Electrohydraulic Formingof Sheet Metal

Rachel SharpCorinne Packard

Isaac FeitlerHao Hu

Third Update

March 6, 2003

Today

Effects of high-velocity metalworking

Update on progress

Design alterations

Next steps

High-Velocity Metalworking

Includes electrohydraulic, electromagnetic, explosive, and pneumatic-mechanical processes

Deformation velocities 100-1000 fpsHydraulic press- 0.2 fpsMechanical press- up to 5 fpsDrop hammer- 1-20 fps

Strain rate and fracture initiation

Low ε-rateNecking in a small areaLittle plastic deformation in bulk

Localized thinning leads to fracture

High strain rate

Necking initiates and area strain-hardens before localized thinning occurs Velocity varies linearly, until necking Velocity gradient forms Non-uniform inertial forces produce tensile

stresses in material outside of neck

Energy is distributed over more of the sample, increasing ductility

Advantages of Electrohydraulic Forming

Over low ε-rate pressing-Some metals can only be formed at high ratesOnly one die neededTubular designs possibleCloser thickness tolerances can be achieved

Over explosive forming-Much slower production rates with explosives

Design Decisions

Cast Metal Dies

Epoxy encased in steel pipe

Rubber flanges

Copper electrodes

Steel plate

Pressure Vessel Design

Hemisphere chosen over cylinder to reduce stress concentrations

To form a hemisphere, embed a spherical glass ornament in epoxy halfway

-Glass can be broken out after cure

-Low cost & readily available

-Temperature not a concern

ConsiderationsEpoxy was chosen for ease of casting, electrical insulation, fracture resistance2-3” of high-strength epoxy needed for

safetyCuring issues —limited suppliers

Cast into steel pipe with 10” inside diameter to convert from tension to compression on the epoxyPipe scrap needed, found in Southampton

Calculations

32

31

31

32

max 2

)(2

rr

rrP y

r

tP y2

Thick-walled sphere

For deforming a workpiece:

y Aluminum=72.5ksi

Considerations

Viton rubber sheet chosen to seal between pressure vessel, workpiece, and die

Cast metal for dies—easier and less expensive than machining entire parts

Copper electrodes—high conductivity and availability

Steel plate—strength and resistance to bending

Die progress

CAD drawings of hemispherical, conical, and automobile mirror shapes

Capacitor bank update

Magnaform electromagnetic former found in Watertown over reuse@mit.edu• Contains a 6kJ capacitor bank that will interface easily with our system• Working out transportation to borrow the ~1 ton machine for the remainder of our project• Contingency remains 1kJ Boomer from Edgerton Center

Progress

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Week

Bre

ak

Pressure Vessel Capacitor Bank Mold Electrohydraulic forming Final Presentation

Vessel design and parts acquisition

Capacitor bank acquisition (at MIT, outside of MIT if needed)

Pressure Vessel assembly

CAD & 3D printing of mold

Casting of mold

Electrohydraulic test

Funnel formation

Final part formation

Presentation preparation

Obstacles

Epoxy is a special formulation and will take 6wks not feasible

Other options investigated and debated Plaster compounds or cast metal Bored steel

Central Machine Shop can have a steel billet bored by the end of next week They have stock on hand, but a donation from

Ohio may be possible

Revised Gantt Chart

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Week

Bre

ak

Pressure Vessel Capacitor Bank Mold Electrohydraulic forming Final Presentation

Vessel design and parts acquisition

Capacitor bank acquisition

Pressure Vessel assembly

CAD

Casting of mold

Electrohydraulic test

Funnel formation

Final part formation

Presentation preparation

Despite setbacks…

Design changes increase safety—pressure vessel will now withstand 18,800psi !Outsourcing pressure vessel frees up laboratory group to focus on die fabrication and other assemblyDecreased time with an apparatus will limit experimental work, but excess time was included in original timeline

Next Steps

Finish calculations for clamping safetyMax. force~170,000lbs. Investigating

bolts, clamps, and hydraulic presses

Investigate casting and cast dies

Transport Magnaform

Assemble apparatus