Laser Zentrum Hannover (LZH) and partners have pioneered the laser-welding of marine gear unit foundations constructed from steel and aluminum foam sandwich material, a demonstration that offers promising results for lightweight shipbuilding construction.
Metal foams are highly porous, lightweight materials which can absorb energy in the form of vibrations and knocks, or noise. They are also heat resistant and provide insulation against electromagnetic waves. Metal foams, embedded in so-called 'sandwich' constructions with aluminum or steel sheets, have a much higher bending stiffness than solid sheets, and due to their lower weight are especially suitable for lightweight construction or for highly stressed parts.
Large-scale metal-foam sandwich materials are especially interesting for highly stressed ship structures, such as foundations for machines or for rudder structures, as a weight reduction of up to 20% is achievable. However, steel-aluminum lightweight structures are difficult to weld, due to an inconsistency and inhomogeneity of the foam core, high stiffness, and material thickness. Thermal effects of foaming the aluminum core in mixed sandwich materials can distort the material, for example, and welding can lead to intermetallic phases and cause cracks in the welding seam.
LZH, Blohm + Voss Naval GmbH, Precitec Optronik GmbH, and the Fraunhofer Institute for Machine Tools and Forming Technology (IWU), are all part of a German government-funded joint project dubbed 'MESCHLAS', which aims to provide lightweight construction using sandwich technology. As part of this work, LZH has developed a process for laser welding large-scale, metal-foam sandwich materials, in which inter-metallic phases do not occur. This process was recently tested on a gear unit foundation at Blohm in Emden, Germany, using a transportable axis system and a mobile diode laser provided by Scientific and Efficient Technologies Ltd and LASER on Demand.
For the first step of mechanically preparing the panel edges, the aluminum foam adhered to the steel top sheets is removed, thus preventing intermetallic phases. The components are then positioned, forming a zero gap, followed by fixing the sandwich panels using conventional spot welding. Butt welds and fillet welds are then laser-welded using a 5 kW diode laser (900-1030 nm wavelength). Tests show that a gap of 0.6 mm can be bridged, and continuous weld seams are possible, with speeds of 0.2 to 1.5 m/min. depending on the material thickness (up to 5 mm).
