Background: The clinching is one of the most common metal joining processes in the manufacturing of metal plate\nbased products (similar or dissimilar, pre-coated or galvanized), especially when the assembly without adding major\njoining elements is required. When the clinched joints work in an aggressive environment, particular attention would be\nplaced on the electrochemical stability and corrosion resistance of the metal constituents (Mizukoshi and Okada 1997). In\njoining design, an appropriate material selection reduces the electrochemical potential differences and prevents\nsignificant galvanic currents (Kruger and Mandel 2011; Calabrese et al. 2014). The durability of the metal joints could be\nheavily influenced in a corrosive environment, whereas the less noble material will tend to increase its corrosion rate;\ninstead the more noble one will reduce its electrochemical dissolution (He et al. 2008; Bardal 2004). Accelerated ageing\ntests (i.e. salt fog test) were carried out to evaluate the durability of the joints in highly aggressive environments\n(Calabrese et al. 2013; LeBozec et al. 2012). Although the durability for a long time of the clinched joint in a corrosion\nenvironment is a known problem, few works focus the attention on the relationship between durability of joints and\nelectrochemical behaviour of the metal constituents. The aim of the present work is to evaluate the durability at long\nageing time in salt spray test (according to ASTM B117) of carbon steel/aluminium alloy joints, obtained by clinching.\nMethods: The investigation has been conducted on one total thickness (2.5 mm) of unsymmetrical joints (i.e. thickness\nsheets of 1.5 mm and 1 mm) to inquire about the effect of corrosion on the two different unsymmetrical configurations\n(St1.5/Al1 and St1/Al.5). The joint resistance has been determined, by means of shear tests of single-lap joints in\naccording to ISO/CD 12996. The samples were exposed to critical environmental conditions following the ASTM B 117\nstandard. To inquire the damage evolution of the samples, 0, 1, 2,3, 5, 7, 10 and 15 weeks of ageing time have been\nchosen. Seven samples for each combination and for each ageing time were realized. A Design of Experiment has been\nperformed, followed by the ANOVA of the results to analyse the influence of the two factors, thickness combinations\nand ageing time, on the mechanical properties of the joints.\nResults: The two sets of joints show a different behaviour at increasing ageing time: the St1.5/Al1 batch shows a\nconstant decay of the load values, instead the St1/Al1.5 set maintains acceptable values of resistance for several weeks\nof ageing, at tenth week the mechanical stability is strongly impaired. In the latter case the presence of the thin oxide\nlayer at the overlapping interface, which behaves as an adhesive interlayer, and the larger thickness of the aluminium\nplate improve the resistance of the St1/Al1.5 joints. Statistical analysis confirms that the two thickness combinations and\nageing time are the significant factors. At zero weeks, neglecting the effect of ageing, the maximum load values of all\nsamples belong to the same population. This means that the resistance of the clinched joints is the same regardless the\ncombination of thicknesses, but by considering both the ageing and thickness, the analysis of variance shows that both\nthickness and weeks are significant parameters distinguishing two different populations in the distribution of loads.\n(Continued on next page)
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