WO1999020461A1

WO1999020461A1 - Method for the fabrication of lightweight structural members of sheet metal with integrated cellular web stiffener and associated structural members

Info

Publication number: WO1999020461A1
Application number: PCT/IT1998/000285
Authority: WO
Inventors: Dario Amidei
Original assignee: Delta Di Amidei Dario & C., S.A.S.
Priority date: 1997-10-22
Filing date: 1998-10-16
Publication date: 1999-04-29
Also published as: IT1295761B1; ITTO970924A1

Abstract

The method for the fabrication of lightweight structural members comprises a first step in which a box structure (10) is manufactured of sheet metal consisting of two half-shells (11, 12), a second step in which a cellular web (16) is manufactured on the surface of which a plurality of three-dimensional cells having hills (17) and valleys (18) are formed, and a third step in which the web (16), placed inside the half-shells (11, 12) so as to bring the hills and the valleys into contact with the half-shells (11, 12), is rendered rigidly and permanently integral with the half-shells (11, 12), and the half-shells in turn are rendered integral with each other; provision is also made for a fourth step, in which the space inside the half-shells (11, 12) is filled with a rigid synthetic resin.

Description

METHOD FOR THE FABRICATION OF LIGHTWEIGHT STRUCTURAL MEMBERS OF SHEET METAL WITH INTEGRATED CELLULAR WEB STIFFENER AND ASSOCIATED STRUCTURAL MEMBERS

TECHNOLOGICAL FIELD OF THE INVENTION The method according to the invention may be used for the fabrication of mechanical parts with structural functions, having a considerable degree of torsional and flexural strength combined with low weight, made of sheet metal.

TECHNICAL BACKGROUND Mechanical parts are known that have the characteristics described above, made typically from the fusion welding or hammer forging of lightweight metal alloys, for example, but not only, aluminium alloys.

The parts made in this way are however expensive, due to the intrinsic cost both of the materials and of the production process; worse still, the latter also has drawbacks of an ecological nature that are difficult to overcome. Also known are mechanical parts having the characteristics described above, produced typically by using semifinished parts consisting of cellular sandwich type laminated sheets or panels of metal alloys, for example thin honeycomb sheet metal or cellular sandwich sheets of the type described in Italian Patent Application No. TO96A000696 and in the corresponding International Patent Applications No. PCT/IT97/000164 and No. PCT/IT97/000165, both the property of the applicant; these semifinished parts, however, have workability limitations which prevent their being used to make complex shapes or for special applications.

This, however, does not solve the problem of producing mechanical parts with structural functions, of unlimited shapes and applications, having a considerable degree of torsional and flexural strength combined with low weight, with materials of low cost and using a fabrication process that is simple, economic and has a low environmental impact.

SUMMARY OF THE INVENTION The object of this invention is that of defining a method for the fabrication of lightweight structural members, including in complex three-dimensional shapes, made of sheet metal with integrated cellular web stiffener, using low-cost materials, for example, but not exclusively, steel sheets, and economic, non-polluting work processes such as, for example, press moulding and laser welding. The method consists in combining an outer box structure made from two half- shells of sheet metal suitably shaped according to a perimeter, with an inner cellular web consisting of a three-dimensional corrugated structure of suitable shape and size, the whole being rendered integral by welding the two half-shells together along the perimeter and by welding the tops of the hills and the valleys of the inner cellular web to the two half-shells.

To endow the structural member without greater strength without significantly increasing its weight, there is also the possibility of filling the inner zone of the box structure with a material, typically a resin, that is rigid at low density or a foam.

Another object of the invention is that of defining lightweight structural members, including in complex three-dimensional shapes, of sheet metal with integrated cellular web stiffener, produced using the method of fabrication described above.

The above-mentioned objects are achieved by means of a method for the fabrication of lightweight structural members of sheet metal with integrated cellular web stiffener and of the relative structural members, characterized as defined in the main claims.

These and other objects, characteristic features and advantages of the invention will become apparent in the course of the following description of a preferred embodiment, provided by way of a non-exhaustive example, in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows in schematic form a cross-sectional view of the pair of half-shells constituting the box of the structural member according to the invention. Fig. 2 shows in schematic form a cross-sectional view of the structural member according to a first embodiment of the invention.

Fig. 3 shows in schematic form a cross-sectional view of the structural member according to a variant of the first embodiment of the invention, wherein the member is made from a semifinished part of cellular sandwich sheet metal. Fig. 4 shows in schematic form a cross-sectional view of the structural member according to the invention in the event that it comprises a functional element.

Fig. 5 shows in schematic form a cross-sectional view of the structural member according to the invention in the event that the two half-shells are of non-uniform thickness.

Fig. 6 shows in schematic form a cross-sectional view of the structural member according to the invention in the event that the cellular web is made of two sheets. Fig. 6bis shows perspectively in schematic form a sectional view of the structural member according to the invention in the event that the cellular web is made of two sheets having corrugations arranged perpendicular to each other. Fig. 7 shows in schematic form a cross-sectional view of the structural member according to the invention in the event that the cells of the cellular web are of non- uniform height.

Fig. 8 shows in schematic form a cross-sectional view of the structural member according to the invention in the event that the cells of the cellular web are made by hydro-forming. Fig. 8bis shows in schematic form a cross-sectional view of the structural member according to the invention in the event that the two half-shells are made of corrugated sheet metal.

Fig. 9 shows in schematic form a cross-sectional view of the structural member according to a second embodiment of the invention, wherein the two half-shells are made of cellular sandwich sheets.

Fig. 10 shows in schematic form a partial cross-sectional view of the structural member according to the second embodiment of the invention, in the event that the two half-shells are micro-drawn.

Fig. 11 shows in schematic form a partial cross-sectional view of the structural member according to the second embodiment of the invention in the event that the cellular web is also micro-drawn.

Fig. 12 shows in schematic form a cross-sectional view of the structural member according to a third embodiment of the invention.

Fig. 13 shows in schematic form a partial cross-sectional view of the structural member according to a variant of the third embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The method for the fabrication of lightweight structural members of sheet metal with integrated cellular web stiffener according to a first embodiment of the invention will be described below, for simplicity's sake, with reference to a substantially flat member or with a slight bend and plan shape bounded by a continuous perimeter, it being possible, though not necessary, for there to be one or more apertures or holes in the said perimeter, this certainly not representing a limiting factor of the invention, as will become apparent hereinbelow.

A first step of the method consists in manufacturing a box structure 10 made from two half-shells, an upper half-shell 11 and a lower half-shell 12, of compact sheet metal having a first thickness s-i, of which a transverse section is illustrated in Fig. 1 . The sections of the two half-shells 11 , 12 have mirroring shapes and comprise a zone 13 and 13' respectively, that is substantially flat and joined all along its perimeter to a wall 14 disposed substantially at right angles to the flat zones 13, 13'; the wall 14 terminates with an edge 15, in turn disposed substantially at right angles to the wall 14.

In the event of structural members of small and average dimensions, the two half-shells 11 , 12 are produced, for example, by pressing with mechanical presses known per se; in the event of large dimensions, they are preferably produced using pressure rollers, also known per se.

A second step of the method consists in manufacturing an inner member or cellular web 16 (see Fig. 2), made from a compact sheet having a second thickness s₂, forming by plastic deformation on its surface a plurality of three-dimensional cells having hills 17 and valleys 18. The cellular web 16, for example, has a plurality of three-dimensional cells of small dimensions, a total height h of between 0.1 and 5 mm, preferably between 0.5 and 3 mm, distributed along a plane at a uniform pitch p according to two substantially perpendicular directions in such a way that the ratio p/h of the pitch to the total height is equal to or greater than 1 , preferably equal to or greater than 2, as described in the Italian Patent Application No. TO96A000696 and in the corresponding International Patent Application No. PCT/IT97/000165, cited earlier, to which reference should be made for a more complete illustration of the characteristic features.

According to another example, the cellular web 16 has a plurality of three- dimensional cells of total height h between 0.1 and 60 mm, preferably between 0.5 and 50 mm, distributed along a plane at a uniform pitch p according to two substantially perpendicular directions in such a way that the ratio p/h of the pitch to the total height is equal to or greater than 1 , preferably equal to or greater than 2, as described in the Italian Patent Application No. PCT/IT97/000164, cited earlier, to which once again reference should be made for a more complete illustration of the characteristic features.

The surface globally of the cellular web 16 has a substantially flat shape or with a slight bend, corresponding exactly to the shape of the zones 13, 13' of the half-shells 11 , 12, and is copymilled, by means of a copymilling process known per se, so as to fit exactly inside the perimeter of the two half-shells 11 , 12, and be placed with the hills 17 and the valleys 18 in contact with the zones 13, 13' respectively of the upper 11 and lower 12 half-shells, the edges 15 of the half-shells 11, 12 being simultaneously in contact with each other.

A third step of the method consists in joining and rendering rigidly and permanently integral the upper half-shell 11 , the lower half-shell 12 and the cellular web 16 inside the two half-shells 11, 12, by means of a joining process, for example welding, and particularly seam laser welding, of the edges 15 together, and welding, for example spot laser welding, of the hills 17 of the cellular web 16 with the zone 13 of the upper half-shell 11 and of the valleys 18 of the cellular web 16 with the zone 13' of the lower half-shell 12; during the joining operation, suitable holding jigs provided with mechanical type elastic systems or hydrostatic pressure rollers, known to those acquainted with the sector art, ensure correct contact between the members to be joined.

Naturally changes may be made to the invention described above, without departing from the scope of the invention itself.

For example, a first change may be made to the method of the invention, particularly useful where the structural member has a substantially flat shape or is slightly curved, consisting in producing the structural member starting from a semifinished piece taking the form of a cellular sandwich sheet with cells of a suitable total height h and suitable pitch p, described in the International Patent Applications No. PCT/IT97/000164 and No. PCT/IT97/000165, cited above, (see Fig 3) in which the second and third steps of the method have already been implemented, the two half-shells 11 , 12 being already joined with the cellular web 16, and the first step of manufacturing the rigid box structure is effected substantially by performing an impact type closing drawing along the perimeter of the member in order to create the edges 15 that are subsequently welded, for example by seam laser welding .

A second change to the method of the invention, useful where the structural member has a complex three-dimensional shape, consists, for example, in effecting on the cellular web 16, in addition to and during forming of the plurality of small-size three-dimensional cells having the hills 17 and the valleys 18, a larger scale forming, corresponding to the same, complex three-dimensional shape assumed by the joining of the two half-shells 11 , 12 along the edge 15. Said larger-scale forming is produced using a known type forming process with a mechanical mould, or, alternatively, using a hydro-forming process, equally known to those acquainted with the sector art.

For particularly complex shapes (for example, with accentuated radii of curvature or drawing), a third change to the method of the invention consists in effecting the larger-scale forming as the first operation after the copymilling, where the semifinished piece used is a compact flat sheet, again produced using a forming process with mechanical mould or, alternatively, using a hydro-forming process; and subsequently effecting forming of the plurality of three-dimensional cells of lower dimensions having the hills 17 and the valleys 18 on the already formed, larger scale sheet, again produced using a forming process with a mechanical mould or, alternatively, using a hydro-forming process.

Among the examples of complex three-dimensional shapes to which the changes to the method described above are applicable, both tubes of any section (circular, elliptical, rectangular, etc.) and also closed or open profiled sections may be cited in particular as examples. Or the method of the invention may also be applied in cases where the two half-shells 11 , 12 are given a shape suitable for containing inside not only the cellular web 16, but also a functional component, where this term is taken generically to mean a component with which the structural member of the invention can cooperate functionally with another member; in Fig. 4 said functional component is shown by way of example as a bushing 20 rendered integral with the two half-shells 11, 12 by welding or other joining process known in the art, where other examples are pins, hubs, shafts, ball bearings, etc.

It is also possible to make yet a fourth change to the method of the invention, consisting in using the two half-shells 11 , 12 in which the first thickness Si is not uniform, but one or both the two half-shells 11 , 12 have zones 22, 23 respectively (see Fig. 5) of a different thickness, for example, as in the figure, thicker than the other zones; said thicker zones 22, 23 may be obtained, for example, by placing a compact sheet of a determined shape having a third thickness s₃ in a suitable position, either on the outside as shown in the figure or on the inside, and rendering it integral with the sheet of thickness s-i of the two half-shells 11 , 12 by seam or spot laser welding or by brazing, in order to create zones having different rigidities.

Yet a fifth change to the method of the invention consists in using a cellular web 16 made of a simple corrugated sheet, with monodirectional corrugations only, of a type known in the sector art; or yet again in using a cellular web 16 made of more than one sheet, for instance two as in Fig. 6, an upper corrugated sheet 24 and a lower corrugated sheet 25 joined together rigidly by spot laser welding or by brazing; or again in using a cellular web 16 made of a compact sheet in which discrete hills are made, for instance by drawing, which extend in a direction substantially perpendicular to a first face and, alternatively, to a second face, opposite the first face, of the sheet itself, the shape of said hills being preferably, though by no means exclusively, that of a spherical bowl, a truncated cone or a truncated pyramid.

Further, within the scope of this fifth change to the method according to the invention, it is possible to make the cellular web from two sheet metals 31 and 32 having corrugations that extend in the longitudinal direction, and which are set one on top of the other so that the respective corrugations are arranged perpendicular to each other. This solution, depicted in Fig. 6bis, has the advantage of defining a cellular structure for the web 16 of the lightweight members that may be fabricated using the method according to this invention, which is easy to make at a low cost, being based on using two corrugated sheet metals easy to find in commerce. More particularly, a web such as this is characterized by having a plurality of cellular regions defined by the intersections made by the corrugations of the two sheet metals and also by having structural properties that are perfectly identical and homogeneous all along its plane. A sixth change to the method of the invention consists again in using a cellular web 16 wherein the total height h of the plurality of cells constituting the small-size three-dimensional forming is not uniform, but variable from zone to zone in order to make the cellular web 16 itself consistent with the two half-shells 11 , 12 comprising the box structure 10, as depicted in Fig. 7, where the total height h varies from a minimum value h_m to a maximum value h_M in relation to the similar variation of the distance between the two half-shells 11 , 12.

Or still a seventh change to the method of the invention consists in using a cellular web 16 comprising a plurality of three-dimensional hollow members 26 (see Fig. 8) made using the hydro-forming technique known per se, starting from a pair of compact flat sheets 27, 28 welded together at the points 29; shape of the hollow members 26 may be any one of those that can be produced using the hydro-forming technique: in the example shown in Fig. 8, the hollow members 26 have a substantially circular shape of diameter d.

Furthermore an eighth change to the method according to the invention consists in using not only flat sheets to make the outer two half-shells 11 and 12, but also drawn sheets with corrugations and typically having an arrangement of hills and/or valleys protruding from a relative middle plane. For example, Fig. 8bis represents a cellular element made in this way, wherein the two corrugated sheets forming the two half-shells are joined rigidly and permanently to a cellular web 16 also made of a sheet drawn and provided with hills and valleys. In particular, the joint between the three drawn sheets is made where the various hills and valleys meet, so as to obtain a complete structural co-operation between the three corrugated drawn sheets.

A second embodiment will now be illustrated of the method for the fabrication of lightweight structural members of sheet metal with integrated cellular web stiffener according to the invention, capable of making the members in question of lighter weight for like flexural and torsional strength.

According to this second embodiment, the method comprises a first step consisting in producing a box structure 60 (see Fig. 9) made of two half-shells, an upper half-shell 61 and a lower half-shell 62, of cellular sandwich sheet metal suitable for bending and drawing and having an overall thickness s_c, of which a transverse cross-sectional view is shown in Fig. 9.

A second step of the method consists in producing the inner member or cellular web 16, and is substantially identical to the second step of the method described as the first embodiment of the invention.

A third step of the method consists in rendering rigidly and permanently integral the upper half-shell 61 , the lower half-shell 62 and the cellular web 16 by seam laser welding the edges 65 together, after they have been forced together compressing the space inside the cellular sandwich sheets; and by spot laser welding of the hills 17 of the cellular web 16 with the lower outer face 63 of the upper half-shell 61 , and of the valleys 18 of the cellular web 16 with the upper outer face 64 of the lower half-shell 62. During welding, suitable holding jigs provided with mechanical type elastic systems or hydrostatic pressure rollers, known in the sector art, ensure correct contact between the members to be joined. Naturally changes may be made to this second embodiment of the invention described above, without departing from the scope of the invention.

In addition to all the changes described above and illustrated with reference to the first embodiment of the invention, which are also substantially applicable to this second embodiment of the invention, a further specific change may also be made consisting in having the laser welding between the hills 17 and the valleys 18 of the cellular web 16 and the half-shells 61 , 62 respectively, scheduled for the third step of the method according to this second embodiment, preceded by an operation of microdrawing of the cellular sandwich sheet constituting the two half-shells 61 , 62 only on those points 66 (see Fig. 10) where the hills 17 and the valleys 18 of the cellular web 16 come into contact with the half-shells 61 , 62 respectively, by means of which the cellular sandwich sheet of the upper half-shell 61 is compacted towards the lower outer face 63 and the cellular sandwich sheet of the lower half-shell 62 is compacted towards the upper outer face 64, thus permitting the creation of more robust welding joints. A further change to the method of the invention, serving to enhance the appearance of the finished structural member, consists in making a variant to the latter-named operation of microdrawing, illustrated in Fig. 11 , consisting of the fact that, while nothing changes as regards the points of contact 66 of the lower half-shell 62, the microdrawing of the upper half-shell 61 is executed by compacting the cellular sandwich sheet towards one of its upper outer faces 67 at the points 68 of contact between the hills 17 of the cellular web 16, in turn homologously drawn substantially with the same shape, to make both the contact between the sheets at the points 68 and the subsequent welding more reliable.

A third embodiment will now be illustrated of the method for the fabrication of lightweight structural members of sheet metal with integrated cellular web stiffener according to the invention, capable of endowing the members in question with greater torsional strength.

According to this third embodiment, the method comprises, in addition to the three steps described previously with reference to both the first and the second embodiment, a fourth step consisting in filling the hollow space 19 (see Fig. 12) between the upper half-shell 11 and the lower half-shell 12 by injecting a synthetic resin through suitable apertures (not shown in the figure) made in one or in both of the two half-shells 11 , 12; the filling may be appropriately facilitated, as is known in the art, by simultaneously applying a pneumatic suction to evacuate the air in the space 19.

Said synthetic resin is preferably a rigid thermoplastic resin, having rigidity characteristics that are heightened by the presence of suitable mineral and/or glass fibre charges; or a thermosetting resin, or again a rigid expanded resin, all synthetic resins well known to those acquainted with the sector art. Naturally changes may also be made to this third embodiment of the invention described above, without departing from the scope of the invention; for example, where the two half-shells 61 , 62 are made of cellular sandwich sheet, it is also possible to fill the hollow spaces 69, inside the upper half-shell 61 , and/or 70, inside the lower half-shell 62 (see Fig. 13), by injecting a synthetic resin through suitable apertures (not shown in the figure) made in the two half-shells 61, 62, to give the finished structural member greater torsional strength.

In short, without prejudice to the principle of this invention, the construction details and forms of embodiment may be significantly altered with respect to what has been described and illustrated, without exiting from the scope of the invention.

Claims

1. Method for the fabrication of lightweight structural members characterized in that it comprises the following steps: producing a box structure (10) of sheet metal comprising a first upper half- shell (11) and a second lower half-shell (12), said box structure (10) having a first perimeter and enclosing a first space (19) within, and said sheet having a determined thickness; producing a cellular web (16) of sheet metal having a surface and a second perimeter, said surface offering a plurality of three-dimensional cells having hills and valleys (17,18) formed by means of a forming process, and said second perimeter being substantially equal to said first perimeter, rendering rigidly and permanently integral by means of a joining process said cellular web (16) inside said box structure (10) with said first upper half-shell (11) and with said second lower half-shell (12), and said first upper half-shell (11) with said second lower half-shell (12).

2. Method according to claim 1 , characterized in that it also comprises the step of filling said first space (19) by injecting a rigid synthetic resin.

3. Method according to claim 1 , characterized in that said first space (19) inside said box structure (10) is suitable for accommodating a functional component (20) in addition to said cellular web (16).

4. Method according to claim 3, characterized in that said functional component (20) is selected from a set consisting of bushings, pins, shafts, hubs and ball bearings.

5. Method according to claim 1 , characterized in that said surface of said cellular web (16) is a substantially flat surface.

6. Method according to claim 1 , characterized in that said surface of said cellular web (16) is a complex three-dimensional surface.

7. Method according to claim 6, characterized in that said complex three- dimensional surface is selected from a set consisting of open profiled sections, closed profiled sections and tubes.

8. Method according to claim 1, wherein said hills and valleys (17,18) have a total height h, characterized in that said total height h is of a value between 0.1 and 60 mm.

9. Method according to claim 8, wherein said hills and valleys (17,18) have a total height h, characterized in that said total height h is of a value between 0.1 and 5 mm.

10. Method according to claim 8, characterized in that said total height h is of a value between 0.5 and 50 mm.

11. Method according to claim 9, characterized in that said total height h is of a value between 0.5 and 3 mm.

12. Method according to claim 8, wherein said hills and valleys (17,18) have a uniform pitch p in two substantially perpendicular directions, characterized in that a ratio p/h of said pitch p to said total height h is equal to or greater than 1.

13. Method according to claim 12, characterized in that a ratio p/h of said pitch p to said total height h is equal to or greater than 2.

14. Method according to claim 8, characterized in that said three-dimensional cells (17,18) have a shape selected from a set consisting of spherical bowl shape, truncated cone shape or truncated pyramid shape.

15. Method according to claim 1 , wherein said hills and valleys (17,18) have a total height h, characterized in that said total height h has a constant value.

16. Method according to claim 1, wherein said hills and valleys (17,18) have a total height h, characterized in that said total height h has a variable value, ranging between a minimum value H_mand a maximum value h_M-

17. Method according to claim 1 , characterized in that said forming process comprises a drawing process.

18. Method according to claim 1, characterized in that said forming process comprises a hydro-forming process.

19. Method according to claim 1 , characterized in that said joining process comprises a laser welding process.

20. Method according to claim 19, characterized in that said laser welding process is seam type.

21. Method according to claim 19, characterized in that said laser welding process is spot type.

22. Method according to claim 1 , characterized in that said joining process comprises a brazing process.

23. Method according to claim 1 , characterized in that said joining process comprises a bonding process.

24. Method according to claim 1 , characterized in that said determined thickness is uniform.

25. Method according to claim 1 , characterized in that said determined thickness is variable.

26. Method according to claim 25, characterized in that said variable thickness is obtained from a sheet metal (22) of determined shape joined rigidly and permanently to at least one of said upper half-shell (11 ) and said lower half-shell (12).

27. Method according to any one of the previous claims, characterized in that said sheet metal of said box structure (10) is a cellular sandwich sheet that is suitable for bending and drawing.

28. Method according to claim 27, characterized in that said joining process comprises a microdrawing process.

29. Method according to claim 28, characterized in that said microdrawing process is effected only on one of said first upper half-shell (11) and said second lower half-shell (12).

30. Method according to claim 28, characterized in that said microdrawing process is effected on both said first upper half-shell (11 ) and said second lower half-shell (12).

31. Method according to claim 29, characterized in that said microdrawing process is also effected on said cellular web (16).

32. Method according to claim 27, characterized in that said cellular sandwich sheet is filled by the injection of a rigid synthetic resin.

33. Method according to claims 2 or 32, characterized in that said rigid synthetic resin is of the type selected from the set consisting of charged thermoplastic resins, thermosetting resins and rigid expanded resins.

34. Method according to claim 1 , characterized in that said upper half-shell (11), said lower half-shell (12) and said cellular web (16) constitute a semifinished part of cellular sandwich sheet metal, and said step of producing a box structure (10) comprises an impact type closing drawing along said first perimeter.

35 - Method according to claim 1 , characterized in that said cellular web (16) comprises at least two sheet metals (31,32) having respective corrugations extending in the longitudinal direction, wherein said sheet metals (31 ,32) are set one on top of the other with the respective corrugations arranged at an incline to each other.

36 - Method according to claim 35, characterized in that the corrugations of said two sheet metals (31,32) are substantially perpendicular to each other.

37 - Method according to claim 1 , characterized in that at least one of said first upper half-shell (11) and said second lower half-shell (12) is made of corrugated sheet metal.

38. Lightweight structural member of sheet metal with a cellular web, characterized in that it is produced according to the method defined in any one of the preceding claims.