Laser Shock Peening Process Research Articles

Understanding the role of monolayer graphene during long range shock strengthening of metal-graphene heterostructure

Graphene has been extensively used in graphene reinforced metal composite, with strengthening effects due to its ability to block dislocation from propagation during the traditional low strain rate deformation process. Laser shock peening has been applied in graphene/metal composites with graphene concentration of 2–10% with a relatively close distance between the graphene layers. This work discovered a strong long-range coupling effects of graphene as a super-strong nanomaterial and shock-wave transmitter during laser shock processing under room temperature LSP and cryogenic temperature (cLSP), under extremely low graphene concentration (1.42 × 10−6% vol.). Compared with simple compressed graphene-copper heterostructure, the yield strength of LSP and cLSP processed samples increases by 40%, and 76% respectively. We found that under laser shock peening (LSP) process, the shock wave can pass through long-distance to generate dislocation transportation from one layer to another graphene with the shock wave interaction between graphene layers separated very far away. Graphene plays an important role not only as a transmitter of shock waves, but also as a strong wall to bounce back shock waves to generate high dislocation density around graphene layers. We have designed experiments to compare the deformation behavior of the laminates under three deformation conditions: compression, LSP, and cLSP, respectively. It was found that the compressed sample has very few parallel dislocation arrays beneath the graphene interface, indicating that graphene blocks the dislocation movement and has very limited strengthening effects. The LSP processed samples contain much high dislocation density, while even higher density dislocation and strength are found in cLSP due to faster shock transportation in graphene/metal layers under cryogenic conditions. Finite element modeling was used to investigate the shock wave interaction with the graphene and metal layer under various conditions, which is consistent with experiments. Molecular dynamics simulation is used to simulate the microstructure of the laminates under various conditions and validated by experiments. This work provides a starting point to understand the long-range strengthening effects of 2D nanomaterials of extremely low concentrations and provide new design strategies for manufacturing graphene-metal nanocomposite and their strengthening approaches.

Evaluation of laser shock peening process parameters incorporating Almen strip deflections

Engineering of component surfaces using Laser Shock Peening (LSP) requires optimisation of key process parameters to enhance performance through the generation of beneficial compressive residual stresses. Quantification of residual stress fields typically requires considerable experimental resources involving multiple complementary techniques to obtain reliable measurements through the depth of the target material. LSP induced plastic strains lead to the formation of residual stresses and cause deformation of thin components. This effect is routinely used in the assessment of mechanical Shot Peening (SP) where distortion coupons, called Almen strips, correlate the peening intensity to process parameters. This study explores the extension of this Almen strip concept to facilitate the quick assessment of LSP parameters for aluminium alloy 6056-T4 aeronautical structural applications. Direct measurements of the resulting LSP residual stresses have been performed using Synchrotron X-Ray Diffraction, Neutron Diffraction, and Incremental Hole Drilling, complemented by analyses of surface integrity and sub-surface modifications. It has been found that the attained near-surface compressive residual stress values and depths are strongly dependent on laser power intensity and spot coverage, whilst surface roughness and microhardness are mostly independent of the LSP parameters. Optimal parameter selection should therefore be primarily focused on the required residual stress field. A direct correlation has been observed between the magnitudes of LSP Almen distortions and the residual stresses. This qualifies extending the deflection based approach to be used as a quick, simple and effective qualitative screening technique of LSP process parameters, such as the laser power intensity saturation.

Hole surface texture reconstructed with laser shock peening and effect on fretting behavior

Laser Shock Peening (LSP) has been widely employed as a post-processing surface treatment technique in improving fatigue resistance of various titanium alloy parts. However, most of the concerns on the LSP application are focused on the alteration in the subsurface layer, rather than the texture properties of the surface layer, which are crucial sensitive factors of the nucleation of fretting induced micro-crack. In this paper, the feasibility and ability of LSP treatment on the reconstruction of surface texture properties in four groups of sequentially milled hole surfaces were investigated. Two different impact routes were set to perform the LSP treatment. The produced surface texture properties, microstructure alteration, surface roughness, residual stress, as well as the fretting fatigue life and fretting behaviors were compared. The results showed that the LSP process could be regarded as a potential technique in constructing the surface texture to introduce beneficial resistance to fretting wear, by which the positive surface skewness value could be altered to negative, and the high surface kurtosis could be changed from leptokurtic to platykurtic or mesokurtic distribution. Moreover, the symmetrical strengthening method (SSM) in LSP treatment (LSPb route) tended to generate surface layer with a more negative skewness, better texture flow trend, numerous low angle grain boundaries (LAGBs), grains with high kernel average misorientation (KAM), and more refined grains, resulted in a lower crack growth rate and approximately doubling the fretting fatigue life. This work provides a method for reconstructing surface texture properties by LSP treatment to meet the performance requirement of components, as well as to expand the application field and ability of the LSP technique.

Online Evaluation of Surface Hardness for Aluminum Alloy in LSP Using Modal Acoustic Emission

This article researches the online evaluation method of surface integrity for aluminum alloy in laser shock peening (LSP) based on multiple-sources modal acoustic emission (MAE). First, a new comprehensive index e.g., sub-surface microhardening rate (SHR) for 7075 aluminum alloy in LSP was constructed to characterize both the impact depth (<inline-formula> <tex-math notation="LaTeX">$150 \mu \text{m}$ </tex-math></inline-formula>) and its hardening rate. Then, a good linear and positive correlation between the SHR of 7075 Al alloy and the AE features in time domain and frequency domain was revealed based on the offline quantitative characterization of SHR and the antisymmetric zero (A0) mode separated from AE signal. In addition, the A0 mode-based Mel frequency cepstrum coefficient (AMFCC) was proposed to deeply extract the time-frequency features of MAE-A0 mode signal. Finally, the SHR classification model based on AMFCC and gradient boosting decision tree for 7075 Al alloy in LSP process was established and was carefully validated using the experimental data. The test results showed the highest mean accuracy of 96.24&#x0025; after the thorough comparison with other tree models and the traditional cepstrum methods. More importantly, deep analysis about the feature importance and their physical interpretation has been conducted from the perspective of non-linear effect inside the LSP material caused by dislocation multiplication as well as the increasing harmonic component of the MAE-A0 signal. This article can provide certain guidance for the development of LSP quality monitoring technology.

Comparisons on localized surface modifications of stainless steels induced by laser shock peening and robotic hammer peening

Localized surface peening processes have been carried out on single spots by employing laser-shock peening (LSP) and robotic hammer peening (RHP) on stainless steels SS304L and SS316L. The LSP process has induced apparent γ-to-ɛ martensite transformations (MTs) in the surface layer of SS304L rather than in that of SS316L. In comparison, the MTs induced by the RHP process in the surface layer of SS304L with the crater depth similar to that produced by LSP is dominated by γ-to-α’ rather than γ-to-ɛ. Further studies of groove-like tracks produced by RHP scanning on SS316L with varied paths provided clues for material migrations on the surface from the center to the edges of the tracks. Detailed surface modifications by LSP and RHP are finally discussed based on the observations under scanning-electron microscope.

A combined experimental-numerical study of residual stress and its relaxation on laser shock peened SiC particle-reinforced 2009 aluminum metal matrix composites

Residual stresses have significant influences on the in-service performance of materials. Laser shock peening (LSP), a technique generating processing-based residual stress, is employed to introduce high-amplitude and large-depth compressive residual stress in SiC particle-reinforced 2009 aluminum (SiC/2009Al) metal matrix composites. Surface residual stresses after single-shot LSP and multiple shots of overlapped LSP were investigated. Single-shot LSP with 10 J, 20 J, 30 J, and 40 J laser energies introduced average surface residual stresses of −126.7 MPa, −129.3 MPa, −85.7 MPa, and −69.3 MPa. Multiple shots with energies of 20 J were inflicted twice on the surface undergoing LSP and exhibited a maximum residual compressive stress of −266.7 MPa. Then, the surface residual stress decreased from −266.7 MPa to −215.3 MPa and further to −212.7 MPa when subjected to 10,000 and 100,000 fatigue cycles. Relaxation also occurred under thermal loads, and the stress decreased from −266.7 MPa to −129.3 MPa after holding at 100 °C for 50 h. Furthermore, simulation of the LSP process was conducted with the SiC/2009Al composite and showed that SiC particles close to the peening surface generated more reverse plastic deformation and resulted in some residual tensile stress in their surroundings. This combined experimental and numerical research seeks to provide deep insight into residual stress distribution and explore the applicability of LSP with metal matrix composites. • Single shot and multiple shots LSP was conducted on SiC/2009Al metal matrix composites. • The presence of SiC particles generates local tensile stresses in the vicinity of SiC particles. • The residual stresses on SiC/2009Al show great resistance to the fatigue cycles. • The residual stresses on SiC/2009Al show poor resistance to the thermal loads.

Microstructures and mechanical properties of α‐SiC ceramics after high‐temperature laser shock peening

AbstractA novel high‐temperature laser shock peening (HT‐LSP) process was applied to polycrystalline α‐SiC to improve the mechanical performance. HT‐LSP prevents microcrack formation on the surface while induces plastic deformation in the form of dislocation slip on the basal planes, which may be caused by the combination of high shock pressure and a lower critical resolved shear stress at 1000℃. A maximum compressive residual stress of 650 MPa, measured with Raman spectroscopy, was introduced into the surface of α‐SiC by HT‐LSP, which can increase the nanohardness and in‐plane fracture toughness of α‐SiC by 8% and 36%, respectively. This work presents a fundamental base for the promising applications of HT‐LSP to brittle ceramics to increase their plasticity and mechanical properties.

Investigation of microstructure and mechanical properties evolution in 7050 aluminum alloy and 316L stainless steel treated by laser shock peening

Laser shock peening (LSP) has been widely applied to enhance the mechanical properties of metallic materials by modifying their sub-surface microstructures. However, controversies still exist on whether grain refinement can be obtained after the LSP process. To investigate the effect caused by LSP in metallic materials, a 7050 aluminum alloy and 316L stainless steel, which are typically high and low stacking fault energy (SFE) materials, respectively, were selected for this study. The microstructure modified by different LSP cycles and energy densities in both materials was illustrated by Electron Backscatter Diffraction (EBSD). The result shows that no grain refinement was observed regardless of the laser cycles and energy density. The most evident change was the increase of dislocation density, and higher dislocation density was observed with the increase of LSP cycles and energy density. The hardness and residual stress measurements around the LSPed areas show that LSP can effectively introduce a plastic deformation layer ranging from 600~1300 μm. It was revealed from the tensile tests that the yield strength of both materials was improved after the LSP process with the scarification of their elongations. Moreover, a potential method to calculate the dynamic yield stress of metallic materials was put forward with the help of the LSP process.

Surface Texturing and Laser Shock Peening Processes on High-Speed Steel Tool for Sustainable Machining

Surface Texturing and Laser Shock Peening Processes on High-Speed Steel Tool for Sustainable Machining

Laser shock peened Ti-6Al-4 V alloy: Experiments and modeling

Systematical microstructure characterization, mechanical testing and constitutive modeling were carried out to quantify the effects of residual stress and grain refinement on tensile properties of Ti-6Al-4 V alloy treated by laser shock peening (LSP). Microstructure characterization showed that grain size is refined to nanoscale in the outmost surface. Meanwhile, LSP treatment introduced a maximum compressive residual stress of 600 MPa in the surface region. The macroscopic tensile test indicated a more prolonged elastic-plastic transition in the LSP-treated Ti-6Al-4 V alloy than the as-received one. As a result, the 0.2% offset yield strength of the LSP-treated Ti-6Al-4 V alloy was reduced by about 52 MPa, while the flow strength at the subsequent strain hardening stage was enhanced. To quantitatively evaluate the individual influence of gradient microstructure and residual stress on the tensile response, we established a deformation-mechanism-based and size-dependent constitutive model. Implementation of the constitutive model demonstrated that the surface grain refinement enhances both the initial yielding and strain hardening of LSP-treated Ti-6Al-4 V alloy. In contrast, the residual stress has a significant weakening effect on the initial yielding but has little influence on its strain hardening behavior. The established microstructure and deformation mechanism-based model can help guide the LSP processing to further improve the mechanical performance of Ti-6Al-4 V alloy.

Reducing scatter in bent angle by a laser shock peening pretreatment

Laser shock peening is a surface treatment technology, which modifies the residual stress state of metal parts. When forming thin sheet metal parts with thicknesses ≤1 mm, locally varying residual stress states are, according to the literature, the main reason for the scatter in the bent angle. Forming processes, such as bending, are often used to manufacture thin sheet metal. Thin sheets are formed in quantities of several hundreds of millions per year. Even scrap rates that are in the ppm range, caused by the scatter in the bent angle, lead to considerable manufacturing costs. To ensure a robust forming process, it is hypothesized that the laser shock peening process can reproducibly change the residual stress state in thin sheet metal parts in such a way that scatter in the bent angle can substantially be reduced. Annealed and 1 mm thick sheet metal made of X5CrNi18-10 were processed with a nanosecond pulsed fiber laser. The influence of laser peening without coating (LPwC) on the bent angle was investigated using bottom bending with a nominal bending angle of 90°. It was revealed that the standard deviation of the bent angle, i.e., the scatter, is reduced by a factor of 2 from ±0.14° down to ±0.07° compared to the initial sheet state. Hence, the possibility to use LPwC as a pretreatment to decrease the scatter in the bent angle for thin sheets is demonstrated.

Laser shock peening Fe-based coatings for enhancing wear and corrosion resistance

Laser Shock Peening (LSP) was introduced to reduce the residual stress and improve the wear and corrosion resistance of Fe-based 43X coating on worn U71Mn steel by laser cladding technology. The tensile stress decreased with increasing of laser power density and impact times, finally, transferred to compressive stress. The microstructure of the coatings was refined due to LSP treatment. The refined microstructure and existence of retained Austenite phase resulted in significant improvement in microhardness and anti-wear performances. The coatings after LSP process presented better corrosion resistance. However, impact three times with a higher power density has negative effect on improving the corrosion resistance.

Effects of laser shock peening on the ultra-high cycle fatigue performance of additively manufactured Ti6Al4V alloy

Although additive manufacturing (AM) allows to fabricate metallic parts with a high static strength comparable to their forged counterparts, the fatigue strength of an AM-ed part is generally inferior, restricting AM from many critical applications. In this study, laser shock peening (LSP) was applied to modify the surface properties of selective laser melting fabricated (SLM-ed) Ti6Al4V titanium alloy. This study performs systematic tests and analyses on the fabricated alloy specimens to characterize their microstructures and mechanical properties including residual stress, tensile strength, ultra-high cycle fatigue (UHCF) strength. The results reveal that LSP can refine microstructure, suppress residual stresses, and delay crack propagation in the affected area. However, the inherent defects in an SLM-ed part, such as unmelted powders, lack of fusion and clusters of α phase, dominate the fatigue failure of the specimens especially in the UHCF regime, resulting in their poor fatigue performance. Meanwhile, The LSP processed specimens showed a lower S-N curve than that of specimens without LSP processing especially in the UHCF regime, which not only results from the inherent defects, but also the increased surface roughness and non-uniform residual stresses.

Laser shock peening regulating aluminum alloy surface residual stresses for enhancing the mechanical properties: Roles of shock number and energy

The enhancement of the mechanical performance of materials is a continuous pursuit in engineering. Laser shock peening (LSP) is an advanced manufacturing method employing extreme conditions, which effectively improves the mechanical properties of the material surface. Herein, LSP was used to treat aircraft 5A06 aluminum alloy, leading to significant enhancement of properties. In the LSP process, the pulse shock wave caused severe plastic deformation of the metal surface, where the strain rate reached 107 s−1. The dislocations multiplied in large numbers and then evolved into large-angle grain boundaries and other structures to achieve grain refinement. Also, LSP significantly improved the residual stress of metals, causing a series of mechanical improvement effects. Under the impact of fine-grain strengthening, the resultant 5A06 aluminum alloy's micro-hardness was increased to 140 HV, a value 65% higher than that of bare aluminum. After the LSP process, the maximum plastic deformation layer depth (PDLD) reached 800 μm. On this basis, the ultimate tensile strength of aluminum alloy was increased by 27%, and the ultimate tensile displacement was also increased. The LSP treatment increased the strength of aluminum alloy without losing ductility. The resultant sample's friction coefficient was reduced by 34%, and the overall surface-displayed uniform wear. This work reveals the structure-activity relationships between the surface residual stress and metal mechanical properties and clarifies the LSP process's strengthening mechanism.

Identification of critical material thickness for eliminating back reflected shockwaves in laser shock peening – A numerical study

Laser Shock Peening (LSP) is one of the emerging surface treatment processes being considered for inducing beneficial compressive surface residual stresses in fatigue critical components. Owing to the ease in handling multiple parameters during optimization of process parameters, simulation based parameterization studies using finite element (FEM) based numerical models are widely gaining importance. Most of the LSP modeling performed so far considered infinite elements in both thickness and lateral directions. However, infinite elements in thickness direction would neglect the deleterious effect of shock wave back reflections for certain sample thicknesses. These back reflections have been reported to result in formation of subsurface cracks in the specimen. Therefore, in this study, using an alternative modeling strategy, effect of thickness on the back reflection of shock waves and its subsequent effect on residual stresses induced are discussed. A 2-D axi-symmetric model with infinite elements in lateral direction and finite elements in thickness direction is developed to simulate a single spot LSP process using ABAQUS/CAE FEM package. It is found that there exists a critical material thickness depending on spot diameter below which the effects of back reflection are predominant.

Compact, diode-pumped, unstable cavity Yb:YAG laser and its application in laser shock peening.

We present the setup of a compact, q-switched, cryogenically cooled Yb:YAG laser, which is capable of producing over 1 J output energy in a 10 ns pulse at 10 Hz. The system's design is based on the recently published unstable cavity layout with gain shaping of the spatial intra-cavity intensity distribution. Using a hexagonal homogenized pump beam, the laser generated an according hexagonal output beam profile. The suitability of such laser properties for the intended use in a laser shock peening process is demonstrated. In the experiment an aluminum plate was treated and the generated residual stresses in the sample subsequently measured. Other applications of this laser system like laser pumping or surface cleaning are conceivable.

Recent Developments and Novel Applications of Laser Shock Peening: A Review

By inducing work hardening and beneficial compressive residual stresses in near‐surface regions, laser shock peening (LSP) improves the fatigue performance of many metallic components. In recent years, LSP has found many new applications in emerging fields: additive manufacturing, ceramics, and metallic glasses. In addition, innovative process development based on LSP has been reported, which includes warm LSP, cryogenic LSP, electropulsing‐assisted LSP, laser peening without coating, femotosecond LSP, and laser peen forming. Herein, a comprehensive review of the LSP process with a focus on the novel applications and innovative process development is aimed for. The history of LSP with key events is briefly reviewed. The fundamental mechanisms of LSP, including the generation of shockwaves by a high‐energy pulsed laser, the generation of compressive residual stresses by the shockwave, and its effect on crack propagation, and how LSP induces grain refinement are also discussed. The effects of compressive residual stresses and grain refinement on the mechanical properties of metallic materials are reviewed. Recent developments in LSP, such as innovative process development based on LSP and its novel applications, are discussed. Finally, the current challenges the LSP technology faces and its future directions are also discussed.

Formation Mechanism and Control Method of Residual Stress Profile by Laser Shock Peening in Thin Titanium Alloy Component.

In the laser shock peening process of titanium alloy thin blades, a shock wave will be repeatedly reflected and coupled in the blades, resulting in the failure of the formation of a gradient residual compressive stress layer, which is the key to improve fatigue performance and resist foreign object impact. This paper takes TC17 titanium alloy sheet as the research object to reveal the influence mechanism on residual stress-strain profile of shock wave reflection-coupling by shock wave propagation and key position dynamic response. Based on the result of influence mechanism, two wave transmission methods are proposed to regulate shock wave in order to reduce the intensity of shock wave reflection. The analysis shows that the high strength stress be formed when the shock wave is reflected and coupled in the sheet, which causes the re-plastic deformation and the decrease of transverse plastic strain. This eventually leads to residual tensile stress up to 410 MPa being formed within a 0.5 mm radial direction and 0.3 mm deep of the spot range. The use of “soft” and “hard” wave-transmitting layers greatly reduces the shock wave reflection intensity, and under the condition of the “hard” wave-transmitting layer, a better impedance matching is achieved, resulting in a residual compressive stress of about 400 MPa.

Machine Learning-Based Prediction and Optimisation System for Laser Shock Peening

Laser shock peening (LSP) as a surface treatment technique can improve the fatigue life and corrosion resistance of metallic materials by introducing significant compressive residual stresses near the surface. However, LSP-induced residual stresses are known to be dependent on a multitude of factors, such as laser process variables (spot size, pulse width and energy), component geometry, material properties and the peening sequence. In this study, an intelligent system based on machine learning was developed that can predict the residual stress distribution induced by LSP. The system can also be applied to “reverse-optimise” the process parameters. The prediction system was developed using residual stress data derived from incremental hole drilling. We used artificial neural networks (ANNs) within a Bayesian framework to develop a robust prediction model validated using a comprehensive set of case studies. We also studied the relative importance of the LSP process parameters using Garson’s algorithm and parametric studies to understand the response of the residual stresses in laser peening systems as a function of different process variables. Furthermore, this study critically evaluates the developed machine learning models while demonstrating the potential benefits of implementing an intelligent system in prediction and optimisation strategies of the laser shock peening process.

Combining manufacturing of titanium alloy through direct energy deposition and laser shock peening processes

Combined manufacturing of direct energy deposition (DED) and laser shock peening (LSP) exhibits great potentials in repairing titanium alloy components and improving their mechanical properties. In this study, a DEDed repaired sample was fabricated by depositing TA15 powders on a forged TC17 plate, and subjected to a following LSP treatment. The effects of LSP on microstructures, residual stress, and tensile properties of DEDed samples were evaluated. After LSP, a high-level compressive residual stress state (maximum =−596 MPa) and microstructure refinement were induced in the treated surface. The average α phase area decreased from 1.88 μm2 to 0.73 μm2, and much more low angle grain boundaries were induced after LSP. Meanwhile, the corresponding ultimate tensile strength (UTS) was improved by 12.46%.