Metal sheet laser welding clamp

A metal sheet laser welding clamp comprising a laser wavelength transparent body having a laser welding portion, the laser welding portion including an upper solid portion for transmitting laser wavelengths to a lower cavity portion for interfacing with a top metal sheet within a stack of metal sheets during a laser welding operation.

TECHNICAL FIELD

The disclosure relates to a metal sheet laser welding clamp and a method for using the same to join metal sheets.

BACKGROUND

Laser welding is a technique used to join multiple metal sheets through the use of a laser beam. The beam provides a concentrated heat source, allowing for narrow, deep welds and high heating and cooling rates. In many applications, for a proper weld to be formed, the metal sheets have to be aligned and in continuous contact along the entire length of the laser weld. To achieve the alignment, various welding clamps have been designed.

SUMMARY

A metal sheet laser welding clamp comprising a laser wavelength transparent body having a laser welding portion, the laser welding portion, including an upper solid portion for transmitting laser wavelengths to a lower cavity portion for interfacing with a metal sheet during a laser welding operation, is disclosed. The lower cavity portion may have an axial direction along its length. The lower cavity portion may have a profile along its length. The lower cavity portion profile may be constant along the length of the lower cavity portion profile. The lower cavity portion profile may be a substantially circular semi-profile. The lower cavity portion profile may be a trapezoidal profile. The lower cavity portion may have a surface including protrusions which may be about 0.1 μm to about 0.015 mm long. The metal sheet laser welding clamp may include an anti-reflective material contacting the surface of the upper solid portion of the laser welding portion. The laser welding portion may include a low-absorptive material.

In another embodiment, a metal sheet laser welding clamp is disclosed. The laser welding clamp may include a laser wavelength transparent body having a laser welding portion and a plurality of perimeter walls. The laser welding portion may include an upper solid portion for transmitting laser wavelengths to a lower cavity portion for interfacing with a metal sheet during a laser welding operation. The laser welding portion may further include one or more channels extending from the lower cavity portion to one or more of the plurality of perimeter walls. The lower cavity portion and the one or more channels each may have a profile along its length. The profiles of the lower cavity portion and the one or more channels may be constant. The cavity portion and the one or more channels each may have an axial direction along its length. The axial directions of the lower cavity portion and the one or more channels may be aligned. The one or more channels may include one or more interconnected channels.

A method of laser welding a plurality of metal sheets is disclosed. The method may include the steps of compressing the plurality of metal sheets with a clamp having a transparent body and a cavity and transmitting laser light through the body and the cavity onto the plurality of metal sheets to form a weld at a welding site of the plurality of metal sheets. The transparent body is a laser wavelength transparent body. The transmitting step may form liquid metal splatter from the plurality of metal sheets. The transmitting step may further include containing the liquid metal splatter within the cavity. The transmitting step may generate gas within the cavity and further include outgassing the gas from the cavity. The outgassing step may include outgassing the gas from the cavity through one or more channels.

DETAILED DESCRIPTION

Except where expressly indicated, all numerical quantities in this description indicating dimensions or material properties are to be understood as modified by the word “about” in describing the broadest scope of the present disclosure.

The first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.

Laser welding is a high energy process used to join multiple sheets of metal using a laser beam. The laser beam is targeted on a weld joint surface. At the surface of the weld joint, the concentration of light energy is converted to thermal energy. Surface melting then occurs and progresses through the weld joint by thermal conductance. To obtain adequate mechanical weld properties in certain applications, the metal sheets should be in continuous contact and precisely aligned along the entire length of the laser weld, thereby avoiding gaps between the metal sheets which may result in a weld with insufficient strength. A variety of laser clamps have been developed to maintain metal sheets in contact with each other. An exemplary laser welding clamp is a clamp in the U.S. Pat. No. 4,847,467, disclosure of which is hereby incorporated by reference.

Previously devised clamps suffer from a number of drawbacks. For example, these clamps do not address formation of weld splatter (spatter) at the weld site. The splatter refers to molten metal droplets that are ejected from the weld site. There has been a significant interest in splatter reduction during laser welding, especially during pulsed laser welding, as the splatter droplets may result in weld defects. The weld defects may include underfill, undercuts, porosity, craters, blowholes, or blowouts, all of which may have a detrimental effect on the mechanical properties of the weld. Typical proposed solutions to weld splatter have involved reduction of power, reduction of weld temperature, increasing nozzle stand-off, defocusing laser beam, changing the pulse shape, or adjusting gas pressure to reduce power density to the weld. However, these solutions have proven inadequate because they may limit the attainable welding speed or weld penetration, and/or result in only a partial prevention of splatter formation.

Previously devised laser welding clamps also do not adequately address outgassing during welding. Gas may be formed when the laser vaporizes metal material and/or various impurities at the weld site. Certain substances such as aluminum alloys outgas and may cause undesirable formation of bubbles, pores, and cracks at the weld site during the welding process. Such formation is undesirable as potentially compromising the quality of the weld.

It is desirable to develop a laser welding clamp which would address formation of splatter at the weld site and/or provide for outgassing of the weld site. It would be also desirable to provide a clamp which would allow for increased flexibility of laser beam focusing, which would not limit attainable welding speed or require reduction of power density.

According to one or more embodiments, a transparent laser welding clamp is provided which includes a laser wavelength transparent body having a laser welding portion. The laser welding portion includes an upper solid portion and a lower cavity portion. The upper solid portion transmits laser wavelengths to the lower cavity portion. The lower cavity portion interfaces with a metal sheet during laser welding process, contains splatter within the cavity portion, and/or provides outgassing. Embodiments disclosed herein also provide a process for laser welding with the clamp of the present disclosure.

FIG. 1depicts a non-limiting example of a laser weld clamp10having a laser wavelength transparent body12having a laser welding portion14. The laser wavelength transparent body12is made from such a material that is capable of transmitting operational laser wavelength(s) through the laser welding portion14to the weld site16. The laser wavelength transparent body12of the clamp10may be transparent to operational wavelengths of different lasers.

The operational wavelength of each laser depends on a laser gain medium. The gain medium is a material excited by a pump source that provides energy to the laser system. The excited gain medium produces spontaneous and stimulated emission of photons, leading to optical gain or amplification. Chemical composition of the gain medium determines the operational wavelength of the laser. The gain media may include liquid such as dye lasers in which the chemical make-up of the dye determines the operational wavelength. The liquids may be organic chemical solvent such as methanol, ethanol, ethylene glycol containing a dye such as coumarin, rhodamine, fluorescein. The gain media may include gas such as CO2, Ar, Kr, and/or gas mixtures such as He—Ne. The gain medium may be metal vapor such as Cu, HeCd, HeHg, HeSe, HeAg, or Au. The gain media may include solids such as crystals and glass, usually doped with an impurity such as Cr, Nd, Er, or Ti ions. The solid crystals may include YAG (yttrium aluminum garnet), YLF (yttrium lithium fluoride), LiSAF (lithium strontium aluminum fluoride), or sapphire (aluminum oxide). Non-limiting examples of solid-state gain media doped with an impurity include Nd:YAG, Cr:sapphire, Cr:LiSAF, Er:YLF, Nd:glass, or Er:glass. The gain medium may include semiconductors having a uniform dopant distribution or a material with differing dopant levels in which the movement of electrons causes laser action. Non-limiting examples of semiconductor gain media may include InGaAs, GaN, InGaN, InGaAsP.

As the operational wavelength depends on the composition of the gain material, the operational wavelengths of different types of lasers significantly differ. The clamp10is therefore made from a material which is transparent to the operational wavelengths of a laser used in a particular application. The laser wavelength transparent body12may be transparent to one or more laser wavelengths from about 238 nm to about 10.6 μm. The laser wavelength transparent body12may be capable of transmitting laser wavelengths in the far ultraviolet, near ultraviolet, visible, near red, and/or far red spectrum. Table 1 below lists operational wavelength(s) of exemplary lasers which the laser wavelength transparent body10may be capable of transmitting.

Any material which is transparent to the operational wavelength(s) of a laser used may be used to produce the laser wavelength transparent body12. Exemplary non-limiting materials may include a variety of optical materials such as borosilicate glass or borosilicate crown glass having a low index of refraction, quartz (silicon dioxide), fused silica (synthetic amorphous silicon dioxide), infrared grade calcium fluoride, magnesium fluoride, zinc selenide, various types of ceramic, or the like. The material may be amorphous or crystalline. It is desirable that the material is substantially free of a variety of impurities, cracks, bubbles, and inclusions which could interfere with transmittance of the laser wavelengths through the clamp10.

The material may have high refractive index homogeneity of about 1.46 to about 1.8 or higher. The refractive index refers to the rate of how much the light slows down when it passes through an optical material. The material may be relatively hard as hardness may have an effect during manufacturing of the material into a certain shape which not only affects the production cost, but also durability of the optical material. The hardness of the material may be about 6-7 or more on Mohs scale of hardness, about 800 to 820 kgf/mm2measured according to Knoop's hardness test, or about 950 to 1000 kgf/mm2or more measured according to Vickers's hardness test. The material should have sufficient hardness so that the clamp10is repeatedly capable of applying sufficient pressure on metal sheets18to be properly welded. The relative hardness also affects scratch resistance of the material. The material may show good scratch resistance, good resistance to mechanical and thermal shock, and/or have overall high damage threshold. The material may have good resistance to cumulative exposure to radiation, especially to ultraviolet radiation. The material may be thermally stable and have a relatively low thermal expansion coefficient of about 0.54 to about 3.2 10−6K−1or lower measured at 20° C. The material may have thermal conductivity of about 1 to about 1.46 W/mK measured at 20° C. In one or more embodiments, the material may have good chemical resistance to a variety of chemical substances such as fluorine. The material may have a relatively low density as not to render the resulting clamp10too heavy. Exemplary density of the material may be from about 1 g/cm3or lower to about 3 g/cm3or higher.

The material should have excellent transmittance in the operational wavelength(s) of the laser used such as about at least 70% or more, 80% or more, 90% or more, or 95% or more. The material may have low coefficient of absorption which renders the material suitable for uses with high power lasers. Exemplary coefficient of absorption of the material may be about 0.01 cm−1to about 0.05 cm−1measured at 190 nm or about 0.03 cm−1to about 0.07 cm−1measured at 2800 nm. The material may have a narrower or wider wavelength range such as from far ultraviolet to far infrared spectrum or any desirable range in between.

The laser wavelength transparent body12and the laser welding portion14may have a variety of shapes, sizes, and configurations, depending on a particular application. A cross-section of the laser wavelength transparent body12and the laser welding portion14may be, but is not limited to, a square, a rectangle, a triangle, or the like. The cross-section of the laser wavelength transparent body12and the laser welding portion14may be angular, regular, irregular, or the like. The laser wavelength transparent body12and the laser welding portion14may have any shape as long as the clamp10is capable of transmitting laser wavelengths.

In at least one embodiment, the laser welding portion14includes an upper portion20and a lower portion22. The upper portion20may be solid. The upper portion20transmits laser wavelength(s) to the lower portion22. When the laser beam24contacts the clamp10, the laser beam24interfaces with the top surface26of the upper portion20.

The lower portion22includes a cavity28to form a lower cavity portion30. The cavity28is located above the weld site16. In at least one embodiment, it is possible to provide two or more weld sites16located under one cavity28. Alternatively, the lower cavity portion30may include more than one cavity28to be located over more than one weld site16.

As can be seen inFIGS. 1-5, the lower cavity portion30may have a profile32along its length or a portion of its length. The profile32defines an outline of a shape to be formed in the clamp material along the length of the lower cavity portion30. The profile32may have any size, shape, or configuration. The profile32may be, but is not limited to be, semi-circular, semi-oval, semi-elliptical, square, trapezoidal, triangular, rectangular, shaped like a parallelogram, rhombus, rhomboid, trapezium, pentagon, hexagon, heptagon, octagon, nonagon, decagon, or have more than ten sides. For example, the profile32may have curved sides and be shaped as a curved rectangle, curved square, curved trapezoid, or the like. The profile32may have rounded or sharp edges. The profile of the cavity28may contribute to increased flexibility in accuracy with which the laser beam24has to be pointed at the weld site16. For example, the semi-circular profile, as is depicted inFIG. 1, curved rectangular profile, as is depicted inFIG. 2A, or trapezoidal profile, as is depicted inFIG. 2B, allow for the laser beam24to be aimed at more than one point at the weld site16while achieving a satisfactory quality of the resulting weld32.

In one or more embodiments, the lower cavity portion30may have an axial direction along its length. The lower cavity portion30comprising the cavity28may extend to one or more perimeter walls34. The cavity28may extend through the entire length of the clamp10. Alternatively, the cavity28may extend through a portion of the clamp10. The cavity28which extends through a portion of the clamp10does not reach one or more perimeter walls. Such cavity28may have any profile, as recited above. For example, the cavity28may be located only in a central portion of the lower cavity portion30and have a semi-circular profile to form a dome capable of containing splatter within the cavity, as is illustrates inFIGS. 6A and 6B.

The laser beam24may be a spot laser beam, as is depicted inFIGS. 6A-6Cproducing a spot weld. Alternatively, the laser beam24may travel along a length of the metal sheet stack18, as is depicted inFIG. 1. The cavity28has dimensions to accommodate a width w1of the laser beam24at the weld site16. AsFIG. 2Aillustrates, the cavity28may be as wide as the width w1of the laser beam24interfacing with the weld site16. Alternatively, as can be seen inFIG. 2B, the cavity may be wider than the width w1of the laser beam24interfacing with the weld site16. The w1of the laser beam24interfacing with the weld site16may be calculated as the focus spot size of the laser beam24. The w1of the laser beam24may be from about several thousands of an inch to several hundreds of an inch in diameter, from about 0.1 mm to about 0.8 mm or about 0.2 mm to about 0.4 mm.

In at least one embodiment, the cavity28has a width wcwhich exceeds the width wwof the weld at the weld site16. The width wcof the cavity28may exceed the width wwof the weld32at the weld site16by about 1% to about 500% or more, by about 50% to about 250%, by about 100% to about 175%. The width wcof the cavity may be about 1.5 times, 2 times, 5 times, or 10 times or more larger than the width wwof the weld32at the weld site16. The wwof the weld32may be about 0.2-2 mm. The wcof the cavity28may be about 0.3 mm to 20 mm. Alternatively, the width of the weld wwmay exceed the width of the cavity wcat the weld site16by about less than 1% to by about 50% or more.

The cavity has such a width wcand height hcthat it can prevent droplets of splatter33from forming underfill, undercuts, porosity, cracks, craters, blowholes, or blowouts at the weld site16and/or to provide sufficient space so that the gas from the weld site16may be effectively released away from the weld32. The cavity28may fulfill the dual function of splatter prevention and outgassing or just one of these functions. The shape and dimensions of the cavity28will determine this. For example, the dome-shaped cavity28formed in the central portion of the lower cavity portion30, as depicted inFIGS. 6A and 6B, may prevent splatter formation while a cavity28extending the entire length of the clamp10, as depicted inFIGS. 1-5, may prevent splatter as well as provide at least one outgassing pathway.

To further facilitate outgassing, the lower cavity portion30of the clamp10may include one or more channels36extending from the lower cavity portion30to one or more of the plurality of perimeter walls34. The one or more channels36provide escape pathways to gases formed at the weld site16. The one or more channels36may provide the only escape pathways to gases formed at the weld site16, such as inFIG. 6C, or additional outgassing pathways, such as inFIGS. 1-5, as the gasses may escape through the cavity28as well.

The one or more channels36may have a profile along their length. The profile of the one more channels36may have one of the shapes of a cavity profile as described above. The profile of one or more channels36may be substantially the same or different than the profile of the cavity28. The profiles of the lower cavity portion30and the one or more channels36may be, but do not have to be, constant. In at least one embodiment, at least some of the channels36have a different profile than the remaining channels36. Both the lower cavity portion30and the one or more channels36each may have an axial direction along its length. The axial direction of the lower cavity portion30and the one or more channels36may be, but do not have to be, aligned. In one or more embodiments, the one or more channels36may include one or more interconnected channels38. The interconnected channels38may be connected with a variety of additional channels having the same or different dimensions as the interconnected channels38. The interconnected channels38may form a lattice. In at least one embodiment, as depicted inFIG. 4, a portion of the lower cavity portion30may include interconnected channels38while another portion may include one or more channels36. Alternatively, the entire lower cavity portion30may include interconnected channels38. Alternatively still, the entire lower cavity portion30may include one more channels36.

In order to provide excellent transmittance of the operational wavelength(s), it may be desirable that at least some of the surfaces of the clamp have a smooth surface that is even, free of ridges, inequalities, projections, breaks, or bumps. The surface may be rough and include protrusions which may be about 0.1 μm to about 0.015 mm long. The surface roughness within this range is especially useful at the interface of the laser with the top surface of the upper portion26of the clamp10, the inner surface of the cavity27, or both. The roughness within this range provides a surface substantially free of protrusions and impurities which may divert, defocus, or absorb the laser beam. Surface quality, and thus roughness, of the clamp may be assessed using a scratch-dig performance specification in accordance with MIL-PRF-13830B; which is hereby incorporated as a reference in its entirety. The scratch designation is determined by comparing scratches on the surface being assessed to a set of standard scratches under controlled lighting conditions. The dig designation relates to a small pit in the surface and is calculated as the diameter of the pit in microns divided by 10. The clamp surfaces should thus have scratch-dig specifications of about 80-50 which is considered standard quality, about 60-40 which is considered precision quality, or about 20-10 which is considered high precision quality.

Because all metals reflect light to some degree, especially metals such as gold, silver, copper, and aluminum, the metal sheets18may be difficult to weld, requiring intense energy usually available from high energy peaking pulses. A choice of a laser with a shorter operational wavelength such as the 1.06 μm operational wavelength of the Nd:YAG laser which is more readily absorbed than longer operational wavelengths such as 10.6 μm of the CO2laser renders certain lasers more suitable for welding of highly reflective metal sheets18. In addition, light absorbing material such as graphite may be applied to the joint surface of the weld32to lower reflectivity of the metal sheets18. The light absorbing material may be applied as a coating. The light absorbing coating is selected according to an operational wavelength of a laser used. Exemplary absorbing materials may have peak wavelengths anywhere within the ultraviolet, visible, and infrared spectra.

Alternatively, in one or more embodiments, it may be desirable to provide a low-absorptive, low-refractive, and/or anti-reflective layer or layers40on at least one surface of the clamp10or make at least some portions of the clamp10such as the upper portion20, the lower portion22, the laser welding portion14, or a combination thereof from a material having low-absorptive, low-refractive, and/or anti-reflective properties to achieve excellent transmission such as above about 90%.FIG. 2Billustrates an anti-reflective material layer40contacting the surface of the upper solid portion20of the laser welding portion14.FIG. 3, on the other hand, illustrates the laser wavelength transparent body12including a low-absorptive material40. The low-absorptive material may be about 0-50% absorptive. The type of material the laser welding portion14is made from will determine the need for low-absorptive, low-refractive, and/or anti-reflective layers. Materials with low index of refraction such as magnesium fluoride may not require an anti-reflective layer. On the other hand, materials such as zinc selenide which have a high index of refraction may require an anti-reflective layer to achieve high transmission. Exemplary anti-reflective coating layers may include materials having reflectance Raveof less than 0.25 to 1.5% at the operational wavelengths of the laser welding portion14.

As was stated above, the clamp10is sufficiently hard to provide sufficient clamping pressure to the metal sheets18. Sufficient pressure is developed if the metal sheets18are aligned and pressed to remain aligned to produce a substantially gap-free joint at the weld site16. The clamping force is sufficient if the metal sheets18remain aligned during the welding process. Lack of intimate contact of the metal sheets18during the welding operation may reduce total area of conductive weld which is detrimental, especially in some applications. Such applications include, but are not limited to, applications that demand about 500 Amp in about 10 sec charging without producing a transient thermal event locally such as DC fast-charge in high voltage batteries. The stack of metal sheets18is substantially gap free if no gaps larger than about 50 μm appear between stacked laser welded sheets. Gaps larger than about 50 μm may result in reduced mechanical properties of the weld32and compromised weld quality along the length of the weld32for the intended purpose.

The metal sheets18may be located between the clamp10and a base plate42. The base plate42may be made from a variety of materials such as materials disclosed for the metal sheets18themselves referenced below. The base plate42may be made from a material requiring higher temperature to melt than the metal sheets18. The base plate42has to be sufficiently hard and temperature resistant to withstand the laser welding process without disintegrating. It is desirable that the base plate42is non-deforming. The base plate42may be attached to a fixture to prevent its displacement and thus prevent misalignment of the metal sheets18during the welding process.

The metal sheets18to be joined by the laser welding may have various dimensions. Exemplary metal sheets18may have thickness of less than about 0.2 mm to more than about 0.8 mm. The metal sheets18may be foil. The clamp10may be used to join one or more metal sheets18of the same thickness, as can be seen inFIGS. 1 and 2A, or different thickness, as is illustrated inFIGS. 2B and 3. The clamp10may be also utilized for microwelding of microelectronics. The metal sheets18may thus have dimensions in microscale.

The material of the metal sheets18may vary, depending on a specific application. Exemplary material of one or more of the metal sheets18includes, but is not limited to, aluminum, silver, gold, copper, tin, nickel, titanium, steel such as stainless steel, the like, or their alloys. While the sheets18are references as metal sheets18, it is contemplated that the clamp10could be utilized during laser welding of sheets involving other materials such as thermoplastics.

The clamp10and the related method of the present disclosure may be used to join the sheets18in a variety of applications such as medical devices, biotechnology, electronics, automotive, aerospace, alternative energy/photovoltaics. An exemplary use of the clamp10may be in a high-voltage battery tab to tab and tab to busbar welding. A non-limiting exemplary use of the clamp10may be laser welding of electrical terminals in a high-voltage battery for battery electric vehicles. It is contemplated that the metal sheet18stacks including a number of sheets18with a thickness of less than 0.2 mm would be laser welded while utilizing the clamp10of the present disclosure. The resulting laser welded cell tab may replace battery ultrasonic welded stacks. While the method of the present disclosure may require relatively high upfront costs, when compared to ultrasonic welding, the present method may result in lower unit cost due to faster and highly repeatable positioning of the laser beam and lower maintenance costs than ultrasonic welding. The clamp10and the related method of its use may be also utilized in fuel cell applications such as in welding of metal separators for fuel cells. Such application may require welding into narrow valleys, on areas about 0.15 mm wide.

The present disclosure further provides a method of laser welding of a number of metal sheets18by compressing the metal sheets18with a clamp10having a laser wavelength transparent body14and a cavity28. The compressing step may include placing a number of metal sheets18between the clamp10and one or more base plates42. The process further includes a step of transmitting laser light through the laser wavelength transparent body14and the cavity28onto the metal sheets18to form a weld32at a welding site16of the metal sheets18. The transmitting step forms liquid metal splatter33from the number of metal sheets18. The process includes a step of containing the liquid metal splatter33within the cavity28. The transmitting step further generates gas within the cavity28. The method further provides a step of outgassing the gas from the cavity28. The outgassing step may include outgassing the gas from the cavity28through one or more channels36or one or more interconnected channels38.

The method may further include forming battery tabs and/or battery busbars while utilizing the clamp10.FIGS. 5A and 5Billustrate forming battery tabs by overlapping metal sheets18.FIG. 5Ashows overlapping of the opposite ends of two metal sheets18to be welded.FIG. 5Billustrates same side overlapping of two metal sheets18to be welded.