# A Numerical Approach for the Efficient Concept Design of Laser-Based Hybrid Joints
###### Abstract  
Laser-based plastic-metal joints have high potential to enable cost-efficient lightweight structures in multi-material design. By an appropriate load-optimized positioning of the microstructure on the joining zone, cost- and strength-optimized joints can be realized. However, there are no design methods and models to efficiently develop these tailored microstructures. Currently, time-consuming experiments are necessary to find the optimum microstructure concepts. These experiments must be repeated when requirements change, e.g., dimensions of the components. To provide a simple and efficient design tool, this paper presents an automated numerical method for the development of cost- and strength-optimized microstructure concepts for laser-based joining zones. The basis for the approach is a new numerical model which generates concepts for microstructures automatically based only on the stress tensor in the joining zone. A new finite element cohesive zone model (CZM) was developed to estimate the joint strength. The CZM parameters were efficiently derived from a finite element model of a single cavity. To determine the costs, a new model is presented that calculates the production time and the cost for any given microstructure. The models were interconnected in a combined optimization procedure and a genetic algorithm was used to determine cost- and strength-optimized microstructure concepts. The approach was applied to a demonstration example where the laser costs were reduced by up to 67% compared with benchmarks with surface-covering parallel linear cavities. The approach shows high potential for the efficient design of cost- and strength-optimal laser-based hybrid joints since it is fully based on simulation models and iterative experiments in the design stage are eliminated. os 20221210649. [[https://doi.org/10.3390/app122010649](https://doi.org/10.3390/app122010649)]([https://doi.org/10.3390/app122010649](https://doi.org/10.3390/app122010649))  
18 October 2022121 October 20221 October 202218 October  
methods (e.g., with milling). Laser manufacturing is also highly suitable for the realization of lightweight-, load-optimized, and multi-material components [13; 14]. Laser-based joining of plastics and metals is carried out in a two-step process:  
In the first step, the metal surface is processed with a highly focused single-mode fiber laser beam. The laser beam is deflected at high speed (\(\sim\)10 m/s) over the metal surface using a galvoscanning system (see Figure 1a) and creates linear cavities. When the laser passes over the same area several times, small undercuts are created on the metal surface. A part of the material evaporates and a part is pushed upwards in the cavity by the melt pressure and partially solidifies at the edges. Depending on the metal, approx. four runs are required to form clear undercuts. The laser system can be configured to generate a variety of patterns for microstructures on the metal surface, such as linear parallel line structures, crossed structures, structures under an angle, or pointwise structures. This work focused on linear parallel structures because these are the easiest to apply in terms of laser process technology and the most robust in terms of process control. After processing the metal, both plastic and metal are joined in a second step under pressure and temperature (see Figure 1b). Various joining methods, such as injection molding, can be used for this process step. During the joining process, the liquefied plastic flows into the cavities and solidifies after cooling. The plastic and metal are connected by mechanical interlocking and, depending on the plastic, also by specific adhesion. The filling of the cavity with the plastic depends on the temperature and solidification behavior of the plastic. The cavities can be usually completely filled if a sufficiently high metal temperature can be ensured to prevent early solidification of the plastic.