Evaporation Recoil Pressure Research Articles

Novel Micro-Welding of Silicon and Glass by Ultrashort Pulsed Laser

The laser welding can provide flexible processing, and ultrashort pulsed laser with high pulse repetition rates enabled locally selective welding of monocrystalline silicon and glass with comparable processing performance to anodic bonding method. The 20ps laser pulse of 1060nm was absorbed at monocrystalline silicon through glass plate, and its surface temperature reached its boiling temperature. In addition, it was considered that the absorption of laser energy to glass was occurred, and the temperature of glass increased more than the forming temperature. Thus the convection of silicon and glass was caused by the recoil pressure of evaporation, which led to mild mixture of silicon and glass. This phenomenon produced anchor geometry at the interface between silicon and glass without gap generation. The number of laser shot in the laser spot had an influence on shearing strength of weld joint, and higher shearing strength of weld joint could be performed at proper number of laser shot in the laser spot.

Through nanohole formation in thin metallic film by single nanosecond laser pulses using optical dielectric apertureless probe

Separate nanoholes with the minimum size down to 35 nm (~λ/15) and nanohole arrays with the hole size about 100 nm (~λ/5) were fabricated in a 50 nm optically "thick" Au/Pd film, using single 532 nm pump nanosecond laser pulses focused to diffraction-limited spots by a specially designed apertureless dielectric fiber probe. Nanohole fabrication in the metallic film was found to result from lateral heat diffusion and center-symmetrical lateral expulsion of the melt by its vapor recoil pressure. The optimized apertureless dielectric microprobe was demonstrated to enable laser fabrication of deep through nanoholes.

Elucidation of Welding Stability Based on Keyhole Configuration during High-Power Fiber Laser Welding

During deep penetration laser welding, a keyhole is formed in the molten pool due to the intense recoil pressure of evaporation. The formation of the keyhole leads to a deep penetration weld with a high aspect ratio and this is the most advantageous feature of welding by high-energy-density beams. The configuration and characteristics of a keyhole are related to the welding stability. In a fiber laser butt-joint welding of Type 304 austenitic stainless steel plate with a high power 10 kW continuous wave fiber laser, an infrared sensitive high-speed video camera was used to capture the dynamic images of the molten pools. The configurations of a keyhole were analyzed through image processing techniques such as median filtering, wiener filtering and gray level threshold segmentation to obtain the edge of a keyhole. The width and the area of a keyhole were defined as the keyhole characteristic parameters, and the deviation between the laser beam and weld center as a parameter reflecting the welding stability. By analyzing the change of the keyhole characteristic parameters during welding process, it was found that these parameters were related to the welding stability. Welding experimental results and analysis of the keyhole characteristic parameters confirmed that the welding stability could be monitored and distinguished by a keyhole configuration during high-power fiber laser welding.

Simulation of Light-Propulsion Acceleration of Powder Particles for Laser Direct Metal Deposition

The results of the numerical analysis of heat- and mass-exchange processes at powder particles motion in a gas flow and radiation during the laser direct metal deposition are presented. The model regarding the particle acceleration due to the lightpropulsion force caused by the vapor recoil pressure from the beamed surface of the particle is proposed. The reactive force results in the noticeable particle acceleration toward the radiation direction, and the particle velocity may exceed significantly the velocity of the carrying gas. The radius of particles slightly varies due to the evaporation; the losses in the clad material mass are negligibly small.

Numerical simulation of transient temperature field during laser keyhole welding of 304 stainless steel sheet

A three-dimensional transient numerical model was developed to study the temperature field and molten pool shape during continuous laser keyhole welding. The volume-of-fluid (VOF) method was employed to track free surfaces. Melting and evaporation enthalpy, recoil pressure, surface tension, and energy loss due to evaporating materials were considered in this model. The enthalpy-porosity technique was employed to account for the latent heat during melting and solidification. Temperature fields and weld pool shape were calculated using FLUENT software. The calculated weld dimensions agreed reasonable well with the experimental results. The effectiveness of the developed computational procedure had been confirmed.

A study on laser drilling of thin steel sheet in air and underwater

In laser drilling of a thin stainless steel sheet in air with Nd:YAG laser pulses of 0.5–1 ms durations it was observed that the 0.5 ms duration laser pulse was more effective in drilling a through-hole than the relatively longer laser pulses with proportionately more energy. Further, laser drilling could be readily done when the sheet was placed at the focal point of the lens and below it but not above the focal point. On the other hand, the underwater laser drilling could be done when the sheet was placed above the focal point. An attempt has been made to explain these experimental observations considering various processes involved in laser drilling in air and underwater. While the recoil pressure of the vapor and plasma played an important role in laser drilling in air; the radial gradient of recoil pressure of evaporation, the Marangoni force induced by the surface tension gradient in melt pool and the cavitation effect of bubble collapse were believed to be responsible for the material removal in underwater drilling process.

Weld pool flows during initial stages of keyhole formation in laser welding

Weld pool transport phenomena during the transition from conduction-mode laser spot welding to keyhole laser spot welding of titanium were studied by numerical simulation. A range of laser powers were simulated and temperature dependent evaporation recoil pressure and cooling were applied as boundary conditions on the weld pool surface. Simulation results predicted a complex time-varying flow pattern during weld pool development. The surface-normal flow at the weld pool centre oscillated between upwards and downwards during the simulation time due to interaction of competing effects of evaporation recoil and surface tension pressures and laser heating and evaporation cooling. The results show that the laser weld pool flow dynamics play a key role during the transition from conduction-mode laser welding to keyhole welding.

Mechanism of keyhole formation and stability in stationary laser welding

The formation and stability of stationary laser weld keyholes are investigated using a numerical simulation. The effect of multiple reflections in the keyhole is estimated using the ray tracing method, and the free surface profile, flow velocity and temperature distribution are calculated numerically. In the simulation, the keyhole is formed by the displacement of the melt induced by evaporation recoil pressure, while surface tension and hydrostatic pressure oppose cavity formation. A transition mode having the geometry of the conduction mode with keyhole formation occurs between the conduction and keyhole modes. At laser powers of 500 W and greater, the protrusion occurs on the keyhole wall, which results in keyhole collapse and void formation at the bottom. Initiation of the protrusion is caused mainly by collision of upward and downward flows due to the pressure components, and Marangoni flow has minor effects on the flow patterns and keyhole stability.

A calculation model for the evaporation recoil pressure in lasermaterial processing

A physical model is proposed to calculate the recoil pressure actingon the surface of an evaporating material irradiated by a high-intensity laserbeam, such as encountered in laser material processing. Laser attenuation inthe laser-induced plasma plume, reflection at the material surface and theenergy consumption for material heating and melting are considered in thismodel. This evaporation model represents a revised form of that previouslyemployed by Chen Xi (1996, 1997, 1998) to calculate the evaporation-addedpressure acting on an evaporating particle exposed to a thermal plasma, buthere the special features associated with laser material processing areconsidered. The calculated recoil pressures compare favourably withavailable experimental data.

Role of beam absorption in plasma during laser welding

The relationship between beam focus position and penetration depthin CW laser welding was studied numerically and experimentally for differentwelding conditions. Calculations were performed using a transient hydrodynamicmodel that incorporates the effect of evaporation recoil pressure and theassociated melt expulsion. The simulation results are compared withmeasurements made on a series of test welds obtained using a 1650 W CO2laser. The simulations predict, and the experiments confirm, that maximumpenetration occurs with a specific location of the beam focus, with respect tothe original sample surface, and that this relationship depends on theprocessing conditions. In particular, beam absorption in the plasma has asignificant effect on the relationship between penetration and focus position.When the process parameters result in strong beam absorption in the keyholeplasma, the maximum penetration will occur when the laser focus is at or abovethe sample surface. In a case of weak absorption however, the penetrationdepth reaches its maximum value when the beam focus is located below thesample surface. In all cases, the numerical results are in good agreement withthe experimental measurements.

Keyhole Behavior during High Power YAG Laser Welding.

In laser welding, power density is very high at the processing points and a cavity called a keyhole is generated by the recoil pressure of evaporation. The authors examined keyhole and plasma/plume behavior using a microfocused X-ray transmission in-situ imaging system and a high speed video camera during spot welding and bead-on-plate welding of SUS304 stainless steel. The following results were observed. The keyhole was generated to a small degree by the first shot of a 26 J pulsed beam and became gradually deeper by increasing the number of shots in spot welding. When a pulsed laser beam of 1.1 ms in rise time and 7.6kW in peak power was used in bead-on-plate welding, the plasma/plume began to be generated in about 0.22ms after the initiation of laser beam irradiation, the keyhole began to form after about 0.6 or 0.7ms, became its deepest after about 1.5ms from the start of laser irradiation, and disappeared after about 1.3ms following the laser termination. The keyhole formation time was 0.6ms in rise time and the average velocity of keyhole growth was about 6m/s at this time.

Temporal evolution of the temperature field in the beam interaction zone during laser material processing

This paper presents a numerical simulation of the temperature field resulting from the impingement of a laser beam on a metal surface. The calculations were based on a hydrodynamic physical model of laser-material interaction, which includes the effect of evaporation recoil pressure on melt flow and the related convective heat transfer. The computations follow an experimentally verified physical model of melt hydrodynamics and the model of laser-induced evaporation developed by Anisimov. The simulations indicate that convective heat transfer which is induced by recoil pressure is significant for absorbed laser intensities from 0.5 MW cm-2 to 10 MW cm-2. This range corresponds to laser welding, cutting and drilling due to melt ejection (hydrodynamic drilling). The simulations also show that the motion of the melt from the centre of the beam interaction zone towards the periphery results in a secondary maxima in the temperature distribution, which in turn can lead to instability of the melt flow and temperature field fluctuations. The model also predicts that the cooling rate in and around the fusion zone is strongly influenced by the recoil-pressure-induced melt flow. At the centre of the beam interaction zone, the cooling rate is much higher (~10 times), and at the periphery, much slower than predicted by a pure heat conduction model.

Excimer laser ablation removal of thin chromium films from glass substrates

The excimer laser has been widely used for the removal of thin metal films . In this work, excimer laser ablation was applied to remove chromium films from glass substrates. To understand the removal mechanism and to seek the optimal processing conditions for pattern generation, a one-dimensional heat flow model was formulated which incorporated melting and vaporization. Morphological investigations were carried out experimentally on samples having different chromium film thicknesses that were irradiated with a KrF excimer laser. The thresholds for film damage, complete removal and glass damage were determined experimentally and compared with the calculated results. The measured single-shot ablation rate is about twice as high as the numerical predictions based on a heat conduction theory. The removal mechanism of the chromium film is found to be divided into two steps of pure vaporization and melt expulsion by vapor recoil pressure. Optimal processing conditions were determined to enhance the accuracy and quality of film removal for pattern generation. Finally, the slit disk of a rotary encoder was manufactured using the optimal processing conditions established.

Dynamics of keyhole and molten pool in laser welding

In laser and electron-beam welding, a deep cavity called a keyhole or beam hole is formed in the weld pool due to the intense recoil pressure of evaporation. The formation of the keyhole leads to a deep penetration weld with a high aspect ratio and this is the most advantageous feature of welding by high-energy-density beams. However, a hole drilled in a liquid is primarily unstable by its nature and the instability of the keyhole also causes the formation of porosity or cavities in the weld metal. In particular, the porosity formation is one of the serious problems in very high-power laser welding, but its mechanism has not been well understood. The authors have conducted systematic studies on observation of keyhole as well as weld pool dynamics and their related phenomena to reveal the mechanism of porosity formation and its suppression methods. The article will describe the real-time observation of keyhole and plume behaviors in the pulsed and continuous-wave laser welding by high-speed optical and x-ray transmission methods, the cavity formation process and its suppression measures.

Hydrodynamic processes in a melt bath under laser–arc interaction conditions

An investigation is reported of the physical features of the hydrodynamic processes taking place in a melt bath in the case of the laser–arc interaction with metals when the laser radiation intensity was in the range 0.5–2 MW/cm2. The velocity of the melt on the surface of the bath was determined experimentally. The dominant role of the vapor recoil pressure in the formation of liquid metal streams was noted. When the laser radiation intensity was ~ 2 MW/cm2 the melt particles participated in a vibrational process which was superimposed on a vortex motion of the melt and complicated the mixing mechanism. A model of the process was proposed and its main characteristics were determined.

Effect of chopping on laser penetration of metal targets

A new technique for increasing the laser penetration efficiency of metals has been developed. By amplitude modulating a free-running neodymium:yttrium aluminum garnet laser pulse, the depth of the crater was increased and the threshold energy for reliable hole drilling was decreased significantly. It was observed that the effect of chopping was optimal at 8–12 kHz with a 70% duty cycle. We believe that this improvement is due to an increase in the vapor recoil pressure and reduced plasma screening. Possible acoustic resonance effects have also been considered and will be discussed.

Formation of periodic surface ripples under the action of pulsed carbon dioxide laser radiation on fused silica

We report theoretical and experimental investigations of the formation of periodic surface ripples (PSR’s) on fused silica under normal incidence of pulsed linearly polarized light of a wavelength-tunable CO2 laser (λ = 10.6, 10.3, 9.6, and 9.3 μm). For the first time to our knowledge, the dielectric permittivity ℰ = ℰ′ + iℰ″ = (n + im)2 of silica under conditions of PSR formation has been measured. PSR’s are generated at all values of λ. It is shown that the inequality ℰ′ 1. A theoretical description of the phenomenon is based on a model, which considers silica under conditions of PSR formation to be a viscous liquid. The fundamental mechanisms of PSR formation are the generation of capillary waves in this liquid owing to vapor recoil pressure and nonuniform removal of the substance from the surface by evaporation. It is shown that good agreement between theoretical and experimental results can be achieved only if a complete theory of PSR is applied, including both electodynamic and thermal processes as well as the calculation of growth rates for capillary waves with allowance made for the values of ℰ′ and ℰ″, which were measured during the PSR formation. We have demonstrated that it is impossible to describe the phenomenon of PSR’s on the fused silica surface by using only the electrodynamic model.