OBSERVATION OF THERMOCAPILLARY EFFECT IN GAS-ASSISTED CO 2 LASER CUTTING OF STEEL

Investigations of melt’s dynamics have been performed with pyrometer in the gasjet-assisted CO2 laser cutting. Pyrometer consists of 4 sensors and local luminosity of metal melt has been measured precisely on different depth along front of cutting with long dimension resolution about 0.1 mm and with time resolution as short as 0.06 ms. The results of data processing of brightness temperature are reported for laser cutting of mildsteel plate 3 mm, 6 mm and 10 mm thick, that allows obtaining spectra of melt’s surface deformation for different values of cutting velocity and assisted gas pressure. The generation of capillary-wave turbulence following thermocapillary effect can be observed in the locations, where intensity of radiation of CO2 laser on melt’s surface exceeds 1 MW/cm. The R-M-S of temperature fluctuations is greater than 10 K in the range of sub-millimeter capillary waves. Thus, an additional mechanism of the anomalous absorption on the front of cutting can compensate the low absorption of the metal in case of 10.6 μm laser in comparison with the absorption of the metal in the near infrared range.


Introduction
An investigation of the dynamics of the laser radiation action on the material and a development of the monitoring systems is one of the directions of quality improvement in laser-thermal technology [1][2][3][4].We have previously carried out the comparison studies on the pulsation spectrum of integrated luminosity of emission from the laser cutting zone [5], the spectrum of brightness temperature under two-color pyrometry [6], and the spectrum of the side edge roughness, resulting from laser cutting [7].This paper reports the results of measuring by a four-channel two-color pyrometer of pulsations of local temperature in the region of exposure of mild-steel plate by radiation of CO 2 laser.

Experimental setup
The experiments were made used "Trumatic L2530" machine (Trumpf GmbH, Germany) with a CO 2 laser of 1500 W power. Cuts of a low-carbon steel sheet of 10 mm thick were made.The "optimal" cutting parameters were used with the lens of 127 mm focus.The cutting velocity was varied in range of ±20 % from the recommended velocity being 20 mm/s.Oxygen was used as an assist gas, its pressure being varied in range of (0.03…0.08)MPa.Experimental cuts of 6 mm-or 3 mm-thickness sheets of low-carbon steel were made with "optimal" cutting parameters too.The cutting velocity was varied (26…40) mm/s for 6 mm steel plate.The pressure of oxygen was varied (0.1…0.6) MPa.The cutting velocity was varied (35…60) mm/s for 3 mm steel plate, and the pressure of oxygen was varied (0.1…0.25) MPa.Measurements of time fluctuations of local luminance of metal melt were performed during cutting (fig.1-3).Thus the brightness temperature T has been taken from small illuminated areas located on different depth on the front of cut: -in the samples of 3 mm thick the depth was 0.6, 1.5 and 2.3 mm; -in the samples of 6 mm thick the depth was 0.6, 1.5, 2.3 and 4 mm; -in the samples of 10 mm thick the depth was 1.5, 2.3, 3.2 and 4.8 mm.

Results and discussion
The spectrum of variance of temperature pulsations we obtained (fig.4) can be divided into three regions.Each of them derived from the different physical process.The low-frequency region I (below 300…700 Hz) coupled with cutting velocity and with transverse striations on the side edge of the plate [2,7].Region II includes frequency of melt ejection f t =v m /h (v m is melt velocity, h -sample thickness).The temperature pulsations associated here with more detail peculiarities of the melt flow.It includes, for instance, the longitudinal striation on the side edge [9].One should note that the melt flow has complicated formation similar to a mountain stream.The spectrum of variance of temperature pulsations in range II is deformed with increasing pressure of assisted gas in experiments.
We will take a special attention in this paper to region III, in which the pulsations originated from capillary-wave turbulence on melt's surface [10].

The energy of melt's surface deformation
It is shown early [8]

The transmission of energy of capillary waves throughout turbulent spectrum
The measured estimation of spectrum ε in some cases does not tend to zero in the area of viscosity dissipation of hydrodynamic turbulence.
where k is capillary -wave number with dispersion 3

( )
k ω ρ σ = (ρ is melt density, σ is melt surface tension).Function Ψ would be constant in case of capillary -wave turbulence with so-called inverse cascade: energy of capillary waves is transferring from short-length wave up to long-length capillary waves due to decay instability [10].Function Θ is near to constant in case of capillary-wave turbulence with low frequency energy input [11].
As we see in our case, fig.7, the inverse cascade of capillary -wave turbulence is realized.

Thermocapillary phenomenon
The spectra of ε do not tend to zero in the area of viscosity dissipation of hydrodynamic turbulence in specific cases of cutting namely in case of present of focused radiation in local zone of measurements.Function Ψ has small value of the same units if it measured in unfocused zone where capillary -wave turbulence weakened.We suppose the thermocapillary phenomenon of abnormal absorption [12] occurs in zone of presence focused, intensity above 1 MW/cm 2 , 10.6 µm radiation.The main reason of thermo-capillary effect is presence of thermal dependence of surface tension as showed in [13].For mild steels: 0.6 mN (m K) T σ ∂ ∂ ≈ − ⋅ , and estimation of Marangoni number is Ma~10, taken into account next data for liquid mild steel: 1.5 N m σ ≈ , melt density 3 7000 kg m ρ ≈ .The phase velocity of capillary waves varies (2-16) m/s in frequency range of 7 kHz to 3 MHz for mild steel, relatively wavelength varies in range of (300-5) µm [the case of "deep" liquid was calculated].In case of using CO 2 laser with focus intensity of 1 MW/cm 2 (estimation is done with laser power 1200 W and lens with F=127 mm) special regime of small-scale capillary waves generation occurs.These waves form so-called capillary-wave turbulence spectrum [10].
Thus, an additional mechanism of the anomalous absorption of radiation can compensate the low absorption of the metal in case of 10.6 µm laser in comparison with the absorption of the metal in the near infrared range.

Conclusion
The measurements of local pulsations of brightness temperature T of melt radiance along the front of the cut of metal sheet by CO 2 laser are carried out.Both spectrum regions of hydrodynamic and capillary turbulence of temperature fluctuations are shown with experimental estimation of threshold of thermocapillary effect of absorption as 1 MW/cm 2 .The results of investigation can be useful for development of monitoring and quality control system of laser cutting process.The R-M-S of temperature fluctuations is greater than 10 K in the range of sub-millimeter capillary waves.Thus, an additional mechanism of the anomalous absorption on the front of cutting can compensate the low absorption of the metal in case of 10.6 µm laser in comparison with the absorption of the metal in the near infrared range.

Figure 2 .Figure 3 .
Figure 2. Temporal oscillations of temperature on depth of 0.8 mm(bottom line); 2.4 mm and 4 mm (upper line).The cutting of 6 mm thick plate of mild steel with cutting velocity 33 mm/s and oxygen pressure of 6 bar
that local fluctuations of T are related to local melt's surface deformations due to unequal radiation absorption.Thus, variance spectrum of temperature fluctuations on local surface ( ) T E ω is related with spectrum of density of energy of surface deformation, caused by forced turbulent pulsation of melt flow (II) and local nonuniformity of surface tension (III)the meaning the density of dissipation of turbulent energy of surface deformation, was calculated from experimental estimation of ( ) T E ω .This function has the maximum in hydrodynamic turbulence on the boundary between inertial and viscosity domains and must tends to zero in high-frequency area.The measurements show the existence of conditions of developed turbulent flow in case of cutting samples of 6 mm thick (fig.5), in contrast to the data measured during cutting samples with thickness of 10 mm (fig.6).

Figure 5 .
Figure 5.The spectrum of density of dissipation energy.The notation used as the same as on fig.4.