Benxin Wu

A self-closed thermal model for laser shock peening under the water confinement regime configuration and comparisons to experiments

Laser shock peening (LSP) is emerging as a competitive alternative technology to classical treatments to improve fatigue and corrosion properties of metals for a variety of important applications. LSP under a water confinement regime (WCR) can produce plasma pressures on the target surface four times higher and two to three times longer than those under direct regime configurations. However, most of the published thermal models for LSP under WCR are not self-closed, and have free variables which have to come from experimental measurements under the same conditions. In this paper, a self-closed thermal model for LSP under WCR configurations is presented. This model has considered most of the relevant physical processes for laser ablation and plasma formation and expansion, and there are no free variables in the model. The simulation results for pressures from the model are compared with the available experimental results in literature under a variety of laser-pulse conditions, and good agreements are found.

From Incident Laser Pulse to Residual Stress: A Complete and Self-Closed Model for Laser Shock Peening

Laser shock peening (LSP) is emerging as a competitive alternative technology to classical treatments to improve fatigue and corrosion properties of metals for a variety of important applications. LSP is often performed under a water confinement regime, which involves several complicated physical processes. A complete and self-closed LSP model is presented in this paper, which requires a sequential application of three submodels: a breakdown-plasma model, a confined-plasma model, and a finite element mechanics model. Simulation results are compared with experimental data in many aspects under a variety of typical LSP conditions, and good agreements are obtained.

Modeling of nanosecond laser ablation with vapor plasma formation

A thermal model for nanosecond pulsed laser ablation is developed, where the heat conduction equation in the target and the gas dynamic equations in the vapor and ambient gas phase are coupled through the Knudsen layer (KL) relations for evaporation/recondensation at the target-vapor interface. The plasma formation and laser-plasma interactions are simulated in the model, which are found to have a significant effect on the laser-induced evaporation process. The shielding effect of the plasma reduces the laser energy reaching the target surface and therefore decreases the surface temperature, and the laser energy deposition in the plasma contributes to the increase of the vapor pressure above the KL. All of these will make the transition earlier from sonic evaporation stage to the subsonic evaporation and then to the recondensation stage, and therefore decrease the laser-induced evaporation depth. The simulation results are compared with experimental data for the plasma transmissivity, plasma front locations and velocities, laser ablation depth, and average plasma temperatures, and reasonably good agreements are obtained. This model is valid when the phase explosion does not occur, that is, when the target surface temperature does not reach or exceed the target material critical temperature.

Laser pulse transmission through the water breakdown plasma in laser shock peening

Laser shock peening (LSP) under a water confinement regime can produce plasma pressures on the target surface four times higher and 2–3 times longer than that under direct regime configurations. However, when the laser power density is above some threshold, a breakdown plasma occurs in water, which screens a significant amount of the incident laser pulse and therefore limits the magnitude and duration of the pressure induced on the target surface. A self-closed numerical model that can simulate the laser pulse transmission through the breakdown plasma generated in water during LSP has rarely been reported in literature. In this work, the breakdown plasma is simulated by solving an electron rate equation coupled with a Maxwell’s wave equation. The peak irradiance and duration of the laser pulse transmitted through the breakdown plasma predicted from the model can be correlated reasonably well with experimental data for 25 ns-1064 nm laser pulses. This model is then coupled with a previously developed therm...

Two dimensional hydrodynamic simulation of high pressures induced by high power nanosecond laser-matter interactions under water

In laser shock peening (LSP) under a water-confinement regime, laser-matter interaction near the coating-water interface can induce very high pressures in the order of gigapascals, which can impart compressive residual stresses into metal workpieces to improve fatigue and corrosion properties. For axisymmetric laser spots with finite size, the pressure generation near the water-coating interface is a two dimensional process in nature. This is in particular the case for microscale LSP performed with very small laser spots, which is a very promising technique to improve the reliability performance of microdevices. However, models capable of predicting two dimensional (2D) spatial distributions of the induced pressures near the coating-water interface in LSP have rarely been reported in literature. In this paper, a predictive 2D axisymmetric model is developed by numerically solving the hydrodynamic equations, supplemented with appropriate equations of state of water and the coating material. The model can p...

A one-dimensional hydrodynamic model for pressures induced near the coating-water interface during laser shock peening

In laser shock peening (LSP) under a water-confinement regime, laser-matter interaction near the coating-water interface can induce very high pressures in the order of gigapascal, which can impart compressive residual stresses into metal workpieces to improve fatigue and corrosion properties. However, self-closed models with spatial distribution considerations for the induced pressures near the coating-water interface in LSP are rarely reported in literature. In this paper, a self-closed model is developed by numerically solving the one-dimensional hydrodynamic equations, supplemented with appropriate equations of state of water and the coating material. The model can produce the one-dimensional spatial distributions of the material responses near the water-coating interface in LSP. The model-predicted pressures have been compared with experimental measurements under a variety of conditions typical for LSP, and good agreements have been found for both the transient pressure history and the peak pressure m...

Absorption coefficient of aluminum near the critical point and the consequences on high-power nanosecond laser ablation

During nanosecond laser ablation, the absorption coefficient determines the laser energy deposition in the target, the accurate knowledge of which near the material critical point is crucial for understanding the fundamental physics of high-power nanosecond laser ablation. In this letter, the absorption coefficient of aluminum near the critical point is calculated through the Drude model based on the measured electrical conductivity data, and its effect on laser ablation is investigated numerically using a heat transfer model. The result supports the experimental observations that phase explosion occurs for the ablation of aluminum by sufficiently intense laser pulses, and the model predicted phase explosion threshold is consistent with experimental measurements.

Parametric Study on Single Shot and Overlapping Laser Shock Peening on Various Metals via Modeling and Experiments

Laser shock peening (LSP) under water confinement regime involves several complicated physical phenomena. Among these phenomena, the interaction between laser and coating material during LSP is very important to the laser-induced residual stress, which has an important effect on the fatigue and corrosion properties of the substrate material. To gain a better understanding of this interaction, a series of experiments, including single shot, single-track overlapping, and multitrack overlapping LSP, has been carried out on various metals with different coatings. A 3D finite element model has also been developed to simulate the LSP process. Combining this with a previously developed confined plasma model, which has been verified by the experimental data from literature, the 3D finite element model is used to predict the residual stresses induced in the substrate material as well as the indentation profile on the substrate surface. The model prediction of indentation profiles is compared with the experimental data. The residual stresses in the depth direction are also validated against the X-ray diffraction measurement data for 4140 steel and Ti-6Al-4V, and good agreements are obtained for both predictions. The effect of process parameters on the residual stress is also investigated both experimentally and theoretically. [DOI: 10.1115/1.4002850]

Engineered and Laser‐Processed Chitosan Biopolymers for Sustainable and Biodegradable Triboelectric Power Generation

Recent advances achieved in triboelectric nanogenerators (TENG) focus on boosting power generation and conversion efficiency. Nevertheless, obstacles concerning economical and biocompatible utilization of TENGs continue to prevail. Being an abundant natural biopolymer from marine crustacean shells, chitosan enables exciting opportunities for low-cost, biodegradable TENG applications in related fields. Here, the development of biodegradable and flexible TENGs based on chitosan is presented for the first time. The physical and chemical properties of the chitosan nanocomposites are systematically studied and engineered for optimized triboelectric power generation, transforming the otherwise wasted natural materials into functional energy devices. The feasibility of laser processing of constituent materials is further explored for the first time for engineering the TENG performance. The laser treatment of biopolymer films offers a potentially promising scheme for surface engineering in polymer-based TENGs. The chitosan-based TENGs present efficient energy conversion performance and tunable biodegradation rate. Such a new class of TENGs derived from natural biomaterials may pave the way toward the economically viable and ecologically friendly production of flexible TENGs for self-powered nanosystems in biomedical and environmental applications.

Ultrasonic Cavitation Peening of Stainless Steel and Nickel Alloy

Ultrasonic cavitation peening is a peening process utilizing the high pressure induced by ultrasonic cavitation in liquids (typically water). However, the relevant previous investigations in the literature have been limited. In this paper, ultrasonic cavitation peening on stainless steel and nickel alloy has been studied, including the observation or characterization of the surface hardness, morphology, profile, roughness and oxygen contamination of treated workpiece samples. It has been found that for the studied situations, ultrasonic cavitation peening (at a sufficiently high horn vibration amplitude) can obviously enhance the workpiece surface hardness without significantly increasing the surface roughness, changing surface morphology observed by scanning electron microscope (SEM), or contaminating the surface by oxygen.© 2013 ASME