WO2016089097A1

WO2016089097A1 - Ultra-high-speed molding method using electroplasticity effect

Info

Publication number: WO2016089097A1
Application number: PCT/KR2015/013027
Authority: WO
Inventors: 김정; 강범수; 노학권
Original assignee: 부산대학교 산학협력단
Priority date: 2014-12-03
Filing date: 2015-12-02
Publication date: 2016-06-09
Also published as: KR101577333B1; US20170326614A1

Abstract

The present invention relates to an ultra-high-speed molding method and, more specifically, to "an ultra-high-speed molding method using an electroplasticity effect", the method enabling a predetermined pulse current to be applied to a molding material to be processed so as to instantly lower the stress of the molding material to be processed, and then enabling a predetermined force to be applied, at an ultrahigh speed, to the molding material to be processed so as to mold the molding material to be processed. The ultra-high-speed molding method using an electroplasticity effect, according to the present invention, comprises the steps of: (a) performing electroplasticity on a molding material to be processed; and (b) molding the molding material to be processed by providing, at an ultrahigh speed, a predetermined molding force to the molding material to be processed. The present invention has an advantage of molding, at an ultrahigh speed (molding time: 150-300 ㎲), a molding material to be processed. In addition, the present invention applies a predetermined molding force to a molding material to be processed, in synchronization with the deterioration of the stress of the molding material to be processed, thereby having an advantage of comparatively saving more energy consumed for the molding force.

Description

Ultra-fast molding method using electric firing effect

The present invention relates to an ultra-high speed molding method, and more particularly, by applying a predetermined current to a workpiece to reduce the stress of the workpiece, and then applying a predetermined force at a high speed to the workpiece. The present invention relates to an "ultrafast molding method using an electrical firing effect" for molding a workpiece.

Recently, the use of lightweight metals is increasing in the automobile or aviation industry to improve fuel efficiency.

For this reason, the use of high strength steel and an aluminum alloy stands out.

However, these special-purpose materials have difficulty in industrial application because they have the same formability (hardening) compared to the original steel alloy in a normal room temperature environment.

For this reason, various techniques have been studied to improve the moldability of the hard alloy.

In order to improve the formability of the metal, a typical method is a hot stamping method of molding at a high temperature.

However, the warm forming method has disadvantages due to high temperature treatment, such as adhesion between the mold and the material, difficulty in lubrication, and life due to the decrease in the strength of the mold.

For this reason, in the case of high strength steel sheet, the development of a molding process suitable for forming a high strength steel sheet is required due to a poor molding dimension accuracy due to spring back after the forming process.

On the other hand, various studies on the properties of metal materials are being conducted, and one of them is revealed that the properties of metal materials change when electricity is applied to the metal. In particular, the fact that the stress of the metal changes when the current is supplied to the metal is known by various sources, which is known in the academic field as electrical baking.

Hereinafter, a brief description will be made with reference to the published papers.

Graduate School of Automotive Ship Technology, University of Ulsan, May 2013 "Study on the Mechanical Behavior of 5052 Aluminum Alloy between Periodic Pulsed Current Conduction" describes the electrical firing as follows.

Since Troitskii explained in 1969 that currents energized by certain materials, such as sodium, changed the properties of materials, Klimov et al. In 1984, and Conrad in 2000, pulsed currents between strains of metallic materials The study found that it lowers.

Through this study, it can be seen that the change of the stress and the crystal structure brought by the current are not caused by the resistance heating.

Conrad's work in 2002 led to plastic phenomena and phase transformations through continuous currents or pulsed currents in various metals and ceramics.

Recent studies, which define this phenomenon as electroplasticity, have shown Electrically Assisted Manufacturing (EAM) technology to improve formability using electroplastic phenomena.

In 2007, Ross and colleagues and Perkins and colleagues found that metal flows were dramatically reduced by energizing a series of currents.

In addition, the comparison between the results of Ross and Perkins showed that the continuous energization between the deformations of the metal decreased the maximum elongation attainable in the tensile test, while the elongation improved dramatically in the compression test.

The reason why the maximum elongation is lowered in this study is that as the cross section of the specimen narrows during the tensile test, the electric energy density per unit area increases, which causes the temperature rise to be excessive enough to cause early failure of the specimen. Because I came.

As a result, the current-carrying processing technology through the continuous current was not available due to the deterioration of the maximum elongation to be applied to the sheet manufacturing process.

The conduction firing phenomenon is a place where the material is applied to generate heat to lower the flow stress, and metal forming has begun to attract attention from research institutes and industrial fields dealing with metal products.

Thus, in 2008, 2009, and 2010, Roth and Salandro attempted to overcome the drawback of decreasing maximum elongation between energized plastics of tensile metals.

In 2008, Roth and coworkers were able to improve the maximum elongation of 5754 aluminum alloy to 400% of typical elongation at break under normal tension by using periodic pulsed current instead of continuous current.

A 2009 study by Salandro et al. Investigated the effects of current duration per unit area and duration of energized pulses using AZ31BO magnesium alloy and suggested process variables that resulted in the greatest loss rate.

In a follow-up study, Salandro's 2010 study examined the effect of increasing the formability of cyclic pulse currents on various 5xxx series and heat-treated aluminum alloys.

Through this study, it was found that the effectiveness of the moldability improvement by energizing molding differs depending on the alloy type and heat treatment conditions.

In addition, a 2009 study by Green et al. Found that a single high current density current could be used as a technique to remove or reduce the elasticity of the restoration just before removing the inter-process load or at the end of molding. Proved to be.

This was an attempt to demonstrate the possibility that energization techniques could be used for more accurate dimensions and shapes, not just for the benefit of further stretching.

Although the effect of changing the mechanical behavior on the various metals brought by current has increased the interest of many researchers and industry, studies on the quantitative evaluation of the effects of pulse variables including duration of current, current density, and current period are very Limited.

In 2011, Salandro investigated the effect of energetic firing on the 304 stainless steel alloy through the pulse current in the bending process and presented an analytical method for determining the load and deformation of the three-point bending process.

In this study, electroplastic bending coefficients were introduced so that the results could be utilized for the design of an electrically assisted bending (EAB) process. The model fits well with the results of the experiment and can predict bending loads within a 10-15% difference.

1 is an exemplary graph showing the electrical firing effect related to the manufacturing-based technology of ultra-high strength steel and Al5000-based body products of more than 1Gpa using the current-carrying molding, published on page 61 of the 2014 Spring Proceedings of the Korean Society for Manufacturing Manufacturing Systems.

As shown in the drawing, when a pulse current of a predetermined magnitude is energized at intervals of 0.01 s, a phenomenon in which the stress of the metal temporarily decreases below a predetermined value appears. In particular, it can be seen that the degree of stress reduction is proportional to the magnitude of the energizing current.

As such, the effect of the electrical calcination (Electroplascity) was confirmed that the flow stress is significantly reduced when the continuous current flows during the plastic deformation of the metal.

However, as shown in the graph of FIG. 1, this effect occurs instantaneously in a short time of about 1 to 2 ms, and there is a problem in that the conventional molding method having a molding time of 1 to 2 seconds is applied as it is due to the insulation problem with the mold. It is necessary to have a molding method capable of applying a predetermined force in a short time.

Meanwhile, explosive forming, electromagnetic forming (EMF), and the like are known as ultrafast molding methods.

Explosive molding is a processing method that uses energy from explosive explosives, and has the characteristics that it can be formed into any shape on a very fast and hard material.

Next, electromagnetic forming (EMF) is a technique of forming a metal at high speed (15 to 300 m / s) by using a high-strength magnetic field, and is a molding method that directly uses energy of a magnetic field to form a metal. One of the representative high-velocity forming processes [1].

This electromagnetic molding method is limited in the molding range of the workpiece because the molding is formed within the range that the magnetic field of the molding coil acts, and a low conductivity electrical material is a high-conductivity driver such as copper to obtain a sufficient forming force for molding (driver) should be used, but since the magnetic pressure generated by the molding coil is applied directly to the workpiece, the molding is performed without any physical contact (non-contact molding), so that problems such as surface defects, lubrication, and abrasion are not generated. This has the advantage of being possible.

In addition, electromagnetic molding is a cold working method, it is possible to maintain the mechanical properties as it is. Therefore, the present invention can be applied to various molding processes such as shaft pipe / expansion, flat plate molding, and joining processes.

In addition, since electromagnetic molding can effectively shape complex shapes, it can be applied to various fields such as the home appliance industry, the automobile industry, and the aviation industry [2].

In Korea, in the early 1990s, the introduction of equipment from abroad was conducted to compare the numerical approach and the experimental results. In 2005, a study was conducted on the joining process of aluminum tubes as a step to apply them to the spaceframe of automobiles. It is not practically used due to lack of technology base and experience [3].

Overseas research on electromagnetic molding has been actively conducted in the United States, Europe, Japan and China, but it is only partially used.

Hereinafter, the principle and method of the electromagnetic molding will be described.

In any closed circuit, when the magnetic flux changes over time, an induced electromotive force equal to the rate of change of the magnetic flux and the opposite direction is induced. This is called Faraday's law and is expressed as Eq. (1).

In Equation (1), ε represents induced electromotive force, Φ represents magnetic flux, and t represents time. In electromagnetic shaping, the discharge of attenuated current to the coil through the capacitor instantaneously within a short time of several hundred μs, as shown in FIG. 2, causes induced electromotive force on the workpieces around due to the change of magnetic flux. This induced electromotive force causes an induced current to flow through the workpiece as a conductor.

The force received by a conductor through which a current flows due to the magnetic field is called Lorentz's force and is represented by equation (2).

F = Idl × B (2)

Where I is the current flowing through the conductor, dl is the length of the conductor, B is the magnetic flux density, and F is the force of Lorentz. Lorentz force is generated as shown in FIG. 3 perpendicular to the plane defined by the conductor length dl and the magnetic flux density B. This force is the forming force in electromagnetic molding [5].

Electromagnetic molding equipment is composed of a high-capacitance capacitor, a molding coil, a control circuit for charging the capacitor, a power supply device, a charge / discharge switch, a mold, and the basic circuit diagram is shown in FIG.

As shown in FIG. 4, a high capacity capacitor connected to the power supply device is charged through a charge switch.

When the capacitor is charged up to the target energy for forming, the impulse current flows through the forming coil by instantaneously discharging through the discharge switch.

The input current applied to the coil attenuates within several hundred μs while generating a strong magnetic field in the forming coil as shown in FIG. 2. The strong magnetic field of the shaping coil generates Faraday's law by inducing current in the opposite direction to the workpiece and shaping force is generated by Lorentz's force.

FIG. 5 is an example of a forming coil used in electromagnetic molding, and FIG. 6 is an external perspective view of the molding coil insulated using epoxy, and FIG. A work piesce is placed between a die and an electromagnetic forming coil.

As is known, electromagnetic molding requires the construction of a control system for controlling the storage energy of the capacitor by controlling the input voltage of the electromagnetic molding equipment, a system for charging and discharging, and a control system for stably charging the capacitor through a constant ammeter. An input voltage regulator can be used to control the storage energy of the capacitor, which can be checked with a voltmeter.

Related prior arts using such an electromagnetic molding method

1. Korean Patent No. 10-0956027, "Electromagnetic Molding Apparatus and Bumper Stay Molded Product Made Using It", 2010.04.27.

2. Korean Patent No. 10-1344867, "Electromagnetic molding apparatus having lower molding means", 2013.12.18.

3. Korean Patent No. 10-1034592, "A plate forming apparatus including a plurality of forming punches and a plate forming method using the same", 2011.05.04.

4. Korean Patent No. 10-1034593, "A stretching sheet forming apparatus including a plurality of forming punches and a stretching sheet forming method using the same", 2011.05.04. Etc.

However, since the forming force using the electromagnetic molding is applied at a short time of several tens to hundreds of micros, a significant forming force that is more than several times the yield stress of the material is sufficient to give sufficient kinetic energy to the workpiece to achieve effective molding. There is a problem that should occur.

For this reason, there is a problem that a considerable amount of power is consumed during electromagnetic molding.

The present invention aims to propose a method capable of reducing the molding force applied to the workpiece under the same molding conditions.

To this end, the present invention proposes a method of applying an electromagnetic molding force after generating an electrical firing phenomenon in the workpiece to reduce the stress of the workpiece.

For reference, the firing treatment for the workpiece to be proposed in the present invention is performed by applying a predetermined current to the workpiece for a predetermined time.

To this end, the present invention, by applying electrical firing to the workpiece to drop the stress of the workpiece immediately and then apply a predetermined molding force to the workpiece for a short time the stress of the workpiece is away from the workpiece A method of molding a molding material is proposed.

In the present invention, the molding force applied to the workpiece is preferably applied within a time when the stress of the workpiece is reduced by electrical firing, and this molding force may be provided by an explosive molding or an electromagnetic molding process. .

Ultra-fast molding method using the electrical firing effect according to the present invention comprises the steps of (a) performing electrical firing on the molding material; (b) forming the workpiece by providing a predetermined molding force at a high speed with respect to the workpiece.

More specifically, the ultra-fast molding method using the electrical firing effect according to the present invention comprises the steps of (a) flowing a current to the workpiece; (b) the stress of the workpiece is lowered to a predetermined value or less; (c) may be formed by applying a predetermined molding force to the workpiece during a time when the stress of the workpiece is separated below a predetermined value.

In the present invention, the intensity of the predetermined pulse current applied to the workpiece is different depending on the type of the workpiece, and the time when the stress of the workpiece is below a predetermined value is the kind of the workpiece. It may differ according to.

The effects of the present invention are as follows.

The present invention has the advantage that the molding to be processed can be molded at a very high speed (molding time: 150 to 300 kPa).

In addition, the present invention is advantageous in that the energy consumed in the molding force can be reduced compared to the other case because a predetermined molding force is applied to the workpiece in synchronization with the time when the stress of the workpiece is lowered.

In addition, the present invention is to combine the ultra-fast electromagnetic molding method having a non-contact molding characteristics with the electrical firing effect in addition to the explosion molding which is an ultra-fast molding method to temporarily lower the flow stress of the workpiece to be processed and to apply the electromagnetic molding instantaneously to ultra-high strength even in cold molding It has the advantage of being able to form steel and hard-forming materials.

Ultra-high-speed molding considering the electrical firing effect proposed in the present invention, for example, electromagnetic molding technology considering the electrical firing effect has not been studied or proposed anywhere in the world at home and abroad, when the present invention is applied to the industrial cold of a product having ultra high strength Due to the molding, not only the production cost can be drastically reduced compared to the existing method but also the molding quality of the product can be improved, and it is expected that it can contribute to the revolutionary development of the next generation ultra high strength steel sheet forming technology.

2 to 4 are diagrams for explaining the concept of electromagnetic molding.

5 is an example of a forming coil used for electromagnetic forming.

FIG. 6 is an external perspective view of a molding coil insulated using epoxy. FIG.

FIG. 7 is a view showing a state in which a work piesce is disposed between a die and an electromagnetic forming coil in order to perform electromagnetic forming.

8 is a view showing a concept of a charge and discharge circuit used in the electromagnetic molding equipment.

9 to 10 are diagrams illustrating a method of performing electromagnetic molding using the electrical firing effect according to the present invention.

11 is a view for explaining an electromagnetic molding method according to the present invention.

Hereinafter, with reference to an embodiment shown in the accompanying drawings will be described in detail the technical configuration of the present invention.

The ultrafast molding method using the electrical firing effect according to the present invention comprises an electric firing step for the workpiece and an ultrafast molding step for the workpiece.

1. First, it will be described in the electrical firing step.

The electrical firing step is a process of lowering the stress of the workpiece under a certain value by flowing a predetermined current to the workpiece.

As is known, when a current of a certain level or more is flowed to the workpiece, a phenomenon occurs in which the stress of the workpiece is momentarily lowered to a predetermined value or less.

Therefore, in the case where a predetermined molding force is applied to the workpiece in synchronization with the time when the stress of the workpiece is lowered to a certain value or less, molding the workpiece with a lower molding force than without applying an electroplastic effect The advantage is that you can.

2. The next ultrafast molding step will be described.

The time that the stress of the workpiece is lowered by the above-described electrical firing effect is different depending on the type of the workpiece, but the time is very short.

Therefore, in order to take advantage of the electroplastic effect, it is necessary to apply a predetermined molding force to the workpiece, within a time when the stress of the workpiece is reduced.

For this reason, the molding force applied to the workpiece is to be applied within the time when the stress is lowered by the electrical firing effect, and it will be desirable to be synchronized.

For example, if the time during which the stress of the workpiece is lowered due to the above-mentioned predetermined effect is about A [sec], the predetermined molding force applied to form the workpiece is to be within A [sec]. .

Since the time for which the electrical firing effect is usually performed is very short, its application is difficult with a general molding method.

Therefore, in the present invention, a method for providing a molding force during a short stress reduction time of the workpiece to be processed according to the electrical firing effect has been devised.

Such molding force may be at least one of explosion molding and electromagnetic molding.

This is because these explosion molding and electromagnetic molding can apply a predetermined molding force to the workpiece to be processed at an extremely high speed.

In the present invention, as one embodiment of the present invention, a method for providing a molding force for a short stress reduction time of the workpiece to be processed using electromagnetic molding will be described.

For reference, the present invention will be described as an example of electromagnetic molding capable of high-speed processing in response to the electrical firing effect, but the embodiment of the present invention is not limited thereto and may be equally applicable to explosion molding.

[Description of Electromagnetic Forming for Implementation of the Present Invention]

As is known in the art, electromagnetic forming (EMF) is a technique of forming a metal at high speed (15 to 300 m / s) using a high strength magnetic field.

In the electromagnetic molding method, electric current is instantaneously discharged by the forming coil, and induced electromotive force is generated in the workpiece due to the change of the surrounding magnetic flux.

When the induced current flows through the workpiece, the workpiece is formed by Lorentz force.

In the electromagnetic molding method, since the magnetic force generated by the molding coil is directly applied to the workpiece, the molding is performed without any physical contact, and thus, there is no problem such as surface defects, lubrication, and abrasion, and thus the molding can be repeatedly performed.

Electromagnetic molding equipment basically includes a resistor, an inductor, a high capacitance capacitor, a molding coil, and a charge / discharge switch as shown in FIG. 8. For reference, the remaining part of the electromagnetic molding equipment illustrated in FIG. 8 except the molding coil is a charge / discharge circuit for supplying a predetermined current to the molding coil (charge / discharge circuit of FIG. 9).

On the other hand, although not shown, such an electromagnetic molding equipment will be understood as a comprehensive concept that further includes a power supply device, a mold as well as a control circuit for controlling the charge-discharge switch.

The high-capacity capacitor connected to the power supply is charged through the charge switch, and when the capacitor is charged to the target energy for molding, the discharge current is instantaneously discharged through the discharge switch to impart a shock current to the forming coil. Will flow.

In general, the input current applied to the coil attenuates within a few hundred μs, generating a strong magnetic field in the forming coil.

The strong magnetic field of the shaping coil generates Faraday's law by inducing current in the opposite direction to the workpiece and forming force by the Lorentz force.

However, in such electromagnetic molding, the molding force applied to the workpiece is required to be above a certain level, and therefore, a considerable amount of energy is consumed in the electromagnetic molding.

Therefore, the present invention proposes a method of applying electromagnetic molding in a state in which the stress of the workpiece is lowered, and proposes a method of relatively reducing energy required for electromagnetic molding.

9 and 10 are conceptual views of an electromagnetic molding apparatus for performing an electromagnetic molding method (an example of an ultrafast molding method) using the electrical firing effect proposed in the present invention.

(A) and (b) of FIG. 9 conceptually illustrate a process in which the workpiece 10 is molded when a current is supplied to the molding coil, and FIG. 10 (a) illustrates the workpiece 10 ) Is a view showing a concept of supplying a current for obtaining the electrical firing effect to the workpiece 10 by connecting a pulse current generator), Figure 10 (b) is the same as that shown in Figure 8 to the molding coil It is a figure explaining the concept of electrically connecting similar charge / discharge circuits.

That is, although not shown in FIG. 9, the pulsed current generator and the charge / discharge circuit shown in FIG. 10 are connected to the workpiece 10 and the molding coil of FIG. 9, respectively.

First, prepare the electromagnetic molding equipment. As shown in FIG. 9, the forming coil, which is a component of the electromagnetic forming apparatus, is maintained in a seating portion, and a mold having a predetermined shape is positioned on the forming coil as shown in the drawing.

Next, the workpiece 10 to be processed is placed on the forming coil. At this time, if necessary, after seating the intermediate member of the ferromagnetic material on the upper part of the molding coil, the workpiece 10 may be seated on the upper part of the intermediate member of the ferromagnetic material, but this may be optional.

Next, an electric firing effect is obtained by conducting a predetermined pulse current to the workpiece by using the pulse current generator shown in Fig. 10A.

As is known, there may be a difference in the time when the stress of the workpiece is lowered below a predetermined value depending on the type of the workpiece, the strength of the applied current, and the duty ratio of the pulse current.

Next, in the present invention, a predetermined pulse current is supplied to the workpiece to control the charge and discharge circuit of the electromagnetic molding equipment to supply a predetermined current to the molding coil so as to be synchronized at a time when the stress of the workpiece falls below a predetermined value. Let's do it.

11 illustrates a concept of processing a workpiece into a predetermined shape by applying a predetermined electromagnetic force to the workpiece by using the electromagnetic molding coil according to the present invention several times.

Therefore, as in the present invention, it is possible to implement the molding of the workpiece more easily by operating the charge / discharge circuit in synchronization with the time when the stress of the workpiece is lowered below a predetermined value.

The ultrafast molding method using the electrical firing effect according to the present invention described above has been described as an embodiment for applying the electromagnetic molding force, but the present invention is synchronized at a point when the stress of the workpiece to be processed is lowered below a predetermined value, such as explosion molding. The same applies to all kinds of molding methods in which a predetermined force can be applied to the workpiece, and the protection scope of the present invention should be interpreted to extend to these technical fields.

Claims

As a super fast molding method using the electrical firing effect,

(a) applying a predetermined pulse current to the workpiece to produce an electrical firing effect that instantaneously lowers the stress of the workpiece below a predetermined value;

(b) provide a high-speed electromagnetic molding force to the workpiece to be synchronized at a time during which the stress of the workpiece is separated below the predetermined value, thereby providing a lower molding force than without the electrical firing effect; Forming the workpiece,

The time point at which the electromagnetic molding force is provided is within a range of 150 to 300 kPa, including a time point at which the stress of the workpiece is lower than a predetermined value.
The method of claim 1,

And the intensity of the predetermined pulse current applied to the workpiece is varied according to the type of workpiece to be processed.
The method of claim 2,

The time point at which the stress of the workpiece is lowered below a predetermined value is different depending on the type of the workpiece.
The method of claim 3, wherein

The ultra-high molding method using the electrical firing effect, characterized in that the electromagnetic forming force is generated using a forming coil using the Lorentz force.