WELDING APPARATUS FOR ALUMINIUM

CROSS-REFERENCE TO RELATED APPLICATION

This application is a completion application of co-pending United States Provisional Patent Application Serial No. 62/216,535, filed September 10, 2015, for Surface Preparation and Resistance Welding Apparatus for Aluminum, the entire disclosure of which, including the drawing, is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1 . Field of the Invention

[0001] This invention relates, in general, to resistance spot welding and, more specifically to the process of resistance spot welding and its associated equipment. More particularly, the present invention concerns the decontamination of the work piece surface prior to welding to enable creation of an acceptable resistance spot weld. Even more particularly, the present invention concerns the apparatus used for both preparing the work piece surface for welding and for producing a spot weld.

2. Description of Prior Art

[0002] Resistance spot welding is used extensively throughout the durable goods and metal fabrication industries for joining sheet metal, including both coated and non-coated steel, as well as non-ferrous metals. It is used to join together two or more thin sheet pieces of metal by electrically inducing localized fusion of the metal, i.e., a weld.

[0003] As is known to those skilled in the art to which the invention pertains, the spot welding process generally involves clamping together the metal pieces or work pieces to be joined, between two axially aligned electrodes and applying high pressure thereto. Then, a high electrical current is passed between the electrodes and the metal pieces. Any resistance point or location in the path of the current becomes heated. With respect to the work pieces, the resistive points, become heated and fused together.

[0004] The heat is generated proportionally at each resistance point throughout the circuit in accordance with the formula Q = l ² R (heat = current squared X resistance).

[0005] Resistance welding electrodes are typically made from a relatively low cost high conductivity copper alloy. These copper alloy electrodes serve three essential functions in the welding process: (1 ) they provide a conduit for carrying a high electrical current to a work piece without significant heating (Joule) losses due to their low electrical resistance; (2) their high thermal conductivity provides a method for conducting heat from the work piece and controlling the cool down process, thereby promoting weld nugget formation and, (3) they properly locate and clamp the work pieces together to establish a good interface and good electrical contact before the weld current is applied. [0006] In general, there are two areas in the weld current circuit where the electrical resistance is high enough to generate substantial amounts of heat. The lower of the two areas is at the interface between the electrodes and an associated work piece and, the higher is at the interface (faying surfaces) between the metal pieces. In most cases, the resistivity of the copper electrodes is much lower than the metal to be joined. Therefore, the greatest amount of heat is generated at the faying surfaces between the sheet metal components. This is true for most metals except for aluminum.

[0007] The presently known processes for spot welding aluminum sheets has always been associated with difficulties, the main cause of which is the material properties of aluminum materials. As is well known, aluminum has both high electrical and high thermal conductivity and an affinity for rapidly developing a thin aluminum oxide coating electrically insulating surface within a very short period of time, when exposed to air.

[0008] Aluminum's high electrical conductivity and, correspondingly, low resistance results, irrespective of other factors, in a need for an excessively high current to produce the heat necessary to create a weld. These factors are detrimental to the resistance welding process as they result in relatively short electrode life, where weld quality is important. Extended electrode life can indeed be achieved at the expense of weld quality.

[0009] Testing has shown that a decrease in electrode life is associated with a successive decline in the shear strength of the welded joint and increasing impairment of surface properties at the weld location, even after a relatively short service life.

[0010] Additionally, because of the relatively low specific resistance of aluminum materials and the comparatively high resistance of the layer of aluminum oxide covering them, only 40-50% of the overall resistance at the point of weld and, therefore, of the heat produced by the current falls to the sheet metal interface and the internal layers of the oxide. The major part of the heat produced by the current (50-60%) arises between the electrodes and the two external oxide coated layers on the aluminum sheets being welded. This is significantly higher than that of steel where only 10-30% of the heat generated to produce a weld falls between the electrodes and the steel sheets.

[0011] It is to be appreciated that in spot welding untreated aluminum sheets, the thermal capacity of the electrodes is, as a rule, not sufficient to take up the relatively large quantities of electrical heat falling to the external layers of oxide, without heating the electrode contact surfaces to temperatures above the "alloying on" temperature. The reasons for this are, first, because heating the metal and the internal layers of oxide to the welding temperature requires, due to the relatively small amounts of electrical heat in these areas, either a correspondingly higher welding current or a correspondingly longer welding time. Therefore, relatively high values of the welding current and welding time are added to the relatively large amounts of electrical heat in the external layers of oxide. Second, because the alloying on temperature of aluminum materials is substantially lower than that of steel, in spot welding untreated aluminum sheets, an alloying on of aluminum to the electrode contact surfaces usually takes place after only a few welds and sometimes even at the first weld. The occurrence of this alloying on, even at the first weld, indicates that the thermal capacity of conventional electrodes is too low. The occurrence of alloying on after several welds indicates inadequate electrode cooling, resulting in successive increases in the initial electrode temperature at the start of individual consecutive welds. It follows that when welding aluminum sheet, the alloying on condition as well as the energy level necessary to produce a quality weld could be mitigated and therefore the entire weld process would benefit by improving the cooling capability of the electrodes.

[0012] Heretofore, a significant amount of effort has been expended toward resolving the issues associated with welding aluminum. Some efforts have been directed toward modifying the shape of the working face of the electrodes. For example, one such attempt was to provide an electrode with a sharply textured end that would pierce the oxidized surface to make electrical contact with the parent metal. This approach is only marginally effective in practice as the textured surface being subjected to both high heat and high compressive loading rapidly wears off.

[0013] The most effective previous efforts have focused on eliminating the oxide surface coating at the electrode/work piece interface prior to welding, such as described in U.S. Patent Nos. 3,992,602 and 4135075, the disclosures of which are hereby incorporated by reference. [0014] These references teach deoxidizing the surface of the aluminum in the immediate vicinity of the weld with a high voltage electrically induced arc being discharged in an inert atmosphere within a gap held between the welding electrodes and the aluminum sheet. This is then immediately followed up by closing the gap and introducing a current high enough to produce a weld.

[0015] While being efficacious for its intended use, the apparatus used to deoxidize the oxide surface is not relevant in today's high volume mass production weld process technology and the modern equipment of the type currently employed in the automotive and aviation industries.

[0016] Further, the welding apparatus, as disclosed in the above aforementioned patents, can be used only in conjunction with a pedestal welder. A pedestal welder is fixed equipment, which is totally inappropriate for use in current high volume automobile assembly operations. Modern high volume motor vehicle production employs portable spot weld guns that can be used in either manual or robotic applications. Standard conventional vehicle design, in the interests of material and weight conservation, keeps weld flanges narrow. This necessitates using a welding apparatus with a minimum size tool envelope.

[0017] Therefore, prior to the present invention, no practical, low cost aluminum spot welding apparatus has been developed for mass production purposes such as the production of automobile bodies, aircraft components and the like. As detailed hereinafter, the present invention provides a compact, low cost apparatus, accommodating the aluminum surface preparation process, with the capability of producing a follow up weld, dimensionally applicable for automotive use, readily adaptable to current weld gun construction and possessing enhanced cooling characteristics.

SUMMARY OF THE INVENTION

[0018] The present invention provides a spot welding apparatus which prepares an aluminum surface to be welded prior to electrode contact and enhances the electrode cooling characteristics as well as energy utilization thereby extending electrode life and promoting improved weld quality.

[0019] A first embodiment of the present invention comprises a fluted electrode shank and an internally finned fluted electrode cap assembly, each of the shank and cap being fluted along their external bodies and in fluid communication with a gas delivery tube for delivering an inert gas to an aluminum work piece surface.

[0020] In a second embodiment hereof, the shank/cap assembly is, instead, based on a standard, commercially available electrode shank/electrode combination in fluid communication with a gas delivery tube having a fluted inner wall.

[0021] For a more complete understanding of the present invention, reference is made to the following detailed description and accompanying drawing. In the drawing, like reference characters refer to like parts throughout the several views, in which :

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Fig. 1 is a side view of a typical portable spot weld gun for either manual or robotic applications used in high volume automotive vehicle and aircraft production; [0023] Fig. 2 is a perspective of a first embodiment of the assembled apparatus of the present invention;

[0024] Fig. 3 is a cross-sectional view taken along 3 - 3 of Fig. 2;

[0025] Fig. 4 is an exploded perspective of the fluted electrode shank and fluted, finned electrode cap assembly used herein;

[0026] Figs. 5a and 5b are, respectively, a perspective view and a side view showing the inert gas delivery to the electrode shank / electrode assembly;

[0027] Figs. 6 is a perspective of the manifold assembly used herein;

[0028] Fig. 7 is a cross-sectional view showing a robot arm having a pair of opposing surface apparatuses in the surface preparation position ;

[0029] Fig. 8 is a cross-sectional view similar to Fig. 7, but showing opposing apparatus in the welding position;

[0030] Fig. 9. is a perspective view of an electrode shank assembly in accordance with a second embodiment of the present invention;

[0031] Fig. 10 is a cross-sectional view of the electrode shank/electrode assembly in the surface preparation position;

[0032] Fig. 1 1 is a cross-sectional view showing the assembly in a weld position;

[0033] Fig. 12 is a partial cross-section view showing the gas flow in a surface preparation position;

[0034] Fig. 13 is a partial cross-sectional view showing the gas shut off position; [0035] Fig. 14 is a partial perspective view of the manifold used in the second embodiment;

[0036] Fig. 15 is a cut away, partial perspective view of the manifold hereof, and

[0037] Fig. 1 6 is an exploded perspective view of the assembly of the second embodiment hereof.

DESCRIPTION OF THE INVENTION

[0038] Referring now to the drawing and, in particular, FIGS. 1 - 9, there is shown a typical portable spot weld gun 10 used in either manual or robotic applications for high volume automotive vehicle production. The relevant gun components comprise a fixed arm 8 and a moveable arm 6. These arms frictionally hold, at their ends, a fixed and a moveable electrode shank/cap assembly, generally, denoted at 12. In accordance with the present invention, the gun 10 is particularly adapted for spot welding together aluminum sheets or work piecesl 1 (FIGS. 7-8).

[0039] With more particularity, the shank/cap assembly 1 2 for use, generally, comprises:

[0040] (a) an electrode shank 14 having a substantially cylindrical body 16 and at least one external flute or channel 18.

[0041] (b) an electrode cap 20 having at least one external flute 22, the cap being mountable onto the shank body 16;

[0042] (c) a manifold 24 encircling the shank 14 medially thereof; [0043] (d) means, such as a set screw 26, for seating the manifold in position on the shank, and

[0044] (e) a gas delivery tube 28 encircling at least a portion of the shank 14 and electrode cap 20, the gas delivery tube 28 being axially movable along the extent of the shank/cap assembly 12.

[0045] The assembly further includes a conventional cooling tube 30. The cooling tube extends through the shank 14 and into an internal cavity 32 of the internally finned electrode cap 20. The tube 30 delivers a cooling fluid, such as cooling water, from a source (not shown) into the cavity 32 in the well known manner.

[0046] As shown in Fig. 3, the electrode shank 14 has a standard tapered end 34 to allow it to be press fitted into a corresponding tapered opening 36 at the terminal end 38 of either the fixed or moveable arms of the spot weld gun 10.

[0047] As shown in Fig. 4, the shank body 16 has at least one and, preferably, a plurality of flutes 18 formed around the exterior periphery thereof. The flutes begin approximately halfway down the length of the shank and extend along an extent 40 to reduced diameter section 42, which defines a shoulder or abutment where the replaceable electrode cap 20 is to be fitted thereonto and be slightly distanced from.

[0048] The replaceable electrode cap 20 has a complementary and similarly externally fluted body 44, and, preferably, the finned internal cavity 32. By being fluted and finned, the cooling capability of the cap is enhanced. This type of electrode cap is similar to the cap described in co-pending PCT Patent Application No. PCT/US16/28259, filed April 19, 2016 which is based on U.S. Provisional Patent Application, Serial No. 62/151 ,682, filed April 23, 2015, the disclosure of which is hereby incorporated by reference.

[0049] Referring particularly to Figs. 2, 3 and 6 the manifold 24 comprises a cylindrical body 46 having an internal cavity 48. The manifold 24 has an internally threaded rear end 50.

[0050] A threaded end cap 52, threaded, as at 54, is used to secure the end cap to the body 46.

[0051] An Ό" ring seal 56 and the set screw 26 are fitted to the end cap 52 to secure the manifold in position on the shank 14. The "0" ring seal 56 seals the manifold 24.

[0052] Means for biasing such as spring 58 is disposed in the cavity 48 of the manifold such that, at a first end 60, the spring is located against the inside wall 62 of the threaded manifold end cap 52 and on the other end of the cap 64 the spring locates on an end surface 66 of the gas delivery tube 28.

[0053] The spring 58 encircles and is moveable along the longitudinal axis of the electrode shank 14 of the assembly 12.

[0054] The manifold body 46 has a port 68 through an which inert gas is introduced into the apparatus and flows to flood the surface of an aluminum sheet that is initially deoxidized with a high voltage electric arc, then shortly thereafter spot welded.

[0055] The cylindrical gas delivery tube 28 is fitted over the electrode shank 14 and within the manifold body 46 in a manner which allows lateral freedom of movement along the center line of the electrode shank with its rest position being maintained by the spring against the internal surface of the manifold body. The gas delivery tube 28 has a first end 70 opposite the end or surface end 66.

[0056] The gas delivery tube 28 is also encircled and captured, at the one end 66, by the manifold body 46.

[0057] The delivery tube 28 contains a series of inert gas access ports 72 formed around the periphery of the first end 70 thereof and in fluid communication with the inert gas port 68. The end 66 is disposed within the manifold cavity 48, when assembled, as noted above.

[0058] The end 70 of the tube 28 includes a series of ports 74 which serve as pressure relief vents through which excess inert gas flooding the work piece surface exits.

[0059] In use, inert gas from a source (not shown) enters the access ports 72 from the inlet 68 and passes to the work piece by way of the flutes 18, 22 along the length of the electrode shank/electrode assembly 12 through the aligned openings during surface preparation. Excess gas is vented to the atmosphere via the ports 74.

[0060] Referring to Fig.7, there is shown two assemblies friction fitted to the weld gun arms in the surface preparation position relative to the aluminum work piece 1 1 . Inert gas, such as argon, is introduced through the ports 72 and flows through the interior of the tube along the flutes 18 and therefrom along the flutes or channels 22 of the cap 20 to the surface of the work piece 1 1 .

[0061] The gas delivery tube, which is touching the work piece surfaces 1 1 from which the electrodes are held a, short distance away (D). This distance D, as well as the abutment of the tube, can be controlled and maintained through suitable sensing equipment (not shown) well known to the skilled artisan.

[0062] At this point a high voltage electric arc is introduced between the electrodes and the work piece to deoxidize and further decontaminate the surface to be welded.

[0063] Shortly thereafter the electrodes are closed (Fig. 8) to allow welding to proceed. Closing the electrodes on the work pieces 1 1 shifts the gas delivery tube rearward, shutting off the flow of the inert gas at the delivery tube access port/electrode shank interface. Thereafter, both the electrodes and the gas delivery tubes are in intimate contact with the work piece and ready to introduce a weld.

[0064] Referring now to Figs. 9-16, there is depicted therein, a second embodiment in accordance herewith and, generally, denoted at 100. According to this embodiment, a standard electrode shank 1 10 comprises a substantially cylindrical body 1 12 having first and second ends 1 14, 1 16, respectively.

[0065] The shank 1 10 has a reduced diameter section 1 18, which terminates at the end 1 16. The junction of the section 1 18 and the body 1 12 of the shank defines a shoulder 120. The assembly 100 also includes a standard domed electrode cap 126 having an internal cavity 128. The first end 1 14 has an entryway 122 through which a cooling tube 124 extends into the internal cavity 1 28 provided in the electrode cap 126, as explained below. [0066] The electrode cap 126 is press fitted onto the end 1 16 of the electrode shank, as shown, and is moveable with the shank 1 10 between a surface preparation position and a weld position.

[0067] The electrode cap 126, comprises a conventional dome-shaped electrode having a substantially cylindrical body 136 and a domed top 132 having a central spot 134. The cavity 128 may or may not have internal fins. Here, the cavity 120 is shown as being finned.

[0068] The assembly 100 further comprises a two-piece manifold assembly 136, which includes a first or front housing section 138 and a stepped end cap 140.

[0069] A front portion 142 of the end cap 140, as shown, has a reduced diameter section 144 and threadably mounts to front section 138. A flange 148 on the front formed on the front portion 142 engages the rear end 148 of the front housing section 138 to define a stop.

[0070] The end cap 140 is internally grooved, as at 150.

[0071] Biasing means, such as a spring 152, is disposed in the groove 150. A seal 154 is used to prevent gas flow rearwardly in the conventional manner.

[0072] An inlet gas delivery port 156 is provided in the end cap 140. The manifold assembly 136 is set in position on the shank 1 10 via a fastener, such as a set screw 158. The shank is inserted and extends through the manifold assembly and is fixed by the set screw 150, as well. [0073] The manifold assembly end cap 140 has the stepped sections because of the position of the inlet port 156.

[0074] Once assembled and in use, a gas delivery tube 160 is used to deliver an inert gas to the work piece surfaces in a manner similar to that described with respect to the first embodiment.

[0075] The tube 160 is a substantially cylindrical open nded tube which encircles the shank 1 10 and the electrode cap 126 when the device is assembled.

[0076] The tube 160 comprises a first end 162 and a second end 164, which end 164 is insertable into the first manifold housing 128 and an elongated portion 166 which is internally fluted or channeled as at 168 and is normally held thereagainst by the spring 152.

[0077] The second end 164 is provided with a plurality of ports 170 circumferentially disposed therearound and which are in fluid communication with the port 156 as well as the flutes 168 to enable gas flow through the manifold and out the first end 162.

[0078] The degree of compression of the spring 152 is limited by the length of the shank/electrode assembly.

[0079] As shown in Fig. 12, when the electrode assembly is in the surface preparation position, the end 163 of the tube 160 is in contact with a work piece 172.

[0080] When the electrode assembly is moved into the weld position, the tube 160 is retracted or compressed by the electrode into contact with a work piece moved by the movable arm of the weld gun, as shown in Figs. 1 1 and 13. Upon contraction or compression, the inserted section 144 of the rear end cap 140 to second end 162 of the tube 16 engages the inserted section of the rear end cap to close off gas flow.

[0081] It should further be noted that the gas tube may be further retracted to expose the electrode cap to facilitate replacement when necessary.

[0082] It can be seen that the above described apparatus for the surface preparation and resistance spot welding of aluminum provides a unique solution for overcoming the difficulties posed by the welding of aluminum in a high volume, mass production application.