DE102020133628A1 - Process for joining at least two joining partners - Google Patents

Abstract

The present invention relates to a method for joining at least two joining partners (3, 8) using ultra-short laser pulses of a laser beam (5) from an ultra-short pulse laser (4), with at least one of the joining partners (3, 8) being transparent for the laser wavelength used, with all joining partners (3, 8) have a thermal expansion coefficient (CTE) and at least one of the joining partners (3, 8) has the highest thermal expansion coefficient (CTEmax) of all joining partners (3, 8), the joining partners (3, 8) being Laser beam (5) are joined together, with a first threshold (CTEt1) of the thermal expansion coefficient being determined, and the joining partners (3, 8) are joined with a sequence of individual pulses of the laser beam (5) when the greatest thermal expansion coefficient (CTEmax) corresponds to or exceeds the first threshold (CTEt1), and the joining partners (3, 8) with a sequence of bursts of at least two Laser pulses of the laser beam (5) are added when the largest thermal expansion coefficient (CTEmax) falls below the first threshold (CTEt1).

Description

technical field

The present invention relates to a method for joining at least two joining partners using ultra-short laser pulses of a laser beam from an ultra-short pulse laser.

State of the art

To join two parts to be joined, it is known to apply a laser beam to the respective parts to be joined in order to generate a melt through energy absorption in the zone exposed to the laser beam, which melt forms a weld seam between the parts to be joined after the melt has solidified. Joining using ultra-short laser pulses enables a stable connection of the joining partners without the use of additional material.

It is known to place the focus or focal zone of the laser beam in the boundary surface or in an area around the common boundary surface of the two joining partners for joining a transparent joining partner with a non-transparent joining partner or for welding two transparent joining partners. The processing laser beam passes through one of the transparent joining partners and generates a melt in the area of the interface between the two joining partners.

If you focus ultra-short laser pulses, i.e. laser pulses in the picosecond range or in the femtosecond range (e.g. 50 fs to 50 ps), in the volume of a material, the high intensity in the focus can lead to non-linear absorption processes. If the time between the successive ultra-short laser pulses is shorter than the heat diffusion time, this leads to heat accumulation or a temperature rise in the material in the focus area. With each of the successive pulses, the temperature can then be increased to the melting temperature of the material and finally the material can be locally melted.

When joining partners, high temperatures occur due to the strong local energy input from the focused laser beam, which are not present in the surrounding material areas. Accordingly, the heat required for processing - for example for the production of a weld seam - leads to tensile and compressive stresses in the surrounding material areas.

These tensile and compressive stresses can also lead to cracking along the weld seam. The reason for this lies in the temperature and pressure gradients induced by the laser beam and the melting process, which create material stresses. The material stresses then relax with a certain probability in the form of one or more cracks.

Presentation of the invention

Proceeding from the known prior art, it is an object of the present invention to provide an improved method for joining two joining partners.

The object is achieved by a method for joining two joining partners with the features of claim 1. Advantageous developments result from the dependent claims, the description and the figures.

Accordingly, a method for joining two joining partners using ultra-short laser pulses of a laser beam from an ultra-short pulse laser is proposed, with at least one of the joining partners being transparent to the laser wavelength used, with all joining partners having a thermal expansion coefficient (CTE) and at least one of the joining partners having a maximum thermal expansion coefficient (CTEmax ) of all joining partners, with the joining partners being joined using ultra-short laser pulses of the laser beam, with a first threshold (CTEt1) of the thermal expansion coefficient being determined, and the joining partners being joined with a sequence of individual pulses of the laser beam if the greatest thermal expansion coefficient (CTEmax ) corresponds to or exceeds the first threshold (CTEt1), and the joining partners are joined with a sequence of bursts of at least two laser pulses of the laser beam if the greatest thermal expansion coefficient ficient (CTEmax) falls below the first threshold (CTEt1).

The ultra-short pulse laser makes the ultra-short laser pulses of the laser beam available, with the individual laser pulses forming the laser beam in the beam propagation direction.

In particular, the ultra-short pulse laser can emit laser pulses with a pulse length of preferably less than 10 ns, preferably less than 500 ps.

Instead of a sequence of individual laser pulses, the laser can also provide bursts, with each burst comprising the transmission of a number of laser pulses. For a certain time interval, the emission of the laser pulses can be very dense, at intervals of a few picoseconds up to hundreds of nanoseconds. The bursts can in particular be so-called GHz bursts, in which the sequence of the successive laser pulses of the respective burst takes place in the GHz range.

A sequence of individual pulses means that the laser emits several individual pulses one after the other. A sequence of individual pulses therefore includes at least two individual pulses. A sequence of bursts means that the laser emits several bursts one after the other. A sequence of bursts therefore includes at least two bursts. In particular, the bursts or individual pulses of the sequence can each be of the same type. The bursts or individual pulses are identical if the laser pulses used have essentially the same properties, ie approximately the same pulse energy, the same pulse length and—in the case of bursts—also the same pulse spacing within the burst.

The transparency of the at least one joining partner has the advantage that the joining laser can be focused through the transparent joining partner, so that the joining area can be localized at the interface between the two joining partners - i.e. on the inside - between the two joining partners.

The first joining partner can be transparent, for example, and the second joining partner can be opaque to the wavelength of the laser. For example, the first joining partner can consist of quartz glass and the second can consist of aluminum. However, both joining partners can also be transparent.

The joining partners are arranged one on top of the other, so that the boundary surfaces of the joining partners, across which the joining partners are to be joined, face one another. In the joining area, heat is accumulated as a result of successive absorption of the ultra-short laser pulses, provided the pulse rate of the laser beam is greater than the rate of heat dissipation through material-specific heat transport mechanisms, in particular through heat diffusion. Due to the increasing temperature in the material of at least the first joining partner from joining pulse to joining pulse or from burst to burst, the melting temperature of the material of the joining partner can finally be reached, which leads to local melting of the material of the joining partner.

The joining area is therefore understood to be that area of the joining partner in which the ultra-short laser pulses are introduced and in which the material is melted. Alternatively, the entirety of the locally melted material in the joining area can also be referred to as a melt bubble. Irrespective of the name, the resulting melt can bridge the common interface of the joining partners and permanently connect the joining partners to one another when cooling down. In particular, the network structure of the joining partners can also change. The cooled melt, which connects the joint partners to one another or produces the weld seam, is then referred to as a joint modification.

In order to melt the material in the joining area, individual pulses and/or bursts can be introduced into the material and successively absorbed. This large number of ultra-short individual pulses and/or bursts introduced at a position is also called a laser spot, with the number of ultra-short individual pulses and/or bursts per laser spot N being given by the product of spot size SG and repetition rate P per feed rate VG: N = SG * P / VG. The spot size describes the spatial area over which the ultra-short laser pulses and/or bursts are emitted into the material.

The expansion of the joining partners per temperature difference is given by the thermal expansion coefficient CTE = ΔX / X * 1 / ΔT. In this case, X can be a volume or ΔX a change in volume, or an area or a change in area, or a length or a change in length. In particular, the CTE has the unit millionths of parts per Kelvin (ppm/K).

If there are different thermal expansion coefficients, the highest thermal expansion coefficient CTEmax of all joining partners can be easily determined. However, if the thermal expansion coefficients of all joining partners are identical, for example because all joining partners are made of the same material, then this thermal expansion coefficient is considered to be the largest thermal expansion coefficient.

The size of the joining area is additionally determined by the beam geometry, in particular the size of the focal zone of the focused laser beam. The beam geometry describes the spatial configuration of the laser beam and other beam properties such as certain diffraction properties of the laser beam, see below.

For example, a volume within a joining partner can be heated in the focus of the laser beam. The heated volume expands accordingly (with a positive CTE), so that the area surrounding the heated volume of the Joining partners a pressure is built up. In particular, this pressure can generate material stresses, which can promote the formation of cracks.

If the CTEmax exceeds a first threshold, then the thermal expansion is greater than allowed by the threshold. This can mean, for example, that thermal expansion can lead to critical material stresses. If the CTEmax corresponds to the first threshold CTEt1 or exceeds the first threshold, then the joining partners are joined with a sequence of individual pulses of the laser beam. This has the advantage that the material can cool down between the individual pulses, so that a crack-free joint of the joint partners is made possible.

If the CTEmax falls below the first threshold CTEt1, the joining partners are joined with a sequence of bursts. The use of bursts leads to improved heat accumulation: the warmer the material, the better additional heat can be accumulated. This applies to glasses, for example, from a temperature of the material of approx. T = 1000°C. Accordingly, bursts enable better control of the heat accumulation in glass or also in opaque materials, so that a crack-free joint connection of the joint partners is made possible. For example, the heat accumulation in the material due to a sequence of bursts can also lead to the weld seam being able to be lengthened compared to a sequence of individual pulses with the same average power. Furthermore, a change in the stress distribution perpendicular to the weld can be achieved, which can lead to a higher strength of the weld.

Depending on the material (and thus depending on the CTE), cracking in the material can be avoided by using a sequence of bursts or single pulses. Improved gap bridging can thus be achieved overall. In particular, materials with higher strength and without additional material layers, such as thermally conductive layers, can be joined.

A second threshold (CTEt2) can be determined and the joining partners can be joined with a sequence of single pulses of the laser beam if the largest thermal expansion coefficient (CTEmax) corresponds to or exceeds the first threshold (CTEt1), the joining partners can be joined with a sequence of Bursts from exactly two laser pulses of the laser beam are joined if the largest thermal expansion coefficient (CTEmax) is between the first threshold (CTEt1) and the second threshold (CTEt2), and the joining partners can be joined with a sequence of bursts from at least three laser pulses of the laser beam if the largest thermal expansion coefficient (CTEmax) corresponds to or falls below the second threshold (CTEt2).

In this case, the second threshold is in particular smaller than the first threshold CTEt2<CTEt1. A finer gradation of the joining parameters is thus achieved, so that an improvement in the joining connection can also be achieved in the area between CTEt1 and CTEt2 by using bursts.

The first threshold (CTEt1) can be 10 ppm/K and preferably the second threshold (CTEt2) can be 1 ppm/K.

For example, a first joining partner can have a CTE of 5 ppm/K and a second joining partner a CTE of 11 ppm/k. Accordingly, CTEmax is equal to 11 ppm/K in this example, so that the joining partners can be joined with a sequence of individual pulses. For example, a first joining partner can have a CTE of 4 ppm/K and a second joining partner a CTE of 6 ppm/k. According to this, CTEmax is equal to 6 ppm/K, so that the joining partners can be joined with a sequence of bursts, each burst comprising two laser pulses. For example, a first joining partner can have a CTE of 0.01 ppm/K and a second joining partner can have a CTE of 0.5 ppm/k. According to this, CTEmax is equal to 0.5 ppm/K, so that the joining partners can be joined with a sequence of bursts, with each burst comprising more than three laser pulses. In particular, in this example, the thermal expansion coefficients are so small that little or no stressing of the materials against one another is to be expected.

The first threshold (CTEt1) and preferably the second threshold (CTEt2) can be determined based on the combination of the materials of the joining partners, preferably in a preliminary test.

Such a preliminary test can consist, for example, of joining the respective material combination with three different weld seams, with a first weld seam being produced by a sequence of individual pulses, a second weld seam being produced by a sequence of bursts with two laser pulses each, and a third weld seam with one Sequence of bursts of three laser pulses is generated. If the third weld cracks, CTEmax is greater than CTE2. If the second weld has cracks, CTEmax is greater than CTE1. If the first weld seam also shows cracks, the average laser power used is too high.

Accordingly, the laser power can be reduced and the preliminary test can be started again.

The first threshold (CTEt1) and preferably the second threshold (CTEt2) can be automatically selected from a database based on the input of the materials involved in the joining partners, and on this basis the joining process can be automatically guided either with single pulses or with bursts.

In particular, the results of various preliminary tests can be entered into such a database. In this case, automatic guidance can mean that the settings of the laser are carried out automatically, for example using a computer.

A database can in particular also be a table or a book, but also an electronically accessible source.

The joining partners can rest against one another at an interface, with the joining being carried out at the interface and the laser beam passing through at least one of the joining partners in the region of the interface and interacting with at least one of the joining partners.

In this way it can be achieved in particular that the melt produced bridges the gap between the joining partners.

The average power of the laser can be between 0.5W and 50W.

The mean laser power L results from the sum of the pulse energies E over a period of time Δt. The average laser power L is calculated accordingly as the product of the pulse energy E and the repetition rate P: L = E * P.

The average power of the laser spot Ls is defined for a single pulse as the product of pulse energy E, repetition rate P _E of the single pulse and number of single pulses N _E : L _S,E = E * P _E * N _E

The average power of the laser spot in a burst is defined as the product of pulse energy E, number of pulses per burst Np, number of bursts N _E and the repetition rate at which the bursts are emitted P _E : L _S,P = E *N _P * P _E * N _E . The average power of the laser spot during a burst is thus scaled only with the number of laser pulses per burst compared to the average power of a single pulse: L _S,P = L _S,E * N _P .

The average power can be defined as the power at which essentially no cracking occurs when the joining partners are joined, in particular less than 1 crack per millimeter of weld seam.

This has the advantage that the joining process does not create any cracks in the material and therefore the material is not damaged.

The pulse energy of the laser pulses in a burst can be modulated for the respective subsequent laser pulse, with the pulse energy of the respective subsequent laser pulse preferably being greater or smaller than the pulse energy of the respective preceding laser pulse.

In this way it can be achieved that the heating of the joining partners can also take place in a controlled manner during a burst.

The pulse energy of the laser pulses can be modulated over time, preferably with the average power remaining the same, with the modulation frequency preferably being between 100 Hz and 10 kHz.

Temporally modulated means that the pulse energy is changed during a modulation period, the modulation period being given by the inverse modulation frequency. The modulation frequency indicates the time scale on which the modulation form is repeated. In particular, a modulation of the pulse energy means that the pulse energy can become larger or smaller during the modulation period. The modulation form indicates which mathematical function the pulse energy follows during the modulation period.

The modulated pulse energy from pulse to pulse means that there are times when less pulse energy is introduced into the joining partner(s) and temperature relaxation can take place. There may also be times when more energy can be injected than without the modulation. Crack formation can thus be controlled and/or avoided.

For example, a temporal modulation can be achieved by varying the intensity of the joining pulses. For example, a strong joining pulse can be emitted followed by two joining pulses with half the intensity. However, the temporal modulation also means that the laser then emits a strong joining pulse, followed by two weakened joining pulses.

The laser pulses of a burst can each have a time interval of at most 1 μs, preferably between 0.02 ns and 1000 ns, and the bursts of the sequence of bursts can each have a time interval of at least 2 μs, preferably between 2 μs and 1000 μs, and/or the individual pulses of the sequence of individual pulses can each have a time interval of at least 0.02 μs, preferably between 0.02 μs and 1000 μs.

This has the advantage that suitable joining parameters can be found for many different materials.

The time intervals between the laser pulses of all bursts used for joining can be the same length and/or the time intervals between the bursts for all bursts used for joining can be the same length and/or all time intervals between all the individual pulses used for joining can be the same length.

The time intervals between the laser pulses can thus be determined by a fixed repetition frequency. In particular, the operation of the laser system can be stabilized by using a fixed repetition frequency.

For example, the laser pulses of a burst can then have a repetition frequency between 50 GHz and 1 MHz and the bursts of the sequence of bursts can have a repetition frequency between 500 kHz and 1 kHz and/or the sequence of individual pulses can have a repetition frequency between 50 MHz and 1 kHz.

The laser wavelength can be between 200 nm and 5000 nm, preferably 1000 nm, and/or the pulse duration of the laser pulses can be between 10 fs and 50 ps. The laser beam can be focused in a focal zone in the joining partners and the fluence in the focal zone can be greater than 0.01 J/cm ² for a single pulse or a laser pulse of a burst.

These parameters make it possible to control the accumulation of heat in the joint partners, to reduce stress and to generate higher strengths in the joint. In particular, it is also possible to adapt the process parameters to the respective materials of the joining partners.

For example, the wavelength of the ultra-short laser pulse can be 1030 nm, the pulse duration of a single pulse being 400 fs, the pulse spacing being 20 ns, which corresponds to a repetition rate of 50 MHz, and the fluence in the focus being 75 J/cm ² , for example.

The laser beam and the joining partners can be moved and/or positioned relative to one another.

Moving relative to each other can mean that either the laser beam or the joining partners or both the laser beam and the joining partners are moved. In this way it can be achieved that the laser beam introduces joints at different locations of the joint partners. In particular, this makes it possible to produce a continuous weld seam between the two joining partners.

The movement can take place with a feed, with laser pulses or bursts being able to be introduced continuously into the joining partners during the feed. A positioning of the joining partners relative to the laser beam consists in bringing the focus zone of the laser beam into the desired penetration depth and into the desired location.

The laser beam can be a quasi-non-diffracting laser beam, preferably a Bessel beam or a Gauss-Bessel beam, and the laser beam can preferably have a focal zone that is elongated in the beam propagation direction. However, the laser beam can also be a Gaussian laser beam.

A Gaussian laser beam is characterized in that the beam profile, ie the cross section through the intensity distribution of the laser beam perpendicular to the direction of propagation of the laser beam, essentially corresponds to a Gaussian bell curve.

The focus zone d ^GF ₀ of a Gaussian beam, the Gaussian focus, or the diameter of the Gaussian beam or the Gaussian profile, is defined on the one hand by the second moments or the variance of the Gaussian curve. On the other hand, the focal zone d ^GF ₀ is defined by the associated characteristic length, the Rayleigh length z _R =π(d ^GF ₀ ) ² /4λ, as the distance from the focal position at which the beam cross section has increased by a factor of 2.

und weisen eine klare Separierbarkeit in eine transversale und eine longitudinale Abhängigkeit der Form $$\text{U}\left(\text{x}\text{,y}\text{,z}\right)={\text{U}}_{\text{t}}\left(\text{x}\text{,y}\right)\text{exp}\left({\text{ik}}_{\text{z}}\text{z}\right)$$

auf. Hierbei ist k=ω/c der Wellenvektor mit seinen transversalen und longitudinalen Komponenten k²=k_z ²+k_t ² und Ut(x,y) eine beliebige komplexwertige Funktion, die nur von den transversalen Koordinaten x,y abhängt. Die z-Abhängigkeit in Strahlausbreitungsrichtung in U(x,y,z) führt zu einer reinen Phasenmodulation, so dass die zugehörige Intensität I der Lösung propagationsinvariant beziehungsweise nicht-beugend ist: $$\text{I}\left(\text{x}\text{,y}\text{,z}\right)={\left|\text{U}\left(\text{x}\text{,y}\text{,z}\right)\right|}^2=\text{I}\left(\text{x}\text{,y}\text{,0}\right).$$

However, a laser beam can also be a quasi non-diffracting beam. Non-diffracting rays obey the Helmholtz equation: $${\nabla }^2\text{u}\left(\text{x}\text{y}\text{e.g}\right)+{\text{k}}^2\text{u}\left(\text{x}\text{y}\text{e.g}\right)=0$$

and show a clear separability into a transverse and a longitudinal dependence of the shape $$\text{u}\left(\text{x}\text{y}\text{e.g}\right)={\text{u}}_{\text{t}}\left(\text{x}\text{y}\right)\text{ex}\left({\text{ik}}_{\text{e.g}}\text{e.g}\right)$$

on. Here k=ω/c is the wave vector with its transversal and longitudinal components k ² =k _z ² +k _t ² and Ut(x,y) is an arbitrary complex-valued function that depends only on the transversal coordinates x,y depends. The z-dependence in the direction of beam propagation in U(x,y,z) leads to a pure phase modulation, so that the associated intensity I of the solution is propagation-invariant or non-diffractive: $$\text{I}\left(\text{x}\text{y}\text{e.g}\right)={\left|\text{u}\left(\text{x}\text{y}\text{e.g}\right)\right|}^2=\text{I}\left(\text{x}\text{y}\text{,0}\right).$$

This approach provides different solution classes in different coordinate systems, such as Mathieu rays in elliptic-cylindrical coordinates or Bessel rays in circular-cylindrical coordinates.

Experimentally, a large number of non-diffracting beams can be realized to a good approximation, i.e. quasi non-diffracting beams, with beam shaping optics, as will be described further below. In contrast to the theoretical construct, these lead only to a finite performance. The length L of the propagation invariance of these quasi non-diffracting rays is also finite.

Furthermore, we define the transversal dimensions of local intensity maxima as the transversal focal zone or as the diameter of the beam profile for quasi-non-diffracting rays d ^ND ₀ as the shortest distance of directly adjacent, opposite intensity minima.

The longitudinal extent of the focal zone in the direction of beam propagation of these intensity maxima, which are almost propagation-invariant, indicates the characteristic length L of the quasi-non-diffracting beam. This is defined by the intensity drop to 50%, starting from the local intensity maximum in the positive and negative z-direction, i.e. in the direction of propagation.

A quasi-non-diffracting ray is present if and only if for d ^ND ₀ ≈d ^GF ₀ , i.e. similar transverse dimensions, the characteristic length L clearly exceeds the Rayleigh length of the associated Gaussian focus, for example if L>10z _R .

Quasi-Bessel rays or Bessel-like rays, also called Bessel rays here, are known as a subset of the quasi-non-diffracting rays. Here, the transversal field distribution Ut(x,y) in the vicinity of the optical axis obeys a Bessel function of the first kind of order n to a good approximation production are widespread. Thus, the illumination of an axicon in a refractive, diffractive or reflective design with a collimated Gaussian beam allows the formation of the Bessel-Gaussian beam. The associated transverse field distribution in the vicinity of the optical axis obeys a good approximation to a Bessel function of the first kind of order 0, which is enveloped by a Gaussian distribution.

Due to the quasi non-diffracting beam, a given laser intensity is homogeneously distributed over an area that is enlarged in particular in the direction of propagation. This prevents the formation of cracks, so that larger connection cross sections or weld seams can be produced, especially in the direction of propagation.

Due to the virtually non-diffracting beam, a significantly larger focus position tolerance can also be achieved during joining. Thus, for example, the influence of local ripples in the material and the focus adjustment is reduced. Accordingly, it can be advantageous to use a virtually non-diffracting beam, in particular a Bessel beam, for joining. The proposed method can thus be used in a further area of application—for example, even when the workpieces to be joined are not perfectly flat on one another in the area of the desired weld seam and there is a corresponding gap between the workpieces. The use of a quasi non-diffracting beam is particularly advantageous for bridging larger gaps, since larger gaps can be bridged in comparison to a diffracting Gaussian beam. In this way, the requirements on the surfaces of the joining partners, for example with regard to their roughness or flatness, can be reduced and the desired joining result can still be achieved.

Typical Bessel-Gauss beams that can be used for joining have, for example, a diameter of the central intensity maximum on the optical axis of d ^ND ₀ =2.5 μm. A Gaussian focus with d ^ND ₀ ≈d ^GF ₀ =2.5 µm, on the other hand, is characterized by a focus length in air of only z _R ≈5 µm at λ=1 µm. In these cases, which are relevant for material processing, even L>>10z _R can apply. Focus zones with a length of between 150 μm and 500 μm are also preferred for joining, with a length of 300 μm being particularly preferred to produce large connection cross sections or to produce wide weld seams.

In particular, the term "focus" can be understood in general as a targeted increase in intensity, with the laser energy converging in a "focus zone". In particular, the term "focus" is therefore used in the following regardless of the beam shape actually used and the methods for bringing about an intensity increase. The location of the increase in intensity along the direction of beam propagation can also be influenced by "focusing". For example, the increase in intensity can be more or less punctiform and the focus zone can have a Gaussian intensity cross section, as is provided by a Gaussian laser beam. The increase in intensity can also be in the form of a line, with a Bessel-shaped focal zone being produced around the focal position, as can be provided by a non-diffracting beam. Furthermore, other more complex beam shapes are also possible, the focus position of which extends in three dimensions, such as a multi-spot profile of Gaussian laser beams.

The laser beam can be focused along the propagation direction by means of appropriate focusing optics. During focusing, the intensity of the laser beam is maximized towards the position of the laser focus. In the direction of beam propagation in front of or behind the position of the focal zone, the intensity of the laser beam is accordingly lower than in the position of the focal zone itself. This applies in particular to Gaussian beams, which enable defined focusing. In the case of quasi-non-diffracting beams, for example Bessel beams or their experimental implementation, focusing does not take place or only to a lesser extent, so that the result here is a focus zone that is extended in the direction of beam propagation rather than a defined focus position.

By shifting the position of the laser focus along the beam propagation direction, or by focusing, the insertion depth of the laser can be determined relative to a surface of a material to be processed, with the insertion depth being given by the distance between the focus position and the surface of the material.

character list

• 1 einen schematischen Aufbau zur Durchführung des Verfahrens;
• 2 verschiedene Laserpulskonfigurationen in Abhängigkeit vom CTE der Fügepartner;
• 3 Schweißnähte mit und ohne an die CTEs der Fügepartner angepassten Laserpulskonfiguration;
• 4 verschiedene mögliche Laserstrahlprofile; und
• 5 zeitlich modulierte Laserpulskonfigurationen.

Preferred further embodiments of the invention are explained in more detail by the following description of the figures. show:

• 1 a schematic structure for carrying out the method;
• 2 different laser pulse configurations depending on the CTE of the joining partners;
• 3 Welds with and without laser pulse configuration adapted to the CTEs of the joining partners;
• 4 various possible laser beam profiles; and
• 5 time-modulated laser pulse configurations.

Detailed description of preferred embodiments

Preferred exemplary embodiments are described below with reference to the figures. Elements that are the same, similar or have the same effect are provided with identical reference symbols in the different figures, and a repeated description of these elements is sometimes dispensed with in order to avoid redundancies.

In 1 1 is a schematic representation of a structure 1 for carrying out the method and a very schematic cross section of two joining partners 3, 8 to be joined together.

An ultra-short pulse laser 4 provides ultra-short laser pulses of a laser beam 5. These can be introduced into the joining partners 3, 8 in the form of a sequence of individual pulses or in the form of a sequence of bursts.

The laser wavelength can be between 200 nm and 5000 nm and/or the repetition rate of the sequence of individual pulses or bursts can be between 1 kHz and 50 MHz and/or the repetition rate of the laser pulses in a burst can be between 1 MHz and 50 GHz. In particular, the average laser power of a laser spot can be between 0.5W and 50W.

The laser beam 5 is focused by suitable focusing optics 6, shown very schematically here, in such a way that the focus zone F, i.e. the area of increased intensity of the laser beam 5, approximately coincides with the common interface 7 of the two joining partners 3, 8.

For example, the fluence in the focal zone F can be at least 0.01J/cm ² . By focusing with the focusing optics 6, in particular the insertion height of the focal zone F can be determined here relative to the first surface of the layer system 8 in the beam direction. In order to focus the laser beam 5 in the common interface 7 of the joining partners 3 , 8 , the first joining partner 8 in the beam propagation direction must be transparent to the wavelength of the laser 4 .

At the boundary surface 7, successive laser pulses are absorbed in the focus zone F in such a way that the material of the joining partners 3, 8 melts and connects across the boundary surface 7 to the other joining partner 8, 3 in each case. As soon as the melt cools down, there is a permanent connection between the two parts to be joined 3, 8. In other words, the two parts to be joined 3, 8 are welded to one another in this area joined. This area, in which the melting and joining of the materials and the subsequent cooling of the melt takes place and in which the actual joining takes place, is also referred to as the joining area. The cooled melt and material connection of the joining partners 3, 8 forms a weld seam.

Due to the strong thermal expansion of the two joining partners 3, 8, cracks can form during or after the joining process, in particular during the cooling process. On the one hand, this can be due to the average power of the laser 4 being too great. However, it is also possible that the configuration of the laser irradiation of the joining partners 3, 8 can enable crack-free bridging of the gap with the same average power.

In 2 different configurations for laser irradiation are shown as examples.

For example, the joining partner 3 can have a first thermal expansion coefficient CTE3 and the second joining partner 8 can have a second thermal expansion coefficient CTE8. The larger of the thermal expansion coefficients CTEmax=max(CTE3, CTE8) is decisive for the type of laser irradiation.

If CTEmax is greater than or equal to a first threshold CTEt1, it is particularly advantageous to introduce the average laser power into the joining partners in the form of a sequence of individual pulses. This is very schematic in 2 a) shown.

If CTEmax is smaller than the first threshold CTEt1, it is particularly advantageous to deliver the mean laser power into the material by means of a sequence of bursts. If CTEmax is greater than CTEt2 and less than CTEt1, bursts are preferably used which include exactly two laser pulses. This is very schematic in 2 B) shown.

If CTEmax is smaller than CTEt2, more than two laser pulses per burst can also be used. This is very schematic in 2c) shown.

For example, CTEt1 can be 100ppm/K and CTEt2 can be 1ppm/K. Depending on the material of the joining partners 3, 8 and the corresponding thermal expansion coefficients CTE, an allocation to the above scheme can then take place.

In 3A the top view of the joining partners 3, 8 is shown, with the first joining partner 8 in the beam propagation direction being visible. By means of a feed system (not shown), a large number of spot welds were introduced into the interface 7 of the layered system of joining partners 3, 8 in order to form a weld seam 9. The joining partners and the laser beam 5 were shifted relative to one another at a feed rate VG in order to produce a continuous weld seam.

For example, the feed speed VG, ie the relative speed of movement between the laser beam 5 and the joining partners 3, 8, can be between 0.01 and 1000 mm/s. For example, in this case the average power was 1W, with CTEmax being 0.5ppm/K, CTEt1 being 100ppm/K and CTEt2 being 1ppm/K. Furthermore, a sequence of single pulses was introduced into the material. Clear cracks 90 along the weld seam 9 can be seen.

In 3B the same joining partners 3, 8 are shown in a top view, again with several welds being inscribed. In contrast to 3A a sequence of bursts was used, each burst comprising four laser pulses. The mean power of the laser was identical too 3A . It's compared to 3A It can clearly be seen that no cracks formed in the area of the weld seam 9, since the changed laser pulse configuration and energy input prevent damage to the joining partners 3, 8. In particular, the joint connection in 2 B less than one crack per millimeter, so that the joint is free of cracks.

In 4A the intensity profile and beam cross section of a quasi-non-diffracting laser beam 5 is shown. In particular, the quasi-non-diffractive beam 5 is a Bessel-Gaussian beam. In the beam cross-section in the xy plane, the Bessel-Gauss beam has radial symmetry, so that the intensity of the laser beam only depends on the distance from the optical axis. In particular, the transverse beam diameter d ^ND ₀ is between 0.25 μm and 10 μm.

In 4B shows the longitudinal beam cross-section, i.e. the beam cross-section in the direction of beam propagation. The beam cross-section has an elongated focal zone that is about 300 µm in size. The focus zone F is thus significantly larger in the direction of propagation than the beam cross section in the xy plane, so that there is an elongated focus zone.

In 4C is analogous to 4A a Bessel beam is shown which has a non-radially symmetric beam cross-section. In particular, the beam cross-section appears stretched in the y-direction, almost elliptical.

In 4D shows the longitudinal focal zone of the Bessel beam, which again has a longitudinal extension of about 300 µm. Accordingly, the Bessel beam also has a focal zone that is elongated in the direction of beam propagation.

In 5A a temporal modulation of the pulse energy is shown. The black bars can be understood as single pulses, so that the pulse energy of consecutive single pulses is modulated. The black bars can also be understood as bursts, so that consecutive bursts have different mean energies. In particular, all of the black bars can also be a burst, so that the temporal modulation takes place during a burst. The modulation rate can be between 100 Hz and 10 kHz, for example 1 kHz. For example, with a 200 kHz modulation rate and bursts comprising 2 pulses, 200 bursts can be provided during one period.

The modulation form is sine ² -shaped, so that, for example, the pulse energy of the successive individual pulses differ from one another according to the sine ² function. Analogous to this is in 5B a temporal modulation of the pulse energy is shown, the modulation shape being triangular here. The laser pulse energy follows a triangular function.

The modulation forms shown make it possible for the joining partners 30, 31 to cool down slightly between the introduction of the pulses with the maximum power shown, so that cracking in the material of the joining partners 30, 31 is prevented.

As far as applicable, all individual features that are presented in the exemplary embodiments can be combined with one another and/or exchanged without departing from the scope of the invention.

Reference List

1

contraption

3

joining partner

4

laser

5

laser beam

6

focusing optics

7

interface

8th

joining partner

9

Weld

90

Crack

f

focus zone

VG

feed rate

Claims (15)

Method for joining at least two joining partners (3, 8) using ultra-short laser pulses of a laser beam (5) from an ultra-short pulse laser (4), with at least one of the joining partners (3, 8) being transparent for the laser wavelength used, with all joining partners (3, 8) have a thermal expansion coefficient (CTE) and at least one of the joining partners (3, 8) has the highest thermal expansion coefficient (CTEmax) of all joining partners (3, 8), the joining partners (3, 8) being connected to one another by means of ultra-short laser pulses of the laser beam (5). are joined, characterized in that a first threshold (CTEt1) of the thermal expansion coefficient is determined, and the joining partners (3, 8) are joined with a sequence of individual pulses of the laser beam (5) if the greatest thermal expansion coefficient (CTEmax) of the first Threshold (CTEt1) corresponds to or exceeds it, and the joining partners (3, 8) with a sequence of bursts of at least two laser pulses de s laser beam (5) are joined when the largest thermal expansion coefficient (CTEmax) falls below the first threshold (CTEt1). procedure after claim 1 , characterized in that a second threshold (CTEt2) is determined and the joining partners (3, 8) are joined with a sequence of individual pulses of the laser beam (5) when the largest thermal expansion coefficient (CTEmax) corresponds to the first threshold (CTEt1). or exceeds this, the joining partners (3, 8) are joined with a sequence of bursts of exactly two laser pulses of the laser beam (5) if the largest thermal expansion coefficient (CTEmax) between the first threshold (CTEt1) and the second threshold (CTEt2) and the joining partners (3, 8) are joined with a sequence of bursts of at least three laser pulses of the laser beam (5) if the largest thermal expansion coefficient (CTEmax) corresponds to or falls below the second threshold (CTEt2). procedure after claim 1 or 2 , characterized in that the first threshold (CTEt1) is 10ppm/K and the second threshold (CTEt2) is preferably 1ppm/K. Method according to one of the preceding claims, characterized in that the first threshold (CTEt1) and preferably the second threshold (CTEt2) based on the combination of the materials of the joining partners (3, 8) are determined, preferably in a preliminary test. Method according to one of the preceding claims, characterized in that the first threshold (CTEt1) and preferably the second threshold (CTEt2) based on the input of the materials involved in the joining partners (3, 8) are automatically selected from a database and on this basis a automatic guidance of the joining process is carried out either with single pulses or with bursts. Method according to one of the preceding claims, characterized in that the joining partners (3, 8) rest against one another at an interface (7), the joining being carried out at the interface (7) and the laser beam (5) passing through at least one of the joining partners ( 3, 8) interacts with at least one of the joining partners (3, 8) in the region of the interface (7). Method according to one of the preceding claims, characterized in that the average power of the laser (4) is between 0.5W and 50W. procedure after claim 7 , characterized in that the average power is defined as the power at which essentially no cracking occurs when the joining partners (3, 8) are joined, in particular less than 1 crack (90) per millimeter of the weld seam (9). procedure after claim 7 or 8th , characterized in that the pulse energy of the laser pulses is modulated in a burst for the respective subsequent laser pulse, preferably the pulse energy of the respective subsequent laser pulse is greater or smaller than the pulse energy of the respective preceding laser pulse. procedure after claim 9 , characterized in that the pulse energy of the laser pulses is modulated in time, preferably with the average power remaining the same, the modulation frequency preferably being between 100 Hz and 10 kHz. Method according to one of the preceding claims, characterized in that - the laser pulses of a burst each have a time interval of at most 1 µs, preferably between 0.05 ns and 1000 ns, and the bursts of the sequence of bursts each have a time interval of at least 2 µs, preferably between 2 μs and 1000 μs to one another and/or the individual pulses of the sequence of individual pulses each have a time interval of at least 0.02 μs, preferably between 0.02 μs and 1000 μs, to one another. Method according to one of the preceding claims, characterized in that the time intervals between the laser pulses of all bursts used for joining are the same length and/or the time intervals between the bursts are the same length for all bursts used for joining and/or all time intervals between all of them single pulses used for joining are of the same length. Method according to one of the preceding claims, characterized in that - the laser wavelength is between 200nm and 5000nm, preferably 1000nm, and/or - the pulse duration of the laser pulses is between 10fs and 50ps, and/or - the laser beam (5) and the joining partners (3, 8) are moved and/or positioned relative to one another. Method according to one of the preceding claims, characterized in that the laser beam (5) is focused in a focal zone (F) in the joining partners (3, 8) and the fluence in the focal zone (F) is greater than 0.01 J/cm ² for a individual single pulse or a laser pulse of a burst. Method according to one of the preceding claims, characterized in that the laser beam (5) is a quasi non-diffracting laser beam (5), preferably a Bessel beam or a Gauss-Bessel beam, and the laser beam (5) is preferably elongated in the direction of beam propagation Focus zone (F) has.

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