SLV – Schweißtechnische Lehr- und Versuchsanstalten
FSW ship - Friction-stir-welded lightweight structures for ship superstructures
Increasing significance is being attached to the development and utilisation of lightweight structures in shipbuilding in view of the rising material prices and energy costs for both the production and utilisation phases of the ships. The problems which relate to the position of the centre of gravity and are associated with the growing ship dimensions particularly in yacht construction and in the passenger ship and ferry segments are also necessitating the utilisation of light materials such as aluminium in upper deck levels (superstructures). The thermal joining processes used until now are technologically demanding and exhibit only limited productivities and high expenditure on remachining. The interfaces between structures made of aluminium and steel also require complicated surface machining and joining processes (utilisation of explosive-clad aluminium/steel connectors, e.g. TRICLAD).
The prerequisite for the economically viable utilisation of friction stir welding for the joining of lightweight structures is the identification of the suitable regions in the ship. For this purpose, these structures must have a design configuration (appropriate for fabrication). Thus, only the cooperation between shipbuilders (shipyards), process technologists and designers leads to the development and application of friction-stir-welded structures in shipbuilding.
Overall aim of the project
The aim of the overall project was to apply and promote the economically viable utilisation of the joining process of "friction stir welding" during the construction of lightweight structures for ship superstructures. The technical feasibility and the economically viable usability were to be proven by considering typical areas of application (application cases). This related not only to the fabrication of these structures and their integration (assembly) on the ship but also to their operating behaviour. The improved product properties in the operating phase of the ships were to be highlighted in addition to the proof of the higher productivity during the fabrication of ship superstructures due to the utilisation of friction stir welding as the joining process.
The integration capacity of lightweight structures made of aluminium (superstructures) into the steel hull was to be improved by the joint between different materials executed by means of friction stir welding. Possibilities of reducing the material thicknesses due to the higher strength values of friction-stir-welded joints were explored at the same time.
Specific requirements of the fabrication of lightweight structures for ship superstructures on the installation technology for friction stir welding were defined as a result of the project processing.
The present project was intended not only to increase the competitiveness of the participating partners (shipyards and engineering offices) for the design and construction of ship superstructures but also to transfer generalisable results for application in the construction of passenger ships and ferries to other German shipyards.
The overall aim thus included not only ship superstructures as a product but also their economically viable fabrication.
Project tasks of GSI SLV Berlin-Brandenburg
Lightweight structures are popular and indispensable in shipbuilding. In this respect, the manufacture of reliable welded joints occupies a key position. FSW technology can meet this demand. The advantages and the associated saving potentials can be demonstrated by the following points:
- High and reproducible weld quality (no pores, low inclination to cracking, low distortion and low loss of strength)
- Low susceptibility to defects - high process stability
- With corresponding backing, it is not necessary to remachine the bottom side of the weld; the weld surface may possibly also be left as it is
- Joining of material combinations (e.g. Al/St) and of materials not suitable for fusion welding (e.g. AlCu and AlLi)
- No special weld preparation required
- No filler or auxiliary materials required
- Great wall thicknesses weldable in one pass
- Depending on the component geometry, welding with milling machines is possible
- Multisheet joints possible
- No trained welders needed
- No environmental impacts caused by dusts, fumes or radiation
The aim of the subproject which was processed by GSI - Gesellschaft für Schweißtechnik International mbH, Niederlassung SLV Berlin-Brandenburg was to prove an increase in the product quality with a simultaneous drop in the scope of the fabrication during the construction of lightweight structures for ship superstructures as a result of the economically viable utilisation of the joining process of "friction stir welding". Further subaims were to permit the elaboration of the saving potential compared with traditional welding processes and the general application of the friction stir welding process in the shipyard sector. A few examples have already been cited (see above).
In the future (i.e. after this research project), large panels, as are customary in shipbuilding at present, should no longer be used exclusively. Thereafter, smaller special elements might be manufactured effectively with the friction stir welding process. GSI SLV Berlin-Brandenburg had the experience and machine technology needed in order to be able to prove the consideration of typical areas of application, the technical feasibility and the economically viable usability. This related not only to the fabrication of such structures, their integration (assembly) on ships and their incorporation into other components but also their operating behaviour. The core of the investigations was the implementation of the design configuration of selected structures in a way appropriate for fabrication. These were elaborated within the framework of the overall project.
In the subproject of GSI SLV Berlin-Brandenburg, investigations were conducted into typical shipbuilding sections and their geometries and applications conducive to friction stir welding. In this respect, the weldability of a component was determined by the fabrication weldability, the welding possibility and the service weldability. Here, investigations were conducted into the suitability for friction stir welding of those utilisation cases of the solutions presently used for lightweight structures or even for new structures which were selected in the subprojects of the other project partners. Any possible changes to these structures were elaborated and general solutions for friction-stir-welded lightweight structures were highlighted in this way. Furthermore, the construction and testing of laboratory specimens of friction-stir-welded lightweight structures served to define important prerequisites for the fabrication process in shipbuilding on the basis of an example structure (mock-up) and to illustrate attainable quality and productivity effects. The parameter windows for the subsequent fabrication were elaborated in the mock-up program.
In summary, the aims of the subproject may be defined as follows:
- Determination of the feasibility
- Elaboration of the process parameters
- Assessment of the friction-stir-welded joints using non-destructive and destructive investigations
Reference of the project to the aims of promotion policies
GSI - Gesellschaft für Schweißtechnik International mbH, Niederlassung SLV Berlin-Brandenburg understood its work within this research project as an effective and necessary contribution to putting the German shipyard industry in a position to become more efficient, more cost-favourable and thus more competitive against the foreign competition (e.g. Asia). This also included:
- Better environmental compatibility
- Saving of energy
- Saving of resources
- Safeguarding of jobs in Germany
Scientific and technical aims of the work
This subproject pursued the aims of the joint project. The following scientific and technical aims of the work were striven for with the processing of this subproject:
- Increase in the productivity and reduction in the required remachining on the basis of optimised tools and process parameters.
- Determination of the parameters required for the joining of the selected lightweight structures by manufacturing and testing laboratory specimens. The testing was carried out with all the common test procedures right up to investigations taking account of maritime aspects.
- Elaboration of optimisation proposals for the design, derivation of the scope of the fabrication and manufacture of mock-ups.
In-house exploitation
As a result of the research project, the participating companies were given access to FSW technology. For GSI SLV Berlin-Brandenburg, new business fields were opened up by implementing the technical and technological innovations. It was possible to extend the activities of GSI SLV Berlin-Brandenburg to the maritime sector and to win over new customers. Furthermore, the expert competence with regard to FSW was intensified and the existing technological know-how could be extended and could be used in other terms of reference. These were, for example:
- Configuration of tools
- Requirements on tools depending on the joining task
- Connection between the weld geometry and the weld quality
The project started on December 1, 2010 and the period ended on November 30, 2012. However, it was intended to continue the cooperation with the project partners beyond the project period in order to recognise further joining technology problems and to be able to process these with the FSW process in advanced projects.