Abstract:
A forming tool for widening an opening includes an expanding device with several expansion elements. The expansion elements are each provided with a curved forming surface extending between two peripheral end edges. The expansion elements are moveable between a first position corresponding to a contracted state where the expansion elements in connection with one another by way of separating surfaces, and a second position corresponding to a widened state where the expansion elements are displaced in radial direction from a center of the expanding device such that the separating surfaces are spaced apart from one another. In the first position the peripheral end edges are spaced further apart from the center than a central region of the forming surfaces so that the fatigue strength of mounting bores can be increased, openings in thick-walled components can expanded more reliably, and/or the formation of cracks is avoided.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
       [0001]    Exemplary embodiments of the invention relate to a forming tool for widening an opening using an expanding device, an expansion element for use in a forming tool according to the invention, and a method for widening an opening. 
         [0002]    In aerospace engineering it is desirable to widen high-loaded mounting bores in metal components using a forming method, which typically involves cold forming. As a result of the plastic deformation of the material in the region of the bore, internal stresses are generated, which contribute to an improvement of the fatigue strength. The propagation of the strain field is thereby subject to the applied strain, and in the case of a widening of about 3% of the initial diameter, corresponds to about two times the initial diameter, for example. 
         [0003]    A plurality of forming tool methods, which are also referred to as mandrel methods, are known from prior art. The object of all these methods known from prior art is a change of the diameter. 
         [0004]    US patent document U.S. Pat. No. 2,672,175 discloses a forming tool used for widening thin-walled pipes, for example, automotive muffler pipes. The tool comprises a conical four-sided block, against which four individual elements slide on the principle of the inclined plane, thus leading to a diameter change. The disadvantage is that the tool is only suited for thin-walled components, and that the forming forces are not transmitted evenly. As a result thereof, there is the risk of component damages. Moreover, the degree of widening cannot be controlled accurately, and larger forces cannot be transmitted. 
         [0005]    US patent document U.S. Pat. No. 3,077,916 discloses a forming tool used for widening thin-walled pipes. The tool is provided with a sleeve having several sleeve elements, which can be displaced in a radial direction. A screw is guided through an inner whole of the sleeve, wherein a conical mandrel is arranged on each end side protruding into the inner hole. By turning the screw, one of the two mandrels is moved toward the other mandrel, wherein the sleeve elements move in a radial direction. Thus, a widening of the thin-walled pipes takes place. A similar tool is also disclosed in US patent document U.S. Pat. No. 3,986,383. However, both forming tools have the disadvantage that they are only suited for thin-walled components, and larger forces cannot be transmitted. Furthermore, the expansion forces are only transmitted non-uniformly so that component damages can occur time and again. In addition, it is not possible to accurately control the expansion degree so that it is often difficult to release the tool again if forces that are too large are applied. 
         [0006]    US patent document U.S. Pat. No. 5,138,863 discloses a forming tool for widening thin-walled pipes for producing pipe connections. The forming tool is provided with a mandrel that can be moved in an axial direction, which can be inserted in a sleeve provided with several sleeve elements, wherein the radial displacement of the sleeve elements is subject to the depth of insertion of the mandrel. The disadvantage hereby is that the forming tool is not suitable for flaring thick-walled structures. 
         [0007]    European patent document EP 1 611 976 A1 discloses a method and a device for reducing internal stresses is disclosed. 
         [0008]    German patent document DE 26 54 102 C2 discloses a forming tool used for widening pipes. The forming tool is provided with a mandrel and a sleeve formed of several sleeve elements that can be moved in a radial direction. The mandrel is displaceably mounted in an axial direction and can be inserted into the sleeve, wherein subject to the depth of insertion of the mandrel, a corresponding radial displacement of the sleeve elements takes place. However, the forming tool has the disadvantage that it cannot be used for widening bore holes of thick-walled structures, for example, thick plates. 
         [0009]    All of the tools for widening of pipes known from prior art are not suited to transmit high forming forces of, for example, 50 kN. This is particularly necessary for high-strength metal components having large wall thickness as used in aerospace applications, in order to introduce internal stresses by means of plastic deformation in the vicinity of the bore. This concerns wall, plate, or stack thickness t, for example, which amount to a multiple of the hole diameter d to be widened, that is, t=0.5 to 5*d. In comparison thereto, the wall thickness of the thin-walled pipes merely corresponds to a fraction of the diameter to be expanded, namely, for example, t=0.05 to 0.3*d. 
         [0010]    It is known to use conventional forming methods for thick-walled components, like the split-sleeve method as disclosed in US patent document U.S. Pat. No. 5,305,627, for example, or the split-mandrel method as disclosed in US patent document U.S. Pat. No. 4,423,619. The forming tool is provided with a sleeve and a mandrel, wherein locally, the mandrel has a diameter that is larger than the bore diameter. The mandrel is pulled through the bore hole, and the diameter of the bore is continuously enlarged over the thickness of the component. However, both expansion methods have the disadvantage that the generated internal stress field is inhomogeneous, and even insubstantial deviations from the production dimensions of the bores and/or the tool result in great fluctuations in the expansion degree. For example, a bore having a diameter of 6 mm, which nominally was widened by 4%, can ultimately have an actual expansion degree of between 2.5% and 4.9%. Furthermore, the methods also have the disadvantage of an inhomogeneous stress and strain distribution. Moreover, with these methods, high-performance materials having highly direction-dependent properties, for example, aluminum alloys as frequently used in aerospace technology, can only be widened to a very small degree, that is, significantly below 3%. Thus, an improvement of the fatigue strength is hardly achieved. In addition, the kind of material deformation and the stress condition in the material resulting therefrom, under which the desired internal stresses are to be generated, lead to early crack formation. Furthermore, the use of sleeves as is done in the split-sleeve method makes the method expensive and uneconomical. In addition, time and again problems occur with the split-mandrel method due to premature tool failure, in particular cold welds between the mandrel and the component. 
         [0011]    International patent publication WO 2007/121932 A1 disclose a forming tool with which a bore can be continuously widened across the entire wall thickness, that is, plate or stack thickness. Due to its introduction of force, the forming tool ensures a favorable multi-axis stress state as compared to the conventional expansion methods. In this way, a homogenous strain field and significantly improved fatigue strength are achieved. The method is particularly advantageous for pronouncedly orthotropic materials, aluminum alloys, for example, in which bores can be widened free of cracks. The bores can hereby be widened up to 5% of their original diameter. 
         [0012]    Exemplary embodiments of the invention are directed to a forming tool and a forming method, with which the fatigue strength of openings in components, for example, mounting bores, is increased, as well as openings in thicker-walled components can be more reliably widened, and/or the formation of cracks is avoided. 
         [0013]    Furthermore, a beneficial expansion element for the tool and/or the method is disclosed. 
         [0014]    The invention, according to an aspect thereof, provides a forming tool for widening an opening using an expanding device comprising several expansion elements, wherein the expansion elements are each provided with a curved forming surface extending between two peripheral end edges, wherein the expansion elements can be moved from the first position into a second position, wherein in the first position, which corresponds to a contracted state, the expansion elements are in connection with one another by way of separating surfaces, and wherein in a second position, which corresponds to an expanded state, the expansion elements are displaced in a radial direction from a center of the expanding device such that the separating surfaces are spaced apart from one another, wherein in the first position, the peripheral end edges are spaced further apart from the center than a central region of the forming surfaces. 
         [0015]    Advantageously, internal stresses in the region of openings can be generated in a targeted manner without damaging the component in the process, for example. 
         [0016]    With a forming tool according to an advantageous embodiment, in the area surrounding a mounting hole, internal compressive stresses, in particular in thick-walled metal components like high-strength aluminum alloys, for example, can be faultlessly introduced, that is, without cracks, so that the fatigue strength of the bore, and ultimately of the component is significantly improved. 
         [0017]    Compared to traditional forming tools, an improvement of 10% to 15% is achieved, for example. The expansion of the bore diameter is hereby 3% to 5% of the initial bore diameter, for example. 
         [0018]    Furthermore, an advantageous embodiment of the forming tool allows a uniform introduction of force into the component so that a markedly improved flow behavior of the material is realized. In particular, internal stresses are uniformly introduced across the component thickness so that the applied internal stresses do not vary across the component thickness but are homogeneous. 
         [0019]    Preferably, with a forming tool according to an embodiment of the invention, no friction occurs on the inner wall of the opening. 
         [0020]    Preferably, the radial displacement of the expansion elements can be controlled so that the proceeding deformation process can be monitored so that a homogeneous strain field is achieved, which is beneficial for an effective increase in fatigue strength. Thus, even high-performance materials can be widened in the area of an opening free of cracks. 
         [0021]    Preferably, the forming tool can be used for widening openings of different diameters. 
         [0022]    Preferably, a forming tool according to an embodiment of the invention facilitates an inspection of the widened opening, because cracks, provided they happen after all, extend from the center to both sides so that cracks can be detected from both sides of the opening. 
         [0023]    Preferably, the forming tool is suited to test new materials with regard to their behavior during a cold forming. In this way, the number of tests of this kind can be reduced, and thus, test expenses associated therewith can be reduced. 
         [0024]    Beneficially, a forming tool according to an embodiment of the invention compensates for dimensional changes as a result of its wear so that it can be used for a longer period of time. 
         [0025]    Preferably, with a beneficial embodiment of the forming tool, no normal force affecting the workpiece surface occurs during the widening of an opening so that no bending moment damaging the workpiece is induced. 
         [0026]    Advantageously, with a forming tool according to the present invention, no cold welding of the sleeve with the workpiece takes place during the widening. Thus, damage to the component by a cold welding is not possible. Moreover, the sleeve can be re-used. 
         [0027]    Preferably, with a forming tool according to an embodiment of the invention, only negligible beads occur in the edge region of an expanded opening. In this way, production costs are reduced because an additional cost-intensive step for removing the bead is eliminated, and the widening process can be integrated into the production process more advantageously. 
         [0028]    Preferably, with a beneficial embodiment of a forming tool, the sleeve can be re-used so that the production costs can be reduced. It is thus no longer necessary to use a new sleeve each time for widening an opening. 
         [0029]    Advantageously, an elaborate and cost-intensive removal of a sleeve is eliminated with a forming tool according to a beneficial embodiment. 
         [0030]    Advantageously, with a forming tool according to an embodiment of the invention, the need of an expensive clamping bushing for widening of an opening is dispensed with. 
         [0031]    Further preferably, a forming tool according to the present invention can be used for widening openings in repair parts, wherein depending on the opening diameter and the desired widening of the opening, the forming behavior of the repair part and the distribution of internal stress in the area of the opening, and the fatigue strength resulting therefrom can beneficially be determined in advance by means of an FEM simulation. 
         [0032]    In order to promote a uniform introduction of force into the component, and thus make a better flow process possible, it is further preferably provided that in the second position, the peripheral end edges and the forming surfaces are equidistantly spaced apart from the center. 
         [0033]    It is further preferred that seen in cross section in radial direction, the forming surface of the expansion elements has the shape of a circular arc. 
         [0034]    Preferably, in the first position, the end edges are located on a circumscribed circle, and a central region of the forming surfaces is at a distance from the circle, and in the second position, both the end edges and the forming surfaces are located on a circumscribed circle. 
         [0035]    The expansion elements can be identical, for example, wherein in the first position, which is corresponding to a contracted state, an outer contour of a cross section, as seen from the expanding device in radial direction, can correspond approximately to a four-leaf clover. 
         [0036]    For example, a diameter in the region of the end edges of the expansion elements can be similar to the initial diameter of the bore to be widened. 
         [0037]    In the second position, which is corresponding to an expanded state, a cylindrical outer radius of the expansion elements can be corresponding to the radius of the opening in an expanded state, that is, the end diameter of the expanded opening. 
         [0038]    In this way, an optimal internal stress distribution in the area of the mounting bore can be beneficially attained. 
         [0039]    In a further advantageous embodiment, a radius of the forming surfaces is greater than a distance of the end edges from the center of the expanding device in the first position, wherein the radius of the forming surfaces is preferably between 2.5% and 10% greater, and in particular, about 5% greater. 
         [0040]    As a result, a markedly homogenous internal stress zone develops during the expansion process, which in turn has a positive effect on the fatigue strength. 
         [0041]    Furthermore, a drive element is preferably provided, by means of which the expanding device can be moved from the first position to a second position. 
         [0042]    Preferably, the drive element is provided with a mandrel, which can be inserted into an inner hole of the expanding device. 
         [0043]    For example, the mandrel is configured as a conical four-sided mandrel and is made of a high-strength metal or ceramic material having a high pressure and flexural strength as well as hardness. 
         [0044]    For example, a hard metal with a strength of more than 4000 MPa and a hardness of more than 60 HRC can be used for the mandrel. 
         [0045]    Preferably, the inner hole of the sleeve is provided with a geometry that is analogous to the geometry of the mandrel. 
         [0046]    Moreover, the mandrel can be of conical shape, wherein the inner hole, that is, an inner side of the expansion elements can, analogous to the mandrel, also be of conical shape. 
         [0047]    The sleeve can be made of a high-strength metal material with high ductility, wherein the sleeve can be made of a nickel-cobalt-based alloy with a tensile strength of over 2000 MPa. 
         [0048]    The inner surfaces of the inner hole and/or the outer surfaces of the mandrel are preferably provided with a coating. The coating reduces the friction coefficient so that there is an improved sliding. Furthermore, the coating protects against abrasion or wear, and in damp surroundings, also against corrosion. Lubricating properties of the coating can be provided by way of intercalated graphite particles. The inner surface of the inner hole and the outer surface of the mandrel can be referred to as functional surfaces of the mandrel and of the expansion elements. The functional surfaces can hereby be coated with an extremely hard layer, for example, for improving the gliding properties, for protection against corrosion, and premature wear and tear. 
         [0049]    Preferably, the coating can be an electrochemically applied nickel layer and/or a diamond-like carbon layer, in particular at a thickness of few nanometers and up to several micrometers. 
         [0050]    The electrochemically applied nickel layer can be 10 μm, or the diamond-like carbon layer can have a hardness of up to 6000 kg/mm 2 . 
         [0051]    In a further advantageous embodiment, an angle between a central axis of the mandrel and an outer surface is between 0.3 degrees and 3 degrees. By way of the angle between the central axis and the outer surfaces, that is, the functional surfaces of the mandrel, the conicity thereof is formed. With the aid of the angle, the axial force is deflected to the radial force required for the expansion. The angle hereby significantly controls the ratio of axial to radial force, the friction losses, and the possible wear on the tool. 
         [0052]    For example, at an angle of 0.3 degrees, for a bore hole of a diameter of 6 mm in an alloy 2000 at a ratio of workpiece thickness t to the bore diameter d of t/d=5, the force can be 20 kN. In comparison thereto, the radial force is 38 kN at an angle of 3 degrees and otherwise same ratio of t/d=5. 
         [0053]    According to a further aspect, preferably an expansion element for use in such a forming tool is formed. 
         [0054]    Preferably, the expansion element is provided with an approximately trapezoidal cross-sectional shape, wherein at least one side extending between the two legs is curved. Further preferably, the curved side has a radius that is corresponding to that of the opening in an expanded state. 
         [0055]    According to a further aspect, the invention provides a method for widening an opening from a first diameter d 1  to a second diameter d 2 , the method comprising the following steps: 
         [0056]    a) Providing several expansion elements, each having a forming surface and two separating surfaces extending between two end edges, wherein in a first position that is corresponding to a diameter of d 1  in a non-expanded state, the expansion elements can be moved into a second position that is corresponding to a diameter d 2  in the desired expanded state, by displacing the expansion elements in a radial direction, wherein the separating surfaces and the forming surfaces are configured such that in the first position, a central region of the forming surfaces, when placed against the non-expanded opening, is further apart from the inner wall of the opening than the end edges, and that in the second position, the forming surfaces altogether correspond to the contour of the inner wall of the desired expanded opening; 
         [0057]    b) assembling the expansion elements to form an expanding device and moving the expansion elements to their first position; 
         [0058]    c) inserting the expanding device into the opening, and 
         [0059]    d) continuously widening the opening by moving the expansion elements into the second position. 
         [0060]    With a continuous widening of the opening, a uniform introduction of force into the component is made possible so that a markedly improved flow behavior of the material is achieved. Advantageously, a homogeneous strain field is attained so that thick-walled metal components, for example, made of high-strength aluminum alloys, can be widened without a formation of cracks. Thus, the fatigue strength of the opening is increased. 
         [0061]    Preferably, the method for widening the opening can be applied from one side so that with two adjacent workpieces, only one of the two openings is widened. Thus, the expanding device can also be used for widening an opening of a metal and non-metal combination, like a hybrid combination with fiber composite materials, for example, where only the metal component is to be widened. The metal portion can hereby be widened without the non-metal part being touched and/or damaged. In this way, a method step referred to as one-step assembly becomes possible because after the drilling for expanding of the metal component, the parts do not need to be separated again but can remain in their already fixed position instead. This makes a saving of time possible. 
         [0062]    Further preferably, the method is used for holding open an opening having a diameter d in workpieces having a plate or stack thickness t, wherein t&gt;d, in particular t&gt;2*d. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0063]    Exemplary embodiments of the invention are described in more detail below with reference to the attached drawings, wherein: 
           [0064]      FIG. 1  is a perspective view of an embodiment of a forming tool; 
           [0065]      FIG. 2  is a perspective view of a mandrel; 
           [0066]      FIG. 3  is a perspective view of an expansion element; 
           [0067]      FIG. 4  is a perspective view of the forming tool inserted into an opening of a workpiece to be widened; 
           [0068]      FIG. 5  is a longitudinal section of the forming tool and the workpiece during a deformation process; 
           [0069]      FIG. 6  is a section of a radial cross section of a forming tool inserted in an opening prior to the widening of the opening; 
           [0070]      FIG. 7  is a schematic illustration of the geometrical ratios of forming tool to opening prior to the widening of the opening; 
           [0071]      FIG. 8  is a schematic illustration of the internal stress distribution in the workpiece after the opening has been widened using a conventional forming tool; 
           [0072]      FIG. 9  is a schematic illustration of the internal stress distribution in the workpiece after the opening has been widened using a forming tool according to an embodiment of the invention; 
           [0073]      FIG. 10  is a longitudinal section of the forming tool according to the invention and a hybrid compound during a deforming process; and 
           [0074]      FIG. 11  is a diagram showing the life span of a workpiece at different load states, wherein openings were not, or were widened using different widening methods. 
       
    
    
     DETAILED DESCRIPTION 
       [0075]    In  FIG. 1 , an embodiment of a forming tool  10  is illustrated. The forming tool  10  is provided with an expanding device  12  and a drive element  14 , which can be inserted into an inner hole  16  of the expanding device  12 . 
         [0076]    As shown in  FIGS. 1 and 2 , the drive element  14  is configured as a mandrel  18 , comprising a first section  20  and a second section  22 . Viewed from a radial direction R, the first section  20  has a polygonal cross section. The first section  20  is of conical design in an axial direction A toward a first end  24  of the mandrel  18 . 
         [0077]    In the present embodiment, the mandrel  18  is configured as a four-sided mandrel, wherein other polygonal embodiments are also feasible. The outer surfaces of the mandrel  18  are configured as functional surfaces  26 , wherein an angle α between the central axis M and the functional surfaces  26  brings about the conicity of the mandrel  18 . 
         [0078]    Ideally, the angle α is between 0.3 degrees and 3 degrees. Via the angle α, the axial force F α  is deflected to the radial force Fr required for the reaming. Thus, the angle α decisively controls the ratio of axial to radial force. 
         [0079]    The mandrel  18  is made of a high-strength metal or ceramic material with high pressure and flexural strength as well as hardness, for example, a hard metal with a strength of more than 4000 MPa and a hardness of more than 60 HRC. 
         [0080]    The functional surfaces  26  of the mandrel  18  can be coated with an extremely hard layer to improve the sliding properties, to protect against corrosion and against premature wear and tear, for example, with an electrochemically applied nickel layer having a thickness of several micrometers, or a diamond-like carbon layer having an extreme hardness of up to 6000 kg/mm 2 , for example. 
         [0081]    The second section  22  is configured as a holding section, by means of which the mandrel  18  can be accommodated in a tool or a receiving device such that the mandrel  18  can be moved in an axial direction and can be inserted into the inner hole  16  of the expanding device  12 . 
         [0082]    According to  FIGS. 1 and 3 , the expanding device  12  comprises several, for example, four expansion elements  28 , which in an assembled state form a sleeve  30 . 
         [0083]    The expansion elements  28  are provided with a first section  32  and a second section  34 . The first section  32  has a larger diameter than the second section  32  and is configured as a flange  36 , which is provided with a bearing surface  38  for positioning on a workpiece  54 . Additionally, a receiving device  40  in the form of a recess is placed into the second section  34 , which in the peripheral direction extends about an outer surface  44  of the first section  32 . 
         [0084]    The second section  34  is provided with a curved forming surface  46 , which if seen in cross section in a radial direction, has the shape of a circular arc. The forming surface is defined between two peripheral end edges  48 . Adjoining each of the end edges  48  is a separating surface  50 , which serves as contact surface for a further expansion element  28 . 
         [0085]    On a side opposite of the forming surface  46 , the expansion element is further provided with a functional surface  52 , which analogous to the functional surface  26  of the mandrel  18  is also of conical design. 
         [0086]    As can be seen in  FIG. 1 , in an assembled state of the expanding device  12 , one expansion element each, by means of its separating surface  50 , abuts a further separating surface  50  of a further expansion element  28  so that the expanding device  12  approximately forms a sleeve  30 . 
         [0087]    In order to fix the expansion elements  28  in their assembled state, an elastic means (not illustrated), for example, in the form of a rubber ring, can be inserted into the recesses  42 . 
         [0088]    As can be particularly seen in  FIG. 6 , in a starting position, only the end edges  48  rest on an inner wall  58 , wherein a central region  60  of the forming surfaces  46  are spaced apart from the inner wall  58  of the opening  56 . In order to achieve this, the separating surfaces  50  in the embodiment are configured such that in the starting position of the expanding device  12 , only the end edges  48  abut the inner wall  58  of the opening  56 , and the central region  60  of the forming surfaces  46  is spaced apart from the inner wall  58 . Thus, in a starting position, the maximal diameter of the expanding device  12  corresponds to the diameter of the opening  56  in a non-expanded state. 
         [0089]    The previously described embodiments are explained in more detail with the aid of an example illustrated in  FIG. 7  for the configuration of the radius r F  of the forming surfaces  46 . For an opening  56 , which in a non-expanded state has a diameter d 1  of 6 mm, and the diameter d 2  thereof is to be widened to 6.3 mm, the radius r F  of the forming surface  46  has a length of 3.15 mm. The inner hole  16  of the expanding device  12  has a polygonal (square) cross section with an inner surface of 3.25 mm lateral length in a non-expanded state, and an inner surface of 4.12 mm lateral length in an expanded state. 
         [0090]    A possible method for widening an opening  56  in a workpiece  54  will now be described in more detail with reference to  FIGS. 4 and 5 . As previously described, the expansion elements  28  are assembled to form an expanding device  12 , and are fixed using an elastic means. Subsequently, the expanding device  12  is inserted into the opening  56 . 
         [0091]    As illustrated in  FIGS. 4 and 5 , for widening the opening  56 , the mandrel  18 , in particular the first section  20  of the mandrel  18 , is introduced into the inner hole  16  of the expanding device  12 . Because the respective functional surfaces  26 ,  52 , are designed to complement each other, a movement of the expansion elements  28  in the radial direction R occurs upon introduction of the mandrel  18  into the inner hole  16 , wherein a displacement of the expansion elements  28  in the radial direction R is subject to the insertion depth of the mandrel  18 . By way of the angle α between the functional surface  26  and the central axis M of the mandrel  18 , the axial force Fa is deflected to the radial force Fr required for the expansion. Thus, angle α decisively controls the ratio of axial to radial force, the friction losses, and the possible wear on the tool. 
         [0092]    As can be seen in  FIG. 5 , a radial displacement of the expansion elements  28  is generated by an axial displacement of the mandrel  18 , whereby a radial force Fr is applied, which widens the opening  56  of the workpiece  54 . In this way, the material in the immediate vicinity of the opening  56  is plastically deformed. This results in the formation of internal stresses in the workpiece, which can occur in the material up to two times the distance of the diameter of the opening. In this way, the fatigue strength of the opening  56  is improved. 
         [0093]    Due to its optimized geometry, the expanding device  12  according to the invention promotes a uniform force introduction into the component, thus making a markedly improved flow behavior of the material possible. As can be seen in  FIGS. 8 and 9 , this results in a significantly more homogenous internal stress distribution after the widening process, which in turn has a positive effect on the fatigue strength. 
         [0094]    In  FIG. 8 , the internal stress distribution after the widening process with an expanding device known from the prior art is illustrated. With this expanding device, the forming surfaces have a radius corresponding to that of the opening in a non-expanded state. As can be seen in  FIG. 8 , an inhomogeneous internal stress distribution is thus achieved, wherein the biggest stress occurs in the central region of the forming surfaces. In the region of the end edges, the internal stresses introduced into the workpiece have barely noticeably penetrated the workpiece. In comparison thereto, with the expanding device  12  according to the embodiment of the invention, a homogenous internal stress distribution is achieved. As shown in  FIG. 9 , an even distribution of the internal stress, that is, an equally long distance of the internal stresses introduced into the workpiece, into the workpiece  54  is attained. 
         [0095]    As can be seen in  FIG. 10 , the expanding device  12  has the advantage of one-sided access to the opening  56  so that the forming tool  10  can also be used for metal-non-metal combinations, like, for example, hybrid compounds with fiber composite materials, where only the metal component is to be flared. The metal workpiece alone can hereby be flared, without the non-metal part being touched or damaged. Thus, a method step referred to as one-step assembly becomes possible because after the drilling for the widening of the opening  56  of the metal component, the parts do not need to be separated again but can remain in their already fixed position. This results in savings in time and costs. 
         [0096]    In  FIG. 11 , the service life of a workpiece for differently introduced stresses is illustrated. The first curve K 1  and the second curve K 2  show the service life curve for a workpiece made of an aluminum alloy, wherein the openings have not been widened. 
         [0097]    The third curve K 3  shows the service life of the same material, wherein the openings were widened by means of a conventional widening method. In this instance, the openings were expanded by 3%. As can be seen, the fatigue strength, and therefore the service life, increased due to the introduced internal compressive stresses. For example, with a non-expanded workpiece, with stress introduced into the workpiece, the number of load cycles is 100000, and with a workpiece, the openings of which have been widened by means of a conventional method, it is 1000000. Accordingly, the material strength was markedly increased. 
         [0098]    By using the expanding device  12  according to the embodiment, the service life can be prolonged once more. The expanding device  12  according to the embodiment allows a greater widening of the opening so that higher internal compressive stresses can be introduced into the workpiece. In the present example, the opening was widened by 4%. As can be seen in the fourth curve K 4 , the service life of the workpiece further increases compared to the service life of a workpiece machined using a conventional widening method. Thus, the forming tool  10  of the present invention makes it possible to further increase the material strength with otherwise like material properties and dimensions. 
         [0099]    The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           10  forming tool 
           12  expanding device 
           14  drive element 
           16  inner hole 
           18  mandrel 
           20  first section 
           22  second section 
           24  first end (of mandrel) 
           26  functional surfaces 
           28  expansion elements 
           30  sleeve 
           32  first section 
           34  second section 
           36  flange 
           38  bearing surface 
           40  receiving device 
           42  recess 
           44  outer surface 
           46  forming surface 
           48  end edge 
           50  separating surfaces 
           52  functional surface 
           54  workpiece 
           56  opening 
           58  inner wall 
           60  central region 
         R radial direction 
         A axial direction 
         M central axis 
         α angle 
         Fα axial force 
         Fr radial force 
         rF radius of the functional surface 
         d 1  diameter of the opening in a non-expanded state 
         d 2  diameter of the opening in an expanded state 
         K 1  first curve 
         K 2  second curve 
         K 3  third curve 
         K 4  fourth curve