Beam shaping system in laser welding processes

Abstract

A beam shaper for transforming an MM beam with a flat-top intensity distribution profile includes an end block fused to the downstream end of an optical fiber from which the MM beam outputs along a path within a laser head. The beam shaper also includes a collimator for the laser head mounted downstream of the end block. The collimated MM beam is then focused onto a working region, wherein the beam waist is characterized by a Gaussian intensity distribution. By positioning the collimator such that the Gaussian region of the flat-top MM beam lies within the end block and within the focal plane of the collimator, a Gaussian region can be provided near the beam waist. Optionally, the Gaussian region can be provided within the waist using diffractive optical elements that transform the flat-top distribution profile into a ring-shaped profile.

Description

Technical Field

[0001] This disclosure relates to laser applications in materials processing. Specifically, this disclosure relates to beam shaping systems incorporated in industrial lasers. Background Technology

[0002] Beam shaping is the process of redistributing the irradiance and phase of a beam of optical radiation. The beam shape is a major factor determining the propagation properties of the beam profile. Applications of beam shaping include metalworking applications, which were previously accomplished using conventional high-flux heat sources such as reactive gas jets, electrical discharges, and plasma arcs. In laser welding, two adjacent or stacked metal sheets are fused together by melting the parts at the weld joint.

[0003] There are three basic welding modes corresponding to the peak power density levels contained within the focused spot size: conduction mode, transition piercing mode, and penetration or piercing mode. Each mode has its advantages and disadvantages. For example, in piercing mode, it is highly desirable for the piercing to have a constant width and depth along the weld area. However, in reality, it is almost impossible to produce uniform piercing due to the so-called piercing collapse phenomenon well known to ordinary technicians. Another detrimental characteristic of the piercing process is the formation of porosity and cracks. Overall, the piercing process is unstable. Conduction mode is known for its stability due to minimal evaporation. However, due to the relatively low power level, the weld penetration is much smaller than that of the piercing process. To achieve the desired results, a very large heat-affected zone must be formed, which leads to high heat input and thus workpiece deformation. In each mode, the weld pool characteristics depend on laser parameters, including energy, flux, and spot size.

[0004] As is known to those skilled in the art, the beam shape is defined by the irradiance distribution of the shaping beam. The irradiance (also known as intensity or power density) of a single-mode (SM) beam is mathematically described by a Gaussian function and therefore has a bell-shaped shape. Many applications can only benefit from Gaussian beams, but it is well known that the power of a single SM laser may not be low enough for processing / welding certain materials.

[0005] To overcome this problem, multiple SM outputs from the corresponding lasers are combined into a single beam with more than one mode, and are therefore further referred to as a multimode (MM) beam. Typically, the M values ​​are between 2 and 10 or even 20. 2 A MM output beam with a mode factor (an indicator of the number of modes) can be referred to as a low-mode (LM) beam. However, since both MM and LM beams have more than one mode, in the context of this disclosure, a flat-top laser beam has an M factor ranging from 2 to 20. 2 Factors, and are still referred to as MM bundles.

[0006] Due to the hopping pattern along the optical path including the MM transmission fiber, the final intensity profile of the MM bundle at the downstream end of the fiber has a flat-top shape. This flat-top intensity profile of the MM bundle is advantageous for many material laser processing operations because the intensity is essentially uniformly distributed across the entire bundle in the focal plane.

[0007] Further propagating along a path including various optical elements such as focusing lenses, the MM beam has multiple beam regions, including a beam waist formed in the focal plane of the focusing lens. The waist is the narrowest beam region and therefore has the highest power density along the beam. While the beam waist is characterized by a similar flat-top intensity distribution, the beam regions preceding the beam waist have individual intensity profiles that may differ from the flat-top shape. One of these pre-waist beam regions is spaced considerably from the waist and is characterized by a quasi-Gaussian intensity profile. The beam region where the beam acquires quasi-Gaussian intensity is further referred to as the (upper)Gaussian region. The propagating beam is symmetrical with respect to its waist. Therefore, a second Gaussian region is spaced downstream of the waist at the same distance as the distance between the upper region and the waist.

[0008] Those skilled in the art of laser-based materials processing know that Gaussian beams are associated with high-quality welds. Due to their bell-shaped intensity distribution profile, the intensity is unevenly distributed across the beam spot, with the highest intensity in the central apex region gradually decreasing towards the base. This profile creates a smooth temperature gradient across the entire surface to be laser-treated, as it allows the irradiated area to be gradually heated first by the front wing, then treated by the intensity peak, and finally gradually cooled by the rear wing. This thermodynamics is attractive to a considerable number of materials processing methods. Regardless of shape, the laser beam is delivered to the welding area through the laser head—the downstream component of any industrial laser system.

[0009] Figure 1 An exemplary laser head 25, typically mounted on a robotic arm, is shown. The laser head 25 includes a beam guiding mechanism that guides the MM flat-top laser beam 10 after it is output from the transmission fiber 22. The optical mechanism includes an end block 15 fused to the downstream end of the transmission fiber 22, which receives the combined beam 10 from a combiner that combines the outputs from individual SM laser sources. As is known to those skilled in the art of lasers, the end block 15 is typically made of quartz and is configured to prevent the fiber end 22 from burning, which is otherwise unavoidable at industrial laser power levels ranging from hundreds of watts to several megawatts. The beam 10 diverges as it propagates through and past the end block 15 before being incident on the collimating lens or collimator 1. The collimator 1 is an optical element that converts the diverging beam 10 from the downstream fiber end 22 into parallel rays. Thus, the downstream fiber end 22 is positioned at a focal point, i.e., spaced from the collimator 1 by a distance equal to the collimator focal length F1.

[0010] A focusing lens 6 with a focal length of F2 focuses the collimating beam 10 onto surface 12, thereby forming a beam waist with a flat-top intensity profile. The Gaussian region 14 of the focused beam is spaced apart from the beam waist.

[0011] An important factor related to beam divergence is the depth of field (DOF), which is closely related to the so-called process window. In the context of materials processing, DOF is the distance at which a laser-processed workpiece can move away from the center of the beam waist while still maintaining the focused beam size. More specifically, it can be defined as the Rayleigh range, well-known to those skilled in the art of optics. In the mechanism disclosed above, the maximum Rayleigh range is located in the beam waist. The Rayleigh range in the Gaussian region 14 is much smaller than the Rayleigh range in the waist. For reasons explained below, a small DOF is inconvenient in laser-based materials processing applications.

[0012] To operate within the Gaussian region 14, the beam 10 should be defocused. This can be achieved by moving the focusing lens 6 and the surface 12 relative to each other. However, the result of defocusing may be unacceptable because each region 14 may have insufficient energy due to the large spot formed on the surface by that region. For example, if the spot size is altered by more than 10% by the defocused beam 10, the power density is fundamentally reduced because density and spot size are quadratically related. Even if the power density is sufficient, the DOF in the Gaussian region 14 is small. This means that part tolerances (the workpieces to be welded are typically not ideally uniform) and / or errors caused by robot motion can severely affect weld quality. Therefore, robot operation is extremely difficult to control during welding using the Gaussian region 14 of the beam 10, leading to complex software and thus high manufacturing costs.

[0013] Configuring laser welding equipment with a beam-shaping system is highly advantageous, as this system can transform the beam 10 such that its Gaussian region 14 lies within the beam waist. This results in increased degrees of freedom, thereby minimizing the detrimental effects of robot motion errors and increasing energy efficiency. The increased degrees of freedom also help minimize damage to expensive but not always uniform workpieces.

[0014] Therefore, it is desirable to provide a beam shaping system in laser-based industrial robot welding equipment, the beam shaping system being configured to form the waist of a flat-top MM beam characterized by a Gaussian intensity distribution on the surface of the workpiece to be laser-treated.

[0015] There is also another need for laser-based material processing that incorporates an improved beam-shaping system. Summary of the Invention

[0016] The disclosed device is configured to take into account at least some of the factors described above. It typically includes a laser source, preferably a fiber laser source or a YAG source, which may include multiple SM continuous wave (CW), quasi-CW, or pulsed lasers, thereby outputting a laser with M… 2 MM laser beams with a power factor (ranging from 2 to 20) and up to 20 kW are used, but higher power is entirely possible. The MM flat-top laser beam is guided along a transmission fiber fused to a quartz block mounted to the laser head and configured to prevent the fiber ends from burning. The flat-top laser beam extends within the quartz block and is guided along a path through the laser head by guiding optics, which may in particular include collimators and focusing lenses. Preferably, but not necessarily, a scanner including a pair of movable mirrors is also mounted in the laser head, as disclosed in detail in US20160368089 and US20180369964, which are incorporated herein by reference in their entirety.

[0017] The laser head is equipped with the beam shaping system of the present invention, which is configured to transform a laser beam having a flat-top intensity distribution into a Gaussian intensity distribution profile. Figure 1 In contrast to the known prior art where the Gaussian region is far from the waistband, the present invention provides a mechanism for placing the Gaussian region near or exactly within the waistband.

[0018] According to one aspect of the disclosed mechanism, this is achieved by providing additional diffraction elements (such as axial prisms, homogenizers, etc.) to the beam-shaping system. Unlike converging lenses, axial prism lenses are designed to focus the light source to a single point on the optical axis, using interference to produce a focal line along the optical axis. Within the beam overlap region, known as the DOF, the axial prism replicates the properties of a Bessel bundle, which is a bundle composed of rings with equal power.

[0019] Unsurprisingly, a Bessel beam can be mathematically described using a Bessel function, which has a cross-sectional intensity profile comprising a set of concentric rings in the focal plane. For example, for a zero-order beam, the Bessel beam intensity profile has an annular cross-section characterized by relatively low energy; for a first-order beam, the cross-section has a spot at the very center.

[0020] Using such a diffraction element allows for the regeneration of a region with Gaussian intensity very close to the beam waist without shifting the focusing lens. In other words, this mechanism forms a Gaussian region almost adjacent to or even within the beam waist. Figure 1 Compared to existing technologies, recreating the Gaussian region near and actually within the waist increases DOF ​​and energy.

[0021] In the aforementioned configuration, the transmission fiber and the downstream end of the collimator are spaced apart by the focal length of the collimator. However, for additional diffractive optical elements, the Gaussian region is located further away from the waist, as shown in the reference. Figure 1 As discussed in the existing technology.

[0022] Another aspect of the invention does not involve additional diffraction elements. In contrast to the optical mechanism disclosed above, the collimator is positioned at a focal length not from the downstream end of the MM transmission fiber, but from the Gaussian region of the top cap bundle. Therefore, the bundle waist now comprises a light spot with a Gaussian distribution profile rather than a flat-top intensity profile on the target surface.

[0023] Both aspects apply to step-index MM fibers. However, the mechanism of the second aspect of the invention is related to gradient fibers. The latter do not use total internal reflection to guide light. Instead, they use refraction. The refractive index of the fiber gradually decreases from its center, eventually falling to the same value as the cladding at the edge of the core, where the refractive index is gradually changed. Relative positions can be established between the fiber end, collimator, and focusing lens, wherein the waist formed on the surface to be treated is characterized by an approximately Gaussian intensity distribution profile associated with increased energy. Attached Figure Description

[0024] Referring to the accompanying drawings, the above and other features will become clear. These drawings are not to scale. They provide illustration and further understanding of various aspects and features and form part of this specification, but do not represent limitation on any particular body or aspect. In the drawings, each identical or nearly identical component appearing in the various figures is represented by similar numbers. For clarity, not every component may be labeled in every figure. In these figures:

[0025] Figure 1 The optical mechanism of a typical known laser head is shown, which is configured to guide a flat-top MM beam to a target.

[0026] Figure 2 The laser head according to the present invention is shown, which is configured to process a workpiece to be laser-treated using one of the Gaussian regions of an MM beam.

[0027] Figure 3 Show Figure 2 The optical mechanism of the laser head of the present invention.

[0028] Figure 4 Show Figure 3 The beam and beam cross-sectional intensity distribution profile downstream of the focusing lens of the optical mechanism.

[0029] Figure 5 Shown by Figure 3 The optical mechanism forms the beam and the penetration depth of each plane of the beam.

[0030] Figure 6 A modified beam-shaping optical mechanism configured according to the concept of the present invention is shown. Detailed Implementation

[0031] The concept of this invention provides a larger processing window in material laser-based processing operations, which typically require the use of high-power and high-quality MM beams. This concept is achieved through the optical mechanism of this invention, which transforms the non-Gaussian intensity profile into a Gaussian intensity profile near the waist of the shaping beam.

[0032] Figure 2 An exemplary laser head 50, configured according to the invention and equipped with the optical mechanism of the invention, is shown. The laser head 50 is a key component of an industrial laser system, located upstream of the workpiece(s) to be laser-treated. The laser head of the invention particularly includes optical components and sometimes also includes electronic components, beam-shaping optical mechanism elements including collimating lenses, which are typically focused downstream of the laser beam transmission fiber 22 and thus spaced apart by a focal length. In its simplest form, collimation ensures that light incident on the input of the collimator 1 propagates parallel to each other downstream from its output. The laser head 50 may optionally have two rotating mirrors 3 and 5, and a fixed mirror 4. Finally, the collimated beam is incident on a focusing lens 6, which focuses the beam onto the surface of the workpiece to be laser-treated. So far, the mechanism shown is similar to... Figure 1 The same organization Figure 1 A workpiece is shown being laser-processed using a flat-top MM beam. The objectives of the disclosed beam-shaping mechanism are: 1. to irradiate the workpiece with a beam having a Gaussian profile; and 2. to place the desired Gaussian region of the beam practically near the waist, i.e., adjacent to or within the waist region. In other words, the illustrated mechanism comprises a combination of optical elements arranged to transform a flat-top or other shaped MM beam into a Gaussian beam, wherein both energy and DOF are increased compared to known prior art.

[0033] Now for reference Figure 2 and Figure 3The invention is realized by introducing a diffractive optical element 2, which is mounted anywhere between the collimator 1 and the focusing lens 6, or at a very short distance downstream of the focusing lens 6, depending on the focal length of the lens. For example, for a focal length of 200 mm, this distance is no more than 10 mm. The combination of the diffractive element 2 and the focusing lens 6 produces a region 20 with a Gaussian intensity distribution of light. In other words, the lens-diffractive element duplex produces a Bessel-Gaussian beam. In the illustrated mechanism, the diffractive element 2 provides a Gaussian beam region 20 that is actually adjacent to or within the beam waist, at a distance F from the focusing lens 6. 21 This distance is only slightly shorter than Figure 1 The focal length of the intermediate lens 6 is F2. In fact, the Gaussian region 14 is so close to the waist that it is considered to be located within the waist including the illuminated surface 12.

[0034] The diffractive element 2 may in particular include a homogenizer, a hologram, and an axon prism. In the illustrated structure, element 2 is an axon prism lens well known to those skilled in the art of optics. In the context of this disclosure, the axon prism 2 transforms the flat-top intensity profile of the beam 10 into a beam shape that can be mathematically described by a Bessel function, and has an annular intensity profile within the waist of the transformed beam. The region of the transformed MM Bessel beam 10 with a Gaussian distribution is not symmetrical, and only the upper region 14 has the desired energy, as will be discussed below. The operating principle of the axon prism is universal for any suitable diffractive optical component.

[0035] Figure 4 Showing has through Figure 2 and Figure 3 The apparatus yields a plan view of the regions of the Bessel beam with varying intensity profiles along the optical path between the diffraction element 2 and the plane downstream of the beam waist, including surface 12. As shown, the maximum top and bottom beam regions, or planes 1 and 9, are 40 mm apart and are symmetrically positioned relative to the waist extending between planes 4 and 5. Planes 1, 2, and 6-9 all show different profiles of the Bessel beam, distinct from the Gaussian profile. Conversely, the profiles in the individual planes 3, and especially 4, closely approximate a Gaussian distribution. Plane 4 is the most intriguing part, as it is actually located within the waist, indicating that the energy there is close to its maximum. As for plane 5, although the profile shown differs slightly from the Gaussian profile, it remains suitable for the intended purpose.

[0036] Figure 5 The diagram shows a DOF equal to 5 mm, where DOF is the difference between adjacent planes 4 and 5 that define the waist. Interestingly, the mechanism is the same, but without the axial prism (such as...). Figure 1The axial prism shown provides only 1 mm DOF in the Gaussian region. Of course, DOF depends on the individual parameters of all optical components of the laser head 50, including the focusing lens 6 (150 mm in this experiment) and the collimator focal length (100 mm), as well as the laser output power. The smallest spot size (i.e., the densest spot size) lies within plane 5 and is equal to 350 μm. Plane 4 has a spot size approximately equal to that of plane 5. Conversely, plane 8 is characterized by a spot size of 2500 mm and is the largest of the planes shown.

[0037] Figure 6 This illustrates another optical mechanism for the beam shaper. Although the latter is similar to... Figure 1 The existing technology shares the same optical elements, specifically including end block 15, collimating lens, and focusing lenses 1 and 6, but it does not have the ability to... Figure 2 and Figure 3 The diffraction element 2 is crucial to the mechanism. Instead, it utilizes the MM flat-top bundle 10 with a region of Gaussian intensity distribution by shifting the collimator 1 downstream from the fiber end 22. The collimator is shifted such that the Gaussian region within the end block 15 is spaced from the collimator by a distance corresponding to the focal length of the collimator 1, rather than from the fiber end 22. As a result, the waist of the bundle 10 on the surface 12 spaced from the lens 6 at the original focal length F2 is characterized by a region of Gaussian intensity distribution.

[0038] The transmission fiber 22 used in all previously disclosed mechanisms is a step-index fiber. However, Figure 4 The mechanism shown can be used in conjunction with gradient-index fiber, which, by definition, is not SM fiber. Using gradient-index fiber... Figure 6 The operation of the mechanism is the same as that of using step-index optical fiber.

[0039] The mechanisms disclosed herein according to the invention are not limited in their application to the details of the construction and arrangement of the components set forth in the following description or illustrated in the accompanying drawings. Other embodiments are possible and can be practiced or implemented in various ways. The examples of specific implementations provided herein are for illustrative purposes only and are not intended to be limiting. Specifically, actions, components, elements, and features discussed in connection with any one or more embodiments are not intended to exclude similar roles in any other embodiments.

[0040] Furthermore, the wording and terminology used herein are for descriptive purposes and should not be considered limiting. Any reference to examples, embodiments, components, elements, or actions of systems and methods mentioned herein in the singular may also include embodiments comprising a plural, and any plural reference to any embodiment, component, element, or action herein may also refer to embodiments comprising only a singular. References in singular or plural forms are not intended to limit the systems or methods, components, actions, or elements of this disclosure. The use of “comprising,” “including,” “having,” “containing,” “involving,” and variations thereof herein means to cover the terms listed thereafter and their equivalents, as well as additional terms. References to “or” may be interpreted as inclusive, such that any term described using “or” may refer to a single, more than one, or any of the terms described.

[0041] Therefore, having described several aspects of the inventive concept, it should be understood that various changes, modifications, and improvements will readily occur to those skilled in the art. For example, the examples disclosed herein can also be used in other situations. Such changes, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the examples discussed herein. Therefore, the foregoing description and figures are by way of example only.

Claims

1. A beam shaper for forming the waist of a flat-top MM beam with a Gaussian intensity distribution profile on a workpiece to be laser-treated, comprising: End block, which is fused to the downstream end of an optical fiber, the optical fiber outputting the flat-top MM bundle along a path; A collimator that receives and collimates the flat-top MM bundle downstream of the end block; A diffractive optical element located between the end block and the workpiece and configured to transform the flat-top MM beam into an MM Bessel beam; as well as A focusing lens, the focusing lens being located in a fixed position and forming the waist of the MM Bessel bundle in the focal plane of the focusing lens on the workpiece, wherein the waist has the Gaussian intensity distribution profile.

2. The beam shaper of claim 1, wherein, The collimator is spaced from the interface between the downstream end of the optical fiber and the end block by a distance equal to the focal length of the collimator, and the MM bundle has a flat-top intensity distribution profile.

3. The beam shaper of claim 1, wherein, The diffractive optical element is located between the collimator and the focusing lens.

4. The beam shaper of claim 1, wherein, The diffractive optical element is located downstream of the focusing lens.

5. The beam shaper of claim 1, wherein, The diffractive optical elements include an axon prism, a hologram, or a homogenizer.

6. The beam shaper of claim 1, wherein, The end block, collimator, diffractive optical element, and focusing lens are mounted into the housing of the laser head of the high-power fiber laser welding system.

7. The beam shaper of claim 1, wherein, The collimator is spaced apart from the end block downstream of the end block, such that the focal plane of the collimator is located within the end block and coincides with the Gaussian beam region of the MM beam having a Gaussian intensity distribution, wherein the Gaussian beam region is focused on the focal plane of the focusing lens on the workpiece to be laser-processed.

8. The beam shaper of claim 1, wherein, The end block, collimator, and focusing lens, fused to the downstream end of the optical fiber, are mounted into the housing of the laser head of a high-power fiber laser welding system.

9. The beam shaper of claim 8, wherein, The optical fiber is a step-index fiber or a gradient-index fiber.

10. The beam shaper of claim 1 further includes a plurality of movable mirrors located upstream of the focusing lens and mounted on the laser head together with the collimator and the focusing lens.

11. The beam shaper of claim 10, wherein, The laser head is mounted on a robotic arm that supports the laser head, and the optical fiber transmits the MM beam from a fiber laser or YAG laser source operating in CW, QCW, or pulsed mode.

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