ELECTROHYDRAULIC FORMING AND ELECTROMAGNETIC FORMING

Electrohydraulic Forming and Electromagnetic Forming
Topics covered
1) Introduction
2) Process Principle
3) Constructional details
4) Advantages and Disadvantages
5) Application�

1. ELECTROHYDRAULIC FORMING AND ELECTROMAGNETIC FORMING Submitted by – Sandeep Kashyap (2015PR21 ) M.Tech . (Prod. Engg .) Submitted to – Dr. Avanish Kumar Dubey Mechanical Engg . Deptt . MNNIT Allahabad

2. Overview Electrohydraulic Forming and Electromagnetic Forming Introduction Process Principle Constructional details Advantages and Disadvantages Application

3. Introduction : In these forming processes large amount of energy is applied for a very short interval of time. These processes are useful to deform materials like Titanium and Tungsten alloys, under high strain rates The parts are formed at a rapid rate, and thus these processes are also called High Velocity Forming (HVF) Processes. There are several advantages of using these forming processes, like die costs are low, easy maintenance of tolerances, possibility of forming most metals, and material does not show springback effect. The production cost of components by such processes is low. The limitation of these processes is the need for skilled personnel.

4. ELECTROHYDRAULIC FORMING (Also Known as Electrodischarge or Electroshape or Electrospark Forming)

5. Process Principle : Electro-hydraulic forming tools and processes, produce a shockwave by creating a high voltage discharge in a liquid that is in contact with the sheet metal blank to be formed. The shockwave in the liquid is propagated towards the blank and causes the blank to be deformed into an open die that has a forming surface. The shockwave forces the blank into engagement with the forming surface to form the metal blank into the desired shape.

6. Constructional Details and Working: A typical configuration of EHF includes a discharge chamber, electrodes, forming die, and a pulse generator which consists of a high-voltage low-inductive bank of capacitors C, a high-voltage/high-current discharge switch D, and a charging/amplifying/rectifying circuit is illustrated in Fig 1. The capacitor bank is capable of producing discharges of 5–25 kV and can store energies up to 100 kJ. A sheet metal blank is placed on top of the discharge chamber. A one-sided die is positioned above the blank. After the air is evacuated from both sides of the blank, the chamber is filled with water, fully immersing the electrodes.

7. After the voltage is applied to the electrodes, an electrical breakdown occurs between the exposed tips of the electrodes which leads to the formation of a stable plasma channel. The channel expands quickly resulting in shock waves of pressure which then continuously transform into flow of liquid. The pressure pulse propagates through the water and applies pressure to the surface of the blank forming it into the die cavity.

8. The deformation can be controlled by applying external restraints in the form of die or by varying the amount of energy released, Fig 2.

9. Process Parameters: Stand off distance : It must be optimum. Capacitor used : The energy of the pressure pulse depends on the size of capacitor. Transfer medium : Usually water is used. Vacuum : The die cavity must be evacuated to prevent adiabatic heating of the work due to a sudden compression of air. Materials formed : Materials having low ductility or having critical impact velocity less than 30 m/s are generally not considered to be good candidate for EHF. All materials that can be formed by conventional forming processes can be formed by EHF also. These materials are aluminum alloys, nickel alloys, stainless steels, titanium, and Inconel 718.

10. Advantages and Disadvantages : Advantages : EHF can form hollow shapes with much ease and at less cost compared to other forming techniques. A single step process (rather than progressive stamping) Extremely fast Enables extremely deep forming (much more than is possible with conventional stamping) Fine details and sharp lines can be easily formed Forming of male and female shapes (negative and positive) The process does not depend on the electrical properties of the work material. Safer in handling than the explosive materials.

11. Disadvantages : Suitable only for smaller works. Need for vacuum makes the equipment more complicated. Proper SOD is necessary for effective process. Each discharge of the electrodes to create the high voltage discharge in the fluid results in the formation of impurities in the water that results in vaporization of the electrodes and may create surface defects in the surface of the part formed in the process. The fluid in the vessel generally must be drained and replaced for each tool cycle. The volume of fluid in the vessel for a larger part tends to be fairly substantial and a considerable portion of the cycle time of the tool is dedicated to draining and refilling the vessel. Advantages and Disadvantages :

12. It include smaller radar dish, cone and other shapes in thinner and small works, Reduction of capital investment for low volume aerospace applications, In Automobile sector such as inside components of a passenger car door, Miniature and fancy equipments having complicated profile for electronic industry, etc. Applications :

13. ELECTROMAGNETIC FORMING (Also Known as Magnetic Pulse Forming)

14. This process is based on the principle stated by lorentz force law that the electromagnetic field of an induced current always opposes the electromagnetic field of the inducing current. In this method a large capacitor bank is discharged producing a current charge through a coiled conductor. If the coil has been placed within a conductive cylinder, around a cylinder or adjacent to a flat sheet of metal, then the discharge induces a secondary current in the workpiece. This secondary current further causes it to be repelled from the coil and conformed to a die or mating work piece. The process is very rapid and is used primarily to expand or contract tubing or to permanently assemble component parts Process Principle :

15. Constructional Details and Working: The setup of EMF consists of the pulsed power generator, the inductor including a fieldshaper , if applicable, the workpiece and application-dependent further tool components such as form-defining dies etc. The process is started by charging and subsequently discharging the capacitor of the pulsed power generator. A sinusoidal current flows through the inductor. This current induces a corresponding magnetic field. If there is an electrically conductive workpiece in direct proximity to inductor, a second opposedly directed current is induced. The energy density stored in the magnetic field between workpiece and inductor acts as magnetic pressure which can reach several hundreds of megapascal and causes the acceleration and deformation of the workpiece. The direction of the movement is always targeted away from the inductor

16. Depending on the geometry and the alignment of tool and workpiece, three process variants can be distinguished. These are : ( i ) electromagnetic compression of tubes and hollow profiles by means of an inductor enclosing the workpiece, (ii) electromagnetic expansion of tubes and hollow profiles by means of an inductor positioned within the workpiece,

17. (iii) electromagnetic sheet forming, for which an inductor is positioned in close proximity of a flat semi-finished part or a preformed component.

18. Workpiece thickness – A higher thickness means that the magnetic field diffuses slower through the workpiece wall. Electrical conductivity – The higher the electrical conductivity of the workpiece, the better the shielding of the magnetic field, the pressure difference in higher. Frequency – A higher frequency of the discharged current can balance a low conductivity or a small wall thickness. Size of the capacitor bank The strength of the current, which decides the strength of the magnetic field and the force applied. Gap between workpiece and tool coil – The smaller the air gap, higher is magnetic field and pressure. Winding of the tool coil – For each pulse generator and each forming task exists an optimum of number of turns. Process Parameters:

19. Advantages and Disadvantages : Advantages : Suitable for small tubes Operations like collapsing, bending and crimping can be easily done. Electrical energy applied can be precisely controlled and hence the process is accurately controlled. The process is safer compared to explosive forming. Wide range of applications.

20. Disadvantages : Applicable only for electrically conducting materials. Not suitable for large work pieces. Rigid clamping of primary coil is critical. Shorter life of the coil due to large forces acting on it. Advantages and Disadvantages :

21. Electromagnetic forming process is capable of a wide variety of forming and assembly operations. Crimping of coils, tubes, wires Bending of tubes into complex shapes Bulging of thin tubes. It has found extensive applications in the fabrication of hollow, non – circular, or asymmetrical shapes from tubular stock. Flat coils have been used on flat sheets to produce stretch (internal) and shrink (external) flanges on ring and disc – shaped work pieces. Electromagnetic forming has also been used to perform shearing, piercing, and rivettting . Applications :

22. References : R.K.Rajawat , “ Electromagnetic Forming – A Technique with Potential Applications in Accelerator” Proceedings of APAC 2004, Gyeongju , Korea. Dhiraj Gayakwad , “ A Review on Electromagnetic Forming Process” 3rd International Conference on Materials Processing and Characterisation (ICMPC 2014) Sergey Golovashchenko , Beverly Hills, MI (US), “Electro-Hydraulic Forming tool having Two Liquid Volumes Separated by a Membrane” Patent Application Publication, Pub. No.: US 2008/0134741 A1 J. Varis , H. Martikka , “Prototyping of 3D sheet metal parts using electro hydraulic forming” ISSN 1392 - 1207. MECHANIKA. 2005. Nr.3(53) NPTEL “http://nptel.ac.in/courses/112107144/Metal%20Forming%20&%20Powder%20metallurgy/lecture9/lecture9.htm”

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