‘* ELECTROFORMING

Published by the EPRl Center for Materials Fabrication Vol. 3, No. 5, 1986

taw Coat, High Enorgy M o d s

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echnque has been in popularity in the

ospace, electrical, m n c e , ordnance and nuclear industries since its introduction about 20 years ago.

This issue of Techcommentary describes how this forming method works and when it can be used, and hdps you e.valuate whether it would be a useful tool for your shop.

Advantages and Limitations

tsdrndogies EMF is: Easy bo we. The process is easy to impkmmt and requires no g>ecW operator skill. Noneontact. There is generaUy no nod for lubricants, and usually no

i surface marring, 90 cleaning and past-finishing are rarely necessary. Flexible. A wide range of shapes can be produced. Fast. Energy release to the work coil takes only microseconds and capacitors recharge in a few Seconds. Repeatable. All process variables can be precisely controlled. Amenable to automation. High production rates are possible. Cleanroom compatible. Parts made in an ultraclean or sterile environment can be sealed in a

Canpard with competing

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plastic bag and formed through the bag. Cost effective. The whole process is performed in one step rather than the two or three typical of competing techniques.

In addition. EMF: “. Joins metals to other m i t a p r nonmetals. Since only pne i

component needs to be conductive, metals also can be joined to rubber, glass, ceramics, plastics and fiberglass. Saves dies. In .assembling, the “die” is often an integral part of the product. Other parts may require only half of a matcheddie set-the electromagnetic force forms the other half. Causes minimal tool wear. Only the coils need to be replaced occasionally. Forms most parts cdd. Product handling is simplifii.

Limitations are few. They include: Workpiece must be electrically conductive, and, preferably, have a low electrical resistivity (high conductivity) Process works best with thinner materials Minimum inside tube diameter is about 2.0 inches for expansion applications Wo# coils are relatively expensive High-resistance workpieces usually require drivers, which can be used only once and may have to be removed Number of equipment suppliers is limited.

Applications EMF is widely used in the

automotive, aerospace, appliance, ordnance, electrical and nuclear industries for:

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Electrohydraulic Forming

Electrohydraulic forming (EHr) IS another process in which the sudden release of stored electrical energy shapes a workpiece. In EHF the energy is released as a shock wave or pressure pulse into the fluid filling a hollow workpiece, and this “liquid punch” forces the workpiece to conform to a die cavity.

Equipment required for EHF includes a high voltage power supply for charging a capacitor bank, switching circuits to discharge the capacitors, a mechanism for coupling the electrical charge to the fluid, and tooling to hold the workpiece.

EHF can be used to form almost any metal or alloy, but its most common application is for unusual part shapes not readily formed by other methods. The technique is used mainly in the aerospace industry, where there is a need for a variety of complexly contoured parts. Its use is likely to remain confined there since there are presently no US. equipment suppliers and almost everyone with much knowledge of the technique works in the aircraft industry.

Fastening clamping rings over rubber sleeves on shock absorbers

PI Attaching reinforcing bands on oil filters Assembling aluminum gas tank filler tubesand plastic gas cap holders Clamping steel covers on aluminum automobile fuel pumps Attaching aluminum drive shafts to

Assembling coaxial- cable Yokes

termination joints

Techcommentarylvd. 31No. 5 1

k make, difficult to inspect and unreliable. BY chanaina to EM between the coil and the worbiece. pulse welding, the manufacturer obtained more consistent, rnetallurgically-bonded joints at a lower cost. The time to manufacture and inspect each fuel element was reduced from 40 to 8 hours.

Tochnicsi Considerations

In deciding whether EMF is the appropriate technique for a particular application you need to consider the following: rn Workpiece characteristics rn Work coils rn Die forms

Energy storage and control.

Wor(tpkc. chmcterlstics - For EMF to work well the workpiece must be electrically conductive, continuous, and fairly thin. EMF works best with materials such as copper, aluminum, lowcarbon and 400 series steels, gold, silver and brass which have relatively high elehtrical conductivity. Lower conductivity materials, such as 300 series stainless steels, titanium and nickel-based alloys, can be formed by using a “driver”-a piece of copper or aluminum that is placed

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The magnetic field exerts pressure on the driver which, in turn, forces the workpiece into the desired shape. After forming, the d r h r is sometimes removed discarded.

Workpiece mrynrwd a b uninterrupted current fW. I f , for example, Q tube is t0.b shaped, its wall must be c o n t h w m ; long & a t % would Inhibit current flow and prevent proper forming.

Material thickness is another important characteristic. EMF works best with materials thinner than about ‘/4 in. There is a t raded among conductivity, yield strength and thickness. Since aluminum is more conductive and requires less energy to form than, say, stainless steel, the same strength electEomagnetic pulse will form a thicker piece of aluminum.

Work calk - EMF operations fall into three main categories- comprehion, expansion, and sheet contouring. The operation performed is determined by the coil design and its placement. The three types of coils are shown in Figure 1.

Compmuion cdls enclose a tubular workpiece and deform it radially inward (Figure 1 a). They are generally used for attaching tubing

to grooves of fittin bing Is compresse

to fittings and for clamping components to produce tight seal as in attaching rubber boots to automobile ball joints. Another application is shaping tubular workpieces to fit tapered components. Compression forming is dane on workpie~es ranging from- about 118 in. to 24 in. in diameter.

a tubular workpiice and deform it radialty outward (Figujy 1 b). They are commonly used ta bulge, flange or shape workpiices ranging from about 2 to 72 in. in diameter and up to 48 in. long. Expansion coils are also used to make contoured ducting intersection parts, and to punch holes in tubing. A coil can also be attached to a rigid or flexible rod for insertion into tubular workpieces where only part of the length is to be formed.

Sheet contorrrlt~ coils are flat, and are placed above or below a flat workpiece (Figure IC). These coils are typically used with a die to form, coin, dimple or blank a workpiece as in, for example, faceted lighting reflectors. Recently, flat coils have found a couple of novel uses-as a magnetic hammer for correcting deformations on very large surfaces, such as aircraft wings, and for electromagnetic riveting. Two facing

tpr#bcr odk are placed inside

No matter which type ofcoil is used, the gap between it and the workpiece should be as small as possible to maximize the pressure pulse for a given energy input. For a shop making a range of products, this could mean a range of relatively expensive work coils. Usually, though, large compression coils are adapted to make smaller products by using a field shaper, an electrically conductive insert placed between the Goll and the workpiece.

14 i h l consideration is coil wear.

component of the final part. For example, in Figure 2, the yoke to which the driveshaft is attached serves as the die. If a die is to be reused, it should be designed for the workpiece material and the number of parts to be produced.. For easily formed materials requiring relatively low impact, dies can be made of aluminum, brass or impact-resistant plastics. For harder materials, especialty in larger lot sizes, steel dies are usual.

If a split die is needed to permit part removal, it should fit well at the split line to avoid mismatch defects on the Droduct. To avoid arcing and

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relies on the force generated by a magnetic field to produce the desired shapes in electrically-conductive metal workpieces. Essential components of an EMF system include a conductive coil, called the work coil, a charging and control system, and energy storage capacitors. A typical setup is shown schematically below.

The capacitors are charged from the line voltage supply, then the entire circuit is isolated from the power source. When the forming circuit switch is closed, charge stored in the capacitors flows as a current through

r

I" Schematic diagram of a compression +ration using EMF

strong magnetic field between the coil and the workpiece. This field in turn induces a current in the conductive workpiece and sets up an opposing magnetic field. The interaction between the two magnetic fields creates a magnetic pressure pulse strong enough to force the workpiece '?to a new shape.

The shape created depends on the type and location of the work coil. A tubular coil around the outside of a workpiece will deform it inward. This is the most common application for EMF since it can be used to attach and assemble a wide variety of components. A tubular coil inside a workpiece will bulge or flange it outward. A flat coil is used most frequently for electromagnetic riveting or for removing dents in sheet products.

The strength of the magnetic force depends on the properties of the work coil and the applied current, both of which can be Dreciselv controlled.

.possible burn marks, split dies should, if possible, be made of a nonconducting material such as impact-resistant plastic.

units are classified by the total amount of energy they can store. Commercially available urtifs have capacitor banks rated at 8, 12 and 16 kiloJoules (kJ). Units can be combined to increase system capacity. The correct site unit for a particular application depends on how much energy is required to form each part, the rate at which magnetic pulses are applied (production rate), and how fast the coil and other electronic components can dissipate heat.

consistent product shape, the electromagnetic pulses must be carefully controlled. Pulse magnitude depends on the amount of energy released from the capacitor bank. The voltage charging the capacitors is carefully measured and is turned off as soon as a preset level is reached. The timing of the electromagnetic pulses is also carefully controlled to ensure maximum production rates. Rates of 200-600 partslhour with manual loading, and up to 12,000 partslhour with automatic loading, are possible.

E m St" and COntd - EMF

To maintain close tolerances and

Safety Considerations EMF equipment operates at high

voltage and current, so appropriate caution is needed to avoid accidents. Points at different potentials, such as the coil and the workpiece, must be adequately insulated to prevent shorting and the insulation checked frequently for signs of wear. All coils fail eventually, usually as a result of insulation breakdown. Since failure may be accompanied bBflying debris, a safety shield is used between personnel and 'the work coil.

Installations doing high-impact forming can be noisy and hearing protectors should be used.

Economic Considerations In deciding whether an EMF

installation is economically feasible, both startup and operating costs have to be considered. Startup costs - Included are capital equipment and training expenses. Equipment cost is determined mostly by energy storage capacity. An 8 kJ unit, the smallest available, costs around $35,000. A =.,. jj. 'F

16 kJ unit, the size most frequently used, costs $60,00085,000. EMF does not require particularly skilled operators so training costs are not great.

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joining metal components to other

products, dies or d?ivers if ihese'ar 'rface flaws that*may have to be ,... ,. necessary, and electric power.'EM moved in a later finishing process.

EMF is a one-step, noncontact and usually produces a product orocess, so surface marrinn is ncll a from surface defects, making it i

equipment places no special demands on the power supply. Energy consumption is 1 kWs per kJ capacity. Therefore, assuming 1 kwh costs $0.06, the cost per form in a 16 kJ system is (16 x 0.06)/3600 or $0.00027.

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Compoting Technologies In compression applications, the

mvin competing technologies are -1 swaging, rotary roll-

problem: Furthermore, the magnetic field passes through nonconducting materials so parts can be formed after anodizing or application of other surface finishes, or after they have been sealed in plastic bags in a clean room. Although research is underway to improve the process, EMF does not yet easily handle thick- walled, high-strength materials. For these applications, contact methods are more cost effective.

for final product assembly. The number of EMF machines in use will probably increase quite rapidly due to the increasing popularity of aluminum and nonmetal automobile parts. EHF, which uses a fluid pressure pulse rather than a magnetic force, works well for forming tubular workpieces into complex shapes.

Techcommentary is designed to acquaint you with the capabilities of EMF and EHF. If you think EMF would be an appropriate technology for your applications, you can obtain additional information in the sources used in this issue of TechCom- mentary, and by talking with the equipment manufacturer.

The information in this

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ng. Most of these

m r hr Materials Fabrication (CMF) is Research Institute, a nonprofit institute

M t y industry. The Center's mission is to assist industry in implementing cost- and energy- efficient, electrichased technologies in metals fabrication and related fields. TechCommentary is one communication vehicle that the Center uses to transfer technology to industry. The Center also conducts research in metal heating, metal removal and finishing, and fabrication. TMS h e of Techcommentary was mads possible through the cooperation of Battelle staff members Robert Fbmntlno, Project Manager, Wttalurorking Section, John Hallowell, Industry Applications Engineer, Laura Cahill, Manager, Marketing and

In Summary In electromagnetic forming, a

magnetic field forces elqcfrically conductk workpiices hto desired shapes. It is an excellent technique for forming fairly thin metals, and for

For further information on Center programs, write or call

CENTER FOR MAT- 'yI.IIIyI An EPFIl R6D A p p r i i s Centw 505 King Awnue Cdumbra Ohio 43201-2893 (614) 424-7737

copyright 0 1988 Battelle Memorial Institute Columbus, Ohio