Laser beam welding for difficult material combinations: As part of the FOLAMI project, researchers at LZH have developed a reliable and efficient process for joining steel and aluminum.
In shipbuilding, lightweight structures made from steel-aluminum joints are becoming increasingly important: the hull is built from steel, while the superstructure is made from aluminum. This lowers the center of gravity while optimizing both stability and weight. The result is reduced fuel consumption and lower CO₂ emissions. Up to now, the two metals have been connected using adapters produced by the complex and risky method of explosive welding.
In the joint project FOLAMI, scientists at LZH further developed laser beam welding of steel-aluminum joints as an efficient and safe alternative.
High Strength through Crossing Laser Beams
In explosive welding, components are joined by a controlled detonation. The colliding parts bond together without melting. In the subproject “Laser beam welding of form-fit steel-aluminum joints using penetration depth control,” LZH researchers established a reliable and efficient method for welding such joints with lasers.
First, elongated grooves are milled into the aluminum sheet and filled with iron powder. Then, steel and aluminum are joined using two crossing laser beams. By reducing the aluminum content in the weld metal, the process becomes more stable and also enables form-fit bonding.
This approach produces a deformable, homogeneous microstructure with few irregularities and high strength compared to non-form-fit joints. In overlap joints, the researchers achieved a maximum tensile shear force of around 12 kN – three times higher than with previous technology. At this load, the yield strength of the 5 mm thick steel is exceeded, resulting in plastic deformation of the steel.
Lightweight Principles in Shipbuilding
To ensure a stable process and compensate for production uncertainties, the partners developed penetration depth control systems. They used both spectral process emission analysis and short-coherence interferometry (OCT, Optical Coherence Tomography). Using the spectrometer-based approach, the scientists measured the emission intensity during welding and correlated it with the penetration depth.
It was demonstrated that intensity and penetration depth are related. This knowledge enables regulation of the penetration depth. When the desired depth is consistently maintained, a weld seam with high load capacity is achieved.
Steel-Aluminum Adapters for Maximum Strength
Thanks to the optimized laser beam welding process, the project partners were able to produce adapters with excellent strength values. Due to form-fit bonding and an adapted microstructure, the laser-welded adapters withstand loads of at least 35 kN – a value exceeding the yield strength of the aluminum alloy used. They also show high cyclic load resistance.
This laser-based joining method offers a reliable and efficient alternative to explosive welding and therefore holds great potential for use in shipbuilding.
The FOLAMI project was coordinated by B.I.G. Technology Services GmbH. In addition to LZH, other partners included Precitec Optronik GmbH, LASER on demand GmbH, the Fraunhofer Institute for Structural Durability and System Reliability, Hilbig GmbH, Fr. Lürssen Werft, MEYER WERFT, Coherent Germany, and DNV SE. It was funded by the German Federal Ministry for Economic Affairs and Climate Action within the framework of the “Maritime Research Program” (funding code 03SX547B), under the supervision of the Project Management Jülich.
