AT504925B1 - POINT WELD ELECTRODE AND POINT WELDING METHOD - Google Patents

Description

2 AT 504 925 B1

The invention relates to an electrode for spot welding sheets with a base body and a contact surface, which comes into contact with one of the sheets to be welded, wherein in the contact surface a concave recess is arranged.

Likewise, the invention relates to a method for carrying out a spot welding with two attached to the spot welding gun electrodes, which are each contacted with an outer at least two sheets to be welded.

From the prior art is known to produce so-called viewpoints. This means that essentially no welding point is visible on the visible side of two welded sheets with the naked eye. For this purpose, a flat electrode is used on the visible side of the sheets to be welded. On the opposite side, a relatively spherical electrode is used to obtain a sufficient current density. The flat contact surface of the flat electrode has the consequence that the current density at the weld is relatively low. The reason for this is that on the side of the flat electrode, ie the visible side, the current transfer occurs at local elevations of the surface, which are determined by the roughness of the sheets. Due to this undefined passage of the current, the spot weld does not form reliably. In some cases, the required spot weld is achieved, in some cases the spot weld is too small and sometimes no weld point is reached at all. Furthermore, in the case of a spot welding with a flat electrode, it can be observed that the surface of the welding spot is incident inward, ie an unevenness is formed. This is explained by the fact that during the welding process, the flat contact surface of the flat electrode is pressed against the viewing panel, so that the heated sheet is hindered in its extension. Upon cooling, shrinkage of the material occurs, causing the surface to sink in on the visible side. Often an attempt is made to counteract these shrinkages with long retention times and high retention forces. However, high retention times increase the cycle time and high retention forces in turn lead to the sheets being deformed on the visible side. Another disadvantage of such flat electrodes for the production of viewpoints is that when welding thick-thin connections, as is usually required at viewpoints, the flat electrode on the visible side sinks into the thin sheet. Thus, the flat electrode itself causes the bumps.

DE 38 44 001 A1 describes a spot welding electrode, which has a depression in the middle part, whereby the pressure concentration on the central part of the electrode surface is avoided and the electric current can be dissipated to the environment.

DE 467 294 C1 describes an electric spot welding method, in which the electrodes are designed to be complementary to one another, so that the metal parts to be welded are correspondingly deformed. This is to superfluous material to be pulled into the wells, so that wrinkling is avoided.

CH 369 726 A describes a welding electrode for welding together the link shell halves of sheet steel element radiators, wherein the electrodes may be complementary to one another, whereby the material to be welded is correspondingly deformed.

Finally, the AT 413 664 B shows a spot welding gun, in which a circulating belt is used to protect the electrodes.

The object of the invention is to provide a spot welding electrode and a spot welding method which enables the production of viewing points without compromising the quality of spot welding.

The object of the invention is achieved on the one hand by a spot welding electrode in which the concave recess has the shape of a spherical cap, the radius of the spherical cap 3 AT 504 925 B1 being between 40 and 150 mm. Such a design of the electrode results in a spot weld at which no post-processing is required. In particular, there is no deformation on the surface of the sheet, since the concave recess creates a space which is available to the molten material in the heating phase during the welding process, so that it can expand in the space. In contrast to known methods, the material thus shrinks on cooling, unlike as described above in the prior art, back to the starting position.

Advantageously, a bearing surface is formed around the concave recess of the electrode, which contacts one of the metal sheets during the welding process, wherein a closed cavity is formed between the concave recess of the contact surface and the metal sheet. As a result, a higher current density is achieved because the support surface is reduced by the concave recess with constant current. Through the cavity, the welding point is completed by the atmospheric oxygen, resulting in no discoloration of the weld.

In order to achieve a surface without unevenness after spot welding, so that no elaborate reworking is required, the ratio of the bearing surface to the base surface of the concave recess is matched to the material of the sheets to be welded according to a further feature of the invention.

It is likewise advantageous if the base area of the concave recess is adapted to the size of a weld nugget required for the welded connection. This ensures that the concave recess can absorb the expanded material due to thermal expansion.

Advantageously, the contact surface and the base of the concave recess is round and formed the support surface around the concave recess by a flat circular ring.

The main body of the electrode is substantially cylindrical, so that it can be easily accommodated by a conventional electrode holder of a spot welding gun.

The main body of the electrode may have a bore for receiving a cooling device or receiving a cooling medium.

The object according to the invention is also achieved by a spot welding method, wherein an electrode with a concave recess in the contact surface is used on the visible side of the metal sheets, so that a closed cavity is formed when contacting the contact surface of this electrode between the concave recess and the metal sheet, and after the spot welding, the electrodes are held in currentless contact with the at least two sheets, so that substantially a plane results on the visible side of the sheet. The resulting advantages can be taken from the above description of the spot welding electrode.

In this case, the electrodes are advantageously kept in contact without current until the sheet, which has been expanded into the closed cavity, is cooled on the visible side and shrunk to its original size.

As the electrode with the concave recess opposite electrode according to a further features of the invention, a known from the prior art electrode with a convex contact surface is used. Through this combination, the current during the spot welding is better focused, resulting in a better material transition and a better welded joint. 4 AT 504 925 B1

By a band between the contact surfaces of the electrodes unevenness of the sheets are better compensated and the entire heat affected zone shielded from atmospheric oxygen, so that no tempering colors. Moreover, the electrodes are protected by such a band and their life is increased.

The present invention will be explained in more detail with reference to the accompanying schematic drawings.

Show:

1 shows a spot welding gun in a schematic representation;

FIG. 2 shows the electrode according to the invention in a three-dimensional representation; FIG.

Fig. 3 shows the electrode according to the invention cut;

4 shows the electrodes and the sheets cut just before the spot welding; and FIG. 5 shows the cut sheets after spot welding with the electrode according to the invention.

By way of introduction, it is stated that identical parts of the exemplary embodiment are given the same reference numerals. Likewise, the following description refers to each of the figures.

In Fig. 1, a spot welding gun 1 for resistance welding of at least two sheets 2, 3 is shown in perspective. The spot welding gun 1 consists of a base body 4 and gun arms 5, on which receptacles 6 for receiving in each case an electrode 7, 24 are arranged. A band 8 can run around the electrode 7, 24. The band 8 is unrolled by a winding device 9, which may preferably be arranged on the base body 4, but also on the gun arms 5 and along the Zangenarmgeometrie on the gun arm 5, the electrode holders 6 and the electrodes 7, 24 and opposite back to Winding device 9 out where the tape 8 is rolled up again. For use of the belt 8, a sliding cap 10 and a pressure element 11 are arranged on the electrodes 7, 24, which are connected in the longitudinal direction of the electrodes 7, 24 movable with this, wherein the pressure element 11 exerts a force on the sliding cap 10. The sliding cap 10 and the pressure element 11 have a guide for the band 8, so that the band 8 can be distanced from the electrodes 7, 24. The sliding cap 10 lifts the band 8 from the electrodes 7, 24 during or after the opening of the spot welding gun 1. In contrast, the sliding cap 10 is pushed back during a welding operation, so that the electrodes 7, 24 or a contact surface 12, 25 of the electrodes 7, 24 comes to rest on the belt 8. Thus, the spot welding gun 1 is closed and the electrodes 7, 24 are pressed against the sheets 2, 3 at a predefined pressure. To carry out the spot welding, a specific electric current, which is supplied by a welding device, is conducted through the electrodes 7, 24. As a result, the sheets 2, 3 are connected to each other by a spot weld welding point 13. By means of such an electrode construction it is achieved that the band 8 does not abut against the contact surface 12 or 25 directly when the spot welding gun 1 is open, so that the band 8 can not rub against the electrodes 7, 24 during displacement. Thus, the life of the electrodes 7, 24 is substantially increased.

In order for the band 8 to be brought to the electrodes 7, 24, devices, in particular deflection rollers and sliding surfaces (not shown), for guiding and deflecting the band 8 are arranged on the forceps arms 5 and / or on the electrode holders 6. As a result, the band 8 can be guided by the winding device 9 to the electrodes 7, 24 and back again. The winding device 9 is coupled to a drive means (not shown), in particular an electronically controllable motor, so that a targeted displacement of the belt 8 is made possible by activation of the drive means.

The winding devices 9 on the gun arms 5 are designed so that a simple and uncomplicated exchange or replacement of the rollers or the belt 8 can be made. Another positive effect of this arrangement of the winding devices 9 is that no disturbing elements or parts are present by the guidance of the belt 8 from the rear, ie from the base body 4 to the electrodes 7, 24, with the result that also in the case of complicatedly accessible metal sheets 2, 3, spot welding can be carried out almost without any problems, since there is no restriction on the accessibility with respect to welding tongs without a belt 8. By this configuration is also achieved that the size of the spot welding gun 1 can be kept low.

For completeness, it is mentioned that the gun arms 5 are adjustably mounted and can be adjusted by an actuating means 14, which may be formed for example by a servo motor.

Thus, according to the described method, a spot welding of sheets 2, 3 can be carried out, for example for the production of so-called view points. This essentially means that the welding point 13 leaves at least on one surface of the sheets 2, 3 to be welded as little unevenness as possible, so that in the optimal case no reworking is required.

For this purpose, it is now provided according to the invention that in the spot welding of viewing points, an electrode 7 is used, which has a concave recess 15 in the contact surface 12.

An embodiment of such an electrode 7 can be seen in FIGS. 2 and 3, wherein FIG. 2 shows a perspective view of an electrode 7 and FIG. 3 shows a longitudinal section through the electrode 7. Finally, FIG. 5 shows the sheets 2, 3 welded to the electrode 7 according to the invention and to the method according to the invention, shown in section.

The electrode 7 consists of a base body 16 with the contact surface 12 and possibly with a bore 17 for cooling purposes. The main body 16 is designed for attachment to the electrode holder 6 accordingly. For the production of view points, a concave recess 15 is provided in the contact surface 12 according to the invention, which preferably has the shape of a spherical cap, whereby a round base of the concave recess 15 results. Similarly, the base body 16 is preferably round or cylindrical, so that due to the recess 15 in the contact surface 12, a bearing surface in the form of a circular ring 18 results, which limits the recess 15 and surrounds. The annulus 18 contacted during the welding process with the sheet on a visible side 23, for example, the sheet 2, and thus forms the contact surface 12 of the electrode 7 with the concave recess 15. Between the concave recess 15 and the sheet 2 is formed a closed cavity 19, which the sheet 2 allows an expansion, as shown in FIG. 4 can be seen. Fig. 4 shows a section through a portion of a spot welding gun 1 in the region of a welding point, wherein the electrodes 7, 24 were pressed against the sheets 2, 3 and no current was passed through the electrodes 7, 24.

For example, the main body 16 has a diameter of 12 mm and the base of the recess 15 has a diameter of 5 mm. Thus results in a width of the annulus 18 of 3.5 mm. The diameter of the recess 15 is preferably matched to the required diameter of a weld nugget 20 (see FIG. 5). The welding lens 20 is the area of the welding point 13, which represents the welded joint. Furthermore, the recess 15 has a certain depth, which results from the spherical radius of the spherical cap. Preferably, the ball radius is between 40 mm and 150 mm. For thin sheets 2 on the visible side with a thickness of about 0.3 mm to 1.5 mm, the ball radius is preferably in the upper range, since the thermal expansion during welding is lower and therefore only a smaller space 19 is required. Conversely, for thicker sheets 2 with a thickness of about 1.5 mm to 4 mm, a larger cavity 19 is required, whereby the ball radius is selected from the lower region. Accordingly, with the diameter of the welding lens 20 remaining the same, the depth of the concave recess 15 is different. So preferably in a range 5 between 0.01 mm and 0.5 mm. Of course, the diameter of the base of the recess 15 can be changed. Accordingly, the support surface must be adjusted so that a correct relationship between base and support surface is guaranteed and the expanded material can be absorbed during the welding process. In this case, the base surface of the recess 15 is adapted to the diameter of the welding lens 20 io, which in turn is dependent on the thickness of the sheet 2. The electrode 24 with the concave recess 15 opposite electrode 24 preferably has a known from the prior art convex or spherical contact surface 25 which behaves in relation to the ball radius opposite to the electrode 7 with the concave recess 15. For example, the spherical radius of the spherical contact surface 25 of the electrode 24 15 is also between 40 mm and 150 mm. This is due to the fact that in a thicker sheet 2, a larger diameter of the weld nugget 20 is required and therefore the support surface larger and thus the spherical radius of the crowned contact surface 25 must be larger. Conversely, with a thinner sheet 2, a smaller diameter of the weld nugget 20 is required, therefore, a small spherical radius of the crowned contact surface 25 is to be used. 20

This information depends strongly on factors such as the thickness of the sheets 2, 3, the thermal expansion and the material of the sheets 2, 3, the welding current, etc., so that different electrodes 7, 24 with the corresponding diameters and radii for different materials welding sheets 2, 3 are required. 25

After the diameters and radii of the electrode 7 with the concave recess 15 have been matched to the sheets 2, 3 to be welded, a visual point can be produced. For this purpose, the band 8 known from the prior art is guided around the electrodes 7, 24. Preferably, a band 8 made of copper or copper alloys is used for the electrode 7 according to the invention with the concave recess 30 15. This has the advantage that little heat is introduced from the outside and can be dissipated by the good thermal conductivity much heat that arises in a spot welding process, which is an essential prerequisite for a point of view. Through the use of such a combination of the electrode 7 and the electrode 24, the required current density and thus a process-reliable welding is ensured. This is due in particular to the smaller contact surface, that is to say the circular ring 18 of the contact surface 12, compared to the flat contact surface known from the prior art, of a flat electrode. 40

In spot welding, the electrode 7 with the concave recess 15 is now pressed against the sheet 2 on the visible side and the electrode 24 with the convex contact surface 25 is pressed against the sheet 3 (FIG. 4). In each case between the electrode 7 and the sheet 2 and between the electrode 24 and the sheet 3, the band 8 is arranged. Subsequently, for a short period of time, tuned to the material to be welded, a high current is conducted through the electrodes 7, 24 and thus the sheets 2, 3, so that they are heated or melted due to their electrical resistance. The concave recess 15 offers the heated material of the sheet 2 a cavity 19 in which the material can expand. The cavity 19 is bounded laterally by the circular ring 18 and down through the sheet 2. In this case, the band 8 with respect to the cavity 19 has no significant effect, since this adapts to the concave recess 15 of the electrode 7. After the current has been passed through the sheets 2, 3, the spot welding gun 1 and thus also the cavity 19 remains closed for a short time. The sheets 2, 3 are thus in the de-energized state of the electrodes 7, 24 further pressed together with a defined force. This time is in practice, for example, in the range of 300 ms. After that will

Claims (9)

Protects the welding point 13 and the weld nugget 20 during the solidification of the melt from the atmospheric oxygen, whereby the formation of so-called tempering colors is prevented 7 AT 504 925 B1. After solidification of the melt, the spot welding gun 1 is opened and the sheets 2, 3 are connected by the welding point 13 form-fitting manner. It is thus achieved that, on cooling, the sheet 2 shrinks back to its initial dimension on the visible side. As a result, a flat surface of the sheet 2 forms on the visible side, which has no bumps (Fig. 5). Likewise, no discoloration occurs around the welding point 13, since this was completed according to the invention of atmospheric oxygen. Thus, with the electrode 7 according to the invention with the concave recess 15, a view point results which has no unevenness on the surface of the sheet 2 on the visible side and therefore does not require post-processing. On the other hand, a recess 21 on the sheet 3 very well, which is due to the spherical contact surface 25 of the electrode 24. In particular, the band used 8 also contributes to preventing the tempering colors, which are formed by oxidation above a certain temperature, around the spot weld 13. The reason for this is that the band 8 can cover a larger area than the contact area 12 alone. The band 8 always covers a so-called heat influence zone 20, which inevitably arises around the diameter of the weld nugget 20, with off. Thus, the heat affected zone 22 is protected from atmospheric oxygen and there is no oxidation and consequently no discoloration. In addition, this effect is supported by the fact that the band 8 dissipates the heat generated by the welding process. This leads to the fact that the heat-affected zone 22 cools faster and thereby falls below the temperature limit, at which oxidation is still possible, faster. This heat dissipation takes place in particular when the electrodes 7, 24 continue to compress the sheets 2, 3 after the spot welding, without current and with a defined force. Of course, the heat dissipation of the belt 8 is matched to the material of the verschwei-30 sequent sheets 2, 3. For materials that heat up strongly, such as aluminum, it is preferred to use a band 8 of very high thermal conductivity, such as a band 8 of copper or copper alloys. Conversely, for materials that tend to heat slightly, such as steel, a lower cost, lower thermal conductivity band 8 may be used. Of course, when selecting the bands 8, care is also taken to ensure that all properties of the band 8 known from the prior art are not neglected so as not to impair the quality of the weld. Likewise, the electrode 7 according to the invention without band 8 can be used if the materials to be welded of the sheets 2, 3 make this possible. In general, it should be noted that for various applications, an electrode 7 with angular, oval, etc. base body 16 may be beneficial. In this case, the concave recess 15 preferably has the same base area as the base body 16. Correspondingly, the bearing surface which contacts the sheet 2 on the visible side results. 50 1. Electrode (7) for spot welding of sheets (2, 3) with a base body (16) and a contact surface (12) which comes into contact with one of the sheets to be welded (2, 3), wherein in the Contact surface (12) a concave recess (15) is arranged, characterized in that the concave recess (15) has the shape of a spherical cap 55, wherein the radius of the spherical cap is between 40 mm and 150 mm. 8 AT 504 925 B1 Second electrode (7) according to claim 1, characterized in that around the concave recess (15) has a bearing surface is formed which during the welding process with one of the sheets (2, 3) contacted, wherein between the concave recess (15) of the contact surface (12) and the sheet (2, 3) a closed cavity (19) is formed. 3. electrode (7) according to claim 2, characterized in that the contact surface (12) and the base surface of the concave recess (15) is round and the support surface around the concave recess (15) by a flat circular ring (18) is formed , 4. The electrode (7) according to any one of claims 1 to 3, characterized in that the base body (16) is substantially cylindrical and for receiving on an electrode holder (6) of a spot welding gun (1) is formed. 5. electrode (7) according to one of claims 1 to 4, characterized in that the base body (16) has a bore (17) for receiving a cooling device. 6. A method for carrying out a spot welding with two attached to the spot welding gun (1) electrodes (7, 24) which are each contacted with an outer at least two sheets to be welded (2, 3), characterized in that on a visible side (23 ) of the sheets (2, 3) an electrode (7) with a concave recess (15) in the contact surface (12) is used, so that when contacting the contact surface (12) of this electrode (7) between the concave recess (15) and the sheet (2, 3) a closed cavity (19) is formed, and after the spot welding, the electrodes (7, 24) with the at least two sheets (2, 3) are kept in contact without current, so that on the visible side ( 23) of the sheet (2, 3) substantially results in a plane. 7. spot welding method according to claim 6, characterized in that the electrodes (7, 24) are kept in contact without current until the in the closed cavity (19) expanded sheet (2, 3) cooled on the visible side (23) and on the Initial size has shrunk. 8. spot welding method according to claim 6 or 7, characterized in that as the electrode (7) with the concave recess (15) opposite electrode (24) uses a known from the prior art electrode (24) having a convex contact surface (25) becomes. 9. spot welding method according to one of claims 6 to 8, characterized in that between the contact surfaces (12, 25) of the electrodes (7, 24) a belt (8) is arranged. For this purpose 3 sheets of drawings