Effects of ageing on mechanical durability of round clinched steel/aluminium joint

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