MIL-J-24445A is the US military specification for bimetallic bonded aluminium-to-steel joints – the transition joints that let shipbuilders weld an aluminium superstructure to a steel deck. Its full title is “Joint, Bimetallic Bonded, Aluminum to Steel”; revision A was issued by the Naval Sea Systems Command in 1977. Although written for the US Navy, it has become the reference specification for structural transition joints worldwide, and shipyards and classification societies routinely ask for it.
What the specification covers
MIL-J-24445A applies to aluminium-to-steel transition joints made by explosion bonding or roll bonding. The joints are cut from bonded plate and can be supplied as bars, strips, discs, rings or rectangles. Joints made by welding or brazing are excluded – aluminium cannot be fusion welded to steel because the two metals form brittle intermetallic compounds. An intermediate layer of aluminium (for example pure aluminium between the steel and the 5083 alloy) is allowed when the bonding process needs it.
The intended use is welded structures where steel must be joined to aluminium, such as an aluminium deck house on a steel deck. The specification itself notes that the joints are for atmospheric and dry structural service and must be coated to prevent galvanic corrosion – see galvanic corrosion between aluminium and steel.
Mechanical requirements
| Property | Requirement | Test |
|---|---|---|
| Bond tensile strength | ≥ 11,000 psi (75.8 MPa) | Ram tensile test, as clad and after the simulated welding cycle |
| Bond shear strength | ≥ 8,000 psi (55.2 MPa) | Triple-lug shear test, as clad (and after the welding cycle for first-article testing) |
| Simulated welding cycle | 315 °C ± 14 °C (600 °F ± 25 °F) for 15 minutes | Applied before re-testing tensile (and shear) strength |
| Bend test | No separation of the bond | Bend test, or triple-lug shear plus chisel test at the manufacturer’s option |
| Ultrasonic inspection | No complete loss of back reflection at the bond | 100% of the bond area of every joint, continuous scanning |
The simulated welding cycle matters in practice: it proves that the bond keeps its strength when the joint heats up during welding to the deck and the superstructure. This is why fabricators are told to keep the interface below the limit stated by the joint manufacturer during welding.
Production lots and testing
- Lot size: a lot is at most 10 bonded plates made at one time under one set of conditions. For explosion bonding, changing plate heats, plate thickness, charge size or type, stand-off distance, cleaning procedure or assembly size starts a new lot.
- Sampling: two sample pieces from diagonally opposite corners of one plate per lot, each giving one tensile and one bend specimen.
- Retests: a rejected lot can be reinstated plate by plate with two bend and two tensile tests per plate; a plate whose specimen fails cannot be retested.
- Ultrasonic testing: every joint, 100% of the bond area; all defective areas are removed.
First-article qualification
Before supplying to the specification, a manufacturer has to prove its process with first-article tests on joints made exactly like production joints:
- Bond tensile and shear tests, as clad and after the simulated welding cycle.
- Axial fatigue tests from compression to tension, up to 1,000,000 cycles, in which any failure in the bond means rejection.
- Welded tensile tests in which the aluminium web is welded on with gas metal arc welding: the joint must fail in the web, or above the load at which the web should fail.
Dimensions and flatness
Bars may not be out of flat by more than 1/16 inch (1.6 mm) over any 12-inch (305 mm) length, and edges must be straight within 1/8 inch (3.2 mm) per 12 inches. These tolerances keep the joint easy to fit along long superstructure seams.
How Triplate® meets MIL-J-24445A
Triplate® is Shockwave’s structural transition joint: shipbuilding steel (grade A, D or DH36), pure aluminium 1050A and seawater-resistant aluminium 5083, explosion welded inside a vacuum chamber. It meets MIL-J-24445A and is approved by Lloyd’s Register, DNV and ABS.
| Property | MIL-J-24445A | Triplate® |
|---|---|---|
| Through-thickness tensile strength | ≥ 75.8 MPa | > 75 N/mm² |
| Shear strength | ≥ 55.2 MPa | > 55 N/mm² (base–interlayer) |
| Bend test | Pass | Pass (tension and compression) |
| Ultrasonic / interface | No loss of back reflection | 100% dense interface, verified by dye-penetrant testing |
| Maximum processing temperature | Bond tested after 315 °C / 15 min | 315 °C |
Vacuum cladding removes the air between the plates before bonding, so the interface is free of oxides and porosity. Shipyards in Korea, the Netherlands, France, Germany, Finland, Italy, Norway, the Philippines and South America use Triplate®, and the Queen Mary 2 alone contains more than 3 km of Triplate® strip. Read more on how to weld aluminium to steel.
Ordering to MIL-J-24445A
The specification asks the buyer to state on the order: the specification number and date, whether first-article testing is required, the base material specifications for the steel and aluminium (and interlayer), the type and thickness of the members that will be welded to the joint, and any packing and marking requirements. Send these details through our contact page and we will quote Triplate® strip, plate or water-jet-cut profiles.
Frequently asked questions
What is MIL-J-24445A?
The US military specification “Joint, Bimetallic Bonded, Aluminum to Steel” (revision A, 1977). It covers explosion-bonded and roll-bonded transition joints used to weld aluminium structures to steel, such as an aluminium deck house on a steel deck.
What strength does MIL-J-24445A require?
A bond tensile strength of at least 11,000 psi (75.8 MPa) and a bond shear strength of at least 8,000 psi (55.2 MPa), also after a simulated welding cycle of 600 °F (315 °C) for 15 minutes.
Does MIL-J-24445A require ultrasonic testing?
Yes. 100% of the bond area of every joint is ultrasonically inspected, and any complete loss of back reflection at the bond is cause for rejection.
Is MIL-J-24445A only for US Navy ships?
No. It was written for the US Navy, but shipyards and classification societies worldwide use it as the reference for aluminium-to-steel structural transition joints.
Does Triplate® meet MIL-J-24445A?
Yes. Triplate® meets MIL-J-24445A and is approved by Lloyd’s Register, DNV and ABS.
Written by Joeri Hofstede, CEO of Shockwave Metalworking Technologies — vacuum explosion welding specialists since 1985.