Laser microwelded part comprising a microweld forming a pattern, method and apparatus for laser welding thereof

Abstract

A weld (1) between a first metallic part (11) and a second metallic part (12), the first metallic part (11) comprising a first metallic material (13), and the second metallic part (12) comprising a second metallic material (14), the first metallic part (11) and the second metallic part (12) overlap by an overlap distance (15) in a region of the weld (1); the weld (1) comprising at least one microweld (5) defined by a width (6) of between 20 μm and 400 μm; the microweld (5) forms a welding pattern (7) defined parallel to a surface (19) of the first metallic part (11); the weld (1) has a width (2) that is at least 0.1mm less than the overlap distance (15); and the weld (1) has a length (3) that is at least 1.5 times the width (2) of the weld (1).

Description

[0001] A Weld

[0002] Field of Invention

[0003] 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, an apparatus for making the weld, and a method for laser welding.

[0004] Background to the Invention

[0005] The joining of bright metals such as gold, copper, aluminium, platinum and silver by laser welding in the near infrared spectrum (800nm to 2500nm) 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’s energy into the metal surface, it is necessary to use laser beams with high power densities.

[0006] 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.

[0007] The challenge is increased when welding thin and low mass work pieces. Such high- power densities are often detrimental, leading to over penetration of the laser beam and resulting unreliable joints. Conversely, if near infrared lasers are operated at lower power densities with beam intensities at or just above the absorbance limits, then this generally results in weak or absent welds as a result of inconsistent and random coupling of the laser beam to the metal surface. 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 use of quasi continuous wave fibre lasers, or the use of nanosecond pulsed fibre lasers. Green lasers have advantages because the reflectivity of bright metals is usually significantly lower at green than at near infrared wavelengths. Quasi continuous wave fibre lasers have advantages because each weld pulse can be produced using a single millisecond laser pulse. Nanosecond pulsed fibre lasers have advantages in that each weld is typically formed using hundreds of pulses, and the total pulse energy used in forming the weld can be accurately controlled thus enabling a reduced heat affected zone. The specific laser chosen is dependent on the application, for example, the reflectivity of the metal being welded, the cosmetic appearance of the weld, how fast the weld can be produced, and the strength of the weld which is often determined by its intermetallic content.

[0008] A particular problem occurs when welding two materials with an overlap weld where the overlap has a narrow width, for example, less than 1 mm. Prior art solutions to form an overlap weld include forming multiple spot welds along the length of the overlap, wobble welding along the length of the overlap, or hatching along the length of the overlap. However, it has been found difficult to produce welds that are sufficiently strong and repeatable using these processes.

[0009] 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.

[0010] There is a need for an overlap weld between bright and / or dissimilar metals and alloys that can be formed on narrow overlaps, and which can be formed reliably and repeatably, and it is an aim of the present invention to provide such a weld. The Invention

[0011] Accordingly, in one non-limiting embodiment of the present invention there is provided a weld between a first metallic part and a second metallic part, the first metallic part comprising a first metallic material, and the second metallic part comprising a second metallic material,

[0012] • the first metallic part and the first metallic part overlap by an overlap distance in a region of the weld;

[0013] • the weld comprising at least one microweld defined by a width of between 20 pm and 400 pm;

[0014] • the microweld forms a welding pattern defined parallel to a surface of the first metallic part;

[0015] • the weld has a width that is at least 0.1 mm less than the overlap distance; and

[0016] • the weld has a length that is at least 1 .5 times the width of the weld;

[0017] 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 m wavelength window, and with pulse energies of around 1 mJ, is being used extensively in the consumer electronics industry. The present invention extends the performance by creating a weld that has important directional strength characteristics. The weld has advantages in applications which have limited space available for the joint or where directional strength is desirable. In addition, the present invention can join a first metallic part and a second metallic part together when the overlap distance is less than 3 mm, less than 1 mm, and even less than 0.5 mm. This is difficult to do with sufficient depth and therefore strength of the weld with prior art methods. A join between the first metallic part and the second metallic part can comprise a single weld or multiple welds. Surprisingly, the weld of the present invention enables much stronger and reliable welds to be formed than can be achieved with multiple circular spot welds, wobble welding, or hatching. The welding pattern may comprise a line in the form of a spiral. A distance between adjacent turns of the spiral may reduce towards the outside of the welding pattern.

[0018] The welding pattern may comprise at least one ring. A distance between adjacent rings may reduce towards the outside of the welding pattern.

[0019] The overlap distance may be less than 3 mm.

[0020] The first metallic material and the second metallic material may remain substantially unmixed in the weld. Advantageously, this can reduce the formation of intermetallics and enables stronger welds to be produced when the first metallic material is different from the second metallic material. 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 by weight, and preferably at most ten percent by weight 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 recrystallization.

[0021] The weld may be inhomogeneous. An inhomogeneous weld is characterised in that its composition varies substantially within the weld.

[0022] The weld may be characterized by a depth which depth varies by less than 50 pm across a proportion of its surface area, wherein the proportion is greater than 75%, preferably greater than 90%, and more preferably greater than 95%.

[0023] The first metallic material may have a reflectivity greater than 90% at an optical wavelength of one micron.

[0024] The first metallic part may have a thickness in a region of the weld of between 0.2 mm to 1 mm.

[0025] The first metallic material may comprise copper, aluminium, nickel, tungsten, iron, bronze, brass, steel, or stainless steel. The second metallic material may comprise copper, aluminium, nickel, tungsten, iron, bronze, brass, steel, or stainless steel.

[0026] Either or both of the first and second metallic materials may be coated, for example with nickel. Nickel coated steel and nickel coated stainless steel is often used in commercial electronic devices.

[0027] The weld is preferably autogenous in order to avoid adding supplementary materials or powders to form the weld.

[0028] The invention also provides an article, characterized in that, the article comprises at least one weld according to the present invention.

[0029] The article may be in the form of 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.

[0030] The invention also provides an apparatus for forming a weld between a first metallic part and a second metallic part, which apparatus comprises a laser, an optical fibre, a collimator, a scanner, a lens, and a controller:

[0031] • the laser is a pulsed laser;

[0032] • the laser is configured to emit a laser beam which is coupled to a surface of the first metallic part via the optical fibre, the collimator, the scanner and the lens;

[0033] • the controller is configured to control the laser and the scanner such that the scanner moves the laser beam on the surface of the first metallic part to form the weld; the apparatus is characterized in that:

[0034] • the apparatus includes a coupler configured to couple an optical signal emitted by the weld from the optical fibre to a detector, which detector feeds back an electronic signal indicative of a temperature of the weld as the weld is being formed; the apparatus being adapted to create a weld between the first metallic part and the second metallic part when:

[0035] • the first metallic part and the second metallic part overlap by an overlap distance in a region of the weld;

[0036] • the weld comprising at least one microweld defined by a width of between 20 pm and 400 pm;

[0037] • the microweld forms a welding pattern defined parallel to a surface of the first metallic part,

[0038] • the weld has a width that is at least 0.1 mm less than the overlap distance, and:

[0039] • the weld has a length that is at least 1 .5 times the width of the weld.

[0040] The apparatus may include an electronic filter configured to filter an output of the detector.

[0041] The controller may control at least one of a power of the laser beam, a focus of the laser beam, a trajectory of the laser beam, and a velocity of the laser beam with respect to the surface of the first metallic part in order to create a weld that has a depth that varies by less than 50 pm across a proportion of its surface area, wherein the proportion is greater than 75%, preferably greater than 90%, and more preferably greater than 95%.

[0042] The apparatus of the invention may alternatively or additionally include parts or configurations as required to utilize the above mentioned optional aspects of the weld of the invention.

[0043] The invention also provides a method for laser welding a first metallic part to a second metallic part, the first metallic part comprising a first metallic material, and the second metallic part comprising a second metallic material, which method comprises:

[0044] • placing the first metallic part onto the second metallic part such that the first metallic part and the second metallic part overlap by an overlap distance;

[0045] • providing a laser for emitting a laser beam in the form of laser pulses;

[0046] • focusing the laser pulses onto a surface of the first metallic part; 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 wherein the microweld is defined by a width of between 20pm and 400pm; and

[0047] • the moving of the laser beam with respect to the metal surface is such that the weld has a width at least 0.1 mm less than the overlap distance and a length that is at least 1 .5 times the width of the weld.

[0048] The welding pattern may comprise a line in the form of a spiral.

[0049] The weld may be started at the innermost end of the spiral welding pattern or from the outside of the weld.

[0050] The weld may be started by scanning the laser beam one or more times around a ring on the outermost side of the welding pattern.

[0051] A distance between adjacent turns of the spiral may reduce towards the outside of the spiral. This is advantageous if the weld manufacturing process starts at the outside of the spiral and continues towards the innermost end of the spiral. The narrower spacing at the outside of the spiral enables heat to be accumulated to form the weld.

[0052] Alternatively, a distance between adjacent turns of the spiral may reduce towards the innermost end of the spiral. This is advantageous if the weld manufacturing process starts at the innermost end of the spiral and continues to the outside of the spiral. The narrower spacing at the innermost end of the spiral enables heat to be accumulated to form the weld.

[0053] The overlap distance may be less than 3 mm.

[0054] The first metallic material and the second metallic material may remain substantially unmixed in the weld.

[0055] The weld may be substantially inhomogeneous.

[0056] The first metallic material may have a reflectivity greater than 90% at an optical wavelength of one micron.

[0057] The first metallic part may have a thickness in a region of the weld of between 0.2 mm to 1 mm. The first metallic material may comprise copper, aluminium, nickel, tungsten, iron, bronze, brass, steel, or stainless steel.

[0058] The second metallic material may comprise copper, aluminium, nickel, tungsten, iron, bronze, brass, steel, or stainless steel.

[0059] Either or both of the first and second metallic materials may be coated, for example with nickel. Nickel coated steel and nickel coated stainless steel is often used in commercial electronic devices.

[0060] The laser beam may be moved with a higher velocity with respect to the surface of the first metal part towards an outside of the spiral than an innermost end of the spiral.

[0061] The method may include the step of monitoring a temperature of the weld as it is being formed. The velocity of the laser beam may be adjusted with respect to the surface of the first metal part such that the rate of heat input into the weld is controlled. This is advantageous because it allows for the control of the weld profile.

[0062] The method may include the step of adjusting a velocity of the laser beam with respect to the surface of the first metal part.

[0063] The weld may be characterized by a depth which depth varies by less than 50 pm across a proportion of its surface area. The proportion may be greater than 75%, preferably greater than 90%, and more preferably greater than 95%. The ability to control the energy in the laser pulses, the size of the laser beam when projected onto the surface, and the velocity of the laser beam as it moves across the surface enables great control of the depth of the microweld or microwelds as they are being formed. Increasing the depth of the microwelds and reducing the variation in the depth increases both the peel strength and the shear strength of the weld.

[0064] The laser pulses may have pulse widths between 100ps and 1000ns. This is advantageous because it provides great control over the heat input into the weld as it is being formed, and enables precise control over the depth and profile of the weld.

[0065] The laser pulses may have pulse energies less than 10mJ. The laser may be characterized by a beam quality M2less than 6, preferably less than 2, and more preferably less than 1 .3.

[0066] The laser may be a nanosecond laser.

[0067] The method of the invention may alternatively or additionally include a step or steps as required to utilize the above mentioned optional aspects of the weld or the apparatus of the invention.

[0068] Brief Description of the Drawings

[0069] Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings wherein:

[0070] Figure 1 shows a weld according to the present invention;

[0071] Figure 2 shows an apparatus for forming the weld according to the present invention;

[0072] Figures 3 to shows a spiral weld in the form of a superellipse and wherein the distance between adjacent spiral turns increases away from the outside of the weld;

[0073] Figure 4 shows a weld comprising five concentric elliptical rings, and wherein the distance between adjacent rings increases away from the outside of the weld;

[0074] Figure 5 shows a weld in the form of a superellipse comprising five concentric rings, and wherein the distance between adjacent rings reduces away from the outside of the weld;

[0075] Figure 6 shows a weld in the form of a superellipse comprising five concentric rings and wherein the distance between adjacent rings increases away from the outside of the weld;

[0076] Figure 7 shows an elliptical weld comprising a spiral and a ring, wherein the distance between adjacent rings increases away from the outside of the weld;

[0077] Figure 8 shows an elliptical weld wherein the weld has a uniform depth across a proportion of its surface area;

[0078] Figure 9 shows two pieceparts welded together with eight circular spiral welds not according to the invention; Figure 10 shows the two pieceparts welded together with four elliptical spiral welds aligned parallel to the weld;

[0079] Figure 11 shows the two pieceparts welded together with four circular welds not according to the invention;

[0080] Figure 12 shows the two pieceparts welded together with four elliptical welds aligned perpendicularly to the weld;

[0081] Figure 13 shows two pieceparts welded together on a circular flange with five circular welds not according to the invention;

[0082] Figure 14 shows two pieceparts welded together on a circular flange with three elliptical welds comprising an elliptical spiral microweld;

[0083] Figure 15 shows two pieceparts welded together on a circular flange with three elliptical welds having concentric elliptical microwelds; and

[0084] Figure 16 shows two pieceparts welded together on a circular flange with four welds having a microweld in the form of a spiral racetrack.

[0085] Detailed Description of Preferred Embodiments of the Invention

[0086] A weld according to the invention will now be described solely by way of example and with reference to Figure 1 . Figure 1 shows a weld 1 between a first metallic part 11 and a second metallic part 12, the first metallic part 11 comprising a first metallic material 13, and the second metallic part 12 comprising a second metallic material 14. The first metallic part 11 and the second metallic part 12 overlap by an overlap distance 15 in a region of the weld 1 . The weld 1 comprising at least one microweld 5 defined by a width 6 of between 20 pm and 400 pm. The microweld 5 forms a welding pattern 7 defined parallel to a surface 19 of the first metallic part 11 . The weld 1 has a width 2 that is at least 0.1 mm less than the overlap distance 15. The weld 1 has a length 3 that is at least 1 .5 times the width 2 of the weld 1 . Figure 2 shows an apparatus 20 that can be used to form the weld 1 . The apparatus 20 comprises a laser 21 which emits a laser beam 22 which is coupled to the surface 19 of the first metallic part 11 via an optical fibre 23, a collimator 24, a scanner 25 and a lens 26. The laser beam 22 may comprise optical pulses 27. The apparatus 20 comprises a controller 28 which controls the laser 21 and the scanner 25. Alternatively, or additionally, the controller 28 may control a focus of the laser beam 22 by adjusting a position of the lens 26. Thus the laser beam 22 can be projected onto the surface 19 of the first metallic part 11 away from focus, and the distance between the lens 26 and the surface 19 can be varied in order to vary the focus of the laser beam 22. This variation may be used to vary both the size of the laser beam 22 on the surface 19, and the profile of the laser beam 22 on the surface 19. The controller 28 may comprise or be a computer, a microprocessor, a microcontroller, or an electronics card. The scanner 25 moves the laser beam 22 on the surface 19 of the first metallic part 11 to form the weld 1 .

[0087] The apparatus 20 may include a coupler 29 configured to couple an optical signal 30 emitted by the weld 1 from the optical fibre 23 to a detector 31 . The detector 31 is used to feedback an electronic signal 32 indicative of the temperature of the weld 1 as the weld is being formed. An electronic filter 33 may be provided to filter the output of the detector 31 in order to improve signal to noise. The electronic filter 33 may comprise a sampler and a phase sensitive detector synchronized with the optical pulses 27. The coupler 29 may be an optical beam splitter, a fused optical fibre tap coupler, or a device that couples optical power propagating in a cladding of the optical fibre 23 to the detector 31 . Other implementations of the coupler 29 are also possible. Preferably the apparatus 20 includes an optical isolator 34 between the coupler 29 and the laser 21 to isolate the laser 21 from back-reflected optical radiation.

[0088] Preferably the controller 28 controls at least one of a power of the laser beam 22, the trajectory of the laser beam 22, and the velocity of the laser beam 22 with respect to the surface 19 of the first metallic part 11 in order to control the depth 4 of the weld 1 across a proportion 81 of its surface area, wherein the proportion 81 is greater than 75%, preferably greater than 90%, and more preferably greater than 95%. The depth 4 is shown with reference to Figure 1 as the depth of the microweld 5, and the proportion 81 is shown with reference to Figure 8. In certain applications, such as welding copper to nickel-coated stainless steel, the control may be such that the depth 4 varies by less than 50 pm across the proportion 81 of the weld 1 . The microweld 5 is typically created by a process similar to key-hole welding, and often contains one or more regions of porosity 45. The regions of porosity may be voids or air holes, or voids containing partial vacuum. Preferably the power of the laser beam 22, its trajectory, and its velocity with respect to the surface 19 of the first metallic part 11 are controlled such that the regions of porosity 45 are contained as deeply as possible within the second metallic part 12 in order not to compromise the strength of the weld 1 . A penetration depth 46, shown with reference to Figure 1 , of the microweld 5 into the second metallic part 12 of more than 100 pm, preferably more than 200 pm, and more preferably more than 300 pm is often desirable.

[0089] The detector 31 may comprise or be an optical detector. Alternatively, or additionally, the detector 31 may comprise one or more acoustic detectors and / or microphones. The detector 31 can be used detect weld quality features and used to provide the feedback signal 32. The feedback of the signal 32 can be in real time, or can be recorded, and the controller 28 programmed such that the controller 28 provides the same control parameters for making the same type of weld 1 . The controller 28 may store more than one set of control parameters, each set of control parameters being used to form a different type of weld 1 .

[0090] The welding process according to the invention differs significantly from conventional welding processes in that it uses many high-frequency, low-energy pulses 27 to form each weld 1 . The laser pulses 27 typically have energies in the range 0.5 mJ - 10 mJ, and preferably approximately 1 mJ. The pulse repetition frequency of the laser pulses 27 is typically 50 kHz to 200 kHz. A typical weld may take 0.1 - 2 s to be made. Thus the number of laser pulses 27 to form each weld 1 can be in the range 5,000 - 400,000. This is very different from conventional welds where a single laser pulse having an energy of 0.1 J - 10 J is used to form each weld, with a typical pulse width of 1 - 100 ms.

[0091] The lower pulse energy used in the present invention gives benefits in achieving joints with lower heat input and significantly lower levels of melt generation. The laser beam 22 is used to create a high-aspect ratio keyhole in order to form the microweld 5. The keyhole typically has an aspect ratio in the range 10 - 100), with a cross-sectional dimension in the range 10 - 30 pm.

[0092] In order to create a stable keyhole, heat energy is required to be accumulated in the first metal part 11 . The required heat accumulation can be achieved by using one or more of: (i) controlling the pulse to pulse overlap by controlling the speed that the laser beam 22 is scanned over the surface 19 of the first metal part 11 and the pulse repetition frequency; (ii) by applying laser pulses 27 having a tailored temporal pulse shape; (iii) defocussing the laser beam 22 prior to forming the first key hole; (iv) by applying laser radiation from a second laser beam (not shown) that may be a continuous wave laser beam at the same or different wavelength prior to forming the first key hole; (v) controlling at least one of the average power, pulse repetition frequency, and pulse energy of the laser beam 22 as it is scanned over the surface 19 of the first metal part 11 ; (vi) controlling the path of the laser beam 22 as the welding pattern 7 is formed; and (vii) selecting a welding pattern 7 that optimizes the desired properties of the weld 1 such as its shear strength, its peal strength, its porosity, and its heat affected zone.

[0093] The laser beam 22 may be focussed with the lens 26 such that the spot size of the laser beam 22 on the surface 19 of the first material 11 is in the order of 30 - 50 pm (1 / e2diameter). An individual straight microweld 5 will typically not be able to produce a weld with sufficient strength and quality. In order to achieve usable welds, the weld area is enlarged by the use of laser beam manipulation, for example by oscillating the laser beam 22 with the scanner 25 in order to widen the microweld 5 and / or by using the scanner 25 to create the welding pattern 7. Preferably the laser beam 22 is scanned over the surface 19 of the first metal part 11 such that successive microwelds 5 or successive parts of the microweld 5 are adjacent to each other. The welding process may then benefit from heat accumulation from the formation of previous microwelds or sections of the microweld 5. Specifically, heat from the formation of the microweld 5 conducts away from the microweld 5, and preheats subsequent microwelds 5 or sections of the microweld 5 thereby modifying the absorption properties of the surface 19 of the first metal part 11 . This heat accumulation benefits the welding process not only in terms of increasing the penetration depth of the key hole and therefore the depth of the microweld 5, but also in increasing the stability of the welding process by helping to stabilise the keyhole. Whether the weld 1 is formed commencing from the inside of the weld 1 , or from the outside of the weld 1 , it is preferable that the relative velocity of the laser beam 22 increases during the welding process in order to allow the build-up of heat required to achieve the required penetration depth.

[0094] In the case of prior art, small diameter, circular, spiral welds (less than 1 mm), it can be difficult to accumulate sufficient heat to be able to give a required depth and strength as well as a depth 4 across the weld 1 . If too much heat is accumulated, parts of the weld 1 can overheat resulting in vapour pressure of the vaporized material blowing a hole out of the weld 1 . Such holes typically occur in the centre of the weld 1 .

[0095] Surprisingly, the inventors have shown that using an elliptical shaped pattern allows the strength of the weld 1 to be increased compared to a circular weld having the same weld area. It is possible to achieve welds having a deeper penetration and a more uniform weld profile with elliptical welds than with circular welds having the same weld area. Surprisingly, as will be shown in the Examples, the elliptical weld may also be stronger when sheared along its major axis than when sheared along its minor axis.

[0096] The weld 1 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 pm wavelength window, and with pulse energies of around 1 mJ, is being used extensively in the consumer electronics industry. The present invention extends the performance by creating a weld 1 that has important directional strength characteristics. The weld 1 has advantages in applications which have limited space available for the joint or where directional strength is desirable. In addition, the present invention can join a first metallic part 11 and a second metallic part 12 together when the overlap distance 15 is less than 3 mm, less than 1 mm, and even less than 0.5 mm. This is difficult to do with sufficient depth and therefore strength of the weld 1 with prior art methods. A join between the first metallic part 11 and the second metallic part 12 can comprise a single weld 1 or multiple welds 1 . Surprisingly, the weld 1 of the present invention enables much stronger and more reliable joints to be formed than can be achieved with multiple circular spot welds, wobble welding, or hatching.

[0097] The welding pattern 7 may be in the form of a spiral 8 as shown in Figures 1 , 2 and 3. Alternatively or additionally, the welding pattern 7 may comprise one or more rings 10 as shown in Figures 4, 5 and 6. The welding pattern 7 may comprise at least one ring 10 and a spiral 8 as shown in Figure 7.

[0098] Adjacent arms of the spiral 8 or adjacent rings 10 are separated by a distance 16. The distance 16 can decrease from an innermost end 17 of the welding pattern 7 as shown in Figures 1 , 2, 3, 4 and 6, increase as shown in Figure 5, or be uniform across the weld 1 . A smaller distance 16 may be preferred where the weld 1 is first formed in order to accumulate sufficient heat to form the microweld 5. The distance 16 may then increase in order to prevent too much heat being accumulated which can distort and potentially ruin the weld 1 . Controlling the distance 16 and the relative velocity of the laser beam 22 with respect to the surface 19 of the first metallic part 11 enables the strength of the weld 1 to be improved and optimized for a particular application. The distance 16 and the velocity can be found by experimentation.

[0099] The weld 1 can be started from either the outside of the weld or its inside. Thus with reference to Figure 7, the weld 1 can be started by scanning the laser beam 22 one or more times around the ring 10 on the outside of the weld 1 until sufficient heat has been accumulated in order to form the microweld 5. The laser beam is then scanned along the spiral 8 in an inwardly directed spiral pattern to form the microweld 5, the velocity of the laser beam preferably increasing to avoid excessive heat accumulation which can cause overpenetration and porosity defects. The innermost end 17 of the welding pattern 7 is shown as being in the geometrical centre of the weld 1 . It is often advantageous to stop the welding process before the laser beam reaches the geometrical centre of the weld 1 in order to prevent accumulated heat causing undesirable overpenetration and / or evaporation of metal at the centre of the weld 1 . Scanning the laser beam several times around the circle 10 enables the microweld 5 to be optimized at the outer extremity of the weld 1 which increases the peel strength of the weld 1 . This is particularly advantageous when the first metallic material 13 is a highly reflective material such as aluminium and copper. The concentric welds shown in Figures 1 to 6 can also be made from the outside of the welds 1 towards the inside of the welds 1 with similar advantages.

[0100] It is sometimes advantageous to form the weld 1 by starting from the inside of the weld 1 . Thus with reference to Figure 7, the weld 1 can be started at the innermost end 17 of the welding pattern 7 by scanning the laser beam 22 relatively slowly until sufficient heat has been accumulated in order to form the microweld 5. The laser beam is then scanned more rapidly along the spiral 8 to avoid excessive heat accumulation. Optionally, the laser beam 22 can be scanned one or more times around the circle 10. Starting from the inside of the weld 1 , but not necessarily its geometric centre, can be advantageous for joining foils and other thin material sections which are more difficult to weld starting from their outside. The welds 1 shown in Figures 1 to 6 can also be made from the inside of the welds 1 outwardly towards the outside of the welds 1 with similar advantages.

[0101] The weld 1 may be elliptical as shown in Figures 1 , 2 and 4. Alternatively, the weld 1 may be a superellipse as shown in Figures 3, 5 and 6. Other shapes are also possible.

[0102] A superellipse may be described parametrically by: x = a cos2 / r(t) y = b sin2 / r(t) where x and y are the cartesian coordinates, a and b are constants related to the overall dimensions of the superellipse, r is a constant that affects the curvature of the superellipse, and t is a parameter which when varied causes the superellipse to be formed. Superellipses with a = b are also known as Lame curves, and the case with a = b and r = 4 is sometimes known as a squircle. The case with a b (“a” not equal to “b”) and r = 2 is an ellipse. The parameters a, b and r that improve or optimize the weld 1 for a particular application can be found by experimentation.

[0103] Preferably the weld 1 is a superellipse, and the constant r is greater than 1 , more preferably between 1 .5 and 5, and more preferably between 1 .5 and 3. Preferably the semiaxes “a”, “b” of the superellipse are not equal. More preferably the modulus of (a-b) / (a+b) is greater than 20%. For a weld 1 that has a microweld 5 in the form of a spiral 8, the magnitude of the semi-axes “a” and “b” reduce continuously as the microweld 5 moves towards the centre of the weld 1 . For a weld 1 wherein the microwelds 5 form concentric rings 10, the semi-axes “a” and ”b” of the weld 1 are uniform in each ring 10, and reduce in magnitude towards the centre of the weld 1 . Other variations are also possible.

[0104] During formation, the weld 1 will normally have a weld pool on the surface 19 of the first metallic part 11 which weld pool will solidify. The weld pool typically contains melt from more than one arm of the spiral 8 or ring 10. The weld pool may extend over the whole surface of the weld 1.

[0105] The first metallic material 13 and the second metallic material 14 may remain substantially unmixed in the weld 1 . Advantageously, this can reduce the formation of intermetallics and enables stronger welds to be produced when the first metallic material 13 is different from the second metallic material 14. By “substantially unmixed” it is meant that the intermetallic content formed by the first metallic material 13 and the second metallic material 14 combined together in single co-mixed alloy phases comprises at most twenty percent by weight, and preferably at most ten percent by weight of the material of the weld 1 . The intermetallic content at interfaces between the first metallic material 13 and the second metallic material 14 may be sufficient to achieve a joint with pre-determined mechanical properties and ohmic resistivity. The intermetallic content at interfaces between the first metallic material 13 and the second metallic material 14 may be small enough to avoid embrittlement such as caused by recrystallization.

[0106] The weld 1 may be inhomogeneous. An inhomogeneous weld is characterised in that its composition varies substantially within the weld. Creating a weld 1 that is inhomogeneous can reduce the amounts of intermetallics formed, and thus increase the strength of the weld 1 .

[0107] The first metallic material 13 may have a reflectivity greater than 90% at an optical wavelength of one micron.

[0108] The first metallic part 11 may have a thickness 9 in a region of the weld 1 of between 0.2 mm to 1 mm.

[0109] The first metallic material 13 may comprise copper, aluminium, nickel, tungsten, iron, bronze, brass, steel, or stainless steel.

[0110] The second metallic material 14 may comprise copper, aluminium, nickel, tungsten, iron, bronze, brass, steel, or stainless steel.

[0111] Either or both of the first and second metallic materials 13, 14 may be coated, for example with nickel. Nickel coated steel and nickel coated stainless steel is often used in commercial electronic devices.

[0112] The weld 1 is preferably autogenous in order to avoid adding supplementary materials or powders to form the weld 1 .

[0113] The invention also provides an article, characterized in that, the article comprises at least one weld 1 according to the present invention.

[0114] The article may be in the form of 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 invention also provides a method for laser welding a first metallic part 11 to a second metallic part 12, the first metallic part 11 comprising a first metallic material 13, and the second metallic part 12 comprising a second metallic material 14, which method comprises: placing the first metallic part 11 onto the second metallic part 12 such that the first metallic part 11 and the second metallic part 12 overlap by an overlap distance 15; providing a laser 21 for emitting a laser beam 22 in the form of laser pulses 27; focusing the laser pulses 27 onto a surface 19 of the first metallic part 11 ; moving the laser beam 22 with respect to the surface 19; focusing the laser pulses 27 with a spot size and a pulse fluence that cause the formation of at least one microweld 5 in the form of a welding pattern 7 defined parallel to the surface 19 wherein the microweld 5 is defined by a width 6 of between 20pm and 400pm; and the moving of the laser beam 22 with respect to the metal surface 19 is such that the weld 1 has a length 3 that is at least 1 .5 times the width 2 of the weld 1 .

[0115] The moving of the laser beam 22 with respect to the surface 19 may be such that the width 2 is at least 0.1 mm less than the overlap distance.

[0116] The welding pattern 7 may be in the form of a spiral 8 or at least one ring 10.

[0117] A distance 16 between adjacent turns of the spiral 8 or rings 10 may reduce towards the outside of the weld 1 .

[0118] The overlap distance 15 may be less than 3 mm, less than 1 mm, or even less than 0.5 mm.

[0119] The first metallic material 13 and the second metallic material 14 may remain substantially unmixed in the weld 1 .

[0120] The weld 1 may be substantially inhomogeneous.

[0121] The first metallic material 13 may have a reflectivity greater than 90% at an optical wavelength of one micron.

[0122] The first metallic part 11 may have a thickness 9 in a region of the weld 1 of between 0.2 mm to 1 mm.

[0123] The first metallic material 13 may comprise copper, aluminium, nickel, tungsten, iron, bronze, brass, steel, or stainless steel. The second metallic material 14 may comprise copper, aluminium, nickel, tungsten, iron, bronze, brass, steel, or stainless steel.

[0124] Either or both of the first and second metallic materials 13, 14 may be coated, for example with nickel. Nickel coated steel and nickel coated stainless steel is often used in commercial electronic devices.

[0125] The laser beam 22 may be moved with a higher velocity with respect to the surface 19 of the first metal part 11 towards an outside of the weld 1 than an inside of the weld 1 .

[0126] The method may include the step of monitoring a temperature of the weld 1 as it is being formed. The velocity of the laser beam 22 may be adjusted with respect to the surface 19 of the first metallic part 11 .

[0127] The laser pulses 27 may have pulse widths between 100ps and 3000ns, and preferably between 1 ns and 1000ns. This is advantageous because it provides great control over the heat input into the weld 1 as it is being formed, and enables precise control over the depth and profile of the weld 1 .

[0128] The laser pulses 27 may have pulse energies less than 10mJ.

[0129] The laser 21 may be characterized by a beam quality M2less than 6, preferably less than 2, and more preferably between 1 .3 and 1 .7.

[0130] The laser 21 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 in the 1 pm, 1 .5 pm, and 2 pm wavelength windows respectively. The laser 21 may use frequency conversion such as frequency doubling to emit laser radiation at shorter wavelengths.

[0131] Preferably, the laser 21 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 1 . An example of such a laser is the TruPulse nano 2010 laser manufactured by TRUMPF Laser UK Limited of Southampton, England which emits laser radiation in the wavelength range 1059nm and 1065nm. The laser is a master oscillator power amplifier having excellent control over laser parameters including peak power, average power, pulse shape, pulse energy, pulse width, and pulse repetition frequency.

[0132] In Examples 1 - 4 provided below, an array of eight prior-art circular welds (Example 1 ) that had been optimised for weld strength were compared to an array of four elliptical welds (Example 2). These were then compared to an array of four larger circular spots (Example 3), and then four elliptical welds (Example 4) at right angles to those in Example 2. The total aggregate area of the welds in each Example was the same.

[0133] The first metallic part 11 was aluminium having a thickness 9 of 0.3mm. The second metallic part 12 was an alloy of aluminium having a thickness of 0.6mm.

[0134] The lasers 21 were a nanosecond ytterbium-doped fibre laser, model TruPulse nano 2010 (Example 1) and TruPulse nano 2020 (Examples 2 - 4) manufactured by TRLIMPF Laser UK Ltd of Southampton, England. The laser beam 22 had a beam quality M2value of approximately 1 .6.

[0135] The scanner 25 was a galvanometer-scanner model no. SS-IV-[Y] SC-[W]-MT-SX manufactured by Raylase of Munich, Germany. The lens 26 was a 163mm focal length F- theta lens.

[0136] The above equipment was used to form and translate the laser beam 22 onto the surface 19 of the first metallic part 11 with a focused spot having a spot size (1 / e2diameter) of 38pm.

[0137] Tensile strength measurements, including peel strength and shear strength, were made using a DBBMTCL-250kgf-1.5-000 manufactured by Testometric of Rochdale, England.

[0138] EXAMPLE 1

[0139] Eight welds 91 were written in a line as shown in Figure 9. The welds 91 were circular spiral welds written perpendicular to the direction 93 in which the shear strength was later measured. The spirals were made by the laser 21 starting from the outside of each weld 91 and moving inwards. The spacing 92 between adjacent ones of the welds 91 was 2 mm, and the welds 91 had a diameter of 0.7 mm. The distance 16 between successive turns of the spiral was 0.04 mm. The parameters of the laser beam 22 were optimised for weld strength based on data accumulated over the past five years and are shown in Table 1 . The laser scanning speed was 80 mm / s. The measured shear strength was 241 N.

[0140] Table 1 - Laser Beam Parameters

[0141] EXAMPLE 2

[0142] Four welds 103 were written in a line as shown in Figure 10. The welds 103 were elliptical spiral welds. The major axes of the spirals and the orientation of the line of the welds 103 were perpendicular to the direction 93 in which the shear strength was later measured. The spirals were made by the laser 21 starting from the outside of each weld 103 and moving towards the inside of the respective weld 103. The spacing 102 between adjacent ones of the welds 103 was 3 mm, and the welds 103 had a length 3 of 2.8 mm, and a width 2 of 0.7 mm. The distance 16 between successive turns of the spiral across its width 2 was 0.04 mm. The parameters of the laser beam 22 were substantially the same as used in Example 1 . The laser scanning speed was 80 mm / s. The measured shear strength was 335 N.

[0143] EXAMPLE 3

[0144] Four welds 113 were written in a line as shown in Figure 11 . The welds 113 were circular spiral welds. The orientation of the line of the welds 113 was perpendicular to the direction 93 in which the shear strength was later measured. The spirals were made by the laser 21 starting from the outside of each weld and moving inwards. The spacing 112 between adjacent ones of the welds 113 was 4 mm, and the welds 113 had a diameter of 1 mm. The distance 16 between successive turns of the spiral was 0.04 mm. The parameters of the laser beam 22 were the same as used in Example 1 . The laser scanning speed was 80 mm / s. The measured shear strength was 341 N.

[0145] EXAMPLE 4

[0146] Four welds 123 were written in a line as shown in Figure 12. The welds 123 were elliptical spiral welds. The major axes of the spirals was parallel to, and the orientation of the line of the welds 123 was perpendicular to the direction 93 in which the shear strength was later measured. The spirals were made by the laser 21 starting from the outside of each weld and moving inwards. The spacing 122 between adjacent ones of the welds 123 was 3 mm. The welds 123 had a length 3 of 2.8 mm, and a width 2 of 0.7 mm. The distance 16 between successive turns of the spiral across its width 2 was 0.04 mm. The parameters of the laser beam 22 were the same as used in Example 1 . The laser scanning speed was 80 mm / s. The measured shear strength was 380 N.

[0147] The results of Examples 1 - 4 are summarized in Table 2 below. It should be noted that the total aggregate area of the welds in each Example was approximately the same. The shear strength is the combined shear strength of the welds in each Example, and the total time is the total time to make all of the welds in each Example.

[0148] Table 2: Results for Examples 1 - 4.

[0149] The four elliptical welds 103 of Example 2 have 39% higher tensile strength in aggregate than the eight circular welds 91 of Example 1 . Additionally, the total time taken to make the four elliptical 103 welds was approximately 10% less than the total time taken to make the eight circular welds 91 of Example 1 .

[0150] The advantages of using elliptical welds over circular spiral welds are both surprising and significant. Examples 1 and 2 showed the advantages when the overlap distance 15, (the space available to make the weld 1 ), was limited to 0.8 mm. The width 2 of the welds was therefore restricted to 0.7 mm. The results demonstrated that the four elliptical welds of Example 2 had a combined tensile strength 39% higher than the combined tensile strength of the eight circular welds of Example 1 , even though the total weld area was the same. In addition, the total time taken to make the four elliptical welds was 10% less than the total time taken to make the eight circular welds. This is because there was less time taken to scan between welds when making the four elliptical welds of Example 2 compared to the eight circular welds of Example 1 .

[0151] The time taken to manufacture the welds in a high-volume consumer electronics environment is commercially important for at least the reasons that it (i) increases productivity and hence reduces costs, (ii) reduces energy consumption, and (iii) may reduce the total number of workstations required. Thus, the advantages of the elliptical welds can be exploited by having stronger welds, or if the strength of the original circular welds was sufficient, by reducing the area of the elliptical welds to further reduce the time taken to manufacture the article.

[0152] The welds produced in Examples 1 and 2 were cross sectioned and examined under a microscope. It was found that the elliptical welds of Example 2 had a depth 4 which was approximately 33% deeper than the depth 4 of the circular welds of Example 1 . This was an unexpected result which, without wishing to limit the scope of the invention, might be attributable to the greater heat accumulation that is possible in a larger elliptical weld due to the extra width available in the fast axis of the ellipse.

[0153] To examine this further, the strength of the elliptical welds 103 of Example 2 was compared in Example 3 with circular welds 113 having the same weld area. This Example would be relevant, for example, if there were no space restriction for making the welds. The results in Example 3 showed a slight increase in weld strength for the circular weld having the same cross-sectional area as the elliptical weld of example 2.

[0154] The weld orientation of the elliptical weld was then rotated by ninety degrees, as described by Example 4, such that the major axes of the four ellipses aligned with the direction of the shear stress measurement. The results show a 11 .4% increase in shear strength over the circular welds of Example 3, and a 13.4% increase in the strength over the elliptical welds of Example 2. This is an unexpected benefit because the shear strength of a weld was believed to be very strongly related to its cross-sectional area which area was the same in Examples 2, 3 and 4. The observed 11 .4% and 13.4% increases were far larger than had been anticipated. Both the circular welds 113 of Example 3 and the elliptical welds 123 of Examples 2 and 4 took the same time to produce. The invention thus provides important benefits for manufacturing design in that the welds of the invention can be aligned to the direction of required strength in mechanical assemblies.

[0155] Figure 13 shows a piecepart 130, such as a camera lens, that has a flange 131 for welding the piecepart 130 to another part 132 such as a case or a housing. When used with the present invention, the flange 131 is the first metallic part 11 , and the part 132 is the second metallic part 12. In this type of assembly there is often a very restricted flange width 135 on which the welds have to be located. In addition, the ability to control the amount of heat used to form the weld is advantageous in order to reduce any distortion or heat affected zone associated with the welding process. In Figure 13, the first metallic part 11 and the second metallic pat 12 are welding together with five welds 133. The welds 133 are circular spiral welds. Each of the welds 133 are written starting from the outside of the weld 133, moving towards its centre. Advantageously, the spacing 16 between successive spiral arms increases towards the centre of the weld 133 in order to control the heat energy input into the weld 133 and prevent a hole forming at its centre. Other ways to control the heat energy input into the weld 133, such as controlling the average output power of the laser, the pulse repetition frequency of the laser can also be used.

[0156] The same pieceparts are shown welded in Figure 14 with three welds 141 according to the present invention. The welds 141 are elliptical spiral welds. Each of the welds 141 are written starting from the outside of the weld 141 , moving towards its centre. Advantageously, the spacing 16 between successive spiral arms increases towards the centre of each weld 141 in order to control the heat energy input into the weld 141 and prevent a hole forming at its centre. Other ways to control the heat energy input into the weld 141 , such as controlling the average output power of the laser, the pulse repetition frequency of the laser can also be used.

[0157] The same pieceparts are shown welded in Figure 15 with three welds 151 according to the present invention. The welds 151 are elliptical welds comprising concentric microwelds. Each of the welds 151 are written starting from the outside of the weld 151 , moving towards its centre. Advantageously, the spacing 16 between successive elliptical microwelds increases towards the centre of each weld 151 in order to control the heat energy input into the weld 151 and prevent a hole forming at its centre. Other ways to control the heat energy input into the weld 151 , such as controlling the average output power of the laser, the pulse repetition frequency of the laser can also be used. The same pieceparts are shown welded in Figure 16 with four welds 161 according to the present invention. The welds 161 are non-circular spiral welds. The non-circular welds have a length 3 greater than their width 2. Advantageously, the non-circular welds are shown having microwelds 5 that curve around the central piecepart.

[0158] The welds 141 , 151 , 161 may have an elliptical or superelliptical shape as described with reference to Figures 1 - 12, and may comprise spirals and / or rings. As shown by the Examples, each non-circular weld may have an approximately 39% increase in shear strength across its minor axis direction compared to two of the circular welds. Furthermore, each non-circular weld may have a 58% increase in shear strength compared to two of the circular welds across its major axis. The result is a product that is quicker and thus cheaper to manufacture, and one that has greater strength. Alternatively or additionally, it may be possible to further reduce the overlap distance 15, and hence the flange width 135, and use smaller non-circular welds, thus enabling greater packing densities and potentially smaller electronic devices or devices of the same size but containing more components and hence features.

[0159] 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.

Claims

Claims1 . A weld (1 ) between a first metallic part (11 ) and a second metallic part (12), the first metallic part (11 ) comprising a first metallic material (13), and the second metallic part (12) comprising a second metallic material (14),• the first metallic part (11 ) and the second metallic part (12) overlap by an overlap distance (15) in a region of the weld (1 );• the weld (1 ) comprising at least one microweld (5) defined by a width (6) of between 20 pm and 400 pm;• the microweld (5) forms a welding pattern (7) defined parallel to a surface (19) of the first metallic part (11);• the weld (1) has a width (2) that is at least 0.1 mm less than the overlap distance (15); and• the weld (1 ) has a length (3) that is at least 1 .5 times the width (2) of the weld (1 ).

2. A weld according to claim 1 wherein the welding pattern (7) comprises a spiral.

3. A weld according to claim 2 wherein a distance (16) between adjacent turns of the spiral reduces towards the outside (18) of the welding pattern (7).

4. A weld according to any one of the preceding claims wherein the welding pattern (7) comprises at least one ring (10).

5. A weld according to claim 4 wherein a distance (16) between adjacent rings (10) reduces towards the outside (18) of the welding pattern (7).

6. A weld according to any one of the preceding claims wherein the overlap distance (15) is less than 3 mm.

7. A weld according to any of the preceding claims wherein the first metallic material (13) and the second metallic material (14) remain substantially unmixed in the weld (1 ).

8. A weld according to any one of the preceding claims where the weld (1 ) is substantially inhomogeneous.

9. A weld according to any one of the preceding claims wherein the weld (1 ) is characterized by a depth (4)10. A weld according to any one of the preceding claims, characterized in that, the first metallic material (13) has a reflectivity greater than 90% at an optical wavelength of one micron.11 . A weld according to any one of the preceding claims wherein the first metallic part (11 ) has a thickness (9) in a region of the weld (1) of between 0.2 mm to 1 mm.

12. A weld according to any one of the preceding claims wherein the first metallic material (13) comprises copper, aluminium, nickel, tungsten, iron, bronze, brass, steel, or stainless steel.

13. A weld according to any one of the preceding claims wherein the second metallic material (14) comprises copper, aluminium, nickel, tungsten, iron, bronze, brass, steel, or stainless steel.

14. An article, characterized in that, the article comprises at least one weld (1) according to any one of the preceding claims.

15. An article according to claim 14 and in the form of 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.

16. An apparatus for forming a weld (1) between a first metallic part (11) and a second metallic part (12), which apparatus comprises a laser (21 ), an optical fibre (23), a collimator (24), a scanner (25), a lens (26), and a controller (28):• the laser (21 ) is a pulsed laser;• the laser (21) is configured to emit a laser beam (22) which is coupled to a surface (19) of the first metallic part (11) via the optical fibre (23), the collimator (24), the scanner (25) and the lens (26);• the controller (28) is configured to control the laser (21 ) and the scanner (25) such that the scanner (25) moves the laser beam (22) on the surface (19) of the first metallic part (11 ) to form the weld (1 ); the apparatus is characterized in that:• the apparatus (20) includes a coupler (29) configured to couple an optical signal (30) emitted by the weld (1) from the optical fibre (23) to a detector (31 ), which detector (31) feeds back an electronic signal (32) indicative of a temperature of the weld (1 ) as the weld (1 ) is being formed; the apparatus being adapted to create a weld (1) between the first metallic part (11) and the second metallic part (12) when:• the first metallic part (11 ) and the second metallic part (12) overlap by an overlap distance (15) in a region of the weld (1 );• the weld (1 ) comprising at least one microweld (5) defined by a width (6) of between 20 pm and 400 pm;• the microweld (5) forms a welding pattern (7) defined parallel to a surface (19) of the first metallic part (11),• the weld (1 ) has a width (2) that is at least 0.1 mm less than the overlap distance (15), and:• the weld (1 ) has a length (3) that is at least 1 .5 times the width (2) of the weld (1 ).

17. Apparatus according to claim 16 and including an electronic filter (33) configured to filter an output of the detector (31 ).

18. Apparatus according to claim 16 or 17 wherein the controller (28) controls at least one of a power of the laser beam (22), a focus of the laser beam (22), a trajectory of the laser beam (22), and a velocity of the laser beam (22) with respect to the surface (19) of thefirst metallic part (11) in order to create a weld (1) that has a depth (4) that varies by less than 50 pm across a proportion (81 ) of its surface area, wherein the proportion (81 ) is greater than 75%, preferably greater than 90%, and more preferably greater than 95%.

19. A method for laser welding a first metallic part (11 ) to a second metallic part (12), the first metallic part (11 ) comprising a first metallic material (13), and the second metallic part (12) comprising a second metallic material (14), which method comprises:• placing the first metallic part (11) onto the second metallic part (12) such that the first metallic part (11) and the second metallic part (12) overlap by an overlap distance (15);• providing a laser (21) for emitting a laser beam (22) in the form of laser pulses (27);• focusing the laser pulses (27) onto a surface (19) of the first metallic part (11);• moving the laser beam (22) with respect to the surface (19);• focusing the laser pulses (27) with a spot size and a pulse fluence that cause the formation of at least one microweld (5) in the form of a welding pattern (7) defined parallel to the surface (19) wherein the microweld (5) is defined by a width (6) of between 20pm and 400pm; and• the moving of the laser beam (22) with respect to the surface (19) is such that the weld (1) has a length (3) that is at least 1 .5 times a width (2) of the weld (1).

20. A method according to claim 19 wherein the moving of the laser beam (22) with respect to the surface (19) is such that the width (2) is at least 0.1 mm less than the overlap distance (15).21 . A method according to claim 19 or 20 wherein the welding pattern (7) comprises a spiral (8).

22. A method as claimed in claim 21 wherein the weld is started from the innermost end of the spiral.

23. A method as claimed in claim 21 wherein the weld is started from the outside of the weld.

24. A method as claimed in claim 22 or 23 wherein the velocity of the laser beam increases during the weld.

25. A method according to any of claims 21 to 24 wherein a distance (16) between adjacent turns of the spiral (8) reduces towards the outside (18) of the welding pattern (7).

26. A method according to any one of claims 19 - 25 wherein the welding pattern (7) comprises at least one ring (10).

27. A method according to claim 26 wherein a distance (16) between adjacent rings (10) reduces towards the outside (18) of the welding pattern (7).

28. A method according to any one of claims 19 - 27 wherein the overlap distance (15) is less than 3 mm.

29. A method according to any one of claims 19 - 28 wherein the first metallic material (13) and the second metallic material (14) remain substantially unmixed in the weld (1 ).

30. A method according to any one of claims 19 - 29 where the weld (1) is substantially inhomogeneous.31 . A method according to any one of the claims 19 - 30, characterized in that, the first metallic material (13) has a reflectivity greater than 90% at an optical wavelength of one micron.

32. A method according to any one of claims 19 - 31 wherein the first metallic part (11) has a thickness (9) in a region of the weld (1) of between 0.2 mm to 1 mm.

33. A method according to any one of claims 19 - 32 wherein the first metallic material (11 ) comprises copper, aluminium, nickel, tungsten, iron, bronze, brass, steel, or stainless steel.

34. A method according to any one of the claims 19 - 33 wherein the second metallic material (14) comprises copper, aluminium, nickel, tungsten, iron, bronze, brass, steel, or stainless steel.

35. A method according to any one of claims 19 - 34 wherein the laser beam (22) is moved with a higher velocity with respect to the surface (19) of the first metallic part (11)towards an outside (18) of the welding pattern (7) than an innermost end (17) of the welding pattern (7).

36. A method according to any one of claims 19 - 35 and including the step of monitoring a temperature of the weld (1) as it is being formed.

37. A method according to any one of claims 19 - 36 and including the step of adjusting a velocity of the laser beam (22) with respect to the surface (19) of the first metal part (11 ).

38. A method according to any one of claims 19 - 37 wherein the weld (1) is characterized by a depth (4) which depth (4) varies by less than 50 pm across a proportion (81) of its surface area, wherein the proportion (81 ) is greater than 75%, preferably greater than 90%, and more preferably greater than 95%.

39. A method according to any one of claims 19 - 38, wherein the laser pulses (22) have pulse widths between 100ps and 1000ns.

40. A method according to any one of claims 19 - 39 wherein the laser pulses (22) have pulse energies less than 10mJ.41 . A method according to any one of claims 19 - 40 wherein the laser (21 ) is characterized by a beam quality M2less than 6, preferably less than 2, and more preferably less than 1.3.

42. A method according to any one of claims 19 - 41 wherein the laser (21) is a nanosecond laser.

43. A method according to any one of claims 19 - 42 comprising providing at least ten of the laser pulses (27) to form the weld (1).

44. A method according to any of claims 19 to 43 wherein the weld is started by scanning the laser beam one or more times around a ring (10) on the outermost side of the weld.

45. A method as claimed in claim 44 where the weld is started by scanning the laser beam one or more times around a ring (10) on the outermost side of the weld until sufficient heat has been accumulated in order to form the microweld.

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