A Study Of Thermal Stability Of Residual Stresses And Fatigue Life Of Laser Shock Peened Ti 6al 2sn 4zr 2mo Alloy

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/cm 2) (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.

Laser shock peening (LSP) is a novel surface treatment process that generates deep compressive residual stresses and microstructural changes and thereby dramatically improves fatigue strength of critical metal aircraft engine parts. In the past, researchers have evaluated the mechanical effects of LSP experimentally through residual strain/stress measurements, microhardness measurements or fatigue life improvement. A number of microstructure characterizations have been done on variety laser shock peened materials. However, getting better view of how LSP brings about changes in the microstructure and establish quantitative relations between LSP parameters and residual strain/stress distributions, microstructure and texture evolution is still challenging. The present study was undertaken to develop a basic understanding of the effects of LSP on the residual strain/stress distributions, texture evolution and deformation microstructural changes in Ti-6Al-4V alloy. Scanning Electron Microscopy, Scanning Probe Microscopy, Conventional X-ray Diffraction, Synchrotron X-ray Diffraction, Electron BackScattered Diffraction, microhardness and nanoindentation have been used to characterize the laser shock peened Ti-6Al-4V alloy samples. The microstructure and surface modification of laser shock peened sample are outlined in terms of laser shock peening processing parameters. Naked laser peened samples show prominent evidence of surface melting and recasting. Little difference between the peened and virgin materials can be found in the taped laser peened samples surface microstructures. Depth-resolved characterization of the residual strains and stresses was achieved using high-energy synchrotron X-ray diffraction as well as by conventional X-ray diffraction. Compressive residual strain at peened surface and tensile residual strain in the interior of the sample are found in taped samples. Naked LSP-treated samples show tensile residual stresses at peened surfaces, then dramatically change to compressive within short depth. Multiple diffraction peaks in the synchrotron X-ray diffraction patterns were used to analyze the residual elastic strain and plastic strain distributions in the LSP-treated Ti-6Al-4V samples. Anisotropic elastic lattice strain response in the hexagonal close-packed alpha titanium was revealed by Williamson-Hall plots of the peak broadening data. The depth profiles of mean diffraction ring width in synchrotron X-ray diffraction and FWHM in conventional X-ray diffraction give evidence of anisotropic plastic strains in the laser peened Ti-6Al-4V samples. Furthermore, using the whole pattern fitting method the Structure-Texture-Microstructure-Phase-Stresss combined analysis was performed based on the synchrotron diffraction data. The evolution of maximum pole intensity values from surface to interior proves that laser shock peening can change the texture in the laser peened samples. The near-surface and through-the-depth changes in strain/stress, texture and microstructure in samples were correlated with the laser processing energy levels applied on the samples. Residual stress relaxation in LSP-treated Ti-6Al-4V alloy due to the sample sectioning was also studied using SXRD and CXRD and was found to be significant to small section widths (to about 8 mm), but not as significant at larger widths, though the sectioning was found to introduce complex gradients. Finally, the local property changes were examined using microhardness and nanoindentation and near-surface hardening due to LSP treatment was noted and related to the plastic strain generated by the process.

Laser shock processing (LSP) is a new and promising surface treatment technique for improving the fatigue durability, corrosion, wear resistance and other mechanical properties of metals and alloys. During LSP, the generated shock wave can introduce a deep compressive residual stress into the material, due to its high-pressure (GPa-TPa), ultra-fast (several tens nanoseconds), ultra-high strain-rate and high-energy. The overall properties and behavior of metal materials subjected to LSP were significantly improved because a refined surface layer was successfully obtained. Nevertheless, up to now, a clear scenery between micro-structure and macro-property of the refined surface layer, especially formation of sub-micrometer grains from coarse grains during severe plastic deformation, is still pending. Therefore, the basic studies of the underlying mechanism for grain refinement by ultra-high strain-rate presented in this book becomes more and more crucial.

Laser shock peening (LSP) for improving fatigue life of IN718Plus superalloy is investigated. Fatigue geometry and LSP parameters were optimized using finite element method (FEM). Residual stress distributions estimated by FEM were validated using Synchrotron XRD and line focus lab XRD, and correlated with microhardness. An eigenstrain analysis of LSP induced edge deflections (measured with optical interferometry) was also conducted. Transmission electron microscopy (TEM) of single-spot LSP coupons shows sudden increase in dislocation density under LSP treated region. Total life fatigue was conducted at R=0.1 at 298K and 923K, with and without LSP. S-N curve endurance limit increases at both temperatures with FEM optimized LSP samples. Based on TEM of fatigue microstructure and LSP coupons, mechanistic description of observed fatigue improvement is attempted. Often need arises to weld components, and weld heat-affected-zone reaches near-solvus temperatures. To simulate this treatment, sub-solvus hot-rolled IN718Plus is aged at 923K. We observe precipitation of thin eta-Ni3(Al, Ti) plates after 1000 hours, making the material susceptible to cracks, and lowering fatigue life. Effect of LSP on fatigue crack growth (FCG) is studied following ASTM guidelines on M(T) geometry at R=0.1. Acceleration in FCG rate with LSP is observed for this geometry and LSP condition. Prior FEM optimization was not conducted for FCG tests, and may account for lower FCG resistance after LSP. FCG results were corroborated with COD compliance based analysis. Crack measurements were done using potential drop method, and crack closure was analyzed. Effect of LSP on overload FCG was investigated by single-cycle 100% overload followed by single-spot LSP on the crack-tip plastic zone. Crack retardation occurs after application of overload+LSP. Effective contribution of overload+LSP to crack retardation is estimated. Fractographic analysis of LSP treated fatigue samples suggests sub-surface crack nucleation, and is analyzed based on stress concentration behavior of small cracks.

There is a strong economic motivation of the aircraft industry to explore novel residual stress-based approaches for the fatigue life extension, repair, and maintenance of the growing fleet of ageing aircrafts, although the effect of residual stresses is not taken into account by the established damage tolerance evaluation methods. Laser shock peening - the most promising life enhancement technique - has already demonstrated great success in regard to the mitigation of fatigue crack growth via deep compressive residual stresses. However, no comprehensive model exists which allows the prediction of generated residual stress fields depending on the laser peening parameters. Furthermore, the hole drilling method - a well-established technique for determining non-uniform residual stresses in metallic structures - is based on measuring strain relaxations at the material surface caused by the stress redistribution while drilling the hole. However, the hole drilling method assumes linear elastic material behavior and therefore, when measuring high residual stresses approaching the material yield strength, plastic deformation occurs, which in turn leads to errors in stress determination. In the light of these two points, the present work aims to optimize the laser shock peening process in regard to high residual stress profiles, their correct measurement by the hole drilling method and demonstration of the fatigue crack growth retardation through the laser peening treatment on the laboratory scale. First, the methodology for the correction of the residual stresses approaching the material yield strength when measuring by the hole drilling is established and experimentally validated. The correction methodology utilizes FE modelling and artificial neural networks. In contrast to the recent studies, the novelty of this methodology lies in the practical and elegant way to correct any non-uniform stress profile for a wide range of stress levels and material behaviors typically used in industrial applications. Therefore, this correction methodology can be applied in industry without changing the procedure of hole drilling measurement. Second, the laser shock peening process is optimized in regard to the generated residual stress profiles using design of experiments techniques. The strategy involves laser peening treatment with different parameters and subsequent measurement of induced residual stress profiles through hole drilling. The measured stress profiles are subjected to correction using the neural network methodology. After that the regression model is fitted into the experimental data in order to find the relationship between the laser peening parameters and the stress profiles' shapes. In the final stage, it is experimentally demonstrated that the established regression model provides an accurate prediction of the residual stress profile when using defined laser peening parameters and vice versa. Third, the regression model obtained in the design of experiments study is used for generating the desired residual stresses in the C(T)50 AA2024-T3 specimens for the fatigue crack propagation test. Significant retardation of the fatigue crack propagation of specimens due to the presence of deep compressive residual stresses is experimentally demonstrated on the laboratory scale.

This work aims to develop a procedure to simulate laser shock peening treatments more efficiently, and to characterize the major differences in laser peening effects for cast and additively manufactured (selective-laser-melted) metallic specimens fabricated from A357 aluminum alloy. In addition, residual stresses (RS) are to be predicted probabilistically as a random field, allowing rigorous determination of RS values for a desired reliability. Laser shock peening (LSP) is a surface treatment technique that induces compressive RS near the surface of target metal components to improve fatigue life. Developing an LSP process using physical experiments is very expensive and time-consuming. To address this issue, finite element methods (FEM) have been widely used to simulate the LSP process and predict RS. Conventionally, almost all material constitutive models used in LSP prediction of RS involve deterministic parameters. Therefore, the predicted RS profiles do not reflect real-world variations in the material or uncertainties in the LSP process. Moreover, prediction of RS as a random field has not been done. While the effect of LSP on cast alloys has been studied extensively, few researchers have investigated the effects of LSP on metallic specimens produced by additive manufacturing processes such as selective laser melting (SLM). Therefore, the objectives of this research are: (1) Develop a procedure to simulate the LSP process with reduced computational time; (2) Conduct experimental and numerical studies to understand the effects of LSP on SLM A357 aluminum alloy; (3) Create a probabilistic approach to quantify the material constitutive model parameters as a joint probability distribution of correlated random variables; and (4) Demonstrate a technique to efficiently generate stochastic maps of the resulting RS random fields, enabling improved reliability analysis for desired RS values. To increase LSP simulation speed, a new systematic procedure is developed using modal analysis and generalized variable damping profiles with the “single explicit analysis using time dependent damping” (SEATD) FEM approach. To begin understanding the effects of LSP on A357 aluminum alloy specimens produced by SLM, true-stress-strain curves of both as-built (AB) and laser shock peened SLM samples are obtained through transverse tensile tests. An initial hypothesis on the effects of LSP during tension testing is formulated and subsequently tested using SEATD approach. To quantify the plasticity-Johnson-Cook (J-C) material model parameters as a joint probability distribution of correlated random variables for heat-treated (HT) and as-built (AB) SLM A357, the Bayesian inference (BI) probabilistic approach is utilized. Also proposed in this work are two BI-quantified-techniques called, respectively, the Multidimensional-BI method and the Spatial-Posterior-Prior-Probability-Mass-Function (SPP-PMF) method. Both can be used to efficiently predict RS as a random field, thus providing far greater insight into the practical ability to attain desired RS. For identical LSP treatments, it is determined that the material models are significantly different for the SLM and the conventional cast A357 aluminum alloys, resulting in much lower overall magnitude of compressive RS in the SLM-alloy. In addition, stochastic maps of the resulting random stress fields for LSP treatments on specific SLM A357 components are generated using the approach described herein.

The surface of any component that undergoes mechanical loading is particularly important in influencing the fatigue life of that part. The factors that contribute to surface fatigue sensitivity are less slip restriction at the surface, the existence of single edge cracks, the surface sees large tensile gradients when the part is exposed to bending and torsion, the surface tends to be an area of multiple defects and the surface is the most sensitive region to environmental effects such as corrosion. Compressive residual stresses are utilized in the turbine engine environment to counteract regions on the surface that must withstand high tensile stresses.

The objective of the program was to demonstrate that laser shock processing is a viable method of improving the fatigue and crack growth performance of mechanically fastened joints. It was shown that a decrease in crack growth rate can be achieved under specified conditions. These conditions involve compressive residual stress (induced by the process) which modify the crack shape and reduce the stress intensity factor. 2024-T3 aluminum alloy reacted better to the process than 7075-T6 aluminum. Results were better in thin (.125 inch) than thick (.250 inch) material. Initial design environmental and cost studies indicate that a laser shock processing system for use on a production line is feasible.