Laser Peening to Improve Material Strength and Hardness
Laser peening strain hardens metals and alloys by cold working the material surface layer. The data below provide hardness mesasurements for several different materials and laser peen processing conditions.
Figure 1 - Surface hardening across a laser shock peened region in 2024-T351 aluminum.
316 Stainless Steel
In this example, the surface hardness of 316 stainless steel was measured in areas that received increasing numbers of laser shots. The hardness increased continuously as additional shots were applied.
Figure 2 - Increasing surface hardness of 316 stainless steel with increasing laser shock peening intensity.
Other alloys, such as 7075 aluminum and 2024-T8 aluminum did not show the same surface hardening under these laser shock peening conditions. This reflects each material's relative strain hardening behavior, and possibly dynamic yield strength. Surface strain hardening will be greater in materials with a higher strain hardening coefficient and lower dynamic yield strength.
In thin sections, through-thickness hardening and over-all strengthening can be obtained after shocking from both sides with the split beam arrangement. This arises from two effects. One is the strain hardening of the surface layer and the other is hardening in the mid-thickness of the section from the local superposition of the shock waves from opposite surfaces. The last effect decreases with increasing thickness, becoming inconsequential in sections thicker than about 0.25 inches (6.3 mm).
Two examples of this effect are shown in Figures 3 and 4. Figure 3 shows the hardness through the thickness of a 0.1-inch (2.54 mm) thick sheet of 2024-T351 after LSP. The base hardness was 140 VHN (Vickers hardness). The hardness gradually decreased below each surface as the peak pressure of the shock wave decreased. The bump in hardness in the mid-section where the shock waves traveling in opposite directions super-imposed is clearly evident.
Figure 3 - Through-thickness hardening in 2024-T351 aluminum after split-beam laser shock peening from both sides simultaneously.
Figure 4 - The strengthening of weld and heat affected zones in aluminum alloys by laser shock peening.
Figure 4 shows the increase in tensile yield strength of weld zones in two aluminum alloys, a work-hardening alloy, 5086-H32, and an age-hardening alloy, 6061-T6, after LSP of the weld and heat affected zones. The specimens were 0.120 inches (3 mm) thick when laser shocked. After LSP the 5086 alloy regained its parent metal strength, as would be expected in a work-hardening alloy. The 6061 alloy regained about half of the strength lost by welding. In this age-hardening alloy, restoring the remainder of the strength lost would require an aging heat treatment. From this response it is clear that LSP could be used to provide localized strain hardening and strengthening in metallic parts for selected applications.
- 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).