Mechanical Behaviour and Corrosion of Interstitial-Free Steel - Aluminium Alloy Self-Piercing Riveted Joints

Abstract

The overall aim of the project is to examine the rivetability of new steels and to investigate the mechanical behaviour of self-piercing riveted (SPR) aluminium-steel hybrid structures for automotive applications. Interstitial Free Steel (I.F.) of 1.2 mm thickness was joined to Aluminium 5754 of 2 mm thickness and Aluminium 5182 (coated and uncoated) of 1.5 mm thickness.

The work began by initially conducting a quality assessment of the various joints that were produced in order to establish the optimum conditions for joining the various sample combinations to be investigated. A relationship was established between the head height and the interlock distance on the one hand and between the interlock distance and the lap shear strength of samples. It was also established that for higher lap shear strength, it is preferable to use the stronger material (I.F. steel) as the pierced sheet and the weaker material (5182) as the locked sheet. However, the results showed that this rule could not be applied for predicting the fatigue behaviour of SPR joints between I.F. steel and 5182. An investigation of the fatigue failure mechanisms was undertaken and possible reasons for this behaviour are discussed. The influence of fretting was also investigated by using scanning electron microscopy and reported.

The fatigue behaviour of Dual Phase (DP600 + 5182) SPR joints was investigated. It was observed that the position of fatigue crack initiation differed with the maximum applied load. An explanation for this observation was provided by considering the failure mechanism of the samples under different load levels. The study also showed how fretting led to the initiation of fatigue cracks.

The corrosion behaviour of (I.F. steel + 5182) samples was investigated by conducting tests in a salt spray according to the ASTM B117-97 standard. Three types of corrosion were observed; galvanic corrosion, differential aeration corrosion, uniform corrosion and are discussed. The weight change with time was monitored and was used to describe the corrosion behaviour. The lap shear strength was measured as a function of corrosion time. The presence of the corrosion product within the overlap was observed to greatly influence the lap shear strength behaviour. A further study was carried out in order to examine the influence of the individual alloys on the corrosion of the SPR samples. In this part the potential influence of pulse current treatment on corrosion was also investigated and was observed to increase greatly the corrosion resistance of the I.F. steel. Principal findings for this observation are also provided.