Impact of cracks on the fatigue behaviour of self-piercing rivet joints in die cast aluminium
Publicerad
Författare
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
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.
Beskrivning
Ämne/nyckelord
Self-Piercing Rivet, fatigue, four-point bending, adhesive SPR joint