WO2001017722A1

WO2001017722A1 - Method and device for welding sheet metal

Info

Publication number: WO2001017722A1
Application number: PCT/FR2000/002419
Authority: WO
Inventors: Bernard Criqui; Ludovic Goby; Patrick Habert; Jean-Paul Hardel
Original assignee: Renault
Priority date: 1999-09-03
Filing date: 2000-09-01
Publication date: 2001-03-15
Also published as: FR2798084A1; FR2798084B1

Abstract

The invention concerns a method for welding a stack of sheet metals comprising at least a top sheet metal (10) and a bottom sheet metal (12) one of which at least is coated with a metal coating. The invention is characterised in that it comprises the following successive steps: stacking tightly two sheet metals (10, 12); heating with a first heat source (26) directed on the outer surface of one of the sheet metals of the stack, a first zone (27) such that the temperature of each coated inner surface (16, 20) is higher than the evaporation temperature of the coating of said surface and such that the temperature of each sheet metal, in the proximity of the interface between the two sheet metals (10, 12) is less than the melting point thereof; then, after the vapours of the coating have been evacuated from a second zone included in the first zone, heating with a second source a third zone (42), at a temperature causing local fusion of each sheet metal.

Description

"Method and device for welding sheets"

The invention relates to a method and a device for welding a stack of coated sheets.

The invention relates more particularly to a method of welding a stack of sheets comprising at least one upper sheet and one lower sheet superposed, at least one of which is coated, at least on its internal face adjacent to the other sheet, by at least one metallic coating having a vaporization temperature lower than or equal to the melting temperature of the sheet of the stack having the lowest melting temperature.

The invention also relates to a device for ^" laser welding of a stack of sheets coated with a metallic coating having a vaporization temperature lower than or equal to the melting temperature of the sheet of the stack having the melting temperature 1a. weaker.

Such stacks are increasingly used in the fight against - corrosion, in particular in the automotive industry. Welding a stack of sheets therefore presents significant challenges, especially when it comes to welding a stack of steel sheets coated with zinc or zinc alloys.

Laser welding has the advantage of heating the materials very locally, and thus only causing slight deformations of the zinc coating and of the sheet metal. Laser welding has become a widely used technique for welding coated sheets. Several techniques can be implemented.

One is to weld the edge of the upper sheet onto the lower sheet, this is the technique known as lap welding. The laser should heat the edge of the top sheet. This operation requires great precision, however the sheet tolerances are important. It is therefore necessary to follow the edges of the upper sheet. A device detects in permanently position the edge of the upper sheet and orient the laser source so that its focal point is located at the right end. This device is very expensive.

Another technique consists in heating the upper face of the upper sheet which will transmit, by conduction, the heat to the lower sheet until the bottom zone extends from said upper face to the lower face of the bottom sheet. This is the so-called transparency welding. With this technique, a problem appears in the weld joint. This is due to the vaporization temperature of the zinc

(about 900 ° C) which is lower than the melting temperature of the steel (about 1500 ° C). Consequently, during the welding, a vaporization of the zinc takes place which is detrimental to the quality of the welding.

Bu lles of zinc vapors are formed, causing craters inside the weld, which affects the aesthetics and ^~ especially the quality of the weld.

In fact, since the weld is porous, its mechanical resistance is lower and its seal is not guaranteed.

In addition, welding causes splashes of molten metal to the outside of the sheets. These projections have two effects.

On the one hand, it is necessary to position the optics associated with the laser devices and which are necessary for the focusing of the beam on the sheets to be welded at a minimum distance which is greater than the maximum distance from the projections of molten metal. This minimum distance, which is on the order of 200 to 300 milimeters, is greater than the distance allowing precise focusing of the beam on the sheets to be welded.

On the other hand, splashes of molten metal reduce the quality of welding due to a lack of material. To overcome these drawbacks, it has been proposed to provide a clearance between the sheets to promote the evacuation of zinc vapors during welding.

To this end, French patent application No. 95.02772 proposes to carry out, before welding, a stamping of the zone to be welded.

The international patent application WO-A1 -99/08829 proposes to form protuberances by local heating on the zone to be welded of the upper surface of the lower sheet. After having installed the upper sheet, the two sheets are welded by transparency.

These solutions are not satisfactory. Indeed, they are complicated to implement. In addition, they increase the working time on the sheets and require costly additional operations.

In order to provide a solution to these problems, the invention provides a method of the type described above, characterized in that it comprises the following successive steps: - stacking without play of the two sheets;

- Heating by a first heat source directed on the external face of one of the sheets of the stack, of a first zone, so that the temperature of each coated internal face is greater than or equal to the vaporization temperature of the coating of this face and so that the temperature of each sheet, in the vicinity of the interface between the two sheets, is lower than its melting temperature;

- Then, after the coating vapors have evacuated a second zone included in the first zone, heating by a second heat source of a third zone, included in the second zone, at a temperature allowing the local melting of each sheet up to 'that the stack of sheets is welded.

According to other characteristics of the process: - a relative movement is caused between the two heat sources and the stack of sheets so that the third zone moves relative to the stack of sheets to form a weld bead; - the two heat sources are connected to each other by means, making it possible to permanently adjust their relative positions, according to the direction of welding, as a function of the thickness of the sheets, of the coating and of the speed of the relative movement of the sources heat with respect to the stack of sheets;

- the coating is an alloy containing zinc;

- the coating is zinc;

- The first heat source is a laser source;

- the second heat source is a laser source; - The first and second heat sources use a single laser source supplying each of the two sources.

The invention also provides a device for laser welding of a stack of sheets coated with a metallic coating having a vaporization temperature lower than or equal to the melting temperature of the sheet of the stack having the melting temperature the weakest, characterized in that it comprises two coupled heat sources, and in that it makes it possible to weld the stack by transparency without a set of coated sheets.

According to another characteristic of the device, the two heat sources use a single laser source supplying each of the two sources.

The invention therefore provides a method and a device for welding a stack of zinc-coated sheets by a weld which has good mechanical strength.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows for the understanding of which reference will be made to the appended drawings in which:

- Figure 1 shows a sectional view of two coated sheets; - Figure 2 shows the first step of u assembly process according to the invention of two coated sheets;

- Figure 3 shows the temperature radient in a section along line 3-3 of Figure 2 of the assembly of the two sheets during the first step of the method according to the invention;

- Figure 4 shows the second step of the assembly process according to the invention of the two coated sheets ^" ;

- Figure 5 shows the welded area in a section along line 5-5 as Figure 4 of the assembly of the two sheets during the second step of the method according to the invention;

- Figure 6 shows a device according to the invention, implemented for welding coated sheets.

In the ^' description an upper, lower orientation is used, without limitation, in accordance with fig ure 1. There is shown on fig ure 1 a first sheet 1 0 and a second sheet 1 2.

The first upper sheet 1 0 has an upper face 14 and a lower face 16 which is coated with a layer of zinc 1 8. The second lower sheet 12 has an upper face 20 and a lower face 22. The upper face 20 is coated a layer of zinc 24.

The two faces 16 and 20 coated with zinc are said to be galvanized. Advantageously, according to a variant not shown, the two faces of each sheet are galvanized.

The thickness of the sheets 10 and 12 varies from 0.6 to 2 millimeters and the thickness of the zinc coating varies between 8 and 12 micrometers in the case of applications in the automotive construction field.

The invention consists in carrying out two successive stages of heating an area of the stack of sheets 1 0 and 12. A first embodiment implements a first heating source 26 and a second heating source 28 to produce a substantially cylindrical weld point.

For welding, the two sheets 1 0 and 12 are superimposed without vertical play so that the zinc layers 18 and 24 are in contact in the areas to be welded.

A first step is illustrated in fig ures 2 and 3 implements the first heating source 26.

It allows preheating by radiation of an energy beam ie a first impact zone 27 of substantially circular shape of diameter D 1 of the upper face 14 of the first sheet 1 0. The heat transfer r is done by conduction rapid thus ensuring a slight temperature difference between that of the zone 27 of the upper surface 14 and that at the welding interface between the faces 16 and 24. Preheating continues until a temperature above the zinc evaporation temperature, that is to say greater than 900 ° C is reached at the interface of the zinc layers 1 8 and 24. The temperature of the sheets 1 0 and 12 near the interface remains , meanwhile, at a temperature below the melting temperature for welding, which in the case of steel is greater than 1500 ° C.

This first step ensures the evacuation of zinc vapors outside the future welding zone.

A temperature gradient exists in the assembly of the two sheets 10 and 12. It is partially represented in FIG. 3. A temperature T1 corresponds to the temperature of vaporization of the zinc and a temperature T2 corresponds to a temperature below the melting temperature of the steel.

The zinc and the zinc vapor are then evacuated from a second zone of diameter D2 greater than the diameter of the welding point to be produced at the inside.

Advantageously, the first heating source 26 is a laser source such as a so-called YAG or C0 ₂ source or a laser diode.

The use of a laser diode is suitable for this first step. Indeed, it makes it possible to heat an area 27 of 2 or 3 mm in diameter, unlike other laser sources which focus on a smaller area.

For this first step the use of a YAG laser source is more suitable than the use of a C0 ₂ laser source. In fact, the absorption coefficient of the steel varies as a function of the wavelength of the incident beam. It decreases when the wavelength of the radiation increases. The absorption coefficient of the sheet for radiation emitted by the YAG laser source of 1 micrometer in wavelength is around 20 to 30%. The wavelength of the radiation emitted by the source C0 ₂ is 10 micrometers or ten times greater than that of the radiation emitted by the source YAG, the absorption coefficient of the sheet then being of the order of 10 to 1 5%.

When the heating source 26 is a first laser source, it emits through an optic 32 a laser beam which is strongly absorbed by the first sheet 1 0.

A second step illustrated in FIGS. 4 and 5 is implemented after a determined time. This determined time must be sufficient to allow cooling and condensation of the zinc vapors beyond the second zone of diameter D2, in accordance with FIG. 5, greater than the diameter of the subsequent welding point. In addition, control of the welding penetration during the second heating step is obtained from a constant value of residual temperature of the first stage in the assembly of the two sheets 10 and 12.

The second heating source 28 allows for example the welding with high energy density ie according to a technique known from the state of the art called "key-hole" welding.

In accordance with FIG. 5, an impact zone 40 of the energy beam 36 on the upper face 14 of the first sheet 10 has a diameter D4 less than the diameter D 1 of the first zone 27 heated during the first step . The transfer by thermal conduction allows the two sheets 10 and 12 to be melted in a third zone 42. This third zone 42 has a diameter D3 at the interface between the first sheet 10 and the second sheet 12 which is less than the diameter D2 corresponding to the zone in which the zinc was removed during the first stage.

The zinc having been evacuated from the third zone 42 no gas bubble coming from its vaporization is found in the molten steel.

As a result, there is no projection of molten steel to the outside of the welded area. As the heating source 28 is not exposed to projections, it is possible to bring it closer to the face 14 so as to obtain a short focal distance, that is to say of the order of 1 00 mm. This makes it possible to increase the energy density ie of the source 28 and consequently to reduce the power and therefore the energy consumption of the heating source 28 for identical heating.

After cooling, the third zone 42 is a weld free from any porosity and its mechanical strength is maximum.

Advantageously, the second heating source 28 is a YAG or C02 laser source having a power of between 2 and 6 kW depending on the welding parameters. The use of one or other of the laser sources is equivalent. Indeed, the implementation of it welding in keyhole overcomes the absorption coefficient of the sheets. The energy yields of the two laser sources are similar. If the second heating source 28 is a second laser source, it emits through an optic 34 a beam 36 with a higher energy density than the first heating source 26.

Advantageously, the heating sources 26 and 28 are a single heating source, the power, the focal distance and / or the diameter of the heated area being varied.

According to a second embodiment illustrated in FIG. 6, the first heating source 26 and the second heating source 28 are used to produce a weld bead 44.

The two heating sources 26 and 28 are fixed to the same robotic arm 46 which enables the shape of the determined weld bead 44 to be precisely monitored. The powers of the two sources 26 and 28 are adjusted so as to carry out the two preceding stages successively in a single pass.

The second heating source 28 is fixed to a slave slide 48. The latter makes it possible to adjust the distance between the first 26 and the second 28 heating source so as to maintain the same parameters for implementing the method by taking into account the variations in curvature of the trajectory of the weld bead 44 and / or the thickness of the two sheets 1 0 and 12. As the control of the penetration of welding during the second step is obtained from a constant value of residual temperature in the stack of sheets 1 0 and 12, the residual temperature value is maintained by setting the distance between the sources to compensate for variations in speed and differences in thickness.

When the welding speed increases, it is necessary to increase the distance between the two heat sources 26 and 28, so that the time elapsed between the heating of an area determined by the first heat source 26 and the heating of this same area by the second heat source 28 remains constant.

Likewise, if the thickness of the stack of sheets 10 and 12 varies, the welding speed and / or the distance between the two heating sources must be modified.

The implementation of this second embodiment allows, as in the first, to remove the zinc from the third welded zone 42. The absence of projections and therefore the possibility of bringing the two heating sources closer together makes it possible to increase the welding speed. Indeed the energy density ie being higher, the temperature rise in the stack is faster. Advantageously, the first zone 27 of the upper face 14 of the first sheet 10 which is heated by the first heating source 26 is optimized from the point of view of thermal transfer. It is then of elliptical shape in accordance with FIG. 6, its major axis can, for example, be parallel to the path of the weld bead. This increases the heating efficiency of the area to be welded.

Advantageously, a single laser source emits a laser beam which feeds the optics 32 and 34. The method thus described makes it possible to produce, by transparency, welding spots or beads without porosity from a stack of two sheets without play so that the sheets are in contact in the area to be welded. This eliminates the costly operations of preparing the sheets and facilitates their positioning relative to each other.

Claims

RESELL I CATIONS

1. Process for welding a stack of sheets comprising at least one upper sheet (1 0) and one lower sheet (12) superposed, at least one of which is coated, at least on its internal face adjacent to the other sheet , by at least one metallic coating having a vaporization temperature lower than or equal to the melting temperature of the sheet of the stack having the lowest melting temperature, of the type which comprises the following successive steps:

- stacking without play of the two sheets (1 0, 12);

- heating a first ^source of heat (26) directed onto the outer one of the sheets of the stack face, a first area (27, D 1) so that the temperature of CHAQ ue inner face (16, 20) coated is greater than or equal to the vaporization temperature of the coating of this face and so that the temperature of each sheet, in the vicinity of the interface between the two sheets, is lower than its melting temperature; - then, after the coating vapors have evacuated a second zone (D2) included in the first zone, heating by a second heat source of a third zone (42, D3), included in the second zone (D2), at a temperature allowing local melting of each sheet until the stack of sheets is welded, characterized in that the two heat sources are connected to each other by means (48), making it possible to continuously adjust their relative positions, according to the direction of welding, as a function of the thickness of the sheets, of the coating and of the speed of the relative movement of the heat sources r (26), (28) with respect to the stack of sheets (10, 12).

2. Method according to claim 1, characterized in that it causes a relative movement between the two sources of heat r (26, 28) and the stack of sheets so q ue the third zone (42) moves relative to the stack of sheets to form a weld bead (44).

3. Method according to one q uelconque of resells ications 1 or 2, characterized in that the coating is an alloy containing zinc.

4. Method according to any one of claims 1 or 2, characterized in that the coating is zinc.

5. Method according to any one of the preceding claims, characterized in that the first heat source (26) is a laser source.

6. Method according to any one of the preceding claims, characterized in that the second heat source (28) is a laser source.

7. Method according to sells ications 5 and 6, characterized in that the first and second heat sources (26, 28) use a single laser source supplying each of the two sources.

8. Device for laser welding a stack of sheets (1 0, 12) coated with a metallic coating having a vaporization temperature less than or equal to the melting temperature of the sheet of the stack having the melting temperature 1a. lower, characterized in that it comprises two heat sources (26, 28) coupled and in that it makes it possible to weld by transparency without play the stack of coated sheets according to resell ication 1.

9. Device according to the preceding claim, characterized in that the two heat sources (26, 28) use a single laser source supplying each of the two sources (26, 28).