# A Numerical Approach for the Efficient Concept Design of Laser-Based Hybrid Joints
###### Abstract
The properties of the joining zone, such as strength, production time, and costs significantly depend on the microstructure. Currently, only the same microstructure with the same pattern is applied on the whole metal surface. Local variations (e.g., stress peaks or low-stress areas) on the joining zone are not addressed with tailored microstructures. The microstructure, i.e., the shape of the cavities and the pattern on the joining zone, can be easily adjusted via the laser software [15]. As a result, there is the potential to develop cost and strength-optimized microstructure concepts [16]. However, this potential has not yet been exploited. In the design, it is not considered where on the joining zone which microstructure would be optimal in terms of cost and strength. The effort to adapt and experimentally test tailored microstructures for each component with different strength and cost requirements is too high [17; 18]. Figure 1: Laser-based joining process of plastics and metals. Today's research activities with regard to the optimized design of laser-based hybrid joints are focused on experimentally adjusting the production properties. The main focus is on the analysis of thermal properties during laser-based joining [19; 20]. The first approaches to model the mechanical properties of laser-based joining with FE simulations exist [21; 22; 23]. However, these approaches do not consider the specific influences of the microstructure on the mechanical properties. Experimentally determining the influence of the microstructure on the joining properties is challenging for laser-based joining. Due to the complex mechanical behavior of the microstructure and the interactions, the experimental effort to generate data-driven models for each specific application (geometry, load case, microstructure) would be too high. Therefore, reliable higher-fidelity numerical models are required to describe the characteristics. Using higher-fidelity numerical models in optimization algorithms is a widely used approach to solve engineering problems. For example, a variety of approaches exist for the multi-objective optimization of the fiber orientation and layup of fiber-reinforced plastics [24; 25; 26; 27]. Furthermore, optimization approaches are increasingly used in the optimization of structural components, e.g., topology optimization or generative design, with respect to multiple domains [28; 29]. The first investigations using a fully simulation-based optimization approach for laser-based hybrids demonstrate the potential for cost- and strength-optimized joining zones by optimally positioning the cavities on the joining zone. In ref. [16], a reduction in laser costs of 82% was achieved by optimally positioning the microstructure on the joining zone using a model-based optimization approach. However, these studies and their models were limited to the 2D case and therefore the cavities always covered the entire length of the surface and only the distance between the cavities could be considered as a design variable. The extension of the approach by adding a further degree of freedom in the 3D case allows exploiting the full potential for tailored microstructures and further enabling the industrial application of the technology. However, systematic design methods and efficient models for developing tailored microstructure concepts as well as determining costs and predicting the joint strength are currently missing for the 3D case. A major obstacle is that laser manufacturing constraints need to be considered in an automated way. Furthermore, strength models do exist for other joining technologies (e.g., adhesives). However, these are mostly reverse-engineered and calibrated from experimental data and are not adapted and tested for the laser-based hybrid joint. Therefore, the research questions that were addressed in this study are as follows:  
* How can tailored microstructure concepts for laser joining zones be automatically generated while considering the laser manufacturing constraints?