Ageing behaviour of folded joints produced by fusion bonding of an outer metal sheet and an inner organic sheet

Adhesive hemming is a process used in the automotive industry to join two metal sheets with good structural performance and good optical appearance of the rim. A new approach is to replace the inner metal sheet by a fibre-reinforced thermoplastic polymer sheet. The joining method used is fusion bonding. This can reduce process time, cost and weight of the finished part. As process route cold hemming is used where the outer metal sheet is first folded around the inner organic sheet at room temperature and afterwards thermally joined by induction. The aim of this work in particular is to investigate the behaviour under accelerated ageing conditions. Especially the cathodic dip coating as corrosion prevention and the air in the fold were varied. The latter can vary in the production process due to manufacturing. In addition, a pre-treatment of the metal surface with a laser was investigated to enhance the mechanical strength of the joint. The samples were produced with a three-step hemming process combined with inductive heating. The applied materials were an organic sheet made of polyamide-6 as matrix material and zinc coated steel. Conditioning according to DIN 1110 and the ageing test VDA 233-102 were applied to laboratory samples. It was shown that conditioning of the specimen reduces the maximal pull out force only slightly while ageing according to VDA 233-102 decreases the pull out force by about 50 %. Furthermore, the air gap has a significant influence on the ageing behaviour of the specimen while the cathodic dip coating only prevents the steel from corrosion but has no influence on the bond strength.