FLEXIBLE LASER WELDING HEAD FOR SLEEVE-TO-TUBE WELDING

Abstract: In one embodiment, a laser welding head is formed of a hollow tubular housing, and includes multiple focusing means for refocusing and realigning the laser beam if it is misaligned relative to the axis of the housing. In another embodiment, the housing is formed of a plurality of flexi­ bly interconnected segments having at least one focusing means in each segment for realigning the beam and for directing the laser beam along interconnected linear paths.

Description: This invention generally relates to laser welding apparatus and more particularly to welding apparatus adapted for welding a sleeve within a curved or deformed tube using a laser beam. Description of the Prior Art : In tube-type heat exchangers, a first working fluid flows internally through a bundle of tubes, a second working fluid flows externally of the tubes and heat exchange occurs between the two fluids through the tube walls. Occasionally a tube may become defective due to corrosion or other known failure modes thereby a leak occurs which allows the two working fluids to mingle. When a leak occurs, not only is heat exchange efficiency diminished, co-mingling of working fluids can cause damage to the heat exchangers as well as equipment connected to it. Accordingly, when a leak occurs it must be either plugged or repaired. Sometimes the tube is taken out of service by plugging both ends of either side of the leak. While such procedure is effective to stop the leak, it causes a reduction in heat exchange capacity because the tube has been removed from service. Sometimes it is possible to repair the tube but often repairs are difficult to effect. In a nuclear reactor, tube-type heat exchangers are commonly used as steam generators. Generally the working fluid flowing through the tubes of a nuclear steam generator is radioactive. When a defect occurs in a tube of a nuclear steam generator resulting in a co-mingling of the working fluids, not only does the leak reduce the effective heat exchange capability of the device and endanger equipment coupled thereto, it also creates a radioactive contamination hazard. Therefore, whatever steps are taken to stop the leak must be effective. If a repair can be made, this is preferred because heat exchange capacity is not significantly affected. There are several methods known in the art for repairing tubes. Many of these methods are not applicable to repair heat exchange tubes because the defective portion of the tube may not be readily accessible. The problem of accessibility is compounded in nuclear steam generators because of the radioactive nature of the environment in which such structures are used. One method used in the repair of heat exchange tubes in a nuclear steam generator involves the use of a metal sleeve having an outside diameter slightly smaller than the inside diameter of the defective tube. The sleeve is inserted into the defective tube and attached to it in such a way as to bridge the defective area of the tube. The ends of the sleeve are welded to the interior surface of the tube to thereby effect the repair. In U.S. Patent 4,694,136 "Laser Welding of a Sleeve Within a Tube", a method of remotely welding a sleeve within a tube of a nuclear steam generator is described. The method therein involves locating the sleeve within the tube at the weld site, positioning a high power laser remote from the weld site, inserting a welding head apparatus at the end of a laser beam transmitting means into the tube near the weld site and communicating laser energy along the transmitting means to the weld site for application to the internal surfaces of the sleeve for welding the sleeve to the tube. The above mentioned US Patents describes the specific arrangement of the laser welding head which includes a straight cylindrical housing which is adapted to extend axially into a steam generator tube and rotate therein for providing a circumferential weld between the sleeve and the tube. U.S. Patent 4,724,298 entitled "Laser Beam Alignment and Transport System" discloses a method and apparatus for aligning the high power laser beam with the desired welding site at the output of the welding head. According to the aforementioned US patents, laser welding has proved to be an effective alternative to other welding methods because it is fast, accurate and produces a smaller heat affected zone. The various methods and apparatus described in the aforementioned patent applications require that the incident laser beam be aligned with respect to the optical axis of the welding head. If misalignment occurs, the laser beam is misdirected resulting perhaps in an inferior weld and possibly damage to the welding head due to absorption of some of the misdirected laser energy. Proper laser welding may also be affected if the tube and sleeve have an axis different from the axis of the welding head. For example, the welding head described in the aforementioned patent applications is adapted for use in a straight section of a tube. If the tube has a bent or curved portion, the welding head may not be effective to communicate laser energy to the weld site because of misalignment of the tube and sleeve axes with the axis of the welding head Figure 1 herein schematically illustrates in a simplified form a welding head 10 similar to the apparatus disclosed in the aforementioned patent applications suitable for welding a sleeve S within a tube T secured within a tube sheet TS of a nuclear steam generator. The welding head has an optical axis A and is adapted to direct the laser beam B (shown in parallel dotted lines) through the cylindrical body 12 which has an inlet 14 and an outlet 16. The welding head 10 projects through tube sheet TS into the tube T and concentric sleeve S that are to be welded together. A lens 18 and mirror 20 serve to focus the laser beam B and direct it through the outlet 16 in the cylindrical body 12 and onto the inner surface of the sleeve S at the weld site W where welding occurs. Means (not shown) is provided to rotate the cylindrical body 12 of the welding head 10 about the optical axis A (shown by the centerline) thereby causing a circumferential weld to be made between the inner surface of the tube T and the outer surface of the sleeve S whereby the sleeve is firmly attached to the tube. There are conditions under which the quality of laser welding performed by the laser welding head 10 described above may be adversely affected. One condition occurs if the axis a b of the incident laser beam B is significantly misaligned relative to the optical axis A of the welding head 10 as shown in Figure 2. In such case the laser beam B is projected towards the cylindrical body 12 at an angle q such that the beam strikes the inside wall 22 of the cylindrical body 12 resulting in scattering and absorption losses. Thus, the laser energy is only partially collected and focused by the lens 18 and mirror 20 resulting in the loss of power delivered to the weld site W, and possibly an inferior weld. Also, it is possible to damage the lens 18, mirror 20 and associated mounting structures (not shown) due to scattering and absorption. It is also possible to damage the inside walls 22 of the welding head 10 due to misdirected laser energy. Another condition that can affect proper laser welding occurs if the tube T and sleeve S to be welded are significantly bent or deformed as shown in Figure 3 herein, in somewhat an exaggerated form for purposes of illustration. In such case, the optical axis A of the welding head 10 is different from the central axis C of the tube T and sleeve S configuration. Thus it may not be possible to insert the welding head 10 sufficiently into the sleeve S to reach the weld site W. Even if it is possible for the weld head outlet 16 to be aligned with the weld site W, the welding head 10 may be too close to the weld site to effect a proper weld. Less severe deformation may permit the welding head 10 to be inserted but may restrict the free rotation of the welding head thereby resulting in damage to the apparatus and possibly an inferior weld. It is the object of the present invention to overcome both of the aforementioned problems. The invention accordingly resides in a laser welding head as defined in claim 1. Described hereinafter is a laser welding head for welding a sleeve within a tube by heating the sleeve and tube to cause fusing of the interface therebetween, using a laser beam. The tube may be bent or deformed. The laser welding head comprises cylindrical housing having a laser beam inlet and outlet and an optical axis. In one embodiment, the cylindrical housing is formed of at least two interconnected hollow cylindrical segments which are adapted to be inserted axially into the tube and sleeve with the laser beam outlet directed at the sleeve to be welded. Flexible joint means interconnects the segments. Focusing means mounted in each respective segment focuses the beam at focal points near each flexible joint and directs the laser beam along the optical axis of the cylindrical housing. A proximal end of the cylindrical housing that is near the laser beam inlet is adapted to allow the cylindrical housing means to rotate axially within the sleeve. In a simplified embodiment the cylindrical housing is formed of a single rigid segment. Plural focusing means within the cylindrical housing directs the laser beam along the optical axis of the housing to correct for possible initial laser beam misalignment. The focusing means within each of the segments directs the laser beam to the next adjacent segment through a convergent zone or beam waist near the focal point. A detector having an aperture therein is located near the beam waist and is radially spaced therefrom. The detector is responsive to laser energy impinging thereon which results from the drift of the laser beam away from the optical axis of the cylindrical housing to produce a signal when the beam drifts. Means aligned with the optical axis of the cylindrical housing is provided for permitting observation of the welding operation and inspection of the weld. BRIEF DESCRIPTION OF THE DRAWINGS A more detailed understanding of the invention may be had from the following description of a preferred embodiment, given by way of example and to be read in conjunction with the accompanying drawing wherein: Figures 1-3 illustrate schematically and in simplified form a rigid welding head arrangement of the type disclosed in US-A-4 694 136 and US-A-4 724 298 assigned to the assignee of the present invention, the teachings of which are incorporated herein by reference. Figure 4 is a schematic illustration in side section of a flexible welding head in accordance with one embodiment of the invention, that is formed of multiple segments and multiple lens elements for directing a laser beam along an optical axis of the welding head. Figure 5 is an illustration similar to Figure 4 but showing the laser beam misaligned with respect to the optical axis of the welding head and illustrating the resulting path of the laser beam through the welding head. Figure 6 is an illustration similar to Figure 4 showing the segments of the welding head disposed with their respective axes out of alignment with the axes of adjacent segments so that the optical axis of the welding head is distorted and not disposed along a straight line, the distortion being as it might be when the welding head is located in a bent tube and further illustrating the path of the laser beam through the welding head. Figure 7 is a schematic illustration in side section of a flexible welding head of the present invention having a large converging lens and a deflecting mirror for reducing and directing the laser beam within the welding head. Figure 8 is a schematic illustration similar to Figure 7 with a built in beam splitter and television camera enabling in-situ inspection of the welding operation and the resulting weld. Figure 9 is a perspective illustration of a partly broken away portion of a welding head having a beam drift detector installed therein, wherein the laser beam is cylindrical in configuration. Figure 10 is a perspective illustration of a partly broken away portion of a welding head having a beam drift detector installed therein, located in the vicinity of the beam waist of a laser beam having converging and diverging portions. Figure 11 is an illustration of a simplified embodiment of the invention having a rigid housing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A flexible laser welding head (welding head) 40 that is constructed in accordance with one embodiment of the present invention is illustrated in Figure 4. The welding head 40 includes a segmented cylindrical housing 42 formed of a number of hollow segments S₁-S₄ having a bore 45. The housing 42 has a beam inlet 44, a beam outlet 46 and an optical axis A. The several segments S₁-S₄ are connected together by flexible joints F₁₂, F₂₃ and F₃₄ having respective centers of rotation C₁₂, C₂₃ and C₃₄ located along the optical axis A at transitions between respective pairs of segments S₁-S₂, S₂-S₃ and S₃-S₄ as shown. The flexible joints F₁₂-F₃₄ can be formed in several ways that are known in the art. One example of such flexible joints include convoluted bellows (as shown). The flexible joints F₁₂-F₃₄ permit the segments S₁-S₄ of the welding head 40 to rotate or become skewed relative to each other along the axis A in order to conform to or follow the configuration of a straight or bent tube, yet such flexible joints F₁₂-F₃₄ are stiff enough to support the segments despite their inclination to the axis A that is required to make a complete circumferential weld between the tube T and the sleeve S. Each segment S₁-S₄ has a corresponding lens element L₁, L₂, L₃ and L₄. The effective length of a segment is measured between centers of rotation at the opposite ends of the segments respectively. For example, the length L₂ of the segment S₂ is the distance between the centers C₁₂ and C₂₃. The effective length of a segment includes the length of the segment plus one-half of the length of each of its two interconnecting joint means, i.e., one joint means at each end of each segment except an end segment having only one joint means. In the operation of the laser welding head 40 (Figure 4), a laser beam B of diameter D₁ collimated along an axis a b is directed at the lens L₁ near the inlet 44. The beam B is brought to a first focus f p1 along the optical axis A of the welding head 40 near the position of the first flexible joint F₁₂. In Figure 4, the laser beam B has an optical axis a b which is colinear with the optical axis A of the welding head 40. Thus, the focal point f p1 lies on the optical axis A of the welding head 40 and coincides with the center of rotation C₁₂ between the segments S₁ and S₂. The beam B propagates from focus f p1 to lens L₂ where the beam has a diameter D₂. The lens L₂ refocuses the beam to a second focal point or focus f p2 near the center of rotation C₂₃ of second flexible joint F₂₃ as shown. Similarly, the beam propagates to lens L₃ and there has a diameter D₃. The lens L₃ focuses the beam B at focal point f p3 near the center of rotation C₃₄ of joint F₃₄. Thereafter the beam B of diameter D₄ passes through the lens L₄, it is reflected off of a mirror 48 and brought to a focus f pw at the contemplated weld site W where welding is to occur. In the embodiment shown in Figure 4, each lens L₁-L₄ is located an integral number of its own respective focal lengths from each adjacent center of rotation. For example, lens L₁ (the first lens) is located in segment S₁ one focal length f L1 from center of rotation C₁₂. If the optical axis A of the cylindrical housing 42 corresponds to the optical axis a b of the beam B, the focus f p1 lies on the optical axis. The lens L4 (the last or nth lens) has a focal length f L4 and is adjustably mounted in segment S₄ by means not shown but known in the art. The lens L₄ is located about two of its focal lengths 2f L4 from the center of rotation C₃₄. The lens L₄ is adjustable in order to provide for fine focusing (at f pw ) of the beam B on the weld site W. The lenses L₂-L₃ (the intermediate lenses) have respective focal lengths f L2 and f L3 and are located at the respective midpoints M₂ and M₃ of the segments S₂ and S₃. The lens L₂ is located two of its focal lengths 2f L2 from each of the respective centers of rotation C₁₂ and C₂₃ of segment S₂. The effective length L₂ of segment S₂ is 4f L2 or the distance between C₁₂ and C₂₃ along axis A. The lens L₃ is similarly configured in segment S₃. In the arrangement illustrated in Figure 4, it is assumed that the cylindrical housing 42 is straight so that the optical axis A is colinear with the individual axes of each segment S₁-S₄. Also, it is assumed that the optical axis a b of the laser beam B is aligned with the optical axis A. If, as hereinafter described, the axis a b of the laser beam B is misaligned with the axis A of the housing 12 the multiple lens system of the present invention corrects such misalignment. If the tube T is bent, the flexibility of housing 42 in combination with the multiple lens system allows for laser beam transport along a series of interconnected straight lines. Figure 5 shows how a misaligned laser beam B will propagate through the laser welding head 40. In this case, tube T is straight and cylindrical housing 42, although flexible, is also straight. The laser beam B, represented by the dashed lines, has its optical axis a b incident on lens L₁ at an angle q₁ with respect to the optical axis A of the welding head 40. The laser beam is initially cylindrically and has a diameter D₁. The lens L₁ focuses the beam B so that it converges conically like an hour glass in a configuration known as a beam waist B₁. The focus of the lens L₁ occurs at f p1 at a distance d₁ off the optical axis A of the welding head 40. The distance d₁ is given by the relation: d₁ = f L1 tan q₁, for small q₁, tan q₁ = q₁, hence d₁ = q₁ f L1 . (1) After passing through the focus f p1 , the laser beam B expands from the beam waist B₁ to a diameter D₂ at the lens L₂, where D₂ = D₁ x 2f L2 /f L1 . (2) At the lens L₂ the center C L of the laser beam B is offset from the optical axis A of the welding head by an amount R₂ = d₁ + q₁ x 2f L2 . (3) (The total offset R₂ between the lenses L₁ and L₂ equals the angle q₁ times the sum of the focal distances f L1 and 2f L2 between the lenses.) The lens L₂ refocuses the beam at focus f p3 at a position d₂ = d₁ off the optical axis of the welding head 40. The center C L of the laser beam B is now misaligned relative to the optical axis A of the welding head 40 by the angle q₂ = (R₂ + d₂)/2f L2 . (4) The laser beam propagates through the remaining lenses L₃-L₄ in a similar fashion, obeying the relations given above and shown in Figure 5. It should be noted that the relationship (R n + D n /2) < D'/2, (5) where n represents the number of an arbitrary nth lens and D′ is the diameter of that lens, must be satisfied to keep the laser beam B within the limits of the lenses. If all of the lenses used in the system have the same diameter, this condition requires that focal lengths of the intermediate lenses must decrease with increasing values of n. In the embodiment illustrated in Figure 5 each intermediate lens L₂-L₄ has a focal length greater than the next lens in the direction of propagation of the beam. Where the housing 42 is flexible, as shown, the length of each corresponding segment S₁-S₄ is adjusted so that each focal point f p1 -f p3 is near a respective center of rotation C₁₂-C₃₄. f L2 > f L3 > f L4 > f Ln-1 (6) It is clear that only a small deviation of the beam axis a b from the optical axis A can be tolerated in the arrangement of Figure 2. Whereas, in Figure 5, although the angle q tends to increase as the laser beam B propagates through the welding head, greater misalignment errors can be tolerated before the beam will strike the side wall. For example, in Figure 2 the maximum error that is acceptable is q′ < (D′ - D₁)/L, (7) where L is the total length of the welding head 10 from the incoming deflecting mirror 20' to lens 18, D′ is the beam diameter at the lens 18, D₁ is the incoming beam diameter, and q′ is the maximum angular misalignment q. The corresponding relation for the laser welding head 40 of the present invention may be derived from Figure 5 wherein q₁′ < (D′ - D₁)/(f L1 + 2f L2 ). (8) Thus, the maximum error is a function of the focal lengths of the first two lenses L₁ and L₂ near the inlet 44. The relationship of equation (8) holds because each time the beam B is refocused along the welding head 40, the focal point f pn is on the opposite side of the optical axis A. Thus, initial misalignment errors tend to cancel each other, and the only error which is significant is the angular error which occurs between the first lens L₁ and the second lens L₂ given by equation (8). In a typical application the maximum error in alignment q₁′ from equation (8) may be 3-5 times the maximum error q′ from equation (7). Another important application of the laser welding head 40 of the present invention occurs in cases where the tube T to be repaired is bent or distorted. Figure 6 shows an exaggerated configuration in which the welding head 40 is located in a bent or non-true tube T that is fitted with an inserted sleeve S. The flexible joints F₁₂-F₃₄ permit the segments S₁-S₄ to adjust to the contour of the tube T and sleeve S configuration about the respective centers of rotation C₁₂-C₃₄. Because the lenses L₁-L₄ are located in the corresponding individual segments S₁-S₄, the optical axis A of the welding head 40 follows the tube T in Figure 6. The segments S₁ and S₂ are misaligned relative to each other by an angle q₁₂. The segments S₂ and S₃ are misaligned relative to each other by an angle q₂₃. Likewise the segments S₃ and S₄ are misaligned by an angle q₃₄. In Figure 6 the laser beam B is focused at f p1 , along the optical axis A of the weld head 40 and near the center of rotation C₁₂ of the joint F₁₂ as shown. The beam B propagates to the lens L₂ but is somewhat displaced by an amount R₂ = q₁₂ x 2f L2 (9) at the location of the lens L₂. The corresponding conditions at the other lenses L₃ and L₄ are indicated in Figure 6. As the welding head rotates about the optical axis A, the flexible joints F₁₂-F₃₄ readjust about the respective centers of rotation C₁₂-C₃₄ to conform to the tube T and sleeve S geometry, and the optical configuration is maintained as the circumferential weld is made. In Figure 6, the optical axis a b of the laser beam entering the weld head 40 at L₁ is shown aligned with the optical axis A of the weld head 40. It should be understood, however, that it is possible for the optical axis a b of the laser beam B to be misaligned relative to the optical axis A of the weld head 40, as shown and described in Figure 5, and achieve satisfactory results in a bent tube T and sleeve S arrangement of Figure 6. Thus, the laser welding head 40 of the present invention may accommodate both a misaligned laser beam B and a deformed tube T and sleeve S configuration at the same time. In the application contemplated in the present invention the angle between segments q₁₂, q₂₃, q₃₄ is typically up to about 3°. However, this should not be construed to limit the invention because greater angles are possible and may be achieved by those skilled in the art. In another embodiment of the present invention shown in Figure 7, the lens L₁ acts as a beam reducing lens. The lens L₂ is located upstream of the inlet 44 of the welding head 40. The beam B, which is reduced by the first lens L₁ is directed at a deflecting mirror 50 and refocused at the first focal point f p1 near the inlet 44 of the weld head 40. The beam B thereafter propagates through the lenses L₂-L₃ etc as illustrated in Figure 7. In another embodiment of the present invention illustrated in Figure 8, a television camera 52 is provided which is adapted to view the welding operation and inspect the weld. In Figure 8 a beam splitter 50′ is substituted for the mirror 50 in Figure 7 and a TV camera 52 and shutter 54 are located in alignment with the optical axis A of the welding head 40 as shown. The beam splitter 50′ has a high reflectivity for laser radiation (incident from the right) and has a high transmission for visible wavelengths. Beam splitters having such characteristics are available in the art and are hence not described in detail herein. In the system described in Figure 8, the laser beam B is deflected into the welding head 40 as before and the TV camera 52 can view the welding region W. In operation, the lens L₄ forms an image of the welding region W at the focal point f p3 . The lens L₃ captures the image which is propagated along the optical axis A towards the beam splitter and the TV camera 52. A shutter 54 is located in front of the TV camera 52 and protects the camera 52 from the intense radiation produced by the laser beam B. The shutter 54 may be a mechanical device or may be a neutral density filter which greatly attenuates the energy reflected from the welding area W. After the weld is completed the shutter 54 may be removed or opened to view the finished weld. An auxiliary source of light (not shown) may be provided to illuminate the weld area. In each of the embodiments of the present invention and as illustrated in Figure 7 and 8, a rotatable connection or joint 56 is located near a proximal end of the weld head for interconnection with portions 58 of the apparatus shown external of the tube T. The joint 56, schematically shown, may have a configuration similar to the arrangement shown in the above referenced related patent applications. For a variety of reasons, the optical axis a b of the laser beam B may drift. For example the beam B may drift because the welding head 40 or the lenses L₁-L₄ have become heated thereby changing the position of the beam B relative to the optical axis A of the weld head 40. Various techniques have been used to detect the position of the beam or a drift or deviation in the position of the beam along the optical axis A. These techniques generally rely on the absorption of the laser energy by a photo detector or a thermal detector. The detector 60 shown in Figure 9 is in the form of an annular ring (sometimes referred to as a wander ring) divided into four quadrant segments or thermal detectors T₁-T₄ each of which produce an output in response to temperature. The detector 60 has a central aperture 62 of a diameters D₀. The laser beam B of diameter D (less than D₀) is directed at the apertures 62 of the detector 60 along the optical axis A. If the laser beam B is centered on the detector 60, it passes through the aperture 62 undisturbed. If the laser beam B drifts off, i.e., becomes misaligned with the optical axis A, part of the beam will strike the detector 60 causing one or more of the thermal detectors T₁-T₄ to become heated and warmer relative to the other segments. The outputs of the thermal detectors T₁-T₄, indicative of the temperature imbalance caused by differential heating, are coupled to means (not shown) which provides indication of the degree of beam drift. In Figure 9, welding head 40 is shown in sectional perspective. The internal diameter D of such a welding head 40 may be, for example, about 9,5 mm (3/8"). The beam B is a cylinder having an exemplary diameter D of about 6,5 mm (1/4"). Because it is usually desirable to have as large a laser beam diameter D as possible, there is usually not much clearance between the beam B and the inside of the welding head 40. Thus, it is difficult to install a detector 60 inside the welding head 40 without either interfering with the propagation of a laser beam or reducing the beam diameter. For example, for a centered beam B, the available clearance S₀ between the beam and the detector 60 is an annular ring of thickness D-D₀ (typically a few hundredths of a mm (a few thousands of an inch)). Thus, there is not much available clearance between the laser beam B and the detector 60. Accordingly, in an arrangement using a cylindrical laser beam of a given diameter, the clearances may be such that it is very difficult to install a detector 60 in the weld head 40 for detecting beam drift. In the present invention, the refocusing technique described herein allows for more available clearance to install the detector 60 in a welding head of the same size as in Figure 9 and in the same beam diameter. In Figure 10 the detector 60 is shown in the weld head 40 near a focus f p of the beam B. The beam converges towards a reduced diameter D w near the focus f p . The convergence of the beam B is known as the beam waist B w . For the same size detector 60 shown in Figure 9, the clearance S w between the beam B and the detector 60 at the beam waist B w is D w -D₀. In the embodiment shown in Figure 10, the beam diameter D w near the beam waist B w is about 2,5 mm (0.1"). Thus, for the same size beam B, welding head 40 and detector 60 as shown in Figure 9, the clearance S w can be increased significantly to a few tenths of a mm (a few hundredths of an inch) or more. Thus, the beam diameter need not be reduced in order to install the detector 60. Although the laser beam B is not reduced to a point source at each focus f p1 -f p4 (the laser beam at the beam waist B w has a finite diameter D w ), the available clearance S w between the laser beam B and the detector 60 is significantly greater than in the arrangement of Figure 9. Therefore, installation of a detector 60 in the narrow confines of the welding head 40 is facilitated as a result of the refocusing technique herein described. Figure 11 is an illustration of a simplified embodiment of the invention. In Figure 11 a laser welding head 140, formed of a rigid cylindrical housing 142 having laser beam inlet 144 and laser beam outlet 146 and an optical axis A, is shown. A plurality of focusing means L₁-L₄ directs a laser beam B having an axis a b along the welding head 140 as shown. In Figure 11, for ease of description, the focusing means L₁-L₄ are located in positions geometrically similar to corresponding focusing means L₁-L₄ in Figure 5. The only difference in the illustrations of Figures 5 and 11 is that in the latter, the housing 142 is rigid having no flexible joint means F₁₂-F₃₄ or individual segments S₁-S₄ as in Figure 5. Figure 11 illustrates the corrective beam aligning feature of multiple focusing means L₁-L₄ whereby a misaligned laser beam B is refocused at focal points f p1 -f p3 along the optical axis A of the housing 142. At each successive focal point f p1 -f p3 refocusing occurs on opposite sides of the axis A from a previous focal point. The geometrical relationship of the focal points f p1 -f p3 and the corresponding offset d₁-d₃ of each respective focal point f p1 -f p3 from the optical axis in Figure 11 is the same as shown and described with respect to Figure 5. The arrangements described herein are examples of a symmetrical simple case where individual lens elements L₁-L₄ relay the beam B along the housing 42, 142 via focal points f p1 -f p3 as shown. The shortest distance between successive focal points is four focal lengths of the particular lens between the two successive focal points. In the case of the flexible housing 42, the focal points are restricted to a zone near the respective centers of rotation C₁₂-C₃₄ of joints F₁₂-F₃₄. Thus, if the length of a particular segment S₁-S₄ is increased, the position of the corresponding lens L₁-L₄ may be changed to increase the distance between focal points to correspond to the distance between successive flexible joints F₁₂-F₃₄ and, where applicable, the length of the segments S₁-S₄ is changed accordingly. In the present invention, the laser beam B is optically relayed along a series of interconnected straight line paths by several discrete lenses, which paths are coterminal with focal points of the respective corresponding lenses. Each focal point is coincident with the focal point of the next adjacent lens along the path, and realignment of the beam is accomplished by this relaying or refocusing technique. In addition, in embodiments employing a flexible housing, the optics are designed to focus the beam B at locations near the centers of rotation of each interconnecting flexible joint to thereby relay the beam along a curved optical axis. Each focusing means may include one or more separate lenses as long as the focal distances satisfy the above conditions so that the beam is relayed from focal point to focal point through the bore of the housing. The symmetry illustrated in the embodiments herein described, wherein each lens L₁-L n is an integral number of focal lengths (e.g., 2) from the opposite ends of adjacent segments, is exemplary only. Asymmetrical arrangements may be provided if desired and such arrangements are within the capacity of those skilled in the art. In an optical system such as described above, a variety of lens materials may be used. For example, ZnSe lenses may be used. Such lenses are not well suited for the transmission of visible radiation as required for TV monitoring of the welding operation. Hence, lenses fabricated of KCl or NaCl which are ideally suited for this application may be used. Such lenses are effectively transparent both to a typical laser wavelength of 10.6 microns and to the visible wavelengths. Since these lens materials are subject to moisture damage, it is usually necessary to provide special coatings to protect them. Such coatings are available in the art. There has thus been provided a laser welding head which is capable of sustaining significant misalignment between the laser beam and the optical axis of the welding head without significantly affecting the quality of the resulting weld or risking damage to the welding head. The flexible welding head is also capable of effecting welds in curved or distorted tubes and means is provided for detecting misalignment or drift of the laser beam within the weld held.

Claims: A laser welding head (40, 140) for interface welding of a sleeve (S) disposed within and contacting a tube (T), by fusing the interface at a weld site using a laser beam (B), said welding head comprising a rotatable cylindrical housing (42, 142) adapted to be inserted axially into the tube and sleeve and having a laser beam inlet (44) and a laser beam outlet (46) for delivering the laser beam to the weld site, and focusing means mounted in the housing along an optical axis (A) thereof for receiving and refocusing the laser beam from said laser beam inlet to said laser beam outlet, characterized in that said cylindrical housing (42) is formed of a plurality of axially interconnected hollow cylindrical segments (S₁-S₄) and flexible joint means (F₁₂-F₃₄) axially interconnecting the segments about a center of rotation (C₁₂-C₃₄) between those segments, and that said focusing means comprise a plurality of focusing elements (L₁-L₄) for directing the laser beam along a central axis (a b ) aligned relatively closely with said optical axis (A) of said housing, with, at least one of said focusing elements being mounted in each cylindrical housing segment and each focusing element mounted in its corresponding housing segment (S₁-S₄) having an associated focal length (f L1 -f L4 ) and being so positioned in its corresponding segment that it refocuses the laser beam at a point near the center of rotation of said flexible joint means. The laser welding head of claim 1, charaterized in that said joint means (F₁₂-F₃₄) comprises a circular radially convoluted bellows for interconnecting each pair of adjacent segments (S₁-S₄), said bellows allowing one segment to pivot relative to its interconnected adjacent segment. The laser welding head of claim 1 or 2, characterized in that the first housing segment (S₁) disposed at the laser beam inlet (44) of the welding head remote from the weld site has its associated focusing element (L₁) mounted therein so as to refocuse the beam to a point (f D1 ) in the flexible joint means between the first segment (S₁) and the next adjacent segment (S₂) of said housing in the direction of propagation of the laser beam, said point being at least one focal length (f L1 ) from said focusing element (L₁). The laser welding head of any of claims 1 to 3, characterized in that said the first housing segment (S₁) at the laser beam inlet (44) has its associated focusing element (L₁) located one focal length (f L1 ) from the center of rotation (C₁₂) in the direction of propagation of the beam; the last segment (S₄) having the beam outlet (46) therein located remote from the first segment and has its associated focusing element (L₄) adjustably located therein at a position at about two focal lengths (2f L4 ) from the center of rotation (L₃₄) of the flexible joint means to the segment (S₃) opposite the direction of propagation of the beam; and at least one intermediate segment (S₂) has its focusing element (L2) located midway between the ends of the segment and about two focal lengths from each center of rotation of the flexible joint means to the adjacent segments. The welding head of any of claims 1 to 4, characterized in that said outlet (46) is in a side wall of the last housing segment (S₄) remote from the beam inlet (44), and a mirror (48) is located in the path of the beam between the focusing element (L₄) for the last segment (S₄) and the outlet (46) for directing the beam radially of the housing towards the outlet. The laser welding head of any of claims 1 to 6, characterized in that said focusing elements (L₁-L₄) comprise a discrete lens for each segment (S₁-S₄). The welding head of claim 6, characterized in that the lenses (L₁-L₄) in the housing segments (S₁-S₄) all have the same diameter and that each intermediate lens (L₂, L₃) in the intermediate segments (S₂, S₃) has a focal length greater than the next lens in the direction of propagation of the beam with the length of each corresponding segment being adjusted so that each focal point is near a respective center of rotation of the respective flexible joint. The laser welding head of any of claims 1 to 5, characterized in that said housing (42) comprises detector means (60) positioned near a focal point (f p1 -f p4 ) for detecting drift of the beam within the housing.

Inventors: HAWKINS, PHILLIP JOSEPH, KASNER, WILLIAM HENRY, TOTH, VINCENT ANDREW
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1988
Main classification: B23K26
IPC codes: B23K26, F22B37