PSI - Issue 25

Tiago Bento et al. / Procedia Structural Integrity 25 (2020) 234–245 Tiago Bento/ Structural Integrity Procedia 00 (2019) 000 – 000

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3. Process description

On account of the plastic deformation and heating, which in aluminum alloys results in temperatures between 400ºC and 480ºC (Chao, Qi, and Tang 2003), the material undergoes a complex recrystallization process which leads to a modified metallographic structure in the joint cross-section. The resultant material can be subdivided into 4 zones with different physical and mechanical properties, base material (BM), heat affected zone (HAZ), thermo mechanically affected zone (TMAZ) and stir zone (SZ) (Chao, Qi, and Tang 2003; R.S. Mishra and Z.Y. Ma 2014). This is especially relevant because the monitoring fibers installed were located on the HAZ in the advancing side of the weld. Whereas the main disadvantage of FSW lap welded joints is the presence of continuous defects, hook and cold lap, along the weld line, adhesive joints ’ mechanical properties are sensitive to environmental co nditions. The hybridization process aims to unite the main advantages of both processes and overcome their drawbacks by creating a more even stress distribution in the joint, while at the same time adding damage tolerance through two competing failure modes. To that purpose, adhesive bonding has been combined with FSW resulting in a hybrid joining technique. This was made possible by the wide versatility of applications and mechanical characteristics of the existent adhesives. The main conclusions drawn in these FSW hybrid studies, (Braga et al. 2015; Maciel, Bento, et al. 2019; Maciel, Infante, et al. 2019; Reynolds 2000), can be summarized as the following: • For aluminum alloys, the ultimate load in hybrid configuration is about 60% higher than in the FSW only configuration for lap joints; • For FSW aluminum lap joints, the fatigue life of hybrid specimens is improved due to the adhesive interlayer which by itself has a very good fatigue performance due to its high ductility and continuous layer with no stress concentrations, • The monotonic strength of the hybrid joint is similar to the one in the adhesive joints, however in the latter, higher ductility was achieved; • The hybrid joints presented higher ductility than FSW only joints; • The hybrid joints do not necessarily improve the mechanical performance of the joints when compared with the adhesive ones. In the best-case scenario, they are equal. However, they present other advantages such as, improved damage tolerance (two failure mechanisms) or ease of manufacturing (welding closes the joint while adhesive cures, there is no need for extended clamping time); • Surface preparation is critical for high strength reliable weld-bonded joints; Another relevant aspect of the coupling between these technologies regarding this paper however is the possibility to integrate strain sensoring fibers in the joint, which will be kept in place by action of the adhesive. In this work, AA6082-T6 aluminum alloy 2 mm sheets were joined together using FSW and a two part epoxy adhesive Araldite 420 A/B. The 6082 alloy is very common in the aerospace industry and is intended for structural applications like rods, bars and profiles (Moreira et al. 2008). The T6 condition is obtained through artificial ageing at a temperature of approximately 180 ºC (Ericsson and Sandström 2003). In Table 1. AA6082-T6 chemical composition the nominal chemical composition is presented whereas the mechanical properties are in Table 2. It is important to consider that during the welding procedure, the mechanical properties are altered due to the high temperatures and deformations, forming different zones. Table 1. AA6082-T6 chemical composition (Braga and December 2017) Al [%] Cr [%] Cu [%] Fe [%] Mg [%] Mn [%] Si [%] Ti [%] Zn [%] Zn [%] 95.2-98.3 ≤0.25 ≤0.10 ≤0.50 0.6-1.2 0.4-1.0 0.7-1.3 ≤0.10 ≤0.20 ≤0.15 3.1. Hybrid technology 3.2. Materials