rus
Issue #3/2022
N. V. Grezev, I. N. Shiganov, A. A. Vasiliev
Parameter Optimization of the Optical Focusing Scheme for a High-Power Optical Fiber Laser Radiation for Heavy Steel Welding
Parameter Optimization of the Optical Focusing Scheme for a High-Power Optical Fiber Laser Radiation for Heavy Steel Welding
DOI: 10.22184/1993-7296.FRos.2022.16.3.198.210
The parameters of the optical scheme for the penetration of materials of large thicknesses by the radiation of high-power fiber lasers are calculated. It is shown that the ratio of the collimating lens C160 with the focusing lenses F250 and F400 gives the best optical characteristics necessary for melting materials of large thicknesses. The optimal parameters of the optical circuit are imple-mented in the focusing system IPG FLW D50. It has been experimentally shown that the selected parameters of the optical scheme make it possible to obtain high-quality through-melting on steels up to 12 mm thick. The preferred optical scheme for welding materials of large thicknesses should be considered the ratio with C160 / F400.
The parameters of the optical scheme for the penetration of materials of large thicknesses by the radiation of high-power fiber lasers are calculated. It is shown that the ratio of the collimating lens C160 with the focusing lenses F250 and F400 gives the best optical characteristics necessary for melting materials of large thicknesses. The optimal parameters of the optical circuit are imple-mented in the focusing system IPG FLW D50. It has been experimentally shown that the selected parameters of the optical scheme make it possible to obtain high-quality through-melting on steels up to 12 mm thick. The preferred optical scheme for welding materials of large thicknesses should be considered the ratio with C160 / F400.
Теги: defocusing of the laser beam faber laser high-power deep-penetration laser welding stability of the laser welding process волоконный лазер мощная лазерная сварка с глубоким проникновением раcфокусировка лазерного пучка стабильность процесса лазерной сварки
Parameter Optimization of the Optical Focusing Scheme for a High-Power Optical Fiber Laser Radiation for Heavy Steel Welding
N. V. Grezev 1, I. N. Shiganov 2, A. A. Vasiliev 1, 2
NTO “IRE-Polus”, Fryazino, Moscow region, Russia
Bauman Moscow State Technical University, Moscow, Russia
The parameters of the optical scheme for the penetration of materials of large thicknesses by the radiation of high-power fiber lasers are calculated. It is shown that the ratio of the collimating lens C160 with the focusing lenses F250 and F400 gives the best optical characteristics necessary for melting materials of large thicknesses. The optimal parameters of the optical circuit are imple-mented in the focusing system IPG FLW D50. It has been experimentally shown that the selected parameters of the optical scheme make it possible to obtain high-quality through-melting on steels up to 12 mm thick. The preferred optical scheme for welding materials of large thicknesses should be considered the ratio with C160 / F400.
Keywords: Faber Laser, stability of the laser welding process, defocusing of the laser beam, high-power deep-penetration laser welding
Received on: 03.02.2022
Accepted on: 18.04.2022
Introduction
The interaction of high-power laser radiation of more than 1.0 kW with metals is accompanied by a number of physical phenomena that ensure deep weld penetration and their connection during welding [1, 2]. In this case, a distinctive feature is the formation of a deep keyhole filled with metal vapors [3]. The phenomena occurring in the keyhole are very complex and diverse, so their understanding and control are of great theoretical and practical importance. A significant influence on the keyhole formation is exerted by the hydrodynamic liquid movement processes [5], gas-dynamic vapor effects [4], plasma processes in the keyhole and above its surface [6, 7], and optical phenomena [8]. Due to their interaction and movement during the welding process, the metal in the keyhole is in an unstable condition, mainly associated with the oscillatory liquid movements on its walls [9].
The formation of such a keyhole requires provision of concentrated laser energy of at least 1 ∙ 106 W / cm2. This energy is transferred deep into the material due to multiple reflections from the keyhole walls [10]. With this mechanism, a metal surface with very low absorptive capacity can behave as an absolute black body, since the laser beam transfers most of its energy to the keyhole surface. In addition, multiple reflection phenomena determine the way in which the laser beam energy is transferred to the metal and, most importantly, affect all other physical processes that occur during laser processing of materials, such as fluid flow, heat transfer and solidification. Therefore, optical phenomena in the keyhole play an important role in obtaining deep weld penetration and formation of the high-quality crystallized metal, for example, in the case of welding [11].
As a rule, laser radiation at the resonator output cannot be directly used for technological purposes, since it does not provide the required energy concentration, the nature of the power density distribution in the radiation beam, and other required specifications. Provision of the power density, re-reflection of beams in the keyhole and their influence on the weld penetration process are determined by the parameters of the laser beam focusing optical system, in particular, the focal distance, the Rayleigh length at the focus (waist), aberrations, etc. In addition, the position of the laser beam waist relative to the surface of the metal being processed has a significant effect [12]. When the position of the laser beam waist is changed, various formation defects are developed in the form of splashes, metal leakage, and cavities that lead to the defect formation during welding [13].
To ensure the stable formation of the weld penetration keyhole and crystallized metal during welding by the high-power optical fiber laser radiation, it is first necessary to calculate and design the optimal optical scheme that will provide the required spot diameter in the laser beam waist and focus depth (Rayleigh length). Depending on the selected optical scheme, it will be possible to determine the focus position relative to the surface of the metal being processed and all other parameters.
The purpose of this paper is to determine the basic principles for selecting optical focusing schemes for the optical fiber laser radiation for welding metals with various thicknesses.
Equipment and materials used
The experimental part of the work was performed using a robotic system that included an IPG YLS‑10000 optical fiber laser with a beam output power of up to 10 kW and a transport fiber diameter of 100 μm, an IPG LC 340 cooling system, and an industrial robot KUKA KR 60 HA. At the transport fiber output, laser radiation has the following specifications: radiation wavelength λ = 1.07 μm, beam intensity divergence angle θ = 0.16 rad, parameter M2 = 1111.03 (the ratio of the product of the waist diameter 2ω0 to the beam intensity divergence angle θ in relation to the product of similar parameters of an ideal beam with a Gaussian distribution), the beam quality parameter BPP = 3.756 mm ∙ mrad.
The laser radiation is converted by the optical head IPG FLW D50-W. The developed optical scheme efficiency was tested during the laser welding process with through weld penetration of plates made of low alloy steel Х52 and Х80 steels with a thickness of 8 to 12 mm.
Optical scheme design
The results obtained in the course of a number of studies have shown [11, 12] that when the optical fiber lasers melt heavy metals, a rather narrow vapor-gas keyhole is formed. The dimensions and shape of the vapor-gas keyhole largely depend on the focusing parameters and the focus location relative to the surface of the workpiece being welded. A graphical analysis of the vapor-gas keyhole shape, obtained from X-ray images, with converging laser beams superimposed on it, is shown in Fig. 1 (source [13]).
Analysis of the obtained schemes has showed that the maximum weld penetration depth occurs at such a focus position (–5 mm), when the front keyhole wall is closest to the vertical. In this case, all radiation penetrates into the keyhole depth. The minimum weld penetration depth of 5.46 mm occurs at the focus position (4 mm), at which a sufficiently large tilt angle is developed on the front keyhole wall that in turn leads to the reflection of a part of the energy into the upper rear part of the keyhole wall. As a result, the splashes and defects are observed in this area that is a consequence of overheating.
Such a high sensitivity of the welding process to changes in the focus position when using an optical fiber laser is based on the fact that the steel used has a higher absorption coefficient of laser radiation with a wavelength of 1.07 μm than, for example, laser radiation with a wavelength of 10.6 μm. Therefore, the effect of multiple radiation re-reflection observed during the laser welding process with gas lasers [14] is noticeably reduced.
In order to make the welding process with the optical fiber lasers more stable and repeatable, it is necessary to reduce the welding process sensitivity to the position of the optical system focus. In order to more evenly distribute the energy over the keyhole depth, it is proposed to use two optical schemes with various focusing lenses, such as С160 / F250 (short-focus) and С160 / F400 (long-focus), where С=160 mm is the focal distance of the collimator, F=250 mm is the focal distance of the short-focus focusing lens and F=400 mm is the focal distance of the long-focus focusing lens (Fig. 2).
The laser radiation parameters were calculated using the following formulas [15].
, (1)
where ВРР (beam parameter product) is a beam quality parameter, mm∙mrad; Øf is the transport fiber diameter, µm; θ is the divergence angle, rad; λ is the wavelength, µm. Next, we will obtain BPP0 for an ideal laser beam and BPP for a beam obtained in real conditions:
; (2)
. (3)
Then a laser beam with a diameter Øс hits the collimator:
, (4)
where C is the focal distance of the collimator, mm. The laser beam waist size at the focus 2ω0 along the focal distance of the lens F is determined by the following formula:
. (5)
The depth of field is determined by the Rayleigh length 2ZR. The ZR value is obtained as follows:
. (6)
In accordance with the calculations, the C160 / F250 optical scheme forms a beam with a diameter of 150 μm at the focus that, at an incident radiation power of 10 kW, has a sufficiently high power density of 52.15 MW / cm2, while the depth of field (Rayleigh length) is 3.25 mm (see Table).
The C160 / F400 optical scheme forms a beam with a waist diameter of 250 mm at the focus, and at a laser power of 10 kW it leads to a lower power density of 20.4 MW / cm2, while the depth of field is 8.32 mm (see Table).
To determine optimality of the designed optical schemes for use in the welding processes, for example, steel plates with a thickness of 12 mm, we propose to consider a few more parameters. First, it is a change in the power density distribution over the depth of the vapor-gas keyhole. Secondly, it is the average power density in the plane of the front keyhole wall and the laser beam incidence and reflection angles from the conditional front keyhole wall (see Figs. 3 and 4).
While using the C160 / F250 short-focus optical scheme, when the laser beam propagates in the direction from the focus position on the metal surface deep into the keyhole, the power density is decreased due to the beam divergence and, accordingly, an increase in the laser spot diameter (Fig. 3). The decrease occurs in the following sequence: at a distance of –2 mm, the power density is 23.6 MW / cm2, at a distance of –4 mm, the density is 12.8 MW / cm2, then, at a depth of –6 mm, the power density is decreased to 8.7 MW / cm2, and at a distance of –8 mm from the focus, the value becomes even smaller and reaches 6.6 MW / cm2, then at –10 mm, respectively, it is 5.2 MW / cm2, and at –12 mm it is 4.4 MW / cm2.
While welding the parts with a thickness of 12 mm, the difference in the power density value at the focus (52.15 MW / cm2 at the surface in accordance with the scheme) and at the output (4.4 MW / cm2) is large, the values differ by 12 times. Such a difference will lead to the splash formation in the focus area due to the metal overheating. Certainly, by varying the focus position relative to the surface of the part, it will be possible to achieve stable keyhole formation.
However, the low power density at the plate output will increase the likelihood of defects in the form of pores and cavities due to insufficient power density and keyhole instability. The average power density in the plane of the front wall is 0.44 MW / cm2 that is clearly insufficient to obtain process stability. The C160 / F250 optical scheme, based on an estimate of the power density distribution through depth, is more suitable for use in welding the metal plates with a thickness of 6–8 mm.
The decrease in the laser radiation power density for the С160 / F400 long-focus system is less intense: at a distance of –2 mm, the power density is 16.6 MW / cm2, then while increasing the depth at a pitch of 2 mm: –4 mm, –6 mm, –8 mm, –10 mm, –12 mm, we get the power density distribution as follows: 11.1 MW / cm2, 8.1 MW / cm2, 6.3 MW / cm2, 5.1 MW / cm2, 4.3 MW / cm2. When welding the parts with a thickness of 12 mm, the difference in density on the part surface and at the output differs by 4.7 times. This is much better than with the short-focus system. In this case, the average power density in the plane of the front wall is higher and equal to 0.56 MW / cm2. This power density distribution is suitable for welding the heavy thick parts.
Figure 5 shows the power density distribution in the focusing range of the laser beam waist from 0 to –12 mm on the welded sample surface using two selected types of focusing optical systems (the maximum laser power for the selected optical fiber laser is 10 kW).
An estimate is made in relation to the laser beam incidence and reflection angles on the front keyhole wall that, in turn, is conditionally determined by the diagonal of the resulting trapezoid over the entire thickness in the area of laser beam impact. For the C160 / F250 optical scheme this angle is 4°, for C160 / F400 it is 3°. The laser beam tracing along such a conditional front wall for the C160 / F250 optical scheme shows that in this case the re-reflection from the front wall is directed to the central area, in the case of the C160 / F400 optical scheme it is directed to the lower part that should be more favorable for obtaining greater weld penetration depth.
In the welding process, the angle of the front wall depending on the welding speed and focus position can be changed, so it is advisable to trace the laser beam reflections for several more positions in increments of 1°. For example, for the C160 / F250 optical scheme, as the inclination angle decreases to 3 and 2°, the re-reflection direction shifts to a depth of 6 and 8 mm. For the C160 / F400 optical scheme, at any position of the inclination angle, the beams are re-reflected to a depth of 10–12 mm that is presumably more favorable for welding with the thicknesses up to 12 mm.
Experimental procedure
The applicability of the calculated optical scheme parameters was evaluated experimentally by melting steel plates with the thickness of 8, 10 and 12 mm made of low-alloy steels (Х52, Х80). The evaluation criterion was the weld stability with through penetration. The focus position when using both focusing lenses was shifted in the range from –9 mm to 9 mm. The welding speeds were 0.6 and 0.9 m / min. The dependences of changes in the required power and the ranges of possible focus position deviations that do not affect the penetration quality and the welding result, were studied.
The ranges of the laser beam focus positions considered on each focusing lens are selected with due regard to the spot diameter on the surface. When the laser beam focus is located on the metal surface, the spot obtained on the F400 focusing lens is larger than on F250. Further, as the beam defocusing is increased to 3 mm, the spots of both focusing lenses will have approximately the same size (Fig. 5). In the case of further magnification, a greater defocusing is required to obtain the same spot size on a long-focus lens.
Figure 6 shows the images of weld macrosections with a through penetration, obtained on the steels with various thicknesses in optimal modes using various optical schemes. As it can be seen on the macrosections, both C160 / F250 and C160 / F400 optical schemes provide high-quality formation with full weld penetration of plates with a thickness of 8.0 to 12.0 mm at the focus position on the surface.
Based on the experimental results, the graphs are plotted that reflect the impact of the focus depth of the focusing lens on the required laser radiation power to obtain through penetration of a plate with a thickness of 8 mm (Fig. 7) and 12 mm (Fig. 8).
Increased thickness of the welded metal requires more laser power when using the short-focus optics (F250) compared to the long-focus (F400) optics due to a deeper focus.
When using the F400 focusing lens with a deeper focus, less laser power is required for through penetration of K60 steel with a thickness of 12 mm.
The focus transition into the metal depth by 3–6 mm reduces the required power for through penetration by 15–17%, and its rise above the surface by 3–6 mm increases the power for penetration slightly by 5–7%. These dependences are preserved for both optical schemes. In the case of an increase in welding speed up to 0.9 m / min, full penetration requires an increase in power by 15–17% when focusing on the surface. However, the dependences remain in the same range when the focus is shifted.
There are also changes in the external formation, splash formation, sagging of the melt, depending on the focus position. It has been established that the range of focus transitions when obtaining the welds without indicated defects for the C160 / F400 optical system is 3 times wider than for the C160 / F250 system. This makes it possible to control the welding parameters over a wider range using the C160 / F400 optical system. Based on this fact, this optical scheme is more suitable for welding thicker materials.
Conclusions
Due to the increased absorption capacity of metals at an optical fiber laser wavelength of 1.07 μm, when welding the materials with great thicknesses, a more extended analysis of the focusing system optical parameters is required.
In addition to calculating the basic parameters (focal spot diameter, power density and focus depth), an assessment of additional parameters is also required:
Using the example of two optical systems C160 / F250 and C160 / F400, the higher applicability of the long-focus system for welding the materials with thicknesses of 10–12 mm has been experimentally confirmed; the optimal welding modes is in wider ranges.
N. V. Grezev 1, I. N. Shiganov 2, A. A. Vasiliev 1, 2
NTO “IRE-Polus”, Fryazino, Moscow region, Russia
Bauman Moscow State Technical University, Moscow, Russia
The parameters of the optical scheme for the penetration of materials of large thicknesses by the radiation of high-power fiber lasers are calculated. It is shown that the ratio of the collimating lens C160 with the focusing lenses F250 and F400 gives the best optical characteristics necessary for melting materials of large thicknesses. The optimal parameters of the optical circuit are imple-mented in the focusing system IPG FLW D50. It has been experimentally shown that the selected parameters of the optical scheme make it possible to obtain high-quality through-melting on steels up to 12 mm thick. The preferred optical scheme for welding materials of large thicknesses should be considered the ratio with C160 / F400.
Keywords: Faber Laser, stability of the laser welding process, defocusing of the laser beam, high-power deep-penetration laser welding
Received on: 03.02.2022
Accepted on: 18.04.2022
Introduction
The interaction of high-power laser radiation of more than 1.0 kW with metals is accompanied by a number of physical phenomena that ensure deep weld penetration and their connection during welding [1, 2]. In this case, a distinctive feature is the formation of a deep keyhole filled with metal vapors [3]. The phenomena occurring in the keyhole are very complex and diverse, so their understanding and control are of great theoretical and practical importance. A significant influence on the keyhole formation is exerted by the hydrodynamic liquid movement processes [5], gas-dynamic vapor effects [4], plasma processes in the keyhole and above its surface [6, 7], and optical phenomena [8]. Due to their interaction and movement during the welding process, the metal in the keyhole is in an unstable condition, mainly associated with the oscillatory liquid movements on its walls [9].
The formation of such a keyhole requires provision of concentrated laser energy of at least 1 ∙ 106 W / cm2. This energy is transferred deep into the material due to multiple reflections from the keyhole walls [10]. With this mechanism, a metal surface with very low absorptive capacity can behave as an absolute black body, since the laser beam transfers most of its energy to the keyhole surface. In addition, multiple reflection phenomena determine the way in which the laser beam energy is transferred to the metal and, most importantly, affect all other physical processes that occur during laser processing of materials, such as fluid flow, heat transfer and solidification. Therefore, optical phenomena in the keyhole play an important role in obtaining deep weld penetration and formation of the high-quality crystallized metal, for example, in the case of welding [11].
As a rule, laser radiation at the resonator output cannot be directly used for technological purposes, since it does not provide the required energy concentration, the nature of the power density distribution in the radiation beam, and other required specifications. Provision of the power density, re-reflection of beams in the keyhole and their influence on the weld penetration process are determined by the parameters of the laser beam focusing optical system, in particular, the focal distance, the Rayleigh length at the focus (waist), aberrations, etc. In addition, the position of the laser beam waist relative to the surface of the metal being processed has a significant effect [12]. When the position of the laser beam waist is changed, various formation defects are developed in the form of splashes, metal leakage, and cavities that lead to the defect formation during welding [13].
To ensure the stable formation of the weld penetration keyhole and crystallized metal during welding by the high-power optical fiber laser radiation, it is first necessary to calculate and design the optimal optical scheme that will provide the required spot diameter in the laser beam waist and focus depth (Rayleigh length). Depending on the selected optical scheme, it will be possible to determine the focus position relative to the surface of the metal being processed and all other parameters.
The purpose of this paper is to determine the basic principles for selecting optical focusing schemes for the optical fiber laser radiation for welding metals with various thicknesses.
Equipment and materials used
The experimental part of the work was performed using a robotic system that included an IPG YLS‑10000 optical fiber laser with a beam output power of up to 10 kW and a transport fiber diameter of 100 μm, an IPG LC 340 cooling system, and an industrial robot KUKA KR 60 HA. At the transport fiber output, laser radiation has the following specifications: radiation wavelength λ = 1.07 μm, beam intensity divergence angle θ = 0.16 rad, parameter M2 = 1111.03 (the ratio of the product of the waist diameter 2ω0 to the beam intensity divergence angle θ in relation to the product of similar parameters of an ideal beam with a Gaussian distribution), the beam quality parameter BPP = 3.756 mm ∙ mrad.
The laser radiation is converted by the optical head IPG FLW D50-W. The developed optical scheme efficiency was tested during the laser welding process with through weld penetration of plates made of low alloy steel Х52 and Х80 steels with a thickness of 8 to 12 mm.
Optical scheme design
The results obtained in the course of a number of studies have shown [11, 12] that when the optical fiber lasers melt heavy metals, a rather narrow vapor-gas keyhole is formed. The dimensions and shape of the vapor-gas keyhole largely depend on the focusing parameters and the focus location relative to the surface of the workpiece being welded. A graphical analysis of the vapor-gas keyhole shape, obtained from X-ray images, with converging laser beams superimposed on it, is shown in Fig. 1 (source [13]).
Analysis of the obtained schemes has showed that the maximum weld penetration depth occurs at such a focus position (–5 mm), when the front keyhole wall is closest to the vertical. In this case, all radiation penetrates into the keyhole depth. The minimum weld penetration depth of 5.46 mm occurs at the focus position (4 mm), at which a sufficiently large tilt angle is developed on the front keyhole wall that in turn leads to the reflection of a part of the energy into the upper rear part of the keyhole wall. As a result, the splashes and defects are observed in this area that is a consequence of overheating.
Such a high sensitivity of the welding process to changes in the focus position when using an optical fiber laser is based on the fact that the steel used has a higher absorption coefficient of laser radiation with a wavelength of 1.07 μm than, for example, laser radiation with a wavelength of 10.6 μm. Therefore, the effect of multiple radiation re-reflection observed during the laser welding process with gas lasers [14] is noticeably reduced.
In order to make the welding process with the optical fiber lasers more stable and repeatable, it is necessary to reduce the welding process sensitivity to the position of the optical system focus. In order to more evenly distribute the energy over the keyhole depth, it is proposed to use two optical schemes with various focusing lenses, such as С160 / F250 (short-focus) and С160 / F400 (long-focus), where С=160 mm is the focal distance of the collimator, F=250 mm is the focal distance of the short-focus focusing lens and F=400 mm is the focal distance of the long-focus focusing lens (Fig. 2).
The laser radiation parameters were calculated using the following formulas [15].
, (1)
where ВРР (beam parameter product) is a beam quality parameter, mm∙mrad; Øf is the transport fiber diameter, µm; θ is the divergence angle, rad; λ is the wavelength, µm. Next, we will obtain BPP0 for an ideal laser beam and BPP for a beam obtained in real conditions:
; (2)
. (3)
Then a laser beam with a diameter Øс hits the collimator:
, (4)
where C is the focal distance of the collimator, mm. The laser beam waist size at the focus 2ω0 along the focal distance of the lens F is determined by the following formula:
. (5)
The depth of field is determined by the Rayleigh length 2ZR. The ZR value is obtained as follows:
. (6)
In accordance with the calculations, the C160 / F250 optical scheme forms a beam with a diameter of 150 μm at the focus that, at an incident radiation power of 10 kW, has a sufficiently high power density of 52.15 MW / cm2, while the depth of field (Rayleigh length) is 3.25 mm (see Table).
The C160 / F400 optical scheme forms a beam with a waist diameter of 250 mm at the focus, and at a laser power of 10 kW it leads to a lower power density of 20.4 MW / cm2, while the depth of field is 8.32 mm (see Table).
To determine optimality of the designed optical schemes for use in the welding processes, for example, steel plates with a thickness of 12 mm, we propose to consider a few more parameters. First, it is a change in the power density distribution over the depth of the vapor-gas keyhole. Secondly, it is the average power density in the plane of the front keyhole wall and the laser beam incidence and reflection angles from the conditional front keyhole wall (see Figs. 3 and 4).
While using the C160 / F250 short-focus optical scheme, when the laser beam propagates in the direction from the focus position on the metal surface deep into the keyhole, the power density is decreased due to the beam divergence and, accordingly, an increase in the laser spot diameter (Fig. 3). The decrease occurs in the following sequence: at a distance of –2 mm, the power density is 23.6 MW / cm2, at a distance of –4 mm, the density is 12.8 MW / cm2, then, at a depth of –6 mm, the power density is decreased to 8.7 MW / cm2, and at a distance of –8 mm from the focus, the value becomes even smaller and reaches 6.6 MW / cm2, then at –10 mm, respectively, it is 5.2 MW / cm2, and at –12 mm it is 4.4 MW / cm2.
While welding the parts with a thickness of 12 mm, the difference in the power density value at the focus (52.15 MW / cm2 at the surface in accordance with the scheme) and at the output (4.4 MW / cm2) is large, the values differ by 12 times. Such a difference will lead to the splash formation in the focus area due to the metal overheating. Certainly, by varying the focus position relative to the surface of the part, it will be possible to achieve stable keyhole formation.
However, the low power density at the plate output will increase the likelihood of defects in the form of pores and cavities due to insufficient power density and keyhole instability. The average power density in the plane of the front wall is 0.44 MW / cm2 that is clearly insufficient to obtain process stability. The C160 / F250 optical scheme, based on an estimate of the power density distribution through depth, is more suitable for use in welding the metal plates with a thickness of 6–8 mm.
The decrease in the laser radiation power density for the С160 / F400 long-focus system is less intense: at a distance of –2 mm, the power density is 16.6 MW / cm2, then while increasing the depth at a pitch of 2 mm: –4 mm, –6 mm, –8 mm, –10 mm, –12 mm, we get the power density distribution as follows: 11.1 MW / cm2, 8.1 MW / cm2, 6.3 MW / cm2, 5.1 MW / cm2, 4.3 MW / cm2. When welding the parts with a thickness of 12 mm, the difference in density on the part surface and at the output differs by 4.7 times. This is much better than with the short-focus system. In this case, the average power density in the plane of the front wall is higher and equal to 0.56 MW / cm2. This power density distribution is suitable for welding the heavy thick parts.
Figure 5 shows the power density distribution in the focusing range of the laser beam waist from 0 to –12 mm on the welded sample surface using two selected types of focusing optical systems (the maximum laser power for the selected optical fiber laser is 10 kW).
An estimate is made in relation to the laser beam incidence and reflection angles on the front keyhole wall that, in turn, is conditionally determined by the diagonal of the resulting trapezoid over the entire thickness in the area of laser beam impact. For the C160 / F250 optical scheme this angle is 4°, for C160 / F400 it is 3°. The laser beam tracing along such a conditional front wall for the C160 / F250 optical scheme shows that in this case the re-reflection from the front wall is directed to the central area, in the case of the C160 / F400 optical scheme it is directed to the lower part that should be more favorable for obtaining greater weld penetration depth.
In the welding process, the angle of the front wall depending on the welding speed and focus position can be changed, so it is advisable to trace the laser beam reflections for several more positions in increments of 1°. For example, for the C160 / F250 optical scheme, as the inclination angle decreases to 3 and 2°, the re-reflection direction shifts to a depth of 6 and 8 mm. For the C160 / F400 optical scheme, at any position of the inclination angle, the beams are re-reflected to a depth of 10–12 mm that is presumably more favorable for welding with the thicknesses up to 12 mm.
Experimental procedure
The applicability of the calculated optical scheme parameters was evaluated experimentally by melting steel plates with the thickness of 8, 10 and 12 mm made of low-alloy steels (Х52, Х80). The evaluation criterion was the weld stability with through penetration. The focus position when using both focusing lenses was shifted in the range from –9 mm to 9 mm. The welding speeds were 0.6 and 0.9 m / min. The dependences of changes in the required power and the ranges of possible focus position deviations that do not affect the penetration quality and the welding result, were studied.
The ranges of the laser beam focus positions considered on each focusing lens are selected with due regard to the spot diameter on the surface. When the laser beam focus is located on the metal surface, the spot obtained on the F400 focusing lens is larger than on F250. Further, as the beam defocusing is increased to 3 mm, the spots of both focusing lenses will have approximately the same size (Fig. 5). In the case of further magnification, a greater defocusing is required to obtain the same spot size on a long-focus lens.
Figure 6 shows the images of weld macrosections with a through penetration, obtained on the steels with various thicknesses in optimal modes using various optical schemes. As it can be seen on the macrosections, both C160 / F250 and C160 / F400 optical schemes provide high-quality formation with full weld penetration of plates with a thickness of 8.0 to 12.0 mm at the focus position on the surface.
Based on the experimental results, the graphs are plotted that reflect the impact of the focus depth of the focusing lens on the required laser radiation power to obtain through penetration of a plate with a thickness of 8 mm (Fig. 7) and 12 mm (Fig. 8).
Increased thickness of the welded metal requires more laser power when using the short-focus optics (F250) compared to the long-focus (F400) optics due to a deeper focus.
When using the F400 focusing lens with a deeper focus, less laser power is required for through penetration of K60 steel with a thickness of 12 mm.
The focus transition into the metal depth by 3–6 mm reduces the required power for through penetration by 15–17%, and its rise above the surface by 3–6 mm increases the power for penetration slightly by 5–7%. These dependences are preserved for both optical schemes. In the case of an increase in welding speed up to 0.9 m / min, full penetration requires an increase in power by 15–17% when focusing on the surface. However, the dependences remain in the same range when the focus is shifted.
There are also changes in the external formation, splash formation, sagging of the melt, depending on the focus position. It has been established that the range of focus transitions when obtaining the welds without indicated defects for the C160 / F400 optical system is 3 times wider than for the C160 / F250 system. This makes it possible to control the welding parameters over a wider range using the C160 / F400 optical system. Based on this fact, this optical scheme is more suitable for welding thicker materials.
Conclusions
Due to the increased absorption capacity of metals at an optical fiber laser wavelength of 1.07 μm, when welding the materials with great thicknesses, a more extended analysis of the focusing system optical parameters is required.
In addition to calculating the basic parameters (focal spot diameter, power density and focus depth), an assessment of additional parameters is also required:
- distribution of power density over the depth of the vapor-gas keyhole;
- average power density on the plane of the front wall;
- estimation of the laser beam incidence and reflection angles from the front keyhole wall.
Using the example of two optical systems C160 / F250 and C160 / F400, the higher applicability of the long-focus system for welding the materials with thicknesses of 10–12 mm has been experimentally confirmed; the optimal welding modes is in wider ranges.
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