1. Introduction
The gas-assisted process represents one of the most prominent methods for laser cutting of metallic parts, in which the molten phase is ejected with the help of an assistance gas flow. Both reactive gases, such as oxygen, and inert gases, such as nitrogen and argon, are commonly used. Oxidizing gas is mainly used for cutting ferrous alloys, such as mild steels, where the ferrous oxides are of low viscosity and the molten dross can be easily removed from the cut edge. In these cases, oxygen exothermically reacts with the metals, adding further energy and increasing the cutting efficiency, allowing operation at an inlet stagnation pressure lower than 5 bar. On the other hand, inert gas is usually selected to operate at a pressure of up to 30 bar for metals that are difficult to cut, such as stainless steels, aluminum, and titanium alloys, in order to avoid retention of molten oxides that are problematic to blow away from the cut kerf because their high viscosity [
1,
2].
The majority of the commercial nozzles are of a conical shape, given the low manufacturing costs. However, this benefit is counterbalanced by a very high gas-consumption rate which increases the operational cost, as well as by a small allowable stand-off distance (X
off, nozzle tip–workpiece distance) that negatively affects the operating window in terms of material thickness. This small X
off condition is imposed due to the occurrence of undesirable aerodynamic phenomena, such as shock waves, through the exit jet flow of the conical nozzles operated at high pressures [
3,
4].
A comprehensive review of the dynamic characteristics of the exit jet from both conical and supersonic nozzles has been illustrated in [
5,
6]. The authors in [
5] demonstrated that the exit jet from the supersonic nozzle is marked by a more uniform density distribution and by a higher momentum thrust if compared with the conical nozzle, resulting in the improvement of the cutting quality (roughness, perpendicularity) and capability (maximum thickness, cutting speed). The same authors in [
6] experimentally proved that the cutting capability in terms of molten material removal and the allowable stand-off distance are enhanced by the use of a supersonic nozzle compared with a conical one.
Several cutting experiments have been conducted to illustrate the effect of nozzle types on the process parameters, namely cutting speed, laser power, stand-off distance, and workpiece thickness [
7,
8,
9,
10,
11,
12,
13,
14]. However, most of these experiments focused only on the cutting capability in terms of maximum achievable thickness rather than on the obtained cutting quality (the measured roughness). Cutting experiments on stainless steel samples using a diode laser were carried out, proving a faster speed for a thickness over 4 mm and a lower roughness with respect to what was achieved with a fiber laser [
8].
In [
9], the effect of the geometrical types of nozzles on the allowable operating tolerance of the stand-off distance was investigated on stainless steel samples of 60 mm thickness, demonstrating the greater operating tolerance for a Minimum Length Nozzle (MLN) and for commercial supersonic nozzles compared with conical subsonic ones.
Shin et al. [
10] conducted several cutting experiments on stainless steel and carbon steel plates of various thicknesses of up to 100 mm by means of 6 kW fiber laser. As a result, an efficient cutting capability of 16.7 mm per kW, accompanied with a high cutting speed, was achieved. In [
11], the cutting performance was improved by preheating the workpiece before the operation, achieving the peak cutting speed with a step-like cutting speed increase technique.
Moreover, Tamura et al. [
12] demonstrated that lasers can be efficiently utilized for cutting in nuclear decommissioning. They successfully executed cutting experiments for carbon steel and stainless steel plates of thicknesses of up to 300 mm using a 30 kW fiber laser.
Authors in [
13] investigated the effects of various operating conditions in an oxygen gas-assisted laser cutting to obtain an optimum kerf width. Steel and mild steel samples with a thickness of 1 to 2 mm were processed using a laser power range of 50 to 170 W, proving that a lower oxygen gas pressure (1 bar) is required to cut a mild steel sample of thickness 1 mm, compared with a pressure of 4 bar to cut steel samples of the same thickness.
The behavior of the exit jet pattern within the cut kerf based on the jet flow–workpiece interaction has been numerically modeled and experimentally investigated in several works reported in literature in order to design different gas injection systems that help to avoid the drawbacks associated with the current conventional cutting nozzles [
15,
16,
17,
18,
19].
Quintero et al. [
15] analyzed and experimentally visualized the gas flow inside the cut kerf to illustrate the main factors controlling the aerodynamic interactions of the assistance gas with the workpiece. They demonstrated that the proper selection of the stand-off distance represents the essential factor in optimizing the molten material removal rate and in avoiding the formation of a recast layer on the cut edge.
Authors in [
16,
17] numerically modeled the exit jet impinging on the cut kerf to analyze the effects of various laser cutting parameters (stand-off distance, exit Mach number, and workpiece thickness) on the formed shock structure. As a result, the direct interaction between the incident shock and the stand-off shock has been found to be mainly affected by the stand-off distance, while a large tolerance variation on this parameter is obtained at high Mach numbers. Similarly, Chen et al. [
18,
19] experimentally studied the achieved cutting quality and capability under various operating conditions and stand-off distances, confirming the dominant effects of the latter parameters.
Moreover, the exit jet from the supersonic nozzle in the free stream has been numerically studied to illustrate the behavior of the exit jet pattern under various operating conditions [
20,
21,
22]. Zhang et al. [
20] numerically modeled and visualized the exit jet patterns from two supersonic nozzles, a Minimum Length Nozzle (MLN) designed according to the Method of Characteristics (MOC) and an identical commercial supersonic nozzle with straight walls diverging by 5°. It was found that the MLN nozzle generates a uniform, bounded, and stable jet, extending for a longer distance (1.4 times) compared to the commercial one. Other authors in [
21] investigated the gas-assisted laser cutting flow through a supersonic nozzle operated at high pressure under various operating conditions (desired-design, over-expansion, and under-expansion). The achieved results have been further investigated and accurately simulated by means of an efficient model developed by the authors of the current work [
22]; the same model was used to simulate the exit jet behavior from the nozzles proposed in this article under various operating stagnation pressures [
23].
Even in view of the existing literatures on the subject, most of the reported experimental work has been focused mainly on the achieved cutting capability (cutting thickness) rather than on the cutting quality (the measured roughness and perpendicularity of the cut edge). In addition, to the best of the author’s knowledge, no previous works reported indications of the effective gas consumption in laser cutting.
5. Experimental Set-Up
The Schlieren technique is widely applied to visualize numerous engineering phenomena. It is recognized that the optical flow visualization is the most suitable method to observe the shock structures in the compressible flow. Authors in [
31] qualitatively investigated and experimentally visualized the shock wave diffraction phenomena around two splitters with spike-shaped structures for different Mach numbers. Schlieren photography was used to obtain an insight into the sequential diffraction processes that take place in different planes and as a result, a complete description of the main flow features was successfully provided. In addition, Schlieren technique represents one of the simplest optical visualizing procedures, especially convenient for observing the intermittent shock structures in the gas jet. For these reasons, the simulation results were compared with experimental characterizations based on the use of the Schlieren method. The scheme of the test facility is shown in
Figure 4; it consisted of a nitrogen (
N2) compressor adapted to provide compressed
N2 up to 25 bar. This compressor was connected to a valve in order to regulate the supply of
N2 into the tested nozzles, to a flow meter to measure the volume flow rate (L/min), and to a pressure sensor to adjust the required pressure. Then, this assembly was connected to the nozzle through the nozzle holder. Furthermore, an optical system was provided to capture the images aimed at investigating the dynamic behavior of the gas flow from the nozzle. This induced a change in gas pressure and temperature and caused a change of the refractive index, which deviated the divergent light beam from the pinpoint light source going through the test area. The non-deviated light beam was filtered by a knife edge placed at a focal point. The deviated light beam was projected onto the image plane. With the best alignment, the undisturbed light beam should form a spot at the knife edge, whose dimensions match the diameter of the pinhole used in the spatial filter. A CMOS camera captured the dynamic behavior of the gas flow from the nozzle.
The whole gas flow was imaged using a spherical mirror with focal length of 2000 mm and diameter of 300 mm. Furthermore, for detailed images including the intermittent shock structures (Mach disks), a smaller spherical mirror with focal length of 650 mm and diameter of 63 mm was used [
23,
31,
32,
33].
During the Schlieren measurements, the mass flow rate was experimentally estimated by the measured volume flow and the knowledge of the gas density. The resulting mass flow rates were calculated for the nitrogen-assisted gas at the sea level condition (15 °C, 1013.25 hPa) and a density (ρ) equal to 1.185 kg/s
3, as summarized in
Table 3.
Laser cutting tests were then conducted by the laser system shown in
Figure 5. The workpiece samples were made by AISI 316 stainless steel plates of different thicknesses: 2, 4, and 6 mm. All the experiments were conducted operating at 3 kW and using a 100 μm delivery fiber core diameter; the laser beam was collimated using a 100 mm lens and focused using a 150 mm focal lens.
A laser cutting head (
Figure 5b), fitted with either the reference conical or the developed supersonic nozzles, was used, and nitrogen was introduced coaxially with the focused beam via these nozzles regulating the gas pressure between 4.5 and 20 bar.
During the laser cutting experiments, the process parameters in the case of the reference conical nozzle were chosen by following the best practices developed under prior experiments. Thus, this nozzle was operated at a fixed stagnation pressure of 20 bar, fixed focal position equal to −4 mm, and cutting speed of 4.5 and 2 m/min for the specimens of thickness 2 and 4 mm, respectively. In addition, the entire cutting experiments were conducted at a low stand-off distance equal to 0.5 mm to avoid the deterioration of the dynamic characteristics and momentum thrust after the formed strong normal shock wave associated with this high operating pressure.
On other hand, the process parameters for the supersonic nozzles were differently specified. The operating stagnation pressure was calculated for each nozzle according to the exact-design condition, while the stand-off distance was fixed at the same reference value of 0.5 mm, although the exit jet from the supersonic nozzle is characterized by higher dynamic characteristics compared to those of the conical one, allowing safer operation at a longer stand-off distance. Finally, the focal position and cutting speed were selected based on several experimental trials, and the reported results were accompanied with the optimal achieved cutting quality.
The cutting-edge quality was investigated by using a stylus profilometer Hommelwerke TESTER T500 (Hommelwerke GmbH, Villingen-Schwenningen, Germany), and the measured values analyzed via the TURBO DATAWIN PC software (Hommelwerke GmbH, Villingen-Schwenningen, Germany). The traverse length (lt), which represents the evaluation length, was set to 15 mm, while the cut-off length (lc) was selected equal to 2.5 mm. The roughness parameters, average roughness (Ra) and average maximum height of the profile (Rz), were measured on each cutting edge at least five times for each sample edge, calculating the mean value and the absolute deviation of the measured roughness values. These measurements of roughness parameters were conducted at the centerline along the cut thickness of the cut edge to unify the comparative assessment of the achieved cutting quality from all nozzles. Moreover, the cutting edge perpendicularity was evaluated with the help of a structured light 3D scanner GOM ATOS Core 2000 (GOM GmbH, Braunschweig, Germany).
Author Contributions
Conceptualization, L.O.; methodology, L.O. and M.D.; software, L.O., M.D. and B.R.; validation, L.O., M.D. and B.R.; formal analysis, L.O., M.D. and B.R.; data curation, L.O., M.D. and B.R.
Funding
This research received no external funding.
Acknowledgments
Authors would like to thanks Libor Mrna from Institute of Scientific Instruments-ISI-Brno-Czech Republic for the experimental flow rate measurements and Marco Montani from Optoprim srl for the support in the laser cutting Experiments.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
Supersonic nozzle outlines: (a) Supersonic nozzle dimensions; (b) Manufactured nozzles using WEDM.
Figure 2.
Mach number distribution of the different nozzles under the desired-design condition: (a) Mach number distribution map; (b) Mach number distribution along the central axes.
Figure 3.
Pressure distribution of the different nozzles under the desired-design condition: (a) Pressure distribution map; (b) Pressure distribution along the central axes.
Figure 4.
Schematic diagram of the Schlieren setup.
Figure 5.
Laser cutting system: (a) Working cabinet and control panel; (b) Laser head.
Figure 6.
Schlieren visualizations: (a) SNA vs. CN0 at (@) 14.25 bar; (b) SNB vs. CN0 @ 7.4 bar; (c) SNC vs. CN0 @ 4.7 bar; (d) The exit jet patterns from SNA, SNB, and SNC at high magnification.
Figure 7.
Measured and calculated mass flow rate.
Figure 8.
Cutting experiments: (a) 2 mm thickness samples (arrows refer to the bottom of the cut kerf); (b) Measured roughness.
Figure 9.
Cutting experiments: (a) 4 mm thickness samples (arrows refer to the bottom of the cut kerf); (b) Measured roughness.
Figure 10.
Cutting experiments: (a) 6 mm thickness samples (arrows refer to the bottom of the cut kerf); (b) Measured roughness.
Table 1.
Supersonic nozzle dimensions and exact operating conditions.
Parameters | Units | SNA | SNB | SNC | CN0 |
---|
Inlet Diameter (Di) | mm | 8.0 | 8.0 | 8.0 | 8.0 |
Throat Diameter (D*) | mm | 1.5 | 1.8 | 2.0 | - |
Exit Diameter (De) | mm | 2.3 | 2.3 | 2.3 | 2.3 |
Nozzle length (L) | mm | 15.5 | 15.5 | 15.5 | 15.5 |
Designed Convergent length (LC1) | mm | 14.0 | 12.5 | 14.0 | 15.5 |
Actual Convergent length (LC) | mm | 13.5 | 12.0 | 13.5 | 15.5 |
Divergent length (LD) | mm | 1.5 | 3.0 | 1.5 | 0.0 |
Entrance (inlet) angle (θI) | degree | 27.07° | 28.97° | 24.89° | 20.85° |
Exit (Divergence) angle (θe) | degree | 29.86° | 9.52° | 9.90° | - |
| Desired-Design Operating Conditions |
Exact Inlet Stagnation Pressure (Po) | bar | 14.3 | 7.5 | 4.5 | 20.0 |
Exact Exit Pressure (Pe) | bar | 1.01325 |
Exact Exit Mach Number (Me) | | 2.37 | 1.96 | 1.63 | - |
Table 2.
Simulations results of the supersonic nozzles under desired-design condition and the reference conical nozzle.
Parameters | Units | SNA | SNB | SNC | CN0 |
---|
Exit Mach No. Me simulated | - | 2.25 | 1.90 | 1.60 | 0.94 |
Quasi 1-D exit Mach no. Me | - | 2.37 | 1.96 | 1.63 | - |
Me difference | - | −6.3% | −3.1% | −1.6% | - |
Exit Pressure simulated Pe,sim | (Pa) | 114,650 | 108,100 | 101,500 | 1,349,400 |
Quasi 1-D exit Pressure Pe,th | (Pa) | 101325 | - |
ΔPe = Pe,sim − Pe,th | - | +13% | +7% | +1% | - |
Exit Velocity Ue at nozzle tip | (m·s−1) | 556 | 495 | 448 | 311 |
Mass flow rate simulated | (g·s−1) | 6.05 | 4.98 | 3.95 | 14.30 |
Table 3.
Schlieren operating conditions, measured volume flow rates, and calculated mass flow rates.
Nozzle | Operating Pressure | Volume flow rates (l/m) | Mass flow rates (g/s) |
---|
SNA | Under-expansion: 20 bar | 458 | 9.05 |
Desired-design: 14.25 bar | 327 | 6.46 |
Over-expansion: 8 bar | 175 | 3.46 |
SNB | Under-expansion: 10 bar | 375 | 7.41 |
Desired-design: 7.4 bar | 274 | 5.42 |
Over-expansion: 4 bar | 171 | 3.38 |
SNC | Under-expansion: 8 bar | 370 | 7.31 |
Desired-design: 4.47 bar | 218 | 4.31 |
Over-expansion: 3 bar | 162 | 3.20 |
CN0 | 20 bar | 852 | 16.84 |
14.25 bar | 677 | 13.38 |
7.4 bar | 300 | 5.93 |
4.47 bar | 258 | 5.10 |
1.89 bar | 140 | 2.77 |
Table 4.
Cutting experiments conditions and measured roughness parameters.
Sample Thickness | Used Nozzle | Operating Conditions | Focal Position [mm] | Roughness Parameters [µm] | Cutting Edge Perpendicularity [°] |
---|
P [bar] | U [m/min] | Ra | Rz |
---|
2 mm | CN0 | 20.00 | 4.5 | −4.0 | 5.04 ± 0.06 | 31.81 ± 0.16 | 1.10 ± 0.09 |
SNA | 14.25 | 8.0 | −6.5 | 5.25 ± 0.16 | 31.44 ± 0.15 | 3.60 ± 0.98 |
SNB | 7.50 | 6.0 | −3.5 | 4.98 ± 0.08 | 28.93 ± 0.24 | 0.40 ± 0.80 |
SNC | 4.70 | 4.5 | −3.5 | 4.82 ± 0.02 | 29.04 ± 0.31 | 2.70 ± 0.70 |
4 mm | CN0 | 20.00 | 2.0 | −4.0 | 5.45 ± 0.04 | 37.72 ± 0.31 | 1.60 ± 0.13 |
SNA | 14.25 | 4.0 | −6.5 | 7.47 ± 0.17 | 40.25 ± 0.61 | 1.20 ± 1.18 |
SNB | 7.50 | 3.5 | −6.5 | 5.53 ± 0.11 | 37.44 ± 0.75 | 0.60 ± 0.22 |
SNC | 4.70 | 5.0 | −5.0 | 9.37 ± 0.39 | 51.58 ± 0.81 | 3.10 ± 0.84 |
6 mm | SNA | 14.25 | 2.5 | −4.0 | 8.65 ± 0.11 | 42.05 ± 0.15 | 1.40 ± 0.18 |
SNB | 7.50 | 2.0 | −4.0 | 8.59 ± 0.33 | 40.07 ± 1.57 | 1.40 ± 0.15 |
SNC | 4.70 | 2.0 | −4.0 | 11.46 ± 1.40 | 60.91 ± 3.32 | 1.30 ± 0.61 |
Table 5.
Cutting and gas consumption efficiency.
Sample Thickness | Used Nozzle | Operating Conditions | Mass Flow Rates [g/s] | Cutting Efficiency (ξ) | |
---|
P [bar] | U [m/min] | (ξ) [mm2/kJ] | (ξ)% w.r.t CN0 | [g/mm2] | % w.r.t CN0
|
---|
2 mm | CN0 | 20.00 | 4.5 | 16.84 | 50 | | 0.112 | |
SNA | 14.25 | 8.0 | 6.46 | 89 | +78% | 0.024 | +78% |
SNB | 7.50 | 6.0 | 5.42 | 67 | +33% | 0.027 | +76% |
SNC | 4.70 | 4.5 | 4.31 | 50 | - | 0.028 | +74% |
4 mm | CN0 | 20.00 | 2.0 | 16.84 | 44 | | 0.126 | |
SNA | 14.25 | 4.0 | 6.46 | 89 | +100% | 0.024 | +81% |
SNB | 7.50 | 3.5 | 5.42 | 78 | +75% | 0.023 | +82% |
SNC | 4.70 | 3.0 | 4.31 | 67 | +50% | 0.021 | +83% |
6 mm | SNA | 14.25 | 2.5 | 6.46 | 83 | - | 0.025 | - |
SNB | 7.50 | 2.0 | 5.42 | 67 | - | 0.027 | - |
SNC | 4.70 | 2.0 | 4.31 | 67 | - | 0.021 | - |
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