Yajun Fan

Laser Forming of Varying Thickness Plate—Part I: Process Analysis

Laser forming (LF) is a non-traditional forming process that does not require hard tooling or external forces and, hence, may dramatically increase process flexibility and reduce the cost of forming. While extensive progress has been made in analyzing and predicting the deformation given a set of process parameters, few attempts have been made to determine the laser scanning paths and laser heat conditions given a desired shape. This paper presents a strain-based strategy for laser forming process design for thin plates with varying thickness, which is utilized in determining the scanning paths and the proper heating conditions. For varying thickness plates, both the in-plane membrane strain and the bending strain need to be accounted for in process design. Compared with uniform thickness plate, the required bending strain varies with not only the shape curvature but also with the plate thickness. The scanning paths are determined by considering the different weight of bending strain and in-plane strain. A thickness-dependent database is established by LF finite element analysis simulation, and the heating conditions are determined by matching the ratio of bending strain to in-plane strain between the required values and the laser forming values found in the database. The approach is validated by numerical simulation and experiments using several typical shapes.

Energy-Level Effects on the Deformation Mechanism in Microscale Laser Peen Forming

Laser microscale peen forming has recently received more and more attention as a viable laser processing technology as it not only imparts desirable residual stress for improvement of fatigue life of the material, but can also precisely control part deformation. In the present study, the effect of energy level on the deformation mechanism in laser microscale peen forming was investigated by both numerical and experimental methods. Deformation curvatures and residual stress distributions of both sides of the specimen, characterized by X-ray microdiffraction, were compared with the results obtained from FEM simulation. The forming mechanism for convex and concave bending was explained in terms of the resulting pressure, compressive stress distribution, and plastic strain. Differences in residual stress distribution patterns were also investigated as a function of the forming mechanism.

Analysis and prediction of size effect on laser forming of sheet metal

Abstract Geometric effects play an important role in laser forming processes; however, few investigations have explored geometric effects other than those induced by sheet thickness. In this paper, the influence of component size or size effect, including variation of sheet width and sheet length, on laser-induced deformation is experimentally, numerically, and analytically investigated. An experimental matrix is designed to cover a wide range of sheet width and length for experiments and numerical simulation under different process conditions. Distinctive trends in bending angle are identified for varying sheet width, length, or both. The results are interpreted in terms of heat sink effect and bending nonuniformity. An analytic model is developed to facilitate size effect prediction. The model is based on the solution to a moving strip heat source over a finite size sheet. It also accounts for the pre-bending effect among consecutive segments on the scanning path. Analytical results are compared with an existing analytical model and numerical simulation.

Microscale laser peen forming of single crystal

As the result of quickly increased requirement in many industrial products resulting from microtechnology, laser thermal microforming and microsurface treatment [microscale laser shock peening (μLSP)] have been well studied. By combining the beneficial effects of these two processes with a controlled bending deformation, microscale laser peen forming (μLPF) attracts more attention recently since it not only improves the fatigue life of the material but also shapes microscale metallic parts at the same time. In the present study, μLSP of single crystal aluminum was presented to study anisotropic material response. Local plastic deformation was characterized by lattice rotation measured through electron backscatter diffraction. Residual stress distributions of both sides of a peened sample, characterized by x-ray microdiffraction, were compared with the results obtained from finite element method simulation. μLPF anisotropic behavior was investigated in three effective slip systems via both the anisotropic ...

Numerical Investigation of Opposing Dual Sided Microscale Laser Shock Peening

Laser shock peening (LSP) is an innovative process which imparts compressive residual stresses in the processed surface of metallic parts to significantly improve fatigue life and fatigue strength of this part. In opposing dual sided LSP, the workpiece can be simultaneously irradiated or irradiated with different time lags to create different surface residual stress patterns by virtue of the interaction between the opposing shock waves. In this work, a finite element model, in which the hydrodynamic behavior of the material and the deviatoric behavior including work hardening and strain rate effects were considered, was applied to predict residual stress distributions in the processed surface induced under various conditions of the opposing dual sided microscale laser shock peening. Thus the shock waves from each surface will interact in different ways through the thickness resulting in more complex residual stress profiles. Additionally, when treating a thin section, opposing dual sided peening is expected to avoid harmful effects such as spalling and fracture because the pressures on the opposite surfaces of the target balance one another and prohibit excessive deformation of the target. In order to better understand the wave-wave interactions under different conditions, the residual stress profiles corresponding to various workpiece thicknesses and various irradiation times were evaluated.

Micro-scale laser peen forming of single crystal

As the result of quickly increased requirement in many industrial products resulting from microtechnology, laser thermal micro-forming and micro surface treatment (microscale laser shock peening (μLSP)) have been well studied. By combining the beneficial effects of these two processes with a controlled bending deformation, microscale laser peen forming (μLPF) attracts more attention recently since it not only improves the fatigue life of the material but also shapes micro scale metallic parts at the same time. In the present study, μLSP of single crystal aluminum was presented to study anisotropic material response. Local plastic deformation was characterized by lattice rotation measured through electron backscatter diffraction (EBSD). Residual stress distributions of both sides of a peened sample, characterized by x-ray micro-diffraction, were compared with the results obtained from FEM simulation. μLPF anisotropic behavior was investigated in three effective slip systems via both the anisotropic slip line theory and numerical method. Also, the work hardening effect resulted from self-hardening and latent hardening was analyzed through comparing the results with and without considering hardening.As the result of quickly increased requirement in many industrial products resulting from microtechnology, laser thermal micro-forming and micro surface treatment (microscale laser shock peening (μLSP)) have been well studied. By combining the beneficial effects of these two processes with a controlled bending deformation, microscale laser peen forming (μLPF) attracts more attention recently since it not only improves the fatigue life of the material but also shapes micro scale metallic parts at the same time. In the present study, μLSP of single crystal aluminum was presented to study anisotropic material response. Local plastic deformation was characterized by lattice rotation measured through electron backscatter diffraction (EBSD). Residual stress distributions of both sides of a peened sample, characterized by x-ray micro-diffraction, were compared with the results obtained from FEM simulation. μLPF anisotropic behavior was investigated in three effective slip systems via both the anisotropic slip li...

Numerical Investigation of Opposing Dual Sided Micro Scale Laser Shock Peening

Laser shock peening (LSP) is an innovative process which imparts compressive residual stresses in the processed surface of metallic parts to significantly improve fatigue life and fatigue strength of this part. In opposing dual sided LSP, the workpiece can be simultaneously irradiated or irradiated with different time lags to create different surface residual stress patterns by virtue of the interaction between the opposing shock waves. In this work, a finite element model, in which the hydrodynamic behavior of the material and the deviatoric behavior including work hardening and strain rate effects were considered was applied to predict residual stress distributions in the processed surface induced under various conditions of the opposing dual sided micro scale laser shock peening. Thus the shock waves from each surface will interact in different ways through the thickness resulting in more complex residual stress profiles. Additionally, when treating a thin section, opposing dual sided peening is expected to avoid harmful effects such as spalling and fracture because the pressures on the opposite surfaces of the target balance one another and prohibit excessive deformation of the target. In order to better understand the wave-wave interactions under different conditions, the residual stress profiles corresponding to various workpiece thicknesses and various irradiation times were evaluated.Copyright

Wave-solid interactions in shock induced deformation processes

A model was developed for material deformation processes induced by laser generated shock waves. The processes include laser peen forming (LPF) and laser shock peening (LSP) of metals. Numerical solutions of the model using finite element method (FEM) were implemented in two steps: (1) explicit step, devoted to shock wave propagation; and (2) implicit step, calculating relaxation of material. A series of LPF and LSP experiments were conducted to validate the model. The residual stress measurements by synchrotron X-ray diffraction and deformation measurements by profilometry showed that the experimental and numerical results were in good agreement. An important aspect of the work is that the numerical results were further analytically explored to gain improved understanding of wave-solid interaction including shock wave attenuation and shock velocity variation.A model was developed for material deformation processes induced by laser generated shock waves. The processes include laser peen forming (LPF) and laser shock peening (LSP) of metals. Numerical solutions of the model using finite element method (FEM) were implemented in two steps: (1) explicit step, devoted to shock wave propagation; and (2) implicit step, calculating relaxation of material. A series of LPF and LSP experiments were conducted to validate the model. The residual stress measurements by synchrotron X-ray diffraction and deformation measurements by profilometry showed that the experimental and numerical results were in good agreement. An important aspect of the work is that the numerical results were further analytically explored to gain improved understanding of wave-solid interaction including shock wave attenuation and shock velocity variation.