rus
Issue #2/2018
A. V. Laskin, V. B. Romashova, S. A. Bolt, N. V. Burov
Laser Beam Intensity Profile Shapers
Laser Beam Intensity Profile Shapers
It often happens that the effectiveness of a particular technology can be significantly improved by converting the intensity distribution of the laser beam. This is one of the reasons for the growing interest in the methods of radiation formation.
Теги: industrial laser technologies laser beam profile shapers slm-technologies slm-технологии индустриальные лазерные технологии формирование лазерных пучков
INTRODUCTION
The inhomogeneity of laser radiation can be a source of difficulty in the use of lasers, for example, in the so-called heat-affected zone (HAZ) in welded or cone-shaped holes [1].
This statement can be illustrated if we consider geometric properties of the single-mode (or TEM00) laser beam intensity profile. Such beam is often used in a variety of materials processing technology, since it provides the greatest energy concentration. Of course, each laser application has its own peculiarities of interaction of material and laser radiation, however there is one feature common to all single-mode lasers – intensity distribution Gaussian function. Therefore, in order to evaluate the efficiency of a laser beam, it is possible to consider only the geometric features of the Gaussian function as a first step without taking into account such effects as combustion, melting, laser torch, etc. accompanying laser processing.
THEORETICAL JUSTIFICATION
The intensity distribution I(r) of single-mode laser beam [2, 3] is described in polar coordinates by the well-known relation:
, (1)
where r is beam radius, ω0 is the constriction radius, Imax is the maximum intensity value. Imax value is selected so that the total beam energy corresponding to figure volume under the surface of the two-dimensional Gaussian function I(r) is equal to a conventional unit (Fig.1).
The goal of most laser technologies is to transfer a certain amount of laser energy to the material being processed, thus, it is logical to use the energy estimate of different parts of the laser beam to analyze the efficiency. For convenience of further consideration, let’s assume that all the processes of laser radiation propagation occur with a time interval equal to one. Amount of energy Er1–r2, concentrated in the annular part of the laser beam, bounded by circles of radii r1 and r2, can be found by integrating the intensity function I(r)
. (2)
Permutation of intensity distribution function (1) into formula (2) and integrating lead to the following formula:
. (3)
Suppose that the technologies for materials processing occur when the intensity of the beam is applied at a certain level Ih (see Fig.1). For convenience of further consideration, let’s introduce the variable: h = Ih / Imaxh. Thus, h is the "working" level of the laser beam, wherein the range of h values is a value between 0 and 1.
Now, let’s consider a three-dimensional geometric figure limited by a horizontal plane and the surface of a two-dimensional Gaussian function I(r) defined in polar coordinates. The volume of this figure has physical sense of the laser beam energy; for the normalization adopted earlier, the value of this energy is equal to one. Let the variables E1, E2 and E3 denote different parts of the figure that can be interpreted as parts of the beam energy:
• E1 is the "apex" of the Gaussian function, where the intensity exceeds the working level h (obviously that this energy is used inefficiently and, in some cases, represents a direct loss, e. g., in the ablation of thin films of material);
• E2 are the "tails" of the Gaussian function where intensity is insufficient for the process, but most often is the cause of HAZ formation (thus, it is almost always the case that corresponds to loss or inefficient use of laser energy);
• E3 is the effective "cylinder" of energy (laser beam "works" with the optimal intensity).
Using equation (3), it is easy to determine the formulas for calculating the values of the "parts of energy":
. (4)
Sum of E1 + E2 is given for the evaluation of losses in the processing of thin films or coatings. The results of calculations using formulas (4) are shown in Fig. 2
These results are very interesting. Unconditional loss of energy E2 can reach very high values. For example, if the working level of energy is half of the maximum (this level is often considered a working one), the energy loss is 50% of the total energy of the laser beam. In the conditions of laser processing of thin films, the energy part E1, the Gauss apex, is also considered as energy loss since this part exceeds the energy working level Ih. Thus, both energy parts E1 and E2 are losses. Their sum E1 + E2 is also shown in the diagram; the minimum of this function is 0.63. In other words, when processing thin films "only" 63% of energy is lost or inefficiently used, and 37% "work" at the best the case.
This is only a geometric interpretation. However, this approach provides an idea of the scale of losses of laser radiation, which can sometimes reach half the total beam energy!
LASER BEAM PROFILE TRANSFORMATION
Undoubtedly, the transformation of the initial Gaussian profile shape into an effective cylinder with uniform intensity would help to efficiently use the laser energy and improve the technologies where homogeneous intensity is most desirable. To implement this transformation, several types of optical systems are suggested, also called shapers (beam shapers) [4]: refractive, diffractive, beam integrators or combined systems. The choice of a suitable solution depends on the power level, wavelength, quality of the beam homogenization and other features of the specific application.
Transformation of the Gaussian beam into a beam with uniform intensity (flat-top or top hat) is the main function of πShaper, refractive family of radiation transformation systems, designed to work with UV, IR visible range lasers common in science and industry.
The concept of the πShaper operation is illustrated in Fig. 3. The Gaussian distribution of the intensity of the collimated beam of the TEM00 laser is transformed into a flat distribution on the apex (similar to the Greek letter π). The output beam is also collimated and has approximately the same dimensions as the input beam.
πShaper is a telescope retaining the integrity of the beam, while the intensity profile is transformed in a controlled manner by lenses with smooth optical surfaces, and one of the basic principles is no wave aberration, which guarantees the preservation of the flatness of the wave front of the output beam. These features distinguish πShaper from integration-type homogenizers, where homogeneous intensity is provided by the use of multi-lens optical components, breaking up the initial beam into parts and "mixing" these parts in a certain working plane. In contrast to integrating homogenizers, the flatness of the output wave front of πShaper ensures its large distance after the device, where the beam properties remain stable. This feature simplifies the use of πShapers in actual systems.
Constructed according to the scheme of Galilean telescope, πShaper implements transformation of laser beams without internal focusing, which is important in the case of applications with powerful and short-pulse lasers. Theoretically, the radiation transformation by πShaper occurs without losses and application of optimized antireflective coatings allows to achieve almost 100% system transmittance – a great advantage over homogenizers diffraction type.
Fig. 4 shows an example of converting a Gaussian beam into a uniform (flat-top) beam using πShaper.
An interesting feature is the fact that one device can form several output profiles depending on the input beam diameter.
Due to this, the πShaper systems of uniform beam formation are applied as useful tools for improving industrial laser technologies and solving some scientific problems. An example of a successful application is the laser ablation of silicon by Ekspla Ltd. (Fig. 5). Here, the hole is made in silicon using a nanosecond laser Ekspla DPSS Nd: YAG using πShaper. The aim was to achieve the "flat bottom" profile of the blind by-pass hole without edge defects – it is important to at manufacture of multi-layer printed circuit boards.
The difference in the results is obvious: if a Gaussian beam is used, the hole shape follows the Gaussian profile, i. e. narrows towards the center (see Fig. 5, left), since almost all the energy is concentrated at the center, and only a small part of it – on the periphery of the hole. The high intensity in the center of the Gaussian beam can exceed the ablation threshold of the copper layer below it.
The shape of the hole changes abruptly when using πShaper. The holes formed through the flat intensity distribution, have clearly edges and flat bottom (see Fig. 5, center and right). The shape of the "flat bottom" of the holes is stable in a wide range of displacements from the image (focal plane). Thus, the use of πShaper helped solve the problem of drilling holes of the required shape.
The formation of the beam profile opens up new possibilities for improving the performance of those laser technologies where the uniform intensity is the most desirable. Moreover, πShaper allows to achieve the optimum result when solving various industrial and scientific tasks in an easy and simple way.
In the practice of laser technologies, two models of laser radiation transformers are used:
• πShaper for the formation of collimated beams of uniform intensity, and
• Focal–πShaper for focused beams and creating uniform or annular spot profiles in the focal plane of the focusing lens.
πShaper
πShaper transforms laser beams with Gaussian or similar intensity distribution in the beam of uniform intensity and with a plane wave front (Fig. 6). Transformation of this type is important in a variety of applications [5]:
• holography,
• interferometry,
• optical systems of modern confocal microscopes and the microscopes implementing various fluorescent technologies,
• optical systems of power lasers built according to the oscillator-amplifier scheme (MOPA-lasers), where the intensity profile control allows to significantly increase the effectiveness and stability of amplification,
• welding,
• surfacing,
• laser hardening.
Some πShaper models have achromatic design, which allows using the same device with different laser sources simultaneously and providing with the same efficiency the transformation of the intensity profile at different wavelengths.
The main properties of πShaper systems:
• innovative optical system for transforming the intensity distribution of laser beams from Gaussian to uniform (flat-top);
• principle of transformation is a controlled change in the wave front within the device – the 1st component introduces the spherical aberration required for the transformation of the intensity profile, the 2nd component compensates for this aberration,
• plane wave front of the output beam, i. e. abrasion-free,
• operation with TEM00 lasers, as well as with multimode lasers, where the intensity distribution is similar to Gaussian, e. g., parabolic,
• collimated output beam, with divergence similar to the input Gaussian beam,
• stability of the output profile at a great distance,
• achromatic design, i. e. the ability to operate multiple lasers simultaneously with the same transformation efficiency at different wavelengths,
• absence of internal focusing.
Focal-πShaper
Focal-πShaper (Fig. 7) operates with single-mode lasers and is designed to form a collimated beam with intensity distribution as a function of the "Airy disk", which, when focused by a lens, is transformed into a spot with a uniform or circular intensity distribution near the focus [6]. Focal-πShaper transformers can be used with any of the focusing optical system, e. g., with scanners with F-Θ lens or microscope lenses; the only condition is that the focusing system must have diffraction-restricted level of focusing which is provided in the modern lenses. Since Focal-πShaper optimizes interference condition when focusing laser beams, and the resulting working spots have a small size, typically less than 100 µm, the main scope of the transformers is associated with the micro-processing of materials.
Main Focal-πShaper applications:
• 3D printing (selective laser melting),
• Laser heating in geophysical studies,
• Marking and engraving,
• Scribing,
• Drilling, including blind holes in printed circuit boards,
• Recording images in printing and electronics,
• Micro-welding,
• Micro-processing of materials,
• Mass-spectrometry,
• Any laser application where it is necessary to focus the radiation, ensuring a uniform or circular intensity distribution.
Main properties of Focal-πShaper systems:
• Afocal lens optical system with smooth optical surfaces,
• Without internal focusing,
• Input beam:
• Collimated or slightly divergent,
• TEM00,
• Output beam:
• Collimated or slightly divergent,
• "Airy disk", optimized for the subsequent focusing into a spot of uniform intensity,
• Profiles of focusing: uniform (flat-top), ring, "inverted Gauss",
• Spectral ranges: UV, visible, IR, CO2 lasers,
• Depth of focus is comparable with Gaussian beams,
• Operation with any focusing optics of the diffraction level,
• Operation with scanners,
• Simple alignment and integration in existing equipment,
• Compactness.
APPLICATION OF SHAPERS IN 3D PRINTING
Recently the technology of selective laser melting (SLM) has been developed actively where the product is layer-formed through the melting of metal powder using focused radiation single-mode (TEM00) lasers. This technology, called SLM (Selective Laser Melting), is used in 3D printers for the production of complex structures and aggregates or integral structures with complex geometry. Its other important advantages [7] are:
• possibility to create details with internal holes,
• effective material consumption (more than 95% of unused powder can be used in the next cycle of production)
• accelerated prototypes production,
• possibility of automation of the production process.
SLM technology (Fig.8) is a thermal process, therefore it is extremely important to control the thermal effects, both in powder melt zone and in the optical path of the 3D-printing equipment [8]. This is especially true today, when the task of increasing productivity is put before the technology perceived by industry, which in turn requires the use of lasers of increased power. The necessary lasers are available, e. g., modern single-mode fiber lasers, whose power reaches several kilowatts. However, one of the serious obstacles to their successful application is Gaussian intensity distribution, resulting to such problems as uneven temperature distribution in the melt zone and subsequent non-uniformity of the melting process, undesirable evaporation or arcing.
Another problem is related to optics – the Gaussian distribution of the laser radiation intensity leads to an uneven heating of the optical components, which in turn causes a thermal shift of the constriction of focused laser beam and wave aberrations; as a result, in the working plane of the 3D-printer there are changes in the size and the intensity distribution from the focused working spot. These thermally induced effects are particularly pronounced on the protective windows of the equipment working chamber, since windows are inevitably covered with a layer of powder and dust in the laser recording process, which increase the uneven heating of the window material. There is a solution to this problem in the form of the windows made of special glass, self-compensating thermal effects, but this topic is beyond the scope of this article.
To equalize the temperature distribution in the melt pool, Focal-πShaper has been successfully applied allowing to manage the distribution of the intensity of the focused spot, an example optical system of 3D- printer is shown in Fig. 9.
Single-mode radiation of fiber laser is transformed by collimating Focal-πShaper into a collimated beam from the Airy disk intensity distribution and is further focused by the lens to a working plane, the image is recorded using a scanner [3]. The spot resulting at a lens focal plane has even distribution intensity (flat-top) or "doughnut". Due to heating properties of heat-conducting materials, it is the "doughnut" which is optimal for SLM technologies because it provides an even temperature distribution and stability of the process.
The optical system given above was applied at experimental installation for tests of SLM technologies with different allocations of intensity and different protective glasses. Let’s compare the image recording processes, obtained from using Gaussian operational spots and spots with "doughnut-like" temperature distribution (Fig. 10). The measurements of distributions and results of steel plate engraving are shown in figure’s bottom.
CONCLUSION
The examples considered allow us to draw the following conclusions:
• significantly lesser arcing in the case of using "doughnut" justifies the necessity of uniform temperature in the powder melt zone;
• "doughnut-like" intensity distribution allows for more efficient use of the laser energy and performance of the process;
• examination of the manufactured parts showed a lower porosity with the "doughnut-like" spot recording.
In general, the study of the SLM process and measurements of actual manufactured parts has confirmed the correctness of the proposed approach to control intensity distribution by means of focused laser spot and to apply protective windows made of special glass in order to enhance the performance and stability of the technological process.
The inhomogeneity of laser radiation can be a source of difficulty in the use of lasers, for example, in the so-called heat-affected zone (HAZ) in welded or cone-shaped holes [1].
This statement can be illustrated if we consider geometric properties of the single-mode (or TEM00) laser beam intensity profile. Such beam is often used in a variety of materials processing technology, since it provides the greatest energy concentration. Of course, each laser application has its own peculiarities of interaction of material and laser radiation, however there is one feature common to all single-mode lasers – intensity distribution Gaussian function. Therefore, in order to evaluate the efficiency of a laser beam, it is possible to consider only the geometric features of the Gaussian function as a first step without taking into account such effects as combustion, melting, laser torch, etc. accompanying laser processing.
THEORETICAL JUSTIFICATION
The intensity distribution I(r) of single-mode laser beam [2, 3] is described in polar coordinates by the well-known relation:
, (1)
where r is beam radius, ω0 is the constriction radius, Imax is the maximum intensity value. Imax value is selected so that the total beam energy corresponding to figure volume under the surface of the two-dimensional Gaussian function I(r) is equal to a conventional unit (Fig.1).
The goal of most laser technologies is to transfer a certain amount of laser energy to the material being processed, thus, it is logical to use the energy estimate of different parts of the laser beam to analyze the efficiency. For convenience of further consideration, let’s assume that all the processes of laser radiation propagation occur with a time interval equal to one. Amount of energy Er1–r2, concentrated in the annular part of the laser beam, bounded by circles of radii r1 and r2, can be found by integrating the intensity function I(r)
. (2)
Permutation of intensity distribution function (1) into formula (2) and integrating lead to the following formula:
. (3)
Suppose that the technologies for materials processing occur when the intensity of the beam is applied at a certain level Ih (see Fig.1). For convenience of further consideration, let’s introduce the variable: h = Ih / Imaxh. Thus, h is the "working" level of the laser beam, wherein the range of h values is a value between 0 and 1.
Now, let’s consider a three-dimensional geometric figure limited by a horizontal plane and the surface of a two-dimensional Gaussian function I(r) defined in polar coordinates. The volume of this figure has physical sense of the laser beam energy; for the normalization adopted earlier, the value of this energy is equal to one. Let the variables E1, E2 and E3 denote different parts of the figure that can be interpreted as parts of the beam energy:
• E1 is the "apex" of the Gaussian function, where the intensity exceeds the working level h (obviously that this energy is used inefficiently and, in some cases, represents a direct loss, e. g., in the ablation of thin films of material);
• E2 are the "tails" of the Gaussian function where intensity is insufficient for the process, but most often is the cause of HAZ formation (thus, it is almost always the case that corresponds to loss or inefficient use of laser energy);
• E3 is the effective "cylinder" of energy (laser beam "works" with the optimal intensity).
Using equation (3), it is easy to determine the formulas for calculating the values of the "parts of energy":
. (4)
Sum of E1 + E2 is given for the evaluation of losses in the processing of thin films or coatings. The results of calculations using formulas (4) are shown in Fig. 2
These results are very interesting. Unconditional loss of energy E2 can reach very high values. For example, if the working level of energy is half of the maximum (this level is often considered a working one), the energy loss is 50% of the total energy of the laser beam. In the conditions of laser processing of thin films, the energy part E1, the Gauss apex, is also considered as energy loss since this part exceeds the energy working level Ih. Thus, both energy parts E1 and E2 are losses. Their sum E1 + E2 is also shown in the diagram; the minimum of this function is 0.63. In other words, when processing thin films "only" 63% of energy is lost or inefficiently used, and 37% "work" at the best the case.
This is only a geometric interpretation. However, this approach provides an idea of the scale of losses of laser radiation, which can sometimes reach half the total beam energy!
LASER BEAM PROFILE TRANSFORMATION
Undoubtedly, the transformation of the initial Gaussian profile shape into an effective cylinder with uniform intensity would help to efficiently use the laser energy and improve the technologies where homogeneous intensity is most desirable. To implement this transformation, several types of optical systems are suggested, also called shapers (beam shapers) [4]: refractive, diffractive, beam integrators or combined systems. The choice of a suitable solution depends on the power level, wavelength, quality of the beam homogenization and other features of the specific application.
Transformation of the Gaussian beam into a beam with uniform intensity (flat-top or top hat) is the main function of πShaper, refractive family of radiation transformation systems, designed to work with UV, IR visible range lasers common in science and industry.
The concept of the πShaper operation is illustrated in Fig. 3. The Gaussian distribution of the intensity of the collimated beam of the TEM00 laser is transformed into a flat distribution on the apex (similar to the Greek letter π). The output beam is also collimated and has approximately the same dimensions as the input beam.
πShaper is a telescope retaining the integrity of the beam, while the intensity profile is transformed in a controlled manner by lenses with smooth optical surfaces, and one of the basic principles is no wave aberration, which guarantees the preservation of the flatness of the wave front of the output beam. These features distinguish πShaper from integration-type homogenizers, where homogeneous intensity is provided by the use of multi-lens optical components, breaking up the initial beam into parts and "mixing" these parts in a certain working plane. In contrast to integrating homogenizers, the flatness of the output wave front of πShaper ensures its large distance after the device, where the beam properties remain stable. This feature simplifies the use of πShapers in actual systems.
Constructed according to the scheme of Galilean telescope, πShaper implements transformation of laser beams without internal focusing, which is important in the case of applications with powerful and short-pulse lasers. Theoretically, the radiation transformation by πShaper occurs without losses and application of optimized antireflective coatings allows to achieve almost 100% system transmittance – a great advantage over homogenizers diffraction type.
Fig. 4 shows an example of converting a Gaussian beam into a uniform (flat-top) beam using πShaper.
An interesting feature is the fact that one device can form several output profiles depending on the input beam diameter.
Due to this, the πShaper systems of uniform beam formation are applied as useful tools for improving industrial laser technologies and solving some scientific problems. An example of a successful application is the laser ablation of silicon by Ekspla Ltd. (Fig. 5). Here, the hole is made in silicon using a nanosecond laser Ekspla DPSS Nd: YAG using πShaper. The aim was to achieve the "flat bottom" profile of the blind by-pass hole without edge defects – it is important to at manufacture of multi-layer printed circuit boards.
The difference in the results is obvious: if a Gaussian beam is used, the hole shape follows the Gaussian profile, i. e. narrows towards the center (see Fig. 5, left), since almost all the energy is concentrated at the center, and only a small part of it – on the periphery of the hole. The high intensity in the center of the Gaussian beam can exceed the ablation threshold of the copper layer below it.
The shape of the hole changes abruptly when using πShaper. The holes formed through the flat intensity distribution, have clearly edges and flat bottom (see Fig. 5, center and right). The shape of the "flat bottom" of the holes is stable in a wide range of displacements from the image (focal plane). Thus, the use of πShaper helped solve the problem of drilling holes of the required shape.
The formation of the beam profile opens up new possibilities for improving the performance of those laser technologies where the uniform intensity is the most desirable. Moreover, πShaper allows to achieve the optimum result when solving various industrial and scientific tasks in an easy and simple way.
In the practice of laser technologies, two models of laser radiation transformers are used:
• πShaper for the formation of collimated beams of uniform intensity, and
• Focal–πShaper for focused beams and creating uniform or annular spot profiles in the focal plane of the focusing lens.
πShaper
πShaper transforms laser beams with Gaussian or similar intensity distribution in the beam of uniform intensity and with a plane wave front (Fig. 6). Transformation of this type is important in a variety of applications [5]:
• holography,
• interferometry,
• optical systems of modern confocal microscopes and the microscopes implementing various fluorescent technologies,
• optical systems of power lasers built according to the oscillator-amplifier scheme (MOPA-lasers), where the intensity profile control allows to significantly increase the effectiveness and stability of amplification,
• welding,
• surfacing,
• laser hardening.
Some πShaper models have achromatic design, which allows using the same device with different laser sources simultaneously and providing with the same efficiency the transformation of the intensity profile at different wavelengths.
The main properties of πShaper systems:
• innovative optical system for transforming the intensity distribution of laser beams from Gaussian to uniform (flat-top);
• principle of transformation is a controlled change in the wave front within the device – the 1st component introduces the spherical aberration required for the transformation of the intensity profile, the 2nd component compensates for this aberration,
• plane wave front of the output beam, i. e. abrasion-free,
• operation with TEM00 lasers, as well as with multimode lasers, where the intensity distribution is similar to Gaussian, e. g., parabolic,
• collimated output beam, with divergence similar to the input Gaussian beam,
• stability of the output profile at a great distance,
• achromatic design, i. e. the ability to operate multiple lasers simultaneously with the same transformation efficiency at different wavelengths,
• absence of internal focusing.
Focal-πShaper
Focal-πShaper (Fig. 7) operates with single-mode lasers and is designed to form a collimated beam with intensity distribution as a function of the "Airy disk", which, when focused by a lens, is transformed into a spot with a uniform or circular intensity distribution near the focus [6]. Focal-πShaper transformers can be used with any of the focusing optical system, e. g., with scanners with F-Θ lens or microscope lenses; the only condition is that the focusing system must have diffraction-restricted level of focusing which is provided in the modern lenses. Since Focal-πShaper optimizes interference condition when focusing laser beams, and the resulting working spots have a small size, typically less than 100 µm, the main scope of the transformers is associated with the micro-processing of materials.
Main Focal-πShaper applications:
• 3D printing (selective laser melting),
• Laser heating in geophysical studies,
• Marking and engraving,
• Scribing,
• Drilling, including blind holes in printed circuit boards,
• Recording images in printing and electronics,
• Micro-welding,
• Micro-processing of materials,
• Mass-spectrometry,
• Any laser application where it is necessary to focus the radiation, ensuring a uniform or circular intensity distribution.
Main properties of Focal-πShaper systems:
• Afocal lens optical system with smooth optical surfaces,
• Without internal focusing,
• Input beam:
• Collimated or slightly divergent,
• TEM00,
• Output beam:
• Collimated or slightly divergent,
• "Airy disk", optimized for the subsequent focusing into a spot of uniform intensity,
• Profiles of focusing: uniform (flat-top), ring, "inverted Gauss",
• Spectral ranges: UV, visible, IR, CO2 lasers,
• Depth of focus is comparable with Gaussian beams,
• Operation with any focusing optics of the diffraction level,
• Operation with scanners,
• Simple alignment and integration in existing equipment,
• Compactness.
APPLICATION OF SHAPERS IN 3D PRINTING
Recently the technology of selective laser melting (SLM) has been developed actively where the product is layer-formed through the melting of metal powder using focused radiation single-mode (TEM00) lasers. This technology, called SLM (Selective Laser Melting), is used in 3D printers for the production of complex structures and aggregates or integral structures with complex geometry. Its other important advantages [7] are:
• possibility to create details with internal holes,
• effective material consumption (more than 95% of unused powder can be used in the next cycle of production)
• accelerated prototypes production,
• possibility of automation of the production process.
SLM technology (Fig.8) is a thermal process, therefore it is extremely important to control the thermal effects, both in powder melt zone and in the optical path of the 3D-printing equipment [8]. This is especially true today, when the task of increasing productivity is put before the technology perceived by industry, which in turn requires the use of lasers of increased power. The necessary lasers are available, e. g., modern single-mode fiber lasers, whose power reaches several kilowatts. However, one of the serious obstacles to their successful application is Gaussian intensity distribution, resulting to such problems as uneven temperature distribution in the melt zone and subsequent non-uniformity of the melting process, undesirable evaporation or arcing.
Another problem is related to optics – the Gaussian distribution of the laser radiation intensity leads to an uneven heating of the optical components, which in turn causes a thermal shift of the constriction of focused laser beam and wave aberrations; as a result, in the working plane of the 3D-printer there are changes in the size and the intensity distribution from the focused working spot. These thermally induced effects are particularly pronounced on the protective windows of the equipment working chamber, since windows are inevitably covered with a layer of powder and dust in the laser recording process, which increase the uneven heating of the window material. There is a solution to this problem in the form of the windows made of special glass, self-compensating thermal effects, but this topic is beyond the scope of this article.
To equalize the temperature distribution in the melt pool, Focal-πShaper has been successfully applied allowing to manage the distribution of the intensity of the focused spot, an example optical system of 3D- printer is shown in Fig. 9.
Single-mode radiation of fiber laser is transformed by collimating Focal-πShaper into a collimated beam from the Airy disk intensity distribution and is further focused by the lens to a working plane, the image is recorded using a scanner [3]. The spot resulting at a lens focal plane has even distribution intensity (flat-top) or "doughnut". Due to heating properties of heat-conducting materials, it is the "doughnut" which is optimal for SLM technologies because it provides an even temperature distribution and stability of the process.
The optical system given above was applied at experimental installation for tests of SLM technologies with different allocations of intensity and different protective glasses. Let’s compare the image recording processes, obtained from using Gaussian operational spots and spots with "doughnut-like" temperature distribution (Fig. 10). The measurements of distributions and results of steel plate engraving are shown in figure’s bottom.
CONCLUSION
The examples considered allow us to draw the following conclusions:
• significantly lesser arcing in the case of using "doughnut" justifies the necessity of uniform temperature in the powder melt zone;
• "doughnut-like" intensity distribution allows for more efficient use of the laser energy and performance of the process;
• examination of the manufactured parts showed a lower porosity with the "doughnut-like" spot recording.
In general, the study of the SLM process and measurements of actual manufactured parts has confirmed the correctness of the proposed approach to control intensity distribution by means of focused laser spot and to apply protective windows made of special glass in order to enhance the performance and stability of the technological process.
Readers feedback
