GB2068814A - Resistance welding of sheets - Google Patents

Abstract

In the electrical resistance of light metal sheets 17, use is made of welding projections 16 having flanks 18, 19 with an internal angle a of from 45 DEG to 75 DEG and an external angle b of from 60 DEG to 100 DEG . The thickness of the material in the region of the flank 18, 19 is approximately 50% greater than the thickness of the neighboring base material. <IMAGE>

Description

SPECIFICATION Resistance welding of sheets This invention relates to resistance welding of metal sheets.

In the mass production of parts by resistance welding, projection welding for ferrous metals is one of the most frequently used welding processes for high quality joins. In contrast to spot welding, the extension of the weld join in projection welding is not determined by the cross-sectional surface of the electrodes, as is known, but by the crosssectional surface of the projection. Because of this fact, the weld surface and, therefore, the current path, the current density and the surface pressure are exactly defined and are constant; furthermore, it is also possible simultaneously to weld a plurality of projections under exactly the same conditions and to prevent shunts.

In the case of non-ferrous metals, in particular aluminium and aluminium alloys, the known projection welding processes cannot be carried out reliably with thin sheets. Thus is due to the fact that aluminium, in contrast to iron, has a far smaller electrical resistance to the welding current and simultaneously conducts the heat generated by the current to a significantly greater extent and consequently leads away from the point to be welded.

An additional problem is the fact that the electrode force necessary for welding which has to be applied to the work piece, must be greater with non-ferrous metals than with iron and the projection collapses before the current may be switched on. If the process is carried out using a lower electrode force, then the projections may burn before a weld is produced, because a too great resistance is produced in the region of contact between the projection and sheet.

Proposals have therefore already been made to take precautions to position the electrode extremely carefully on the work piece and only to activate the full electrode force, via an adjustable time switch, immediately before the welding current appears.

In known processes, it has not been possible to obtain a satisfactory, randomly recproducible projection welding for the mass production of parts even on machines with a programme control for the mutual temporal co-ordination of current and pressure.

Reference is also repeatedly made in the more recent relevant literature (e.g. "Resistance Welding Manual", Vol. 1, p. 44; Aluminium-Taschenbuch 1 3th edition, 1 974, p.

581) to the fact that the projection welding of aluminium is not reliable. Another publication by Pfeifer: "Fachkunde des Widerstandsschweissens" does indicate on page 44 that the projection welding of light-metals is "possible using suitable machines". However, a teaching for carrying out such a welding is not provided.

It is known to weld aluminium using a stamped projection and it has proved to be successful. Stamped projections may only be provided on solid parts, e.g. handles for pans; solid projections cannot be produced at all cheaply on sheets or other thin-walled parts.

An object of the present invention is to provide a projection with which sheets of aluminium aluminium alloys and the like may be reliably projection welded and in particular, may also be multiple projection welded.

Another object of this invention is to provide a process for the production of the projection which is suitable for the mass production of parts.

A further object of this invention is to weld aluminium and aluminium alloys using a minimum of energy, or with as low a demand on the mains as possible.

A further object of this invention is to provide a way of ensuring the production of projection welded joins of aluminium and aluminium alloys with a metallurgically perfect design and reproducibility.

According to the invention, there is provided a projection for the electrical resistance welding of sheets, having flanks with an internal angle a of from 45 to 75 and an external angle of from 60 to 1 00', the thickness of the material in the region of the flanks being approximately 50% greater than the thickness of the neighbouring base material.

Some embodiments of the invention will be described with reference to the accompanying drawings, in which: Figure 1 illustrates a section through a stamped projection, Figure 2 illustrates a section through a conventional round projection, Figure 3 illustrates a cross-section through a longitudinal projection, Figure 4 illustrates a section through an annular crimped projection, Figures 5 and 6 are current/power diagrams, Figure 7 illustrates a section through a conventional projection tool.

Figure 8 illustrates a section through a projection tool for the production of a projection according to Figs. 3 or 4, and Figure 9 illustrates a section or a welding spot of an aluminium annular-crimped projection weld.

Fig. 1 illustrates a solid projection 1 as it may be produced on a solid light metal work piece 2 by forming, for example, by stamping.

Projections 1 of this type resist the electrode force necessary for maintaining a regular electrical transition resistance on one side between the surface 3 of the electrodes and the work pieces 2 and 4 to be welded and on the other side between the work pieces 2 and 4 themselves.

Fig. 2 illustrates a conventionsl, hollow stamped round projection 5 as is generally used for iron sheets.

With light metals, the projection 5 collapses before the welding current can be switched on and a usable weld join is not produced.

Fig. 3 illustrates a cross-section through a linear wedge-shaped projection 16 for aluminium and aluminium alloys which is used for welding narrow flanges, in which there is not enough room for an annular projection. The projection has two flanks 18 and 19 which have a thickness which is approximately 50% greater than the base material 17. This thickness is obtained by the large plastic forming of the base material 17 in the forming tools.

The flanks 18 and 19 of the wedge-shaped projection 16 are compression moulded from the base material i.e. the sheet 17 to include an internal angle a of approximately 60 and an external angle ss of approximately 90 .

Fig. 4 illustrates a design of an annular or annular-crimped projection 7 which has proved to be particularly advantageous for welding aluminium and aluminium alloys. The projection 7 illustrated in this figure differs externally from the annular projections customary for steel, particularly by the lower ratio ,d" of from 1.5 to 3, which produces in this case, if required, a completely through-welded spot according to Fig. 9.

An increase in the thickness of the base material by approximately 50% is achieved in regions 11 and 12 of the flanks in this projection form as well by the forming process. Not only the increase in thickness in the projection flanks 11 and 12 but in this form, the centre area 13 enclosed by the annular projection remains unchanged locally with respect to, i.e. remains flush with the surrounding material 10, may in particular also transfer a substantial portion of the electrode force to the inside flank 12 of the projection. Consequently, the total electrode force is distributed regularly in the projection 7 over both flanks 11 and 12. The force lines in the projection 7, 16 extend approximately parallel and are only slightly inclined to the perpendicular through the welding plane.The incline of the force lines is noticeably reduced when the projection 7, 16 starts to be crushed under the effect of the electrode force F and under the effect of heating by the welding current I.

The amount of the specific surface pressure between the work pieces is substantially maintained.

The diameter d of the projection 7 may naturally not be provided as any value, since the welding current demand increases approximately in square to the diameter d. For a sheet thickness of 1.05 mm, a diameter d of 3 mm and projection height h of 0.7 mm have proved to be advantageous, for example with the material AIMS 0.4 Si 1.2. A comparison with a round projection with a height h of 1.1 mm and with the same diameter d showed that with a load with the force F of 200 daN, a remaining deformation of only 8% of h occurred with the projection according to Fig. 4; however, with the round projection, there was a deformation of 54% of h.

The projections according to the invention are compression moulded (Fig. 8) in contrast to the known production method of projections with ferrous materials, in which case these are freely flow formed (Fig. 7). The geometrical shape of the projections, in particular of the flanks 11. 12, 18, 19 is thereby identical for all projections on one workpiece.

The base material is plastically formed by using a shaping die and a shaping die-plate 20, 21.

On one hand, the angles a and ss may not only be observed in particular with the forming tools 20, 21 according to Fig. 8, but may firstly be obtained at all and on the other hand, the required forming and consequently, the consolidation of the material in the flanks 11, 12, 18 19 may be achieved. It is significant that in the case of an annular projection, the centre circular area 13 remains unchanged as compared with the area of the sheet adjacent to the projection.

It is obvious to a skilled operative that qualitatively good weld does not depend on the projection form alone, as the material to be welded and the temporal course of the electrode force and, in particular, the course of the welding current also play an essential part. The high thermal conductivity of aluminium and its alloys require the welding energy, i.e. the welding current to be introduced with a very short time. In addition, aluminium materials require greater currents than ferrous materials, as is known, for the reasons mentioned (high electrical conductivity, low internal electrical resistance).

Weld joins according to the section in Fig.

9 may surprisingly be produced with are qualitatively extremely outstanding and may be reproduced in any manner using the design of the projections described, in the form according to Figs. 3 and 4 together with a welding current/electrode force path according to Figs. 5 and 6.

In Figs. 5 and 6, reference number 14 denotes the current path in the time t1 t2 or t, t3; reference number 15 denotes the path of the electrode force in the time to t4. The current curve 14 in Fig. 6 has a so-called afterglow phase of t2t3, compared to the curve in Fig. 5. Such an afterglow phase may prove to be useful or necessary with certain materials in order to prolong the cooling time (improved recrystallisation). An additional increase in the welding pressure (dash-dotted path of the electrode force 15 in Figs. 5 and 6) at the end of the welding phase contributes to a prevention of cavitations and crackings in the welding spot.

Fig. 9 illustrates a cross-section of a welding spot 22 of an annular projection welding between two sheets 23 and 24 made of different aluminium alloys. The surfaces 25 were not freed from the oxide layer in either of the two sheets 23 and 24. Nevertheless, the welding spot is completely homogeneous, symmetrical and without inclusions. Another factor to be observed is the design of the electrode, or the moving mass thereof, on which the so-called recharging behaviour depends. The electrode must be able to follow the melting projection without delay during the very short welding time t, t2 in order to prevent liquid material spraying away.

The electrodes are preferably designed with a large area so that a low current density occurs between the work piece and the electrode and as a result thereof, the electrode is exposed to extremely low wear. The use of a frequency converter welding machine is particularly advantageous for carrying out the welding described. By using a direct currentlike single pulse welding, the welding energy may be introduced into the workpiece in the shortest time possible.

When simultaneously welding a large number of projections, the electrodes are advantageously movable and spring-mounted, so that each individual projection may be charged by the electrode force or the welding current under exactly the same conditions.

A weld join according to the present invention has a large number of advantages compared to the type of weld join of aluminium and aluminium alloys and the like, spot welding, which hitherto, has been carried out reliably to some extent. The advantages of the invention are as follows: 1. a large increase in production, because several projections may be welded at the same time without shunts occurring and without uneconomically high welding currents being required, consequently; 2. there is a regular mechanical strength of the joins, 3. a small change in thickness where there are fluctuations in the current, 4. an additional increase in production, on account of a slight, negligible electrode soiling and wear owing to the slight current density at the contact points, 5. a constantly uniform welding quality, owing to the omission of alloyings on the electrodes, 6. the correct position of the weld join on the workpiece due to the previously applied projection, and 7. no cleaning of the oxidised sheet surfaces by pickling, brushing before welding.

Claims (9)

1. A projection for the electrical resistance welding of sheets, having flanks with an internal angles (a) of from 45 to 75 and an external, (ss) of from 60 to 100 the thickness of the material in the region of the flanks being approximately 50% greater than the thickness of the neighbouring base material.

2. A projection according to claim 1, which is of annular shape, the centre circular area enclosed by the annular shape being flush with the neighbouring base material.

3. A projection according to claim 1, which is of linear shape.

4. A projection according to claim 1, which is of annular shape, the centre circular area enclosed by the annular shape projecting slightly behond the plane of the neighbouring base material.

5. A projection according to any preceding claim, wherein the angle a is 60 and the angle ss is 90 .

6. A projection according to claim 2, wherein the ratio of the average diameter d of the annular projection to the flange width b is smaller than three.

7. A projection according to any preceding claim, wherein the average thickness of the flanks is approximately 50% greater than that of the base material.

8. A process for the production of an annular or longitudinal projection on sheets and thin-walled parts according to claim 1, wherein the projection is produced by a plastic forming process of the base material and receives the dimensionsl-accurate shaping by compression moulding the material flowing between a die and die-plate of the forming tool.

9. A welding projection for the electrical resistance welding of aluminium sheets, the said projection having a recess on one side and an overhang on the other side, an electrode supporting force reducing the transition resistances at the electrodes, the overhang of the projections is substantially not deformed (or formed back) before welding current is turned on.