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A B S T R A C T This study investigates the fracture behaviour of refill friction spot welding welds under shear tensile loading. Overlap joints of 6181-T4 aluminium were produced in 1.7-mm sheets by varying the rotational speed from 1900 to 2900 rpm and the welding time from 2 to 3.4 s while keeping the plunge depth constant at 1.75 mm. After shear tensile tests, the samples were analysed using optical microscopy and scanning electron microscopy. The strength of the weld and its ductile/brittle behaviour are associated with the nucleation, growth and propagation of two types of cracks: circumferential cracks and annular cracks. Welds produced with longer welding times (≥3 s) and slower rotational speeds (1900 rpm) had higher strengths, low scattering and high energy absorption prior to failure, while welds produced with short welding times (2 to 2.4 s) resulted in poor joints, especially when they also used high rotational speeds.

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The main objective of the current work was to produce sound Refill FSSW joints between AA6181-T4 aluminum and DP600 steel plates. The steel plates were used in two different surface conditions: with and without galvanized surface layer. The Taguchi statistical method was used to find out the set of parameters indicated to produce joint with higher mechanical resistance. Then, the possibility of joining these dissimilar metals using the Refill FSSW process was verified. Tool rotation speed and welding time were varied to observe its effect over the joint behavior. The results of lap shear tests showed that galvanized layers do not cause any substantial change on the final joint mechanical resistance, even though different joining mechanisms had been observed.