Lesson 4.5DeformChapter 4 · Sheet and tube forming

Hydroforming

Fluid pressure pushes tube or sheet into a die

Hydroforming uses high-pressure fluid, usually water, instead of a solid punch to push metal into the shape of a die. A tube is blown out like a balloon inside a closed die, or a sheet is pressed against a single die half. Car frame rails, aluminum bicycle frames and saxophone tubes are made this way, light and stiff in one piece.

tube hydroforming · 3 stagesThe three panels follow one tube through the cycle, cut along its length; circled numbers match the steps below.

How it works

Load. A tube, often pre-bent to the rough path of the part, goes into the lower half of a split die (a two-part steel mold).
Close and seal. A press closes the upper die and holds it shut with great force. End punches (plugs pushed in from each side) seal both tube ends.
Fill. Water, usually with additives against rust, fills the tube through one punch and drives the air out.
Pressurize and feed. The pressure rises, up to a few thousand bar, and the wall bulges out until it fills the cavity. Meanwhile the end punches push the tube ends inward, feeding in metal so the wall does not thin too much; then the pressure drops and the die opens.

Sheet hydroforming works the same way with a flat sheet: fluid, often behind a rubber membrane, presses the sheet onto one solid die half.

Key facts

Materials: Ductile steels, including high-strength grades, stainless steel, aluminum, brass and copper, as tubes or sheets
Typical parts: Car frame rails, engine cradles and exhaust parts, bicycle frame tubes, pipe T-joints; sheet panels and dishes
Part size: Tubes up to a few meters long, such as car frame rails; sheet-formed satellite dishes up to about 6 m across
Precision: Typically within a few tenths of a millimeter, because the part is pressed against a solid die
Surface finish: The outside copies the die; no tool marks on the inside
Batch size it suits: Mostly high volume (cars, bikes); sheet versions also suit small batches
Tooling and machine cost: Dies cost tens to hundreds of thousands of dollars; a large press with its high-pressure system is typically a multi-million-dollar investment
Speed: Typically from about 20 seconds to a minute per part, slower than stamping

Strengths and limits

Strengths

Complex hollow shapes in one piece, replacing welded assemblies
Lighter, stiffer parts with fewer welds
Even wall thickness and extra strength from work hardening
Sheet hydroforming needs only one solid die half

Limits

Longer cycle times than stamping
High-pressure equipment and dies are expensive
The tube bursts if it is stretched too far, so expansion is limited
Tubes often need bending and pre-forming first

Where it's used

AutomotiveFrame rails, engine cradles, roof rails, exhaust parts
Bicycles and mobilityAluminum bike frame tubes, wheelchair frames
MusicThe brass body tube of some saxophones
Oil and gasLarge T-shaped pipe joints
Aerospace and lightingSheet panels, satellite antennas, lighting housings

How to recognize it

Hollow parts whose cross-section changes along their length, from round to square to oval, with no extra seams.
One-piece frame members where you would expect several stamped halves welded together.
Smooth bulges and crisp shaped profiles on aluminum bike tubes.