METHOD AND APPARATUS FOR WELDING THIN METAL SHEETS

Abstract: There is decribed a method for welding thin metal sheets (A, B) by the localised heating of limited juxtaposed zones of the metal sheets to be welded to cause their fusion and to form spot welds (S). In each weld zone, the metal sheets (A, B) are heated and fused along a closed line so that each weld spot (S) has a substantially annular configuration in cross section in a plane parallel to the metal sheets. This welding may be achieved for example with the use of a laser (2) produced by a laser source (1), or by electrical resistance welding with a pair of opposed electrodes (6, 7) the ends whereof which are intended to grip the metal sheets (A, B) are suitably shaped.

Description: The present invention relates to a method of welding superimposed metal sheets. In particular, the invention relates to a method according to the preamble of appended claim 1. For the firm connection of metal sheets (for example to form box or shell structures) it is known to effect welding with "full" spot welds that is of the type illustrated by way of example in Figures 1 and 2 of the appended drawings. In these Figures two juxtaposed metal sheets A and B are joined by spot welds C. The spot welds C may be achieved, for example, by electrical resistance welding using a pair of electrodes (not shown) between which the metal sheets are gripped. Each spot weld C is substantially cylindrical and forms a monolithic body. The spot weld of known type described above presents a zone of stress concentration when the welded structure of which it forms part is subject to stress: in general, the state of tension at the spot weld can reach values up to two to three times the magnitude of the maximum stress foreseeable under static conditions. When a structure having "full" spot welds of conventional type is subject to vibrational loads (and, in general, to cyclic dynamic loads) the problem arises of possible failure of the spot welds or of their rupture by fatigue. Indeed, when the location of use involves repeated impacts, vibrations, etc., these stresses generate oscillations which give rise to resonances and relative modal forms of oscillation of the structure formed by the metal sheets. The bending corresponding to several modal forms may have an amplitude of the order of the thickness of the metal sheet as is seen in Figure 2. Clearly under these conditions strong concentrations of oscillating tensions which are variable in sign are created and cause fatigue of the spot welds. The metallic and metallurgical characteristics of the material of the spot weld are usually poorer than the characteristics of the surrounding zones of the structure. Furthermore typical phenomena of erosion of an electrolytic nature due to the accumulation of scales condensed residues and contaminants of various types facilitate micro-erosion and/or micro-cracks and their propagation, since the spot weld is usually more sensitive to the electrochemical phenomena indicated above. The spot welds are thus more readily inclined to break and hence are intrinsically not reliable in certain critical applications from the safety point of view or where very severe stresses arise under the conditions of use. This generally results in the need to make use of an excessive number of weld spots to ensure safety conditions. In structural calculations, the "full" spot weld of conventional type is considered as a beam element having an infinite rigidity perpendicular to the metal sheets and tightly fixed thereto. If we consider two metal sheets joined by a series of successive spot welds disposed for example along a line, and if these metal sheets are stressed so that a spot weld at the end of the series of welds is loaded with a considerable shear force, this weld is prone to fracture ("fragile fracture") and the successive spot welds are likely to break in a chain. Indeed, after the breakage of a first spot weld the shear force is transmitted to the successive spot "rigidly" or totally, that is, without being at least partly absorbed in deformation work. The other spot welds may then yield, that is break in succession, so that the fracture propagates like a chain (domino effect). The disadvantages of the full spot weld may be overcome by a spot weld having a substantially annular, preferably circular, configuration in cross section. The weld thus effected, together with the metal sheets attached thereby constitute a type of "box" which is able to absorb part of the stress applied to the structure of which it forms part in an elastic-plastic manner. Thus for example, in the case of two metal sheets joined by a series of tubular spot welds disposed in succession along a line, the welds are able to absorb part of the applied stress in an elastic-plastic manner and thus are not subject to fragile fracture, and furthermore do not display the phenomenon of rigid or total transmission of the stress from one spot weld to the other and chain yielding. The article "Erste Erfahrungen beim widertandspunktschweißen mit Elektroden ringförmige Arbeitsfläche", Schweißen & Schneider, 26 (1): pages 11-13; Jan 1974 discloses a method according to the preamble of claim 1. The present invention proposes an improved method of the previously specified kind, which comprises additionally the features defined in the characterizing portion of the attached claim 1. Further characteristics and advantages of the invention will become apparent from the detailed description which follows, given with reference to the appended drawings, provided purely by way of non-limiting example in which: Figures 1 and 2, already described, show two sheet metal portions joined by "full" spot welds according to the prior art, Figure 3 shows two sheet metal portions joined by "tubular" spot welds on an enlarged scale, Figures 4 and 5 show the different shear behaviour of a tubular element and of a full element with the same section, Figure 6 shows schematically apparatus which can be used to effect spot welding of the type shown in Figure 2, Figures 7 to 10 show variants of execution of the welding method according to the invention. Figure 3 shows two tubular spot welds S. The metal sheets A and B shown in this drawing have been heated locally and fused along a clodes line which is substantially circular so that each weld spot S is constituted essentially by a tubular cylindrical element with an average radius of, for example, 2 mm. In order to effect the spot weld S of Figure 3 the apparatus shown in Figure 6 may be used. The improved capacity of the spot welds of Figure 3 to withstand knocks and impact illustrated in Figures 4 and 5 which show how a thinwalled tubular structural element (Figure 4) is much more difficult to break by a concentrated impact, for example with a chisel, than a solid cylindrical structure having the same resistant section strength (Figure 5). The tubular spot welds shown in Figure 3 are formed by means of an electrical resistance welding process, as shown in Figure 6. In this Figure, two opposing electrodes 6 and 7 are shown, between which the metal sheets A and B to be welded are gripped. The electrodes 6 and 7 are connected to a power voltage supply 8 of conventional type. When this supply is energised a current is established between the electrodes 6 and 7 and passes through the metal sheets A and B. The region of the metal sheets A and B through which the current passes is heated strongly by the Joule effect and the material constituting this region is fused. The facing ends of the electrodes 6 and 7 have respective axial cavities 6a, 7b such that these ends have an essentially annular cross section. As a result of this shape of the ends of the electrodes, the current flowing through the metal sheets A and B is essentially in a tubular region: in this region the material and the metal sheets are fused and subsequently cooled, forming a monolithic structure constituting the tubular spot weld itself. With the arrangement of the electrodes shown in Figure 6, if problems of shunting arise or if the current paths in the metal sheets A, B tend to move towards the interior of the spot weld, a pulsed current can be supplied to the electrode 6, 7 so that the greater electromagnetic repulsion caused by the higher values of the average current means that the lines of current tend to move radially outwardly. In order to avoid shunting problems, that is, movement of the current paths towards the centre of the spot weld (and hence to avoid the formation of a core in the zone which should otherwise be empty) according to the invention recourse is made to the solutions illustrated in Figures 7 and 8: in each weld zone, holes 10A, 10B of a diameter less than that of the spot welds which it is desired to form are previously preformed. These holes may also be only blind holes, as shown in Figure 8, that is limited to the zones of interface between the metal sheets A, B. Alternatively bosses 70 may be pre-formed in (at least) one of the metal sheets during the pressing of the metal sheets in the zones of welding. This solution is illustrated in Figure 9. Figure 10 illustrates another alternative solution for achieving spot welds by means of electrical resistance welding. According to this solution. each electrode 6, 7 is constituted by an array of small needle or wire electrodes 17, 17 disposed along a closed line. The electrodes 16. 17 may be supplied with pulses sequentially or simultaneously by means of an electronic control circuit. Each electrode 16, 17 forms a small weld point: the spacing between the electrodes must be such as to allow the small weld points to be joined so as to form together the desired spot weld.

Claims: 1. A method for spot-welding metal sheets (A, B) by localised heating and melting the sheets (A, B) in at least one weld zone along a closed line, the method including: superimposing the sheets (A, B) so as to bring them into contact with each other at least along and around said closed line; providing a pair of electrodes (6, 7), the ends of which have a substantially tubular configuration; the ends of the said electrodes (6, 7) having respective edge portions with the same shape of said closed line; gripping the sheets (A, B) in the weld zone from opposite sides between the said electrodes (6, 7); the electrodes (6, 7) being gripped onto the metal sheets (A, B) so that the said edge portions thereof are wholly in contact with the sheets (A, B) along the said closed line; and applying an electric voltage across the electrodes (6, 7) to cause the flow of an electric current through the sheets (A, B), whereby a current flows through the metal sheets (A, B) essentially in a tubular region thereby forming a tubular spot weld; characterised in that, in order to prevent the occurrence of current shunting in the weld zone upon application of said voltage between the electrodes (6, 7), at least one of said sheets (A, B) is previously provided with a local deformation (10A, 10B; 70) defining a cavity in the weld zone within said closed line; the electrodes (6, 7) being gripped onto the metal sheets (A, B) outside said local deformation. 2. The method according to claim 1, wherein a hole (10A, 10B) is preformed in each welding zone of the metal sheets (A, B), the hole having a cross-section of dimensions less than the dimensions of the spot weld to be effected. 3. The method according to claim 2, wherein the said holes (10A, 10B) are through holes. 4. The method according to claim 2, wherein the said holes (10A, 10B) are blind holes. 5. The method according to claim 4, wherein the said holes (10A, 10B) are each formed in a metal sheet (A, B) from the face intended to be located next to the other metal sheet (B, A). 6. The method according to claim 1, wherein a boss (70) is preformed in each welding zone in at least one of the metal sheets (A, B); the boss (70) and the facing portion of the other metal sheet (B, A) defining the said cavity. 7. The method according to claim 1, wherein each electrode (6, 7) includes a plurality of micro-electrodes (16, 17) located along a closed line to form a substantially tubular configuration.

Inventors: LA ROCCA, ALDO VITTORIO
Applicants: FIAT AUTO S.P.A.
Application year: 1985
Main classification: B23K26
IPC codes: B23K26, B23K11