Patent Application
20110097596
The present invention relates to a method for hydroforming placing a metal tube in a mold, clamping the mold, then applying internal pressure in the tube and a pushing action in the tube axial direction (hereinafter referred to as an “axial pushing action”) to form the tube into a predetermined shape and to a hydroformed product formed by the same.
In recent years, applications for hydroforming have been growing—particularly in the field of auto parts. The advantages of hydroforming are that it is possible to form an auto part, which used to be made from several press-formed parts, from a single metal tube, that is, combine parts and thereby reduce costs, and reduce the number of welding locations and thereby lighten the weight.
However, the metal tube used as a material is generally uniform in cross-section, so a shape with a large expansion rate (ratio to circumferential length of tube of circumferential length after hydroforming) was difficult to work.
Further, the difficulty of hydroforming is not only affected by the expansion rate, but is also affected by the cross-sectional shape or presence of any bending. In particular, the length of the location expanded has a large effect.
For example, with the T-shape such as in
In hydroforming of a long expanded location, unless applying a considerably large axial pushing action, the tube will become thin in wall thickness and end up cracking, but the larger the axial pushing action, the easier the tube will buckle or wrinkle in the tube axial direction.
Further, a long expanded location means that in that region, in the initial state, the metal tube and the mold will not yet be in contact, so buckling or wrinkles will occur more easily.
So far as the inventors know, in the region of an expansion rate of 1.35 or more, hydroforming to 3.5 times or more the outside diameter of the original metal tube is not seen.
In general, to prevent buckling or wrinkles in the hydroforming, it is important to test different load paths of internal pressure and axial pushing action (hereinafter referred to as simple a “load path”) by trial and error to find the suitable load path.
A general example of the load path is shown in
Among these, finding a suitable path for stage 2 consumes the most effort and has relied heavily on the skill of the hydroforming workers.
Patent Document 1 introduces an example of this, but this method is a method of preparing in advance a crack limit line and a wrinkle limit line and selecting a load path between the two limit lines.
However, in actuality, it is difficult to prepare these two limit lines. Usually, a large number of experiments and trial and error in analysis of numerical values are required. Further, the limit lines are often broken lines. If so, the number of parameters for determining the broken lines becomes greater and therefore tremendous labor becomes necessary for the trial and error.
Further, Patent Document 2 proposes a method cyclically changing the internal pressure along with the axial pushing action. For example, this is a method of changing the internal pressure to a square wave (a) or sine wave (b) such as shown in
This method is proposed as a method for preventing cracking, but later research reports that it is also effective in suppressing wrinkles (see Non-Patent Document 1). However, the load path of this method increases in variables such as the waveform, period, amplitude, etc. compared with the variables in the above-mentioned broken line load path, so finding a suitable load path method for becomes even more difficult.
As a method when hydroforming a shape with a long expanded region, other than the above method of using a load path, there is also the method of specially designing the mold.
For example, Patent Document 3 jointly uses movable molds and a counter to realize expansion in the long region while preventing buckling of the metal tube.
However, the mold structure of this method is extremely complicated, so the mold costs become higher. Further, the items controlled during working are not limited to the internal pressure and axial pushing actions (axial pushing actions by movable molds). Facilities enabling control of the retracted position of the counter also become necessary. Further, since the items controlled increase, finding a suitable load path requires greater skill and trial and error.
The present invention proposes a method of working able to work a hydroformed product with a long expanded region without any buckling or wrinkles remaining, which method of working does not requiring skilled labor or trial and error much at all. Further, it proposes a hydroformed product worked by that method of working.
To solve such a problem, the present invention has as its gist the following:
(1) A method for hydroforming feeding an inside of a metal tube with a pressure medium to apply internal pressure and applying an axial pushing action from the two ends of the metal tube so as to form the metal tube into a predetermined shape,
the method for hydroforming characterized by performing a first step of raising the internal pressure in a state with the metal tube fixed in position at the two ends or a state applying an axial pushing action of 10% or less of the total amount of axial pushing action, then applying an axial pushing action while holding the internal pressure at a constant pressure so as to expand the metal tube near the ends, then performing a second step of raising only the internal pressure without applying an axial pushing action so as to thereby expand a center of the metal tube, then performing a third step of lowering only the internal pressure to the value of the constant pressure without applying an axial pushing action, then repeating the first to third steps one or more times, then raising the internal pressure in the state not applying an axial pushing action or applying an axial pushing action of 10% of the total axial pushing action amount or less.
(2) A method for hydroforming feeding an inside of a metal tube with a pressure medium to apply internal pressure, applying an axial pushing action from the two ends of the metal tube, and simultaneously applying an axial pushing action to the movable molds so as to form the metal tube into a predetermined shape,
the method for hydroforming characterized by performing a first step of raising the internal pressure in a state with the metal tube fixed in position at the two ends or a state applying an axial pushing action of 10% or less of the total amount of axial pushing action, then simultaneously applying an axial pushing action to the two ends of the metal tube and the movable molds while holding the internal pressure at a constant pressure so as to expand the metal tube near the ends, then performing a second step of raising only the internal pressure without applying an axial pushing action to the two ends of the metal tube and an axial pushing action to the movable molds so as to thereby expand a center of the metal tube, then performing a third step of lowering only the internal pressure to the value of the constant pressure without applying an axial pushing action to the two ends of the metal tube and an axial pushing action to the movable molds, then repeating the first to third steps one or more times, then raising the internal pressure in the state not applying an axial pushing action or applying an axial pushing action of 10% of the total axial pushing action amount or less.
(3) A hydroformed product produced using a method for hydroforming as set forth in the (1) or (2), the hydroformed product characterized in that a region where a circumferential length of an expanded cross-section of the metal tube is expanded by 1.35 times or more compared with the circumferential length of the cross-section of the original metal tube continues in the tube axial direction of the metal tube for at least 3.5 times the outside diameter of the original metal tube.
Note that “near the end of the metal tube” in the present invention is defined as the region within 35% or more from the end of a metal tube compared with the length of the metal tube before applying the axial pushing action by a fixed internal pressure. Further, the “pressure medium” is a liquid, gas, or solid and includes rubber, low melting point metal, steel balls, and all other media which can transmit pressure.
According to the present invention, hydroforming a shape with a long expanded region becomes easy. Due to this, the scope of application of hydroforming becomes greater and parts can be merged and weight reduced.
a: example of T-forming
b: example of hydroformed product with long expanded location
a: example of square wave
b: example of sine wave
a: example of state with metal tube set inside mold
b: example of state of metal tube finished being worked
a: example of state 1, b: example of state 2, c: example of state 3
a: example of state with metal tube set inside mold
b: example of state of metal tube finished being worked
a: example of state with metal tube set inside mold
b: example of state of metal tube finished being worked
a and 4b show an example of setting a circular cross-section metal tube 1 in hydroform molds 2 and 3 and expanding it by hydroforming to shape it into a hydroformed product 4 having a rectangular cross-section. For example, a steel pipe of an outside diameter of 63.5 mm and a wall thickness of 2.0 mm (steel type: JIS STKM13B) is expanded to a rectangular cross-section of 63.5 mm×84 mm (corner roundness R=10 mm). The expansion rate in that case is 1.39. Further, the length of the region with an expansion rate of 1.39 is 320 mm (5 times the outside diameter of 63.5 mm).
Below, using as an example working by that hydroform mold, embodiments of the present invention will be explained along the flow path shown in
First, at stage 1, in the same way as the above method, without applying an axial pushing action, a pressure medium (for example, water) 6 is fed into the metal tube 1 to raise the pressure by only the internal pressure. However, in some cases, to prevent seal leakage from the tube ends, sometimes slight axial pushing actions of 10% of the total axial pushing amount or less are applied. This initial pressure PH is the pressure at which the metal tube plastically deforms without cracking and is found relatively easily by calculation or experiments.
For example, the present inventors engaged in research and as a result learned that a yield start pressure Pp in a planar strain state of a metal tube (see following formula (1)) can be used as a yardstick for the initial pressure PH (see Non-Patent Document 2). Note that the “D” on the formula indicates the outside diameter of the original tube (mm), “t” the wall thickness (mm), and “r” the r value, and “YS” and “YSp” indicate the 0.2% yield strengths in the single-axis tension state and planar strain state.
However, when the shape is complicated etc., the error from the above formula becomes larger, so it is more reliable to find the initial pressure PH experimentally. Specifically, the initial pressure PH is set with reference to the pressure when cracking when raising the internal pressure until the metal tube cracks without applying any axial pushing action. For example, it is set to a pressure of 0.7 to 0.8 time the pressure at the time of cracking.
In the above way, the internal pressure is raised until the initial pressure PH found by calculation or experiment. This state corresponds to the stage 1 in
Next, the stage 2 where the internal pressure and axial pushing actions are applied is entered.
First, while holding the internal pressure at the initial pressure PH, the axial pushing punches 5 are made to advance to apply only axial pushing actions. This operation is called the “first step” as shown in the enlarged view of the load path of
As a result of research of the inventors, even with application of only axial pushing actions without increasing the internal pressure, the metal tube is expanded, but in this case, the expansion proceeds not from the center part, but near the ends (part N1 of state 2 of
As opposed to this, with this method, the value of the internal pressure remains as held, so by expansion during an axial pushing action, there is almost no possibility of cracking. Further, the only variable in the load path is the amount of the axial pushing action, so the method is extremely simple.
The axial pushing action amount δS (mm) up to the state 2 has to be suppressed to an amount of axial pushing action of an extent enabling elimination of wrinkles in the later steps. As the method for finding the suitable axial pushing action amount δS, it is sufficient to stop changing the amount of axial pushing action in the middle, obtain a sample, and select an amount of axial pushing action of an extent not resulting in large wrinkles. The value of the suitable axial pushing action amount δS differs depending on the formed shape and the dimensions and strength of the material tube, but from the results of research of the inventors, about 0.2 to 4 times the wall thickness of the material tube is preferable. Further, about 3 times is preferable.
Next, the axial pushing actions are stopped and only the internal pressure is raised. This operation is called the “second step”. In this step, no axial pushing actions are applied, so the expansion proceeds again at the center part (part M of state 3 of
After this, while stopping the axial pushing actions, the pressure is lowered once to the initial pressure PH. This process is called the “third step”. Even if setting a load path of a step shape applying axial pushing actions without lowering the internal pressure at the pressure PT, the pressure is too high, so the metal tube immediately ends up cracking. Accordingly, the third step of increasing the pressure to the peak pressure PT, then lowering it once to the initial pressure PH has extremely important meaning in the method of the present invention.
If similarly repeating the first step to the third step in the above way, the tube is alternately expanded at the center part and near the ends and becomes a uniformly expanded shape in the tube axial direction. Further, as shown in
If repeating the above first to third steps one or more times, finally, as shown in
Above, an embodiment of the method for hydroforming proposed in the above (1) was explained. Application of this method to hydroforming using movable molds however corresponds to the method proposed in (2).
Below, an embodiment of this method will be explained.
In this method, as shown in
Even when using the movable molds 9, in the same way as the case of applying the axial pushing action to only the tube ends, it is possible to use the load path explained using
The metal tube set as in
Next, at stage 2, first, a first step is performed of holding the internal pressure at a constant pressure while simultaneously applying axial pushing actions to the two ends of the metal tube 1 and the movable molds 9 to thereby expand the metal tube 1 near the ends, then a second step is performed of raising only the internal pressure to thereby expand the center part of the metal tube 1, then a third step is performed of lowering the internal pressure to the value of the constant pressure. Further, the first to third steps are repeated one or more times to form the tube into the product shape, then, in a state without applying any axial pushing action or applying axial pushing actions of 10% or less of the total amount of the axial pushing action, the internal pressure is raised to obtain the hydroformed product 4 such as in
This method using movable molds, compared with the method of pushing only the tube ends, enables the reduction of the wear resistance of the non-expanded parts, so a large expansion rate can be achieved. However, with this method, at the time of the initial start of working, there will be an expanded region longer than the shape of the worked part desired to be finally obtained, so the conventional method had the problem that buckling or wrinkles in the tube axial direction occurred more easily than with a usual hydroformed product.
As opposed to this, according to the present invention, by using the load path explained above, it is possible to eliminate the above problems of buckling or wrinkles even if using movable molds, so further great effects can be exhibited.
If using such a series of hydroforming methods (usual hydroforming method and hydroforming method using movable molds), even a part long in the tube axial direction will not be left with buckling or wrinkles and a part with a large expansion rate can be obtained. Specifically, it is possible to obtain a hydroformed product with a region of an expansion rate of 1.35 or more continuing in the tube axial direction for a length of 3.5 times or more the diameter of the material tube—impossible with the conventional method. However, in the above, the explanation was given of the example of an extremely long region with an expansion rate of 1.35 or more or 5 times the diameter of the material tube.
Below, examples of the present invention will be shown.
For the tube, steel pipe of an outside diameter of 63.5 mm, a wall thickness of 2.0 mm, and a length of 600 mm (steel type: JIS standard STKM13B) was used. The material characteristics were a YS of 385 MPa and an r value of 0.9. For the hydroform mold, the mold of
The load path of the hydroforming is shown in
First, if using the above-mentioned formula (I) to calculate the yield start pressure Pp in the planar strain state, it was 28.4 MPa. However, when actually raising the internal pressure without applying an axial pushing action until that steel pipe cracked, it cracked at 26.5 MPa. Accordingly, the initial pressure PH was set at 20 MPa or 0.76 time the actual cracking pressure of 26.5 MPa, while the top peak pressure PT was set at 25.5 MPa or 0.96 time the 26.5 MPa. Next, the amount of axial pushing action δS per cycle was set at 6 mm or 3 times the wall thickness of 2 mm of the material tube. Therefore, when running a test comprising several cycles of an initial pressure PH: 20 MPa, top peak pressure PT: 25.5 MPa, axial pushing action amount δS: 6 mm, the tube contacted the mold over substantially the entire length at 10 cycles. Therefore, the operation was repeated for a total of 10 cycles, that is, until the final axial pushing action amount of 60 mm, then the axial pushing action was stopped and only a high internal pressure was applied. The final pressure was set at 135 MPa as a sufficient pressure for the radius of curvature R of the corner to become R=10 mm in the same way as the mold.
The suitable load path such as shown in
In the hydroformed product obtained by the present invention, the circumferential length of the cross-section expanded into a rectangular shape was 278 mm. This corresponds to an expansion rate of 1.39 times the 63.54 tube. Further, the length in the tube axial direction in the cross-section having that expansion rate was 320 mm or 5.0 times the 63.5 mm outside diameter of the tube. In this way, a long hydroformed product with a large expansion rate, impossible by the conventional hydroforming, could be obtained by the method of the present invention.
Further, for comparison, the inventors attempting hydroforming by a cyclically changing load path as described in the above Patent Document 2. The load path is shown in
However, when actually performing the hydroforming, the tube ended up immediately cracking at the first cycle. This is believed to be because, unlike the method of the present invention, the pressure during the axial pushing action is high. For safety's sake, the pressure was lowered by 3 MPa as a whole and similar working applied, whereupon cracks could be prevented, but large wrinkles remained after the end of the work. This is believed to be because, unlike the method of the present invention, at the time of raising the pressure in the cycle, an accompanying axial pushing action is applied, so wrinkles easily form.
Using the same material tube as in Example 1, a hydroformed product of the same shape as in Example 1 was attempted to be worked by a hydroform mold using movable molds shown in
As a result, in the hydroformed product obtained by the present invention, the circumferential length of the cross-section expanded into a rectangular shape was 278 mm. This corresponds to an expansion rate of 1.39 times the 63.5φ material tube. Further, the length in the tube axial direction in the cross-section having that expansion rate was 320 mm or 5.0 times the 63.5 mm outside diameter of the material tube. In the same way as in Example 1, a part with no buckling or wrinkles or other working defects was obtained. Further, Example 2 was able to utilize the load path of Example 1 as is, so no trial and error at all was required.
Using the same metal tube and the same mold as in Example 1, hydroforming was performed by the load path shown in
As a result, in the hydroformed product obtained by the present invention, the circumferential length of the cross-section expanded into a rectangular shape was 278 mm. This corresponds to an expansion rate of 1.39 times the 63.5 φ tube. Further, the length in the tube axial direction in the cross-section having that expansion rate was 320 mm or 5.0 times the 63.5 mm outside diameter of the tube. Even if using this load path, in the same way as Example 1, a long hydroformed product with a large expansion rate could be obtained by the method of the present invention.
Using the load path of
As a result, in the hydroformed product obtained by the present invention, the circumferential length of the cross-section expanded into a rectangular shape was 278 mm. This corresponds to an expansion rate of 1.39 times the 63.5 φ tube. Further, the length in the tube axial direction in the cross-section having that expansion rate was 320 mm or 5.0 times the 63.5 mm outside diameter of the tube. With this working method as well, a long hydroformed product with a large expansion rate could be obtained by the method of the present invention.
According to the present invention, hydroforming of a shape with a long expanded region becomes easy. Due to this, the range of application of hydroformed products is expanded and parts can be combined and weight reduced. In particular, application to auto parts will enable vehicles to be reduced in weight more and therefore improved in fuel economy and as a result will contribute to suppression of global warming. Further, greater application in industrial fields not applied to much in the past, for example, household electrical applications, furniture, construction machinery parts, motorcycle parts, building materials, etc. can be expected.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-175764 | Jul 2008 | JP | national |
| 2009-122181 | May 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/062260 | 6/30/2009 | WO | 00 | 12/29/2010 |
