In-Depth Residual Stresses Induced by Laser Peening
Laser peening creates a distribution of residual stresses below the surface that is much deeper than shot peening. The actual depths of the laser peening induced stresses will vary depending on the type and intensity of the processing conditions chosen and the material properties. In general, the depths will range from 0.020 to 0.040 inches (0.5 to 1 mm) deep and can be driven significantly deeper (measured up to 5mm deep). For comparison, the depths from shot peening will generally be under 0.010 inches (0.25 mm) deep. When attempting to obtain deeper residual stresses with shot peening and other surface enhancements, there is often a degradation of the surface caused by the severe impact conditions required to drive stresses inward. In addition, laser peening seldom produces the “hook” in the compressive residual stress profile just below the surface that is observed after shot peening. The “hook” is a result of highly coldworked material near the surface that fades quickly with increasing depth. The high amount of coldwork leads to instability in the residual stresses but is not a concern with laser peening. In contrast, laser peening creates the highest residual stress at the surface that gradually decreases with distance below the surface.
The deep penetration of the compressive residual stresses produced by laser shock peening is illustrated for 2024-T351 aluminum in Figure 1, where the residual stresses are still high 0.040 inches (1 mm) below the surface. Figure 1 also shows that the in-depth stress uniformity across the shock peened spot is maintained. The stress measurements were made at two points within the laser shocked peened spot: one in the center and one at the mid-radius. The good agreement between these results demonstrates that the surface dip in the residual stress in the center of the spot extends to no more than between 0.003 to 0.008 inches (0.075 to 0.2 mm) deep, and more importantly, that the in-depth stress distribution is uniform to at least the mid-radius of the spots.
Figure 1 - In-depth residual stress profiles are deep and consistent from center to at least the mid-radius of the laser shock peened spot.
The laser peening process is highly customizable to each application and the desired results. Increasing the laser shock peening intensity drives the compressive stresses deeper below the surface. Figure 2 shows this for a titanium alloy laser peened as 0.5 inch-thick plate. Lower intensity shock peening produces both a lower surface compressive stress and a shallower depth. The compressive stress extends to only about 0.010 inches (0.25 mm) below the surface. An intermediate intensity shock peening creates a higher surface compressive stress and this stress extends down to about 0.020 inches (0.5 mm). After a higher intensity peening, the compressive stresses extend down to about 0.030 to 0.040 inches (0.75 to 1 mm). The stresses after LSP generally lie in the range of -100 to -120 ksi (689 to 827 MPa) for this alloy, although higher surface stresses have been observed. Figure 2 is also representative of the response of other titanium alloys and titanium to laser peening: that being, depending on the particular alloy properties and processing conditions, the stresses may be somewhat higher or lower, and could extend deeper below the surface.
Figure 2 - In-depth residual stresses in Ti-6Al-4V at three different laser shock peening intensity levels
Titanium alloys are just one example of the many materials that benefit from laser peening. The compressive residual stresses also extend far into the nickel-base superalloy, IN718, as shown in Figure 3 for 0.5 inch-thick plate. Steels also respond well to LSP. Figure 4 shows the residual stresses in a steel used for aircraft landing gear. The compressive residual stresses extend for a significant distance below the surface and far deeper than conventional surface enhancement methods.
Figure 3 - In-depth residual stress in nickel-based super alloy IN718.
Figure 4 - In-depth residual stresses in AF 1410 steel.
The residual stresses in the preceding figures were measured in relatively thick sections, 0.25 to 0.5 inches (6.3 to 12.7 mm) thick. In thinner sections the depth of the residual stresses will be affected by the interaction of the shock waves reflecting from opposite surfaces. This is shown in Figure 5 for 0.060-inch (1.5-mm) thick 4340 steel sheet hardened to 54 RC. In this case, a higher intensity treatment does not necessarily drive the compressive residual stresses deeper into the material (since the thickness limits this), but it does increase their magnitude. It is likely that the residual stress profile shown is symmetrical around the mid-thickness at 0.030 inches (0.75 mm).
Figure 5 - Residual stress profiles in 4340 steel sheet. Mid-thickness of the sheet is 0.030 inches (0.75mm).
- A. H. Clauer, B. P. Fairand and B. A. Wilcox, “Laser Shock hardening of weld Zones in Aluminum Alloys”, Metallurgical Transactions A, 8A, 1871-1876 (December, 1977).
- A. H. Clauer and B. P. Fairand, “Interaction of Laser-Induced Stress Waves With Metals”, Applications of Lasers in Materials Processing, ed. by E. Metzbower, ASM, Metals Park, OH (1979).
- A. H. Clauer, B. P. Fairand, and J. Holbrook, “Effects of Laser-Induced Shock Waves on Metals”, Shock Waves and High Strain Rate Phenomena in Metals, ed. by M. A. Meyers and L. E. Murr, Plenum Press, New York (1981), pp. 675-702.
- A. H. Clauer, C. T. Walters, and S. C. Ford, “The Effects of Laser Shock Processing on the Fatigue Properties of 2024-T3 Aluminum”, Lasers in Materials Processing, ASM, Metals Park, OH (1983) pp. 7-22.
- T. R. Tucker and A. H. Clauer, Laser Processing of Materials, Report MCIC-83-48, Metals and Ceramics Information Center, Columbus, OH (November, 1983).
- Laser Shock Processing Increases the Fatigue Life of Metal Parts, Materials and Processing Report, 6, (6), 3 (September, 1991).