Abstract
Organic sheets made of fiber-reinforced thermoplastics can make a crucial contribution to increase the lightweight potential of a technical design. They show high specific strength- and stiffness properties as well as good damping characteristics, while being able to show a higher energy absorption capacity than comparable metal constructions. In addition, organic sheets provide good recycling capabilities. Nowadays, multi-material designs are an established way in the automotive industry to combine the benefits of metal and fiber-reinforced plastics (FRP). Currently used technologies for the joining of organic sheets and metals in large-scale production are mechanical joining and adhesive technologies. Both require large overlapping areas to achieve the desired joint strength and stiffness of the technical design. Additionally, mechanical joining is usually combined with “fiber-destroying” pre-drilling and punching processes. This will disturb the force flux at the joint zone by causing unwanted fiber- and inter-fiber failure and inducing critical notch stresses. Therefore, the multi-material design with fiber-reinforced thermoplastics and metals needs optimized joining techniques that don’t interrupt the force flux, so that higher loads can be induced and the full benefit of the FRP material can be used. This article focuses on the characterization of a new joining technology, based on the Cold Metal Transfer (CMT) welding process, that allows to join organic sheets and metals in a load path optimized design. This is achieved by realigning the fibers around the joint zone by the integration of a thin metal pin. The alignment of the fibers will be similar to load paths of fibers inside structures found in nature. A tree with a knothole is always going to align its fibers in principle stress direction. As a result of the bionic fiber design, high joining strengths can be achieved. The increase of the joint strength compared to blind riveting was performed and proven with stainless steel and orthotropic reinforced composites in tensile shear-tests, based on the DIN EN ISO 14273.
Highlights
To achieve weight reduction in the automotive sector, mainly materials such as aluminum alloys or high strength steels are used
Multi-material designs are an established way in the automotive industry to combine the benefits of metal and fiber-reinforced plastics (FRP)
This article focuses on the characterization of a new joining technology, based on the Cold Metal Transfer (CMT) welding process, that allows to join organic sheets and metals in a load path optimized design
Summary
To achieve weight reduction in the automotive sector, mainly materials such as aluminum alloys or high strength steels are used. Applied technologies for the joining of FRP-parts with metal structures are frictional and form-locking joints as well as adhesive joints, whereby for these technologies the joining components need to be prepared in a complex way. Drilling and punching with cut section leads to unwanted fiber-destruction and inter-fiber failure [4] This interrupts the force flux within the fibers and induces critical notch stresses into the joining areas. A new joining concept, derived from the “Cold Metal Transfer Welding” (CMT), gives the decisive solution approach This process allows the one-sided, fault-tolerant and fiber-fair joining of the different material groups in short cycle times. With this new joining technology, overlapping areas can be reduced as compared to usual riveting techniques. The increase of the joint strength when using pins instead of blind rivets is tested with specimens made of glass fiber-reinforced plastics (GFRP) with a thermoset epoxy resin matrix using the hand lay-up technique
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