ULTRA-HIGH-SPEED FORMING METHOD USING ELECTROPLASTICITY EFFECT

Abstract

A method for forming or deforming a workpiece at a high speed using electroplasticity effect includes: applying a first level of pulse current to the work piece to cause the electroplasticity effect and lower a flow stress of the work piece; and applying a first level of electromagnetic force to the workpiece for 150˜300 μs to form or deform the work piece in a desired shape while the flow stress of the work piece is maintained at the lowered level, wherein the first level of electromagnetic force is lower than a second level of electromagnetic force which is required to form or deform the workpiece in the desired shape without applying the first level of pulse current.

Description

TECHNICAL FIELD

The present invention relates to a method of deforming a workpiece in a high speed, and more particularly, to a method of deforming a workpiece using electroplastic effect by applying a certain amount of pulsed current on the workpiece to momentarily lower the flow stress of the workpiece and then applying a certain amount of force in a high speed onto the workpiece to deform the workpiece.

BACKGROUND OF INVENTION

Recently in automobile or aviation industries, light weight metal is popularly employed for improving energy efficiency. Because of this, high strength steel and aluminum alloy are being prominently used. However, since these special-purpose materials have limited formability at room temperature compared to primary steel alloy, it is difficult to utilize the special-purpose materials at an industrial and commercial level. For this reason, various techniques are being studied for improving the formability of metal alloy sheets.

A typical method of improving the formability of metals is a hot stamping method which forms metals at a high temperature. However, the hot stamping method causes various drawbacks including adhesion between molds and materials, insufficient lubrication, and a short lifespan problem a degrading strength of molds. Because of these drawbacks, a process for deforming high strength steel plates is being required these days.

Meanwhile, various researches on properties of metallic materials are ongoing. It is known that when electricity is applied to metals, the characteristics of metallic materials change. Particularly, various researches show that when electric current is applied to metals, the flow stress of metals changes. This is academically called electroplasticity.

Electro-plastic effect will be described below in reference to an article entitled “Mechanical behaviors of 5052-H32 aluminum alloys under pulsed electric current” published on May 2013, University of Ulsan Institute of e-Vehicle Technology. Troitskii explained in 1969 that the electric current applied to specific materials like Natrium or Sodium changes the properties of materials in 1969. Klimov et al. and Conrad released, respectively in 1984 and in 2000, research results that electric current pulse during deformation of metallic materials lowers the flow stress.

According to these researches, it is found that the changes of flow stress caused by applied electric current and the changes of grain structure are not resulted from joule heating. In 2002, Conrad found that transformation plasticity and phase transition can occur in various metals and ceramic by applying continuously current or electric current pulse to the metals and ceramic.

Recent studies defined this phenomenon as electroplasticity suggested a technique called Electrically Assisted Manufacturing (EAM) that can advance formability by using electroplastic phenomenon. According to studies in 2007 by Ross and colleagues, and Perkins and colleagues, flow stress of metals can be rapidly decreased by applying continuous current.

Also, comparing research results of Ross and Perkins, it was found that the maximum tensile strength ratio decreases when electric current pulse is continuously applied while the maximum compressive strength ratio rapidly increases.

The reason why the maximum tensile strength ratio decreases in the research is that the electric energy density per unit area which applies to a test subject (or specimen) gets higher as the section part of specimen gets narrower during tensile test. As a result, temperature of specimen rises excessively high enough to cause premature fracture of specimen.

According to this result, electrically assisted forming or deforming process through continuous currents could not be applied to sheet metal manufacturing processes due to decrease in the maximum tensile strength ratio (or the maximum elongation percentage). Electroplastic phenomenon lowers the flow stress of a workpiece by applying electric currents to the workpiece and hence generating heat. The electroplastic phenomenon received attention form research institutes and industry fields that deal with metal products

Solutions have been suggested by Roth and Salandro in 2008, 2009, and 2010 which could overcome the disadvantages due to decrease in maximum elongation percentage of extending metal. Roth and his colleagues in 2008 were able to increase the maximum elongation percentage of 5754 aluminum alloy by applying periodic electric current pulse, not continuous currents, by 400% compared with fracture elongation ratio of typical general extension.

In the research by Salandro and his colleagues in 2009, they also investigated electric current pulse duration time and the effects of pulse parameter per unit area by using AZ31BO magnesium alloy and proposed process parameters that bring the biggest elongation percentage. The following research by Salandro in 2010 studied the relation between periodical pulsed current and an increase of formability, targeting many kinds of 5xxx series and aluminum alloy of heat treated condition.

Through this research, it could be known that how much the formability increases by the electrically assisted forming process varies depending on the kinds of alloy and heat treated conditions. In addition, according to the research by Green in 2009, the technique can be used to remove elastic restoration or to decrease elastic restoration by applying a single high density current at the last step of the forming or deforming process or just before removing a forming load.

These research shows that the electrically assisted forming technique can potentially be used not only for increasing, but for more accurate sizing and forming Despite the increasing attention of many researchers and industrial circles on the effect of electric current brings which changes mechanical behavior of various metals, research on quantitative assessments about pulse variable effects including duration, density and cycle of currents are very limited.

In 2011, Salandro investigated the effects of electroplasticity through pulsed current targeting 304 alloy-stainless in bending process and suggested a method for obtaining loading and deformation of 3-point bending test. This research introduced electroplastic bending coefficient and designed electrically assisted bending (EAB) process using the electroplastic bending coefficient. This model fits well with experimental results and was able to predict bending loads with a deviation as low as 10˜15%.

FIG. 1 is an exemplary graph that shows the effects of electroplasticity to steels with a high ultimate tensile strength above 1 Gpa and shows the effects of electroplasticity to Al5000 Family automotive body products. FIG. 1 is found in 2014 Korean Society of Manufacturing Technology Engineers Spring Conference journal at page 61. As aforementioned, when a certain amount of current applies at intervals of 0.01 s, the strain of metals temporarily gets down below a certain value. Particularly, the degradation degree is proportional to the amount of through-current.

Likewise, due to electroplastic effect, when continuous current is passed through during plastic deformation of metals, the flow stress significantly decreases. However, as shown in FIG. 1, these effects occur momentarily in a short time period, e.g., about 1˜2 ms and thus it is difficult to directly employ it to existing general forming methods. In existing general forming methods, molding time lasts only about 1˜2 seconds due to insulation problems with molds. Thus, a new forming or deforming method is necessary to apply a certain amount of force in a short time.

Meanwhile, there are explosive forming and electromagnetic forming (EMF) in high-velocity forming method. Explosive forming is a forming method using energy resulting from explosion of gunpowder which has characteristics of fast forming speed and ability to form or deform hard materials in arbitrary shapes.

Next, an electromagnetic forming (EMI) process is another high-speed metal forming method which forms and deforms in a high speed as fast as 15˜300 m/s using a high level of magnetic field. This method directly uses magnetic field energy for metal forming. This is one of typical high-speed forming processes along with the above-mentioned explosive forming process. This electromagnetic forming method can only be applied to a limited size of workpiece because the magnetic field of a formed coil is limited in size or in distance.

Another disadvantage of the electromagnetic forming (EMF) process is that a high conductive driver such as copper is employed to form or deform a material with low conductivity. However, the electromagnetic forming (EMF) process is advantageous in that it forms or deforms material by utilizing the magnetic pressure induced by a forming coil without any contact between the material and the forming coil. Thus, surface defects can be prevented, lubrication issue or erosion issue do not occurs, and a repeated formation or deformation is possible.

Also, electromagnetic forming process is a cold processing method which allows the workpiece to sustain its mechanical properties. Therefore, it can be applied to various forming processes including tube reducing/expansion, sheet forming, or bonding process. Furthermore, electromagnetic forming process can be applied to various fields including electronics industries, automotive industries and aviation industries because it is possible to effectively form materials in complicated shapes.

In South Korea, in early 1990s, researches were conducted comparing theoretical and numerical estimate with experimental data using equipment imported from abroad. In the year 2005, research on a soldering process of aluminum tubes was conducted to apply to a spaceframe of automobiles. However, due to lack of technology base and experiences, it failed to put it to practical use. Looking abroad, although researches on the electromagnetic forming process are being actively carried in the US, Europe, Japan and China etc., it is merely partially successful to put it to practical use until now. Hereinafter, principles and methods of the electromagnetism forming will be described.

When magnetic flux changes over time in a given closed circuit, induced electromotive force is induced that is the same in strength as time-changing rate of magnetic flux and the opposite thereto in direction. This is called Faraday's law and it can be expressed by the following Formula (1).

$\begin{array}{cc}\varepsilon =-\frac{d\Phi }{\mathrm{dt}}& \mathrm{Formula}\left(1\right)\end{array}$

In Formula (1), ε is induced electromotive force, Φ is magnetic flux, and t is time. As shown in FIG. 2, in the electromagnetic forming process, when current that decays within a short time period of hundreds of μs is discharged through a capacitor to the coil, induced electromotive force applies to a workpiece located nearby due to a change of magnetic flux. Because of this induced electromotive force, induced current flows through the workpiece which is a conductor.

The force received by the conductor carrying current due to magnetic field is called Lorentz force and this is expressed by the following Formula (2).

F=Idl×B  Formula (2)

Here, I is the current that passes through the conductor, dl is the length of conductor, B, is magnetic flux density, and F is Lorentz force. As shown in FIG. 3, the Lorentz force is generated perpendicularly to a plane defined by the length of conductor dl and the magnetic flux density B. This force serves as a plasticity force in electromagnetic forming process [5].

An apparatus for electromagnetic forming process may include a high-capacity capacitor, a formed coil, a control circuit, a power feeding equipment for charging the capacitor, a charge/discharge switch, and molds. An example of basic configuration is shown in FIG. 4.

As shown in FIG. 4, the high-capacity capacitor connected to the power feeding equipment charges energy through the charge switch. When the capacitor is charged up to a targeted energy level, an impulsive current passes through the forming coil by current is momentarily discharged through the discharging switch.

The current applied to coil decays within hundreds of μs, generating a strong magnetic field onto the forming coil. The strong magnetic field of forming coil applies induced current to a subject workpiece in the opposite direction by Faraday's Law. As a result, plasticity generates by Lorentz force and the forming process is performed.

FIG. 5 is an embodiment of the forming coils that are used for an electromagnetism forming process, FIG. 6 is a perspective view of a forming coil insulated by Epoxy, and FIG. 7 shows an exemplary arrangement of equipment to perform an electromagnetism forming in which the workpiece is located between a die and an electromagnetic forming coil.

As is known, to properly conduct an electromagnetism forming process, a system controlling stored energy through input voltage regulation of electromagnetic forming apparatus, charging and discharging for capacitor is necessary. And a control system for stably charging a capacitor through an ammeter is also required. The energy stored in the capacitor can be controlled through input voltage regulation apparatus, and the voltage of capacitor may be monitored through a voltmeter. The documents considered to be relevant are as follows.

• 1. Korean Patent NO. 10-0956027 which is entitled “ELECTROMAGNETIC FORMING DEVICE, AND FORMED ARTICLE OF BUMPER STAY BY USING THE SAME,” Apr. 4, 2010
• 2. Korean Patent NO. 10-1344867 which is entitled “Electromagnetic forming apparatus having lower die,” Dec. 18, 2013
• 3. Korean Patent NO. 10-1034592 which is entitled “FLEXIBLE SHEET FORMING APPARATUS WITH MULTIPLE FORMING PUNCHES AND FORMING METHOD USING THE SAME,” May 4, 2011
• 4. Korean Patent NO. 10-1034593 which is entitled “FLEXIBLE SHEET STRETCHING FORMING APPARATUS WITH MULTIPLE FORMING PUNCHES AND STRETCHING FORMING METHOD USING THE SAME,” May 4, 2011

However, the plasticity force using this electromagnetic forming method has works to a workpiece within a short time of tens or hundreds of μs. For effective formation or deformation, considerable plasticity force as much as several times or more of yield stress of the workpiece is necessary to give sufficient kinetic energy to the workpiece. To provide such amount of plasticity force, considerable amount of power is consumed. That is, large power consumption is a drawback of the electromagnetic forming method.

DETAILED DESCRIPTION OF INVENTION

Problems to be Solved

The present invention provides a method for forming or deforming a workpiece with a reduced forming force under a given condition. To this objective, the present invention suggests a method that applies electromagnetic plasticity after decreasing flow stress of workpiece by generating electroplastic phenomenon on workpiece.

According to an embodiment, the plasticity process to a workpiece is performed by applying electric currents for a given amount of time period. For example, a certain amount of plasticity is applied onto the workpiece during a given short time period to lower flow stress of the workpiece and then applying electroplastic force onto the workpiece while the reduce flow stress is maintained.

According to the present invention, it is desired to apply the plasticity to the workpiece while its flow stress is at a reduced level through electroplastic effect. The plasticity can be provided by an explosive forming process or or an electromagnetic forming process (EMF).

SUMMARY OF INVENTION

In an embodiment, a high speed forming method using electroplastic effect may include (a) a step of applying electroplastic effect onto work piece; and (b) a step of forming or deforming the workpiece by providing a certain amount of plasticity onto the workpiece at a high speed.

More specifically, high speed forming or deforming method using electroplastic effect according to an embodiment of the present invention may include (a) a step of applying electric currents onto workpiece; (b) a step of dropping flow stress of the workpiece below a certain level; (c) a step of applying a certain amount of plasticity onto the work-piece while the flow stress of the work piece is maintained at the reduced level.

Advantages of Invention

The present invention may provide many advantages including the following: the forming or deforming process applied to the workpiece can be performed at a high speed, e.g., taking molding time of 150˜300 μs.

According to an embodiment of the present invention, energy consumption can be reduced compared with prior art since it is synchronized at the point when a flow stress of work-piece drops.

In another embodiment, an explosive forming process, which is one of high speed forming methods, is combined with a non-contact electromagnetic forming process to obtain electroplastic effect, temporarily lower flow stress of work-piece using the electroplastic effect, and apply an electromagnetic forming process in a moment. This method can relatively easily form or deform high-tensile-strength-steels or materials with low formability even under cold forming conditions.

The high speed forming process using electroplastic effect according to the present invention, such as an electromagnetic forming technique with electroplastic effect, is neither studied nor suggested. When applied in a cold-forming process of ultrahigh-strength products, the present invention may not only substantially reduce production expenses but also improve quality of the product. Therefore, it can contribute to develop steel boards with a high ultimate tensile strength for next generation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing electroplastic effects on steels with a high ultimate tensile strength and Al5000 Family automotive body products in an electrically assisted forming process. The gragh shown in FIG. 1 is found in 2014 Korean Society of Manufacturing Technology Engineers Spring Conference journal at page 61.

FIG. 2 to FIG. 4 are schematic illustrations to explain a concept of an electromagnetic forming process.

FIG. 5 is an embodiment of a formed coil in use for an electromagnetic forming process.

FIG. 6 is a perspective view of a forming coil insulated by Epoxy.

FIG. 7 shows an exemplary arrangement of equipment for an electromagnetism forming process in which awork-piece is located between a die and an electromagnetism forming coil.

FIG. 8 shows a concept of charging and discharging circuit in use for an apparatus for an electromagnetic forming process.

FIG. 9 to FIG. 10 are schematic illustration explaining an electromagnetic forming method using electroplastic effect according to an embodiment of the present invention.

FIG. 11 is a schematic illustration explaining an electromagnetic forming process according to an embodiment the present invention.

EMBODIMENTS

Hereinafter, the present invention will be described in more detail referring to the drawings attached hereto

High speed forming method using electroplastic effect according to the present invention is configured to a step of electroplastic effect on work piece and a step of high speed forming on work piece.

1. First, a step of generating electroplastic effect will be explained.

Electroplastic effect generating step is a process of lowering flow stress of a work-piece below a certain level by applying a certain amount of electric current to the work-piece. As is well known, when a certain level of electric current is applied to a workpiece, flow stress of the workpiece drops below a certain level for a short time period.

When a certain amount of plasticity is applied to the workpiece while the flow stress of the workpiece is maintained at a reduced level, it is possible to form or deform the workpiece with a relatively lower level of plasticity compared with a case where no electroplastic effect is employed.

2. Next, a step of performing formation or deformation at a high speed will be explained.

The time when the flow stress of work piece gets lower by the aforementioned electroplastic effect varies depending on workpieces, but, in general, the time is very short.

To use electroplastic effect, it is necessary to apply a certain amount of plasticity while a flow stress of the workpiece is maintained low. Thus, the timing of applying the plasticity to the workpiece is desirable to be synchronized with the timing when the flow stress is maintained low.

For example, if the time duration when the flow stress is maintained low by electroplastic effect is A seconds, it is preferred that the time duration when the certain amount of plasticity is applied to the workpiece is A seconds or less.

Any conventional forming method can be employed since the time duration when the electroplastic effect lasts is very short. Therefore, in the present invention, a method should be selected which is capable of providing plasticity during such a short time period electroplastic effect lasts. An explosive forming method or an electromagnetic forming method may satisfy such condition.

Either the explosive forming or the electromagnetic forming method may apply a certain level of plasticity to the workpiece at a high speed. Hereinafter, an embodiment of the present invention employing an electromagnetic forming method will be explained. The method may provide plasticity during such a short time period and while the low-flow stress of the workpiece is maintained.

The present invention is not limited to be applicable to or combinable with an electromagnetic forming method (EMF) which is performed at a high speed using the electroplastic effect. Instead, the present invention is also applicable to or combinable with an explosive forming method.

[Embodiment Employing an Electromagnetic Forming Method (EMF)]

As is known in the industry, an electromagnetic forming method (EMF) is a technology for forming or deforming metals at high speed, e.g., at 15˜300 m/s, by using a high strength magnetic field. In the electromagnetic forming method (EMF), induced electromotive force applies onto the workpiece due to current discharged by a forming coil and due to a change of magnetic flux. When the induced current flows on the workpiece, the workpiece is subject to formation or deformation by Lorentz force.

The electromagnetic forming method (EMF) is performed without a physical contact with the workpiece and this is advantageous in that surface defects can be prevented, lubrication issues, abrasion issues, etc. can be prevented. Thus, a repetitive forming process is possible without deterioration in quality.

As shown in FIG. 8, an apparatus for performing an electromagnetic forming method may include a resistance, an inductor, a high-capacity capacitor, a formed coil, charging/discharging switches, etc. All components or parts shown in FIG. 8, except for a formed coil, are charging and discharging circuits for providing a certain amount of current to the formed coil. See the charging/discharging circuit shown in FIG. 9.

Although not shown in FIG. 8, the electromagnetic forming apparatus may further include a control circuit for controlling the charging and discharging switches, a power supply, a mold, etc.

A high-capacity capacitor connecting to power supply unit is charged through the charging switch. The capacitor is charged up to a targeted energy level and then be abruptly discharged during a very short time period through discharging switch so that an impulsive current starts to flow to the forming coil.

The current applied to the coil decays within hundreds of μs, thereby applying a strong magnetic field onto the forming coil. The strong magnetic field applies an induced current to the workpiece in the opposite direction by Faraday's Law, generating Lorentz force which serves as a plasticity force to the workpiece. To form or deform the workpiece, the plasticity force applied to the workpiece should be above a certain level. Thus, the electromagnetic forming method consumes a considerable amount of energy.

To reduces power consumption, in an embodiment of the present invention relatively the electromagnetic forming process is performed in in a state when the flow stress of the workpiece is lowered.

FIG. 9 and FIG. 10 are illustrations of an apparatus performing an electromagnetic forming method according to an embodiment. Referring to (a), (b) of FIG. 9, a process of forming a workpiece (10) is performed when current is supplied to a forming coil. Referring to (a) of FIG. 10, current is supplied to the workpiece (10) for obtaining electroplastic effect by connecting the workpiece (10) to a pulsed current generator. Referring to (b) of FIG. 10, a multiple number of charging and discharging circuits each of which may be identical or similar to what is shown in FIG. 8 are connected to the forming coil.

Although not shown in FIG. 9, a pulsed current generator and the charging and discharging circuit shown in FIG. 10 are connected respectively to the workpiece (10) and the forming coil shown in FIG. 9.

First, an apparatus for performing an electromagnetic forming method is prepared. As shown in FIG. 9, the forming coil tis placed on the right position and a certain shape of mold is placed over the forming coil.

Next, the work piece (10) is placed over the forming coil. In another embodiment, an intermediate member of ferromagnetic material may be additionally located between the forming coil and the workpiece (10).

Next, electroplastic effect is obtained by applying a certain amount of pulsed current using a pulsed current generator, for example, as shown in FIG. 10.

As is known, the time necessary for a flow stress of the workpiece to reach down to a certain level varies depending on the kinds of workpieces, intensity of the current that is applied to, a duty ratio of the pulsed current, etc.

Next, in an embodiment of the present invention, a certain level of current is provided to the forming coil to drop a flow stress of the workpiece to a certain level. Then, the electromagnetic forming apparatus controls the charging and discharging circuits so that the timing of providing a certain amount of pulsed current onto the workpiece is synchronized with the timing at which the flow stress of the workpiece is maintained at the reduced level.

FIG. 11 is a schematic illustration explaining an electromagnetic forming process according to an embodiment the present invention. A certain level of electromagnetic force is intermittently and repeatedly applied to the workpiece to form or deform the workpiece in a desired shape.

According to an embodiment of the present invention, the charging and discharging circuits is synchronized with the timing at which the flow stress of the workpiece is at a reduced level. Thus, the workpiece can be more easily formed or deformed.

The present invention using electroplasticity is not limited to a combination of the electroplasticity effect with electromagnetic force. Instead, the electroplasticity effect according to the present invention can be combined with any kind of methods, for example, an explosive forming method that can apply a certain amount of force to a the workpiece. The timing of applying the certain amount of force is synchronized with the timing at which a flow stress of the workpiece is reduced to a certain lower level and maintained at the certain lower level. Thus, the scope of the present invention should not be interpreted to be limited to the combination of the electroplasticity effect and the electromagnetic force.

Claims

1. A method for forming or deforming a workpiece at a high speed using electroplasticity effect, comprising:

applying a first level of pulse current to the work piece to cause the electroplasticity effect and lower a flow stress of the work piece; and

applying a first level of electromagnetic force to the workpiece for 150˜300 μs to form or deform the work piece in a desired shape while the flow stress of the work piece is maintained at the lowered level,

wherein the first level of electromagnetic force is lower than a second level of electromagnetic force which is required to form or deform the workpiece in the desired shape without applying the first level of pulse current.

2. The method of claim 1,

wherein the first level of pulse current varies depending on the workpiece.

3. The method of claim 2,

wherein the flow stress of the workpiece varies depending on the workpiece.

4. The method of claim 1,

wherein the first level of electromagnetic force is Lorentz force.

5. The method of claim 1:

wherein the first level of electromagnetic force is applied using a high-capacity capacitor.

6. The method of claim 1,

wherein the first level of electromagnetic force is generated using a forming coil.

7. The method of claim 6,

wherein the forming coil discharges currents, changes a magnetic flux, and applies an induced electromotive force to the workpiece.

8. The method of claim 7,

wherein the forming coil creates a strong magnetic field, and

wherein the strong magnetic field applies an induced current to the workpiece by Faraday's Law.