DE102015121064B3 - Joining by fusion bath displacement - Google Patents

Abstract

A method of joining joining partners of dissimilar composition, comprising: arranging a first joining partner in mechanical contact with a second joining partner; - Applying a first laser beam through a recess in the first joining partner on a first surface region of the second joining partner; Laser-induced generation of a melt between the first surface region and a second surface region of the second joining partner lying opposite the first surface region, wherein the first surface region is melted at least in sections into the melt; - Moving the melt at least in a portion of the recess and holding the melt at least partially within the recess; - Solidification of the melt, whereby the two joining partners are anchored to each other via the solidified in the recess melt.

Description

The invention is in the field of mechanical engineering and relates to joining techniques for metallic materials, in particular the joining of dissimilar materials in an overlap arrangement.

A variety of engineering designs require joining dissimilar materials to combine different material properties (eg, weight, strength, ductility, thermal conductivity). The publication DE 694 22 087 T2 describes a semiconductor electric heater and method of making the same, wherein a molten semiconductor material flows into a hole in a metal material, reacts with the metal material, and solidifies to form a bond having a substantially resistive electrical resistance between the metal member and the semiconductor element. The publication DE 10 2010 018 354 A1 describes a method and an arrangement for influencing the weld pool in a weld seam. In particular, the combination of steels and aluminum alloys in automotive body construction is desired in order to operate lightweight while ensuring crash safety can. However, the use of mixed material construction methods is made more difficult by the fact that conventional fusion welding methods for joining can not be used here. For example, gas metal arc welding, laser beam welding and resistance spot welding are based on the generation of a molten phase which, with dissimilar materials, causes the formation of brittle intermetallic phases. These intermetallic phases reduce the fatigue strength of the compound and are therefore to be avoided.

Against this background, a method according to claim 1 and the use of the described method according to claim 16 is proposed.

• – Anordnen eines ersten Fügepartners in mechanischem Kontakt mit einem zweiten Fügepartner;
• – Applizieren eines ersten Laserstrahls durch eine Ausnehmung im ersten Fügepartner auf einen ersten Oberflächenbereich des zweiten Fügepartners;
• – Laserinduziertes Erzeugen einer Schmelze zwischen dem ersten Oberflächenbereich und einem dem ersten Oberflächenbereich gegenüber liegenden zweiten Oberflächenbereich des zweiten Fügepartners, wobei der erste Oberflächenbereich des zweiten Fügepartners zumindest abschnittsweise zu der Schmelze aufgeschmolzen wird;
• – Bewegen der Schmelze zumindest in einen Abschnitt der Ausnehmung und Halten der Schmelze zumindest teilweise innerhalb der Ausnehmung;
• – Abkühlen und/oder Erstarren der Schmelze, wodurch die beiden Fügepartner über die in der Ausnehmung erstarrte Schmelze miteinander verankert werden.

According to a first aspect, therefore, a joining method for joining partners made of dissimilar materials is proposed, which comprises the following steps:

• Arranging a first joining partner in mechanical contact with a second joining partner;
• - Applying a first laser beam through a recess in the first joining partner on a first surface region of the second joining partner;
• Laser-induced generation of a melt between the first surface region and a second surface region of the second joining partner lying opposite the first surface region, the first surface region of the second joining partner being melted at least in sections into the melt;
• - Moving the melt at least in a portion of the recess and holding the melt at least partially within the recess;
• - Cooling and / or solidification of the melt, whereby the two joining partners are anchored to each other via the solidified in the recess melt.

Thus, the joining of the two joining partners takes place mainly via the material of the second joining partner which melts at a lower temperature than that of the first joining partner. Through a recess in the first joining partner by means of laser melted material of the second joining partner is moved into the recess of the first joining partner and solidifies there. If the solidified material at least partially encloses an undercut or an edge of the recess in the first joining partner, this results in a mechanical anchoring. Alternatively or additionally, and independently of the thus achievable form-fitting, a material bond can be formed (depending on the material and the process). An advantage of this embodiment is achieved with only a few process steps training a reliable joint connection. The movement of the melt is advantageously carried out under the action of an externally controllable force, which acts directly on the molten material. This force can, for example, be a compressive force of a gas or a compressive force of the surrounding atmosphere when a negative pressure is applied to the opposite side of the joining partners brought into mechanical contact with one another. Likewise, centrifugal forces can be used, for example if a laser spot moves in a corresponding manner with a mechanically contacted and continuously moved material composite, and the melt produced by the laser spot as the two joining partners pass through a deflection roller into one or more recesses in the outer (upper) Joining partner is thrown and solidified therein by the abrupt withdrawal of heat on contact with the wall of the recess. As described in more detail below, but also an electromagnetically induced force for directional movement of the melt can be used.

Namely, according to a preferred embodiment, the moving and / or holding comprises generating an eddy current in the melt, wherein the melt is at least partially moved into the recess and held in the recess.

Advantageously, the generation of the eddy current can be done without contact and only requires a temporary effort to fix the two joining partners with little effort. The two joining partners must be stabilized during the joining process only briefly to each other and kept in mechanical contact with each other. Particularly characteristic of the above-described embodiments of the proposed method is thus in contrast to other techniques - apart from the contact of the joining partners to each other - performed without contact training of the joining zone.

According to a typical embodiment, the eddy current is generated in the melt by means of an electrical coil placed in the vicinity of the second surface region of the second joining partner and / or a pole shoe associated with the coil.

Advantages of using a coil and / or associated with the coil ferromagnetic molding, such as a ferromagnetic coil core, pole piece or more pole pieces resulting from the focusing of the magnetic field generated by the coil. By suitable design of pole shoes, the force acting on the melt force can be distributed so that the recess is largely uniformly filled with the melt.

According to a further embodiment, the coil is electrically driven such that an oscillating magnetic field is generated and a Lorentz force directed at least partially in the direction of the recess at least partially moves the melt into the recess and / or holds it therein.

Advantages of this embodiment result from the relatively easily controllable strength of the force acting on the melt and the thus controllable volume of the melt, which can be held in the recess and solidified there.

According to a further embodiment, a density of the generated magnetic flux is in the range of 1 mT to 1 T, in particular between 50 mT and 750 mT, for example between 100 mT and 500 mT.

Advantages of this embodiment are that magnetic flux densities of this order of magnitude can be achieved with conventional coil systems and materials, and / or are reproducibly adjustable.

According to a further embodiment, the frequency of the oscillating magnetic field is less than 5 kHz for material thicknesses of the melting joining partner below about 5 mm and less than 1 kHz for material thicknesses of the melting joining partner greater than about 5 mm. As is known, the penetration depth of an alternating field behaves inversely proportional to the frequency of the alternating field. For higher penetration depths (or material thicknesses), relatively lower frequencies are preferred over thinner materials. The frequency of the oscillating magnetic field for material thicknesses of the low-melting composite partner of 5 mm or below 5 mm is preferably between 60 Hz and 1 MHz, in particular 100 Hz to 10 kHz, preferably between 500 Hz and 5 kHz or less than 5 kHz and less than 1 kHz for material thicknesses> 5 mm, in particular less than 750 Hz.

Advantages of this embodiment are therefore based on the fact that the penetration depth of the oscillating magnetic field over the frequency of the alternating electric field is adaptable to the material thickness of the joining partner to be melted, so that a heating of the joining partner having the recess (which is typically the upper joining partner) is avoided.

According to a further embodiment, the application of the first laser beam through the recess in the first joining partner precedes the production of the recess itself with the same laser beam, with another laser beam or with an electron beam. Laser beam (s) and electron beam can be applied continuously or discontinuously, or pulsed.

Advantages of this embodiment arise with the extended process freedom. For example, a laser which is used in parallel to the joining or subsequently to it - for example for the cutting of the material - can be used partially to form the joint connection. The combination of lasers of different power (density) extends the range of materials that can be used as first and second joining partners. The use of an electron beam makes it possible, due to the action of the low-melting second joining partner as a base material, to produce precise through-holes of advantageous geometry in the higher-melting first joining partner. In addition, eliminates the need for accurate detection of Position of an already existing shaping, as long as between a first and a second laser use the joining partners are not moved in relation to each other.

According to a further embodiment, the recess has a diameter which is reduced in the direction of the second joining partner and / or an undercut.

Advantages arise with the improved positive locking and the achieved mechanical anchoring of the two joining partners with each other.

According to a further embodiment, a preferred melt temperature is set and / or maintained when moving over an applied power of the first or second laser and / or by the strength of the induced eddy current.

Advantages of this embodiment are obtained with an improved temperature control in the melt, resulting in a reduced brittleness of the solidified melt and thus increased strength of the achieved joining connection.

According to a further embodiment, the holding and / or targeted solidification of the melt comprises a timed control of a laser power (or the power of an electron beam when such is used to produce the melt) and / or an electrical signal used to generate the eddy current.

Advantages arise with a possible energy saving by the energy input remains limited to the time required to produce the joint connection. In addition, a cooling curve thus defined can advantageously influence the solidification behavior by reducing the formation of shrinkage stresses and avoiding formation of cracks.

According to a further embodiment, a volume of the melt moved into the recess in a solidified state is smaller than a volume of the recess itself. In other words, the solidified melt does not completely fill the recess.

Advantages result from the fact that a total of for the operation of at least one laser and at least one coil spent electrical power can be minimized and still a reliable joint connection can be achieved.

According to another embodiment, the recess in the first, e.g. higher-melting or difficult to be machined to be drilled joint partner produced by drilling and thus includes a circular through-hole, such as a hole.

Typically, a bore is easier to produce than a possibly consuming to be milled recess with a complex contour having different radii of curvature.

According to a further embodiment, the first joining partner comprises a material which melts at a higher temperature (which is higher melting) than a material covered by the second joining partner.

Especially for the joining of such different materials, the joining method proposed here is attractive due to short process times and high durability. A component produced according to the method is stabilized by a combination of material bond and positive connection at discrete sections. In this case, the said combination of material bond and positive connection is realized by the previously described introduction of a melt of a portion of the lower melting joining partner into a recess of the higher melting joint partner and the solidification of the melt in the recess. Advantageously, can be dispensed with additional components or tools - such as screws and rivets - as well as the use of appropriate tools or other steps. Overall, this results in a comparatively lower mass and a shorter production time of the component.

According to a further embodiment, the first joining partner comprises
a carbon fiber reinforced plastic, a steel, titanium, a titanium alloy or a ceramic and the second joining partner comprises a light metal, a light metal alloy, a polymer or a polymer composite, in particular a polymer filled with carbon or metal particles. Be the beginning Of course, unfilled (pure) plastics in the molten state, eg due to the action of gas, can be forced into the recess.

It is advantageous to use the proposed joining method especially for combinations of dissimilar materials, since the use of additional aids, such as, for example, the use of washers on the side of the softer material can be dispensed with. Such washers (O-rings) are typically indispensable for the hitherto conventional screw or rivet connections.

According to a further embodiment of the method, at least one of the two joining partners is a metal sheet.

Especially when using sheet metal, for example in vehicle construction in general (aircraft, automotive construction, shipbuilding ...), in mechanical and plant engineering or in the technical building equipment sheets for purposes of sound or thermal insulation are typically provided over a large area with a dissimilar material , Since lightweight construction offers energetic advantages in said fields of application, the use of the method for the purpose described, or the use of the components accessible according to the method, is of economic advantage.

According to a further aspect, the use of the method described above for producing a composite metal sheet comprising one of the combinations aluminum / steel, titanium / aluminum, titanium / copper, steel / copper, tungsten / copper is proposed.

Specific advantages of this use accrue with the particular application of the named material pairings and typically include reduced mass, reduced manufacturing time, and high reliability.

The described embodiments can be combined with each other as desired.

The connection of dissimilar materials is currently done in industrial application by non-melting joining methods. Essentially, in industrial practice, the joining methods gluing, mechanical joining (clinching, riveting, screwing, also in combination with gluing), as well as soldering are common [1]. In soldering processes, the challenge is, in particular, to control the energy input to the extent that the growth of intermetallic phases is largely avoided. In particular for the combination of the joining partners aluminum and steel method approaches have been proposed for this, which are based on a one-sided melting of the aluminum and the use of additional materials for wetting the steel side. It is proposed to use hybrid processes such as, e.g. to use the common use of laser beam and MIG torch [2] or a combination of continuous and pulsed laser beam [3]. Here "MIG" stands for "metal welding with inert gases".

In contrast to previously known methods, the proposed method provides for certain applications immediately sufficient or immediately completely sufficient strength without having to rely on extensive preparation steps. Advantageously, the method proposed here is thus time and cost saving. With the stated advantages, it provides access to industrial practice without any procedural restrictions. In contrast to adhesive bonds, which harden only after a long waiting time and are typically only able to be loaded with complete curing, further processing or also the end application is possible here directly with the production of the welded connection. Additional mechanical joining methods are not required. While adhesive bonds are cumbersome to use and may be weakened by aging effects, the proposed process typically provides a composite of durable stability in only one step.

While mechanical connections require expensive joining elements, such as screws or rivets, and adapted pre-machining (e.g., drilling) or high application forces (e.g., riveting), which in turn require support and retention devices, the proposed method does not require this extra effort. In addition, even high strength or brittle materials (eg, press-hardened boron-manganese steels) can be easily joined if the molding is produced by a laser process. The resulting composites are lighter overall, since no joining elements are required, which would otherwise increase the weight of the construction. In contrast to the previously used only on a laboratory scale method of soldering with partial melting or welding a bimetallic strip, both of which have not found an industrial application in the case of the proposed method is the immediate application in industrial practice or in large-scale production possible.

The invention comprises a novel joining method which is able to overlap non-identical compounds, in particular between a high-melting and a low-melting material, by virtue of the low-melting joining partner being entangled with the latter via a recess in the high-melting joining partner. The formation of intermetallic phases in the joining zone is largely avoided, since only the low-melting joining partner forms the melt, with which the recess is filled. In this case, it is secondary whether the recess was already present in the higher melting (arranged above) joining partner before contacting the two joining partners or is produced directly by means of laser drilling. The presence of intermetallic phases in a joining zone, for example because of deviating thermo-mechanical behavior and the formation of cracking peaks, involves risks with regard to the reliability of the joining zone comprising the intermetallic phase.

As is known, in laser drilling, material is melted and forced out of the resulting bore by evaporation. The recess formed in this case can have an arbitrary contour, depending on whether the laser beam or a path along which it is guided, is continuously maintained. The recess may e.g. a circular bore or an elongated slot or have a polygonal shape. The introduction of such continuous recesses is hereinafter referred to simplifying as drilling. The higher-melting material makes essentially no contribution to a melt formed after drilling below the bore / recess in the low-melting material. The joint achieved with the solidification of the melt pressed into the bore is based on a combination of form-fit and material bond.

The novel joining method is suitable for lap joints of two joining partners, e.g. of different metals, in which the joint partner below has the lower melting point (eg aluminum in a steel-aluminum arrangement). The method is thus based on the introduction of a recess, in particular a through-hole, for example a hole in the upper joint partner (either by way of Vorbearbeitungsschrittes or laser beam, immediately before melting) and a subsequent heating of the underlying joining partner by means of laser through them Recess (eg bore) through. In this case, the lower joining partner is melted. By a suitable choice of the process parameters laser intensity and Einstrahldauer the volume and expansion of the molten bath can be controlled and overheating be avoided, so that the upper joining partner itself is not or only partially melted. The formation of intermetallic phases is thereby largely avoided.

Subsequently, the molten bath is subjected to an alternating (oscillating) magnetic field by means of an electromagnet arranged below the joining connection, which leads to the induction of eddy currents in the material. The generated eddy currents cause the formation of Lorentz forces within the changing magnetic field. The joint now results from these forces pushing the melt upwards against the force of gravity into the passage opening, thus at least partially filling the previously introduced recess (for example a bore) with the melt. The degree of volume displacement can be influenced by the frequency of the magnetic field and its strength. A preferred magnetic flux density for the process is in the range of 1 mT to 1 T, in particular between 10 mT and 750 mT, preferably between 10 mT and 500 mT, for example between 50 mT and 500 mT.

A preferred frequency is between 60 Hz and 1 MHz, in particular 100 Hz to 10 kHz, preferably between 500 Hz and 5 kHz. The respectively preferred frequency depends on the particular circumstances, in particular on the material thickness of the joining partners. Thus, the preferred frequency for material thicknesses of the joining partners is <5 mm below 10 kHz and is typically less than 5 kHz. For material thicknesses of the joining partners (sheets) above 5 mm, the frequency of the magnetic field used for the molten bath displacement is typically less than 1 kHz.

In addition, the laser power can be adjusted during the molten bath displacement to prevent overheating or evaporation of the molten bath. Suitable lasers are commercially available, and for example selected from a CO ₂ laser, a solid-state laser (disk, fiber, rod designs) and a diode laser. Typical industrially available lasers reach powers of 500 W to 100 kW, can be operated continuously (in CW mode) or pulsed, for example up to 20 kHz. For the electromagnetic displacement of the melt, it is advantageous if the melt to the bottom of the lower joining partner sufficient for thicker material thickness (metals) may thereto advantageously be used a deep welding effect, what for an energy density approximately from 10 ⁵ W / cm ² Case is. However, the power required in the specific application depends on many parameters (material, material thickness, thermal conductivity, diameter of the molding and thus melt pool volume, etc.). The advantage is therefore the Power and / or a pulse rate of the laser and / or an exposure time of the surface of the second joining partner to the laser energy controlled so that a molten pool of the corresponding extent is generated. An average power of the laser used is at least 100 W, for example 200 W, preferably 1 kW. These and higher powers in the kW range, in combination with pulsed operation, advantageously permit short process times, so that within one second several, preferably two, more preferably at least five joints are formed using only one laser.

If the supply of energy is terminated by the laser beam, the melt solidifies, whereby a suitable time reduction of the laser power can influence the solidification behavior. The magnetic field continues to act on the melt in order to keep the melt pressed into the joining zone in the joining zone even when the temperature-induced volume shrinkage is used, and to prevent the formation of cracks. Due to the solidified melt, a joint is produced both by mechanical positive engagement and by a metallurgical adhesion at the interface of the artificially similar materials of the overlapping joining partners. The positive connection can be promoted by a conical design of the hole in the higher melting (upper) joint partner.

A conical shape of the recess, for example a conical bore, i. A narrowing of the bore in the upper joining partner to the lower joining partner can be achieved either by known cutting methods or by suitable laser drilling techniques (for example percussion drilling, helical drilling) and is advantageous for both method variants a) and b) described here. Likewise, the introduction of the recess by punching is possible. An undercut leads to the reliable anchoring of the solidified melt in the recess.

As already mentioned, different forces come into consideration for directionally moving the molten bath. The above-described electromagnetic fusion bath displacement represents a prominent embodiment of the proposed method. Likewise, with a suitable arrangement of the joining partner a displacement of the molten bath of the second joining partner in the recess in the first joining partner, for example, under the influence of a centrifugal force.

For an estimation of whether the melt produced in a bore can leave the bore alone under the action of its own weight against the surface tension and the capillary force, the material thickness and the melt bath diameter must also take into account the material thickness and thus the hydrostatic pressure in the melt. For the hypothetical case of using the gravitational force to move the melt of a joining partner in a (then) arranged below recess of the other joining partner of the just yet to be held against the gravitational force melt diameter for aluminum as a low-melting ("second") joining partner roughly as follows : material thickness 10 mm ≤ 15 mm maximum melt pool diameter 15 mm ≤ 10 mm 20 mm ≤ 7.5 mm

For the named material thicknesses of the joining partners in the overlapping area, a respective larger melt bath diameter for "sagging" of the melt in the direction of gravity would lead to a recess in the other joining partner arranged below the melting joining partner. However, these melt bath diameters require unfavorably high laser power. The only gravitational movement and then timely stopped by solidification outflow of a locally produced melt in a recess of the joining partner arranged underneath therefore not practical for the material combinations envisaged here and set parameters. The situation is similar in the case of the joining of plastics by one-sided generation of a melting zone: Although plastics have a significantly lower surface tension, but their flow is hindered due to the significantly higher viscosity. Using the principle of unidirectional molten bath displacement described herein, a force directly acting on the melt can be used to make a joint.

According to the invention, for the application of the Lorentz force as the force acting on the melt, an arrangement of the low-melting joining partner below the higher-melting joining partner, in particular below a recess in the same and in close mechanical contact with that provided.

In addition to the use of laser energy to melt the low-melting joining partner, its molten bath can alternatively also be generated by an electron beam or under its participation. However, a synchronous displacement of the molten bath can only be done by a non-magnetic force, since an immediate combination of magnetic field and electron beam processing precludes each other due to mutual interference. This of course does not preclude the use of an electron beam to produce the recess, as well as an electron beam can be used together (synchronously), alternately or sequentially with a laser beam to first create a recess and then, through the recess, a melt to create.

The accompanying drawings illustrate embodiments and, together with the description, serve to explain the principles of the invention. The elements of the drawings are relative to one another and not necessarily to scale. Like reference numerals designate corresponding parts accordingly.

1 shows a first process variant (a) with laser drilling.

2 shows a second process variant (b) with prefabricated bore.

3 shows a metallographic image (reflected light microscopy) of a ground specimen of the joining result using the example of the materials AlMg ₃ (2 mm, below) and St12 (1 mm, above) according to process variant b). A laser power of 5 kW had an impact duration of 400 ms.

4 shows a modified embodiment of the achieved joint connection

In particular shows 1 schematically the course of the proposed method according to variant a) with the introduction of a hole in the upper joining partner with the aid of a laser.

The first joining partner 1 is in mechanical contact with the second joining partner 2 , Both each comprise a different material and are thus dissimilar.

Preferably, the two joining partners are releasably fixed to each other, for example by a clamping device (not shown). In the first method step / partial image A becomes a first surface area 11 of the first joining partner under the action of a laser beam 4 provided with a recess. After making the recess, the laser beam in the subsequent method step / partial image B is now directed directly onto the exposed surface area of the second joining partner. Under its action, the material of the second joining partner is heated locally, successively melted and a melt 5 educated. When the melted section 5 , or the melt extends to the opposite surface of the second joining partner, sets - according to the subsequent process step / partial image C, the effect of an external force on the melt. In the present case, a Lorentz force F _{L is} induced by suitably controlling a coil with electrical signals. This will cause the magnetic field 7 the coil 6 directly or via pole shoes associated with the coil 6 so on the melt 5 directed that induced eddy currents and / or the Lorentz force F _L press the melt in the recess. Under the action of the Lorentz force, a gentle curvature in the remelted material is formed at the rear side of the joining zone, ie at the side of the low-melting joining partner opposite the recess, as the melt solidifies (cf. 3 ). Unlike a sharp notch, this curvature has a mean radius of curvature which is at least as great or at least 1.5 times as great as the average diameter of the recess in the higher-melting joining partner on the side facing the low-melting joining partner. Preferably, this radius of curvature is at least 2.5 times, preferably more than 3 times, the mean diameter of the recess on the side facing the second joining partner. This radius of curvature thus largely corresponds to the radius of a circular section, the surface of which largely corresponds to the cross-section of the concavity produced. In this context, "largely" means an inequality of at most 20% of the two surfaces. Preferably, an inequality of the two surfaces is less than 10%. Advantages of such large radii of curvature, or of such gentle curvature arise with the high achievable load capacity of the joint connection against a vibration load. At the same time, the laser beam becomes 4 so controlled that the melt 5 remains liquid or at least partially liquid. For this purpose, the area of the melt which is held in a flowable manner preferably has a diameter which corresponds at least to twice the diameter of the recess in the higher-melting joining partner on the side facing the low-melting joining partner. In parallel, the magnetic field or the applied force is maintained until the melt is solidified. In the final process step (see part D), the melt forms a solidification reaching into the recess area 3 from, which causes a local material and preferably an additional local form fit between the two joining partners. Under the material connection, or a cohesive connection is understood in this context, the metallurgical connection of the low-melting joining partner on him facing surface portions of the higher melting joint partner.

2 shows the procedure for the process variant b) using a pre-drilled pre-drilled, first joining partner.

In a process step O preparing the joining, the first joining partner becomes 1 comprising a recess 10 and the second joining partner 2 brought into mechanical contact with each other. Preferably, their position is fixed to each other, for example by a clamping device (not shown). In the following method step / partial image B is a laser beam 4 through the recess to a first surface area 21 of the second joining partner 2 directed. It causes the formation of a melt 5 between the first surface area 21 and the opposite second surface area 22 of the second joining partner. In the subsequent process step / sub-picture C is under the action of an external force, here again a Lorentz force under the influence of a magnetic field 7 the melt pressed into the recess where it is solidified. According to the final method step / partial image D results in a fabric and - in particular in the presence of an undercut recess - an additional form-fitting, which ensures a local connection of the two art mating joining partners.

3 shows the joining result in the practical embodiment in a steel-aluminum connection according to process variant b). The first joining partner 1 here includes a steel sheet. The second joint partner 2 includes aluminum. The solidified in the counterbore melt 3 forms a bulge, which guarantees a local form closure in addition to the achieved material bond. By a partial melting of the upper joining partner in the edge region of the bore an undercut and thus the additional anchoring of the joining partners can also be achieved. In this case, the expansion of the melting is locally limited in order to avoid the formation of intermetallic phases. The illustrated example was generated with 5 kW at an exposure time of 400 ms.

4 shows a joint connection between the two joining partners 1 and 2 in a non-continuous melting of the second joining partner 2 , The solidified molten bath fills the recess 10 only incomplete, but at least in the area of a circumferential undercut. The two joint partners are stapled to each other as it were. Advantages of such an embodiment are the detachable nature of this compound.

When joining flat joining partners (eg metal sheets, overlapping flanges), a high total loading capacity of the composite can be achieved by introducing a large number of individual joining joints and their suitable arrangement. Even with only partial filling can be produced depending on the requirements of a sufficiently strong connection. An exact "filling" of the shape is technically more complex. Against this background, preference is given to only partial replenishment. It is expressly emphasized that the joining method proposed here does not produce a butt joint, that is to say no weld, but leads to the formation of a joining zone in the case of overlapping arrangement of at least two materials of identical construction, comprising a material and / or positive connection of the two joining partners in the joining zone. It is irrelevant whether one of the two joining partners or in the region of or adjacent to the joining zone are additionally cranked or folded.

Alternatively, the displacement of the molten material by means of a gas (stream) it can be done with positive pressure or by displacement by negative pressure. For displacing a plastic melt, a directional jet of high boiling fluid may be used.

According to typical embodiments, for the other parameters described here, preferred material thicknesses of the melting ("second") joining partner are in the range of 0.5-20 mm, while the material thickness of the non-melting (first) joining partner is in the range of 0.5-10 mm lies.

Characteristic of the proposed method is therefore the use of an external force, such as a magnetic field and the utilization of molten bath dynamic effects for targeted displacement of a molten bath generated by laser radiation and producing a joint by forming a positive and / or cohesive connection, after the solidification of a Melt primarily a low-melting joining partner in a recess of a higher melting joint partner.

Advantageously, the method is non-contact and acts without great forces on the joining partners. The effect of the forces used is locally limited and is effectively almost directly limited to the melt. Advantageously compared to mechanical joining no additional materials or joining components are needed. Compared with gluing, the proposed method leads to immediately durable or load-bearing connections and, in the case of simultaneous laser drilling, does not require a pre-processing step. Laser drilling can also penetrate high-strength and ultra-high-strength steels so that these materials can also be joined. This represents a further advantage over mechanical joining. Likewise, when drilling through the upper joining partner, any coating (eg galvanizing) of the material is carried along, thus preventing mixing of the coating material with the melt or in the resulting joining zone. The only one-sided melting of the low-melting joining partner avoids the formation of intermetallic phases. This represents a significant advantage over all other fusion welding processes. Furthermore, a gap bridging capability can be observed, which permits positional tolerances of the joining partners. In addition, the method with simultaneous adjustment of the bore is position independent and does not require such as e.g. Soldering, a precise guide along the joint. By controlling the strength of the magnetically induced eddy currents with simultaneous control of the time energy output by the laser (temporal pulse shaping), it is also possible to control the solidification behavior or the cooling rate of the molten joining partner and thus favorably influence the joining zone properties with respect to reduced residual stresses.

references

• [1] F.M. D. Wit, J.A. Poulis (2012) Joining technologies for automotive components, in: J. Rowe (Ed.) Advanced Materials in Automotive Engineering, Woodhead Publishing Ltd., ISBN / EAN: 9781845695613; pp. 315-329.
• [2] C. Thomy, A. Wirth, M. Kreimeyer, F. Wagner, F. Vollertsen (2007) Laser MIG Hybrid Joining of Aluminum-Steel Lightweight Structures, Laser Engineering Journal, 4: 36-40.
• [3] S. Frank (2015) Flux-free laser joining of aluminum and galvanized steel, J Mater Process Tech, 222: 365-372.

Claims (16)

Method for joining joining partners ( 1 . 2 ) of dissimilar composition comprising: - arranging a first joining partner ( 1 ) in mechanical contact with a second joining partner ( 2 ); - Applying a first laser beam ( 4 ) through a recess ( 10 ) in the first joining partner ( 1 ) to a first surface area ( 21 ) of the second joining partner ( 2 ); Laser-induced production of a melt ( 5 ) between the first surface area ( 21 ) and a second surface area opposite the first surface area ( 22 ) of the second joining partner ( 2 ), the first surface area ( 21 ) at least in sections to the melt ( 5 ) is melted; - moving the melt ( 5 ) at least into a portion of the recess ( 10 ) and holding the melt ( 5 ) at least partially within the recess ( 10 ); - solidification of the melt ( 5 ), whereby the two joining partners ( 1 . 2 ) over in the recess ( 10 ) solidified melt ( 3 ) are anchored together. The method of claim 1, wherein the moving and / or holding comprises: generating an eddy current in the melt ( 5 ), wherein the melt ( 5 ) at least partially into the recess ( 10 ) is moved and / or in the recess ( 10 ) is held. The method of claim 2, wherein generating the eddy current in the melt ( 5 ) by means of a close to the second surface area ( 22 ) of the second joining partner ( 2 ) placed electrical coil ( 6 ) and / or one with the coil ( 6 ) associated pole piece ( 6 ) he follows. Method according to claim 3, wherein the coil ( 6 ) is electrically driven so that an oscillating magnetic field ( 7 ) and a Lorentz force (F _L ) the melt ( 5 ) moves and / or holds directionally.  A method according to claim 4, wherein a magnetic flux density of the magnetic field is in the range of 1 mT to 1 T, between 10 mT and 750 mT, or between 50 mT and 500 mT. Method according to claim 4 or 5, wherein a frequency of the oscillating magnetic field ( 7 ) between 60 Hz and 1 MHz, between 100 Hz to 10 kHz, between 500 Hz and 5 kHz or less than 5 kHz for material thicknesses ≤ 5 mm and less than 1 kHz for material thicknesses> 5 mm. Method according to one of the preceding claims, wherein the application of the first laser beam ( 4 ) through the recess ( 10 ) in the first joining partner ( 1 ) directly generating the recess with the laser beam ( 4 ), with a second laser beam and / or with an electron beam. Method according to one of the preceding claims, wherein the recess ( 10 ) one towards the second joint partner ( 2 ) has reduced diameter or an undercut. Method according to one of the preceding claims, wherein when moving a temperature of the melt ( 5 ) by an applied power of the first or the second laser ( 4 ) and / or maintained by the induced eddy current. Method according to one of the preceding claims, wherein the holding and / or solidification of the melt ( 5 ) is effected by a timed control of a laser power and / or an electrical signal used to generate the eddy current. Method according to one of the preceding claims, wherein a volume of the in the recess ( 10 ) moving melt ( 5 ) in a solidified state is smaller than a volume of the recess. Method according to one of the preceding claims, wherein the recess ( 10 ) is produced by drilling. Method according to one of the preceding claims, wherein the first joining partner ( 1 ) comprises a material which is higher melting than one of the second joining partner ( 2 ). Method according to one of the preceding claims, wherein the first joining partner ( 1 ) comprises a carbon fiber reinforced plastic, a steel, titanium, a titanium alloy or a ceramic, and the second joining partner ( 2 ) comprises a light metal, a light metal alloy, a polymer or a polymer composite. Method according to claim 13 or 14, wherein at least one of the joining partners ( 1 . 2 ) is a metal sheet.  Use of a method according to any one of claims 1 to 15 for producing a composite sheet comprising one of the combinations aluminum / steel, titanium / aluminum, titanium / copper, steel / copper, tungsten / copper.

Images