Laser-processing of hybrid automotive parts

Fraunhofer ILT and its project partners successfully demonstrate production of a hybrid composite-metal roof bow for BMW 7 series.

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For multi-material lightweight applications, processing of different materials such as composites and metals and their hybrids is necessary. In the HyBriLight Project, funded by the Federal Ministry of Education and Research (Bonn, Germany), a consortium of 9 companies developed an innovative hybrid roof bow which demonstrates the successful implementation of novel laser-based processes for lightweight production. All basic information like dimensions and mechanical requirements of the demonstrator roof bow are based on the original part of the BMW 7 series.

The roof bow consists of a fiber-reinforced plastic bar connected to two metal connecting plates made of steel which can be spot-welded to the car body. The connection between composite and metal is changed from adhesive joining and riveting to an innovative laser-based joining approach. Therefore the metal connecting plates are pretreated with ultrafast lasers in order to create a spongy surface topography.

The joining process itself takes place during a compression molding process of the plastic bar. The molten polymer is pressed into the generated structures, hardens and creates a strong and reliable joint based on mechanical interlocking between the two materials. The laser-based hybrid joint withstands shear stress of nearly 50 MPa. The joining process is enabled by a special variothermal mold which allows preheating the metal connection plates and process control via integrated sensors during the process.

In order to save costs the carbon fiber reinforced thermoset material from the original part is replaced by a thermoplastic PA6 Matrix with long glass fiber reinforcement. To maintain the stiffness and strength of the part, the roof bow is partially reinforced with carbon fiber UD-tapes. The cycle time for the compression molding process is about 75 seconds. Afterwards the demonstrator is trimmed with a single mode fiber laser using a novel high speed scanning system. A multi-pass cutting approach enables cutting the multi-material mix in a single process step with minimized heat-input. The demonstrator part shows the successful integration of novel laser-based processes into a close-to-industry process chain.

Fraunhoffer ILT is part of the AZL international composites network. This post is courtesy of the CompositesWorld and AZL Aachen GmbH media partnership.