Joining steel hull and aluminum superstructure in the yachts construction
by Bering Yachts 27 Jan 2021 07:46 PST
Joining steel hull and aluminum superstructure in the yachts construction © Bering Yachts
It is widely known that both steel and aluminum possesses certain characteristics that are extremely demanded in the marine industry. But from the very beginning of marine usage of aluminum alloys, the question was how to join two such different metals so the advantages of both can be utilized to the maximum without endangering the whole structure?
Traditional methods of joining, like riveting and bolted joints have fallen in favor due to the fact that in a few years considerable corrosion can occur aided by capillary action caused mainly by the widely differing thermal expansion coefficients of aluminum and steel. In spite of efforts to prevent it, this phenomenon allows seawater to seep into the dissimilar metal joint, thereby resulting in severe corrosion. In many cases, the only way to maintain the ship in a seaworthy condition is to completely replace the aluminum-steel transition or, in some cases, to replace the complete superstructure.
Recognizing the need for a marine industry application, Du Pont started development in 1966 of a clad sheet that would facilitate attaching the aluminum superstructure to the steel deck. The resulting bonded sheet is a Triclad employing aluminum alloy 5ì56 bonded to A5l6 Grade 55 steel with a commercially pure 1100-series aluminum interface between the structural materials.
Explosion bonding explained
As the process uses a large quantity of explosives, the actual cladding operation is generally carried out in a remote place. The two (or more) metals to be joined are first prepared for cladding. The faces are cleaned and the plates set up one above the other with a predetermined gap.
Generally, the thinner metal (the cladder) is uppermost and slightly larger than the base metal. The overhang is again determined by the thicknesses of the metals. As the gap is critical, it is important that the plates are flat (generally better than 3mm/m). The plates are placed on a firm sand base, such that they are evenly supported, care being taken to ensure that no foreign material enters the gap.
A frame is positioned around the periphery of the cladder, the depth of which is designed to ensure that the quantity of explosive (the loading) per unit area is consistent with the loading prescribed for the metal combination and the cladder thickness. The explosive loading ensures that the cladder is accelerated to the optimum speed for bonding, and the velocity of the explosion front across the plate ensures that the angle between the cladder and the base is optimum for the metals.
The extremely high pressure generated at the point where the metals initially meet vaporizes the surface contaminants (oxides) which are ejected, thus producing the molecular bond between the two virgin surfaces. The layer of metal removed is only microns thick. Some localized work hardening occurs, but in general, the properties of the two metals remain unchanged. The metal temperature after cladding is such that you can place your hand on the surface. In most cases, the interface is slightly wavy. Whilst in most cases, the bond between the two metals is consistent, there are some, where the metal structures differ considerably. The bond quality can then be improved by using an interlayer.
What is Triclad
One such case is between aluminum alloy and steel, where the interlayer generally used is commercially pure aluminum. This is the origin of the name Triclad. For specialist applications titanium may be used. Each plate is ultrasonically examined for defects and leveled as appropriate for its duty.
Triclad is the trademark of Merrem & la Porte aluminum/steel structural transition joint material. Merrem and la Porte's involvement in the marketing of al/st transition joints stems from having been originally appointed as agents in the Benelux countries for explosion bonded plates manufactured by Nobelclad, France subsidiary of Dynamic Materials Corporation (DMC). When it was perceived that the Netherlands should be a significant market for STJ's, Merrem & la Porte was asked to trial market the product. The sales were so successful that it was a logical step to source STJ's for world markets. Merrem & la Porte has since been the global and exclusive outlet for DMC produced STJ's.
As indicated, Triclad is a special clad, designed generally to facilitate the joining of marine-grade aluminum structures to steel structures. It is produced as a standard-sized "parent plate" of 1.5 x 4 m, with a useable area of 1300 x 3800 mm from which strips or other shapes can be cut. The metal grades chosen are designed to be compatible with the commonly used marine grades of aluminum and steel.
DMC has standardized on the following grades and nominal metal thicknesses:
- ASTM A516 Gr.55 / Aluminum 1050A/Aluminum 5086 (or 5083). Thickness: 19 + 9.5 + 6 mm Shear strength: min 60 MPa / typical 94 MPa. Tensile strength: min 76 MPa / typical 126 MPa
- Shipbuilding steel Gr.D / Aluminum 1050A /Aluminum 5083 Thickness: 15 + 3 + 10 mm and 20 + 3 + 10 mm. Shear strength: min 70 MPa / typical 94 MPa. Tensile strength: min 80 MPa / typical 181 MPa
- ASTM A516 Gr.55 / Aluminum 1050A /Aluminum 5083 Thickness: 10 + 5 + 4 mm. Shear strength: min 60 MPa / typical 94 MPa. Tensile strength: min 76 MPa / typical 126 MPa.
Mechanical properties of Triclad
The MIL-J-24445A specification, which Triclad conforms to, calls for the mentioned minimum properties in both the as-clad condition as well as after a simulated welding cycle (heat treatment 15 minutes, 315 degreesC + Air cool). DMC releases parent plates on the basis of these criteria.
However, typical values for Triclad are considerably higher. After simulated welding cycle:
- Through thickness tensile strength 120 MPa
- Bond shear strength 88 MPa.
First article testing for compliance with the MIL-J 24445A specification also included axial fatigue strength testing as well as tensile strength determination on welded specimens. Again all criteria were well met. Rigorous quality procedures and regular authority verification have earned Triclad product/use approval from the following authorities:
- Lloyd's Register of Shipping (LRS)
- Det Norske Veritas (DNV)
- Bureau Veritas (BV)
- American Bureau of Shipping (ABS)
Corrosion resistance
Considering the lower galvanic potential of the steel, extreme corrosion of the aluminum may be anticipated; particularly near the interface. This is the area where the metal has been heavily worked and the anode is in close proximity.
Initial corrosion tests on unpainted samples of approximately equal aluminum to steel areas revealed however a natural insulating effect. As expected, slight penetration began at the interface as the aluminum started to corrode. But, instead of acting as a latent area of high ion concentration and thereby accelerating corrosion, the penetration area gradually filled with an extremely hard and inert corrosion product, aluminum oxide hydrate.
The oxide acted as a seal and rendered the system passive after only a minor penetration; the exact level dependent upon the severity of the initial corrosive environment. Accelerated salt-spray tests, simulating years of exposure, further demonstrated that corrosion became negligible after the initial barrier had been built up. Painted samples, whose interface had been scratched so as to expose only a small area, were subjected to the same testing environments. With these, the only interface corrosion was a slight pinpoint area beneath the scratch. The solid metallurgical bond restricted the electrolyte from penetrating the interface, while the buildup of corrosion product prevented extensive pitting.
Conclusion
Triclad is not unique specifically designed for shipbuilding although this segment is mainstream. The rails production, automotive industry, oil and gas industry, electrochemical processing, energy sector and many more utilize the benefits of different metal clad. This is why the quality control and welding procedures, developed by the Triclad producers, are important to adhere to strictly. In that case, Triclad will serve almost carefree for long years.