Optimization Of Laser Shock Peening Parameters For Elimination Of Residual Stress In Additive Manufacturing

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 book highlights the fundamentals and latest progresses in the research and applications of laser shock peening (LSP). As a novel technology for surface treatment, LSP greatly improves the resistance of metallic materials to fatigue and corrosion. The book presents the mechanisms, techniques, and applications of LSP in a systematic way. It discusses a series of new progresses in fatigue performance improvement of metal parts with LSP. It also introduces lasers, equipment, and techniques of newly developed industry LSP, with a detailed description of the novel LSP blisk. The book demonstrates in details numerical analysis and simulation techniques and illustrates process stability control, quality control, and analysis determination techniques. It is a valuable reference for scientists, engineers, and students in the fields of laser science, materials science, astronautics, and aeronautics who seek to understand, develop, and optimize LSP processes.

Laser shock peening (LSP) is a process for inducing compressive residual stresses using shock waves generated by laser pulses. It is a relatively new surface treatment for metallic materials that can greatly improve their resistance to crack initiation and propagation brought on by cyclic loading and fatigue. This book, the first of its kind, consolidates the scattered knowledge about LSP into one comprehensive volume. It describes the mechanisms of LSP and its substantial role in improving fatigue performance in terms of modification of microstructure, surface morphology, hardness, and strength. In particular, it describes numerical simulation techniques and procedures that can be adopted by engineers and research scientists to design, evaluate, and optimize LSP processes in practical applications.

This book introduces the fundamentals and principles of laser shock peening (LSP) for aeronautical materials. It focuses on the innovation in both theory and method related to LSP-induced gradient structures in titanium alloys and Ni-based alloys which have been commonly used in aircraft industries. The main contents of the book include: the characteristics of laser shock wave, the formation mechanism of gradient structures and the strengthening-toughing mechanism by gradient structures. The research has accumulated a large amount of experimental data, which has proven the significant effectiveness of LSP on the improvement of the fatigue performance of metal parts, and related findings have been successfully applied in aerospace field. This book could be used by the researchers who work in the field of LSP, mechanical strength, machine manufacturing and surface engineering, as well as who major in laser shock wave and materials science.

Laser peening (LP) is a surface enhancement technique that has been applied to improve fatigue and corrosion properties of metals. The ability to use a high energy laser pulse to generate shock waves, inducing a compressive residual stress field in metallic materials, has applications in multiple fields such as turbomachinery, airframe structures, and medical appliances. In the past, researchers have investigated the effects of LP parameters experimentally and performed a limited number of simulations on simple geometries. However, monitoring the dynamic, intricate relationships of peened materials experimentally is time consuming, expensive, and challenging. With increasing applications of LP on complex geometries, these limited experimental and simulation capabilities are not sufficient for an effective LP process design. Due to high speed, dynamic process parameters, it is difficult to achieve a consistent residual stress field in each treatment and constrain detrimental effects. With increased computer speed as well as increased sophistication in non-linear finite element analysis software, it is now possible to develop simulations that can consider several LP parameters. In this research, a finite element simulation capability of the LP process is developed. These simulations are validated with the available experimental results. Based on the validated model, simplifications to complex models are developed. These models include quarter symmetric 3D model, a cylindrical coupon, a parametric plate, and a bending coupon model. The developed models can perform simulations incorporating the LP process parameters, such as pressure pulse properties, spot properties, number of shots, locations, sequences, overlapping configurations, and complex geometries. These models are employed in parametric investigations and residual stress profile optimization at single and multiple locations. In parametric investigations, quarter symmetric 3D model is used to investigate temporal variations of pressure pulse, pressure magnitude, and shot shape and size. The LP optimization problem is divided into two parts: single and multiple locations peening optimization. The single-location peening optimization problems have mixed design variables and multiple optimal solutions. In the optimization literature, many researchers have solved problems involving mixed variables or multiple optima, but it is difficult to find multiple solutions for mixed-variable problems. A mixed-variable Niche Particle Swarm Optimization (MNPSO) is proposed that incorporates a mixed-variable handling technique and a niching technique to solve the problem. Designing an optimal residual stress profile for multiple-location peening is a challenging task due to the computational cost and the nonlinear behavior of LP. A Progressive Multifidelity Optimization Strategy (PMOS) is proposed to solve the problem. The three-stage PMOS, combines low- and high- fidelity simulations and respective surrogate models and a mixed-variable handling strategy. This strategy employs comparatively low computational-intensity models in the first two stages to locate the design space that may contain the optimal solution. The third stage employs high fidelity simulation and surrogate models to determine the optimal solution. The overall objective of this research is to employ finite element simulations and effective optimization techniques to achieve optimal residual stress fields.

METAL ADDITIVE MANUFACTURING A comprehensive review of additive manufacturing processes for metallic structures Additive Manufacturing (AM)—also commonly referred to as 3D printing—builds three-dimensional objects by adding materials layer by layer. Recent years have seen unprecedented investment in additive manufacturing research and development by governments and corporations worldwide. This technology has the potential to replace many conventional manufacturing processes, enable the development of new industry practices, and transform the entire manufacturing enterprise. Metal Additive Manufacturing provides an up-to-date review of all essential physics of metal additive manufacturing techniques with emphasis on both laser-based and non-laser-based additive manufacturing processes. This comprehensive volume covers fundamental processes and equipment, governing physics and modelling, design and topology optimization, and more. The text adresses introductory, intermediate, and advanced topics ranging from basic additive manufacturing process classification to practical and material design aspects of additive manufacturability. Written by a panel of expert authors in the field, this authoritative resource: Provides a thorough analysis of AM processes and their theoretical foundations Explains the classification, advantages, and applications of AM processes Describes the equipment required for different AM processes for metallic structures, including laser technologies, positioning devices, feeder and spreader mechanisms, and CAD software Discusses the opportunities, challenges, and current and emerging trends within the field Covers practical considerations, including design for AM, safety, quality assurance, automation, and real-time control of AM processes Includes illustrative cases studies and numerous figures and tables Featuring material drawn from the lead author’s research and professional experience on laser additive manufacturing, Metal Additive Manufacturing is an important source for manufacturing professionals, research and development engineers in the additive industry, and students and researchers involved in mechanical, mechatronics, automatic control, and materials engineering and science.

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.

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.