New Paths in Shipbuilding: Friction Stir Welding with Aluminium

For thousands of years, the sea has proven a formidable challenge to the construction of efficient and reliable means of transportation. Even today, men and vessels alike are exposed to considerable strain. Depending on the specific mission requirements, the fashion in which ships are designed, what materials are made use of, and how they are processed, may vary. The decisive parameters are, essentially, deadweight, maximum tonnage limit and selected speed, further factors being maintenance cost and fire resistance.

Steel has been the dominant material for the construction of ships due to its excellent strength and toughness as well as its cheap manufacturing and processing costs. In 1895 aluminium was made use of by Yarrow & Co. Shipbuilders for the first time. Application on an industrial scale, however, could only commence after the discovery of Metal Inert Gas Welding during the 1940s and the production of aluminium-magnesium alloys (5000 series) with enhanced resistance to corrosion.

Today aluminium is extensively utilised in the maritime environment: in the construction of high-performance passenger ferries, in a growing number of landing crafts and warships and on oil rigs. The latter have to sustain the progressively higher weights of complex equipment. Thanks to Friction Stir Welding which was developed in the early 1990s new options are now available to shipbuilders.

Friction Stir Welding (FSW)

Friction Stir Welding was invented in 1991 by the Welding Institute in Cambridge, UK. Unlike in fusion welding, the joining occurs not during the liquid phase but below the melting point of an alloy. A rotating tool with a wear-resistant tool traverses the joint, generating frictional heat which plasticates the material. The weld is formed solely by means of mechanical stirring and forging. As opposed to other welding procedures, FSW involves neither filler materials nor any protective gas. The required temperatures are considerably lower than in arc welding. This results in first-rate mechanical properties and low distortions.

Because of the relatively low melting point of aluminium, the light metal is particularly well suited for friction stir welding, although the method has successfully been applied to steel, titan, lead and copper, too. Pieces of dissimilar thickness as well as different aluminium alloys can be joined, for instance 2000 to 7000 series. Even the joining together of different materials (e.g. steel with aluminium) has become a viable proposition and produces welds of remarkable quality. The joint lengths can reach up to 16m in length. Yet another advantage is the very clean and robust weld which requires no subsequent grinding or polishing. The strength and corrosion resistance of the friction stir welds is equal or even superior to results achieved through arc welding.

The very high forces acting on the material which is to be welded render handheld operations impossible. This applies all the more to large plates and profiles, which may involve several tools simultaneously, for example for double-sided welding or welding in opposite directions. Savings in labour costs are currently offset by comparatively high expenses for equipment. For this reason, numerous shipyards in the US have been drawing on the services of specialised friction stir welding providers. Even so, the benefits of this technology clearly outweigh its potential disadvantages in many cases. FSW greatly facilitates the welding of butt joints and allows for new designs in construction. The consistently high quality of welds is not achievable by use of manual welding, which inevitably has to be resorted to during arc welding procedures. The significantly more precise results enable the prefabrication of plates, panels and extruded profiles as well as castings outside the shipyard, thereby permitting more efficient manufacturing processes. This reduces the time a given construction occupies the shipyard. Last but not least, fewer emissions are generated thanks to the reduced use of arc welding.

Considerable interest from the military sector

The first commercial deployment of FSW in the maritime context was undertaken in Haugesund, Norway, in 1995, where initially aluminium panels were assembled for cooling tanks on fishing boats. Meanwhile, also shipbuilders in the Far East have availed themselves of friction stir welding, including the rapidly expanding Chinese Navy. For instance, China’s futuristic-looking fast attack craft, Type 022 “Houbei”, is believed to have been made from friction stir welded aluminium. A great deal of interest in fast aluminium constructions is also being expressed by the US Navy these days. Inter alia, the superstructure of littoral combat ship 2, USS Independence, utilises FSW material. As in the case of the Chinese Type 022, radar absorbant shaping requirements demanded a high level of ingenuity from engineers. Likewise, the experimental prototype FSF-1 “Sea Fighter” and the “Joint High Speed Vessel” rely on FSW aluminium, the latter being a multipurpose platform for the transport of troops and combat equipment.

Further advances such as portable FSW for repairs and new joint geometries, may see additional gains in efficiency for what is still considered a very expensive welding procedure. This notwithstanding, friction stir welding has developed into a technology which modern shipbuilding using aluminium is hard to imagine without.