm, demonstrating the thermal stability of laser peening benefits.

Thermal Stability of Shot Peening

A similar experiment performed on shot peened titanium alloy samples reveals contrasting results¹. 

With exposure to higher temperatures, thermal relaxation occurs and the compressive residual stress magnitudes diminish. Shot peening creates more dislocations than laser peening due to the random distribution of shot media impacts across the surface. This leaves the microstructure in a less organized state that will trend towards equilibrium when heated. This effect is detrimental for components operating at high temperatures, as they won’t retain the benefits of surface enhancement and they become susceptible to fatigue failure.
Laser peening produces compressive residual stresses with less cold work than shot peening. As a result, the microstructure of the treated part retains enhanced characteristics at higher temperatures. Laser peening is an optimal surface enhancement treatment for components that operate in high-temperature environments.

1. Xie, L., Jiang, C., Ji, V., 2011. Thermal relaxation of residual stresses in shot peened surface layer of (TiB + TiC)/Ti-6Al-4 V composite at elevated temperatures. Mater. Sci. Eng. A 528, 6478–6483.

Laser peening is superior to shot peening in many ways.  Contact LSPT to learn more.