Fiorenzo Castellini, info@tubetechmachinery.com

It reduces the heat input and the crackability tendency

The transport sector, especially passenger transport with trains and ships, shows an ever growing trend of the use of components made of aluminium alloys used for the interesting combinations of their characteristics like low density resulting in light weights combined with high corrosion resistance, high mechanical characteristics, good weldability, etc. In addition, aluminium alloys have high deformability when properly heated making them a primary choice for extrusion, suitable also for forging and hot press moulding. These characteristics make them particularly appreciated by planners who can choose between multiple engineering solutions resulting from the multiple forms of profiles, forged parts, extruded products, castings, foils, plates, etc.
In the marine sector, to which the product manufactured by the proposed system is destined, the reduction of the weight of the structural components is one of the main objectives to pursue choosing lighter metal materials whose mechanical properties guarantee performance and reliability.
Extruded aluminium products welded to each other to obtain self-supporting structures of great dimensions are a notable technological opportunity for the reduction of the weight-rigidity ratio. For large passenger ships, the greatest interest is directed toward the structures of the bridges, especially those located higher up (as many as 18 bridges can be found on a passenger ship) because, in addition to the general reduction of the weight of the hull, there is a reduction of the barycentre with great benefits in terms of floatation stability. For the construction of these bridges the plans in welded extruded aluminium are a valid alternative to the traditional solution comprised of metal sheet with vertical reinforcements. The weight goes from about 50 kg per square metre of bridge constructed of steel to less than 20 kg per square metre for the structure in extruded aluminium. The following figures show a typical extruded aluminium product, an enlargement of the welded joint and the structure comprised of multiple profiles welded to each other. The welding of the profiles is traditionally conducted by means of conventional processes (MIG, TIG, etc.). These processes have reached full maturity and are generally used in the shipyards but involve a degradation of the mechanical properties because of the known crackability tendency of aluminium alloys when heated and the formation of diffuse porosity.
These phenomena are dependent on a combination of mechanical (nature of the shrinkage stress due to the high thermal expansion coefficient) and metallurgical (presence of alloy elements that tend to form low boiling point components and entrapment of hydrogen in the molten weld pool) factors: both of these factors make it advisable to use welding techniques that give the welded joint low heat input values. From this point of view the bundle laser welding technique drastically reduces the overall heat input and decreases the crackability tendency when heated.
The system that we propose, by means of a completely automatic longitudinal laser welding process, joins the 12-metre long extruded components. The result is a self-supporting product that is part of the bridge the dimensions of which, for transport reasons, are limited to 2.5 m of width (normal transport) by 12 m of length (dimension of the extruded profile).
The aluminium alloys used for the production of the extruded profiles are those most commonly used in shipyards: alloys belonging to the 6xxx family (aluminium-magnesium-silicon).
They are absolutely the most frequently used for nautical and marine purposes given the interesting combinations they offer including:
- high hot formability so much as to be considered extrusion alloys par excellence that allow the realization of complex, open or closed forms, thin thicknesses even at high speeds of extrusion - simple and economic cycle of recovery
- good corrosion resistance
- optimal weldability
- high mechanical characteristics
- good cold deformability
- high quality surface finishing
The two alloys used for the production of the welded components are:
- AlMgSi 0,7 (EN AW 6005): this aluminium alloy, especially in its variation 6005A, has very good mechanical resistance and can be used for heavy duty structures
- AlMgSiMn (EN AW 6082): in the T6 version (that is solubilised, tempered and aged) this alloy has the best mechanical characteristics among the class 6xxx alloys.
For quality welding (full penetration, no cracks, acceptable porosity, good weld geometry, etc.) a series of welding and laboratory tests were conducted which allowed the arrangement of different combinations of parameters that were found to be satisfactory. A series of tests were executed with and without filler metal using 1.2 mm diameter AlMg5 and AlSi5 wire.