Volkher Onuseit

Heat accumulation during pulsed laser materials processing

Laser materials processing with ultra-short pulses allows very precise and high quality results with a minimum extent of the thermally affected zone. However, with increasing average laser power and repetition rates the so-called heat accumulation effect becomes a considerable issue. The following discussion presents a comprehensive analytical treatment of multi-pulse processing and reveals the basic mechanisms of heat accumulation and its consequence for the resulting processing quality. The theoretical findings can explain the experimental results achieved when drilling microholes in CrNi-steel and for cutting of CFRP. As a consequence of the presented considerations, an estimate for the maximum applicable average power for ultra-shorts pulsed laser materials processing for a given pulse repetition rate is derived.

Heat accumulation effects in short-pulse multi-pass cutting of carbon fiber reinforced plastics

The formation of a matrix evaporation zone (MEZ) in carbon fiber reinforced plastics during multi-pass laser cutting with picosecond laser pulses is studied for a wide range of pulse frequencies (fpu2009=u200910–800u2009kHz) and feed rates (vfu2009=u20090.002–10u2009m/s). Three regimes of the formation of the MEZ are found and related with different heat accumulation effects: (i) small MEZ (<2u2009μm) with negligible heat accumulation, (ii) moderate-size MEZ (up to a few hundred microns) determined by heat accumulation between pulses, and (iii) large MEZ (up to a few millimeters) caused by heat accumulation between scans. The dependence of the size of the MEZ on the number of scans and the scan frequency was studied to distinguish the two heat accumulation effects (between pulses and between scans), which occur on different time-scales. A diagram to illustrate the boundaries between the three regimes of the formation of the MEZ as a function of feed rate and pulse frequency is proposed as a promising base for further studies and as ...

Oxygen-assisted multipass cutting of carbon fiber reinforced plastics with ultra-short laser pulses

Deep multipass cutting of bidirectional and unidirectional carbon fiber reinforced plastics (CFRP) with picosecond laser pulses was investigated in different static atmospheres as well as with the assistance of an oxygen or nitrogen gas flow. The ablation rate was determined as a function of the kerf depth and the resulting heat affected zone was measured. An assisting oxygen gas flow is found to significantly increase the cutting productivity, but only in deep kerfs where the diminished evaporative ablation due to the reduced laser fluence reaching the bottom of the kerf does not dominate the contribution of reactive etching anymore. Oxygen-supported cutting was shown to also solve the problem that occurs when cutting the CFRP parallel to the fiber orientation where a strong deformation and widening of the kerf, which temporarily slows down the process speed, is revealed to be typical for processing in standard air atmospheres.

Influence of laser parameters on quality of microholes and process efficiency

To enable the direct-spinning process of super-micro fibres (< 0.5 dtex) suitable for novel medical, hygienical and technical products microhole arrays with diameters down to 25 μm in very high quality are required. Using ultrashort pulses together with a helical drilling optics microholes with high accuracy were manufactured in metals of a thickness in the range of 0.3 mm. However, the required process time for a single microhole ranges up to several ten seconds. Simple energy balance considerations show that higher averaged powers - either achieved with larger pulse energies or an increased repetition rate - considerable reduce the process time. In this case plasma formation and heat accumulation show an increased formation of melt and recast. Thus, the objective is to increase the productivity while maintaining consistent quality of the microholes. With this aim, the influence of pulse energy and repetition rate on the borehole geometry, processing quality and process efficiency was investigated for helical drilling. In the present research work a TruMicro 5250 laser source (tp = 8 ps, λ=515 nm, fR=800 kHz) was used. To determine the process time of the microhole the transmitted laser radiation was recorded. A systematic evaluation of the process quality and process time dependent on pulse energy and repetition rate will be presented in this contribution. First laser manufactured spinning nozzles with microhole diameters down to 25 μm processed in 0.24 mm thick AuPt alloy were used to fabricate unique super-micro fibres with yarn counts down to 0.2 dtex.

High-efficiency laser processing of CFRP

Industrial laser processing of carbon fibres is very promising for large-volume production of CFRP lightweight parts. Yet reduced quality and the large process energy needed for processing carbon requiring high average powers actually limits the use of laser technology.However, choosing appropriate laser parameters and processing strategies the thermal damage caused by the laser radiation can be utilized to realize a very efficient processing of CFRP: In the first step only small kerfs are created by sublimating the carbon material. By producing two close kerfs fibre fragments are created which can be removed in a second step by just sublimating the matrix material. The detached fiber fragments are either removed by the ablating material pressure or by an additional process gas jet.The present paper compares the required absorbed energy densities for the different processing strategies of CFRP. The theoretical considerations are compared with experimental results achieved with ns-drilling of blind holes.Industrial laser processing of carbon fibres is very promising for large-volume production of CFRP lightweight parts. Yet reduced quality and the large process energy needed for processing carbon requiring high average powers actually limits the use of laser technology.However, choosing appropriate laser parameters and processing strategies the thermal damage caused by the laser radiation can be utilized to realize a very efficient processing of CFRP: In the first step only small kerfs are created by sublimating the carbon material. By producing two close kerfs fibre fragments are created which can be removed in a second step by just sublimating the matrix material. The detached fiber fragments are either removed by the ablating material pressure or by an additional process gas jet.The present paper compares the required absorbed energy densities for the different processing strategies of CFRP. The theoretical considerations are compared with experimental results achieved with ns-drilling of blind holes.

Ablation dynamics and shock wave expansion during laser processing of CFRP with ultrashort laser pulses

Carbon fibre reinforced plastics (CFRP) have a large potential in the automotive lightweight construction due to their low density and high mechanical stability. Compared with today’s laser processing methods of metals the main issues in laser processing of CFRP are the very differing thermal, optical and mechanical properties of the components. To understand the process in detail, the ablation process of CFRP with ultrashort laser pulses was investigated. The shock wave and the vapor resulting from processing with single laser pulses were recorded. Shadow photography and luminescence photography with an ultra-high-speed camera was used to show the ablation process with a temporary resolution of up to 3 ns. The field of view was 250 μm × 250 μm. An ultrashort laser pulse with pulse duration of 4 ps and a wavelength of 800 nm was focused onto the workpiece. The energy content of the shock wave was calculated from the resulting images. The energy content of the shock wave was about 20 % of the incident energy and the speed of propagation of the shock wave was more than 2000 m/s. The high intensities in the range of 1013 W/cm2 lead to formation of a plasma plume which was clearly seen in the shadow photography images.

Influence of cut front temperature profile on cutting process

Laser cutting of metals is one of the most common applications in industrial manufacturing. Due to this, every improvement of process efficiency and the consequential increase of cutting velocity while maintaining the quality implies a high economic potential. A key factor influencing the maximum cutting speed is the distribution of the absorbed laser power which depends on the polarization state of the incident laser beam.This work presents experimental results obtained with a radially polarized CO2 Laser with an output power of 3700W. These results are compared with results from cutting with a standard circularly polarized beam. The investigation is focused on the different behavior of the cutting process due to the different polarization states. The emission spectra of the cutting front were measured with a spectrometer with high spatial resolution. The temperature was determined by fitting a Planck black-body radiation to the calibrated continuum emission. The temperature measurement has an accuracy of ±100K and a spatial resolution of 136 measurement points along the cut front.The temperature profiles of the cutting front are shown to be different for the two polarizations. This difference is supposed to be the reason for the observed difference in the resulting cutting edge surface.Laser cutting of metals is one of the most common applications in industrial manufacturing. Due to this, every improvement of process efficiency and the consequential increase of cutting velocity while maintaining the quality implies a high economic potential. A key factor influencing the maximum cutting speed is the distribution of the absorbed laser power which depends on the polarization state of the incident laser beam.This work presents experimental results obtained with a radially polarized CO2 Laser with an output power of 3700W. These results are compared with results from cutting with a standard circularly polarized beam. The investigation is focused on the different behavior of the cutting process due to the different polarization states. The emission spectra of the cutting front were measured with a spectrometer with high spatial resolution. The temperature was determined by fitting a Planck black-body radiation to the calibrated continuum emission. The temperature measurement has an accuracy o...

Efficient processing of CFRP with a picosecond laser with up to 1.4 kW average power

Laser processing of carbon fiber reinforce plastic (CFRP) is a very promising method to solve a lot of the challenges for large-volume production of lightweight constructions in automotive and airplane industries. However, the laser process is actual limited by two main issues. First the quality might be reduced due to thermal damage and second the high process energy needed for sublimation of the carbon fibers requires laser sources with high average power for productive processing. To achieve thermal damage of the CFRP of less than 10μm intensities above 108 W/cm² are needed. To reach these high intensities in the processing area ultra-short pulse laser systems are favored. Unfortunately the average power of commercially available laser systems is up to now in the range of several tens to a few hundred Watt. To sublimate the carbon fibers a large volume specific enthalpy of 85 J/mm³ is necessary. This means for example that cutting of 2 mm thick material with a kerf width of 0.2 mm with industry-typical 100 mm/sec requires several kilowatts of average power. At the IFSW a thin-disk multipass amplifier yielding a maximum average output power of 1100 W (300 kHz, 8 ps, 3.7 mJ) allowed for the first time to process CFRP at this average power and pulse energy level with picosecond pulse duration. With this unique laser system cutting of CFRP with a thickness of 2 mm an effective average cutting speed of 150 mm/sec with a thermal damage below 10μm was demonstrated.

High ablation rate laser processing of CFRP for repair purpose

The use of Carbon Fiber Reinforced Plastics (CFRP) in industrial mass production has been rising dramatically in the last few years due to its light weight and high mechanical strength. However, like any other material, structures made of CFRP may get damaged at high service load or by unpredictable impacts. The aim of this study was to maximize the ablation rate in order to optimize the removing of the damaged material for industrial applications. A nanosecond laser with an average power of 21u2005W and a galvano scanner were used to treat CFRP surface. The strategy presented in this paper comprised two consecutive steps of grooving and removing. In the grooving step, the fibers cut into small fragments. In the removing step, the defocused beam scanned between two adjacent grooves with sufficient energy to sublimate the plastic matrix. By implementing the “grooving/removing” strategy, an ablation rate of 1.8u2005mm3/s was achieved which is about four times higher than ablation with pure sublimation.The use of Carbon Fiber Reinforced Plastics (CFRP) in industrial mass production has been rising dramatically in the last few years due to its light weight and high mechanical strength. However, like any other material, structures made of CFRP may get damaged at high service load or by unpredictable impacts. The aim of this study was to maximize the ablation rate in order to optimize the removing of the damaged material for industrial applications. A nanosecond laser with an average power of 21u2005W and a galvano scanner were used to treat CFRP surface. The strategy presented in this paper comprised two consecutive steps of grooving and removing. In the grooving step, the fibers cut into small fragments. In the removing step, the defocused beam scanned between two adjacent grooves with sufficient energy to sublimate the plastic matrix. By implementing the “grooving/removing” strategy, an ablation rate of 1.8u2005mm3/s was achieved which is about four times higher than ablation with pure sublimation.

Heat accumulation during pulsed laser materials processing: erratum

With this erratum we aim to correct a transcription error that occurred in our previous paper: In Eq. (3)a)-(3c), Eq. (5), and Eq. (6) the Greek characters were not converted correctly. The properly formatted formulae are listed below. All other contents, calculations and conclusions of the original paper remain unchanged.