Transition Joints for Dissimilar Metals: Explosion-Bonded, Roll-Bonded, and Friction-Welded Inserts

Some metal combinations cannot be fusion welded. Aluminum to steel, aluminum to copper, and titanium to steel all form brittle intermetallic compounds when melted together. Bimetallic transition inserts solve this problem by creating a pre-bonded junction between the two metals through a solid-state process that keeps the interface below melting temperature. You then weld each side of the insert to its matching structure using standard fusion welding procedures.

Three manufacturing processes produce transition inserts: explosion bonding (most common for structural applications), roll bonding (for clad plate and sheet), and friction welding (for round and tubular shapes). Each process creates a different type of bond with different capabilities and limitations.

How Transition Inserts Work

The transition insert is a factory-made strip, plate, or tubular piece with Metal A on one side and Metal B on the other. The bond between A and B was created at temperatures below the melting point of either metal, so no intermetallic compounds formed (or the intermetallic layer is thin enough to be non-critical, typically under 10 microns).

In the field, you make two separate welds:

1. Weld the Metal A side of the insert to your Metal A structure using the standard welding procedure for Metal A.
2. Weld the Metal B side of the insert to your Metal B structure using the standard welding procedure for Metal B.

The pre-bonded interface handles the transition between unlike metals. Each weld is a standard same-metal joint with no dissimilar metal issues.

Explosion Bonding

Explosion bonding (explosive welding, explosive cladding) uses a controlled detonation to accelerate a “flyer” plate into a “base” plate at velocities of 650-1,600 ft/s. The collision creates enormous pressure at the interface (millions of psi), producing a metallurgical bond through plastic deformation and localized adiabatic heating.

The Bond Interface

The interface has a characteristic wavy pattern visible in cross-section. This wave pattern:

• Mechanically interlocks the two metals, like a sinusoidal dovetail joint
• Increases bond area by 20-50% over a flat interface
• Contains a thin diffusion layer (typically 1-5 microns) where atoms from each metal have interdiffused

The intermetallic compound layer at the interface is discontinuous and extremely thin (under 10 microns for properly bonded material). Contrast this with fusion welding, where the intermetallic layer is continuous and can be hundreds of microns thick. The thin, discontinuous nature of the explosion-bonded intermetallic layer prevents brittle behavior.

Bond Strength

In most properly bonded combinations, the bond is stronger than the weaker parent metal. A tensile test on an aluminum/steel explosion bond typically fails in the aluminum (the weaker material), not at the bond interface.

Common Configurations

Explosion-bonded transition inserts come in several standard configurations:

Welding Rules for Explosion-Bonded Inserts

These rules are critical and non-negotiable:

1. Maximum temperature at the bond interface: 600F. Exceeding this causes intermetallic compound growth that weakens the bond. Calculate heat transmission from your weld location to the bond line. Use thermal modeling or conservative distance rules.

2. Minimum weld distance from bond line: 1/2 inch. Most specifications require that no weld (including tack welds) be placed closer than 1/2 inch to the visible bond line.

3. Weld the aluminum side first. Aluminum welding produces lower heat input than steel welding. By welding the aluminum side first, you minimize the heat that reaches the bond interface.

4. No preheat on the transition insert. Preheat raises the temperature of the entire insert, including the bond interface. Weld without preheat and use higher amperage if needed for fusion.

5. No torch cutting through the bond interface. Cut transition inserts with a saw, waterjet, or cold-cutting method. Flame cutting through the bond zone destroys it.

6. Inspect before welding. Check the insert for delamination (visible separation at the bond line edges) before installation. Ultrasonic testing per ASTM A578 or equivalent confirms bond integrity on critical applications.

Roll Bonding

Roll bonding creates clad plate by passing two stacked plates through heavy rolling mills at elevated temperature. The combination of heat and pressure creates a metallurgical bond across the entire interface.

Process

1. Clean and prepare the mating surfaces of both plates.
2. Stack the plates with a clean separator (flux or vacuum atmosphere) to prevent oxidation.
3. Roll the assembly through multiple passes at temperatures below the melting point of either metal.
4. The severe plastic deformation breaks up surface oxides and creates fresh-metal contact across the interface.

Characteristics

Roll-bonded clad plate is common for pressure vessels: a carbon steel shell for structural strength with a stainless steel or nickel alloy lining for corrosion resistance. The entire vessel is made from clad plate, not just the transition zone.

For transition insert applications, roll-bonded material is cut into strips and used the same way as explosion-bonded inserts. The same welding rules apply: keep heat away from the bond interface.

Friction Welding

Friction welding produces transition joints between round bars, tubes, and other rotationally symmetric parts. One piece rotates at high speed while pressed against the other. Friction generates heat at the interface, plasticizing the metal without melting it. When rotation stops, forge pressure completes the bond.

Process Variants

Rotary friction welding (RFW): One piece rotates, the other is stationary. Standard for bar stock and tubular transitions.

Friction stir welding (FSW): A rotating tool plunges into the joint line and plasticizes the surrounding material. Works on flat plate and sheet. FSW can join aluminum to steel in butt or lap configurations, producing a narrow (under 5 microns) intermetallic layer.

Linear friction welding (LFW): One piece oscillates linearly against the other. Used for non-round cross-sections (turbine blades, structural members).

Applications

Friction-welded transition pieces are typically machined after bonding to remove the flash (upset material) at the interface. The finished piece looks like a solid bar or tube with a visible bond line at the transition point.

Shipbuilding: The Flagship Application

The largest user of bimetallic transition inserts is the shipbuilding industry. Modern warships and fast ferries use aluminum superstructures on steel hulls to reduce topside weight and improve stability. The aluminum-to-steel connection at the deck edge uses explosion-bonded 5083/A516 transition strips.

Installation procedure (typical):

1. Transition strip arrives from the manufacturer as a flat bar, typically 3 inches wide with 1/4 inch aluminum on top and 3/8 inch steel on the bottom.
2. Cut strips to length with a cold saw or waterjet. No flame cutting.
3. Position the strip along the deck edge with the steel side facing the steel hull and the aluminum side facing the superstructure.
4. Tack and weld the aluminum side first. TIG or MIG with ER5356 filler, per the aluminum WPS. Keep welds at least 1/2 inch from the bond line.
5. After the aluminum side is complete and cooled, tack and weld the steel side. MIG or stick with the appropriate steel filler.
6. Inspect both weld lines by visual and NDT per the classification society requirements (ABS, DNV-GL, Lloyd’s, Bureau Veritas).

This connection method has been in service since the 1960s and has a proven track record on thousands of vessels worldwide.

Quality Assurance

Transition inserts are inspected before installation:

• Ultrasonic testing (UT): Scans the bond interface for delaminations. ASTM A578 or equivalent for clad plate, manufacturer specification for strip inserts. Minimum 95% bond area is typical.
• Shear testing: Destructive test on sample coupons from the same production lot. Per ASTM A264 or manufacturer specification.
• Bend testing: Sample coupons bent to 180 degrees without separation at the bond interface.
• Metallographic examination: Cross-section sample showing the bond interface, intermetallic layer thickness, and wave pattern (for explosion-bonded).

Cost and Availability

Explosion-bonded transition inserts are a specialty product. Lead times range from 4-12 weeks depending on the combination, size, and quantity. Costs vary widely:

• Simple aluminum/steel strips: $15-40 per linear foot
• Large custom plates: $50-200 per square foot
• Tubular or ring transitions: $200-2,000+ per piece depending on diameter and wall thickness

For small quantities, some distributors stock common configurations (5083/A516 ship strips, 304L/A516 clad plate). Custom combinations require a quote from a manufacturer.

For the overview of why dissimilar joints fail and how to select filler metals, see the dissimilar metal welding guide. For the buttering technique used to create buffer layers on weldable dissimilar combinations, see the buttering guide.

Back to the dissimilar metals welding category.