Vladimir V. Semak

Transient model for the keyhole during laser welding

A novel approach to simulating the dominant dynamic processes present during concentrated energy beam welding of metals is presented. A model for the transient behaviour of the front keyhole wall is developed. It is assumed that keyhole propagation is dominated by evaporation-recoil-driven melt expulsion from the beam interaction zone. Results from the model show keyhole instabilities consistent with experimental observations of metal welding, metal cutting and ice welding.

Effect of surface tension on melt pool dynamics during laser pulse interaction

A previously developed model for simulation of recoil-pressure induced melt displacement during laser pulse interaction has been upgraded to include the restraining effect of surface tension. The results of numerical simulations of melt displacement/ejection during laser welding and drilling using this enhanced model are presented. In particular, the dependences of the threshold pulse energies for melt displacement and melt ejection as functions of laser pulse duration, beam radius and beam intensity distribution are computed and analysed.

On the possibility of microwelding with laser beams

We define microwelds as having fusion zone dimensions of <100 μm. At such small dimensions the required laser irradiance to produce melting is at or above conventional estimates of the irradiance required to produce drilling. The question thus arises if such small microwelds can be made via laser processes. A theoretical criterion defining the threshold pulse energy and beam intensity required for melt displacement (necessary for penetration-mode welding or drilling) is proposed. The results of numerical simulation present dependences of the threshold pulse energy and beam intensity as functions of laser pulse duration and beam radius. Experimental verification of the proposed criterion is described and a comparison of theoretical predictions and measurements is presented.

Drilling of steel and HgCdTe with the femtosecond pulses produced by a commercial laser system

The results of interaction of single and multiple 200 fs laser pulses with thick stainless steel and HgCdTe samples are reported. The threshold laser energy density required to produce surface melting is measured. The melt dynamics and evolution of surface morphology are observed for different pulse energies and number of laser pulses. It is observed that, as with a long laser pulse interaction, a layer of melt can be produced at the sample surface. Melt ejection in the radial direction towards the periphery of the interaction zone is observed when the pulse energy is increased. The observed melt dynamics resembles the evaporation recoil melt removal typical of the laser interactions in the range from nanoseconds to continuous wave (CW). The observed melt ejection is attributed to a nanosecond component of the laser pulse with an estimated energy of approximately 25% of the total laser pulse energy. The measured melting threshold energy density for stainless steel is comparable with the published theoretically predicted threshold for nickel computed using a two-temperature model.

Role of beam absorption in plasma during laser welding

The relationship between beam focus position and penetration depth in CW laser welding was studied numerically and experimentally for different welding conditions. Calculations were performed using a transient hydrodynamic model that incorporates the effect of evaporation recoil pressure and the associated melt expulsion. The simulation results are compared with measurements made on a series of test welds obtained using a 1650 W CO{sub 2} laser. The simulations predict, and the experiments confirm, that maximum penetration occurs with a specific location of the beam focus, with respect to the original sample surface, and that this relationship depends on the processing conditions. In particular, beam absorption in the plasma has a significant effect on the relationship between penetration and focus position. When the process parameters result in strong beam absorption in the keyhole plasma, the maximum penetration will occur when the laser focus is at or above the sample surface. In a case of weak absorption however, the penetration depth reaches its maximum value when the beam focus is located below the sample surface. In all cases, the numerical results are in good agreement with the experimental measurements.

On the possible effect of pedestal pulse on material removal by ultrahigh intensity laser pulses

The results of the interaction of 200?fs and 4?ns laser pulses produced by a Ti?:?Sapphire commercial laser system with various materials are reported. The drilling rates were measured as a function of laser pulse energy and material thickness. Saturation of the material removal rate was observed for high laser pulse energy. The dependence of the material removal rate on the sample thickness was measured. The results of numerical simulation of laser drilling with femtosecond and nanosecond laser pulses using a theoretical model which includes phase transition at the critical temperature are reported. The computed drilling rates are in agreement with the data of measurements. The effect of a suspected nanosecond pedestal pulse containing approximately 30% of total pulse energy on the rate of drilling with femtosecond laser pulses is numerically simulated and the computed results are discussed.

Effect of power losses on self-focusing of high-intensity laser beam in gases

A theoretical study of power loss from periphery of an ultrashort pulse laser beam and temporally resolved defocusing produced by laser-induced plasma are performed using paraxial approximation. Our analysis incorporates consideration of spatial distribution of the laser beam irradiance and the results show that substantial power losses (10–80%) occur from the beam periphery limiting the length of a filament. It was also shown that the generally accepted concept of self-focusing critical power is inconsistent with consideration of self-induced refraction of spatially distributed laser beam. A new criterion for self-focusing and hypothesis for multiple filamentation are proposed.

Theoretical analysis of supercontinuum and coloured conical emission produced during ultrashort laser pulse interaction with gases

We use a conceptually new approach to theoretical modelling of self-focusing in which we integrated diffractive and geometrical optics in order to explain and predict emission of white light and coloured rings observed in ultrashort laser pulse interaction. In our approach, laser beam propagation is described by blending the solution of the linear Maxwells equation and a correction term that represents nonlinear field perturbation expressed in terms of paraxial ray-optics (eikonal) equation. No attempt is made to create an appearance of exhaustive treatment via use of complex mathematical models. Rather, emphasis is placed on elegance of the formulations leading to fundamental understanding of the underlying physics and, eventually, to an accurate practical numerical model capable of simulating white light generation and conical emission of coloured rings produced around the filament.

Application of melt ejection criterion in simulation of micromachining with laser

The theoretical criterion defining the threshold pulse energy and beam intensity required for melt ejection is proposed. The results of numerical simulation present dependencies of the threshold pulse energy and beam intensity as functions of laser pulse duration and beam radius. The experimental verification of proposed criterion is described and the comparison of theoretical predictions and measurements is presented. The criterion is applied for simulation of laser drilling metal foil with thickness in the range 25 μm - 125 μm using laser beam with 12 μm beam radius and pulse durations 10 ns and 100 ns. The computational results are used to interpret the results of experimental study of laser drilling of 125 μm aluminum foil using a single mode beam of a XeCl laser performed at the Nederlands Centrum voor Laser Research (NCLR) and the University of Twente. Additional results on Nd:YAG spot welds in pure Ni are also presented.

Ultrashort pulse laser micromachining performance enhancements

Lasers have long been promised to be a revolutionary technology for manufacturing, but have not yet achieved the anticipated market penetration. Major hurdles still exist in bringing new technologies, such as ultrashort pulse lasers, to the manufacturing floor, the first being easily repeatable, high quality laser micromachining. We explored methods to improve the spatial and temporal beam quality of a femtosecond laser system in an effort to reduce melt and heat affected zone using percussion and trepanning drilling techniques in the semiconductor HgCdTe. Drilling results will be presented of experiments conducted using spatial filters, beam homogenizers, and second harmonic generation at various laser pulse energies. We report the process parameters that produced the cleanest holes with minimal melt and ejected material.Lasers have long been promised to be a revolutionary technology for manufacturing, but have not yet achieved the anticipated market penetration. Major hurdles still exist in bringing new technologies, such as ultrashort pulse lasers, to the manufacturing floor, the first being easily repeatable, high quality laser micromachining. We explored methods to improve the spatial and temporal beam quality of a femtosecond laser system in an effort to reduce melt and heat affected zone using percussion and trepanning drilling techniques in the semiconductor HgCdTe. Drilling results will be presented of experiments conducted using spatial filters, beam homogenizers, and second harmonic generation at various laser pulse energies. We report the process parameters that produced the cleanest holes with minimal melt and ejected material.