WO2000069593A1

WO2000069593A1 - Process and device for two-stage arc welding of a stud to a metallic structure and associated stud

Info

Publication number: WO2000069593A1
Application number: PCT/GB2000/001712
Authority: WO
Inventors: Hermann Roeser
Original assignee: Emhart Inc.; Black & Decker Europe
Priority date: 1999-05-18
Filing date: 2000-05-04
Publication date: 2000-11-23
Also published as: AU4590100A; DE19922679A1

Abstract

The present invention relates to a process and a device for arc welding a stud (3) to a metallic structure (1). The stud (3) is guided on to the structure (1) until the tip (7) touches the structure (1) in a weld-on region (2). When the tip (7) comes into contact with the structure (1) it is vaporised explosively by a surge of power from a welding power source (11), wherein an explosion wave thus generated and/or the plasma thus generated cleanse the weld-on region (2) of the structure (1) of coatings, impurities and/or oxides (2a). The welding power source (11) is switched off or controlled in such a way that for a short time after the explosion there is no arc between structure (1) and stud (3), so the point of contact (2) can solidify again. Then in a second stage of the process an arc welding procedure known per se for welding the stud (3) to the structure (1) is carried out, in particular according to the stroke ignition process.

Description

PROCESS AND DEVICE FOR TWO-STAGE ARC WELDING OF A STUD TO A METALLIC STRUCTURE AND ASSOCIATED STUD

The present invention relates to a process for arc welding a stud to a metallic structure, in particular to a structure made of aluminium or an aluminium alloy.

The welding of a metallic stud to a metallic structure by means of arc welding is a widespread process, which is employed particularly in the automotive industry. Substantially two different welding processes are differentiated with corresponding design of the associated studs.

In one welding process, the so-called stroke ignition process, the weld-on end of the weld stud has a flange, which usually also has a ridge of material on its underside, which can be arched or have the shape of a very blunt cone. In the stroke ignition process the stud is first guided toward a structure with the ridge of material, until an electrical contact results. A high electric power is now conducted from a welding power source through stud and structure. Then the stud is raised again slightly from the structure, giving rise to an arc between stud and structure, which melts the structure in a weld-on region and also parts of the ridge of material. After a set time the stud is lowered into the molten metal, which has been created in the weld-on region, resulting, on subsequent cooling, in a very stable weld connection between stud and structure.

A different process for arc welding of studs to a metallic structure is the so- called tip ignition process. Here the weld-on end of the weld stud has a protruding or projecting tip, which is firstly guided toward the structure. The tip is dimensioned in such a way that when struck by a welding power it is abruptly vaporised, forming a hot plasma, by which the remaining weld-on end and the structure are joined. The stud is directly lowered at great speed into the molten metal. A process of this kind is described, for example, in the DE 42 36 527. The high energy required for this process leads to a very loud explosion noise. Additionally, the structure must generally be supported at the back of the weld-on region, since the impact of the weld stud at high speed can otherwise cause vibrations, which impair the quality of the welding. In any case the quality of the welding greatly depends on the quality of the tip.

The increasingly wide application of stud welding processes also results in cases of application that cannot be satisfactorily resolved by either of the two processes. There are cases of application, for instance, where the structure is coated or soiled in the weld-on region, in particular provided with a layer of oil, and cases where a material which has an oxide layer has been used for the structure. Combinations of both cases also occur.

For such cases of application it is the object of the present invention provide a process for arc welding a stud to a metallic structure, which enables qualitatively high-grade and repeatable weld connections to surfaces provided with an oxide layer and/or a different surface covering, in particular oil. Also a corresponding device and a weld stud particularly suitable for the process is to be provided by the invention.

A process according to claim 1, a device according to claim 8 and a weld stud according to claim 10 serve to meet this object. Advantageous developments are given in the respective dependent claims.

The process for arc welding a stud to a metallic structure, wherein the stud has a flange on its weld-on end, from which at least one tip projects, is according to the invention in two stages and has the following steps:

a) the stud is guided toward the structure until the tip touches the structure in a weld-on region,

b) when the tip comes into contact with the structure it is vaporised explosively by a surge of power from a welding power source, wherein an explosive wave thus generated and/or the resultant plasma cleanse the weld-on region of the structure of impurities and/or oxides, c) the welding power source is switched off or controlled in such a way that for a short time after the explosion there is no arc between structure and stud, so the point of contact can solidify again,

d) then in a second stage of the process an arc welding procedure known per se for welding the stud to the structure is carried out, in particular according to the stroke ignition process.

Unlike with the prior art, the tip on the weld-on end of the stud is not used for joining the weld-on end of the stud and the weld-on region of the structure, but only made to explode, in order to effect cleansing of the surface of the structure in the weld-on region. The abrupt vaporising of the tip and/or the plasma thus resulting not only push aside or burn any possible oil film, but can also break up any coating or oxide layer on the surface and thereby generate a metallic surface, which assists a subsequent welding process. Irrespective of the nature of the surface before the explosion, the nature of the surface after the explosion is always smooth, enabling good consistency of welding.

A particular field of application of the invention is the welding of studs to structures made of aluminium or an aluminium alloy. Typically, a layer of aluminium oxide forms on the surface of such materials, which makes contact with a weld stud difficult and does not always allow weld connections to be reproduced by conventional welding processes. The stud to be welded can in this case also be made of aluminium or an aluminium alloy.

Since the energy of the first explosion can already melt the weld-on region to some extent, it is of advantage, firstly, before the actual welding process, to produce again defined conditions, in which for a period of about 10 to 200 msec, preferably 50 to 100 msec, after the explosion the creation of an arc is prevented, to allow the point of contact to solidify. It can be important here, to maintain a distance between stud and structure after the explosion, so that no particularly undesired spot welding connection results accidentally, which hinders or makes impossible the subsequent welding process. It is most favourable if the length of the tip exactly corresponds to the desired distance after the explosion. In this case the stud with tip only needs to be guided on to the structure and held in this position until the actual welding procedure begins.

Since, after the explosion of the tip and extinguishing of the arc, ignition of the tip is no longer possible, according to the invention, following the explosion a conventional stroke ignition process, as described above, is carried out in a second stage of the process. After the explosion of the tip the flange of the stud has a shape which is suitable for the stroke ignition process, for example with a ridge of material at the weld-on end. This can be brought into contact with the weld-on region of the structure and then lifted again, resulting in an arc for the final welding process. Owing to the fact that the metallic surface in the weld-on region has been previously cleansed in the first stage, a high quality of welding can be achieved.

A device for carrying out the process must be suitable firstly to generate an initial strong surge of power, which makes the tip explode, and a short time later the second welding power required for the actual stroke ignition process. A welding power source with a corresponding control device is needed for this, which preferably should contain two capacitor batteries, one for the explosion of the tip and one to carry out the welding process. The device must also have all the typical devices for carrying out a stroke ignition process, in other words, a stud holding device, a linear drive to move the stud holding device and a fast control, which can regulate with great accuracy the movement cycle of the stud and the course of the welding power, in particular with precise timings in the range of 2 to 20 msec.

A linear drive, in particular with an electromagnetic drive, has proved to be especially suitable for the present invention. Such drives, as described, for example, in the DE 4437 264 Al, allow very precise control or regulation of the position and the speed of the stud during the welding process. A weld stud with a flange to be welded to a structure by means of arc welding serves to carry out the process according to the invention, wherein at least one tip protrudes from the flange, which is shaped in such a way that it can be vaporised explosively by a surge of electric power, wherein the length of the tip corresponds to a minimum distance between flange and structure, which must be maintained during the cooling down stage after the explosion. Conventional studs with tip for the tip ignition process are dimensioned in such a way that they are vaporised by a surge of power and thus release the energy required for welding, wherein the length of the tip exactly corresponds to the desired length of the arc at the beginning of the welding process. Weld studs according to the present invention have at least one tip, which is dimensioned in such a way that a small explosion to cleanse the weld-on region can be achieved, the length of the tip having nothing to do with the second arc, which is to be ignited later and serves for the actual welding, but only needs to be big enough for no undesired spot welding to take place after the explosion, with the given dynamics of the movement device.

It can be especially advantageous for the present invention if two or more tips protrude from the flange, which can be made to explode at the same time. In this way the blast wave of the explosion can be influenced in its effect to achieve a particularly good cleansing of the surface of the structure. If there are several tips these are preferably arranged in the peripheral region of the flange, preferably distributed approximately evenly over same region. In this way a broad effect of the blast wave can be achieved with a distinctive maximum in the centre of the weld-on region.

To produce tips on the weld studs according to the invention it is possible to design these as trimmings, making the production process very easy and cheap.

Alternatively tips of this kind can be produced by cutting appropriately into the flange in the vicinity of the tip.

To illustrate the invention two embodiments of the invention are shown partially schematically in the drawings, but the invention is not restricted to these. Figure 1 shows in cross-section a weld stud according to the invention above a structure, the rest of the welding device being shown schematically.

Figure 2 shows a different embodiment of a weld stud according to the invention with several tips in cross-section.

Figure 3 shows a perspective view of the weld stud from Figure 2.

Figure 1 shows a structure, preferably made of aluminium or aluminium alloy, with a weld-on region 2, which can be covered with a surface coating 2a, in particular with an oxide and/or a layer of oil. A weld stud 3 with a stud shank 4, a flange 5, which has a ridge of material 6 at its lower end, is arranged above the structure 1 in a stud holder 8. The stud holder 8 can be moved axially by means of a linear drive, precisely regulated or controlled. A control 10 is connected to the linear drive 9 via a control lead 13. Additionally, the control 10 is also connected to a welding power source 11 via a lead 14, the welding power source 11 supplying the stud holder 8 with the required welding power via a power lead 15. The welding power source 11 preferably contains a first capacitor battery 12a and a second capacitor battery 12b.

The weld stud 3 has a tip 7 on its underside, which can, for example, be cylindrical with a diameter of 1 - 3 mm and a length of 2 - 5 mm and provided with a pointed lower end.

To carry out the process according to the invention the stud 3 is lowered in the stud holder 8 far enough for the tip 7 to come into electrical contact with the structure 1 in the weld-on region 2. By discharging the first capacitor battery the tip 7 is made to vaporise explosively, the surface coating in the weld-on region 2 being broken up and removed. For a period of about 100 msec the stud 3 remains in the position it was in when the tip 7 exploded, so there is a distance between structure 1 and stud 3. To avoid an arc directly after the explosion either the welding power source must be switched off or the distance of the stud 3 from the structure 1 must be sufficiently great. As soon as the weld-on region 2 has become firm again after the explosion the stud 3 is brought into contact with the weld-on region 2, which is now metallically clean. After the explosion of the tip 7 the stud 3 has the typical structure of a weld stud suitable for the stroke ignition process, i.e. preferably a flange 5 with a ridge of material 6 arranged underneath. In this way a normal welding process can be carried out after the stroke ignition process, in particular with the aid of the second capacitor battery 12b, at the end of which there is a firm weld connection between stud 1 and weld stud 3. The exact succession in time of explosion, cooling time and stroke ignition process demands a precise regulation of the movements of the stud holder 8 and the welding power source 11. A drive with a stepping motor 9a and a spindle 9b, as already used in conventional stroke ignition processes, is particularly suitable for precise regulation.

Figures 2 and 3 show an alternative embodiment of a weld stud according to the invention with several tips 17a, 17b, 17c, which are distributed evenly over the periphery of the weld-on end.

The present invention is especially suitable for automated welding processes, in which a plurality of weld studs with weld connections of high quality and repeatability is to be attached to structures, in particular made of aluminium or aluminium alloys.

Claims

1. Process for two-stage arc welding of a stud (3) to a metallic structure (1), wherein the stud (3) has a flange (5) at its weld-on end, from which at least one tip (7) projects, with the following steps:

a. the stud (3) is guided toward the structure (1) until the tip (7) touches the structure (1) in a weld-on region (2),

b. when the tip (7) comes into contact with the structure (1) it is vaporised explosively by an initial surge of power from a welding power source (11), wherein an explosive wave thus generated and/or the resultant plasma cleanse the weld-on region (2) of the structure (1) of impurities and/or oxides (2a),

c. the welding power source (11) is switched off or controlled in such a way that for a short time after the explosion there is no arc between structure (1) and stud (3), so the point of contact (2) can solidify again, wherein during this time the stud (3) does not touch the structure (1),

d. then in a second stage of the process an arc welding procedure known per se for welding the stud (3) to the structure (1) is carried out, in particular according to the stroke ignition process.

2. Process according to claim 1, wherein the metallic structure (1) consists of aluminium or an aluminium alloy.

3. Process according to claim 1 or 2, wherein the stud (3) consists of a similar material to the structure.

4. Process according to claim 1 or 2, wherein the stud (3) consists of steel and the structure (1) is also of steel, preferably a steel provided with a surface coating.

5. Process according to one of the previous claims, wherein for a period of 10 to 200 ms, preferably 50 to 100 ms, after the explosion of the tip (7) no arc is present.

6. Process according to one of the previous claims, wherein a distance is maintained between stud (3) and structure (1) after the explosion.

7. Process according to one of the previous claims, wherein to weld the stud (3) a contact with the structure (1) is produced for a short time to re-ignite a second arc, the stud (3) is then lifted again while the arc is burning and finally lowered into the resulting molten metal at the point of contact (2).

8. Device for carrying out the process according to one of the previous claims with a welding power source (11), preferably containing two capacitor batteries (12a, 12b), a stud holding device (8), a linear drive (9) to move the stud holding device (8) and a fast control (10), which can regulate the cycle of movement and the course of the welding power with a precision in the range of 2 to 20 msec.

9. Device according to claim 8, wherein the linear drive (9) comprises an electromagnetic linear motor (9a).

10. Weld stud (3) with a flange (5) for welding to a structure (1) by means of two- stage arc welding, wherein at least one tip (7) protrudes from the flange, characterised in that the tip (7) is formed in such a way that it can be vaporised explosively by a surge of electric power, the length of the tip (7) corresponding to a minimum distance between stud (3) and a structure (1).

11. Weld stud according to claim 10 for a device according to one of claims 8 or 9, characterised in that the length of the tip (7) is dimensioned in such a way that after its explosion, with a given movement dynamic of the device, no contact of the flange (5) with the structure (1) results during an initial stage of the welding process.

12. Weld stud according to claim 10 or 11, characterised in that two or more tips (17a, 17b, 17c) protrude from the flange (5).

13. Weld stud according to claim 11, characterised in that the tips (17a, 17b, 17c) are arranged in the peripheral region of the flange (5), preferably approximately evenly distributed over the periphery.

14. Weld stud according to claim 10, 11 or 12, characterised in that the tip(s) (17a, 17b, 17c) is (are) designed as trimmings.

15. Weld stud according to claim 10, 11 or 12, characterised in that the tip(s) (17a, 17b, 17c) is (are) produced by cutting in to the flange (5).