On the mechanical behaviour of BFRP to aluminum AA6086 mixed joints

The aim of this work is to analyze the possibility to join aluminum alloy AA6086 and composite laminates reinforced with basalt fibers, an innovative material which use is growing in several applications as an alternative to glass fibers. To this goal, three joining techniques were investigated: mechanical by Self Piercing Riveting (in the next called SPR), adhesive by co-curing technique and mixed in which the joining techniques (i.e. adhesive and mechanical) were combined. Two manufacturing technologies (i.e. hand lay-up and vacuum bagging) were used both to produce composite substrates and to realize co-curing adhesion between the substrates to be joined. Mixed joints were realized by inserting the rivets in co-cured joints after 48 h than the initial phase of the curing process (i.e. the phase of mixing the resin with own hardener). Overall, six lots of joints were realized (two for each joining technique). All joints were characterized by performing single lap joint tests. The mechanical results were analyzed through a two way analysis of variance. The experimental results show that adhesive joints, realized by vacuum bagging method, show average failure load 22.9% higher and standard deviation 70.6% lower than those realized by hand lay-up, respectively. This means that the vacuum bagging technology allows to increase the adhesion strength of the interface between metal sheet and Basalt Fiber Reinforced Polymer (in the next BFRP), allowing the above failure load growth. Furthermore the failure mechanisms change from adhesive mode to partially cohesive one for the adhesive joints realized by hand lay-up and vacuum bagging, respectively. By comparing mixed joints, different results are obtained: i.e. the hand lay-up joints show both higher average failure load (+42.9%) and standard deviation (+208.3%) than those realized by vacuum bagging. The poor performances of the mixed joints realized by vacuum bagging can be considered due to the excessive value of the chosen riveting load. Statistically, two variables were investigated: i.e. joining technology (i.e. mechanical, adhesive and mixed) and manufacturing process (i.e. hand lay-up and vacuum bagging). These can influence the properties of the joints. In particular, the joining technology results a significant factor. Moreover, an interaction between the two variables exists.