Constitutive Modeling Of Laser Shock Peening On Additive Manufactured Ti 6al 4v

Laser shock peening (LSP) is a mechanical surface treatment that induces plastic deformation and compressive residual stress in the material similar to shot peening. Pressure generated by the pulsed laser is far above the yield stress of the material and pulse duration is on the order of nanosecond. Finite element analysis is applied to simulate LSP and predict plastic deformation and residual stress through different constitutive models. The development of analytical model including model details, loading time history and constitutive model equations and parameters is discussed. Finally, results are presented for different constitutive models and experiment results are discussed.

Fatigue in Additive Manufactured Metals provides a brief overview of the fundamental mechanics involved in metal fatigue and fracture, assesses the unique properties of additive manufactured metals, and provides an in-depth exploration of how and why fatigue occurs in additive manufactured metals. Additional sections cover solutions for preventing it, best-practice design methods, and more. The book recommends cutting-edge evidence-based approaches for designing longer lasting additive manufactured metals, discusses the latest trends in the field and the various aspects of low cycle fatigue, and looks at both post-treatment and manufacturing process-based solutions. By providing international standards and testing procedures of additive manufactured metal parts and discussing the environmental impacts of additive manufacturing of metals and outlining simulation and modeling scenarios, this book is an ideal resource for users in industry. Discusses the underlying mechanisms controlling the fatigue behavior of additive manufactured metal components as well as how to improve the fatigue life of these components via both manufacturing processes and post-processing Studies the variability of properties in additive manufactured metals, the effects of different process conditions on mechanical reliability, probabilistic versus deterministic aspects, and more Outlines nondestructive failure analysis techniques and highlights the effects of unique microstructural characteristics on fatigue in additive manufactured metals

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 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.

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.

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.

Mots-clés de l'auteur: Selective Laser Melting ; Laser Shock Peening ; 3D LSP ; residual stresses ; microstructure ; recrystallization ; microhardness ; distortion ; fatigue life ; crack density.

"Additive manufacturing (AM) is an attractive solution to reduce processing cost of complex components. The interest of the aerospace industry into Ti-6Al-4V resides in its excellent strength to weight ratio and corrosion resistance properties. As of today, control of the process to structure to property relationship for AM technologies remains challenging. This research investigates the effects of some key laser wire deposition (LWD) parameters and various post deposition heat treatments on the developed microstructure and subsequent properties of the deposited Ti-6Al-4V materials. The effect of two different travel speeds on the structural development of thin Ti-6Al-4V deposits was investigated. A travel speed set at 1.4 mm/s promoted recrystallization of columnar prior [beta] grains into horizontal prior [beta] grains and a diffusion-controlled type of microstructure. A travel speed set at 7.2 mm/s resulted in higher cooling rates that produced a refined microstructure while no recrystallization of the prior [beta] grains has been observed. Next the effect of five different post deposition heat treatments were investigated. A stress relief preserved the morphology of the deposited samples while annealing or HIP, followed or not by aging, coarsened the developed microstructure. The lower travel speed exhibited a strong anisotropy in elongation with lower values generated along the build direction as opposed to the isotropic results in elongation at higher travel speed. Moreover, samples processed at 7.2 mm/s produced high strength meeting the minimum wrought requirements as set by the AMS4911. Eventually, the grain boundary strengthening mechanism with regards to the [alpha] platelets thicknesses was found to be travel speed dependent. Thick samples were characterized by the deposition of multiple lateral beads within one layer. Thermal history was first investigated through the development of a FEM model reproducing the deposition process. Typical macrostructures and microstructures that developed post deposit were then investigated. Subsequent static tensile properties were found to be hardly meeting the cast minimum requirements as set by the ASTM F1108. No anisotropy in the impact toughness properties was observed. Eventually, HIP did not induce any major improvement of the mechanical properties for both thin and thick specimens and is not required for LWD applications. All the previous findings were eventually applied for the printing of a large Ti-6Al-4V aerospace component. The deposition strategy was presented. Destructive and non-destructive testing were done to check the printed part geometry and the process to structure to properties relationship. The printed part was characterized with a buy-to-fly ratio (BTF) of about 3.2:1 as opposed to a BTF ratio of 30.8:1 when using conventional subtractive processes." --