This is a study of the effects of laser shock peening (LSP) on residual stress generation, thermal stability of the LSP induced residual stresses and the associated fatigue strength enhancement in the temperature range of (315°C-538°C;600°F-1000°F) the aerospace alloy Ti-6Al-2Sn-4Zr-2Mo (Ti-6242). LSP is a novel surface treatment process that has proven to enhance he fatigue strength/life of metallic components is a mechanical surface treatment process that has proven to enhance the fatigue life of components. The findings of this study are useful to assess the application of LSP process for industrial and aerospace components that operate at elevated temperatures.
Initial experiments were performed to identify the LSP process parameters (energy) range on Ti-6242 to induce the maximum compressive stress. Results showed that compressive residual stresses increased with increasing energy and remained constant at 800 Mpa, above 8J (8GW/cm2) (saturation).
Thermal relaxation studies of the LSP induced residual stresses were conducted on specimens processed at 8GW/cm2 (at 8 Joules). LSP processed samples were exposed to temperatures of 315°C (600°F), 482°C (900°F) and 538°C (1000°F) for durations of 10 minutes to 100 hours. No appreciable relaxation was observed at 315°C even after long exposures. Stresses relaxed to about half the initial value when exposed to 482°C for 24 hrs and 538°C for 1hr. This suggests that the benefits of LSP can be realized even at 538°C. Zener-Wert-Avarami analysis of the kinetics of residual stress relaxation gave an activation enthalpy of 116 KJ/mol, which is in the range reported for self diffusion in Ti-alloys. This suggests that residual stress relaxation is associated with a creep-like mechanism involving rearrangement and annihilation of dislocations by climb.
LSP simulations were successfully used to optimize LSP treatment for obtaining through-thickness compression in fatigue test coupons.
Fatigue testing in three-point ending was conducted on notched coupons LSP-treated with the optimized parameters. LSP is found to have a very beneficial effect on fatigue life; this enhancement is partially retained even after thermal exposure to temperatures as high as 482°C for 24 hours. For example, the unpeened sample reached the run out condition at a stress value of about 100 Mpa; compared to 200 Mpa and 175 Mpa for the LSP’d and LSP’d followed by heat treated (900°F) respectively. At 225 Mpa loading the LSP’d sample showed an average increase in life of 6 times compared to unpeened sample. LSP’d + heat treated (900°F) showed an increase in life by 1.34 times. At 200 Mpa loading, the LSP’d sample showed an average increase in life of 2.33 times compared to unpeened sample (estimated cycles to failure are 43,000). The LSP’d + heat treated (900°F) sample showed an increase in life by 1.46 times.
Assessment of fatigue crack growth rates from SEM-based striation spacing data in samples tested at room temperature reveal that the stress intensity range required to achieve a given crack growth rate is much higher following LSP. This reduction in crack growth rates is partially retained even after thermal exposure of the LSP-treated samples.
The above results point to the benefit of use of LSP processing to extend life of high temperature components, in this case those made of the Ti-6242 alloy.