CATALYTIC CONVERTER SUBSTRATE AND ASSEMBLY

Abstract: A substrate (10) for a catalytic converter is formed of steel sheets (16) embossed with corrugations (34,36) in a chevron pattern such that the metal of each sheet (16) buckles at vertices (32) of adjoining corrugations (34,36) to form projections (38) extending out of the plane of the sheet (16). The sheets (16) are stacked with the projections (38) registered to form weld points and capacitive-­ discharge welding secures the sheets (16) together. The substrate (10) is formed in two halves (12) which are clamped together and then secured by end rings (14).

Description: Field of the Invention This invention relates to a welded metal substrate for a catalytic converter and particularly to a metal substrate assembly for a catalytic converter. Background of the Invention In catalytic converters for automotive vehicles, catalysts are supported on substrates having a large area and there are exhaust flow passages over that area so that intimate contact between the exhaust gas and the catalyst can occur. Honeycomb substrates formed of foil strips have been proposed but they are typically deficient in some respects and/or present problems from a manufacturing and/or functional standpoint. For example, it is known to form the honeycomb substrate by spirally winding strips of smooth and corrugated foil together but there results the problem of relative telescoping or sliding occurring between the layers which first abrades the catalyst and can end in structural failure of the substrate. With such an arrangement, it is difficult to maintain the integrity of the metal substrate without some form of mechanical strengthening or bonding of the layers. Then there remains another major problem of allowing design flexibility in the shape of the metal-layered substrate cross-section to meet various space allocations whilst maintaining a curved profile for housing strength reasons. This is particularly important in meeting certain vehicle under-floor space requirements where a low profile cross-section of, for example, an oval shape is desired rather than a circular one which requires a larger height for the same frontal area. Moreover, there is the difficulty and expense of manufacture in completely forming a metal-layered substrate so as to be suitable for a final step of applying a catalyst coating thereon. There have been several proposals for containing the corrugated foil in the desired profile using some form of welding, brazing or mechanical detention. The latter approach is disclosed by U.S. Patents Nos. 4,619,912 and 4,559,205. In both of these U.S. patents, alternating layers of smooth foil and corrugated foil of varying widths are stacked together to achieve the desired oval profile and then are sheathed in a retainer comprising a pair of metal shells which are assembled by pressing them over the foil layers and then welding the shells together along mating plane surfaces. The U.S. patents described below utilize welding of the foil. U.S. Patent No.4,186,172 shows a spirally-wound structure of circular cross-section having alternating smooth and corrugated sheets. The welding is done by laser-beam or electron-beam welders either during the winding process or after winding. U.S. Patent No.4,282,186 also shows a spirally-wound structure but a structure which has a hollow centre so that it may be flattened to form an oval, racetrack profile in cross-section. Spot-welds are made in the end faces of the foil matrix by electron-beam welders; or brazing of the end faces may be used instead. In another version, instead of welding the end faces of the foil, semi-elliptical half-shells are pressed around the matrix and welded together to maintain a desired oval shape. It is known to form chevron patterns in metal foil sheets and to stack the sheets into assemblies having a large surface area for supporting catalysts and having gas flow passages to promote contact of the exhaust gas with the catalyst. U.S. Patent No.4,382,323 shows such a chevron pattern foil wound in a spiral but the manner of retaining it in a wound shape is not disclosed. Ridges in the adjacent layers of the foil abut and cross one another at their peaks to form passages between the sheets. U.S. Patent No.4,753,919 discloses a method for optimizing the number of contact points between confronting surfaces of an accordion-folded metal foil corrugated in a chevron pattern by specifying a certain mathematical relationship of the chevron and foil dimensions. The manner of securing the layers of the stack is not discussed. Summary of the Invention It is therefore an object of the present invention to provide a substrate for a catalytic converter especially configured for welding together and adapted for welding many layers of foil by a single welding event. It is another object of the invention to provide such a substrate having weld projections that make contact points between adjacent sheets to facilitate capacitor-discharge welding of the sheets. Another object is to provide such a substrate by pre-assembling each of two half-substrates, and then welding and securing the two halves together by welded end rings. The invention is carried out by an assembly of corrugated metal foil sheets for a substrate comprising: a stack of corrugated sheets secured together so that a plurality of fluid flow paths are defined between the sheets, each sheet having a centreline and a plurality of rows of corrugations extending transverse to the centreline and each row having corrugation-ridges in a chevron pattern with the ridges of adjacent rows having different directions, the corrugation-ridges having peaks which meet at vertices at the boundaries of adjacent rows, the peaks defining planes, each sheet having projections located on at least some of the vertices and extending outwardly from said planes into contact with an adjacent sheet in the stack, and the sheets being welded together at the projections. The invention is further carried out by a corrugated metal foil substrate for a catalytic converter comprising a plurality of corrugated foil sheets made by the steps of: forming each of a plurality of corrugated sheets by crimping a plain sheet with rows of angled corrugations to form a chevron pattern with vertices at adjoining rows, the angle between the corrugations at the vertices being sufficient to form outstanding projections normal to a median plane of the sheet, stacking a plurality of corrugated sheets with the projections of one sheet extending towards, and in contact with, an adjacent sheet, the stack being bounded by upper and lower sheets, and the stack being united by passing a pulse of weld-current through the stack from electrodes at the upper and lower sheets to resistance-weld the sheets together at the projections. Brief Description of the Drawings The above and other advantages of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings wherein like references refer to like parts and wherein: Figure 1 is an isometric view of a fully-assembled substrate according to the invention. Figure 2 is an exploded view of a half-substrate and associated half-rings, Figure 3 is a schematic view of apparatus for welding together corrugated sheets to form a half-substrate according to the invention, Figure 4 is a plan view of a corrugated sheet for a substrate according to a first embodiment of the invention, Figure 5 is an end view of the sheet of Figure 4 taken along line 5-5 of Figure 4, Figure 6 is an enlarged sectional view of the sheet of Figure 4 taken along line 6-6 of Figure 4 which follows along a valley of the sheet, Figure 7 is a sectional view of a sheet similar to that of Figure 6 but modified according to a second embodiment of the invention, Figure 8 is a plan view of a corrugated sheet for a substrate according to a third embodiment of the invention, Figure 9 is a partly broken-away plan view of two adjacent super-imposed sheets according to the third embodiment, Figure 9a is an enlarged sectional view of a portion of Figure 9, Figure 10 is a partly broken-away front view of tooling for producing the corrugated sheets according to the invention, and Figure 11 is an enlarged view of a tool tooth of the tooling of Figure 10. Description of the Preferred Embodiment Corrugated sheets for a catalytic converter substrate are advantageously secured together by resistance-welding to form a secure core assembly. Because of the large number of sheets involved in each substrate the prior-art practice of separately welding pairs of adjacent sheets is time-consuming and expensive. Thus the welding of all or of at least many sheets at once is an attractive alternative. It has been found that capacitive-discharge welding is capable of making satisfactory welds when the sheets have been properly formed with weld projections which define precise weld points of a number and size to present the proper weld resistance. Figure 1 shows a converter substrate 10 which is made up of two half-substrates 12 held together at their ends by half-rings 14 which are assembled to the halves as shown in Figure 2 and welded at their ends to complete the full substrate assembly. The rings 14 are L-shaped in cross-section to overlie a portion of the substrate outer surface as well as a marginal portion of the end faces. A tang 15 depending from the centre of each half-ring 14 pierces the outer layers of sheets to hold the rings in place. As is evident in Figure 1, the assembly is formed of a stack of corrugated sheets 16 of equal lengths but of varying widths so that an oval profile is attained. Each sheet has a centreline 17 parallel to the longitudinal axis of the substrate. The corrugations in the sheets have a modified chevron configuration so that adjacent sheets define exhaust gas passages in a direction to foster generally longitudinal gas flow. Such corrugations, as is well known, provide extensive areas for the residence of catalyst material and contact of the gas with the catalyst material. As will be further described, the chevron corrugations can be formed with ridges meeting at vertices so that the sheet metal buckles a controlled amount at the vertices to form weld projections. The sheets are laid up in a stack so that the projections on adjacent sheets make contact. As shown in Figure 3, the stack of sheets for one half of the substrate are placed between electrodes 18 and 20 of a capacitive-discharge welder 22 and compressed together sufficiently to assure good contact at the weld projections. The upper electrode 20 is manipulated by an air-pressure operated actuator 24 for opening and closing and for applying the welding pressure under control of a pressure control 26. The welder 22 is capable of delivering a single pulse of current to effect welding at the projections. As is well known, a capacitive-discharge welder comprises a transformer with a bank of large capacitors in the primary circuit and means for charging the capacitors to a high voltage, and weld electrodes in the secondary circuit for delivering a discharge current at a low voltage. For a stack of 48 sheets approximately 44.45 mm (1 3/4 inches) thick and measuring 152.4 x 152.4 mm (6 x 6 inches) for the largest and 76.2 x 152.4 mm (3 x 6 inches) for the smallest sheet, the weld current pulse is 1 to 1.5 milliseconds long and delivers 28,000 joules energy at 150 volts. The modified chevron pattern shown in Figures 4, 5 and 6 is a series of peaked ridges 28 which define planes 30 and are arranged at angles to one another so that they meet at vertices 32. A median plane 33 of the corrugated sheet is shown halfway between the planes 30 of the peaks. The corrugation-ridges are arrayed in parallel rows of like ridges, the rows extending transverse, i.e., perpendicular to the sheet centreline 17. Narrow rows have short ridge segments 34 whilst wide rows have long segments 36. The segment lengths between vertices include short segments 34 or lands with ridges running parallel to the centreline 17 of the sheet and long segments 36 which extend at an angle to the centreline. Preferably the width of the row with corrugations parallel to the centreline is less than half the width of the rows with angled corrugations. The long segments 36 come together at one set of vertices 32 and the short segments 34 are interposed between long segments 36 to form another set of vertices. In either case the metal buckles to project beyond the peak plane 30 in a direction perpendicular to the plane 30 or 33 to form projections 38 at some of the vertices. Figure 6 shows that the projections 38 at the short segments extend upwardly and the projections 38 between the long segments extend downwardly. When the sheets are stacked together for welding, alternate sheets are flipped over to register the points of adjacent sheets. In an alternative configuration the projections 38 may extend up and down from each vertex as shown in Figure 7. One effect of the short segments is to increase the number of projections compared to a pattern having long segments only. In the embodiment of Figures 8 and 9 all long segments 36 are separated by short segments 34 so that short and long segments alternate. In addition, projections 38 extend from every vertex 32. This pattern optimizes the number of projections 38. The lay-up of sheets 16 in a stack is illustrated in Figure 9 for a pair of adjacent sheets 16a and 16b where the upper sheet 16a is broken away to reveal the lower sheet 16b. Figure 9a is a cross-section which shows how the projections from adjacent sheets are registered to form a weld site. The chevron angle c (Fig. 9) between the long segment 36 and the short segment 34 is optimized to enhance the formation of rounded, somewhat conical projections 38. For stainless steel foil 0.051 mm (0.002 inch) thick having a corrugation pitch of 2.26 mm (0.089 inch) and a corrugation height (peak-to-peak) of 1.02 mm (0.04 inch), the preferred angle c is 10 degrees. An acceptable range for angle c is 8 to 14 degrees. The required projection is not obtained at angles less than 8 degrees. At an angle c of 10 degrees the projections 38 are large enough to form a generally circular contact area having a diameter of 0.127 mm (0.005 inch) when like projections are pressed together.Each projection has a height of the order of 0.0254mm (0.001 inch). Where a curved substrate profile is desired, sheets made according to the patterns of any one of the above described embodiments are cut in varying widths to conform the desired core profile when assembled. If a rectangular core section is preferred, the sheets will all have the same width. Adjacent sheets are oriented with projections registered for contact. A stack of sheets for a half substrate 12 is placed between the electrodes 18 and 20 of the welder and weld pressure applied. Then a single pulse of weld current is directed through the stack to complete the welds at the contacting projections. Two such welded half-substrates are clamped together and the half-oval rings 14 are applied to the ends of the substrate halves 12 with the tang 15 piercing the outer sheets. The abutting ends of the half-rings 14 are welded together to complete the assembly of the substrate. As an alternative to welding substrate halves and holding them together by end rings, the sheets may be all welded together at the contacting projections: then the end rings will not be necessary. One approach to the all-welded structure is to weld all the sheets at once instead of in halves. Depending on the desired profile, stacking the sheets with the projections maintained in registry may be somewhat involved or obtaining adequate welds through the stack may be difficult. Another approach is to weld the sheets into small packets and then weld two, three or more packets together to make the whole core. The choice of assembly methods depends on the ability to weld a large stack of sheets in the desired profile or to weld together the packets of sheets. Tools 40 for corrugating the foil sheets are shown in Figure 10. Gear-like rotary tools mesh together to roll the corrugations in a plain foil. The corrugation pattern of Figures 5 and 6 is produced by the illustrated tools 40. Teeth 42 on each wheel 44a, 44b, 44c have alternate right and left pitches to form rows of corrugations of opposite angles in the foil. Spacers 46 between wheels 44b and 44c separate those wheels. The corrugations parallel to the centreline are formed in this space without benefit of a toothed wheel to positively form those corrugations. The metal-bending action at the adjacent wheels is sufficient to form the corrugations in the space. A further benefit is that the projections 38 at the vertices of the corrugations are also formed here due to the buckling at the bend of the metal peaks. A relief in the form of a chamfer 48 on each tooth edge adjacent to a spacer 46, as best shown in Figure 11, enhances the projection formation. A 45 degree chamfer 48 of 1.27 mm (0.05 inch) width provides satisfactory relief. While the projections 38 are best formed at the chamfer in the regions adjacent a short segment 34, they also occur at the vertices of two long segments 36 as formed by wheels 44a and 44b. If the pattern of Figures 8 and 9 is desired, a spacer will be used between every pair of wheels 44a, 44b, etc., and both ends of each tooth 42 will be chamfered.

Claims: A method of making a corrugated metal foil substrate (10) for a catalytic converter, containing an assembly of corrugated metal foil sheets (16) characterised in that the method comprises the steps of: forming each corrugated sheet (16) by crimping a plain sheet with rows of angled corrugations (36) to form a chevron pattern with vertices (32) at adjoining rows, the angle (c) between the corrugations (36) at the vertices (32) being sufficient to form outstanding projections (38) normal to the median plane (33) of the sheet (16); stacking a plurality of said corrugated sheets (16) with the projections (38) of one sheet (16) extending towards and in contact with the projections of an adjacent sheet (16), the stack (12) being bounded by upper and lower sheets (16); and uniting the stack (12) by welding the sheets (16) together at the projections (38) thereof. A method of making a corrugated metal foil substrate for a catalytic converter according to claim 1, characterised in that the sheets (16) of the stack (12) are welded by passing a pulse of weld current through the stack (12) between the upper and lower sheets (16) to weld the sheets (16) together at the projections (38) thereof. A method of making a corrugated metal foil substrate for a catalytic converter according to claim 1, characterised in that the sheets (16) vary in width and are stacked one upon the other to form, when assembled together, a stack (12) having a half-oval contour in cross-section; and a complete substrate (10) which is oval in cross-section is formed by assembling two of said half-oval stacks (12) together and then welding a retaining collar (14) around each end of the assembled stacks (12). A method of making a corrugated metal foil substrate for a catalytic converter according to claim 1, characterised in that the chevron pattern on each sheet (16) is formed with three interdigitated sets of corrugations comprising a set with corrugations (34) parallel to the centreline (17) and two sets with corrugations (36) extending at an angle (c) in either direction from the centreline (17), and the sheets (16) are stacked with the projections (38) on each sheet (16) substantially in contact with projections (38) on adjacent sheets (16). An assembly of corrugated metal foil sheets for a substrate comprising: a stack of corrugated sheets (16) secured together so that a plurality of fluid flow paths are defined between the sheets, each sheet (16) having a centreline (17) and a plurality of rows of corrugations extending transverse to the centreline (17) and each row having corrugation-ridges (28) in a chevron pattern with the ridges of adjacent rows having different directions, characterised in that the corrugation-ridges (28) have peaks which meet at vertices (32) at the boundaries of adjacent rows, the peaks defining planes (30); each sheet has projections (38) located on at least some of the vertices (32) and extending outwardly from said planes (30) into contact with the projections of an adjacent sheet in the stack; and the sheets are welded together at the projections (38) thereof. An assembly of corrugated metal foil sheets according to claim 5, characterised in that the projections (38) of each sheet are aligned with and in contact with the projections (38) of an adjacent sheet. An assembly of corrugated metal foil sheets according to claim 5, characterised in that some rows have corrugations (36) extending at an angle (c) to the centreline (17) in either direction and other rows have corrugations (34) extending parallel to the centerline (17) with adjacent rows having different directions. An assembly of corrugated metal foil sheets according to claim 7, characterised in that the rows of corrugations alternate, with two adjacent rows having corrugations (36) at an angle (c) to the centreline (17) in either direction and a third row having corrugations (34) extending parallel to the centreline (17). An assembly of corrugated metal foil sheets according to claim 7, characterised in that the rows of angled corrugations (36) alternate in direction and are separated by a row with corrugations (34) parallel to the centreline (17). An assembly of corrugated metal foil sheets according to any one of claims 6, 7 or 8, characterised in that said angle (c) to the centreline (17) of the corrugations (36) is in the range of 8 degrees to 14 degrees, and the width of the row with corrugations (34) parallel to the centreline (17) is less than half the width of the rows with angled corrugations (36). An assembly of corrugated metal foil sheets according to any one of claims 6, 7 or 8, characterised in that the foil is stainless steel approximately 0.051 mm thick, and said angle (c) to the centreline (17) of the corrugations (36) is about 10 degrees. A catalytic converter substrate (10) formed from assemblies of corrugated metal foil sheets (16) according to any one of the preceding claims, in which said substrate (10) comprises: upper and lower half-substrates (12) secured together to form a whole substrate; each half-substrate (12) comprises an assembly of corrugated metal foil sheets (16) according to any one of the preceding claims; half-rings (14) conforming to the half-substrate contour are attached to each end of each half-substrate (12); and the half-rings (14) at each end of the substrate (10) are welded together to secure the complete substrate (10).

Inventors: LUNKAS, MICHAEL JOHN, SPILKER, MATTHEW WAYNE
Applicants: GENERAL MOTORS CORPORATION
Application year: 1990
Main classification: F01N3
IPC codes: F01N3, B01J35, B01D53