Lightweight materials, such as aluminum die castings, are used more and more for automotive applications. Due to the limited weldability, joining these materials by self-pierce riveting has been established. The challenge in this regard is that these materials, especially new high strength aluminum die castings, have a limited ductility, while the joining processes locally induce large plastic deformations. Consequently, joining by forming of these materials can be accompanied by cracks, which develop during the forming operation. This paper shows the experimental and numerical investigation of a new die concept for self-pierce riveting materials with limited ductility. At the new tool concept the riveting die is separated and a movable die element is used. This element allows that the parts are superimposed with compressive stresses during the self-pierce riveting process. In the paper it can be shown, that in contrast to the conventional process crack-free joints can be generated by using the new tool concept. Determination of the joining parameters and the die design was supported by simulative investigations. Additionally, the new and the conventional self-pierce riveting process are compared on the basis of results from the experimental investigations.
This paper shows an overview of different analyses regarding current challenges at self-pierce riveting with solid rivets as well as semi-tubular rivets of lightweight materials like aluminum die casting, carbon fiber reinforced plastic and 7xxx series aluminum alloy. The joining process analyses will demonstrate the cause and the development as well as the influence on joint quality of individual joining process-induced defects. In addition, methods are described how these imperfections can be avoided or reduced.
The importance of environment friendly mobility strengthens the need of lightweight design in the automotive industry. New electric car models, like the BMW i3, already have car body with a high amount of carbon fiber reinforced plastics (CFRP) to allow, as a result of the low vehicle weight, appropriate ranges without reloading the battery. Methods for joining materials like CFRP play a key role to implement lightweight designs into car body production. Conventional joining methods like spot welding cannot be used for such material combinations. Due to the good automation and possible combination with adhesive, mechanical joining techniques such as self-pierce riveting (SPR) are very relevant for joining these lightweight materials. While generally self-pierce riveting of CFRP with aluminium is possible, different damages e.g. delaminations, fiber or matrix fractures in the CFRP can occur during the joining process and have to be considered.
This paper shows an analysis of these process induced damages when self-pierce riveting CFRP compound with aluminum sheet metal and investigates their influence on the joint strength. In our research the conventional SPR process of CFRP-aluminium joints is compared to the application of a new die concept for SPR in which a separated die is used to reduce the process induced delaminations in the CFRP. Additionally, these joining results are contrasted to SPR joints with pre-drilled CFRP components. Through the pre-drilling the damages in the CFRP can nearly be avoided completely and so these joints can be used as a reference. The results of the three processes to produce CFRP-aluminum joints are compared by micrographs, computed tomography and strength tests.
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