Patent Description:
Joining of bright metals such as gold, copper, aluminium, platinum and silver by laser welding in the near infrared spectrum (<NUM> to <NUM>) presents a challenge, as the surface of bright metals are 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.

The function of the 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 <NUM>% reflectivity to a lower value, which for some metals, can be less than <NUM>% 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 sections. 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 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 <NUM> lasers that emit at <NUM>. This is because the reflectivity of bright metals is usually significantly lower at <NUM> 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 <NUM> with a second laser at <NUM> 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 the 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. A large intermetallic region is 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 in which 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, generating 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, and the weld can therefore break along this intermetallic layer.

<CIT> describes a laser beam irradiation apparatus which can accurately perform a linear welding. A series of spot welds are formed using a flash pumped YAG laser. The shape of the cross section of the laser beam is rectangular.

German patent application, <CIT> describes a method for joining at least two workpieces of identical or dissimilar metallic materials to a component by means of a continuously emitting laser beam by forming a weld along a joining surface by partially absorbing the laser beam in an area of the joining surface and a molten bath. A key hole is formed in the workpiece, and the key hole is oscillated in a circular motion when forming the weld.

European patent application <CIT> describes a method for welding thin plates of different metals. A series of spot welds are formed using either laser beams or electron beams.

Japanese patent application, <CIT> describes a laser welding process requiring two lasers. A first pulse laser is used to melt the surface of the workpiece, and a second laser is used for welding the workpiece.

There is a need for a simpler solution for joining bright and dissimilar metals and alloys without problems caused in the joint interface. The method should be able to produce consistent and predictive results on each joint. The resulting weld should have no reliability issues associated with intermetallics.

There is a need for an apparatus and method for laser welding that avoids the aforementioned problems.

The invention provides a method for laser welding a first metal part to a second metal part, which method comprises: placing the first metal part on the second metal part; providing a laser arranged to emit a laser beam in the form of laser pulses that have pulse widths in the range 1ns to 1000ns; providing a scanner for scanning the laser beam with respect to a metal surface of the first metal part; providing an objective lens for focusing the laser pulses onto the metal surface; providing a controller that is adapted to control the scanner such it moves the laser beam with respect to the metal surface, causing the laser to emit the laser pulses, focusing the laser pulses that have the pulse widths in the range 1ns to 1000ns with a spot size and a pulse fluence that cause the formation of a plurality of melt pools in the first metal part and heat stakes in the second metal part, wherein each heat stake extends from a different one of the melt pools and has a distal end, 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, and wherein the first metal part has a reflectivity greater than <NUM>% at a wavelength emitted by the laser.

The invention is particularly attractive because the weld can be formed from two dissimilar metals, one of which can be a bright metal, and can be formed through direct interaction between the materials and a laser beam. The metals can also have different melting points. The resulting weld is robust, repeatable, can be electrically conductive, and has no weaknesses caused by intermetallics.

The spot size may be between <NUM> and <NUM>. The spot size may be between <NUM> and <NUM>.

The laser may be configured to provide a pulse energy of 10mJ or less. The pulse energy may be 1mJ or less.

The laser may provide between ten to one hundred pulses on the focussed spot.

The heat stake may have a width that is less than or equal to half its depth.

The first metal part may comprise multiple layers.

The second metal part may comprise multiple layers.

The first metal part and the second metal part may be formed from the same metal. The first metal part and the second metal part may be formed from different metals.

The first metal part may comprise copper or a copper alloy.

The first metal part may comprise a metal selected from the group comprising copper, aluminium, gold, silver, platinum, nickel, titanium, stainless steel, and an alloy containing one of the preceding metals such as bronze, brass, nickel-titanium, and amorphous alloys.

The first metal part may have a reflectivity greater than <NUM>%.

The first metal part may melt when exposed to a pulse energy of 10mJ or less.

When the first metal part comprises copper, the second metal part may comprise nickel plated steel. When the first metal part comprises aluminium, the second metal part may comprise steel.

The first metal part may have a thickness in a weld region of no more than <NUM>. The thickness may be less than <NUM>. The thickness may be less than <NUM>.

The second metal part may have a thickness in a weld region of at least <NUM>. The thickness of the second metal part in the weld region may be less than <NUM>.

A Young's modulus of the first metal part may be less than a Young's modulus of the second metal part.

The first metal part may comprise a first metal and the second metal part may comprise a second metal, and the first metal may be substantially more ductile than the second metal.

The heat stakes may be in the form of a spiral.

The distance may be such that the focussed spots overlap with each other in at least one direction.

Examples of articles that can be produced are beverage cans, tabs on beverage cans, mobile phones, tablet computers, televisions, machinery, and jewellery.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:.

<FIG> shows apparatus for laser welding a first metal part <NUM> to a second metal part <NUM>, which apparatus comprises a laser <NUM>, a scanner <NUM>, an objective lens <NUM> and a controller <NUM>. The laser <NUM> emits a laser beam <NUM> in the form of laser pulses <NUM>. The laser beam <NUM> is shown being delivered to the scanner <NUM> via an optical fibre cable <NUM> and a collimation optic <NUM>. The collimation optic <NUM> expands and collimates the laser beam <NUM> and inputs the laser beam <NUM> into the scanner <NUM>. The scanner <NUM> is for moving the laser beam with respect to a metal surface <NUM> of the first metal part <NUM>. The objective lens <NUM> focuses the laser beam <NUM> onto the metal surface <NUM>. The controller <NUM> controls the scanner <NUM> such that it moves the laser beam <NUM> with respect to the metal surface <NUM> to form a plurality of focussed spots <NUM> on the metal surface <NUM>.

As shown with reference to <FIG>, the laser pulses <NUM> can be characterized by an instantaneous peak power <NUM>, an average power <NUM>, a pulse shape <NUM>, a pulse energy <NUM>, a pulse width <NUM>, and a pulse repetition frequency FR <NUM>. It is important to select the laser <NUM> such that sufficient peak power <NUM> can be obtained to overcome the reflectivity of the metal surface <NUM> in order to ensure sufficient coupling of the pulse energy <NUM> with the metal surface <NUM> is achieved in order to melt the metal surface <NUM>.

<FIG> shows a spot <NUM> having a spot size <NUM> formed by focussing the laser beam <NUM> onto the metal surface <NUM>. The optical intensity <NUM> is the power per unit area of the laser beam <NUM>. The optical intensity <NUM> varies across the radius of the spot <NUM> from a peak intensity <NUM> at its centre, to a <NUM>/e<NUM> intensity <NUM> and to zero. The spot size <NUM> is typically taken as the <NUM>/e<NUM> diameter of the spot <NUM>, which is the diameter at which the optical intensity <NUM> falls to the <NUM>/e<NUM> intensity <NUM> on either side of the peak intensity <NUM>. The area <NUM> of the spot <NUM> is typically taken as the cross-sectional area of the spot <NUM> within the <NUM>/e<NUM> diameter.

Pulse fluence <NUM> is defined as the energy per unit area of the spot <NUM> on the surface <NUM>. Pulse fluence is typically measured in J/cm<NUM>, and is an important parameter for laser welding because weld quality is highly influenced by the pulse fluence <NUM>. The optimum pulse fluence <NUM> for a particular weld varies between different materials and material thicknesses. The optimum pulse fluence <NUM> for welding a metal piece part can be determined through experimentation.

Referring again to <FIG>, the apparatus focuses the laser pulses <NUM> with a spot size <NUM> and a pulse fluence <NUM> that causes the formation of a plurality of melt pools <NUM> in the first metal part <NUM> and heat stakes <NUM> in the second metal part <NUM>. Each heat stake <NUM> extends from a different one of the melt pools <NUM> and has a distal end <NUM>. The controller <NUM> controls the scanner <NUM> such that the focussed spots <NUM> are spaced apart by a distance <NUM> that is small enough to cause the melt pools <NUM> to overlap and that is large enough to ensure the distal ends <NUM> of the heat stakes <NUM> are distinct and separate from each other in at least one direction <NUM>.

Each heat stake <NUM> is formed by at least one of the pulses <NUM>, the number of pulses <NUM> being dependent on the pulse fluence <NUM>. Ten to one hundred pulses <NUM> are typically used for a laser with 1mJ pulse energy <NUM>. The distance <NUM> between the centres of the focussed spots <NUM> will approximate the distance between the centres of the respective heat stakes <NUM>. The controller <NUM> can cause the scanner <NUM> to hold the focussed spot <NUM> still during the formation of each of the heat stakes <NUM>. Alternatively, the controller <NUM> can cause the scanner <NUM> to dither the focussed spot <NUM> during the formation of each of the heat stakes <NUM>, preferably by an amount less than the distance <NUM>. The distance <NUM> is typically <NUM> to <NUM>, and preferably <NUM> to <NUM>.

The overlapping melt pools <NUM> and the heat stakes <NUM> form a composite weld <NUM>. For clarity, <FIG> shows the focussed spots <NUM> as black circles, and the weld <NUM> in cross section within a three dimensional depiction. The melt pools <NUM> are shown melted together without boundaries between them, and an interface is shown between the melt pools <NUM> and the heat stakes <NUM>. Metallurgical studies have demonstrated that both the melt pools <NUM> and the heat stakes <NUM> may comprise material that is from both first metal part <NUM> and the second metal part <NUM>. Good mixing of the metals can be achieved. There is generally no well defined boundary between the melt pools <NUM> and the heat stakes <NUM>. The distal ends <NUM> of the heat stakes <NUM> are shown as ending in a sharp point. However this is not necessarily so; the distal ends <NUM> may be substantially curved and may be fragmented such that they have more than one end.

Successive focussed laser spots <NUM> may be separated as shown in <FIG> such that the separation <NUM> between the centres of the laser spots <NUM> is greater than the spot size <NUM>. Alternatively, successive focussed laser spots <NUM> may overlap as shown in <FIG> such that the separation <NUM> is less than the spot size <NUM>. In <FIG>, the focussed laser spot <NUM> may represent a single laser pulse <NUM> or multiple laser pulses <NUM>.

By "distinct and separate from each other", it is meant that the distal ends <NUM> of the heat stakes <NUM> do not form a substantially smooth weld in all directions; the heat stakes <NUM> may be at least partially separate from each other in at least one direction <NUM>. Alternatively, the heat stakes <NUM> may be at least partially separate from each other in all directions substantially parallel to the metal surface <NUM>. By "weld" it is meant a connection made by welding or joining.

A shield gas <NUM> may be applied over the weld <NUM> from a gas supply <NUM> in order to prevent the weld <NUM> oxidising or to keep the weld <NUM> clean. The shield gas <NUM> can be argon, helium, nitrogen, or other gases commonly used in laser welding. The shield gas <NUM> may be mixtures of the aforementioned gases. The gas supply <NUM> may comprise a gas bottle, a nozzle, and a flow control regulator.

The weld <NUM> has a substantially jagged surface at the distal ends <NUM> of the heat stakes <NUM>. 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 <NUM> are in synchronism with a control signal <NUM> used to control the scanner <NUM>. This may be achieved by using a synchronisation signal into the controller <NUM>, or by adapting the controller <NUM> such that the controller also controls the laser <NUM>.

The scanner <NUM> can be the galvanometric scan head shown in <FIG>. Alternatively or additionally, the scanner <NUM> can be a moveable two-dimensional or three-dimensional translation stage, or a robot arm. The scanner <NUM> is shown as comprising a first mirror <NUM> for moving the laser beam <NUM> in a first direction <NUM>, and a second mirror <NUM> for scanning the laser beam <NUM> in a second direction <NUM>. The first and the second mirrors <NUM>, <NUM> would typically be attached to galvanometers (not shown). The scanner <NUM> and the objective lens <NUM> 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.

The laser <NUM> 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. The laser <NUM> can be a nanosecond laser. The laser <NUM> is preferably a rare-earth-doped nanosecond pulsed fibre laser, such as a ytterbium doped fibre laser, an erbium-doped (or erbium ytterbium doped) fibre laser, a holmium-doped fibre laser, or a thulium doped fibre laser. These lasers emit laser radiation in the <NUM>, <NUM>, <NUM> and <NUM> wavelength windows respectively. By a nanosecond pulsed laser, it is meant a laser that can emit pulses having pulse widths <NUM> in the range 1ns to 1000ns. 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 welding. Millisecond lasers can generally form a weld by emitting a single pulse.

A method according to the invention and for laser welding the first metal part <NUM> to the second metal part <NUM>, will now be described solely by way of example and with reference to <FIG>. The method comprises: placing the first metal part <NUM> on the second metal part <NUM>; providing the laser <NUM> for emitting the laser beam <NUM> in the form of the laser pulses <NUM>; providing the scanner <NUM> for moving the laser beam <NUM> with respect to the metal surface <NUM> of the first metal part <NUM>; focussing the laser beam <NUM> onto the metal surface <NUM>; providing the controller <NUM> that is adapted to control the scanner <NUM> such it moves the laser beam <NUM> with respect to the metal surface <NUM>, configuring the apparatus to focus the laser pulses <NUM> with the spot size <NUM> and the pulse fluence <NUM> that cause the formation of the plurality of the melt pools <NUM> in the first metal part <NUM> and the heat stakes <NUM> in the second metal part <NUM>, wherein each heat stake <NUM> extends from a different one of the melt pools <NUM> and has a distal end <NUM>, and adapting the controller <NUM> to space the focussed spots <NUM> apart by the distance <NUM> that is small enough to cause the melt pools <NUM> to overlap and that is large enough to ensure the distal ends of the heat stakes <NUM> are distinct and separate from each other in at least one direction <NUM>.

The method may include the step of providing the shield gas <NUM> and the gas supply <NUM>, and applying the shield gas <NUM> over the weld <NUM>. The shield gas <NUM> can be argon, helium, nitrogen, or other gases commonly used in laser welding. The shield gas <NUM> may be mixtures of the aforementioned gases. The gas supply <NUM> may comprise a gas bottle, a nozzle, and a flow control regulator.

In the following, frequent reference will be made to "reflective metals", which is meant to mean metals having a reflectivity greater than <NUM>% at an emission wavelength <NUM> of the laser <NUM> at the temperature at which the first metal part <NUM> is processed.

The laser weld <NUM> formed by the apparatus or the method of the invention may be autogenous, that is, no other materials other than the first and the second metal parts <NUM>, <NUM> are added to form the weld.

The first metal part <NUM> may have a thickness <NUM> in a region of the weld <NUM> of no more than <NUM>. The thickness <NUM> may be less than <NUM>. The thickness <NUM> may be less than <NUM>. The thickness <NUM> may be less than <NUM>. The second metal part <NUM> may have a thickness <NUM> in the region of the weld <NUM>. The thickness <NUM> may be at least <NUM>. The thickness <NUM> may be less than <NUM>.

Referring to <FIG>, the heat stake <NUM> may have a width <NUM> that is at most half its depth <NUM>. This is advantageous because it allows the heat stake <NUM> to penetrate further and may allow the first metal part <NUM> to grip the second metal part <NUM> better.

As shown in <FIG>, the first metal part <NUM> may comprise a metal part <NUM> which is coated with a coating <NUM>. The coating <NUM> may be a metal plating such as nickel or chrome, or may be a chemically-induced coating formed by processes such as anodization. The coating <NUM> may be a polymer coating.

The first metal part <NUM> may comprise multiple layers <NUM> as shown with reference to <FIG>. The multiple layers <NUM> 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 <NUM> may comprise multiple layers <NUM>. The multiple layers <NUM> may be folded sheets of the same metal, layers of the same metal, or layers of different metals. The layers <NUM> may comprise the same metal as the layers <NUM>, or different metals. The weld <NUM> is shown joining the first metal part <NUM> to the second metal part <NUM>. The weld <NUM> is shown partially penetrating the second metal part <NUM>.

<FIG> shows a laser weld <NUM> between the first metal part <NUM> and the second metal part <NUM> using prior art techniques, including for example, laser welding with a green laser using a single high-energy pulse of 100mJ or more. The weld <NUM> has a much larger mass than one of the individual melt pools <NUM> plus its associated heat stake <NUM> shown with reference to <FIG>, and consequently takes a longer period to cool down. This results in metallic mixing in a weld pool <NUM>, the formation of an associated boundary layer <NUM>, and an area around the weld <NUM> that is affected by the heat but where the metals have not flowed - the so-called heat affected zone (HAZ) <NUM>. The mechanical properties of the heat affected zone <NUM> can be substantially degraded as a result of thermal heat tempering, which tempering should generally be minimized. The heat affected zone <NUM> is generally visible (for example after etching with acid) on both the top surface <NUM> of the first metal part <NUM> and the bottom surface <NUM> of the second metal part <NUM>.

The boundary layer <NUM>, when welding steel to steel, can result in carbon formation along grain boundary interfaces, thereby providing a pathway for fracturing the weld <NUM>. Similarly, the boundary layer <NUM> 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 <NUM>. Thus the existence of the large boundary layer <NUM> and the heat affected zone <NUM> are not desirable in either the welding of similar metals or the welding of dissimilar metals.

Whether the weld <NUM> is formed from similar metals or dissimilar metals, the mechanical properties of the material comprising the weld pool <NUM> are likely to be weaker than the properties of the base materials that comprise the first metal part <NUM> and the second metal part <NUM>. Heat affected zones <NUM> are also of a concern if they affect the appearance or chemical composition of the first and second metal parts <NUM>, <NUM>.

The problems associated with intermetallic layers <NUM> and heat affected zones <NUM> increase when welding thin sheet metals. 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 <NUM>, <NUM>.

<FIG> depicts a top view of the weld <NUM> shown in <FIG>. A heat affected zone <NUM> is usually visible (possibly after chemical etching). However, with proper selection of the laser <NUM> and the laser pulse parameters shown with reference to <FIG>, there is generally no heat affected zone visible on the bottom surface. This is because the heat stakes <NUM> each have significantly less mass than the weld <NUM>, and consequently cool more rapidly. Similarly, there is little or no evidence of intermetallic layers <NUM> surrounding the heat stakes <NUM>. The lack of intermetallic layers and a heat affected zone that extends through the second metal part <NUM> provide great advantages over prior art welding techniques.

The second metal part <NUM> shown in <FIG> may comprise a metal part <NUM> which is coated with a coating <NUM>. The coating <NUM> 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 <NUM> may be a tab <NUM> such as found in beverage cans. The tab <NUM> is shown welded to the second metal part <NUM> with the weld <NUM>.

Beverage cans are often made from thin sheets of aluminium that are less than <NUM> in thickness. In a beverage can, the coating <NUM> would be a polymer coating usually applied before the weld <NUM> is formed. It is important that the method of forming the weld <NUM> does not degrade the coating <NUM>. The apparatus and method of the present invention achieves this by virtue of the heat stakes <NUM>, shown with reference to <FIG>, as there is less heat generated in the second metal part <NUM> compared to a prior art weld.

Referring again to <FIG>, the first metal part <NUM> and the second metal part <NUM> may be formed from the same metal. The metal may be aluminium or copper, or alloys thereof. Alternatively, the first metal part <NUM> and the second metal part <NUM> may be formed from different metals.

The first metal part <NUM> may comprise copper or a copper alloy.

The first metal part <NUM> may comprise a metal selected from the group comprising copper, aluminium, gold, silver, platinum, nickel, titanium, stainless steel, and an alloy containing one of the preceding metals such as bronze, brass, nickel-titanium, and amorphous alloys.

The first metal part <NUM> may have a reflectivity <NUM> greater than <NUM>% at the wavelength (A) <NUM> emitted by the laser <NUM>. <FIG> shows the wavelength <NUM> being <NUM>; this is intended to be non-limiting. Ytterbium pulsed fibre lasers are especially attractive to use as the laser <NUM>; these emit in the wavelength range from approximately <NUM> to approximately <NUM>. The laser <NUM> can also be an erbium doped, or erbium ytterbium co-doped fibre laser, each emitting at around <NUM>, or a holmium or thulium doped fibre laser emitting at around <NUM>. The use of lasers emitting at <NUM> and <NUM> provide eye safety advantages that are important in certain applications. There are also many other laser types that emit in the near infra-red wavelengths.

The spot size <NUM> may be <NUM> to <NUM>, and preferably <NUM> to <NUM>.

The first metal part <NUM> may melt when exposed to a pulse energy <NUM> of 10mJ or less. The pulse energy <NUM> may be <NUM> mJ or less. The pulse energy <NUM> may be 1mJ or less. The pulse energy may be 100µJ or less. The pulse energy may be 10µJ or less. Thicker materials require larger pulse energies <NUM> than thinner materials.

The first metal part <NUM> may comprise copper. The second metal part <NUM> may comprise nickel plated steel.

The first metal part <NUM> may comprise aluminium. The second metal part <NUM> may comprise steel.

The first metal part <NUM> may be defined by a Young's modulus which is less than a Young's modulus of the second metal part <NUM>.

The first metal part <NUM> may comprise a first metal and the second metal part <NUM> may comprise a second metal. The Young's modulus of the first metal may be less than a Young's modulus of the second metal. Advantageously, the first metal may be substantially more ductile than the second metal. This has important advantages if the weld <NUM> is repeatedly strained since the heat stakes <NUM> will be more resistant to metal fatigue resulting in failure.

The heat stakes <NUM> are preferably formed in a line that is not linear in order to increase the shear strength of the weld <NUM>. For example, the heat stakes <NUM> may be formed in the form of a spiral <NUM> as shown with reference to <FIG>. The spiral <NUM> is formed by causing the controller <NUM> to move the laser beam <NUM> in a trajectory <NUM> that is in the form of the spiral <NUM>, and which has a first location <NUM> shown as being in the inside of the spiral, and a second location <NUM>, shown as being on the outside of the spiral. It is generally preferred that the spiral trajectory <NUM> starts from the first location <NUM>, but may alternatively start from the second location <NUM>. <FIG> shows a cross section through the resulting weld <NUM>, which cross section is beneath the overlapping melt pools <NUM>, shown with reference to <FIG>. As shown with reference to <FIG>, the successive focussed spots <NUM> are separated by the distance <NUM> which is greater than the spot size <NUM>. It is preferred that the laser <NUM> is pulsed at least once, and preferably between ten to one hundred times, on each of the focussed spots <NUM>. By this means, it is possible to control the amount of heat being injected into each part of the weld <NUM> very precisely, thus allowing the strength of the weld <NUM> to be optimised. The choice of whether to commence from the first location <NUM> or the second location <NUM> can be determined experimentally from the strength of the resulting weld <NUM>. In <FIG>, a distance <NUM> is shown between the centres of two of the heat stakes <NUM>, and a distance <NUM> between centres of adjacent spiral arms <NUM>. The distance <NUM> can be less than <NUM>, less than <NUM>, and preferably less than <NUM>. The distance <NUM> can be less than <NUM>, less than <NUM>, less than <NUM>, and preferably less than <NUM>. Optimizing the distances <NUM>, <NUM> can be achieved experimentally by measuring physical parameters such as peel strength, shear strength, and electrical contact resistance.

<FIG> shows the first metal part <NUM> welded to the second metal part <NUM> with three of the welds <NUM>. The welds <NUM> can be the spiral weld <NUM> shown with reference to <FIG>. The welds <NUM> can have a diameter <NUM> of between <NUM> to <NUM>, and preferably between <NUM> to <NUM>. By using a plurality of the welds <NUM>, more strength and rigidity is obtained.

<FIG> shows a graph of pulse fluence <NUM> and absorbed energy density <NUM>, where the absorbed energy density <NUM> is the total pulse energy <NUM> absorbed by the first and the second metal parts <NUM>, <NUM> per unit surface area by the laser pulses <NUM>. In order to initiate the weld <NUM> shown with reference to <FIG>, it is necessary to use a pulse fluence <NUM> that is at least equal to the first pulse fluence threshold <NUM>. This is in order to initiate coupling of the laser beam <NUM> to the metal surface <NUM>, and the melting of the metal surface <NUM>. Once the metal surface <NUM> has begun to melt, the remaining pulses <NUM> should have a pulse fluence that is at least equal to the second pulse fluence threshold <NUM>. The second pulse fluence threshold <NUM> can be substantially less than the first pulse fluence threshold <NUM>. For a first metal part <NUM> with high reflectivity, that is, reflectivities at the wavelength of the laser beam <NUM> greater than <NUM>%, the second pulse fluence threshold <NUM> can be between two and ten times smaller than the first pulse fluence threshold <NUM>. As each of the pulses <NUM> is absorbed, they contribute to the absorbed energy density <NUM>. The absorbed energy density <NUM> absorbed at each of the focussed locations <NUM> should be at least equal to the first energy density threshold <NUM> at which the laser stake <NUM> begins to penetrate the second metal part <NUM>, but less than the second energy density threshold <NUM> at which the weld <NUM> becomes unacceptably brittle. It can be seen that by varying the pulse parameters shown with reference to <FIG>, the number of pulses, and the distances <NUM> between focussed spots <NUM>, there is a great controllability of the weld <NUM>, and moreover, greater control over its formation, and therefore mechanical properties, than prior art techniques.

<FIG> shows a key hold weld <NUM> that joins the first metal part <NUM> to the second metal part <NUM>. In this process, the laser beam <NUM> not only melts the first and the second metal parts <NUM>, <NUM> to form molten metal <NUM>, but also produces vapour (not shown). The dissipating vapour exerts pressure on the molten metal <NUM> and partially displaces it. The result is a deep, narrow, vapour filled hole called the keyhole <NUM>. Such a process may be involved in the formation of the heat stakes <NUM> in the apparatus and method of the invention.

The key hole <NUM> is surrounded by the molten metal <NUM>, and moves with the laser beam <NUM> in the direction <NUM> that the laser beam <NUM> is scanned. The molten metal <NUM> solidifies behind the key hole <NUM> as it moves, forming a weld seam <NUM>. The weld seam <NUM> is deep and narrow. The weld depth <NUM> may be up to ten times greater than the weld width <NUM> shown with reference to <FIG>. The laser beam <NUM> is absorbed with high efficiency in the key hole <NUM> as it is reflected multiple times.

The apparatus and method of the invention extend to the case in which the heat stake <NUM> forms a continuous weld <NUM>, as shown with reference to <FIG>. Here the controller <NUM> has controlled the scanner <NUM> to scan the laser beam <NUM> in the spiral <NUM> shown with reference to <FIG> such that successive focussed spots <NUM> shown with reference to <FIG> overlap. The focussed spots <NUM> have resulted in the heat stake <NUM> that is continuous in the direction of the spiral arms <NUM>, but which is at least partially separate in a radial direction <NUM>. The radial direction <NUM> can be the direction <NUM> in <FIG>. As shown by the cross section in <FIG>, it is preferred that the melt pools <NUM> overlap. The weld <NUM> shown with reference to <FIG> and <FIG> can be the continuous weld <NUM> of <FIG>. The weld <NUM> may be formed with the key hole <NUM> described with reference to <FIG>.

The method of the invention will now be described with reference to the apparatus and method described with reference to <FIG> and to the non-limiting examples set out below. The laser <NUM> was a nanosecond ytterbium-doped fibre laser, model SPI G4 70EP-Z manufactured by SPI Lasers UK Ltd of Southampton, England. The laser <NUM> is a master oscillator power amplifier having excellent control over the laser parameters shown in <FIG>, namely the peak power <NUM>, the average power <NUM>, the pulse shape <NUM>, the pulse energy <NUM>, the pulse width <NUM>, and the pulse repetition frequency FR <NUM>. The scanner <NUM> was a galvanometer-scanner model Super Scan II manufactured by Raylase of Munich, Germany with a <NUM> beam aperture (not shown). The controller <NUM> comprised a desktop computer with a Windows <NUM> operating system on which SCAPS scanner application software licensed by SCAPS GmbH of Munich, Germany was used to program, operate, and store the code for the scanner <NUM> for steering the laser beam <NUM>. The lens <NUM> was a <NUM> focal length F-theta lens. The collimator <NUM> had a <NUM> focal length. The lens <NUM>, the collimator <NUM>, and the scanner <NUM> were configured to form and translate the laser beam <NUM> onto the surface <NUM> of the first metal part <NUM> with a focused spot <NUM> having a spot size <NUM> of <NUM> and an area <NUM> of <NUM> x <NUM>-<NUM> cm<NUM>.

With reference to <FIG>, the first metal part <NUM> was copper grade C110 with a <NUM> thickness, and the second metal part <NUM> was aluminium grade <NUM> with a <NUM> thickness. Following experimentation to determine the peak power <NUM>, the pulse shape <NUM>, the pulse energy <NUM>, the pulse width <NUM>, and the pulse fluence <NUM>, it was decided to scan the laser beam <NUM> at a linear speed of <NUM>/s over the metal surface <NUM> and with the distance <NUM> (shown with reference to <FIG>) between successive of the focussed spots <NUM> of <NUM> (measured centre to centre). This corresponds to the pulse repetition frequency <NUM> of <NUM>. The appropriate control parameters were then fed into the controller <NUM> and the laser <NUM> set up accordingly, The laser beam <NUM> was repetitively pulsed at the pulse repetition frequency <NUM> of <NUM>, and scanned over the metal surface <NUM> in the spiral <NUM> shown with reference to <FIG>. The spiral was formed with a <NUM>/s linear speed. The total lengh of the spiral <NUM> was <NUM>, and was formed from the first location <NUM> to the second location <NUM>. The diameter <NUM> of the weld <NUM> was <NUM>. The pulse width <NUM> was <NUM> ns at full width half maximum FWHM and <NUM> ns at <NUM>% of instantaneous peak power <NUM>. Total pulse energy <NUM> was <NUM> mJ with an average power <NUM> of <NUM> W and a peak power <NUM> of <NUM> kW. Each laser pulse <NUM> had a peak power intensity of <NUM> x <NUM>+<NUM>W/cm<NUM> with a pulse fluence <NUM> of <NUM> J/cm<NUM>. A shield gas mixture <NUM> was used of <NUM>% Argon and <NUM>% Helium supplied thorough a flow control regulator at <NUM> cubic feet per hour from a <NUM> diameter copper nozzle <NUM> over the weld <NUM>. The weld <NUM> that was formed is of the type shown in <FIG>. The heat stakes <NUM> form a continuous line along the spiral, and are at least partially separated in a radial direction <NUM> across the spiral, corresponding to the direction <NUM> shown in <FIG>. The weld pools <NUM> are continuous across the entire surface area of the weld <NUM>, though as shown in <FIG>, the surface of the weld <NUM> is not smooth. Observation of the welds <NUM> revealed aluminium colouring on its top surface, <NUM>, indicating that the metals have mixed in the weld. The welds <NUM> were observed to be extremely strong for their size.

With reference to <FIG>, the first metal part <NUM> was copper grade C110 with a <NUM> thickness, and the second metal part <NUM> was also copper grade C110 with a <NUM> thickness. After experimentation, it was determined that the same process parameters could be used as described with reference to Example <NUM>. The resulting welds were observed to be extremely strong for their size.

With reference to <FIG>, the first metal part <NUM> was stainless steel grade <NUM> with a <NUM> thickness <NUM> and the second part <NUM> was grade stainless steel <NUM> with a <NUM> thickness <NUM>. Following experimentation to determine the peak power <NUM>, the pulse shape <NUM>, the pulse energy <NUM>, the pulse width <NUM>, and the pulse fluence <NUM>, it was decided to scan the laser beam <NUM> at a linear speed of <NUM>/s over the metal surface <NUM> and with the distance <NUM> (shown with reference to <FIG>) between successive of the focussed spots <NUM> of <NUM> (measured centre to centre). This corresponds to the pulse repetition frequency <NUM> of <NUM>. The appropriate control parameters were then fed into the controller <NUM> and the laser <NUM> set up accordingly, The laser beam <NUM> was repetitively pulsed at the pulse repetition frequency <NUM> of <NUM>, and scanned over the metal surface <NUM> in the spiral <NUM> shown with reference to <FIG>. The spiral <NUM> was formed with a <NUM>/s linear speed. The spiral <NUM> was formed from the first location <NUM> to the second location <NUM>. The diameter <NUM> of the weld <NUM> was <NUM>. The pulse width <NUM> was <NUM> ns at full width half maximum FWHM and <NUM> ns at <NUM>% of instantaneous peak power <NUM>. Total pulse energy <NUM> was <NUM>µJ with an average power <NUM> of <NUM> W and a peak power <NUM> of <NUM> kW. Each laser pulse <NUM> had a peak power intensity of <NUM> x <NUM>+<NUM> W/cm<NUM> with a pulse fluence <NUM> of <NUM> J/cm<NUM>. A shield gas mixture <NUM> was used of <NUM>% Argon and <NUM>% Helium supplied thorough a low control regulator at <NUM> cubic feet per hour from a <NUM> diameter copper nozzle <NUM> over the weld <NUM>. The weld <NUM> that was formed is of the type shown in <FIG>. The heat stakes <NUM> form a continuous line along the spiral, and are at least partially separated in a radial direction <NUM> across the spiral, corresponding to the direction <NUM> shown in <FIG>. The weld pools <NUM> are continuous across the entire surface area of the weld <NUM>, though as shown in <FIG>, the surface of the weld <NUM> is not smooth. Because of the different parameters being used, the weld <NUM> resembled a traditional lap weld, with excellent mixing of the metals, but almost negligible heat affected zone <NUM> (shown with reference to <FIG>). However, the continuous heat stakes <NUM> did extend from the weld, resulting in an uneven surface as shown in <FIG> across the radius <NUM> of the weld <NUM>. However the extension of the heat stakes <NUM> from the weld <NUM> was substantially less than observed for the copper aluminium and copper copper welds of Examples <NUM> and <NUM> respectively. The welds <NUM> were observed to be extremely strong for their size.

Claim 1:
A method for laser welding a first metal part (<NUM>) to a second metal part (<NUM>), which method comprises:
• placing the first metal part (<NUM>) on the second metal part (<NUM>),
• providing a laser (<NUM>) arranged to emit a laser beam (<NUM>) in the form of laser pulses (<NUM>) that have pulse widths (<NUM>) in the range 1ns to 1000ns,
• providing a scanner (<NUM>) for scanning the laser beam (<NUM>) with respect to a metal surface (<NUM>) of the first metal part (<NUM>),
• providing an objective lens (<NUM>) for focusing the laser pulses (<NUM>) onto the metal surface (<NUM>),
• providing a controller (<NUM>) that is adapted to control the scanner (<NUM>) such that it moves the laser beam (<NUM>) with respect to the metal surface (<NUM>),
• causing the laser (<NUM>) to emit the laser pulses (<NUM>),
• focusing the laser pulses (<NUM>) that have the pulse widths (<NUM>) in the range 1ns to 1000ns with a spot size (<NUM>) and a pulse fluence (<NUM>) that cause the formation of a plurality of melt pools (<NUM>) in the first metal part (<NUM>) and heat stakes (<NUM>) in the second metal part (<NUM>), wherein each heat stake (<NUM>) extends from a different one of the melt pools (<NUM>) and has a distal end (<NUM>),
• adapting the controller (<NUM>) to space the focussed spots (<NUM>) apart by a distance (<NUM>) that is small enough to cause the melt pools (<NUM>) to overlap and that is large enough to ensure the distal end (<NUM>) of the heat stakes (<NUM>) are distinct and separate from each other in at least one direction (<NUM>), and
• wherein the first metal part (<NUM>) has a reflectivity (<NUM>) greater than <NUM>% at a wavelength (<NUM>) emitted by the laser (<NUM>).