Patent Application
20070170158
The present invention relates to the field of welding and, more particularly, to a method and system for welding together two or more metal foils.
A support plate 110 supports the metal foils 102, 104 and a weld plate 112 positioned on top of the thin metal foil 102 holds the metal foils 102, 104 in place. In addition, the weld plate 112 shields portions of the thin metal foil 102 from the laser beam 108 and acts as a heat sink to assist in controlling the portions of the thin metal foil 102 that melt during the welding process. The weld plate 112 typically includes a recess 113 for receiving a thermocouple (not shown) that acquires thermal readings during the welding operation.
The set up illustrated in
Additional defects may occur from a lack of stability and alignment of the laser beam as it moves along a desirable weld path. There may also be the lack of an adequate melt-pool, thereby leading to beading of the thin foil without drawing a desirable amount of material from the thick foil into the melt-pool to produce a desirable weld. Further, the use of pulsed lasers may introduce a lack of beam uniformity, and pulse-to-pulse stability, and may thereby cause pin-hole defects and thin foil material injecting inward resulting in an undesirably rough surface and an inadequate weld.
Imperfect and non-uniform welds are undesirable for certain applications, e.g., medical applications. For example, thin metal foil welds in devices for use within the human body, e.g., pace makers, must be free from rough edges to avoid rejection by the body. Therefore, welds that are uniform and free from imperfections are desirable for use in such applications.
One embodiment of the present invention is method for welding together a first foil and a second foil by positioning the first foil having a first thickness adjacent the second foil having a second thickness, the first thickness being greater than or equal to the second thickness, and applying a laser beam to the first foil to weld at least a portion of the first foil and the second foil together. In a further embodiment, a high thermal conductivity top-plate is positioned adjacent the first foil.
In a further embodiment, the step of applying the laser beam includes activating a continuous wave high power direct diode laser with a predetermined wavelength and power along a predetermined weld line at a predetermined slew rate. Additionally, a measure of temperature proximate the predetermined weld line may be obtained to desirably vary the predetermined power and/or the predetermined slew rate of the laser. In another embodiment, the method is performed in the presence of an assist/process gas or while an assist/process gas is being blown proximate the weld line.
An additional embodiment of the present invention is a system for laser welding foils, the system including a continuous wave laser for applying a laser beam along a weld line on the foils, a thermally conductive plate with a continuous edge placed proximate the weld line, and a linear movement stage that moves either the laser or the foils so that the laser beam passes over the weld line at a predetermined slew rate.
A further embodiment includes a thermocouple placed within a recess of the thermally conductive plate for measuring the temperature proximate the weld line, and whereby a controller varies the slew rate and/or the power of the laser beam responsive to the temperature measurements. Additionally, the thermally conductive plate may have an angled edge to function as a laser beam block and/or reflector. In another embodiment, a process gas injection system is also included to supply a desirable process gas or blow the process gas along the weld line.
The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:
a-b are perspective drawings of exemplary welding setups, in accordance with the present invention;
a-c are perspective drawings of exemplary welding setups including exemplary thermally conductive plates, in accordance with embodiments of the present invention;
In
In their article MICRO-WELDING OF THIN FOIL WITH DIRECT DIODE LASER (Proceedings of SPIE Vol. 5063 Fourth International Symposium on Laser Precision Microfabrication, pgs. 287-291, 2003), Nobuyuki Abe, Yoshinori Funada, and Masahiro Ishide disclose an empirical finding for laser-welded butt joints that when the melt pool surface diameter exceeds approximately three times the foil thickness, the joint often fails due to excessive beading of the foil material. In the exemplary embodiment of
In this exemplary embodiment, laser 206 may desirably be activated to apply laser beam 208 as a continuous wave having a constant fluence. The laser power and the spot size of the laser beam are desirably controlled to vary the fluence, though the spot size may also be varied to affect the melt pool diameter. The desired constant fluence is based on the foil thicknesses and the thermal properties of the foils. It is also noted that the fluence may be varied by varying the slew rate with which the beam spot is scanned along the weld line.
Alternately, laser beam 208 may be applied at a lower fluence at edge locations along weld line 212 that are located near the side edges of top foil 202, and ramped up to a predetermined steady-state fluence along locations of weld line 212 that are located at a distance from the side edges. The depth of these edge locations may vary depending on the thickness and/or thermal properties of the foil. For a typical thin metal foils (100-300 μm thick) these edge locations may extend about 1 mm from side edges of top foil 202.
The laser may desirably be operated at a wavelength that is substantially absorbed by the material used for top foil 202. For example, 808 nm light couples energy efficiently into steel.
In an exemplary embodiment of the present invention, laser 206 may be a High-Power Direct Diode Laser (HPDDL) or other continuous wave laser used for laser welding.
In one embodiment, laser 206 may be scanned along weld line 212 at a welding slew rate, where laser 206 is accelerated to the welding slew rate along a region of weld line 212 that is not incident on top foil 202. In an alternate embodiment, support plate 210 may be swept while laser 206 is held stationary so that the beam spot of the laser beam 208 passes over top foil 202 along weld line 212. This alternative scanning method may be desirable to allow mass manufacture in which foils are scanned one after another through the laser beam while traveling on a conveyor belt.
It is noted that the slew rate also affects to total fluence incident on a portion of the foil surface and, thus, the size of the melt pool formed. Therefore, the desired slew rate may be related to the desired laser power level and the desired spot size. Often the highest practical slew rate is desired to increase productivity.
In an experiment, two steel foils were laser welded using this exemplary equipment. Top foil 202 and bottom foil 204 were 150 microns and 100 microns thick, respectively, and had exemplary widths of 34 mm and exemplary lengths of 52 mm. Laser 206 was operated between 150 and 200 watts of output power and focused to a spot size of approximately 700 microns diameter. The slew rate was 35 mm/s.
It is noted that foils to be used in the exemplary embodiments of the present invention may be formed of one or more: metals such as copper, gold, silver, steel, aluminum, molybdenum, tungsten, iron, tantalum, and nickel; polymer materials; and plastic materials.
As is shown in
In a further embodiment of the invention, thermally conductive plate 304 may be placed proximate the weld line as shown in
In
b shows thermally conductive plate 306 overhanging only a first side edge of top foil 302, proximate weld line 312. Such a configuration may be used to apply a laser beam (not shown) to weld only a portion of top foil 302 to one or more bottom foils (not shown).
Additionally, the laser beam may be generated by a continuous wave HPDDL at a predetermined wavelength and power along a weld line at a welding slew rate. Further, the thermally conductive plate may be placed proximate the weld line. The desired wavelength and power may be chosen based on the thicknesses and type(s) of foil material being welded, including the electromagnetic coupling wavelength and thermal characteristics of the foil material. In an exemplary embodiment, the foil material is 50-300 micron thick steel and the wavelength and power are 808 nm and 150-200 watts, respectively.
In a further embodiment, the laser beam may be applied at the predetermined slew rate by moving the first foil and the second foil on a linear movement stage along the weld line at the welding slew rate. Alternately, the beam spot of the laser beam may be moved on a linear movement stage to move the laser beam along the predetermined weld line. This may be accomplished optically, such as with rotating mirrors, or it may be accomplished mechanically by coupling the laser beam into an optical fiber and translating the output head of the fiber optical assembly. A combination of these two methods may be used to scan beam spot along the weld line.
In another embodiment, a thermocouple or other temperature sensor Imay be used to measure of the temperature of the foil(s) proximate to the weld line. A controller may then implement algorithms for varying the power, beam spot size, and/or the slew rate according to the measured temperature. For example, an increase in temperature beyond a predetermined threshold may prompt the controller to lower the laser power and/or to increase the slew rate or spot size. Alternately, a drop in temperature may cause the controller to increase the laser power and/or decrease the slew rate or spot size. Those skilled in the art will recognize that many alternate means for measuring a temperature may be used.
Additionally, the welding processes described above may take place in the presence of a process gas, or alternately while the process gas is blowing across the weld line. The process gas may desirably reduce oxidation, or other chemical activity, of the foil(s) during welding. Additionally, the process gas may help rapidly cool the foil(s) following the welding. The process gas may include, for example at least one of nitrogen, carbon dioxide, or a noble gas. It is also noted that the foils may alternatively be placed in a chamber of the process gas rather than having the process gas blown on them during welding.
The exemplary welding setup is now described in detail. In an exemplary embodiment, bottom foil 402 is a metal foil, such as steel foil, having of thickness of between approximately 50 microns and 1 mm, for example. In the exemplary embodiment, the top foil 414 may also be a metal foil such as steel having a thickness of between approximately 50 microns and 1.5 mm, for example. In another exemplary embodiment, the suitable ratio of bottom foil thickness to top foil thickness may be between approximately 0.25 and 1.5, for example. In an exemplary embodiment, the bottom and top foils 402, 404 are the same type of metal (e.g., steel). In an alternative exemplary embodiment, the foils may be formed of different materials.
The laser 406 produces laser beam 408 for welding together the top foil 404 and the bottom foil 402. In an exemplary embodiment, the laser may be a continuous wave (CW) laser that produces a continuous laser beam 408 with a substantially consistent energy output rather than a pulsed laser beam that is typically used in the art for welding metal foils. Using a CW laser may allow the power supplied to the top foil 404 to be carefully controlled such that energy from the laser beam 408 does not penetrate through both top foil 404 and bottom foil 402. In an exemplary embodiment, the power output of laser 406 is controllable to compensate for differences in the foil thicknesses, the foil types, and the rate at which the laser beam 408 moves relative to the surface of the top foil 404. In addition, the wavelength of the laser 406 is selected to ensure efficient coupling of the foils 402 and 404 during welding.
In one embodiment of the invention, laser 406 may be a Nuvonyx HPDDL having a 1000 watt fiber-coupled unit operating at 808 nm specifically terminating in a coaxial-gas assisted cutting head available from Nuvonyx, Incorporated of Bridgeton, Mo., USA. In an exemplary embodiment, this diode laser is operated between about 150 and 200 watts. In alternative exemplary embodiments, it is contemplated that a laser that produces a pulsed laser beam may be employed.
Further, a support structure or robot arm (not shown) may support the laser 406 and move the laser 406 and the support plate 410 relative to one another such that the laser beam 408 travels along a weld line 413 along the surface of top foil 404 during welding. Alternatively, this support structure or robot arm may be used to manipulate optics, possibly including fiber optics through which laser beam 408 has been coupled. Such a support structure may enable laser beam 408 to be positioned with six degrees of freedom (i.e., along orthogonal X, Y, Z axes with rotation around each axis) relative to top foil 404. In an exemplary embodiment, the support structure moves the support plate 410 and the laser 406 remains stable during welding. In an alternative exemplary embodiment, during welding, the support structure may move the laser 406 and the support plate 410 remain stable. In a further alternative embodiment, the support structure concurrently moves both the laser 406 and the support plate 410. A suitable support structure for use in the present invention will be understood by those of skill in the related arts.
Support plate 410 supports the bottom foil 402 and the top foil 404 for welding. Support plate 410 is sized to support at least the portions of the foils 402, 404 that are to be welded by the laser and, in the illustrated embodiment, supports the entire surface of the bottom foil 402 opposite the top foil 404. The support plate may desirably be formed of any solid material with a high enough melting point to remain unaffected by the welding process.
Additionally,
In the illustrated embodiment of
In an alternative exemplary embodiment, the setup depicted in
At block 604, the second foil and the first foil are positioned on top of one another. In one embodiment, the front edge of the second foil is positioned such that it is substantially flush with a front edge of the first foil to form an edge weld when welded together. In an alternative embodiment, the front edges of the metal foils are offset with respect to one another to form a lap or tee weld when welded together.
At optional block 606 (shown in phantom), a decision may be made (during manufacture or prior to manufacture) to add one or more additional foils to the first two foils. A positive decision leads to block 607, which positions the next foil on top of the previous foil and transfers control back to block 606. These steps may be performed for any number of additional foils.
At optional block 608 (shown in phantom), a weld plate is positioned on top of the previously stacked foil to maintain the position of the previously stacked foil relative to all other foils beneath it and prevent buckling of the foils. As described above, the weld plate may also shield portions of the highest stacked foil from the laser beam and further may act as a heat sink to limit the amount of material drawn into a melt pool (i.e., the melt pool does not form under the weld plate).
In one embodiment, the front (desirably continuous) edge of the weld plate is positioned a minimum distance from the front edge of the highest stacked metal foil to allow formation of an adequate weld (e.g., a distance from the front edge of the highest stacked foil about two to four times total thickness of the stacked foils) up to essentially any distance that allows the top plate to remain in contact with the highest stacked foil while preventing buckling (e.g., about 12 mm or more).
In an alternative exemplary embodiment that does not include optional block 608, the laser beam produced by the laser may be controlled such that the foils do not buckle and/or bead. Thus, a heat sink and shield are not used. In this alternative embodiment, the weld plate may be anything capable of maintaining the positioned relationship of the metal foils and to prevent buckling, or may be eliminated (e.g., the weight of the highest stacked foil may be sufficient to maintain its position on the foil(s) beneath it and/or may be thick enough to prevent buckling).
At optional block 610 (shown in phantom), a process gas may be supplied to the welding environment surrounding the foils, and/or may be blown over the weld line.
At block 612, the laser beam irradiates the highest stacked foil to weld together at least a portion of the highest stacked foil and all foils beneath it. When the laser beam irradiates the highest stacked foil, the absorbed energy of the laser beam melts the highest stacked foil and, in turn, also melts the foils beneath. The melted portions of foils form a melt pool that, when cooled, binds them together.
During welding, the laser beam moves along the weld line on the surface of the highest stacked foil at a welding slew rate (e.g., 35 mm/sec). In an exemplary embodiment, the velocity of the beam spot of the laser beam relative to the highest stacked foil is ramped up (i.e., to the welding slew rate) and ramped down outside of the welding region (i.e., not on the surface of the highest stacked foil). Typically, the angle of incidence of the laser beam may be substantially normal to the surface of the highest stacked foil, however, it is contemplated that satisfactory results may be obtained by varying the angle at which the laser beam is applied to the highest stacked foil.
The process ends with block 644.
An increased process window may be achieved using an embodiment of the present invention due to the effectiveness of energy transfer from larger to smaller thermal masses. In addition, optimizing the melt characteristics of the thick metal foil may reduce defect rates due to ablation of the thin metal foil or beading of the melt pool due to surface tension.
Exemplary process parameters for welding thin and thick metal foils together using a 1000 Watt Fiber Coupled Diode laser are set forth in Table 1.
Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. For example, although an embodiment may depicts the foils positioned on top of a support with a laser beam directed downward onto the first foil, the foils may have essentially any orientation as long as they are positioned adjacent one another and the laser beam is directed at the first foil. Also, it is noted that in the exemplary figures of the present invention the weld lines are shown as straight merely for ease of illustration and are not intended to be limiting. Various other modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US04/26513 | 8/13/2004 | WO | 12/13/2006 |
| Number | Date | Country | |
|---|---|---|---|
| 60501337 | Sep 2003 | US |
