Hongqiang Chen

Characterization of Plastic Deformation Induced by Microscale Laser Shock Peening

Electron backscatter diffraction (EBSD) is used to investigate crystal lattice rotation caused by plastic deformation during high-strain rate laser shock peening in single crystal aluminum and copper sample on (I 10) and (001) surfaces. New experimental methodologies are employed which enable measurement of the in-plane lattice rotation under approximate plane-strain conditions. Crystal lattice rotation on and below the microscale laser shock peened sample surface was measured and compared with the simulation result obtained from FEM analysis, which account for single crystal plasticity. The lattice rotation measurements directly complement measurements of residual strain/stress with X-ray micro-diffraction using synchrotron light source and it also gives an indication of the extent of the plastic deformation induced by the microscale laser shock peening.

SPATIALLY RESOLVED CHARACTERIZATION OF RESIDUAL STRESS INDUCED BY MICRO SCALE LASER SHOCK PEENING

Single crystal aluminum and copper of (001) and (110) orientation were shock peened using laser beam of 12 micron diameter and observed with X-ray micro-diffraction techniques based on a synchrotron light source. The X-ray micro-diffraction affords micron level resolution as compared with conventional X-ray diffraction which has only mm level resolution. The asymmetric and broadened diffraction profiles registered at each location were analyzed by sub-profiling and explained in terms of the heterogeneous dislocation cell structure. For the first time, the spatial distribution of residual stress induced in micro-scale laser shock peening was experimentally quantified and compared with the simulation result obtained from FEM analysis. Difference in material response and microstructure evolution under shock peening were explained in terms of material property difference in stack fault energy and its relationship with cross slip under plastic deformation. Difference in response caused by different orientations (110 and 001) and active slip systems was also investigated.

Predictive Modeling for Glass-Side Laser Scribing of Thin Film Photovoltaic Cells

Laser scribing of multilayer thin films is an important process for producing integrated serial interconnection of minimodules, used to reduce photocurrent and resistance losses in a large-area solar cell. Quality of such scribing contributes to the overall quality and efficiency of the solar cell and therefore predictive capabilities of the process are essential. Limited numerical work has been performed in predicting the thin film laser removal processes. In this study, a sequentially-coupled multilayer thermal and mechanical finite element model is developed to analyze the laser-induced spatio-temporal temperature and thermal stress responsible for SnO2:F film removal. A plasma expansion induced pressure model is also investigated to simulate the non-thermal film removal of CdTe due to the micro-explosion process. Corresponding experiments on SnO2:F films on glass substrates by 1064nm ns laser irradiation show a similar removal process to that predicted in the simulation. Differences between the model and experimental results are discussed and future model refinements are proposed. Both simulation and experimental results from glass-side laser scribing show clean film removal with minimum thermal effects indicating minimal changes to material electrical properties.

Modeling Schemes, Transiency, and Strain Measurement for Microscale Laser Shock Processing

In this paper, a coupled modeling scheme, which considered the dynamic evolution of laser-induced plasma and the complete physical interaction between the plasma, confined medium, coating layer, and processed metal, is compared with two decoupled modeling schemes in which shock pressure was first determined and used as loading in subsequent FEM-based stress/strain analysis. The relative merits and limitations of these schemes are evaluated in terms of their ability to describe process transiency such as shock pressure, shock velocity, and dynamic deformation history and to predict the stress/strain to be imparted into a target material. Both bulk and thin-film samples of copper were studied. Model predictions were investigated together with strain measurements based on an X-ray microdiffraction technique.

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...

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.

Systematical Characterization of Material Response to Microscale Laser Shock Peening

The response of materials after microscale laser shock peening ( mLSP) was experimentally characterized and compared with the theoretical prediction from FEM analysis in microlength level. Since mLSP is predominantly a mechanical process instead of a thermal process, the characterization focuses on mechanical properties and associated microstructures. An X-ray microdiffraction technique was applied on the postpeened single crystal aluminum of (001) and (110) orientations, and an X-ray profile was analyzed by subprofiling and Fourier analysis method. Spatially resolved residual stress and strain deviation was quantified and explained in terms of the heterogeneous dislocation cell structure. In-plane crystal lattice rotation induced bymLSP were measured by electron backscatter diffraction (EBSD) and compared with the FEM simulation. Average mosaic size was evaluated from X-ray profile Fourier analysis and compared with the result from EBSD. Surface strength increase and dislocation cell structure formation were studied. The systematical characterization helps develop more realistic simulation models and obtain better understanding in microlength level.@DOI: 10.1115/1.1811115#

Removal Mechanism and Defect Characterization for Glass-Side Laser Scribing of CdTe/CdS Multilayer in Solar Cells

Laser scribing is an important manufacturing process used to reduce photocurrent and resistance losses and increase solar cell efficiency through the formation of serial interconnections in large-area solar cells. High-quality scribing is crucial since the main impediment to large-scale adoption of solar power is its high production cost (price-per-watt) compared to competing energy sources such as wind and fossil fuels. In recent years, the use of glass-side laser scribing processes has led to increased scribe quality and solar cell efficiencies, however, defects introduced during the process such as thermal effect, micro cracks, film delamination, and removal uncleanliness keep the modules from reaching their theoretical efficiencies. Moreover, limited numerical work has been performed in predicting thin film laser removal processes. In this study, a nanosecond (ns) laser with a wavelength at 532nm is employed for pattern 2 (P2) scribing on CdTe (Cadmium telluride) based thin-film solar cells. The film removal mechanism and defects caused by laser-induced micro-explosion process are studied. The relationship between those defects, removal geometry, laser fluences and scribing speeds are also investigated. Thermal and mechanical numerical models are developed to analyze the laser-induced spatio-temporal temperature and pressure responsible for film removal. The simulation can well-predict the film removal geometries, generation of micro cracks, film delamination and remaining materials. The characterization of removal qualities will enable the process optimization and design required to enhance solar module efficiency.

Response of thin films and substrate to micro scale laser shock peening

Micro scale laser shock peening (µLSP) can potentially be applied to metallic structures in micro devices to improve fatigue and reliability performance. Copper thin films on single-crystal silicon substrate are treated by using µLSP and characterized using techniques of X-ray micro-diffraction 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 µLSP. The experimental result is favorably compared with FEM simulation based on single crystal plasticity.Micro scale laser shock peening (µLSP) can potentially be applied to metallic structures in micro devices to improve fatigue and reliability performance. Copper thin films on single-crystal silicon substrate are treated by using µLSP and characterized using techniques of X-ray micro-diffraction 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 µLSP. The experimental result is favorably compared with FEM simulation based on single crystal plasticity.

Experimental Characterization and Simulation of Three Dimensional Plastic Deformation Induced by Microscale Laser Shock Peening

Different experimental techniques and 3D FEM simulations are employed to characterize and analyze the three dimensional plastic deformation and residual strain/stress distribution for single crystal Aluminum under microscale laser shock peening assuming finite geometry. Single pulse shock peening at individual locations was studied. X-ray micro-diffraction 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 (EBSD). The result is experimentally quantified and compared with the simulation result obtained from FEM analysis. The influence of the finite size effect, crystalline orientation are investigated using single crystal plasticity in FEM analysis. The result of the 3D simulations of a single shock peened indentation are compared with the FEM results for a shocked line under 2D plain strain deformation assumption. The prediction of overall character of the deformation and lattice rotation fields in three dimensions will lay the ground work for practical application of μLSP.Copyright