EP0593799B1 - Method of and device for shaping a blank of sheet - Google Patents

Description

The invention relates to a method for deforming a sheet metal blank from a material with an exponential tensile stress-strain behavior and a device suitable for carrying out this method.

In the aerospace industry, titanium and its alloys are increasingly used for fuel tanks or the like because of their low weight and good corrosion resistance. The titanium-β alloys that are particularly suitable for this purpose, however, are insufficiently cold-formable. These alloys have an exponential stress-strain behavior, as is illustrated by a schematic stress-strain diagram in FIG. 1. The diagram shows that the titanium-β alloys do not have the usual strain hardening, so that when the tensile test is carried out below room temperature above the yield point, in the plastic range, the necking and then breaking occurs without further increase in tensile stress. This has a major impact on the cold formability of these materials. Even with a very low percentage of cold forming, there is a risk that fatigue cracks will occur or the material will bulge out in an uncontrolled manner if the material is not subjected to heat treatment after every very small forming step. Cold forming, which is the easiest for these materials, is cold rolling, but can only be used to produce flat sheets.

In particular, shells with a larger diameter (over 600 mm), a small wall thickness (under 3 mm) and / or a high curvature (hemisphere) have so far only been produced by hot forming processes, after which the desired small wall thickness had to be turned off.

At higher temperatures, however, titanium and its alloys have a high affinity for air components, which on the one hand form a corrosion layer on the surface of the material and on the other hand make the material brittle due to hydrogen absorption. Both are highly undesirable and can only be prevented or eliminated if either the heating (for hot forming or heat treatment) takes place in a protective gas atmosphere or the corroded layer is removed mechanically or the embrittlement is reversed by heat treatment.

The invention has for its object to provide a simple and inexpensive method and a device for cold forming a material with exponential tensile stress-strain behavior to hollow shells of low wall thickness.

The object is achieved by the method specified in claim 1.

It was found that the above-mentioned defects, such as fatigue cracks or bulges, do not occur in the cold forming of these materials according to the invention, even at high degrees of deformation, if the material has no tensile forces in the plastic Area is exposed and the deformation is only caused by pressure forces exerted by the two opposing pressure rollers on the workpiece. With the method according to the invention, it is possible to produce hollow shells with a large diameter and relatively thin wall thickness to the final dimension by cold forming, without fatigue cracks or bulging being able to be determined and without the problems associated with heating of the material occurring. The high degree of cold forming that can be achieved with the method according to the invention brings about a grain refinement in the structure of the titanium-β alloy, which in turn results in higher strength and toughness, so that the load-bearing cross section and thus the weight can be further reduced. In addition, the high degree of cold deformation in the circumferential direction leads to a change in the texture of the original rolling direction of the cold-rolled sheet metal blank, so that the risk of inherent stress distortion associated with this texture is reduced. The pressure forces to be applied via the pressure rollers can be metered very precisely, so that not only shells with a constant wall thickness but also wall thickness changing over the circumference of the shell can be easily produced. In addition, by using two rollers, the springback that occurs when sheet metal is arched can be controlled so precisely that the shells can be manufactured with a very high degree of dimensional accuracy. Since neither a protective gas atmosphere nor repeated intermediate annealing are necessary, the process according to the invention can be carried out simply and quickly.

Advantageous further developments of the invention Process can be found in claims 2 to 4.

The object is further achieved by a device according to claim 5.

With the device according to the invention, the workpiece is rotated and the spinning rollers are driven in a path-controlled manner. This division of the relative movements helps to prevent tensile stresses in the plastic area during the deformation. From US-PS 3 815 395 it is already known to form tank bottoms with the aid of two pressure rollers acting on opposite sides of the workpiece, but in the device described there the workpiece is clamped in the center and freely rotatable, while the pressure rollers are both driven in rotation as well as over a predetermined radial path (which causes the workpiece to rotate). This superimposition of the motion control means that local tensile stresses cannot be avoided. The device is therefore only usable either for hot forming or for materials with normal work hardening.

Advantageous developments of the device according to the invention can be found in subclaims 6 to 11.

Fig. 1

ein wahres, schematisch dargestelltes Zugspannungs-Dehnungs-Diagramm einer Titan-β-Legierung, und

Fig. 2

eine schematische Darstellung einer erfindungsgemäßen Vorrichtung im Teilschnitt.

An embodiment of the invention is explained below with reference to the drawings. Show it:

Fig. 1

a true, schematically illustrated tensile-strain diagram of a titanium-β alloy, and

Fig. 2

a schematic representation of a device according to the invention in partial section.

2 shows a device 1 for cold forming sheet metal blanks 2 '(shown in dashed lines) into hollow shells 2 which, in addition to the hemispherical shape shown, can also be shaped as a spherical cap, conical, elliptical or with other cross-sectional shapes. The sheet metal blank 2 'is in the form of a sheet metal blank made of a material with the exponential tensile stress-strain characteristic shown in FIG. 1. These materials include the titanium-beta alloys Ti-15V-3Cr-3Al-3Sn (Ti15-3) and Ti-3Al-15Mo-2.7Nb-0.2Si (Beta-21S). The sheet thickness of the blank 2 'is normally above the desired sheet thickness of the finished shell, but can already have final dimensions in certain areas (near the opening, pole). If the finished shell 2 is to have larger wall thickness differences, it may be advisable to contour the sheet metal blank beforehand with different raw wall thicknesses, e.g. by turning or grinding. The diameter of the sheet metal blank 2 'is selected in accordance with the desired opening width of the finished shell plus the clamping dimension. With the present method, shells with an opening width of more than 600 mm can be produced, which previously could not be produced by cold forming. Even opening widths of 1500 or 2500mm and above are possible. The method according to the invention is preferably used for thin-walled shells with wall thicknesses between 0.3 and 3 mm.

The sheet metal blank 2 'is held in the device 1 by a clamping device 3, which has a clamping ring 4 for uniformly clamping the circumference of the sheet metal blank 2 '. The clamping device 3 is optionally adjustable in order to allow the clamping of blanks 2 'with different diameters. The clamping ring 4 is rotatably mounted about a center line 6 in the direction of arrow 6a via a rotary bearing 5 designed as a roller bearing. The rotation is carried out by a drive 7, which has a motor 8 and a drive pinion 9, which meshes with a correspondingly formed toothing on the clamping ring 4.

There is a tool carrier 10 and 11 on each side of the clamping device 3. Each of the tool carriers 10, 11 is parallel to the center line 6 in a first direction in the direction of the double arrows 10a and 11a, and in a direction in the direction of the double arrows 10b and 11b in a second direction linearly displaceable perpendicular to the center line 6. The direction of movement 10a and 10b or 11a and 11b lie in one plane. At the end of each tool carrier 10, 11 facing the clamping device 6, an arm 12 or 13 can be rotated about an axis 12 'or 13' in the direction of the double arrows 12a or 13a. The axes 12 'and 13' are perpendicular to the plane of movement of the linear displacements 10a, 10b and 11a, 11b, so that the twisting movement 12a, 13a takes place in the plane of the linear movements 10a, 10b and 11a, 11b. For pivoting the arms 12 and 13 in the direction of the double arrows 12a, 13a, a suitable actuator 14 or 15 is provided, which at the same time exerts the deformation force. There is also a drive (not shown) for moving each tool carrier 10, 11 in the direction of the double arrows 10a, 10b or 11a, 11b.

At the free ends of each arm 12, 13 there is one Press roller 16, 17 freely rotatable about an axis 16 'or 17'. The axes 16 'and 17' extend perpendicular to the pivot axis 12 'and 13' of the respective arm 12, 13 and are arranged such that each of the pressure rollers 16, 17 projects with its circumference over the respective arm 12, 13 and with the above part of its circumference can be brought into contact with the workpiece 2 ', 2. The pressure rollers 16, 17 are further arranged in the direction of rotation of the workpiece 2 ', 2, so that they can be rotated about their axes 16', 17 'by the rotating workpiece.

The first, on the inside of the curvature to be produced pressure roller 16 is relatively narrow and provided with a rounded circumference, so that even with narrow curvatures, only the circumference of the first pressure roller 16 comes into contact with the workpiece 2 ', 2. The second pressure roller 17 arranged on the outside of the curvature to be produced is designed as a counter roller against which the first pressure roller 16 works.

The drive of the clamping device 3, the actuators 14 and 15 and the drives, not shown, for moving the tool carriers 10, 11 in the directions 10a, 10b or 11a, 11b are connected to a common control, also not shown. The controller can be a CNC controller, a template copy controller, or any other known controller. By this control, the pressure rollers 16 and 17 are guided synchronously during the forming process, so that both pressure rollers 16, 17 always work against each other at the location of the deformation. Both pressure rollers 16, 17 are thereby moved by a combined linear movement along the double arrows 10a, 10b or 11a, 11b and a pivoting movement along the double arrows 12a, 13a each controlled in the direction of their axes 16 ', 17' via a path along the double arrows 16a and 17a, which follows the contour of the curvature to be formed in this deformation step. The tracks 16a and 17a of the pressure rollers 16 and 17 extend radially to the sheet metal blank 2 'or over a meridian of the curvature, the common plane in which the tracks 16a and 17a lie intersecting the center line 6. The direction of the deformation takes place from the area near the clamping ring 4 to the point of intersection of the center line 6 through the workpiece 2 ', 2 at the pole and back, the rollers 16, 17 in the position shown in solid lines in FIG. 2 near a point of reversal Path control and in the position shown in dashed lines near the other reversal point of the path control. The path control is carried out in such a way that the two pressure rollers 16, 17 can pivot relative to one another only about the center of the curvature of their circumferential surfaces (radius R) in order not to generate any frictional forces.

The control further effects an infeed movement of the pressure roller 16 in the direction of the counter roller 17 and away from it in order to adjust the distance between the two pressure rollers 16 and 17 to the wall thicknesses of the workpiece 2 ', 2 which decrease in the course of the deformation process. This feed movement can take place during the shaping process and can be controlled, for example, by pressure sensors on the pressure rollers. In addition, a predetermined control of the roller spacing is also possible if regions of the workpiece 2 ', 2 are to be deformed to different degrees, for example to provide the shells 2 with different wall thicknesses.

The device 1 according to the invention operates as follows: after the sheet metal disc 2 'has been clamped in, the clamping ring 4 is rotated by the drive 7 about the center line 6 in the direction of the arrow 6a. Then the pressure rollers 16 and 17 are brought to the blank 2 'at a predetermined distance from one another from opposite sides and are guided radially to the blank 2' in a path 16a and 17a, respectively, which is predetermined for the first deformation step, so that in connection with the rotation of the blank 2 'results in a spiral line around the center line 6. The speed of the clamping ring 4, the distance between the pressure rollers 16, 17 and the shape and speed of the path control in the direction of arrows 16a and 17a are matched to one another and to the material used so that the pressure rollers 16 and 17 alone compressive forces to deform the material are exerted, while any tensile forces that remain may remain below the yield strength of the material and thus make no contribution to plastic deformation. The material is thus merely squeezed between the pressure rollers 16 and 17, the material being allowed to elongate essentially perpendicular to the direction of the pressure forces. The path control of the pressure rollers 16, 17 ensures that this material elongation does not lead to bulging, but rather forms the desired curvature without the material having to be stretched by tensile stresses, as in conventional pressure methods.

For example, a tank half-shell was formed from the titanium alloy Ti 15-3 using the method according to the invention. As blank, a sheet blank with a diameter of 510mm was cut from a cold-rolled sheet in solution-annealed and with a cutting roller machine quenched condition, sheet thickness 2.08mm, cut out. With the help of two pressure rollers, the sheet metal blank was only converted into a hemispherical shell with an opening diameter of 444.8mm, an unchanged wall thickness of 2.08mm directly at the pole, and a wall thickness of 2mm at an angular distance of approximately 5 ° using 28 forces in 28 forming steps without intermediate annealing to the pole, a wall thickness of 1.32 mm directly adjacent to the clamping point at the shell opening and a wall thickness curve that continuously drops down to about 0.76 mm and then rises again continuously between the shell opening and the pole. No fatigue cracks or shape discontinuities such as folds or bumps were found in the finished tank half-shell. The dimensional deviations from the specified shape and wall thickness (lowest achieved wall thickness 0.76; specification 0.8mm; opening diameter 444.8mm, specification 445mm) were within the tolerance range. These dimensional deviations were due to the fact that in the device used, the pressure rollers 16, 17 could not be pivoted, ie were not supported on the tool carrier 10, 11 with the axes 12a, 13a. In addition, the pressure roller 16 acting on the inside of the shell was path-controlled by an inductive copying device according to a copying template and the counter roller 17 by hand via hydraulic valves.

A tank half-shell with an opening diameter of 950mm was also manufactured. A pre-contoured sheet blank was used as the starting material, the sheet thickness of which was 3.2 mm near its center and 2.1 mm in the remaining outer edge area. The transition between the two wall thickness areas was rounded out. The contouring was carried out using grinding or turning processes specially developed for titanium alloys. This pre-contoured sheet metal blank was cold-formed into a tank half-shell with an opening diameter of 950 mm using the method according to the invention and without intermediate annealing. The material in the pole area of the shell was also deformed and thus stretched, so that the wall thickness in the pole was reduced to 3.0 mm. The wall thickness in the opening area of the bowl was 1.2 mm. Between the shell opening and the pole, the wall thickness initially narrowed to 0.8 mm and then rose again continuously. The change in thickness of the pre-contoured sheet blank was compensated somewhat, but was still visible. This tank half-shell also showed neither fatigue cracks nor shape discontinuities such as folds or bumps after the deformation.

By means of a suitable path control of the spinning rollers, in addition to the shells described with continuously changing wall thicknesses, shells with an almost constant wall thickness can also be produced.

In a modification of the exemplary embodiment described and drawn, as already mentioned, the pressure rollers can also be moved linearly only in two axes if larger manufacturing tolerances are permitted. The shape and size of the spinning rollers can be changed according to the work to be done. Both pressure rollers can have the same shape. Under certain circumstances, a preformed blank can also be used instead of the sheet metal blank.

Claims (11)

1. A method for forming a sheet-metal blank (2') made from a material with exponential tensile stress/extension behaviour, more particularly a titanium-β alloy, in order to produce a thin-walled hollow shell (2), more particularly with a wall thickness between 0.3 and 3 mm, the sheet-metal blank (2') being clamped at its periphery, rotated so as to turn about its centre line (6), and cold-formed purely by means of localised pressure forces between first and second pressure rollers (16, 17) controlled by a continuous path control system and engaging on opposed sides of the blank (2'), thus producing the shell (2), the relative speed between the workpiece (2', 2) and the pressure rollers (16, 17) and the force exerted by the pressure rollers (16, 17) on the workpiece (2', 2) being matched to one another in such a manner that the tensile forces introduced into the workpiece (2', 2) are below the yield point of the material.
2. A method according to claim 1, characterised in that the continuous path control of the pressure rollers (16, 17) takes place in at least two directions (10a, 10b, 11a, 11b).
3. A method according to claim 1 or 2, characterised in that during forming, the pressure rollers (16, 17) are displaced from the periphery of the workpiece (2', 2) in the direction (16a, 17a) of the centre line (6) and back again, in a plane extending substantially through the centre line (6).
4. A method according to one of claims 1 to 3, characterised in that the pressure rollers (16, 17) are freely rotatable about their axes (16', 17').
5. A device for carrying out the method according to one of claims 1 to 4, with:

   - an annularly arranged clamping device (3) for clamping a peripheral region of a sheet-metal blank (2');

   - a drive unit (7) for rotational driving of the clamping device (3) about a centre line (6) extending perpendicularly to the plane of the sheet-metal blank (2');

   - first and second pressure rollers (16, 17) controlled by a continuous path control system, one roller being arranged on one side of the clamping device (3) and the other being arranged on the other side of the clamping device (3); and

   - a control means for the continuous path control of the pressure rollers (16, 17).
6. A device according to claim 5, characterised in that the pressure rollers (16, 17) have at least two directions of movement (10a, 10b, 11a, 11b).
7. A device according to claim 5, characterised in that the pressure rollers (16, 17) have at least three directions of movement (10a, 10b, 12a, 11a, 11b, 13a).
8. A device according to claim 7, characterised in that each of the pressure rollers (16, 17) is arranged on a tool support (10, 11) via an arm (12, 13) which is movable about an axis (12', 13') of rotation, the said tool support being linearly displaceable both parallel and transversely to the centre line (6) in two mutually perpendicular directions (10a, 10b, 11a, 11b).
9. A device according to one of claims 5 to 8, characterised in that the pressure rollers (16, 17) are [each] arranged so as to be freely rotatable on the arm (12, 13), via a roller axis (16', 17') extending perpendicularly to the axis (12', 13') of rotation of the arm (12, 13).
10. A device according to one of claims 5 to 9, characterised in that the control means is a CNC control.
11. A device according to one of claims 5 to 9, characterised in that the control means comprises a tracer control using templates.

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