EP0633075B1 - Method and apparatus for forming hollow metallic profiles supported by internal pressure - Google Patents

Abstract

In the method described for forming hollow metallic profiles with the assistance of internal pressure to give workpieces (1) provided with branches or the like (12), a liquid pressure medium is fed to the interior of the blank, which is mounted in a die (2) and is brought into engagement at its two ends with movable press rams (3, 4), and the blank is pressed into at least one cavity (7) in the die (2) with the aid of the press rams, expanding in the process; during this procedure, the pressure (pi) of the pressure medium in the interior of the blank, which is closed off by the press rams (3, 4) and is heat-treated before the pressing operation, is controlled as a function of the progress made in pressing, the pressure (pi) being set in such a way in process phases in which there is little stretching of the material of the blank that essentially the yield strength of the material is just reached, whereas, in process phases in which the stretching is great, the pressure (pi) is increased in order to raise the breaking elongation of the material. <IMAGE>

Description

The invention relates to a method for the internal pressure-assisted forming of hollow metal profiles in tools for workpieces provided with bulges, neckings, branches or the like. The inside of a blank attached to the tool, the end faces of which have at least one movable pressing die and a stationary counter-holding tool part , preferably with two movable press rams, is brought into engagement, a liquid pressure medium is supplied, the blank is pressed with the help of the press ram (s) with bulging into at least one cavity in the tool and the pressure of the pressure medium inside the through the Press ram and stationary counter-tool part or the ram closed blank depending on the process progress during pressing, preferably depending on the ram path is controlled.

Furthermore, the invention relates to a device for performing this method, with a tool and a pressure medium supply device which is connected to a pressure medium source and which comprises a pressure medium supply line through the or one of the press rams or, if appropriate, through the stationary counter-holding tool part, and which has a pressure setting device is assigned, and to a workpiece obtained by the method according to the invention.

It is known to use internal pressure-based forming processes for tubular workpieces made of ductile steels with a circular cross section in order to produce T-pieces from these tubular blanks. In this type of forming technology, the workpiece is supported from the outside by a specified tool (also called a die or external shape) and from the inside by high pressure medium pressures. The deformation is carried out with the aid of press rams, and in the course of this deformation or pressing, parts of the tubular blank are bulged into a cavity of the tool and molded into it. For example, FR-A-1 048 482 and US-A-3 350 905 show such forming techniques, wherein according to FR-A-1 048 482 the pressure of the pressure medium inside the workpiece is equal to the pressure with which the press rams are applied , held or limited to a smaller value with the help of a reducing valve; according to US-A-3 350 905, on the other hand, this internal pressure can be changed with the aid of fixed control cams, which are scanned by a roller for the purpose of actuating a pressure medium pump, depending on the position of the press rams in order to avoid excessive internal pressure and thus destruction of the workpiece as the forming process progresses. Finally, EP-A-36 365 discloses a method and a device of the type mentioned at the outset, with regard to the structural irregularities and wall thickness differences in the outlet pipes which are always present, the least possible rejection of the pressure deformation is to be achieved in that at least once in one Predetermined value of the longitudinal compression, i.e. the press ram path, the pressure of the pressure medium in the interior of the workpiece and / or the transverse deformation, i.e. the path of a counter-ram in the tool cavity into which the workpiece is bulged, is measured and regardless of the measurement result, the ratio of internal pressure to longitudinal compression is redefined, based on experiments previously carried out on different blank types.

However, the known techniques have hitherto only been able to be used for very specific workpieces or blanks, in particular with a circular cross section, and moreover only if they consist of very specific materials, namely ductile steels with a brittle fracture ^{toughness of} 100 ^{MN.m -3/2} ; In the case of other blanks with non-round cross-sections, for example square, rectangular cross-sections, and / or in the case of other materials, such as high-strength aluminum alloys (which have a brittle fracture ^toughness of less than 30 ^{MN.m -3/2} ), such internal pressure-supported forming has hitherto been the case because of the low ductility or because of the difficulties that occur when the material material flows into molded parts with a large curvature (ie with a small radius of curvature), this was practically impossible since the material breaks prematurely.

ist.It is therefore an object of the invention to remedy this and to provide a technology with which blanks with a non-circular cross-section or made of such brittle materials, such as high-strength aluminum alloys, can also be subjected to an internal pressure-assisted forming process, in order in this way to produce T-pieces, connecting pieces, Make branching pieces, knot pieces and the like to be able to. In particular, the aim is to produce workpieces that have a surface radius of curvature in the corresponding areas that is less than 10% of the replacement diameter D _{ers of} the outer cross section Q of this blank, this replacement diameter ${\text{D}}_{\text{first}}\text{= 2}\sqrt{\text{Q / π}}$

is.

The inventive method of the type mentioned is accordingly characterized in that in process phases with small expansions of the material, the pressure of the pressure medium is set so that the comparative stress formed in the workpiece from the spatial stress state essentially reaches the deformation resistance of the material, whereas in process phases with large expansions, the pressure of the pressure medium is increased in order to increase the elongation at break of the material sufficiently by lowering the mean stress. With such a pressure control of the pressure of the - liquid - pressure medium inside the blank during the pressing process, depending on the type and dimension of the workpiece to be produced, in particular depending on the wall thickness of the blank, on the cross section of the blank, etc., the flow of the material can be ensured without tearing , whereby the internal pressure guide is of crucial importance when the material flows into molded parts with a large curvature. It generally applies that the internal pressure is to be set higher, the greater the wall thickness of the blank, and / or the smaller the radii of curvature which must be followed when the blank is deformed. Even with small workpiece or blank diameters, the internal pressure must be dimensioned comparatively high. The reason for this is the large comparative strains that occur in such cases (according to Tresca or Mises) at individual points on the workpiece, so that by lowering the hydrostatic stress component (increase in hydrostatic pressure) the possible comparative fracture elongation must be increased (compressive stresses are calculated negatively).

When using the technique according to the invention, it is possible for the first time to produce, in particular, thin-walled, high-strength aluminum parts with a non-round cross-section with bulges of virtually any shape. Such workpieces can be used, for example, as nodes in so-called "space frames" (frame structures made of aluminum or aluminum alloys which are covered with aluminum plates) in vehicle body technology are used, in which case straight or curved rods or profiles are inserted into knot pieces and welded or glued therein.

The pressure of the pressure medium during the forming process will be controlled in more detail in such a way that in process phases in which an initial bulging, possibly a shaping of the workpiece onto variable (movable) tool parts, and the occurring comparative strains are small, the pressure together with The stresses in the workpiece caused by the press ram to the uniaxial stress state by forming the so-called comparative stress (according to Tresca or Mises) reaches the deformation resistance of the material (given the stress conditions, the next point on the so-called hyper flow surface in the stress space, which in plastic Deformation according to the flow condition of Tresca (oblique hexagonal prism) or Mises (oblique elliptical cylinder) includes all conceivable stress states during flow); in contrast, in process phases in which large strains occur, for example to replenish material in molded workpiece parts, the pressure is kept at least at such a time at all times that the resulting hydrostatic stress component (negative), which is formed in the workpiece from the spatial state of tension (negative), related to the Strain resistance according to the corresponding material characteristic, has an elongation at break that is greater than the maximum comparative elongation occurring on the workpiece at this point in time.

In the context of the pressure control according to the invention during the pressing of the blanks, it has proven to be particularly advantageous for an effective pressing process if the pressure medium at the beginning of the pressing process is at a low excess pressure, for example in the order of 300 to 600 bar, and towards the end of the pressing process the maximum value, for example in the order of a few kbar, is brought. The internal pressure of the blank, ie the pressure of the pressure medium, for example during approximately the first half of the pressing process (ie half of the path of the ram or the ram) can be more or less constant at about one third to one fifth of that provided towards the end of the pressing process Maximum values can be kept, and then from about half the pressing process the internal pressure can be increased gradually, for example linearly, up to the maximum pressure at the end of the pressing process. In any case, during the initial bulging due to the set internal pressure, the reference stress resulting from the internal pressure and the stresses from the pressing and frictional forces must reach the material's resistance to deformation.

Furthermore, it has proven to be advantageous if the pressure medium is brought to a maximum pressure of at least 80% of the tensile strength of the material of the blank during the pressing process. As such, when determining the value for the maximum pressure, not only the material itself, of course, but also the material thickness and also the cross section of the workpiece must be taken into account, as has already been mentioned above. The pressure control depends in detail on the type and shape of the workpiece to be produced, and it can advantageously be provided here that the pressure of the pressure medium in pressing phases in which the blank material flows around small radii of curvature of the tool, depending on the wall thickness, to 5%. -40% of the maximum pressure during pressing is set (the reference stress reaches the deformation resistance). The maximum internal pressure is reached outside of these phases.

For the workpieces or materials to be processed according to the invention (which have a lower ductility than ductile steels), it is then of particular advantage if the pressure medium is already at an increased preload pressure, preferably 5% -20%, in particular before the blank is pressed about 10% of the maximum pressure reached during pressing, e.g. is brought to a value in the order of 100 to 1000 bar. Such prestressing of the pressure medium is particularly important for the initial problem-free bulging of the blank, the value of the prestressing pressure being selected as a function of the buckling stress of the material to be deformed; in particular, the internal pressure is to be determined in such a way that there is no buckling at the point of the bulge to be produced, which could happen if the pressure medium is not preloaded, in particular when processing wall thicknesses which are small in relation to the other dimensions of the hollow profile.

To ensure exact pressure control during the pressing process enable, it is also advantageous if the blank is filled with the pressure medium free of gas inclusions before the pressing process, and if the pressure medium is brought to an increased pressure by a pump. If there were gas inclusions in the - liquid - pressure medium, the compression and expansion of the gas inclusions would not make it possible to achieve the desired pressure increase or reduction and thus impair the exact pressure control sought.

In order to ensure the desired exact pressure control from another point of view, namely the point of view of closing the blank during the pressing process, in order to ensure the increased internal pressure, the blank must be sealed as well as possible on its end faces. For example, press seals with step-shaped shoulders on the front edges can be used to seal the blank tightly, which have a height of 5% -100% of the mean wall thickness of the blank, and with which the material of the blank is plasticized to produce a metallic seal.

The blanks are preferably subjected to a heat treatment before the pressing process, in order to thereby make the material more ductile. Because of its microstructure, it then has a significantly higher elongation at break. In this context, an advantageous embodiment of the method according to the invention is characterized in that, in the case of blank materials which have alloy components at the grain boundaries, in particular high-strength aluminum alloys, a heat treatment is carried out before the pressing process in such a way that by storing these alloy components in the grain lattice increases the ductility of the material, and that the pressing process is carried out within a predetermined time window, for example 1 hour to 4 hours after the heat treatment, but before the alloy components are again outsourced to the grain boundaries. For example, it has been shown that it is advantageous if blanks made of AlMgSi alloys are heated at about 400 ° C. for about 100 minutes in an annealing furnace, then cooled in air and pressed in about 3 hours after the end of the annealing furnace heat treatment. The duration of the actual pressing process is in the range of one or a few minutes, often less one minute, including the previous filling with the pressure medium. The pure pressing time can therefore sometimes be as little as 20 s to 30 s.

With some workpieces to be produced, it is conceivable to simply deform the bulge to be produced into a corresponding cavity in the die or in the tool, ie to bulge it; for an exact molding process, especially in the case of longer bulges or branches, however, when the blank is pressed into the (respective) tool cavity into which the material of the blank is deformed, a variable counter tool part is used, which has a cross-sectional shape corresponding to the (respective ) has to be bulged, held against it, and it is particularly advantageous here according to the invention if the (respective) variable tool part is held in a pressure-controlled and / or displacement-controlled manner and the force introduced into the workpiece is dimensioned such that the total occurring in the workpiece mean stress (the hydrostatic stress component) is reduced to such an extent that the elongation at break thereby determined becomes greater than the greatest strain occurring in the workpiece part in question supported by the variable tool part. The exact pressure or displacement of the variable counter-holding tool part (s) for supporting bulged workpiece parts is all the more important, the more brittle the material and the smaller the wall thickness. The counter support creates a hydrostatic stress component in the adjacent outer fiber of the workpiece, which increases the possible, exploitable elongation at break (especially in the case of brittle materials), so that the greatest comparative strains occurring in these areas become smaller than the elongation at break. This prevents the workpiece from bursting. Such holding back is particularly advantageous if the blank has a wall thickness less than 10% of the equivalent diameter D _ers (as defined above) or if materials with a brittle fracture ^toughness less than 50 MN · m ^-3/2 or materials with a Tensile strength Rm less than 350 N / mm ² can be used. Such countermeasures are also favorable in the case of bulges which are to be produced unevenly with respect to the cross section and which have small radii of curvature on parts of the surface, in particular less than 10% of the named replacement diameter D _ers . In particular, according to the invention, as defined above, control of the force directed against the workpiece and transmitted by the variable counter-holding tool parts is to be aimed for, in the case of the fiber (the outer fiber) of the workpiece which is in contact with the respective variable counter-holding tool, a hydrostatic force related to the deformation resistance Stress component is caused, the elongation at break according to the corresponding material characteristic is greater than the greatest comparative elongation occurring there at any time.

It is also advantageous if the force to be exerted by the variable counter-holding tool part is at least 5% of the force exerted by the internal pressure on the part to be bulged out at any point in the process, in particular even at the start of the process.

Furthermore, it has proven to be advantageous if, in the case of the simultaneous production of several bulges, branches or the like, and accordingly the use of several variable pressure-controlled counter-holding tool parts, a path monitoring or measurement is carried out in addition to their pressure control, in order to ensure that the speed is not the same Movements of the variable counter tool parts to adjust their pressure control.

In molded parts with a large curvature, e.g. Rounded corners of a rectangular or acute-angled bulge part, the process lubrication is also important, since there is a large friction surface in relation to the volume of material formed. A corresponding application of lubricant is therefore particularly important at the edges of the semi-finished products. It has proven to be advantageous here if in zones with a small radius of curvature of the blank or tool, in particular on rounded edges, at least 50% more lubricant is applied than on the other parts of the blank or tool.

To carry out the method according to the invention with the specified internal pressure control, a pressure medium source per se could be used, from which the pressure medium is delivered at a controllable pressure, this pressure medium of variable pressure being supplied to the interior of the blank. However, complications could arise in the event of a decrease in the internal pressure during the course of the process, and it would be in the A corresponding changeover valve device with a return to a sump or the like is generally required.

A particularly simple and effective device of the type mentioned at the outset, however, is characterized according to the invention in that a pressure medium discharge line leads through the (other) press ram or, if appropriate, the stationary counter-holding tool part, and in that the pressure setting device is accommodated in this pressure medium discharge line. The pressure medium is therefore supplied from a pressure medium source with a constant, high pressure per se, and the pressure setting is carried out in the discharge line, a controlled pressure valve preferably being used for this purpose. For the pressure medium supply, a pump supplying the pressure medium with a predetermined high pressure can simply be provided as the pressure medium source, and then a check valve is advantageously accommodated in the pressure medium supply line, so that the pressure medium cannot be pushed back from the interior of the blank to the pump.

As mentioned, a workpiece made of brittle material, for example with a brittle fracture toughness of less than 30 MN.m ^-3/2 , in particular made of an aluminum alloy, can be obtained for the first time with the method according to the invention, and such a workpiece is thus also the subject of the invention.

• Fig.1 schematisch das innendruckgestützte Umformen von kreisrunden rohrförmigen Werkstücken aus duktilen Stählen in einem Werkzeug, wie dies an sich bekannt ist;
• Fig.2 in einem schematischen Detailschnitt die Dehnung eines Werkstückes in einer Zone großer Oberflächenkrümmung;
• Fig.3 schematisch eine Vorrichtung zum innendruckgestützten Umformen mit Innendrucksteuerung gemäß der Erfindung;
• Fig.4 eine schematische Seitenansicht des dabei verwendeten Werkzeuges, in Richtung des Pfeils IV in Fig.3;
• Fig.5 einen schematischen Querschnitt durch dieses Werkzeug, gemäß der Linie V-V in Fig.4;
• Fig.6 ein Diagramm zur Veranschaulichung einer möglichen Innendruck-Steuerung während des Prozeßverlaufes, abhängig vom Weg der Preßstempel;
• Fig.7 ein Schema zur detaillierteren Veranschaulichung einer Vorrichtung ähnlicher jener von Fig.3, wobei beispielsweise ein Werkstück mit zwei Abzweigungen hergestellt wird;
• die Fig.8 bis 10 je eine Flußdiagramm zur Veranschaulichung von verschiedenen Steuer- und Regelungsmöglichkeiten während des innendruckgestützten Umformens von Werkstücken mit Innendrucksteuerung; und
• Fig.11 schematisch eine modifizierte Umform-Vorrichtung.

The invention is further explained below using exemplary embodiments with reference to the drawings. The individual shows:

• 1 shows schematically the internal pressure-based forming of circular tubular workpieces from ductile steels in a tool, as is known per se;
• 2 shows a schematic detail section of the elongation of a workpiece in a zone of large surface curvature;
• 3 schematically shows a device for internal pressure-based forming with internal pressure control according to the invention;
• 4 shows a schematic side view of the tool used, in the direction of arrow IV in FIG. 3;
• 5 shows a schematic cross section through this tool, along the line VV in Figure 4;
• 6 shows a diagram to illustrate a possible internal pressure control during the course of the process, depending on Way of the ram;
• FIG. 7 is a diagram for a more detailed illustration of a device similar to that of FIG. 3, for example a workpiece having two branches being produced;
• FIGS. 8 to 10 each show a flow chart to illustrate various control and regulation options during the internal pressure-based forming of workpieces with internal pressure control; and
• 11 schematically shows a modified forming device.

According to FIG. 1, a blank is shaped in a conventional manner from a tube with a circular cross-section to a workpiece 1 with a bulge in the form of a T-piece, the workpiece 1 in a stationary tool 2 with the aid of press dies 3 placed on the end face, 4 is pressed. The pressing force that is exerted on the workpiece 1 with the pressing dies 3, 4 is illustrated schematically with arrows 5 and 6, respectively. The workpiece 1 is filled with a liquid pressure medium before the forming or pressing, after which the two press rams 3, 4 are brought into engagement with the end faces of the workpiece 1 and the pressing process begins. During pressing, a corresponding internal pressure builds up in the workpiece 1, which supports the material of the workpiece 1 from the inside, whereas the tool 2 provides external support. In this way, the material of the workpiece 1 is caused to flow into the cavity 7 in the workpiece 2.

Dabei bezeichnet w wie erwähnt die Wandstärke des Werkstückes 1, und mit r ist der Krümmungsradius bezeichnet, um den der Werkstoff herum am Werkzeug 2 entlang fließt. Die neutrale Faser ist im übrigen in Fig.2 bei 11 angegeben.In the past, such internal pressure-assisted forming could only be carried out for ductile steels as a material and also for blanks with a circular pipe cross-section, whereas brittle materials, such as aluminum materials in particular, and workpieces with other cross-sectional shapes, such as square or rectangular cross-sectional shapes, did not undergo such a deformation because the material has previously buckled, torn or burst. Above all, it was not possible to make the material of the workpiece flow around comparatively sharp edges with small radii of curvature. In this context, reference is made to the schematic illustration in FIG. 2, in which a section of the workpiece 1 is illustrated in an edge region of the tool 2, where a relatively small radius of curvature given is. The workpiece 1 is supported from the inside with the aid of the pressure medium that has been filled into the workpiece 1, as is schematically illustrated by arrows 8 in FIG. The following relationship applies to the strain ϕ in the material of the workpiece 1 in the area of the outer fiber 9 or the inner fiber 10: $$\begin{array}{c}\begin{array}{c}\text{Inner fiber: ϕi = ln (1 + w / (2r + w))}\\ \text{Outer fiber: ϕa = ln (1 - w / (2r + w))}\end{array}\end{array}$$

As mentioned, w denotes the wall thickness of the workpiece 1, and r denotes the radius of curvature around which the material flows along the tool 2. The neutral fiber is otherwise indicated at 11 in FIG.

From these relationships, as also from FIG. 2, it can be seen that the smaller the radius of curvature r, the greater the stretch on the inner fiber 10. Accordingly, there is a high risk of tearing or the like at this point, but this risk can be counteracted by appropriately dimensioned internal pressure support.

In the case of internal pressure-supported forming, the knowledge is exploited that the deformability of a material depends not only on the material itself, but also to a large extent on the stress conditions. This results, for example, from the relationship for the flow condition k _f (according to Mises) known in the literature in connection with the theory of plastic deformation $${\text{k}}_{\text{f}}\text{=}\sqrt{\text{3/2}}\text{·}\sqrt{{\text{(σ}}_{\text{x}}{\text{- σ}}_{\text{m}}{\text{)}}^{\text{2nd}}{\text{+ (σ}}_{\text{y}}{\text{- σ}}_{\text{m}}{\text{)}}^{\text{2nd}}{\text{+ (σ}}_{\text{e.g.}}{\text{- σ}}_{\text{m}}{\text{)}}^{\text{2nd}}}$$

In this, σ _x , σ _y and σ _{z are} the stresses in the main stress directions (principal stress axes) x, y and z, and σ _m is the mean stress as follows: $${\text{σ}}_{\text{m}}{\text{= (σ}}_{\text{x}}{\text{+ σ}}_{\text{y}}{\text{+ σ}}_{\text{e.g.}}\text{) / 3}$$

The mean stress σ _m is also referred to in the literature as the hydrostatic stress component.

The deformation resistance k _f is usually only from the change in shape as well as the temperature during forming, but in special cases the rate of deformation can also affect the resistance to deformation k _f .

Essential to the invention is the physical effect that the pressure expansion (that is the greatest possible positive or negative expansion, which allows material, temperature and stress state) of a plasticizable material by lowering the hydrostatic stress component (possibly related to the strain resistance, in order to increase the deformation history) capture) can be increased significantly. The more negative the hydrostatic stress component (all-sided pressure), the better it supports the plastic forming effects (grain boundary sliding, dislocations, etc.) through structural order. Purely theoretically, it would be possible to achieve any desired expansion, whereby the state of stress at every point on the workpiece must always lie on the so-called hyper flow surface, which is defined by the above equation for k _f according to Mises. So the greater the strains that occur, especially when material has to be pushed into bulged parts, the greater the internal pressure of the liquid support medium (it is included as a negative tension in the mean tension in the workpiece).

As already mentioned, there are great differences in the forming behavior between steel materials and high-strength aluminum alloys, which in particular have a brittle fracture ^toughness below 30 MN · m ^-3/2 . Nonetheless, such brittle materials and also workpieces with non-circular cross-sections can be subjected to internal pressure-assisted forming if, based on the findings indicated above, an appropriate internal pressure control is carried out - in particular in connection with a previous heat treatment suitable for the material (soft annealing).

An arrangement for internal pressure-assisted forming with such a pressure control is now illustrated very schematically in FIG. In FIG. 3 - just like in FIG. 7 to be explained - components corresponding to the respective components of FIG. 1 are designated with the same reference numerals. In detail, there is again a workpiece 1 inside a tool 2, and press rams 3 and 4 are brought in from above and below in order to press the workpiece 1 and thereby in bulge a tool cavity 7, as illustrated in Figure 3. A movable tool part 13 for counter-holding is arranged in the tool cavity 7 to support the bulged workpiece part 12, this movable or variable counter-holding tool part 13 being referred to below as a counter-holding stamp for the sake of simplicity. The counter-holding force of this counter-holding die 13 is indicated by Fg, whereas the pressing force of the pressing dies 3, 4 is indicated by Fp1 and Fp2. The internal pressure in the interior of the workpiece 1 is indicated by pi, this internal pressure pi being controlled as a function of the path s of the press rams 3, 4 during the course of the process. Accordingly, the press rams 3, 4 are assigned in the drawing, not illustrated in more detail, of conventional design per se, so as to detect the path s of the press rams 3, 4 at any time. Depending on this path s, the internal pressure pi (s) is set with the aid of a pressure valve 14 which is arranged in a pressure medium discharge line 15 leading through the upper press ram 3, a control valve 16 being connected upstream of it and having two positions - filling or Presses - owns.

A pressure medium supply line 17 runs through the other - lower - press ram 4, through which a controllable pump 18, which is driven by a motor 19, supplies the liquid pressure medium - preferably simply water - to the interior of the workpiece 1. In order to prevent the pressure medium from being driven back to the pump 18, a check valve 20 is furthermore arranged in the pressure medium supply line 17.

4 and 5 in connection with FIG. 3 the shape of the die or tool 2, also called shape, can be seen in more detail, whereby it can be seen that the externally circular tool 2 has an elongated interior or molding space 21 of a square shape Cross-section, for the workpiece 1 having a square cross-section, and furthermore the cavity 7 of a rectangular cross-section branching therefrom for the bulge or branch 12 of the workpiece 1 to be produced.

In operation, a blank, ie a hollow profile with a square cross-section, from which the workpiece 1 with the branch 12 is to be formed, is inserted into the tool 2, after which the two press rams 3, 4 are brought into engagement with the end faces of the blank, a tight seal on these end faces being brought about with the aid of the press rams 3, 4. For this purpose, the press rams 3, 4 can be stepped on their end faces on the outer circumference, ie have step-shaped shoulders, which in particular have a height in the range from 5% to 100% of the mean wall thickness of the blank, and with which the material of the blank during pressing is plasticized, whereby a metallic seal is brought about.

After this pressing of the press rams 3, 4 onto the blank, the pump 18 is switched on in order to supply pressure medium, in particular water, to the interior of the blank via the pressure medium supply line 17. The valve 16 on the outlet side is in the "fill" position as shown, it being easy to determine in this position when the pressure medium exits on the outlet side (as is schematically illustrated in FIG. 3 with the outlet line 22).

The control valve 16 is then adjusted upward into its “pressing” position, as shown in FIG. 3, in which a connection to the pressure valve 14 instead of to the outlet line 22 is established on the outlet side. After the hollow profile has been sealed by the press rams, the pressure of the pressure medium is now increased with the aid of the pump 18, and the pressing process can begin by driving the press rams 3, 4 with the pressing force Fp1 or Fp2. This results in the aforementioned tight sealing of the end faces of the blank by the press rams 3, 4, by plasticizing the blank material, and during the pressing, the internal pressure pi of the pressure medium is increased by the upsetting process. The internal pressure pi (s) is continuously adjusted with the help of the pressure valve 14, starting with a preload pressure at the beginning of the pressing, a pressure increase to approximately twice the preload pressure can be provided, after which, for example, a linear pressure increase in a next section of the process about ten times the pressure value - up to about the middle of the entire pressing process, ie half way the press ram 3, 4 - is provided; the internal pressure pi is then kept constant, for example, at this maximum value that has now been reached.

During the pressing process, which is controlled by internal pressure of the workpiece 1, the bulge or branch 12 is produced, with the counter-punch 13 being held here with a counter-holding force Fg, which is preferably also controlled as a function of the path, in order to prevent the bulge 12 from bursting or bursting.

6 shows a diagram of the internal pressure pi (in bar) over the path s (in mm) of the press rams 3, 4, the internal pressure pi generally being controlled as a function of the path as set out above. Specifically, a preload pressure pv of, for example, 130 bar is set at the beginning of the pressing process, and when the pressing begins, the internal pressure pi increases to a value pa of e.g. 250 bar, which is kept constant up to a press ram path s of 7.5 mm (phase of the molding on the punch 13 and beginning bulge with small strains). Then the internal pressure pi is linearly increased to the final value or maximum value pe of, for example, 2000 bar, this section of the pressure increase ending at approximately half of the pressing process (press ram travel s = 15 mm), after which the final value pe from 2000 bar to is held at the end of the pressing process (s = 30 mm). (Phase of the large strains during the replenishment of material in the formed bulge).

A workpiece 1 'with two branches 12', 12 '' can also be produced with a fundamentally similar internal pressure curve as shown in FIG. 6, as is illustrated in the diagram of FIG. In this case, the tool 2 'has two cavities 7', 7 '' for the bulges or branches 12 ', 12' ', with corresponding counter-holding punches 13', 13 '' as variable tool parts in these cavities 7 ', 7' ' a contact pressure Fg1 (s) or Fg2 (s) are kept movable. The press rams 3, 4 are applied to the workpiece 1 'with a pressing force Fp1 or Fp2. To fill the workpiece 1 'with water, a supply line 23 is provided, to which the pressure medium supply line 17 is connected via a directional control valve 24 and on the one hand via the pump 18 or on the other hand directly via a check valve 25.

On the outlet side, a pressure valve 14 'is in turn connected to the pressure medium discharge line 15, which in the present case is controlled via a hydraulic control circuit Pump 26 and a control line 27, in which a check valve 28 is received, can be adjusted in pressure.

Furthermore, there is a control and regulating unit 30 which controls the process water preload pump 18 and the pump 26 which can be adjusted with regard to the outlet pressure in the control circuit for the pressure valve 14 'for the purpose of setting the respective internal pressure pi. Input variables for the control and regulating unit 30 are the path s of the press rams 3, 4 (it is usually sufficient to take the path of a press ram, for example 3, and feed them to the unit 30) and the internal pressure pi (see also for safety reasons) ). For the press punches 3, 4 and the counter-punches 13 ', 13' ', a control or regulation can be provided as desired, and accordingly the sizes Fp1, Fp2, Fg1 and Fg2 according to Fig. 7 are both recorded and set, e.g. simply path-dependent or with an additional readjustment (actual value-target value comparison), as will be explained in more detail below with reference to the flow diagrams from FIGS. 8 to 10.

In the simplest case, the unit 30 could be a PLC (programmable logic controller) unit, with which the pressing pressures (pressing forces Fp1, Fp2, Fg1, Fg2) and in particular the internal pressure pi can be set simply as a function of the path. A corresponding flow chart is shown in FIG. 8, which includes such a simple path-dependent pressure control and counterforce control.

In detail, according to FIG. 8, after starting the process at 31 in block 32, the inputs for v1 (valve for the punch 3, for the pressing force Fp1), for v2 (valve for the pressing punch, 4, for the pressing force Fp2) are made, for the internal pressure pi (s), for the counterforce Fg1 (s) and for the counterforce Fg2 (s).

; nach Erreichen des Vorspanndrucks pv werden - gemäß Block 35 in Fig.8 - die Ventile v1 und v2 gestellt. Damit beginnt der eigentliche Preßvorgang, währenddessen der Weg s des oder der Preßstempel 3, 4 gemessen wird, s. Block 36 in Fig.8. Dabei wird gleichzeitig das Überdruckventil 14' für den Innendruck pi abhängig vom Weg s laufend gestellt, Block 37 in Fig.8. Weiters erfolgt ein Stellen der Ventile für die Gegenhaltestempel 13' und 13'', für die Gegenhaltekraft Fg1(s) und Fg2(s), s. Block 38 in Fig.8. Anschließend wird in Block 39 zyklisch abgefragt, ob der Preßstempel-Weg s den Endwert hiefür erreicht hat, d.h. ob der Preßvorgang beendet werden kann oder nicht. Wenn dies nicht der Fall ist, wird bei 40 wieder zum Block 36 etc. zurückgekehrt, und es erfolgt eine neuerliche Wegmessung, Überdruckventilstellung etc. Ist der Preßstempelweg an seinem Endwert sEnde angelangt, dann folgt nach dem Block 39 das Ende des Prozeßablaufes, wie in Fig.8 bei 41 angedeutet ist.After this entry, the workpiece 1 'is filled in block 33 until the pressure medium emerges at the outlet of the pressure valve 14'. After this process, according to block 34, the pressure medium in the workpiece 1 'is pretensioned to the desired pretensioning pressure by means of the pump 18 $\text{pv = pi (s = 0)}$

; After the preload pressure pv has been reached, the valves v1 and v2 are set in accordance with block 35 in FIG. This starts the actual pressing process, during which the path s of the press ram 3, 4 is measured, see Block 36 in Fig. 8. At the same time, the pressure relief valve 14 'for the internal pressure pi is continuously set depending on the path s, block 37 in FIG. 8. Furthermore, the valves for the counter-holding punches 13 'and 13''are set, for the counter-holding force Fg1 (s) and Fg2 (s), s. Block 38 in Fig. 8. Subsequently, in block 39 it is queried cyclically whether the press ram path s has reached the end value for it, ie whether the pressing process can be ended or not. If this is not the case, the system returns to block 36, etc. at 40, and a new path measurement, pressure relief valve position, etc. is carried out 8 is indicated at 41.

In a further flow chart, FIG. 9 illustrates a somewhat modified process flow compared to FIG. 8, in that an internal pressure-dependent regulation for the counter-stamping forces Fg1 and Fg2 takes place. Steps 31 to 37 are the same as in the flow chart according to FIG. 8, so that a new description of the same can be omitted. After step 37 (setting the internal pressure pi), the internal pressure pi, the counter-holding force Fg1 of the one counter-holding stamp 13 'and the counter-holding force Fg2 of the second counter-holding stamp 13' 'are measured (block 42 in FIG. 9). This is followed by a control of Fg1 and Fg2, i.e. Depending on the internal pressure pi, a setpoint Fg1 (pi) and a setpoint Fg2 (pi) are defined and compared with the actual value Fg1ist or Fg2ist. Depending on the comparison of the actual value and the setpoint, the valves for actuating the counterhold punches 13 'or 13' 'are set (block 43 in Fig. 9). Then in block 39 it is again queried whether the press punches have already reached their end of travel, and if not, the process returns to block 39, if so, the end 41 is reached.

Another modification is provided in the process sequence illustrated by the flow diagram according to FIG. 10, in that a control loop for the pressing force Fp1 or Fp2 of the pressing dies 3 or 4 is provided depending on the pressing die path s. In detail, steps 31 to 35, which in turn are identical, are followed by a modified measuring step at 36 ', where not only the ram path s but also the ram forces Fp1 and Fp2 are measured. After setting the Internal pressure pi (s) with the help of the pressure valve 14 'in step 37 - as in step 37 according to FIG. 8 - the control of Fp1 and Fp2 illustrated in block 44 follows depending on the ram path s, with a path-dependent setpoint for Fp1 and Fp2 is determined and an actual value / setpoint comparison is carried out, the valves for Fp1 and Fp2 being set accordingly.

Following this control step 44, the valves for the counter-holding punches 13 'or 13' '(Fg1, Fg2) are again set, analogously to FIG. The rest of the procedure is the same as that of Fig. 8 or 9.

11 schematically shows a device with a tool 2 without variable tool parts (counter-punch) in a cavity 7, for example a hemispherical bulge 12 on the workpiece 1 'during the pressing using the press punches 3, 4.

The technique of tightly guiding pressure medium lines through press rams, as in the present case the pressure medium supply line 17 and the pressure medium discharge line 15 through the pressure rams 4 and 3, is per se, e.g. of tube bending machines, known and does not require any further explanation here.

If a heat treatment of the blank to be pressed is desired before the pressing process - as a rule, such a heat treatment is preferred for soft annealing - the duration and the degree of heat treatment (temperature) naturally depend on the respective blank, in particular on the respective material. Usually heat treatment will have to be carried out in such a way that the blank is sufficiently long, e.g. 1 to 2 hours, heated to a few 100 ° C in an annealing furnace, after which it is allowed to cool in air; Before the so annealed material would lose ductility again due to the removal of alloy components to the grain boundaries, the pressing process - e.g. about 2 or 3 hours after the heat treatment.

The upright arrangement of the tool shown in FIGS. 3 and 7 with the supply of the pressure medium from below has the advantage that when filling, air bubbles or gas bubbles in general can rise upwards, so that the interior of the blank or workpiece 1 or 1 ' can be filled without gas bubbles can.

Adequate lubrication is also important when pressing, particularly in places with a strong curvature where the material has to flow around tight radii or into tight radii. Appropriate lubricant application is therefore particularly important on edges - including the blank. In particular, it is advantageous to apply around 50% more lubricant in the edge area.

The internal pressure-based forming according to the invention with internal pressure guidance is to be explained in more detail below using a specific example.

Example:

A hollow profile blank with a square cross-section was pressed from an AlMgSi 0.5 material, the dimensions of the blank being as follows: external dimensions 30 × 30 mm ² , wall thickness 2 mm, corner radius 2 mm, length 155 mm. The tensile strength for AlMgSi 0.5 is approx. 200 N / mm ² .

Before being pressed, the blank was heated to 400 ° C. in an annealing furnace for 100 minutes and then cooled in air. The press processing took place after approx. 3 hours. For this purpose, a preload pressure of 130 bar was set (at the beginning of the pressing process), and the maximum internal pressure that was reached in the second half of the process phase, where large expansions of the material of the workpiece were required, was 2000 bar. The pressing speed was 5 mm / s, and the counter-holding force of the one counter-holding die for the production of a workpiece with a branch, square as the initial profile, was 10 kN.

As mentioned, the workpiece produced was a T-piece, similar to that shown in FIG. 3, with a branch length of 53 mm (measured against the rear wall), and the workpiece had a final length of 90 mm. In the area of the junction, the corner radius was 3 mm.

If the invention was explained in more detail above with the aid of particularly preferred exemplary embodiments, further modifications and modifications are of course possible within the scope of the invention. Thus, in particular, it is also conceivable to provide process phases in which the internal pressure pi is modified, as shown in the diagram in FIG. 6, in which the internal pressure is again reduced somewhat in between, for example, if the material of the workpiece is to be caused to flow in the branch around an additional corner area. Furthermore, it is of course possible to provide the control illustrated in FIGS. 9 and 10 on the one hand for the counter-holding force of the counter-holding punches and on the other hand for the pressing force of the pressing punches combined in the process sequence, that is to say, for example, a step according to block 43 would be carried out in the sequence shown in FIG. 10 9 before or after block 44.

A wide variety of conventional pressure valve types can be used as the pressure valve 14 and 14 ', in particular, in addition to the hydraulically operated pressure valve 14' shown in FIG. 7, also electromagnetically operated pressure valves.

The control and regulating unit 30 can furthermore be implemented in a conventional manner with corresponding electrical circuits, such as, in particular, A / D converters, D / A converters, comparators and / or microprocessors. Such regulating and control units are basically known per se in conventional presses, so that a detailed explanation can be omitted here.

If the invention was explained above on the basis of particularly preferred exemplary embodiments, modifications and modifications are nevertheless possible within the scope of the invention. For example, it is conceivable to produce indentations on the workpieces at the same time as the bulges, neckings, branches, etc.

Claims (16)

1. A method for internal pressure supported forming of metallic hollow sections in tools to work pieces (1) provided with bulges, necks, branchings or the like, wherein a liquid pressure medium is supplied to the interior of a blank arranged in the tool (2) which, at its front sides, is engaged with at least one movable press die and one stationary counter-holding tool part, preferably with two movable press dies (3, 4), the blank is pressed, under bulging, into at least one cavity (7) in the tool (2), by aid of the press die(s) (3, 4), and the pressure of the pressure medium in the interior of the blank sealed off by the press die and the stationary counter-holding tool part or by the press dies (3, 4) is controlled in dependence on the process progress at pressing, preferably in dependence on the distance (5) passed by the press die, characterised in that in process phases with slight elongations (ϕ) of the material, the pressure (pi) of the pressure medium is adjusted such that the comparative strain formed on the basis of the spatial state of stress in the work piece (2) substantially just reaches the mean tensile strength (k_f) of the material, whereas in process phases with strong elongations (ϕ), the pressure (pi) of the pressure medium is increased so as to sufficiently raise the elongation at break of the material by lowering the mean tension.
2. A method according to claim 1, characterised in that the pressure medium is adjusted to a slight overpressure (pa), e.g. in the range of from 300 to 600 bar, at the beginning of the pressing procedure, and to a maximum value (pe), e.g. in the range of a few kbar, towards the end of the pressing procedure.
3. A method according to claim 1 or 2, characterised in that during the pressing procedure, the pressure medium is adjusted to a maximum pressure (pe) of at least 80% of the tensile strength of the material of the blank.
4. A method according to any one of claims 1 to 3, characterised in that in pressing phases, in which the material of the blank flows around small radii of curvature of the tool (2), the pressure (pi) of the pressure medium is adjusted to 5% to 40% of the maximum pressure during pressing, depending on the wall thickness (w).
5. A method according to any one of claims 1 to 4, characterised in that already before pressing the blank, the pressure medium is adjusted to an increased bias pressure (pv), preferably 5% to 20%, in particular approximately 10% of the maximum pressure (pe) reached during pressing, e.g. to a value in the range of from 100 to 1000 bar.
6. A method according to any one of claims 1 to 5, characterised in that the blank is filled with pressure medium before the pressing procedure so as to be free from gas bubbles, and that the pressure medium is adjusted to an increased pressure (pi) by means of a pump (18).
7. A method according to any one of claims 1 to 6, characterised in that in case of blank materials exhibiting segregations of alloying components to the grain boundaries, in particular of high-strength aluminum alloys, a heat treatment is carried out before the pressing procedure such that the ductility of the material is increased by incorporation of these alloying components into the grain lattices, and that the pressing procedure is carried out within a given time-window, e.g. 1h to 4h after the heat treatment, yet before the occurrence of a renewed segregation of the alloying components to the grain boundaries.
8. A method according to claim 7, characterised in that blanks of AlMgSi alloys are heated in an annealing furnace at approximately 400°C for, approximately, 100 minutes, subsequently are cooled in air and are pressed approximately 3 hours after termination of the annealing furnace heat treatment.
9. A method according to any one of claims 1 to 8, wherein, when compressing the blank in the (respective) tool cavity (7), into which the material of the blank is moulded, a variable counter-holding tool part (13) having a cross-sectional shape corresponding to the (respective) portion to be bulged, is counter-held, characterised in that counter-holding with the (respective) variable tool part (13) is effected under the control of pressure and/or distance and the force introduced thereby into the work-piece is selected such that the mean tension (the hydrostatic tension portion) occurring in sum in the work piece (1) is reduced so much that the elongation at break determined thereby becomes greater than the greatest elongation occurring in the respective work piece portion supported by the variable tool part (13).
10. A method according to claim 9, characterised in that the force (Fg) applied by the variable counter-holding tool part (13) is adjusted to at least 5% of the force exerted by the internal pressure in the blank on the portion to be bulged.
11. A method according to claim 9 or 10, characterised in that when simultaneously making several bulges, branchings or the like (12, 12') and thus when using several variable pressure-controlled counter-holding tool parts (13, 13'), a distance control or measurement, respectively, is carried out in addition to the pressure control thereof so as to adapt their pressure control when the variable counter-holding tool parts (13, 13') perform movements of un-equal speeds.
12. A method according to any one of claims 1 to 11, characterised in that the lubricant applied in zones having a small surface-radius of curvature of the blank or of the tool, respectively, in particular at rounded-off edges, is by at least 50% more than that applied on the remaining portions of blank and tool (2), respectively.
13. An apparatus for carrying out the method according to any one of claims 1 to 12, with a tool (2) as well as with a pressure-medium supply device (23-25) connected to a pressure medium source and comprising a pressure-medium supply duct (17) through the or through one of the press dies (4) or optionally through the stationary counter-holding tool part, and which has an associated pressure adjustment device (14; 14'), characterised in that a pressure-medium drain duct (15) leads through the (other) press die (3) or optionally through the stationary counter-holding tool part, and that the pressure adjustment device (14, 14') is incorporated in this pressure-medium drain duct (15).
14. An arrangement according to claim 13, characterised in that the pressure adjustment device is formed by a controlled pressure valve (14; 14').
15. An arrangement according to claim 13 or 14, characterised in that a nonreturn valve (20) is incorporated in the pressure-medium supply duct (17), in case a pump (18) supplying the pressure medium at a pre-selected pressure is provided as the pressure medium source.
16. A metallic work piece having at least one bulge, neck, branching or the like, characterised in that is has been moulded from a hollow section by the method according to any one of claims 1 to 12, and that it consists of a brittle material, such as an aluminum alloy.

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