In the research project “FOLAMI,” the Fraunhofer Institute for Structural Durability and System Reliability LBF focused on evaluating the mixed steel-aluminum joints produced by form-fit laser beam welding for use in fatigue-resistant semi-finished products in shipbuilding. Alongside the development of the welding process, numerical and experimental investigations were carried out to verify the fatigue strength first at the specimen level and subsequently for specifically optimized adapters serving as connecting elements between steel and aluminum structural components.
In shipbuilding—particularly in yacht construction—the lightweight material aluminum is already widely used in combination with conventionally processed steel materials. Up to now, steel-aluminum joints have typically been realized using so-called explosion-welded adapters, also known as explosion cladding. However, alternatives to this manufacturing process are often associated with higher costs or design and functional limitations. Laser beam welding, as a novel technique, can offer an alternative production method in this regard.
Optimization and Evaluation Using Simulation and the Peak Stress Method
As part of the welding process development, the institute initially supported the optimization of weld seam geometries—and thus process parameters—through numerical load simulations. The research focused on developing a methodology for evaluating steel-aluminum laser weld joints and implementing it in a welded adapter. For this purpose, a local assessment method—the so-called “Peak Stress Method”—was adapted to the application case of steel-aluminum hybrid joints, using digital image correlation for strain validation.
AI-Based Fatigue Life Prediction of Laser Welds Between Aluminum and Steel
The evaluation procedure developed by the institute is designed to correlate weld seam parameters—such as bonding width or penetration depth of the laser weld between EN AW-6082 T6 aluminum and S355 steel—with the fatigue strength derived and experimentally verified using the Peak Stress Method. To expand the data base, parameter variations of the weld seam geometries and finite element load simulations were conducted to determine “peak stresses.” These data—divided into training and test datasets—were then used to predict fatigue life through a neural network.
Transferability to Other Metallic Multi-Material Joints
The research results not only provide an evaluation approach for this specific steel-aluminum hybrid joint but also demonstrate the potential to apply this method to other joined metallic multi-material combinations. In this context, the methodology can be transferred beyond shipbuilding applications to other industrial sectors.
Funding and Project Partners
The joint project “Form-fit Laser Beam Welding of Steel-Aluminum Joints for Fatigue-Resistant Semi-Finished Products in Shipbuilding (FOLAMI) – Subproject: Evaluation of Steel-Aluminum Joints Produced by Form-fit Laser Beam Welding for Fatigue-Resistant Semi-Finished Products in Shipbuilding” (Funding code: 03SX547C), coordinated by Forschungszentrum Jülich GmbH (PtJ), was funded under the research program “Maritime Technologies of the Next Generation” by the German Federal Ministry for Economic Affairs and Energy based on a resolution of the German Bundestag. The project duration was from June 1, 2021, to December 31, 2024.
Project partners included B.I.G. Technology Services GmbH, Coherent (Germany) GmbH (associated), Fr. Lürssen Werft GmbH & Co. KG (associated), Hilbig GmbH (associated), LASER on demand GmbH, Laser Zentrum Hannover e.V., MEYER WERFT GmbH & Co. KG (associated), and Precitec Optronik GmbH.
