JP2009113054A - Press-drawing method for aluminum alloy plate, and panel press-drawn by the same method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize increase in machining speed in press-drawing of aluminum alloy plate, in which the increase is substantially not practiced because of generation of forming defects, such as cracks and wrinkles, though the increase is strongly desired from both view points of improvements of formability and productivity. <P>SOLUTION: A slide is not lowered at a constant speed through the whole stroke from a top dead center to a bottom dead center, but the relative speed of a slide and a cushion pad is lowered when an upper die is brought into contact with a work. Thereby, blank-holding force is surely generated by clamping a part of the work with a blank holder connected to the cushion pad and the upper die. Further, the lowering speed of the slide is raised after the contact, so that the machining speed in the press-drawing, which is performed with a lower punch fixed below the work and with the upper die, is raised. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method of press-drawing an aluminum alloy sheet and a molded panel thereof.

  In recent years, weight reduction of the vehicle body has been promoted in order to reduce the fuel consumption of automobiles. As one of the countermeasures, a press-drawn panel of a lighter aluminum alloy plate material has been used for an automobile body sheet that has conventionally used a press-drawn panel such as a mild steel plate. However, aluminum alloy sheet is inferior in press drawability compared to mild steel sheet, and it is impossible to integrally form parts having complex shapes and parts having deep side walls in automobile body sheets. There were limitations. For this reason, development of a processing method for improving the press drawability of the aluminum alloy sheet is strongly desired.

  FIG. 1 shows a Stribeck diagram that is well known in the field of friction wear and lubrication (Non-Patent Document 1 below). The horizontal axis represents a value obtained by dividing the product of the viscosity η of the lubricating oil and the relative speed V of the sliding surface by the surface pressure P. In the region from A to D in this diagram, the friction coefficient μ can be reduced by increasing the relative speed V. Generally, it is said that the lubrication state in press molding is in the range from boundary lubrication to mixed lubrication.

  In press molding, reducing the coefficient of friction between the die or blank holder and the workpiece is extremely effective as a means for improving the moldability. For these reasons, increasing the processing speed in press molding is expected as an effective means for improving moldability.

  Also, increasing the processing speed is desirable from the viewpoint of improving productivity.

For the reasons described above, it is strongly desired to increase the processing speed in press-drawing of an aluminum alloy sheet. The present invention has been made in order to achieve this object. As a result, as described later, a servo press machine is particularly focused on among press forming apparatuses as will be described later. Examples of various patent documents and non-patent documents related to the servo press include the following patent documents 1 to 4 and non-patent document 2.
Japanese Patent Laid-Open No. 10-277797 JP 2004-17098 A JP 2006-122997 A JP 2006-255787 A “Physical Chemistry of Lubrication” Toshio Sakurai, Yuki Shobo, 1974, p. 19 “Plasticity and Processing”, Vol. 47, No. 546, 2006, p. 18

As described above, it is strongly desired to increase the processing speed in the press drawing process of an aluminum alloy sheet.
On the other hand, if the drawing speed is increased, molding defects such as cracks and wrinkles are likely to occur.

  Therefore, in the drawing of aluminum alloy sheets, it is strongly desired to increase the processing speed from the viewpoint of improving the formability and improving the productivity as described above. In reality, the drawing speed cannot be increased so much due to the problem of occurrence.

The present inventors have studied a method for solving the above problems and increasing the drawing speed of an aluminum alloy sheet while suppressing the occurrence of forming defects such as cracks and wrinkles.
As a result of diligent investigations on the cause of forming defects such as cracks and wrinkles that are likely to occur as the drawing speed increases, the inventors have found that the mechanism is as follows. In other words, when the workpiece is drawn, the slide descends at a high speed and the upper die and the workpiece placed on the blank holder come into contact with each other at a high speed. At this time, it is transmitted to the cushion pad, and at this time, a large slide and the cushion pad collide with each other, and a large impact load momentarily acts on the cushion pad. This is because it fluctuates greatly.

In order to reduce the impact load, it is conceivable that the relative speed of the slide and the cushion pad is lowered (soft touch) when the upper die and the work come into contact with each other.
However, if the slide lowering speed is kept low, the above-described drawability cannot be improved, and the number of strokes is reduced, thereby reducing productivity.

  Therefore, it is preferable that the relative speed between the slide and the cushion pad when the upper die and the work come into contact with each other is reduced, so that the impact load is reduced (soft touch) and the blank holder connected to the cushion pad is The wrinkle pressing force of the workpiece by reliably pinching a part of the workpiece with the upper die is surely generated, and the slide after the contact is increased by increasing the descending speed of the slide after the contact. The lower mold is fixed at the lower position of the workpiece while holding the workpiece wrinkle pressing force by sandwiching a part of the workpiece with the blank holder connected to the cushion pad that is lowered at the same speed as the upper die. It is to increase the processing speed of the drawing of the workpiece performed by the punch and the upper die. In other words, the slide does not lower the entire stroke from the top dead center to the bottom dead center at a constant speed, but slowly lowers the slide when the upper die and the workpiece come into contact with each other, so that the lower die punch and the upper die Thus, the present invention has been completed by conceiving that it is preferable to increase the descending speed of the slide when drawing the workpiece.

  That is, the first invention of the present application lowers the slide attached with the upper die from the top dead center, and sandwiches a part of the workpiece between the blank holder connected to the cushion pad and the upper die. After generating the work wrinkle pressing force, the lower die punch fixed at the lower position of the work while holding the work wrinkle pressing force by simultaneously lowering the slide and the cushion pad and the upper die In the press forming method of the aluminum alloy sheet material that performs the drawing process, the slide lowering speed B when the slide and the cushion pad are simultaneously lowered is lower than the slide lowering speed A when the upper die and the workpiece are in contact with each other. It is a press forming method of an aluminum alloy sheet characterized by being raised.

The second invention is the ratio R of the maximum value Bm of the slide lowering speed B when the slide and the cushion pad are simultaneously lowered with respect to the slide lowering speed A when the upper die and the work are in contact with each other. = Bm / A is 1.2 or more. This is a press forming method of an aluminum alloy sheet.
A third invention is a press molding method of an aluminum alloy plate material according to the second invention, wherein R is 1.7 or more.
A fourth invention is a press molding method of an aluminum alloy sheet material according to the third invention, wherein R is 3.0 or more.
The fifth invention is a molded panel characterized in that the aluminum alloy sheet of the first invention is a 5000 or 6000 series aluminum alloy sheet having a breaking elongation of 20% or more and a tensile strength of 200 MPa or more.

  According to the present invention, in press drawing forming of an aluminum alloy sheet, it is possible to improve forming workability while suppressing the occurrence of defects such as cracks and wrinkles, and increase the processing speed, thereby improving productivity. I can do it.

  In order to specifically realize the first to fifth inventions, a press capable of controlling the pressing conditions as desired may be used. For this, a mechanical press provided with a drive motor, a flywheel, and a clutch brake in the slide drive mechanism can be used to control the slide lowering speed. However, they generally become very complex in structure and are often difficult to control the pressing conditions as desired.

  Therefore, as a result of investigations from various viewpoints, the present inventors have found that using a servo press that uses a servo motor for the slide drive mechanism is an extremely effective means for achieving the above object. I found. In the servo press, attention is paid to the point that the use of a flywheel and a clutch brake can be avoided and the pressurization condition can be easily controlled as desired (Patent Document 1 and Patent Document 2).

The embodiment of the present invention will be further described in detail.
For example, Patent Document 4 proposes a method of relaxing the impact load by controlling the lifting and lowering operations of the slide and the cushion pad. Specifically, the slide is lowered from the top dead center, the slide is stopped when the upper mold and the work come into contact with each other, or the cushion pad is gradually raised while the slide is stopped. In this way, the relative speed of the slide and the cushion pad is made substantially zero to make contact.

FIG. 2 is a schematic diagram showing the configuration of a single-acting press (FIG. 5 of Patent Document 4).
In the press machine, a slide located at the upper part and a bolster located at the lower part are provided so as to face each other. The slide moves up and down by receiving power from a slide drive mechanism built in the upper crown. An upper die is attached to the lower part of the slide. On the other hand, the bolster is fixed to the upper part of the bed, and a lower punch is attached to the upper part of the bolster. The bolster and the lower die punch are provided with a plurality of holes penetrating in the vertical direction, and cushion pins are inserted into the holes. The upper end of the cushion pin contacts the lower part of the blank holder provided in the concave portion of the lower die punch, and the lower end of the cushion pin contacts the cushion pad of the die cushion provided in the bed. A beam is provided between the inner wall surfaces of the bed, and the die cushion is supported by the beam.

  In press working, a work (work object: blank sheet) is placed on a blank holder. When the upper die is lowered together with the slide, the convex portion of the upper die is brought into contact with the workpiece. The workpiece is held between the convex portion of the upper die and the blank holder, and the wrinkle is pressed. When the slide and the upper die are further lowered, the work is drawn by the upper die and the lower die punch. At this time, the cushion pad, the cushion pin, and the blank holder are integrally lowered while generating an upward biasing force.

  According to Patent Document 1 and Patent Document 2, it is shown that slide motion can be freely controlled by using a servo press, and that productivity improvement and moldability improvement can be expected by multi-step processing and composite processing. Yes. In Patent Document 3, it is proposed that the machining time can be shortened by a combined machining of drawing and piercing. Furthermore, it has been introduced that the equipment load can be reduced by performing piercing while the slide is raised and lowered again from the bottom dead center after drawing. In Non-Patent Document 2, it is introduced that the formability of the steel sheet is improved by the slide motion control.

Hereinafter, the present invention will be described in more detail.
In FIG. 3, the slide motion when the present invention is carried out using a servo press is schematically shown with the vertical axis representing the slide position and the horizontal axis representing time. In addition, in the same figure, the slide motion when it is normally pressed using a crank press irrespective of the present invention is also shown for reference.

  In FIG. 3, the descent speed in the slide descent start region from the top dead center a to the b point can be set according to the press specifications.

  Control is performed so that the slide lowering speed starts to decrease from the deceleration start point b, and when reaching the point c where the die and the blank holder come into contact, the slide lowering speed decreases to a predetermined lowering speed. As a result of investigations by the present inventors, the desired relative speed A between the slide and the cushion pad when the upper die and the workpiece contact each other has an optimum value depending on the weight of the blank holder connected to the cushion pad and the wrinkle presser load. However, it is not determined uniquely. For example, when a blank holder having a weight of 200 kg is used with a 3000 kN press and a wrinkle pressing load is 75 kN, a speed of 100 m / s or less is appropriate. When the relative speed is 100 mm / s or more, the wrinkle presser load fluctuates greatly and becomes unstable immediately after pinching a part of the workpiece with the upper die and the blank holder, and cracks and wrinkles are caused by drawing. A malfunction occurs. Desirably, it is 80 mm / s or less.

  About the lower limit of the relative speed, it is desirable to slow down as long as productivity is not affected. According to the present invention, it is possible to start drawing immediately after sandwiching a part of the workpiece with the upper die and the blank holder. However, by decelerating the slide and bringing the die into contact with the workpiece, it is possible to suppress the wrinkle presser load's impact rise and large load fluctuations, but it is impossible to instantaneously reduce the wrinkle presser load fluctuations to zero. is there. Therefore, it is desirable to provide a time lag (from the point c to the point d) as needed between the time immediately after the part of the work is held between the upper die and the blank holder until the start of drawing. In this section, it is possible to further suppress fluctuations in the wrinkle presser load.

  Next, after the wrinkle pressing step, the slide and the cushion pad are simultaneously lowered, and the work is drawn by the upper die and the lower die punch. In this drawing process (from point d to point e), it is effective that the slide and cushion pad descending speed, that is, the drawing speed is 100 mm / s or more, preferably 150 mm / s or more. Here, the drawing speed is the maximum speed Bm during the drawing process.

  As a result of extensive studies by the inventors, it has been confirmed that the formability is improved by increasing the forming speed in the drawing of the aluminum alloy plate. In drawing molding, lubricating oil is generally used to prevent the occurrence of cracks and scratches. As described above, when the molding speed is increased, it is considered that the lubricity is improved by the effect of reducing the friction coefficient between the tool and the workpiece when the lubricating oil is used, and the drawability is improved. If it is less than 100 mm / s, the effect of improving lubricity is small, and the effect of expanding the forming limit of the aluminum alloy sheet is small.

  The present inventors experimentally studied the optimum conditions with a servo press (trade name: Komatsu Hybrid AC Servo Press H2F) with a pressurizing capacity of 3000 kN. Since it varies depending on the mold conditions, etc., optimization is required according to individual conditions. As a guideline for speed setting, it is desirable to set the speed at which the upper die and the workpiece come into contact with each other to the maximum speed at which the wrinkle pressing force does not fluctuate depending on the press capacity and mold conditions. Try and error is required.

  As for the speed increase at the time of drawing, a slight increase in the speed when the upper die and the cushion pad abut can be effective, but the speed should be as fast as possible depending on the capacity of the press machine used. desirable. Specifically, the ratio R = Bm / A of the maximum value Bm of the slide lowering speed B when the slide and the cushion pad are simultaneously lowered with respect to the slide lowering speed A when the upper die and the work are in contact with each other is set to 1. .2 or more is desirable. Preferably, R is set to 1.7 or more, and more preferably, R is set to 3.0 or more, so that fluctuation of the wrinkle pressing force immediately after contact between the upper die and the workpiece is suppressed, and the improvement of drawability is also effective. Can be realized.

  The larger the maximum value of Bm, the more effective, but the current press machine has a limit of several hundred mm / s. The larger R is, the more power is required to accelerate the slide after the upper die and the workpiece are in contact (to increase the slide lowering speed when the slide and the cushion pad are simultaneously lowered to a desired value). Therefore, it is realistic that the value of R is kept to the minimum necessary level according to each processing object.

  The return process (from bottom dead center e to top dead center f) may be arbitrarily controlled, but productivity can be improved by setting the maximum pressing speed.

As for the aluminum alloy, 5000 series or 6000 series is suitable in terms of formability and strength.
The 5000 series aluminum alloy sheet is used for various applications because of its high strength and good formability. The 6000 series aluminum alloy sheet is slightly inferior in formability to the 5000 series, but has a characteristic that the strength is increased at the time of coating baking, and has recently been adopted as an automotive panel such as a hood. Since all have inferior formability compared to steel plates, improvements are desired. In order to ensure good moldability, an elongation at break of 20% or more and a tensile strength of 200 MPa or more are required. If the elongation at break is less than 20%, cracking tends to occur in the molding requiring high stretchability. In molding that requires deep drawability, it is advantageous that the tensile strength is high, and cracking tends to occur at less than 200 MPa. Therefore, when higher formability is required, a breaking elongation of 20% or more and a tensile strength of 200 MPa or more are required.

Next, examples of the present invention will be described.
Using a JIS5182-O material with a plate thickness of 1 mm, a rectangular cylinder was formed with a servo press of 150 mm × 300 mm × depth 45 mm. (For reference, TS (tensile strength), YS (yield strength), and EL (elongation) of JIS5182-O material are shown in Table 1.)

In the crank press mode, the maximum speed is 80 mm / s, 150 mm / s, and in the variable speed mode, the initial descent speed of the slide is 600 mm / s, the descent speed is 80 mm / s when decelerating, and the maximum speed during drawing is 80 mm / s. The commercially available rust preventive oil R303P for automobile materials was used as the lubricating oil with three conditions of (R = 1.0), 150 mm / s (R = 0.53), and 200 mm / s (R = 0.40).

  The results are shown in Table 2.

When forming with the maximum speed of 80 mm / s and changing the wrinkle presser load, it was possible to form in the crank press mode and the variable speed mode with the wrinkle presser load of 50 kN. At the wrinkle presser loads of 75 kN and 100 kN, cracks occurred at the punch corners under any conditions.
When the maximum speed at the time of drawing was set to 150 mm / s (R = 0.53), molding was possible in the variable speed mode with soft touch with a wrinkle presser load of 75 kN. On the other hand, in the crank press mode, cracks did not occur, but constriction occurred at the punch corner. Further, in the molding in the variable speed mode in which the maximum speed was increased to 200 mm / s (R = 0.40), the molding was possible even with a wrinkle presser load of 100 kN.

The same square tube molding was performed using AA6022-T4 material having a plate thickness of 1 mm. (For reference, TS (tensile strength), YS (yield strength), and EL (elongation) of AA6022-T4 material are shown in Table 1.)
The results are shown in Table 3.

At a maximum speed of 150 mm / s, molding was possible both in the crank press mode and in the variable speed mode (R = 0.50) when the wrinkle presser load was 50 kN. In the case of the crank press mode, cracks occurred in the punch corner portion at wrinkle presser loads of 75 kN and 100 kN. In the case of the variable speed mode (R = 0.50), cracking occurred at a wrinkle presser load of 100 kN, but constriction occurred at 75 kN. Molding in the variable speed mode with the maximum speed increased to 300 mm / s (R = 0.25) can be molded up to a wrinkle presser load of 75 kN. Necking occurred at 100 kN, but improved compared to the crank press mode.

Stribeck diagram known in the field of friction wear and lubrication Schematic diagram showing the configuration of a single-acting press Schematic diagram showing slide motion when the present invention is implemented using a servo press

Claims (5)

  After the slide with the upper die attached is lowered from the top dead center and a part of the workpiece is clamped by the blank holder connected to the cushion pad and the upper die, the work wrinkle pressing force is generated. An aluminum alloy plate material that performs drawing processing of a workpiece with a lower die punch that is fixed at a lower position of the workpiece and the upper die while maintaining a workpiece wrinkle pressing force by simultaneously lowering the slide and the cushion pad. In the press forming method, an aluminum alloy characterized in that a slide lowering speed B when simultaneously lowering the slide and the cushion pad is made higher than a slide lowering speed A when the upper die and the workpiece are in contact with each other. Press molding method for plate materials.   In Claim 1, the ratio R = Bm / A of the maximum value Bm of the slide lowering speed B when the slide and the cushion pad are simultaneously lowered with respect to the slide lowering speed A when the upper die and the workpiece contact each other is set to 1. A method for press-forming an aluminum alloy sheet, characterized in that it is 2 or more.   3. The press forming method for an aluminum alloy sheet according to claim 2, wherein R is 1.7 or more.   2. The press forming method for an aluminum alloy sheet according to claim 1, wherein R is 3.0 or more.   2. The molded panel according to claim 1, wherein the aluminum alloy sheet is a 5000 or 6000 series aluminum alloy sheet having a breaking elongation of 20% or more and a tensile strength of 200 MPa or more.

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