A recently completed European CRAFT programme, led by Henrob and managed by TWI, involved the development of a lightweight self-piercing riveting gun and an extensive study of the process in comparison with resistance spot welding.
Self-piercing riveting is used for high speed mechanical fastening of sheet material components. It is suitable for joining dissimilar materials, as well as coated and painted materials, and can also be used to join some plastics to metals. The process has seen rapid development in recent years and is being used in a range of industrial and automotive applications. Some 1800 rivets are included in the manufacture of Audi's new aluminium A2 car for example.
A lightweight gun frame based on carbon fibre composite material was designed and constructed by Flexprop, Sweden. This achieved adequate stiffness and fatigue life for setting forces up to about 55kN, a throat depth of 85mm and a weight of 75kg. The prototype gun underwent rigorous factory trials and endurance testing at Henrob before a refined version was produced. The gun was mounted on an ABB robot and successfully proved under automotive production conditions at Volvo in Olofström, Sweden.
During the gun development, TWI and the Technical University of Dresden conducted supporting work on joint properties for self-piercing riveting for a range of materials, with comparison to resistance spot welding where applicable. The steels ranged from 0.7mm to 2.5mm thickness, and included low carbon, high strength and zinc coated materials. 1.2 and 3.0mm aluminium alloys were used, and also metal/plastic composites and an SMC plastic. Adhesives were also used in selected material combinations in association with riveting or spot welding.
The influence of selected process factors on joint performance was also examined. This highlighted the important factors but showed the riveting process to be a robust production process. A best practice guide to self-piercing riveting was produced, and this will be made available on TWI's corporate website.
The partners in the two-year programme were: Henrob Ltd (UK), TWI, The Technical University of Dresden (Germany), Flexprop Production AB (Sweden), ABB Body-in White AB (Sweden), Hanteck Ltd (UK), Nordisk Industrirobot Scandria AB (Sweden), Corus (The Netherlands) and Permabond Ltd (UK).
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