Explosion welding (cladding)

Solid state: a blast slams one plate onto another

Explosion welding bonds a thin plate of one metal onto a thick plate of another by slamming them together with a controlled blast. The impact is so fast and violent that the clean surfaces join solidly without melting. It makes cladded plates (a cheap structural metal covered with a costly corrosion-resistant one), such as titanium on steel, that no other welding method can join.

Photo: Public domain, via Wikimedia Commons

Explosion welding · side view in the middle of the blastCircled numbers match the steps below; the drawing freezes the blast halfway, with the bonded part on the left.

How it works

Stack the plates. The thick base plate lies on a solid bed of sand or steel. The thinner cladding plate sits above it on small spacers, leaving a gap of typically a few millimeters.
Spread the explosive. An even layer of a slow, granular explosive, often a few centimeters thick, covers the cladding plate.
Detonate at one end. The detonation front sweeps across the plate at typically about 2,000–3,000 meters per second.
Fling the cladding down. Behind the front, the cladding plate bends and hits the base plate at several hundred meters per second, at a small angle.
Scrub with a jet. At the moving collision point, the enormous pressure squirts a thin jet of surface metal forward. It strips away oxides and dirt from both plates.
Bond without melting. The freshly cleaned surfaces are pressed together so hard that their atoms bond, leaving a typical wavy interface. The whole weld takes about a thousandth of a second.

Why it is special. Metals such as titanium and steel form brittle compounds when melted together, so fusion welding them fails. Because explosion welding never melts them, it joins pairs that nothing else can, over areas of many square meters at once.

Key facts

Materials: Almost any pair of ductile metals: steel with stainless steel, titanium, zirconium, nickel alloys, copper or aluminum
Typical parts: Clad plates for pressure vessels, heat-exchanger tube sheets, aluminum-to-steel transition joints, electrical connectors for smelters
Part size: Cladding typically a few millimeters to a few centimeters thick; plates up to several meters across
Precision: Low: plates are flattened, cut and machined after bonding
Surface finish: Rough and marked by the blast; the outer face is ground or polished afterward
Batch size it suits: One-off plates and small batches made to order
Tooling and machine cost: Little machinery, but a licensed remote blast site, explosives and specialist staff; each plate costs thousands of dollars or more
Speed: The weld takes about a millisecond; preparing and finishing a plate takes hours to days

Strengths and limits

Strengths

Joins metal pairs that cannot be fusion welded
Bonds very large areas in one shot
No heat-affected zone: each metal keeps its properties
Bond is often stronger than the weaker of the two metals

Limits

Needs explosives, a remote site and strict safety rules
Mostly limited to flat plates and simple tubes
Noise and vibration restrict where and when it can be done
Brittle metals crack under the impact

Where it's used

Chemicals and oilReactor vessels and columns with titanium or zirconium linings
Power and desalinationHeat-exchanger tube sheets in condensers
ShipbuildingAluminum-to-steel transition strips for joining aluminum decks to steel hulls
Aluminum smeltingAluminum-to-steel electrical connectors
ArtSculptures made by blasting layered metal sheets

How to recognize it

A plate with two different metals bonded face to face, with a perfectly sharp boundary on the cut edge.
Under a magnifier, that boundary is wavy, like a frozen ripple, instead of straight.
No weld beads anywhere: the joint is the whole face of the plate.

Compare with

Friction stir welding7.6also solid-state, but joins seams along an edge rather than whole faces.Brazing and soldering7.3can also join dissimilar metals, but with a weaker filler layer and only small joints.Explosive forming4.9uses the same kind of blast to shape a single sheet instead of joining two.

Watch it

NobelClad · What explosion welding is and why it bonds

NobelClad · Plate preparation, standoff, explosive layer and detonation, step by step

NobelClad · The industrial process from start to finished plate

Sichuan Jinglei Science and Technology Co., Ltd. · A real outdoor blast cladding a large plate

See real photos

Wikipedia: Explosion welding — history, materials and limits; few photos
Wikimedia Commons photo search

Remember

A blast flings one plate onto another so fast that their clean surfaces bond without melting.
A jet of metal at the collision point scrubs both surfaces clean just before they touch.
It is the go-to method for cladding: joining costly corrosion-resistant metals onto cheap steel over large areas.