DE102014105941A1 - Laser beam welding process for the reduction of thermo-mechanical stresses - Google Patents

Abstract

The invention relates to a method for reducing thermomechanical stresses when a weld pool solidifies during welding with a laser beam by subsequently influencing the temperature of the weld metal, the laser beam executing a spatially oscillating movement parallel and / or perpendicular to the weld seam during welding, with solidification of the weld pool is controlled by an additional and synchronous to the spatial oscillation performed time oscillation of the laser beam intensity and / or the laser beam collimation.

Description

The invention relates to a method for laser beam welding, which expands the process window with respect to the hot crack-free welding of metal alloys.

Welding of aluminum wrought alloys in the combination AlMg (non-hardenable, inter alia 5154; 5654; 5454; 5554; 5052) and AlMgSi (hardenable, inter alia 6014; 6061; 6082) is in practice always with the alloying of the melt by additional wire connected. This is due to the fact that aluminum alloys with Si contents of 1% and / or as binary systems with Mg-Si contents of less than 3% have a pronounced hot cracking sensitivity. This is due to the chemical composition of the joining partners, a large temperature interval between liquidus and solidus temperature. In conventional laser beam welding, the low eutectic leads to solidification cracks within the weld and fissuring cracks due to the re-melting of low-melting eutectics and the simultaneous occurrence of thermomechanical stresses.

The welding of components made of high-strength steel (defined as steel grades with a yield strength> 250 MPa) is characterized by two problems. On the one hand, material properties such as springback behavior after forming processes, due to the difficult adjustment of microalloying elements during steelmaking, vary to a certain extent; T. are necessary for the manufacturing process (for example, AlSi coatings for press-hardened steel grades are called), considerable problems for downstream thermal joining process. The combination thus leads to component offsets and gaps between the components to be joined and, secondly, to reduced weldability due to the coating elements penetrating into the molten bath, the chemical composition and thus the viscosity of the melt and affinity for surrounding gases and consequently the oxidation behavior of the melt Influence melt. This is accompanied by a reduction in component strength.

In the prior art, the hot crack sensitivity can be influenced by alloying filler materials so that the solidification interval is reduced to a non-critical level, such. In DE 102 14 949 C1 described. This requires a suitable supply of filler and therefore the use of wire-fed processing optics.

DE 196 30 429 C1 describes a laser beam method without wire feed for joining structural components, which consist of silicon-containing aluminum alloy, in which at least one component is firmly connected before joining in the joint area with a seam-influencing welding filler. However, this method requires additional work steps in the production of the parts to be welded, since a wire-like welding filler has to be attached. As a result, this method is unsuitable for repair welding work.

Especially with steels and superalloys based on nickel, it is known to avoid hot cracking by quenching the molten bath. DE 197 13 701 A1 discloses z. As welding under water. A disadvantage of this method is that an additional cooling medium is required for quenching, which is to bring (targeted) to a position to be quenched.

DE 38 51 702 T2 describes a method of preventing cracking in nickel alloyed substrates whereby a supply of coating particles is dispersed within a molten pool of laser beam and rapid solidification of the bath is effected by performing rapid relative movement between laser beam and bath. However, special coating particles are required to perform this process, and a reduction in hot cracking is limited to nickel alloys with a gamma prime phase.

A downstream thermal treatment to reduce the risk of hot cracking has already been introduced DE 697 32 000 T2 . DE 100 55 950 B4 . DE 103 02 458 B4 and DE 100 42 197 B4 suggested, according to these proposals is in each case an additional component for heat generation in the respective processing device is necessary.

A subsequent thermal treatment without additional heat treatment device is in DE 103 01 445 B4 for the laser welding of lightweight structural components in aircraft. Here, the laser beam is split into two beams lying one behind the other in the feed direction, whereby the two power components are divided in the ratio 60:40. A disadvantage of this method are the fixed ratio of the power components and the fixed distance between the two laser focuses.

The hot crack sensitivity can be further reduced by geometrical component measures, such. In DE 100 31 510 A1 disclosed. These are the example of the lap joint the situation the weld seam to the component edge, the seam shape and position of the beginning and end of the seam on the component (ie, for example, to place the seam beginning on the sheet and not to start at the sheet edge with the weld). In the further course of these measures referred to as clamping of the component, in the form of either releasably interconnected clamping tools as well as by provided with distances to the edge of the component welds and to the effect of the internal clamping action of the component called.

The object of the invention is to reduce the hot cracking sensitivity of weld seams during laser beam welding of metallic alloys with critical solidification interval (relative to the material-specific TFR terminal freezing range) by means of thermal after-treatment, the after-treatment being adapted to the requirements (eg material composition of to be joined workpieces, type of weld, feed rate during welding, etc.) of the welding process to be customizable. This dynamic adjustment should be possible during the welding process and carried out with the laser beam used for welding.

The solution of this task is carried out according to claim 1; expedient embodiments of the invention are located in the subclaims.

According to the invention, a reduction or avoidance of the hot crack sensitivity during laser welding of metallic alloys with critical solidification interval (based on the material-specific TFR) by thermal measures for a targeted solidification, wherein the thermal measures downstream temperature influence of the molten bath by means of a spatial deflection and a temporal energy adaptation of the Include laser beam.

These spatial deflection and temporal energy adaptation of the laser beam effecting the downstream temperature influencing are referred to below as modulation in each case. According to the invention, the modulations occur in a recurring cycle, i. that is, the position of the laser beam focus on the weld metal or the energy of the laser beam oscillate (oscillate) periodically, and the parameters of the oscillation (for example, amplitude and frequency) during a welding operation should be variable; d. h., The oscillation parameters can be changed during welding or adapted to the welding conditions.

The spatial oscillation (i.e., oscillation of the deflection) of the laser beam during the welding process may be one-dimensional (1D-along a line), two-dimensional (2D-in-plane) or even three-dimensional (3D-in-space). For this purpose, the laser beam is deflected by means of suitable devices in the three spatial directions. A deflection of the laser beam along and transverse to the feed direction can be caused by galvanometer scanner. According to the invention it is provided that the spatial dimensions, d. H. the amplitude, the spatial oscillations in the range between 0.2 times and three times the focus diameter, d. H. the spatial extent of the laser light spot on the workpiece or the weld, lie.

The temporal oscillations of the laser beam energy are achieved by a variation of the laser power of the beam source and / or by a collimation adjustment in the axial beam direction (i.e., a widening or focusing of the light beam). The laser power can be done by controlling the power supply or a pulse frequency with pulsed lasers. The collimation adjustment can be motorized, piezoelectric, hydraulic or pneumatic driven (in the axial beam direction).

According to the invention, the spatially and temporally oscillating modulations of the laser beam are synchronized with each other. On the hardware side, this can be done by a laser power adaptation of the beam source synchronized to the galvanometer scanner positioning. These synchronized modulations are referred to hereinafter as 4D modulation.

It is envisaged that the oscillation frequency of the 4D modulation (or the respective individual modulations) is above 100 Hz. The frequency is varied according to the invention during the welding process, with the individual modulations of the 4D modulation always oscillating synchronously (i.e., their respective periods are identical or integer multiples of each other).

For example, it can be provided that the spatial oscillations of the laser beam are performed one-dimensionally along the feed direction, wherein the laser energy in the front (with respect to the feed direction) maximum point of spatial oscillation is maximum and is reduced in the rear turning point, for example, to 75% of the maximum energy. As a result, at the point of welding, a molten bath is produced whose cooling rate can be controlled by 4D modulation.

In addition, it can be provided by 2D oscillations of the deflection of the laser beam, ie perpendicular and parallel to the weld, to increase the volume of the molten area (melting zone), resulting in a reduction the temperature gradient can lead to both sides of the melting zone and thus to reduce the hot cracking tendency.

The advantage of the method according to the invention is that a heat treatment downstream of the welding process of the weld can be performed with the laser used for welding, whereby a targeted solidification of the molten bath to avoid hot cracking can be performed by selecting the oscillation frequency and / or amplitude. Since the parameters (eg oscillation frequency or amplitude) of the 4D modulation can be freely selected, the temperature influence for the reduction of thermomechanical stresses can be adapted and adjusted dynamically during the process or modified accordingly for different weld geometries and / or consideration of the clamping of the component become.

A further advantage of the method according to the invention is that for a continuous weld seam transition (eg fillet weld to I seam) only the process parameters are to be adapted via the 4D modulation. Only the 4D modulation allows the necessary adjustment of the line energy to achieve such a process change.

It can be provided that the oscillations, i. H. the temporal curve of the oscillation amplitude, the shape of a sine curve, a triangle (sawtooth) or a rectangle have. In particular, the rectangular shape of the oscillation may also include switching on and off, for example the laser power. It can also be provided that the different modulations differ in their forms of oscillation. Thus it can be provided that the oscillation form of the spatial modulation is different from that of the temporal modulation. For example, the oscillation of the spatial deflection of the laser beam perpendicular to the feed direction may be done with a sawtooth and square-wave feed, while at the same time the collimation oscillates sinusoidally (and synchronously, i.e., with an integer multiple of the period of spatial modulation).

Furthermore, it can be provided that the parameters of the 4D modulation are selected such that, on the one hand, thermo-mechanical stresses are reduced, taking into account the degree of clamping of the component, and, on the other hand, a defined welding depth is achieved. Thus, by beam modulation, wherein the laser beam is deflected during the welding process such that the necessary for the process beam intensity for controlled power input into the workpiece is always achieved, a controlled weld depth of any seams on the lap joint can be achieved.

It can also be provided that the parameters of the 4D modulation are selected as a function of the feed rate, ie. h., the parameters of the 4D modulation are adapted during the welding process to a changing feed rate. For example, to achieve different feed speeds, the degree of overlap of the oscillatory movement can be selected as a function of the viscosity of the melt.

The fields of application of 4D modulation are not limited to the areas just described. Due to the unrestricted flexibility of the process, the process parameters can be adjusted depending on the requirements of welding, also dynamically during the welding process.

The invention will be explained in more detail with reference to embodiments. These show in a schematic representation of the

1 the joining of a fillet weld at the lap joint; and

2 four design variants of heat post-treatment.

1 shows the laser head with laser optics 1 Which at a predetermined feed velocity v _s of the overlap joint along 2 the two plates 3 is moved from ferritic steel. By means of laser optics 1 integrated deflecting mirror (not shown) becomes the laser beam 4 for producing the weld 5 along the lap joint 2 moved, taking at the position 5.1 a new section of the weld is created. The welding speed v _w is greater than the feed speed v _s . For heat post-treatment, the laser beam 4 to the already created section 5.2 the weld 5 repositioned. This is done with a repositioning speed v _REP , which is much larger than the welding speed v _w . At the same time the laser power is reduced. The laser beam 4 So oscillates periodically along the weld between two sections 5.1 and 5.2 wherein the laser power is varied with the same period.

In 2 are shown on the top of the weld four variants of heat post-treatment. For the sake of clarity, only the lap is 2 the two steel plates 3 displayed. On the representation of the weld was omitted in this figure. 2 shows the laser swept area 6 during the thermal measures for a downstream temperature influence and the isotherms 7 ( 7.1 denotes the isotherm for 200 ° C and 7.2 the for 400 ° C).

In the method according to 2a the laser is in the first section 5.1 the weld seam operated at maximum power. Subsequently, the laser is (back) on the previously welded section 5.2 The laser power is reduced to 75%. This will heat again in the subsection 5.2 introduced the weld to influence the isothermal formation and thus the cooling behavior.

Since it is necessary to influence the cooling behavior superficially laterally of the weld seam, the in 2 B illustrated variant of the method advantageous over the variant according to 2a , The laser beam is defocused by a change in position of the collimator or focusing unit, whereby the area influenced by the laser beam 6 on the workpieces 3 is enlarged. Thus, the area performance (intensity per area) is reduced, which is why in this specific case, no laser power adjustment is required. A defocused heat aftertreatment hinders the heat flow in the component and thus spreads the temperature gradients over a larger area, as at the isotherms 7.1 and 7.2 can be clearly seen.

Another variant is in 2c to see. By means of suitable optical measures with a cylindrical lens and axicon, locally limited heat traces are generated, which are the thermal after-treatment on those zones 6 limited, in which a welding process downstream heat input is necessary. Due to the special limitation of the area swept by the laser beam, the range of high temperatures (up to 1000 ° C) is increased, which contributes significantly to the setting of fine-grained microstructures.

2d shows an embodiment variant of the method in which the laser beam is deflected for downstream heat treatment targeted on that side of the sheet, which induced heat dissipation due to hot cracks. In addition, the laser beam is defocused. The result is the formation of the isothermal field 7 limited to the untreated seam, the isothermal field 7 is shifted and thus counteracts one-sided, component-related heat dissipation.

LIST OF REFERENCE NUMBERS

1

Laser head with laser optics

2

lap

3

steel plate

4

laser beam

5

Weld

6

Irradiated area

7

isothermal

7.1

200 ° C isotherm

7.2

400 ° C isotherm

v _s

 feed rate

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10214949 C1 [0004]
• DE 19630429 C1 [0005]
• DE 19713701 A1 [0006]
• DE 3851702 T2 [0007]
• DE 69732000 T2 [0008]
• DE 10055950 B4 [0008]
• DE 10302458 B4 [0008]
• DE 10042197 B4 [0008]
• DE 10301445 B4 [0009]
• DE 10031510 A1 [0010]

Claims (7)

Laser beam welding process for reducing thermomechanical stresses during solidification of a molten bath during welding by a subsequent temperature control of the weld metal, wherein the laser beam performs a spatially oscillating movement during welding, characterized in that the spatially oscillating movement of the laser beam is parallel and / or perpendicular to the weld and the solidification of the molten bath is controlled by an additional temporal oscillation of the laser beam intensity and / or the laser beam collimation. A method according to claim 1, characterized in that the spatially oscillating movement of the laser beam takes place synchronously with the temporal oscillations of the intensity and / or the collimation. Method according to one of the preceding claims, characterized in that the spatial oscillations have an amplitude in a range between 0.2 times and three times the diameter of the laser focus. Method according to one of the preceding claims, characterized in that the oscillations are carried out with a frequency of more than 100 Hz. Method according to one of the preceding claims, characterized in that the time dependence of the amplitude of the spatial oscillation of the laser beam, the intensity oscillation and / or the collimation oscillation have a sine, rectangular, sawtooth or triangular shape, wherein the synchronous oscillations each may have a different shape. Method according to one of the preceding claims, characterized in that the frequency, the maximum amplitude and / or the shape of the oscillations are variable during welding. Method according to one of the preceding claims, characterized in that the frequency and / or the amplitude and / or the shape of the oscillations is selected so that a predefined welding depth is achieved.

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