Impact of cracks on the fatigue behaviour of self-piercing rivet joints in die cast aluminium

Abstract

In the current push towards more electric vehicles new ideas of manufacturing within the automotive industry are being developed at a rapid pace. A few years ago the idea of exchanging many welded parts with one giant die cast aluminium component came to fruition. The process came to be known as megacasting in many circles and since then a number of car manufacturers have invested in the technology. The use of these brittle, cast aluminium components poses a problem however when using joining methods which are typical in the automotive industry, such as self-piercing rivets (SPR). The SPR process requires large plastic deformation resulting in often unavoidable cracks in these cast components. Therefore it is important to investigate the impact of cracks on the fatigue behaviour of SPR joints into die cast aluminium. Using a soft 240LA, low alloy steel as a top sheet and die cast AlSi7MnMg as the bottom sheet fatigue samples were prepared using relatively soft rivets. Two different dies were used to induce samples which had no cracks visible on the joint button and samples which had severe cracking visible on the joint button. One version was prepared without adhesive and the other had material milled off to achieve a fixed gap thickness when adhesive was applied. Both were tested in fatigue using a four-point bending fixture. A large variation was observed in the non-adhesive fatigue samples with no significant impact from cracks. The adhesive samples all went until runout. A number of investigations were done into the microstructure of the aluminium, void concentration and cross-sectional analysis of SPR joints as well as fracture surface analysis. It is unclear if cracks on the SPR joint button do impact fatigue strength as there were a large number of variables in the samples such as thickness, amount of oxides in the fracture surface, cracking severity on the button, and voids inside the material. However, adhesive does seem to increase the fatigue strength, without knowing quantitatively how much.