Youneng Wang

Wave-solid interactions in laser-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 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 was 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. It is the first time to numerically and experimentally study the novel process of micro-scale LPF. 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.

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.

EFFECT OF PULSING PARAMETERS ON LASER ABLATIVE CLEANING OF COPPER OXIDES

The characteristics of copper oxide removal are comparably investigated under different pulsing strategies. A two-dimensional model is utilized to numerically simulate the laser ablative cleaning process. In the model, property discontinuity and Stephan and kinetic boundary conditions are taken into account, and the moving phase interface in the material is evaluated with the enthalpy method. Experiments are carried out on copper samples having different oxide layer thicknesses. The copper oxide layer thicknesses determined by ellipsometer and the chemical constituents of the copper oxide layer determined via x-ray photoelectron spectroscopy are incorporated into this numerical model. Under the single-pulse irradiation strategy, a higher laser intensity threshold is determined by the model based on the criterion of removing the oxide film as much as possible without damaging the substrate. Under the multipulse irradiation strategy, a lower threshold is employed to remove the oxide layer, while the appropr...

High gain and wide dynamic range punchthrough heterojunction phototransistors

In this article, we propose and demonstrate a novel punchthrough heterojunction phototransistor (HPT). The base of the transistor is lightly doped and completely depleted under the operating condition. The collector bias current can be applied without the base terminal. The transistors exhibit optical conversion gain as high as 1240 at an incident optical power as low as 0.5 μW, and the gain changes less than 15% over a 20 dB range of incident optical power. The transient measurements showed that the transistor has a high response speed than that of conventional two or three terminal HPTs. This represents the best performance of HPTs with similar dimensions. The results of simulation showed that the punchthrough HPTs have much lower noise characteristics than conventional HPTs. The principle reported here can be applied to HPTs made from other material systems, such as AlGaSb/GaSb and InP/InGaAs, for long wavelength optical communications.

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 ...

Spatially Resolved Characterization of Geometrically Necessary Dislocation Dependent Deformation in Microscale Laser Shock Peening

As the laser spot size in microscale laser shock peening is in the order of magnitude of several microns, the anisotropic response of grains will have a dominant influence on its mechanical behavior of the target material. Furthermore, conventional plasticity theory employed in previous studies needs to be re-examined due to the length scale effect. In the present work, the length scale effects in microscale laser shock peening have been investigated. The crystal lattice rotation underneath the shocked surface was determined via electron backscatter diffraction. From these measurements, the geometrically necessary dislocation (GND) density that the material contains has been estimated. The yield strength increment was then calculated from the GND distribution by using the Taylor model and integrated into each material point of the finite element method (FEM) simulation. Finite element simulations, based on single crystal plasticity, were performed for the process both with and without considering the GND hardening, and the comparison has been conducted.

Study of anisotropic character induced by microscale laser shock peening on a single crystal aluminum

The beam spot size used in microscale laser shock peening is of the same order as grain size in many materials. Therefore, the deformation is induced in only a few grains so that it is necessary to treat the material as being anisotropic and heterogeneous. In order to investigate the corresponding anisotropic features, different experimental techniques and three-dimensional finite element simulations are employed to characterize and analyze anisotropic responses for single crystal aluminum under single pulse shock peening at individual locations. X-ray microdiffraction techniques based on a synchrotron light source affords micron scale spatial resolution and is used to measure the residual stress spatial distribution along different crystalline directions on the shocked surface. Crystal lattice rotation due to plastic deformation is also measured with electron backscatter diffraction. The result is experimentally quantified and compared with the simulation result obtained from finite element analysis. The...

Dynamic Material Response of Aluminum Single Crystal Under Microscale Laser Shock Peening

The process of laser shock peening induces compressive residual stresses in a material to improve material fatigue life. For micron sized laser beams, the size of the laser-target interaction zone is of the same order of magnitude as the target material grains, and thus the target material must be considered as being anisotropic and inhomogeneous. Single crystals are chosen to study the effects of the anisotropic mechanical properties. It is also of interest to investigate the response of symmetric and asymmetric slip systems with respect to the shocked surface. In the present study, numerical and experimental aspects of laser shock peening on two different crystal surfaces (110) and (114) of aluminum single crystals are studied. Lattice rotations on the top surface and cross section are measured using electron backscatter diffraction, while residual stress is characterized using X-ray microdiffraction. A numerical model has been developed that takes into account anisotropy as well as inertial terms to predict the size and nature of the deformation and residual stresses. Obtained results were compared with the experimental finding for validation purpose.

Response of Thin Films and Substrate to Micro-Scale Laser Shock Peening

Micro-scale laser shock peening ({mu}LSP) can potentially be applied to metallic structures in microdevices to improve fatigue and reliability performance. Copper thin films on a single-crystal silicon substrate are treated by using {mu}LSP and characterized using techniques of X-ray microdiffraction and electron backscatter diffraction (EBSD). Strain field, dislocation density, and microstructure changes including crystallographic texture, grain size and subgrain structure are determined and analyzed. Further, shock peened single crystal silicon was experimentally characterized to better understand its effects on thin films response to {mu}LSP. The experimental result is favorably compared with finite element method simulation based on single-crystal plasticity.

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.