Patent Publication Number: US-2018045232-A1

Title: A Weld

Description:
FIELD OF INVENTION 
     This invention relates to a weld. The weld may join one or more reflective materials. The weld may have a low ohmic resistance, a high shear strength, and a high peel strength. This invention also relates to an article comprising the weld and a method for laser welding. 
     BACKGROUND TO THE INVENTION 
     The joining of bright metals such as gold, copper, aluminium, platinum and silver by laser welding in the near infrared spectrum (800 nm to 2500 nm) presents a challenge. This is because the surface of the bright metals is highly reflective with poor absorbance. To overcome the surface reflectivity and initiate coupling of the laser&#39;s energy into the metal surface, it is necessary to use laser beams with high power densities. 
     The function of the laser beam on a bright material approximates a discreet function with a very narrow operating window from beam hold-off (reflection) and absorption. At first the surface reflects substantially all of the laser light. However, once the surface reflectivity is overcome by sufficient laser intensity, a melt of the surface is initiated. The reflectivity then almost immediately transitions from its original highly reflective condition of more than 80% reflectivity to a lower value, which for some metals, can be less than 50% reflectivity. This causes the melt pool on the surface to grow extremely rapidly. It is consequently very difficult to control. 
     The challenge is increased when welding thin and low mass work pieces. High power densities are often detrimental, leading to over penetration of the laser beam, which results in unreliable joints. Conversely, if lasers are operated at lower power densities that are just above the absorbance limits, then the pulse duration has to be increased. Thermal heat sinking of the absorbed energy into the regions surrounding the weld can then cause overheating of the work piece, resulting in weak or absent welds. 
     The present known preferred method of laser welding of copper and other bright metals such as gold and silver, involves the use of lasers that emit at visible green wavelengths. The most common lasers are frequency doubled 1064 nm lasers that emit at 532 nm. This is because the reflectivity of bright metals is lower at 532 nm than at near infrared wavelengths. The laser joining of bright metals with such lasers produces welds that are repeatable and consistent but at the cost of efficiency, complexity, and costs associated with frequency doubling. In some applications, it is necessary to combine a laser emitting at 532 nm with a second laser at 1064 nm in order to increase efficiency and productivity. Such dual wavelength systems require closed loop monitoring of the laser welding process using sophisticated beam monitoring and real time analysis in order to analyze and tailor the structure of the weld. Such diagnostic devices use video analysis of the back reflected light and the weld pool characteristics in order to provide feedback to a laser controller. These systems are complex and expensive. 
     The use of green lasers has been adopted to perform weld joints of bright metals without specifically addressing the application of joining dissimilar metals. Conventional welding of dissimilar metals relies on specific control of the dilution of the metals at the interface and resulting thermal conditions to minimize mixing of the dissimilar metals which results in so-called intermetallics in the joint. Large intermetallic regions are prone to fracture from stresses acting on the joint, and the fracture propagates through the entire joint until failure. 
     Laser welding with continuous wave and pulsed lasers is well known, with either a continuous weld front, or overlapping spot welds wherein the weld forms a continuous seam. Defects in the materials caused by the welding process create weaknesses, and are unacceptable in the majority of applications. Pulsed welds are typically formed using microsecond and millisecond pulses. The pulse causes the material to melt which resolidifies to form the weld. When welding dissimilar materials, the weld interface can contain intermetallics, which are a compound formed from the two materials being joined, and are typically brittle and undesirable in nature. The weld can therefore preferentially fracture along this intermetallic layer under mechanical load. 
     Forming low ohmic resistance welds between highly reflective materials has important applications in the electronics and electrical engineering industries, including in the manufacture of batteries, solar cells, semiconductor packaging, and electronic printed circuit boards. Various techniques are used, including laser welding. However the high reflectivity can require relatively expensive visible lasers. In addition, the welding equipment, process and the resulting welds do not meet current requirements of fast manufacturing speeds, low ohmic resistance, high shear strength, and high peel strength. Consequently, processes other than laser welding are often used. 
     Laser welds in work pieces comprising one or more reflective metals, for example gold, copper, aluminium, platinum and silver, are often unreliable and weak. Laser welds in articles comprising dissimilar materials are typically brittle and undesirable in nature. 
     There is a need for a weld between bright and/or dissimilar metals and alloys that does not have reliability issues and it is an aim of the present invention to provide such a weld. 
     THE INVENTION 
     Accordingly, in one non-limiting embodiment of the present invention there is provided a weld between a first material and a second material, the first material being a first metallic material, and the second material being a second metallic material, the weld has a width between 0.5 mm and 7 mm, the weld comprises at least one microweld, the microweld forms a welding pattern defined parallel to a surface of the first material, and the microweld has a characteristic feature size of between 20 μm and 100 um. 
     The weld of the present invention has important applications in the electronics and electrical engineering industries. The ability to create welds in reflective metals using nanosecond fibre lasers, emitting in the 1 μm wavelength window, and with pulse energies of around 1 mJ, is new and unexpected. Moreover, the welds can have greater strength and reliability than prior art welds. The weld may be used in articles such for example as batteries, solar cells, semiconductor packaging, and electronic printed circuit boards. 
     The weld comprises at least one microweld. The microweld forms the welding pattern. The welding pattern may be formed of a plurality of the microwelds. Alternatively, the welding pattern may be formed from a single microweld. The welding pattern may comprise a line in the form of a spiral. Alternatively or additionally, the welding pattern may comprise a plurality of hatch lines. The hatch lines may be in the form of a grid. The hatch lines may form a rectangular grid. The hatch lines may form a triangular grid. The welding pattern is preferably a two dimensional welding pattern. 
     The first material and the second material may remain substantially unmixed in the weld. By “substantially unmixed” it is meant that the intermetallic content formed by the first material and the second material combined together in single co-mixed alloy phases comprises at most twenty percent, and preferably at most ten percent of the material of the weld. The intermetallic content at interfaces between the first material and the second material may be sufficient to achieve a joint with pre-determined mechanical properties and ohmic resistivity. The intermetallic content at interfaces between the first material and the second material may be small enough to avoid embrittlement such as caused by recrystalization. 
     The weld may be substantially inhomogeneous. The weld may comprise discrete zones of the first metallic material and the second metallic material. 
     The first material may have a reflectivity greater than 90% at an optical wavelength of one micron. 
     The first material may have a different melting temperature than the second material. 
     The microweld may comprise a hole formed in the first material. The first material may be contained within the second material. At least one of the first and the second material may have flowed into the hole. The first material may have a top surface and a bottom surface. The bottom surface may be closer to the second material than the top surface. The hole may have a width at the top surface and a width at the bottom surface, wherein the width at the top surface is wider than the width at the bottom surface. The hole may be a countersunk hole, and the microweld may resemble a rivet. 
     The microweld may comprise a zone of the first material within the second material. 
     Surprisingly, the weld provides a simpler solution for joining bright and dissimilar metals and alloys, producing consistent and predictive results on each joint formed by the weld. Arranging for one of the first and the second materials to flow into the hole without substantially mixing with the other material, helps prevent intermetallics from forming, and avoids the reliability issues associated with intermetallics such as brittleness and weak welds. Consistent and predictive results are obtainable with a range of alloys, including amorphous metal alloys, castings, sintered alloys, and injection formed alloys. They are also obtainable with refractory metals, including iridium, tungsten, molybdenum, niobium, and tantalum. Refractory metals are chemically inert, have a higher density and higher hardness than metals such as iron, copper, and nickel, and are characterised by melting temperatures above 2000° C. The increased surface area of the weld provides more contact area, which in turn reduces ohmic resistance. Reducing ohmic resistance is an important consideration for increasing efficiencies of batteries and solar panels. Examples of parts that may be connected include: electrical connections, such as copper to aluminium connections, inside batteries; low profile electrical connections between flexible circuit elements and thin-section busbars; metallic enclosures for medical electronic devices; electromagnetic interference and radio frequency shielding of electrical components; attaching leads, filaments, and wires to electrical connections and circuit boards; other electrical connections in consumer electronics such as mobile phones, laptop computers, televisions, and other consumer electronic devices; metallic labels and tags; silver, platinum, and gold parts in jewellery; and medical devices, sensors and other electrical circuits. Amorphous metal alloys are used in additive manufacturing, a form of three dimensional printing, wherein metal powders are sintered with a laser. 
     The first material may comprise a metal selected from the group consisting of copper, aluminium, iron, nickel, tin, titanium, iridium, tungsten, molybdenum, niobium, tantalum, rhenium, silver, platinum, gold, and an alloy comprising at least one of the foregoing materials. 
     The second material may comprise a metal selected from the group consisting of copper, aluminium, iron, nickel, tin, titanium, iridium, tungsten, molybdenum, niobium, tantalum, rhenium, silver, platinum, gold, and an alloy comprising at least one of the foregoing materials. 
     Other metals for the first material and the second material may be employed. The first material and the second material may be the same or different. 
     The width may be between 0.5 mm and 2.5 mm. 
     The characteristic feature size may be a width of the microweld. The characteristic feature size may be between 40 μm and 100 um. 
     The present invention also provides an article comprising a weld according to the invention. Examples of articles are a smart phone, a mobile phone, a laptop computer, a tablet computer, a television, a consumer electronic device; a battery; a solar cell; an integrated electronic circuit component; a printed circuit board; an electrical connection; a low profile electrical connection between flexible circuit elements and thin-section busbars; a metallic enclosure for a medical electronic device; and an electrical connection in consumer electronics devices; metallic labels and tags; silver, platinum, and gold parts in jewellery. 
     The present invention also provides a method for laser welding a first material to a second material, which method comprises:
         placing a first metal part comprising the first material on a second metal part comprising the second material,   providing a laser for emitting a laser beam in the form of laser pulses,   providing a scanner for scanning the laser beam with respect to a surface of the first metal part,   providing an objective lens for focusing the laser pulses onto the surface, and   providing a controller that is adapted to control the scanner such that the scanner moves the laser beam with respect to the surface,
 
characterized by
   moving the laser beam with respect to the surface,   focusing the laser pulses with a spot size and a pulse fluence that cause the formation of at least one microweld in the form of a welding pattern defined parallel to the surface;   the moving of the laser beam with respect to the metal surface is such that the weld has a width between 0.5 mm and 7 mm; and   wherein the microweld has a characteristic feature size of between 20 μm and 400 μm.       

     The laser may be operated to form a plurality of melt pools in the first metal part and a plurality of heat stakes in the second metal part. Each heat stake may extend from a different one of the melt pools and may have a distal end. The method may include adapting the controller to space the focussed spots apart by a distance that is small enough to cause the melt pools to overlap and that is large enough to ensure the distal end of the heat stakes are distinct and separate from each other in at least one direction. 
     The controller may be operated to select a first laser signal to create a melt pool on the surface, a second laser signal to initiate welding of the first metal part to the second metal part, and a third laser signal to weld the first metal part to the second metal part to form the microweld. The first and the second laser signals may be same or different from each other. The first, second, and third laser signals may be provided in a single pass of the laser beam across the surface, or in a plurality of passes of the laser beam across the surface. The first and the second laser signals may be provided in a first pass of the laser beam across the surface, and the third laser signal may be provided in a second pass of the laser beam across the surface. 
     The second laser signal may be selected to have a plurality of pulses characterized by a pulse width that is greater than 100 ps. 
     The second laser signal may be selected to have a peak power which is substantially greater than a peak power of the third laser signal. 
     At least one of the first, second and third signals may be selected to inhibit the formation of intermetallics. 
     At least one of the first, second and the third signals may be selected to improve the smoothness of a surface of the laser weld. 
     The welding process may be one that forms a key hole. The method may include providing a fourth laser signal which is selected to close the key hole. 
     The first material may be substantially more ductile than the second material. 
     The laser may be characterized by a beam quality M 2  less than 4, preferably less than 2, and more preferably less than 1.3. 
     The laser may be a nanosecond laser. 
     The laser may be characterized by a wavelength between 1000 nm and 3000 nm. 
     The laser may be a rare-earth doped fibre laser. 
     The method may comprise forming a hole in the first material with the laser, melting at least one of the first and the second material with the laser, and flowing at least one of the first and the second material into the hole. 
     The first material and the second material may remain substantially unmixed in the weld. 
     The hole may be formed by pulsing the laser such that at least some of the first material is injected into the second material. 
     The hole may be formed by first forming a hole that does not penetrate through the first material, and then pulsing the laser such that at least some of the first material is injected into the second material. 
     The first material may have a top surface and a bottom surface. The bottom surface may be closer to the second material than the top surface. The hole may have a width at the top surface and a width at the bottom surface, wherein the width at the top surface is wider than the width at the bottom surface. The hole may be a countersunk hole. 
     The method may include a step of remelting at least one of the first material and the second material with the laser. 
     The weld may comprise at least one void in at least one of the first material and the second material. 
     The pulse repetition rate may be greater than 10 kHz, may be greater than 100 kHz, and may be greater than 200 kHz. The spot size, the pulse fluence, the pulse width, and the pulse repetition frequency may be selected such that at least one of the first material and the second material resolidifies between successive laser pulses thereby inhibiting the formation of an intermetallic phase in the weld. Selecting a pulse waveform that ensures that at least one of the first material and the second material is quenched rapidly substantially reduces intermetallic growth, and thereby avoids the reliability issues associated with intermetallics such as brittleness and weak welds. 
     The spot size may be less than 100 μm. The spot size may be less than 60 μm. 
     The first material may have a higher melting temperature than the second material. 
     The first material may have a reflectivity greater than 90% at an optical wavelength of one micron. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings wherein: 
         FIG. 1  shows a weld according to the present invention; 
         FIG. 2  shows a weld in the form of a continuous spiral; 
         FIG. 3  shows a weld in the form of rectangular hatching; 
         FIG. 4  shows a weld in the form of rectangular hatching; 
         FIG. 5  shows a weld in the form of triangular hatching; 
         FIG. 6  shows a laser system for producing a weld according to the present invention; 
         FIG. 7  shows a hole cut in the first material by the laser; 
         FIG. 8  shows second material that has been melted by the laser; 
         FIG. 9  shows the finished weld wherein the molten second material has flowed into the hole formed in the first material by the laser; 
         FIG. 10  shows a hole that does not pass through the first material; 
         FIG. 11  shows molten second material underneath the hole; 
         FIG. 12  shows the finished weld wherein molten second material has flowed into the hole formed in the first material by the laser; 
         FIG. 13  shows a weld being formed; 
         FIG. 14  shows a weld having zones of the first material within the second material; 
         FIG. 15  shows a laser system for producing a weld according to the present invention; 
         FIG. 16  shows parameters of a pulsed laser waveform; 
         FIG. 17  shows parameters of a focussed laser spot; 
         FIG. 18  shows two focussed laser spots spaced apart; 
         FIG. 19  shows two focussed laser spots that are overlapping; 
         FIG. 20  shows a stitched pattern of microwelds; 
         FIG. 21  shows a laser system wherein a pulsed laser output is varied while making the weld; 
         FIG. 22  shows a microweld being made using keyhole welding; 
         FIG. 23  shows a cross section of a microweld; 
         FIG. 24  shows a waveform that is used to close a keyhole; 
         FIG. 25  shows a first material that is coated with a coating; 
         FIG. 26  shows a first material welded to a second material, wherein the first and the second material comprise layers; 
         FIG. 27  shows a prior art weld comprising intermetallics and a heat affected zone; 
         FIG. 28  shows a weld according to the present invention comprising a heat affected zone; 
         FIG. 29  shows a tab welded to a second metal part with a weld; 
         FIG. 30  shows a graph of pulse fluence and absorbed energy density; 
         FIG. 31  shows an example of a weld made according to a method of the present invention; 
         FIG. 32  shows the results of a shear test of the weld shown in  FIG. 31 ; 
         FIG. 33  shows two sheets of aluminium foil connected by copper foil using welds according to the present invention; 
         FIG. 34  shows a weld formed of brass and copper; 
         FIG. 35  shows evolution of pulse shape with pulse repetition frequency in a nanosecond pulsed fibre laser based on a master oscillator power amplifier configuration; and 
         FIG. 36  shows two pulse waveforms having the same average power in the nanosecond pulsed fibre laser. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     A weld according to the invention will now be described solely by way of example and with reference to  FIG. 1 .  FIG. 1  shows a weld  3  between a first material  1  and a second material  2 , the first material  1  being a first metallic material, and the second material  2  being a second metallic material, the weld  3  has a width  4  between 0.5 mm and 7 mm, the weld comprises at least one microweld  8 , the microweld  8  forms a welding pattern  5  (shown enlarged) defined parallel to a surface  6  of the first material  1 , and the microweld  8  has a characteristic feature size  7  of between 20 μm and 400 um. 
     By parallel to the surface  6  of the first material  1 , it is meant either on the surface  6  in the vicinity of the weld  3 , or beneath the surface  6 , for example, below a weld pool. The welding pattern  5  is preferably a two dimensional welding pattern. By width  4  of the weld  3 , it is meant the smallest transverse dimension of the weld  3  on the surface  6 . 
     The welding pattern  5  shown in  FIG. 1  comprises a plurality of microwelds  8  in the form of a spiral. The characteristic feature size  7  of the microwelds  8  is the width or the diameter of the microwelds  8 . The arms  9  of the spiral are separated by a first separation  10 . The microwelds  8  are separated by a second separation  11  within the arms  9  of the spiral. The second separation  11  can be 50 μm to 450 μm. Preferably, the second separation  11  is between 50 μm and 200 μm. The spiral may be circular, or may be elongated such as in the form of a race track. Other patterns may also be used. 
     The weld  3  can be in the form of the welding pattern  20  shown in  FIG. 2 , which welding pattern  20  comprises a single microweld  21  that is in the form of a spiral  22 . The characteristic features size  7  of the microweld  21  is the width of the microweld  8 . The arms  9  of the spiral are separated by the first separation  10 . 
     The welding pattern  5  may comprise a plurality of hatch lines  31  as shown in  FIGS. 3, 4 and 5 , each hatch line  31  comprising at least one microweld  8 . The welding pattern  5  may comprise a perimeter ring  33  comprising at least one microweld  8  as shown in  FIGS. 3 and 5 . Advantageously, the perimeter ring  33  can help to relieve stress in the weld  3 . The characteristic feature size  7  of the microweld  8  is the width of the microweld  8 . The hatch lines  31  may comprise a rectangular grid, as shown in  FIGS. 3 and 4 , with individual hatch lines  31  being separated by the first separation  10  and by a third separation  32 . The hatch lines  31  may also form a triangular grid as shown with reference to  FIG. 5 . Other grid patterns are also possible. 
     The first separation  10  in  FIGS. 1 to 5  can be in the range 20 to 2000 μm. The first separation  10  can be in the range 50 μm to 500 μm. Preferably the first separation  10  is the range 50 μm to 250 μm. More preferably the first separation  10  is in the range 50 μm to 125 μm. 
     The third separation  32  in  FIGS. 3 to 5  can be in the range 20 to 2000 μm. The third separation  32  can be in the range 50 μm to 500 μm. Preferably the third separation  32  is the range 50 μm to 250 μm. More preferably the third separation  32  is in the range 50 μm to 125 μm. The third separation  32  can be the same as the first separation  10 . 
     The weld  3  can be made using the apparatus shown in  FIG. 6 . The apparatus comprises a laser  61  coupled to a laser scanner  67  by beam delivery cable  69 . The laser  61  emits a laser beam  62  which is focussed onto the surface  6  with an objective lens  68 . 
     The laser  61  is preferably a nanosecond laser that emits at a wavelength of approximately 1060 nm. Various options for the laser  61  will be described later. 
     By a nanosecond pulsed laser, it is meant a laser that can emit pulses having pulse widths in the range 1 ns to 1000 ns. Such lasers may also be able to emit shorter pulses, and longer pulses, and may also be able to emit continuous wave radiation. Such lasers are different from prior art millisecond lasers that are conventionally used for producing welds. Millisecond lasers generally form a weld by emitting a single pulse, and the welds that are formed by millisecond lasers have a very different visual appearance from the welds  3  of the present invention. Surprisingly, the welds  3  of the present invention can be formed in highly reflective metals and refractory metals, and by virtue of the shorter pulses that contain less energy, the welds  3  are extremely strong, even when using dissimilar metals, highly-reflective metals. At least one of the first material  1  and the second material  2  may cool down very rapidly between pulses, leaving insufficient time for intermetallic formation within the microweld  8 . Welds  3  can also be formed in combinations of metals, such as aluminium and stainless steel, in which strong, reliable and predictive welds have been difficult to achieve with prior art techniques. 
     As shown in  FIGS. 7 to 9 , the first material  1  and the second material  2  may be substantially unmixed in the microweld  8 .  FIG. 7  shows a hole  71  that has been formed with the laser  61 .  FIG. 8  shows molten second material  81  that has been melted with a laser.  FIG. 8  shows the microweld  8  that is formed after the molten second material  81  has flowed into the hole  71  and resolidified. The flow may occur because of capillary action, by vapour pressure caused by the rapid expansion of vapourized material by the laser pulse, or by the Marangoni effect, which is the mass transfer along an interface between two fluids due to surface tension gradient. In the case of temperature dependence, this phenomenon may be called thermo-capillary convection (or Bénard-Marangoni convection). 
     The weld  3  shown with reference to  FIGS. 7 to 9  has a top surface  72  and a bottom surface  73 . The hole  71  has a width  74  at the top surface  72  which is wider than a width  75  at the bottom surface  73 . Importantly, such an arrangement can increase the peel strength of the microweld  8 . The hole  71  is a countersunk hole and the microweld  8  resembles a rivet. The width  74  may be less than 200 μm. The width  74  may be less than 50 μm. The width  74  may be less than 20 μm. 
       FIG. 10  shows a hole  76  that does not penetrate through the first material  1 . The hole  76  can be formed by ensuring that the energy in the pulse is not sufficient to raise the vapour pressure in the first material  1  to a level in which the hole  76  penetrates to the bottom surface  73  of the first material  1 . This can be achieved by selecting the laser  61  such that it can deliver lower energy pulses such as pulses with lower peak powers, or pulse widths that are less than 20 ns. The scanner  67  can be used to scan the laser beam  62  on the first material  1  in order to obtain a predetermined shape of the hole  71 . For high reflectivity materials (for example, reflectivity greater than around 90% at 1 μm wavelength) picoseconds lasers (lasers that emit pulses having pulse widths between 1 ps and 1000 ps) may be used advantageously.  FIG. 11  shows molten second material  81  that has been melted by the laser  61 . The laser  61  can then be pulsed such that the hole  76  now penetrates to the second surface  73 , creating the hole  71 , thus allowing at least some of the second material  2  to flow into the hole  71  as shown with reference to  FIG. 12 . At least some of the first material  1  may be injected into the second material  2 , as shown by the zones  121  of the resulting microweld  8  shown in  FIG. 12 . At least one void  122  may also occur in the second material  2 . The void  122  may assist the flow of the second material  2  through the hole  71  by vapour pressure. 
       FIGS. 13 and 14  shown a microweld  8  formed with a laser  61  that has sufficient peak power to overcome the reflectivity of the first material  1 , and sufficient energy to form a key hole  133  in the second material  2 . Vapour pressure caused by the rapid heating of the first material  1  causes at least some of the first material  1  to be injected into the hole  71  or ejected from the hole  71 . This is shown by the material  131  being injected into the key hole  133  formed in the second material  2 , and the material  132  being emitted out of the hole  71 . The materials  131  and  132  may be in the vapour phase, fluid phase, solid phase, or a combination of at least two of the forgoing material phases. Molten second material  81  can then flow into the hole  71  as shown with reference to  FIG. 14 . Zones  121  of the first material  1  and voids  122  may be present in the microweld  8 . 
     The microwelds  8  shown with reference to  FIGS. 1 to 5  can be one or more of the microwelds  8  shown with reference to  FIGS. 9, 12 and 14 . 
     The microweld  8  may be substantially inhomogeneous. Unlike prior art welds, the microweld  8  may be substantially unmixed. By “substantially unmixed” it is meant that the intermetallic content formed by the first material  1  and the second material  2  combined together in single co-mixed alloy phases comprises at most twenty percent, and preferably at most ten percent of the material of the microweld  8 . The intermetallic content at interfaces between the first material  1  and the second material  2  may be sufficient to achieve a joint with pre-determined mechanical properties and ohmic resistivity. The intermetallic content at interfaces between the first material  1  and the second material  2  may be small enough to avoid embrittlement such as caused by recrystallization. Advantageously this avoids the problems of brittle or weak welds arising from intermetallics that can occur when forming a weld between dissimilar metals. The result is a weld  3  capable of joining bright and dissimilar metals and alloys, producing consistent and predictive results on each weld. 
     The first material  1  may have a different melting temperature than the second material  2 . This enables one of the first and the second materials  1 ,  2  to resolidify prior to the other material, and to flow, thus avoiding substantial mixing of the first and the second materials  1 ,  2 . In order to optimize the performance of the microweld  8 , the parameters of the laser  61 , such as pulse width, pulse repetition frequency, pulse energy, and peak power can be adjusted. The first material  1  may have a melting temperature that is at least 50% higher or lower than a melting temperature of the second material  2 . 
     The first material  1  may be defined by a Young&#39;s modulus which is less than a Young&#39;s modulus of the second material  2 . Advantageously, the first material  1  may be substantially more ductile than the second material  2 . This has advantages if the weld  3  is repeatedly strained since the microwelds  8  will be more resistant to metal fatigue. 
     The first material  1  may have a reflectivity  145  greater than 90% at an optical wavelength  140  of one micron. The reflectivity  145  can be defined at 20 C. 
     With reference to  FIGS. 1 to 5 and 7 to 14 , the first material  1  can comprise a metal selected from the group consisting of copper, aluminium, iron, nickel, tin, titanium, iridium, tungsten, molybdenum, niobium, tantalum, rhenium, silver, platinum, gold, and an alloy comprising at least one of the foregoing materials. The alloy can be bronze, brass, a nickel titanium alloy, or an amorphous alloy. The second material  2  can comprise a metal selected from the group consisting of copper, aluminium, iron, nickel, tin, titanium, iridium, tungsten, molybdenum, niobium, tantalum, rhenium, silver, platinum, gold, and an alloy comprising at least one of the foregoing materials. Other metals for the first material  1  and the second material  2  may be employed. The first material  1  and the second material  2  may be the same or different. 
     Surprisingly, a weld  3  between bright and dissimilar metals and alloys has consistent and predictive qualities. Arranging for one of the first and the second materials  1 ,  2  to flow into the hole  71  without substantially mixing with the other material, helps prevent intermetallics from forming, and avoids the reliability issues associated with intermetallics such as welds which are brittle and weak. The increased surface area of the weld  3  provides more contact area, which in turn reduces ohmic resistance. Reducing ohmic resistance is an important consideration for increasing efficiencies of batteries and solar panels. 
     The width  4  may be between 0.5 mm and 2.5 mm. Preferably the characteristic feature size  7  is between 40 μm and 100 μm. 
     The present invention also provides an article comprising at least one weld  3  according to the Figures disclosed. Examples of articles are smart phones, mobile phones, laptop computers, tablet computers, televisions, and other consumer electronic devices; batteries; solar cells; integrated electronic circuit components; printed circuit boards; electrical connections, such as copper to aluminium connections, inside batteries; low profile electrical connections between flexible circuit elements and thin-section busbars; metallic enclosures for medical electronic devices; and electrical connections in consumer electronics devices; metallic labels and tags; silver, platinum, and gold parts in jewellery. 
     A method according to the invention for laser welding a first material  1  to a second material  2 , will now be described with reference to  FIG. 15 . The method comprises:
         placing a first metal part  151  comprising the first material  1  on a second metal part  152  comprising the second material  2 ,   providing a laser  61  for emitting a laser beam  62  in the form of laser pulses  161 ,   providing a scanner  67  for scanning the laser beam  62  with respect to a surface  6  of the first metal part  151 ,   providing an objective lens  68  for focusing the laser pulses  161  onto the surface  6 , and   providing a controller  153  that is adapted to control the scanner  67  such that the scanner  67  moves the laser beam  62  with respect to the surface  6 ,
 
characterized by
   moving the laser beam  62  with respect to the surface  6 ,   focusing the laser pulses  161  to form a focussed spot  12  with a spot size  174  and a pulse fluence  176  (shown with reference to  FIG. 17 ) that cause the formation of at least one microweld  8  in the form of a welding pattern  5  defined parallel to the surface  6 ;   the moving of the laser beam  62  with respect to the metal surface is such that the weld  3  has a width  4  (shown with reference to  FIG. 1 ) between 0.5 mm and 7 mm.   wherein the microweld  8  has a characteristic feature size  7  of between 20 μm and 400 μm.       

     The laser radiation  62  is directed to the scanner  67  via an optical fibre  147  and a collimation optic  142 . 
     The laser beam  62  is preferably moved in two dimensions with respect to the surface  6  such that the resulting welding pattern  5  is a two dimensional welding pattern. 
       FIG. 15  shows the laser  61  emitting at a wavelength  140  and a beam quality  146  defined by an M 2  value. The wavelength is shown as being 1060 nm and the beam quality  146  as being 1.6; this is intended to be non-limiting. 
     The first metal part  151  can have a thickness  143  in a region of the weld  3  of no more than 5 mm. The thickness  143  may be less than 2 mm. The thickness  143  may be less than 1 mm. The thickness  143  may be less than 0.5 mm. The second metal part  152  can have a thickness  144  in a region of the weld  3  of at least 100 μm. The thickness  144  may be less than 0.5 mm. The first metal part  151  can have a reflectivity  145  greater than 80%. Other reflectivities are also possible. 
       FIG. 16  shows pulses  161  defined by a peak power  162 , an average power  163 , a pulse shape  164 , pulse energy  165  (shown as the shaded area under the pulse), a pulse width  166 , and pulse repetition frequency F R    167 . The average power  163  is equal to the product of the pulse energy  165  and the pulse repetition frequency  167 . The pulse width  166  is shown as the full width half maximum value (FWHM) of the peak power  162 . Also shown is a pulse width  168  measured at 10% of the peak power  162 . The pulse  161  comprises a pre-pulse  160  that can be followed by a lower power region  169 . 
       FIG. 17  shows a spot  12  having a spot size  174  formed by focussing the laser beam  62  onto the surface  6 . The optical intensity  172  is the power per unit area of the laser beam  62 . The optical intensity  172  varies across the radius of the spot  12  from a peak intensity  179  at its centre, to a 1/e 2  intensity  173  and to zero. The spot size  174  is typically taken as the 1/e 2  diameter of the spot  12 , which is the diameter at which the optical intensity  172  falls to the 1/e 2  intensity  173  on either side of the peak intensity  179 . The area  175  of the spot  12  is typically taken as the cross-sectional area of the spot  12  within the 1/e 2  diameter. Pulse fluence  176  is defined as the energy per unit area of the spot  12  on the surface  6 . Pulse fluence is typically measured in J/cm 2 , and is an important parameter for laser welding because weld quality is highly influenced by the pulse fluence  176 . 
     The laser  61 , the collimation optic  142  and the objective lens  68 , should be selected such that sufficient optical intensity  172  and pulse fluence  176  can be obtained to overcome the reflectivity of the surface  6 . The pre-pulse  160  can be used for overcoming the reflectivity of the first material  1 , and for forming the hole  71  shown with reference to  FIGS. 7 to 14 . The lower power region  169  can be used to melt the second material  2 . The laser parameters shown with reference to  FIG. 16  can be adjusted to optimize desired characteristics of the weld  3 . The optimum pulse fluence  176  for a particular weld varies between different materials and material thicknesses. The optimum pulse fluence  176  for welding a metal piece part can be determined through experimentation. 
     The laser  61  in  FIG. 15  can be operated to form a plurality of melt pools  19  in the first metal part  151  and a plurality of heat stakes  17  in the second metal part  152 . Each heat stake  17  extends from a different one of the melt pools  19  and has a distal end  154 . The method includes adapting the controller  153  such that the laser  61  and the scanner  67  cause the focussed spots  12  to be spaced apart by a distance that is small enough to cause the melt pools  19  to overlap and that is large enough to ensure the distal end  154  of the heat stakes  17  are distinct and separate from each other in at least one direction  155 . 
     By “distinct and separate from each other”, it is meant that the distal ends  154  of the heat stakes  17  do not form a substantially smooth weld in all directions; the heat stakes  17  may be at least partially separate from each other in at least one direction  155 . Alternatively, the heat stakes  17  may be at least partially separate from each other in all directions substantially parallel to the metal surface  6 . By “weld” it is meant a connection made by welding or joining. 
     Successive focussed laser spots  12  may be separated as shown in  FIG. 18  such that the separation  181  between the centres of the laser spots  12  is greater than the spot size  34 . Alternatively or additionally, successive focussed laser spots  12  may overlap as shown in  FIG. 18  such that the separation  181  is less than the spot size  34 . If the laser spots  12  are separated as shown in  FIG. 18 , then the heat stakes  17  can be distinct and separate from each other from each other in more than one direction  155 . If however the laser spots  12  overlap, as shown in  FIG. 19 , then the resulting microwelds  8  can be linear welds such as shown in  FIG. 20 . The pattern  5  can either be formed from a plurality of such microwelds  8  as shown, or be formed by a pattern  5  of a single microweld  8 . In the latter case, the heat stakes  17  are distinct and separate from each other in only one direction  155 . In  FIGS. 17 and 18 , the focussed laser spot  12  may represent a single laser pulse  161  or multiple laser pulses  161 , and the above discussion extends to the case in which the laser spot  12  is dithered to increase the characteristic feature size  7  of the microweld  8 . 
     Each heat stake  17  is formed by at least one of the pulses  161 , the number of pulses  161  being dependent on the pulse fluence  176 . Ten to one hundred pulses  161  are typically used for a laser with 1 mJ pulse energy  165 . The distance  181  between the centres of the focussed spots  12  will approximate the distance  18  between the centres of the respective heat stakes  17 . The controller  153  can cause the scanner  67  to hold the focussed spot  12  still during the formation of each of the heat stakes  17 . Alternatively, the controller  153  can cause the scanner  67  to dither the focussed spot  12  during the formation of each of the heat stakes  17 , preferably by an amount less than the distance  18 . The distance  18  is typically 20 μm to 150 μm, and preferably 40 μm to 100 μm. 
     The weld  3  can be a composite weld formed by the overlapping melt pools  19  and the heat stakes  17 . For clarity,  FIG. 15  shows the focussed spots  12  as black circles, and the weld  3  in cross section within a three dimensional depiction. The melt pools  19  are shown melted together without boundaries between them, and an interface is shown between the melt pools  19  and the heat stakes  17 . Metallurgical studies have demonstrated that both the melt pools  19  and the heat stakes  17  may comprise material that is from both the first material  1  and the second material  2 . 
     Good mixing of the metals can be achieved, which can be advantageous when both the first and the second materials  1 ,  2  are stainless steel. In this case there is generally no well defined boundary between the melt pools  19  and the heat stakes  17 . 
     The distal ends  154  of the heat stakes  17  are shown as ending in a sharp point. However this is not necessarily so; the distal ends  154  may be substantially curved and may be fragmented such that they have more than one end. 
     As shown with reference to  FIG. 15 , the method may include the step of providing a shield gas  155  from a gas supply  156 , and applying the shield gas  155  over the weld  3 . Shield gases can be used to keep to prevent the weld  3  oxidising or to keep the weld  3  clean. The shield gas  155  can be argon, helium, nitrogen, or other gases commonly used in laser welding. The shield gas  155  may be mixtures of gases. The gas supply  156  may comprise a gas bottle, a nozzle, and a flow control regulator. 
     The weld  3  has a substantially jagged surface at the distal ends  154  of the heat stakes  17 . This is in direct contrast with conventional welding practice in which a smooth distal end of the weld is thought to be advantageous. A weld line that is not smooth is believed to be a cause for concern in the prior art. 
     The apparatus is preferably such that the laser pulses  161  are in synchronism with a control signal  157  used to control the scanner  67 . This may be achieved by applying a synchronisation signal into the controller  153 , or by adapting the controller  153  such that the controller also controls the laser  61 . 
     The scanner  67  can be a galvanometric scan head. Alternatively or additionally, the scanner  67  can be a moveable two-dimensional or three-dimensional translation stage, or a robot arm. The scanner  67  is such that it can move the laser beam  62  in a first direction  158  and a second direction  159 . The scanner  67  and the objective lens  68  may be part of a processing optics known by persons skilled in the art. The processing optic may have additional optical elements like tiled mirrors, additional focus control and/or beam shaping optics. 
     As shown in  FIG. 21 , the method of the invention may comprise operating the controller  153  to select a first laser signal  201  to create the melt pool  19  on the metal surface  6 , a second laser signal  202  to initiate welding of the first metal part  151  to the second metal part  152 , and a third laser signal  203  to weld the first metal part  151  to the second metal part  152  to form the microweld  8 . The first, second and third laser signals  201 ,  202 ,  203  are depicted comprising the laser pulses  161 . Preferably, the controller  153  controls the laser  61  such that the first, second and third laser signals  201 ,  202 ,  203  are in synchronism with the scanner  67 . 
     A first cross section  221  shows the melt pool  19  caused by absorption of the first laser signal  201  by the first material  1  during a first time period  204 . When welding reflective metals, the absorption of the metal can increase significantly when the melt pool  19  is created. To optimize the weld properties, it can therefore be important for the controller  153  to select the second laser signal  202  once the reflectivity  145  changes. 
     A second cross section  222  shows the initiation of welding in a second time period  205 . The second laser signal  202  has caused the melt pool  19  to extend through the first metal part  151  and into the second metal part  152 . The distal end  226  of the melt pool  19  is shown penetrating the second metal part  152 . The melt pool  19  will then begin to contain metal from both the first metal part  151  and the second metal part  152 . Alternatively or additionally, metal from the first metal part  151  may penetrate into the second metal part  152 . In either case, welding can be said to have been initiated. A key hole  133  is shown as being present. The key hole  133  was described with reference to  FIG. 13 , and will be further described with reference to  FIGS. 22 and 23 . The key hole  133  may not occur during the second time period  205  and may not occur at all. If the key hole  133  is present, then most of the laser beam  62  may be absorbed by the key hole  133 . When welding reflective metals, it may therefore be beneficial that at least one of the peak power  162  and the pulse energy  165  of the second laser signal  202  reduces with the increasing absorption of the laser beam  62  in order to limit eruptions occurring from the key hole  133 . If the welding process continues without the controller  153  changing to the third laser signal  203 , then there can be too much energy being absorbed by the first and the second metal parts  151 ,  152 , which can result in violent eruptions of material from the key hole  133 , and consequently, rough surfaces that are undesirable, especially for such as jewellery and medical devices for insertion into humans. 
     A third cross section  223  shows the first metal part  151  being welded to the second metal part  152  in a third time period  206  by the third laser signal  203 . This may occur in the same pass of the laser beam  62  across the surface  6  in which the first and the second laser signals  201  and  202  were applied, or in a subsequent pass. If the first material  1  is highly reflective, then the peak power  162  of the third laser signal  203  may be selected such that it is less than the peak power  162  of the second laser signal  202 ; this has the effect of causing less violent eruptions of molten material from the key hole  133 . In certain circumstances, it may be preferred that the third laser signal  203  is a continuous wave signal. The melt pool  19  is shown as being larger than the melt pools  19  in the first and second cross sections  221 ,  222 , but this is meant to be non limiting. The laser beam  62  is shown focussed into the key hole  133 . The distal end  226  of the weld pool  19  is shown extending further into the second metal part  152 . The key hole  133  may not be present during the third time period  206 . 
     Key hole welding is shown in more detail in  FIG. 22 . In this process, the laser beam  62  not only melts the first and the second metal parts  151 ,  152 , but also produces vapour. The dissipating vapour exerts pressure on the molten metal  225  and partially displaces it. The result is a deep, narrow, vapour filled hole called the keyhole  133 . Such a process may be involved in the formation of the microweld  8  and the heat stakes  17  (if present) in the apparatus and method of the invention. 
     The method may be one in which the key hole  133  is surrounded by the molten metal  225 , and moves with the laser beam  62  in the direction  226  that the laser beam  62  is scanned. The molten metal  225  solidifies behind the keyhole  133  as it moves, forming the microweld  8 . The microweld  8  can be deep and narrow. The laser beam  62  is absorbed with high efficiency in the key hole  133  as it is reflected multiple times. As shown in  FIG. 23 , the microweld  8  may have a depth  228  that is greater than its width  229 . The weld depth  228  can be up to ten times greater than the weld width  229 . Alternatively, the weld depth  228  can be greater than ten times greater than the weld width  229 . 
     The heat stake  17  shown with reference to  FIGS. 15 and 21  can form at least part of the microweld  8  shown in  FIG. 23 . The width  229  can be the characteristic feature size  7  shown with reference to  FIGS. 1 to 5  and  FIG. 15 . By heat stake  17 , it is meant a weld that penetrates into the second metal part  152 . The heat stake  17  may resemble a spike penetrating the second metal part  152 . Alternatively, the heat stake  17  may be a deep penetration weld that may be linear or curved along its length. The first and second materials  1 ,  2  may be mixed together in the heat stake  17 , or they may be substantially unmixed. Alternatively the heat stake  17  may mainly comprise the first material  1 . 
     In certain cases, such as for example when welding materials having substantially different melting temperatures, the key hole  133  may not close properly, leaving a void  122  in the weld  3 . This can be resolved by providing a fourth laser signal  240 , shown with respect to  FIG. 24 , which laser signal  240  is selected to close the key hole  133 . The average power  153  of the fourth laser signal  240  may be reduced with time. In  FIG. 24 , the fourth laser signal  240  comprises a plurality of pulses  161 , with a smaller pulse repetition frequency  167  than the pulse repetition frequency  167  of the third laser signal  203 . In addition, the peak power  162  is reduced with time. Other fourth laser signals  240  are also possible. 
     Referring again to  FIG. 21 , the microweld  8  is shown in cross section after it has cooled down. The microweld  8  is shown as comprising an optional heat stake  17  extending into the second metal part  152 . Also shown is material  132  on the surface of the weld  3 , and a void  122  within the second metal part  152 . The material  132  and the void  122  were previously described with reference to  FIG. 13 . As described with reference to  FIGS. 1 to 14 , pattern  5  can comprise a plurality of the microwelds  8  shown in  FIG. 21 , or a single microweld  8  which forms the pattern  5 . 
     The welding method can be improved or optimized with respect to one or more of the following criteria: (i) the elimination or reduction of the material  132 , (ii) the elimination or reduction of the voids  122 , (iii) reduction of surface roughness or the improvement of a surface of the weld  3 , (iv) reduction of time taken to form the weld  3 , (v) strength of the weld  3 , and (vi) reliability of the weld  3 . The optimization can be achieved through the selection of one or more of the first, second, third and fourth laser signals  201 ,  202 ,  203 , and  240 , the selection and focussing of the objective lens  68 , and the selection of scanning speeds of the scanner  67 . The optimization can be achieved through experimentation. For example, at least one of the first, second and third signals  201 ,  202 ,  203  may be selected to inhibit the formation of intermetallics. This should increase the strength and the reliability of the weld  3 . Parameters for optimizing welds in different materials and thicknesses  143 ,  144  can be stored in the controller  153  and the laser  61 . 
     The microweld  8  may be formed by a single pass of the laser beam  62  over the surface  6 , or in multiple passes of the laser beam  62  over the surface  6 . The first, second and third laser signals  201 ,  202 ,  203  may be provided in a single pass of the laser beam  62  as it forms the microweld  8 . Alternatively, the first and the second laser signals  201 ,  202  can be provided in a pass of the laser beam  62  over the surface  6 , and the third laser signal  203  in another pass of the laser beam  62  over the surface  6 . 
     In certain cases, it is important that the method for forming the weld  3  is as simple as possible, and preferably uses the same steps for different materials. In this event at least two of the first, second, third, and fourth laser signals  201 ,  202 ,  203 , and  240  can comprise pulses  161  having the same waveforms. 
     The method of the invention described with respect to  FIGS. 15 and 21  can comprise the steps described with reference to  FIGS. 7 to 14 . The method can include forming the hole  71  in the first material  1  with the laser  61 , melting at least one of the first and the second materials  1 ,  2  with the laser  61 , and flowing at least one of the first and the second materials  1 ,  2 . The first and the second materials  1 ,  2  may be flowed into the hole  71  The first material  1  and the second material  2  may remain substantially unmixed in the microweld  8  as shown in  FIG. 8 . The hole  71  may be formed by pulsing the laser  61  such that at least some of the first material  1  is injected into the second material  2  as shown in  FIGS. 12 and 13 . 
     The step of forming the hole  71  may include cutting the first material  1 . By cutting, it is meant cutting or engraving. The step may include cutting the second material  2 . 
     The steps of melting and flowing at least one of the first and the second materials  1 ,  2  may be provided in an additional pass of the laser beam  62  over the microweld  8 . 
     The step of forming the hole  71  may include forming a microweld  8  between the first material  1  and the second material  2 . However, the microweld  8  may not have the required strength, structure or appearance. The steps of melting at least one of the first and the second materials  1 ,  2 , and flowing at least one of the first and the second materials  1 ,  2  may improve the strength, structure or appearance of the microweld  8 . Preferably some or all of the laser parameters described with reference to  FIG. 16  are selected to inhibit the formation of intermetallics  281  in the microweld  8  when melting and flowing at least one of the first and the second materials  1 ,  2 . 
     The step of melting at least one of the first and the second materials  1 ,  2  may include the step of operating the laser  61  such that the pulse fluence  176  preferentially melts one of the first and the second materials  1 ,  2  in preference to the other one of the first and the second materials  1 ,  2 . Preferentially melting one of the first and the second materials  1 ,  2  can inhibit the formation of intermetallics  281 . 
     The step of melting at least one of the first and the second materials  1 ,  2  may include the step of operating the laser  61  with a pulse fluence  176  and a pulse repetition frequency  167  that melts both the first and the second materials  1 ,  2 . Preferably, the pulse fluence  176  and the pulse repetition frequency  167  are selected such that at least one of the first and the second materials  1 ,  2  solidifies between successive pulses  161 . This can inhibit the formation of intermetalics in the microweld  8 . 
     The first material  1  may melt when exposed to a pulse energy  165  of 10 mJ or less. The pulse energy  165  may be 4 mJ or less. The pulse energy  165  may be 1 mJ or less. The pulse energy  165  may be 100 μJ or less. The pulse energy  165  may be 10 μJ or less. Thicker materials require larger pulse energies  165  than thinner materials. 
     As shown in  FIGS. 10 to 12 , the hole  71  may be formed by first forming the hole  76  that does not penetrate through the first material  1 , and then pulsing the laser  61  such that at least some of the first material  1  is injected into the second material  2 . 
     The step of forming the hole  71  may include pulsing the laser  61  with at least one pulse  100  having a pulse width  166  defined by a full width half maximum value that is less than or equal to 100 ns. The pulse width  166  may be less than or equal to 10 ns. The laser  61  may be a nanosecond pulsed laser. 
     The step of forming the hole  71  or the hole  76  may include pulsing the laser  61  with at least one pulse  161  having a pulse width  166  that is less than or equal to 20 ns. The pulse width  166  may be less than or equal to 1 ns. The pulse width  166  may be less than or equal to 100 ps. The pulse width  166  may be less than or equal to 10 ps. The laser  61  may be a picosecond pulsed laser. Preferably the laser  61  is such that it can emit both picosecond pulses (less than 1 ns) and nanosecond pulses (less than 1 μs). An advantage of having pulse widths  107  less than 1 ns is that less energy is provided in the pulse  161 , and this can assist cutting the hole  76  in the first material  3  without surface roughness or penetration through the first material  1 . Multiple pulses  161  may be employed to cut the hole  71  or the hole  76 . 
     The laser weld  3  formed by the apparatus or the method of the invention may be autogenous, that is, no additional (filler) materials are added in forming the weld  3 . 
     Referring to  FIGS. 6, 15 and 21 , the laser  61  can be a fibre laser, a solid state rod laser, a solid state disk laser, or a gas laser such as a carbon dioxide laser, or a combination thereof. The laser  61  may be a laser source with external optical modulators such as an acousto-optic modulator for creating the pulses  161 . The laser  61  may be a Q-switched laser, a modulated continuous wave laser, or a quasi continuous wave laser. The laser  61  is preferably a master oscillator power amplifier. The laser  61  is preferably able to output laser pulses  161  as well as a continuous wave output. 
     The laser  61  may be defined by a beam quality M 2  value  109  that is between 1 and 25. The M 2  value  109  may be in a range 1 to 10, 1 to 5, or 2 to 5. Preferably the M 2  value  109  may be in a range 1.3 to 2. The M 2  value  109  may be less than 1.3. 
     The laser  61  is preferably a rare-earth-doped nanosecond pulsed fibre laser, such as a ytterbium doped fibre laser, an erbium-doped fibre laser, a holmium-doped fibre laser, or a thulium doped fibre laser. These lasers typically emit laser radiation at the wavelength  140  in the 1 μm, 1.5 μm, 2 μm and 2 μm wavelength windows respectively. 
     The laser  61  may be a laser that can emit the laser pulses  161  that have the pulse widths  166  between approximately 10 ps and 3000 ns, preferably in the range 100 ps and 1000 ns, and more preferably in the range 1 ns to 1000 ns. The laser  61  may also be able to emit a continuous wave laser signal. Preferably, the laser  61  has a wide variety of pulse shapes and pulse parameters that can be selected in order to optimize the properties and cost of producing the weld  3 . An example of such a laser is the nanosecond ytterbium-doped fibre laser, model SPI G4 70 EP-Z manufactured by SPI Lasers UK Ltd of Southampton, England. The laser emits at a wavelength  140  in the range 1059 nm and 1065 nm. Table 1 shows pulse parameter data for 36 waveforms (wfm0 to wfm35) that are selectable by the operator of the laser. Each waveform has a minimum pulse repetition frequency PRF 0  at which maximum pulse peak power is obtained, and a maximum pulse repetition frequency PRFmax at which the minimum pulse peak power is obtained. The maximum pulse energy Emax is obtained at the minimum pulse repetition frequency PRF 0 , and is not increased if the laser is operated below the minimum pulse repetition frequency. The peak power obtainable at the minimum pulse repetition frequency PRF 0  is the peak power that corresponds to Emax, and is shown in the right hand column. 
       FIG. 35  shows how the pulse shape  164  varies with pulse repetition frequency  167  for waveform WF 0  shown in Table 1. As the pulse repetition frequency  167  increases, the peak power  162  reduces, and the full width half power (FWHP) pulse width  166  increases from approximately 20 ns at 10 kHz to approximately 220 ns at 560 kHz. The average power  163  is approximately 70 W for each pulse waveform, the pulse energy  165  reducing with increasing pulse repetition frequency  167 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Pulse parameters of the laser used in Examples 1, 2,  
               
               
                 and 11 to 13. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Max. 
                 Typ. 
                   
                 Typ.  
               
               
                   
                   
                   
                 pulse 
                 FWHM 
                 Pulse 
                 peak 
               
               
                   
                   
                   
                 energy, 
                 pulse width 
                 width at 
                 power 
               
               
                   
                 PRF0 
                 PRFmax 
                 Emax 
                 at Emax 
                 10% 
                 at Emax 
               
               
                 wfm 
                 (kHz) 
                 (kHz) 
                 (mJ) 
                 (ns) 
                 (ns) 
                 (kW) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 0 
                 70 
                 1000 
                 1.0 
                 46 
                 240 
                 13 
               
               
                 1 
                 88 
                 1000 
                 0.87 
                 45 
                 220 
                 10 
               
               
                 2 
                 95 
                 1000 
                 0.76 
                 42 
                 200 
                 10 
               
               
                 3 
                 102 
                 1000 
                 0.71 
                 40 
                 175 
                 10 
               
               
                 4 
                 105 
                 1000 
                 0.69 
                 38 
                 160 
                 11 
               
               
                 5 
                 112 
                 1000 
                 0.64 
                 40 
                 145 
                 10 
               
               
                 6 
                 119 
                 1000 
                 0.61 
                 35 
                 130 
                 11 
               
               
                 7 
                 126 
                 1000 
                 0.57 
                 33 
                 120 
                 11 
               
               
                 8 
                 130 
                 1000 
                 0.56 
                 32 
                 115 
                 11 
               
               
                 9 
                 137 
                 1000 
                 0.53 
                 35 
                 105 
                 10 
               
               
                 10 
                 144 
                 1000 
                 0.50 
                 30 
                 100 
                 10 
               
               
                 11 
                 151 
                 1000 
                 0.48 
                 36 
                 90 
                 10 
               
               
                 12 
                 158 
                 1000 
                 0.46 
                 37 
                 80 
                 11 
               
               
                 13 
                 168 
                 1000 
                 0.43 
                 26 
                 65 
                 10 
               
               
                 14 
                 179 
                 1000 
                 0.40 
                 33 
                 58 
                 10 
               
               
                 15 
                 189 
                 1000 
                 0.38 
                 27 
                 60 
                 10 
               
               
                 16 
                 200 
                 1000 
                 0.36 
                 34 
                 55 
                 10 
               
               
                 17 
                 214 
                 1000 
                 0.34 
                 34 
                 50 
                 10 
               
               
                 18 
                 228 
                 1000 
                 0.32 
                 33 
                 45 
                 10 
               
               
                 19 
                 245 
                 1000 
                 0.29 
                 32 
                 40 
                 10 
               
               
                 20 
                 266 
                 1000 
                 0.27 
                 26 
                 36 
                 10 
               
               
                 21 
                 291 
                 1000 
                 0.25 
                 26 
                 33 
                 10 
               
               
                 22 
                 315 
                 1000 
                 0.23 
                 25 
                 30 
                 10 
               
               
                 23 
                 350 
                 1000 
                 0.21 
                 23 
                 26 
                 10 
               
               
                 24 
                 403 
                 1000 
                 0.18 
                 19 
                 23 
                 9 
               
               
                 25 
                 490 
                 1000 
                 0.15 
                 16 
                 20 
                 9 
               
               
                 26 
                 600 
                 1000 
                 0.12 
                 13 
                 16 
                 9 
               
               
                 27 
                 850 
                 1000 
                 0.08 
                 9 
                 10 
                 8 
               
               
                 28 
                 1000 
                 1000 
                 0.07 
                 9 
                 10 
                 7 
               
               
                 29 
                 70 
                 900 
                 1.0 
                 72 
                 270 
                 8 
               
               
                 30 
                 70 
                 800 
                 1.0 
                 75 
                 295 
                 8 
               
               
                 31 
                 70 
                 600 
                 1.0 
                 85 
                 320 
                 7 
               
               
                 32 
                 70 
                 600 
                 1.0 
                 90 
                 350 
                 7 
               
               
                 33 
                 70 
                 600 
                 1.0 
                 95 
                 380 
                 6 
               
               
                 34 
                 70 
                 600 
                 1.0 
                 100 
                 420 
                 6 
               
               
                 35 
                 70 
                 500 
                 1.0 
                 110 
                 470 
                 6 
               
               
                 36 
                 70 
                 500 
                 1.0 
                 115 
                 520 
                 5 
               
               
                   
               
            
           
         
       
     
       FIG. 36  shows the pulse shape  164  for two different pulse waveforms shown in Table 1 at the minimum pulse repetition frequency PRF 0 . The average power  163  is approximately 70 W for each pulse waveform. 
     The laser can also provide a continuous wave (cw) laser beam  62 , which can be selected as the third or fourth laser signal  203 ,  240 . 
     The ability to weld highly reflective metals using nanosecond fibre lasers, emitting in the 1 μm wavelength window, and with pulse energies  165  of around 1 mJ, is new and unexpected. 
     Referring to  FIG. 21 , the second laser signal  202  may be selected to have a plurality of the pulses  161 . The pulse width  166  may be greater than 100 ps. 
     The second laser signal  202  can be selected to have a peak power  162  that is substantially greater than the peak power  26  of the third laser signal  203 . 
     The second laser signal  202  can be selected to have a pulse repetition frequency  167  which is substantially less than the pulse repetition frequency  167  of the third laser signal  203 . The average power  163  of the second laser signal  202  may be characterized by an average power which is substantially equal to the average power  163  of the third laser signal  203 . The third laser signal  203  may be a continuous wave signal; this can be advantageous when welding a reflective metal as it avoids rapid absorption of pulse energy  165  that increases vapour pressure in the first material  1  and results in eruptions of material from the microweld  8 . The second and the third laser signals  202 ,  203  can be applied in the same pass of the laser beam  62  over the first material  1 , or in different passes. 
     The peak power  162  of the first laser signal  201  may be selected to have a peak power  162  that is greater than a peak power  162  of the second laser signal  202 . This can assist coupling of the laser beam  52  to the first material  62  as high peak power  162  is needed to overcome the reflectivity  145  of the first material  1 . 
     The pulse energy  165  of the first laser signal  201  may be selected to a have a pulse energy  165  that is less than the pulse energy  165  of the second laser signal  202 . 
     The pulse width  166  of the second laser signal  202  may be selected to be less than 2.5 ms, preferably less than 1 ms, and more preferably less than 100 ns. 
     The pulse repetition frequency  167  of the second laser signal  202  may be selected to be greater than 1 kHz, preferably greater than 10 kHz, and more preferably greater than 100 kHz. 
     The welding process that is optimised may be one that improves a smoothness of a surface  231  of the laser weld  3 . Alternatively or additionally, the welding process that is optimised may be one that increases the strength of the laser weld  3 . Alternatively or additionally, the welding process that is optimised may be one that reduces the time taken to form the laser weld  3 . 
     As shown in  FIG. 25 , the first material  1  may be coated with a coating  251 . The coating  251  may be a metal plating such as nickel or chrome, or may be a chemically-induced coating such as anodization. The coating  251  may be a polymer coating. 
     The first metal part  151  may comprise multiple layers  231  as shown with reference to  FIG. 26 . The multiple layers  231  may be folded sheets of the same metal, layers of the same metal, or layers of different metals. Alternatively or additionally, the second metal part  152  may comprise multiple layers  232 . The multiple layers  232  may be folded sheets of the same metal, layers of the same metal, or layers of different metals. The layers  231  may comprise the same metal as the layers  232 , or different metals. The weld  3  is shown joining the first metal part  151  to the second metal part  152 . The weld  3  is shown partially penetrating the second metal part  152 . 
       FIG. 27  shows a laser weld  275  comprising a weld pool  270  between the first metal part  151  and the second metal part  152  using prior art techniques, including for example, laser welding with a green laser using a single high-energy pulse of 100 mJ or more, or welding with a quasi continuous wave fibre laser. The weld  275  has a similar overall size as the weld  3  shown in  FIG. 1 . Consequently, the weld pool  270  is considerably larger than the microwelds  8  when molten shown with reference to  FIGS. 1 to 5 and 7 to 14 , has a higher thermal mass, and will take a longer time to cool down. This results in metallic mixing. However if the mixing is not good enough, then this results in the formation of an associated boundary layer  271 , which when welding dissimilar metals, contains intermetallics that can be brittle. There is also an area around the weld pool  270  that is affected by the heat but where the metals have not flowed—the so-called heat affected zone (HAZ)  272 . The mechanical properties of the heat affected zone  272  can be substantially degraded as a result of thermal heat tempering, and should generally be minimized. The heat affected zone  272  is generally visible (eg after etching with acid) on both the top surface  273  of the first metal part  151  and the bottom surface  274  of the second metal part  152 . 
     The boundary layer  271 , when welding steel to steel, can result in carbon formation along grain boundary interfaces, thereby providing a pathway for fracturing the weld  3 . Similarly, the boundary layer  271  when welding dissimilar metals may comprise intermetallics with a grain structure reflecting the cooling time from fusion to solidification. Such intermetallics are often brittle in nature, and therefore represent a weak point in the weld pool  270 . Thus the existence of the boundary layer  271  and heat affected zone  272  are not desirable in either the welding of similar metals or the welding of dissimilar metals. 
     Whether the weld  275  is formed from similar metals or dissimilar metals, the mechanical properties of the material comprising the weld  275  are likely to be weaker than the properties of the base materials that comprise the first metal part  151  and the second metal part  152 . Heat affected zones  272  are also of a concern if they affect the appearance or chemical composition of the first and second metal parts  151 ,  152 . 
     The problems associated with intermetallic layers  271  and heat affected zones  272  increase when welding thin sheet metals (less than 1 mm). Other issues concerning the time taken for welds to cool down include damage to coatings such as polymers on the first and second metal parts  151 ,  152 . 
       FIG. 28  depicts a top view of the weld  3  shown in  FIG. 1 . Here the weld  3  is circular, achieved by rastering the laser beam  62  around on the metal surface  6 . A heat affected zone  281  is usually visible (possibly after chemical etching). However, with proper selection of the laser  61  and the laser pulse parameters shown with reference to  FIGS. 16 and 17 , there is generally no heat affected zone visible on the bottom surface. This is because the microweld  8  has significantly less mass than the weld pool  270 , and consequently cools more rapidly. Similarly, there is little or no evidence of intermetallic layers  271  surrounding the microwelds  8 . These features provide great advantages over prior art welding techniques. 
     Referring to  FIGS. 16 and 17 , the method of the invention can be one in which the pulse repetition frequency  167  is greater than 10 kHz, and the spot size  174 , the pulse fluence  176 , the pulse width  166 , and the pulse repetition frequency  167  are selected such that at least one of the first material  1  and the second material  2  resolidifies between successive laser pulses  161  thereby inhibiting the formation of an intermetallic phase in the weld  3 . The pulse repetition frequency  167  may be greater than 100 kHz and may be greater than 200 kHz. The pulse repetition frequency  167  may be greater than 500 kHz. 
     The spot size  174  may be less than 100 μm. The spot size  174  may be less than 60 μm. The first or the second material  1 ,  2  may have a higher melting temperature than the other material. The first material  1  may have a reflectivity  145  greater than 90% at an optical wavelength  140  of one micron. 
     The second metal part  152  shown in  FIG. 29  may comprise a metal part  292  which is coated with a coating  293 . The coating  293  may be a metal plating such as nickel or chrome, or may be a chemically-induced coating such as an anodization. The first metal part  151  may be a tab  291  such as found in beverage cans. The tab  291  is shown welded to the second metal part  152  with the weld  3 . 
     Beverage cans are often made from thin sheets of aluminium (the second metal part  152 ) that are less than 250 μm in thickness. In a beverage can, the coating  293  would be a polymer coating usually applied before the weld  3  is formed. It is important that the method of forming the weld  3  does not degrade the coating  293 . The apparatus and method of the present invention achieves this by virtue of the microweld  8 , shown with reference to  FIGS. 1 to 24 , as there is less heat generated in the second metal part  152  compared to a prior art weld. 
       FIG. 30  shows a graph of pulse fluence  176  and absorbed energy density  303 , where the absorbed energy density  303  is the total pulse energy  165  absorbed by the first and the second metal parts  151 ,  152  per unit surface area by the laser pulses  161 . In order to initiate the weld  3  shown with reference to  FIGS. 1 to 5, 7 to 15, 18 to 24, and 25 , it is necessary to use a pulse fluence  176  that is at least equal to the first pulse fluence threshold  301 . This is in order to initiate the melting of the metal surface  6 . Once the metal surface  6  has begun to melt, the remaining pulses  161  should have a pulse fluence  176  that is at least equal to the second pulse fluence threshold  302 . The second pulse fluence threshold  302  can be substantially less than the first pulse fluence threshold  301 . As each of the pulses  161  is absorbed, they contribute to the absorbed energy density  303 . The absorbed energy density  303  absorbed at each of the focussed locations  16  should be at least equal to the first energy density threshold  304  at which the microweld  8  begins to penetrate the second metal part  152 , but less than the second energy density threshold  305  at which the weld  3  becomes unacceptably brittle. If too much energy is absorbed by the weld  3 , there will be excessive heating of the first and the second materials  1 ,  2 , resulting in sufficient time for intermetallics to form and a weak weld  3 . It can be seen that by varying the pulse parameters shown with reference to  FIGS. 16 and 17 , the number of pulses  161 , and the distances  181  between focussed spots  12 , there is a great controllability of the weld  3 , and moreover, greater control over its formation, and therefore mechanical properties, than prior art techniques. The preferred values will vary for different materials, and thicknesses of materials, and can be found by experimentation. 
     The method described with reference to  FIGS. 15 and 21  may include the step of remelting at least one of the first and the second materials  1 ,  2  with the laser  61 . This can improve the cosmetic appearance of the weld  3 , and also improve physical characteristics such as shear strength, peel strength, porosity, and ohmic resistance. 
     In Examples 1 and 2, provided below, the laser  61  was a nanosecond ytterbium-doped fibre laser, model SPI G4 70 EP-Z manufactured by SPI Lasers UK Ltd of Southampton, England. The laser  61  is the master oscillator power amplifier described with reference to  FIGS. 35 and 36 . The beam quality  146  had an M 2  value of approximately 1.6. The scanner  67  was a galvanometer-scanner model Super Scan II manufactured by Raylase of Munich, Germany with a 10 mm beam aperture (not shown). It can be controlled with a controller (not shown) such as a desktop computer with a Windows 8 operating system on which SCAPS scanner application software licensed by SCAPS GmbH of Munich, Germany. This can be used to program, operate, and store code for steering the laser beam  62 . The lens  68  was a 163 mm focal length F-theta lens. 
     The above equipment can be used to form and translate the laser beam  62  onto the top surface  6  of the first material  1  with a focused spot having a spot size  174  (1/e 2  diameter) of 40 μm and an area  175  of 1.256×10 −5  cm 2 . 
     Example 1 
       FIG. 15  shows an artistic impression of a cross-section through a weld  310  formed between copper having a thickness  143  of 100 μm and aluminium having a thickness  144  of 400 μm. The weld  310  was in the shape of the spiral, shown with reference to  FIG. 2 , with a first separation  10  of 50 μm between the spiral arms  9 , and a diameter  4  of 1 mm. The width  74  of the hole  71  was approximately 5 μm to 20 μm. The weld  310  was formed using multiple pulses  161  from the laser  61 , which pulses  161  overlayed each other on the first material  1  by approximately 95% to 98% in area. The laser  61  has cut the first material  1 , which is copper, and the second material  2  (aluminium) has flowed into the hole  71 . At least some of the first material  1  has been injected into the second material  2 , as evidenced by the zones  121  that comprise the first material  1 . The zones  121  extend to approximately 300 μm to 400 μm into the second material  2 . Voids  122  are also present. A heat affected zone  281 , shown by the approximately triangularly-shaped dashed line of depth  311 , is present under the holes  71 . Only one of the heat affected zones  281  is shown for clarity. This heat affected zone  281  resembles a heat stake that is commonly seen when welding thermoplastic parts together. 
     The weld  310  has excellent shear resistance, as evidenced by a shear test.  FIG. 32  illustrates the failure mode when three welds  310  of the type shown in  FIG. 31  were sheared. The first material  1  failed around the welds  310 , and not through the welds  310 , thus indicating that the welds  310  were stronger than the surrounding material. This is an unexpected result, and shows the importance of being able to flow the second material  2  into the hole  71  without forming characteristically brittle intermetallics. 
     The weld  310  has surprisingly good shear resistance, and excellent ohmic resistance. This makes the welding process of the invention as described with reference to  FIGS. 15, 21 and 31 , suitable for joining sheets of first material  1  and second material  2  with welds  3 , wherein the weld  3  provides electrical contact between the first material  1  and the second material  2 . In the example of  FIG. 33 , the first material  1  is copper, and the second material  2  is aluminium, a combination of materials that is often found in batteries. 
     Additional peel strength would be obtainable by increasing the countersinking of the hole  71  as shown in  FIG. 7 . 
     Example 2 
       FIG. 34  shows an artistic impression of a cross section of a weld  340  between a first material  1  copper and a second material  2  brass. The weld  340  was also formed in a similar spiral to the weld  310  shown with reference to  FIG. 13 . It is surprising that the brass has flowed into the copper material to form the weld  340  with very little intermetallic mixing. The weld  340  is substantially inhomogeneous. The copper and the brass have flowed, but have not mixed together to form new homogeneous material phases. The material phases of the copper and the brass are largely unmixed, with the copper and the brass being in their original material phases. This is particularly surprising given that brass is an alloy of copper and zinc. There are zones  121  of the first material  1  contained within the second material  2 . There are also voids  122 . The resulting joint formed by the weld  340  has excellent shear strength. 
     Examples 3 to 10 
     The laser  61  used in Examples 3 to 10 was a nanosecond ytterbium-doped fibre laser, model SPI G4 70 W HS-H manufactured by SPI Lasers UK Ltd of Southampton, England. The laser is substantially similar to the laser used in Examples 1 and 2, though with a poorer beam quality  146 , which was increased from approximately M 2 =1.6 to approximately M 2 =3. The spot size  174  was approximately 80 μm, which is approximately twice as large as obtained with the higher brightness laser used in Examples 1 and 2. Similar waveforms are provided with the laser as were described with reference to Table 1 and  FIGS. 25 and 36 . 
     Table 2 shows details of the welds  3  in Examples 3 to 10. The first metal listed in each example was the first material  1 , and the second metal listed was the second material  2 . 
     The welding pattern  5  was the rectangular hatching of  FIG. 3 . The first separation  10  and the third separation  32  were both equal to each other, and were varied between 0.2 mm and 2 mm. The optimum value was found to be approximately 0.5 mm in each of the Examples 3 to 10. 
     The characteristic feature size  7  of the microweld  8  was the width of the microwelds, which was approximately 60 μm to 250 μm depending on the materials used. 
     The width  4  of the welds  3  was between 1.5 mm and 5 mm, depending on the metals and their thickness. Larger widths were used on the thicker metals. 
     Argon was used as the shield gas  155  in Examples 5 to 10. There was no shield gas used in Examples 3 and 4. The nickel alloy was an austenite nickel-chromium iron alloy that is sold under the trade name INCONEL 718. The stainless steel was a molybdenum-bearing grade, austenitic stainless steel under the trade name SS316. 
     In Table 2, the first column shows the materials that were welded together. In each Example, the first metal stated was the first material  1 , and the second metal stated was the second material  2 . The thicknesses  143 ,  144  of the first and second materials  1 ,  2  are shown as the size in mm. 
     In each Example, there were two passes of the laser beam  61  in the same pattern  5  shown with reference to  FIG. 3 . The parameters of the first pass are shown in the first line of each Example, and the parameters of the second pass are shown in the second line of each Example. The parameters were varied to optimize the appearance and the strength of the welds  3 , and the optimized parameters are shown in the table. 
     The first pass had a higher peak power  162  than the peak power  162  of the second pass. The first pass created holes  71  in the first material  1  as shown with reference to  FIG. 8 . The holes  71  may also extend into the second material  2 . The first pass can also create a weld  3 . However most of the welds  3  created by the first pass could easily be broken, were in general not strong, and had poor appearance. The second pass melted at least one of the first and the second materials  1 ,  2 . If the second material  2  melted in preference to the first material  1 , then the second material  2  flowed into the hole  71  as described with reference to  FIG. 9 . However if the first material  1  melted in preference to the second material  2 , then the first material  1  flowed into the hole  71 , which hole  71  may extend into the second material  2 . The result in each of the Examples 3 to 10 was a weld  3  that was substantially stronger than achieved with the first pass. It is believed that this is because the formation of intermetallics was inhibited. In addition, the second pass cleaned the surface  6  giving the weld  3  a smooth and clean appearance. 
     In each of the Examples save for Example 9, the first pass had a peak power  162  of 13 kW at a pulse repetition frequency  167  of 266 kHz. For Example 9, the first material  1  was copper, and the first pass was performed with a slower scan speed and at a peak power  162  of 25 kW. A slower scan speed was also required in Examples 4 and 10 where the second material  2  was copper; copper has a high reflectivity  145 . It was not necessary to decrease the scan speed in Example 7. Without wishing to limit the scope of the invention, it is believed that this may be because titanium has a higher melting point than copper. 
     The laser parameters used in the second pass were varied in order to optimize the strength and appearance of the welds  3 . Surprisingly, good welds could be produced with continuous wave signals in each case. However, a higher frequency waveform produced stronger welds in Examples 3 to 9. In Examples 4 to 9, the pulse repetition frequency  167  was 600 kHz, resulting in pulses  161  having approximately 44% of the pulse energy  165  than in the first pass. In Example 3, as a result of the lower average power used, the pulse energy  165  in the second pass was 32% of the pulse energy  165  of the first pass. It is believed that the lower pulse energies resulted in less vapour pressure being generated when the laser beam  62  was absorbed during the second pass. The second pass for Example 10 was made using a continuous wave signal having a peak power  162  equal to the average power  163  of 50 W. The scan speed was 20 mm/s, which was lower than the scan speed of 30 mm/s of the first pass. It was necessary to use a relatively slow scan speed (20 to 25 mm/s as compared to 75 to 80 mm/s) for the second pass in Examples 4, 9 and 10, all of which involved welding copper. The scan speed for the second pass was 80 mm/s when welding titanium to copper, Example 7. 
     The welds  3  produced by Examples 3 to 10 have a very different appearance from prior art welds. By taking advantage of the variety of pulse waveforms obtainable from the laser, it was possible to obtain strong welds from materials, such as stainless steel to aluminium, that have hitherto been difficult to weld. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Process Parameters used in Examples 3 to 10 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 FWHM 
                 10% 
                   
                   
                 Aver- 
               
               
                   
                   
                 Scan 
                 Pulse 
                 Pulse 
                 Peak 
                   
                 age 
               
               
                   
                 Size 
                 speed 
                 Width 
                 Width 
                 Power 
                 PRF 
                 Power 
               
               
                 Example 
                 (mm) 
                 (mm/s) 
                 (ns) 
                 (ns) 
                 (kW) 
                 (kHz) 
                 (W) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                  3. Aluminium 
                 0.1 
                 100 
                 20 
                 30 
                 13 
                 266 
                 70 
               
               
                 to Brass 
                 0.3 
                 80 
                 12 
                 10 
                 8 
                 600 
                 50 
               
               
                  4. Aluminium 
                 0.1 
                 30 
                 20 
                 30 
                 13 
                 266 
                 70 
               
               
                 to Copper 
                 0.4 
                 25 
                 12 
                 10 
                 8 
                 600 
                 70 
               
               
                  5. Stainless 
                 0.15 
                 100 
                 20 
                 30 
                 13 
                 266 
                 70 
               
               
                 Steel to 
                 0.5 
                 75 
                 12 
                 10 
                 8 
                 600 
                 70 
               
               
                 Aluminium 
                   
                   
                   
                   
                   
                   
                   
               
               
                  6. Titanium to 
                 0.12 
                 160 
                 20 
                 30 
                 13 
                 266 
                 70 
               
               
                 Aluminium 
                 0.5 
                 120 
                 12 
                 10 
                 8 
                 600 
                 70 
               
               
                  7. Titanium to  
                 0.12 
                 160 
                 20 
                 30 
                 13 
                 266 
                 70 
               
               
                 Copper 
                 0.4 
                 80 
                 12 
                 10 
                 8 
                 600 
                 70 
               
               
                  8. Aluminium 
                 0.1 
                 120 
                 20 
                 30 
                 13 
                 266 
                 70 
               
               
                 to Nickel alloy 
                 0.5 
                 40 
                 12 
                 10 
                 8 
                 600 
                 70 
               
               
                  9. Copper to 
                 0.1 
                 30 
                 24 
                 250 
                 25 
                 55 
                 70 
               
               
                 Nickel alloy 
                 0.5 
                 20 
                 12 
                 10 
                 8 
                 600 
                 70 
               
               
                 10. Stainless 
                 0.15 
                 30 
                 20 
                 30 
                 13 
                 266 
                 70 
               
               
                 Steel to Copper 
                 0.4 
                 20 
                 CW 
                 CW 
                 50 W 
                 CW 
                 50 
               
               
                   
               
            
           
         
       
     
     Example 11 
     Other than as stated below, the welds described in Examples 11 to 13 were made using the same apparatus as used for Examples 1 and 2. With reference to  FIG. 15 , the first material  1  was copper grade C110 with a 150 μm thickness, and the second material  2  was aluminium grade 5052 with a 500 μm thickness. Following experimentation to determine the peak power  162 , the pulse shape  164 , the pulse energy  165 , the pulse width  166 , and the pulse fluence  176 , it was decided to scan the laser beam  62  at a linear speed of 50 mm/s over the metal surface  6  and with the distance  181  (shown with reference to  FIG. 18 ) between successive of the focussed spots  12  of 0.7 μm (measured centre to centre). This corresponds to the pulse repetition frequency  167  of 70 kHz. The appropriate control parameters were then fed into the controller  153  and the laser  61  set up accordingly, The laser beam  62  was repetitively pulsed at the pulse repetition frequency  167  of 70 kHz, and scanned over the metal surface  6  in the spiral  22  shown with reference to  FIG. 10 . The spiral  22  was formed with a 50 mm/s linear speed. The total length of the spiral  22  was 15.8 mm, and was formed from the inside  22  to the outside  24  of the spiral. The diameter  4  of the weld  3  was 1 mm. The pulse width  166  was 115 ns at full width half maximum FWHM. The pulse width  169  was 520 ns at 10% of peak power  162 . Total pulse energy  165  was 1 mJ with an average power  163  of 70 W and a peak power  162  of 5 kW. Each laser pulse  161  had a peak power intensity of 3.98×10 +8  W/cm 2  with a pulse fluence  176  of 79.6 J/cm 2 . A shield gas mixture  155  was used of 50% Argon and 50% Helium. The gas supply  156  was a 6 mm diameter copper nozzle that was placed over the weld  3 . The gas was supplied through a flow control regulator at 10 cubic feet per hour. The weld  3  that was formed is of the type shown in  FIGS. 2 and 15 . The heat stakes  17  form a continuous line along the spiral  22 , and are at least partially separated in a radial direction  25  across the spiral  22 , corresponding to the direction  155  shown in  FIG. 15 . The weld pools  19  are continuous across the entire surface area of the weld  3 , though as shown in  FIG. 15 , the surface of the weld  3  is not smooth. Observation of the welds  3  revealed aluminium colouring on the top surface  6 , indicating that the aluminium has melted and has flowed. The copper and aluminium have at least partially mixed in the weld  3 . The welds  3  were observed to be extremely strong for their size. 
     Example 12 
     With reference to  FIG. 15 , the first material  1  was copper grade C110 with a 150 μm thickness  143 , and the second material  2  was also copper grade C110 with a 150 um thickness  144 . After experimentation, it was determined that the same process parameters could be used as described with reference to Example 11. The resulting welds were observed to be extremely strong for their size. 
     Example 13 
     With reference to  FIG. 15 , the first material  1  was stainless steel grade 304 with a 250 μm thickness  143  and the second material  2  was grade stainless steel 304 with a 250 μm thickness  144 . Following experimentation to determine the peak power  162 , the pulse shape  164 , the pulse energy  165 , the pulse width  166 , and the pulse fluence  176 , it was decided to scan the laser beam  62  at a linear speed of 225 mm/s over the metal surface  6  and with the distance  181  (shown with reference to  FIG. 18 ) between successive of the focussed spots  12  of 0.225 μm (measured centre to centre). This corresponds to the pulse repetition frequency  167  of 1 MHz. The appropriate control parameters were then fed into the controller  153  and the laser  61  set up accordingly, The laser beam  62  was repetitively pulsed at the pulse repetition frequency  167  of 1 MHz, and scanned over the metal surface  6  in the spiral  22  shown with reference to  FIG. 2 . The spiral  22  was formed with a 225 mm/s linear speed. The spiral  22  was formed from the inside  22  to the outside  24 . The diameter  4  of the weld  3  was 1 mm. The pulse width  166  was 9 ns at full width half maximum FWHM. The pulse width  168  was 9 ns at 10% of the peak power  162 . Total pulse energy  165  was 7 μJ with an average power  163  of 70 W and a peak power  162  of 8 kW. Each laser pulse  161  had a peak power intensity  179  of 6.36×10 +8  W/cm 2  with a pulse fluence  176  of 5.6 J/cm 2 . A shield gas mixture  155  was used of 50% Argon and 50% Helium supplied thorough a low control regulator at 10 cubic feet per hour from a 6 mm diameter copper nozzle over the weld  3 . The weld  3  that was formed is of the type shown in  FIGS. 2 and 15 . The heat stakes  17  extended from the weld  3  in the form a continuous line along the spiral  22 , and are at least partially separated in a radial direction  25  across the spiral, corresponding to the direction  155  shown in  FIG. 15 . The weld pools  19  are continuous across the entire surface  6  of the weld  3 , though as shown in  FIG. 15 , the surface of the weld  3  is not smooth. The top surface of the weld  3  resembled a traditional lap weld, with excellent mixing of the metals, but almost negligible heat affected zone  272  (shown with reference to  FIG. 27 ). However the extension of the heat stakes  17  from the weld  3  was substantially less than observed for the copper aluminium and copper welds of Examples 11 and 12 respectively. The welds  3  were observed to be extremely strong for their size. 
     The present invention also provides a weld  3  according to the method of the invention. 
     The present invention also provides an article when welded according to the method of the invention. Examples of articles are a smart phone, a mobile phone, a laptop computer, a tablet computer, a television, a consumer electronic device; a battery; a solar cell; an integrated electronic circuit component; a printed circuit board; an electrical connection; a low profile electrical connection between flexible circuit elements and thin-section busbars; a metallic enclosure for a medical electronic device; and an electrical connection in consumer electronics devices; metallic labels and tags; silver, platinum, and gold parts in jewellery. 
     It is to be appreciated that the embodiments of the invention given above with reference to the Figures and the Examples have been given by way of example only and that modifications may be effected. Individual components shown in the Figures and individual values shown in the Examples may be used in other Figures and other Examples and in all aspects of the invention.