Peen Forming Research Articles

The scaling of laser peen forming: A two-experiment finite similitude approach

Laser peen forming (LPF) utilizes laser-induced shock waves to bend and shape metal plates in what is effectively a non-thermal metal-forming process involving no hard tooling. A difficulty with the process, arising from the rapid localized physics involved, is the determination of process conditions for the establishment of desirable process outcomes. The nanosecond physical behaviors induced by the pulsed laser can make simulation impractical, effectively restricting investigations to experiments as the only practical recourse. This paper focuses on the use of scaled experimentation for LPF with the objective of making experimental outcomes more broadly applicable to a wider range of process conditions. Understanding how processes scale can in principle aid in the establishment of process parameters through timely and cost-effective experiments. Scaled experimentation has recently undergone a paradigm shift with the arrival of the finite similitude scaling theory. The theory provides extra degrees of freedom and facilitates the use of unlimited numbers of scaled experiments and allows for anisotropy in plate thickness. It is demonstrated in the work through experimental tests and simulation at two different scales, that geometric and loading similarities can be broken, yet the behavior of LPF can be quantified to good accuracy.

Achieving high strength and ductility of Al-Zn-Mg-Cu alloys via laser shock peening and spray forming

In order to further enhance the strength and ductility of ultra-high strength aluminum alloys, the laser shock peening technology was applied to ultra-high strength Al-Zn-Mg-Cu alloy. The ultimate tensile strength, elongation and hardness can reach to 751 MPa, 11 % and 208.3 HV by combining spray forming, secondary extrusion, solid solution, retrogression and reaging as well as laser shock peening. The high strength and hardness of the alloys is mainly attributed to the fine-grained layer on surface as well as new grain boundaries, dislocation cells and high-density dislocations introduced by laser shock peening, uniform nano-sized strengthening phases with high density precipitated during heat treatments. The excellent ductility of the alloys is mainly ascribed to multiple structures including fine-grained layer on surface and slip lines inside different grains introduced by laser shock peening, smaller size of fibrous grains and Al7Cu2Fe phase produced by secondary extrusion and spray forming. The aging treatment after laser shock peening can lead to the annihilation of high-density dislocations as well as significantly promote the formation of stable and coarse η phase, which can greatly reduce the strength of the studied alloys.

Inverse method of eigenstrain determination of laser shock peening

The determination of eigenstrain from residual stress is a crucial aspect in process predicting and optimizing of laser peen forming. However, direct measurement of eigenstrain is not possible, and therefore an indirect method based on the Fredholm integral equation is proposed for determining eigenstrain from residual stress. The residual stress of full-coverage laser shock peening (LSP) is analyzed, and the governing equation of eigenstrain is derived from the elastic deformation of a linear elastic plate under full-coverage LSP. The governing equation is a second-kind Fredholm integral equation, which can be solved analytically using the weight function method. Since residual stress is difficult to measure accurately in LSP, an inverse method based on the integral governing equation is proposed for determining eigenstrain from an incomplete set of measurements. The validity of this method is demonstrated through simulation and experimental results, highlighting its high efficiency and engineering application value.

Process planning for laser peen forming of complex geometry: An analytical-based inverse study

Laser peen forming (LPF) is a promising process for the flexible manufacturing of complex thin-walled structures. However, process planning for LPF remains challenging due to the deformed geometry relying on the cumulative effect of thousands of laser shocks without dies. The optimization-based planning method is adaptable but easily gets trapped in local optima, leading to insufficient robustness and efficiency. To overcome these limitations, this study introduces an analytical-based inverse study to complement the optimization-based method. Firstly, an inherent and straightforward analytical model between eigen-moment and surface curvature is proposed, enabling a direct inverse determination of the eigen-moment field from the desired geometry shape. Subsequently, a clustering method is employed to achieve aggregation control over the analytically determined eigen-moment field, avoiding extensive optimization iterations. Moreover, a physical-based surface decomposition method is devised to formulate double-side LPF strategies for surfaces with negative or zero Gaussian curvature. To validate the proposed analytical-based process planning method, an elliptic paraboloid-like surface and a hyperbolic paraboloid surface are selected as the objectives. Experimental results show high conformity between the deformed and objective surfaces, validating the proposed method. The analytical-based planning method facilitates the efficient and robust determination of forming strategies, complementing the optimization-based method and providing a comprehensive solution for LPF process planning under small deformations.

Effect of Scanning Velocity on Bending Deformation per Pulse in Sheet Metal Bending by Laser Peen Forming

Effect of Scanning Velocity on Bending Deformation per Pulse in Sheet Metal Bending by Laser Peen Forming

An equivalent numerical model for calculating shot peening bending deformation based on the laser forming method

Shot peen forming is an essential method for forming large-size thin-walled structural parts in the aerospace field. However, the complex elastic–plastic deformation involved in the forming process presents a challenge in calculating directly the deformation response under the specified peening parameters. The energy of the shot beam in the shot peen forming process is equivalently converted to that of the laser beam in the laser forming process, enabling the equivalent simulation of macroscopic deformation of the shot-peened workpiece using the laser-induced temperature field. It is proven that the essential factors affecting the laser forming process are the laser power, scanning speed, the width and thickness of the plate. When other parameters remain constant, the equivalent laser energy can be determined by the impact energy of the shot beam and the equivalence factor. The average error of the proposed model is 5.068%, with a maximum error of less than 10%, demonstrating a well agreement between the simulation and test results. Furthermore, the equivalent numerical model is used to examine the influence of process parameters on shot peen forming, indicating that the radius of curvature produced by shot peen forming increases with nozzle speed and plate thickness, but decreases with air pressure.

Effects of peening velocity and coverage on peen forming

As an effective forming technique, peen forming is widely used in sheet metal components with large dimensions. However, the production efficiency is seriously limited as several peening tests are required before actual production. To address this issue, a numerical simulation method of peen forming was proposed. In this work, an elastoplastic peening finite element model was developed to study the relations between process parameters and forming effects. A set of peening forming tests with different coverages and velocities were conducted to validate the model. Results indicate that the maximum errors of converted arc height and forming curvature are less than 5% and 14%, respectively. Additionally, the saturation phenomenon can be observed as the peening coverage reaches 60%. The corresponding depth of the maximum compressive residual stress varies almost linearly with shot velocity. In addition, the best peening forming effect can been achieved a radius of curvature as low as 500 mm, which peening velocity selected 60 m/s.

Design and analysis of longitudinal–flexural hybrid transducer for ultrasonic peen forming

Ultrasonic peen forming (UPF) is an emerging technology that exhibits great superiority in both its flexible operating modes and the deep residual stress that it produces compared with conventional plastic forming methods. Although ultrasonic transducers with longitudinal vibration have been widely studied, they have seldom been incorporated into UPF devices for machining in confined spaces. To meet the requirements of this type of machining, a sandwich-type piezoelectric transducer with coupled longitudinal–flexural vibrational modes is proposed. The basic structure of the transducer is designed to obtain large vibrational amplitudes in both modes. Experimental results obtained with a prototype device demonstrate the feasibility of the proposed transducer. The measured vibrational amplitude for the working face in the longitudinal vibrational mode is 1.0 μm, and electrical matching increases this amplitude by 40%. The flexural vibration characteristics of the same prototype transducer are also tested and are found to be slightly smaller than those of longitudinal mode. The resultant working strokes of the UPF impact pins reach 1.7 mm and 1.2 mm in the longitudinal and flexural modes, respectively. The forming capability of the prototype has been evaluated via 15-min machining on standard 2024-T351 aluminum plates. After UPF, an improved surface morphology with lower surface roughness is obtained. The aluminum plate test piece has an apparent upper deformation with an arc height of 0.64 mm. The measured peak value of the compressive residual stress is around 250 MPa, appearing at a depth of 100 μm. The proposed longitudinal–flexural hybrid transducer thus provides a high-performance tool for plate peen forming in confined spaces.

Eigenstrain‐based analysis of why uniformly shot peened aluminium plates bend more in the rolling direction

AbstractShot peen forming is a process widely used to shape aircraft components such as wing skins, yet its fundamental working is still crudely understood. It is understood that a light conventional peen forming treatment applied uniformly over an initially flat plate will induce isotropic in‐plane stretching of the surface layer and will thus lead to a panel curving with identical curvatures in all directions. However, [1] made the startling observation that uniformly peen formed aluminium plates of different aspect ratios all bent along their laminating direction irrespective of the peening direction. This experimental result is counterintuitive because the residual stresses due to the lamination process are 1 order to 2 orders of magnitude smaller than those induced by the shot peen forming treatment. In the present study, we apply the eigenstrain theory to estimate the effect of the different sources of anisotropy on uniformly peen formed aluminium plates. Potential sources of anisotropy included the plastic anisotropy of rolled aluminium, nonequibiaxial initial stresses that redistribute when their equilibrium is disturbed by peening, the geometry of the specimens and externally applied prestress. For the alloy and peening conditions considered, we show that plastic anisotropy had no discernible influence on the resulting shape of the peen formed specimens. Initial residual stresses, on the other hand, caused slightly larger bending loads in the rolling direction of the alloy. Although the magnitude of these loads was approximately 30 times smaller than peening‐induced loads, it was sufficient to overcome the geometric preference for rectangular sheets to bend along their long side and cause all unconstrained specimens to bend along the rolling direction instead. Our analysis highlights the importance of the history of the material that is being peened. Residual stresses already present in the part before peening must be considered to ensure good simulation predictions.

Peening pattern optimization with integer eigen-moment density for laser peen forming of complex shape

Peening pattern optimization with integer eigen-moment density for laser peen forming of complex shape

Process-based surface flattening method for laser peen forming of complex geometry

Laser peen forming (LPF) is a novel flexible forming process with several advantages, such as noncontact and high controllability, but it lacks a method to obtain the initial blank geometry based on its specific deformation behavior, limiting the application of the process. This study proposes a process-based surface flattening method to determine an optimal initial blank of complex geometry to accommodate the precise forming requirement. The process-based surface flattening method is developed based on the deformation behaviors of the laser-peened (LPed) and non-LPed zones with equal-strain and equal-area criteria, respectively. The objective surface is first flattened with the equal-area criterion, then the in-plane strain in LPed zones is loaded into the equal-area flattened blank. Because the in-plane strain is unknown before flattening, an optimization model is conceived to solve the in-plane strain in LPed zones based on the proposed equal-strain flattening method. Experiments are designed to validate the surface flattening methods regarding the LP-formed saddle surface as the objective with a known initial blank. The flattening blanks are quantitatively compared with the initial blank. The process-based flattening method presents the blank geometry with a mean absolute error (MAE) of 0.027 mm, while the geometry-based flattening method presents the result with a MAE of 0.398 mm, illustrating the process-based method can provide a more effective initial blank shape.

Process-based deep learning model: 3D prediction method for shot peen forming of an aircraft panel

Shot peen-forming is a more precise method of forming aircraft panels than conventional methods. The traditional method of acquiring the process parameters relies mainly on prior theoretical knowledge and trial-and-error. Despite the finite element method's ability to replace some experimentation, it still cannot realize the design of shot peen forming processes parameters of an aircraft panel based on a known contour. This study uses an innovative model-based deep learning approach to predict aircraft panel deformation and active design the shot peening parameters. The prediction time is less than 1 second, resulting in a significant reduction in computational time. The shot peen forming process parameters and the geometric structure characteristics of the aircraft panel are divided into independent channels to establish a high-dimensional feature map, which are used to train the deep learning model. The forming contours of the 2024-T351 high-strength aluminum alloy panel are predicted under different shot peening processes. In addition, the process parameters are designed according to the known contour of the forming process. To verify the precision of the proposed method, the designed shot peen forming process is used to manufacture a single curvature aircraft panel with a curvature radius of 3500 mm. There is good agreement between the forming contour and the theoretical design contour. The maximum deformation error is less than 1 mm and its mean error is 7.8%. The mean curvature radius error is 5.668%. The proposed method provides a new and practical reference to the precise design of the shot peen-forming process.

A study on bending deformation behavior induced by shot peening based on the energy equivalence

Shot peening is an essential process for forming large thin-walled components in aerospace industries. However, it is challenging to determine directly the mapping relationship between shot peening parameters and deformation response because of the complex elastic–plastic deformation of shot-peened components. In this paper, a computational model of bending deformation of peened plate samples is established by examining the shot peening behavior from the aspect of energy equivalence, in which the deformation energy of the shot-peened plate is determined by the total kinetic energy input due to the shot impacts and a correction factor that takes into account the influences of oblique impact, shot interaction and shot overlap on the peen forming process in practice. It is proven that the square of curvature radius is proportional to the cube of the thickness but inversely proportional to the indentation coverage and the average kinetic energy input of a single shot. For the effect of a single shot impact in peen forming, the average kinetic energy input of a single shot is a power function with the indentation diameter.

Experimental study on laser peen forming of aluminium alloy 2024-T351 plate

Laser peen forming technology LPF is proposed on the basis of laser peen strengthening technology and mechanical peen forming technology. It is an advanced high-efficiency and long-life manufacturing technology for panels. This forming technology has a wide application prospect in the field of aerospace manufacturing. In the forming process, there are many factors affecting the forming process, such as laser pulse energy, shot peening times, shot peening path, material, structure and so on. At present, there are few studies on the influence of laser peen forming process parameters on the shape of parts, which can not meet the technical requirements of forming process. In this paper, based on the single curvature bending forming of al2024-T351 aluminum alloy, the elastic-plastic deformation of metal surface formed by laser shot peening is investigated. The spot morphology formed by laser shot peening is analyzed. The influence law of different forming process parameters on the forming shape of parts is established. The results show that the thickness is the main influencing factor for the curvature radius of the impact area. The radius of curvature increases with the increase of thickness, and decreases with the increase of pulse energy and coverage. Using the regression analysis method, the mathematical model of laser peen forming curvature radius is established. The predicted results are consistent with the experimental results, and the maximum relative deviation is 2.6%.

Effect of Shot Peen Forming on Corrosion-Resistant of 2024 Aluminum Alloy in Salt Spray Environment

The effect of shot peen forming on the corrosion-resistant of 2024 aluminum alloy in a salt spray environment was studied with an electrochemical workstation. The surface morphology and cross sectional morphology of the original and shot peen-formed sample were studied by a scanning electron microscope. After shot peen forming, the salt spray corrosion resistance of 2024 aluminum alloy was worsened (the corrosion rates of the original alloy and the shot peen-formed alloy were 0.10467 mg/(cm2·h) and 0.27333 mg/(cm2·h), respectively, when the salt spray corrosion time was 5 h). The radius of capacitive reactance arc of the sample subjected to shot peen forming was smaller than that of the original sample. When the salt spray corrosion time was 5 h, the doping density (NA) of the original alloy was 2.5128 × 10-13/cm3. After shot peen forming, the NA of the alloy increased to 15 × 10-13/cm3. For the shot peen-formed sample, pitting corrosion first occurred in the crater lap zone and became severe with salt spray time. The cross sectional morphology of both original and the shot peen-formed samples shows that severe intergranular corrosion occurred in the salt spray environment. However, for the original sample, the intergranular corrosion distribution was lamellar. For shot peen-formed sample, the intergranular corrosion distribution was network.

Corrosion Behavior and Microstructure of 2024 Aluminum Alloy Sheets by Shot Peen Forming in a Salt Spray Environment

Corrosion Behavior and Microstructure of 2024 Aluminum Alloy Sheets by Shot Peen Forming in a Salt Spray Environment

Closed-loop shot peen forming with in-process measurement and optimization

Shot peen forming is relied upon in the aerospace manufacturing industry to form thin metal panels into complex geometries. The accuracy and repeatability of the process depend on the initial stress state of the part. The measurement of residual stresses being costly or destructive leaves the field in want of a more efficient method to produce accurate parts. This work proposes a closed-loop method, based on in-process measurements, a finite element model, and optimization to reduce shot peen forming errors by iterative replanning. We demonstrate in an experimental test case of three sample plates that the proposed method can reduce the forming error below 0.2 mm, while the standard open-loop planning yielded errors from 0.333 mm to 0.667 mm, for samples of 1.04 mm thickness.

Numerical Investigation of Influence of Spot Geometry in Laser Peen Forming of Thin-Walled Ti-6Al-4V Specimens

The aviation industry demands thin-walled structures of high dimensional accuracy. Varying radii and individual use-cases, e.g. for repair purpose, require flexible forming techniques. Laser peen forming (LPF) represents such a forming process providing precise energy input by a pulsed laser over a wide energy range. Among adjustable parameters such as laser intensity and focus size, the spot shape, i.e. square and circular, is usually fixed for a specific laser system. As the spot shape is a crucial parameter, this work focuses on the effect of the spot shape on structural deformation after LPF application. Therefore, models for laser peen forming of thin-walled Ti-6Al-4V strips for LPF systems with circular and square focus shapes are set up. Geometric conditions on both focus shapes ensure equal energy input during the laser processing. The numerical simulation relies on the so called eigenstrain method, leading to a cost-efficient calculation of resulting deformation after the dynamic LPF process. Square-based peening pattern exhibit higher deflection. For increasing spot size, the deflection difference between square and circle-based patterns increase slightly.

Picosecond Laser Shock Micro-Forming of Stainless Steel: Influence of High-Repetition Pulses on Thermal Effects.

A study of the peen forming of thin stainless steel metal foils (50 thick) using a solid-state ps-pulsed laser, emitting at a wavelength of was conducted. The pitch distance between consecutive laser pulses was kept constant by tuning the laser repetition rate from 0.4 to 10 , and subsequently the scanning speed. The induced bending angle and the radius of curvature were used to measure the effect of the treatment. Their dependence on the pulse energy, the treated area, the distance between lines, and the laser repetition rate was studied. High repetition rates do not allow the sample to cool down, affecting the bending to the point of being negligible. An FEM simulation and experiments were carried out to prove that the increase in temperature due to high repetition rate can relax the stresses induced by laser peen treatment, thus preventing bending in the sample.

Effect of saturation peening on shape and residual stress distribution after peen forming

Effect of saturation peening on shape and residual stress distribution after peen forming