Influence of Heat Treatment on the Corrosion Resistance of Welded Joints of Aluminum Alloys of the Al-Mg-Si-Cu System

We present the results of complex investigations of the influence of full cycle of heat treatment (HT) including the procedures of hardening and artificial aging on the corrosion and corrosion-mechanical resistance of welded joints of an aluminum alloy of the Al-Mg-Si-Cu system with a thickness of 1.2 mm. Welded joints were prepared with the use of a nonconsumable electrode by the free arc method in the protective atmosphere of gaseous argon. The results of electrochemical investigations demonstrate that the procedure of HT promotes weakening of the electrochemical heterogeneity between the base metal and the weld. This leads to a decrease both in the drop of potential from 18 to 9 mV and in the current of anodic dissolution. Moreover, HT does not lower the resistance to the layer corrosion of this welded joint, which can be estimated as equal to grade 2-3 in the initial state and to grade 1 after HT. As the signs of layer corrosion, one can use darkening of the surface and the formation of local delaminations with diameters <= 1.5 mm whose area on each surface does not exceed 1%. For the base metal, we observe an insignificant lowering of the resistance to intercrystalline corrosion (increase in the depth of fracture of the grain boundaries from 0.084 mm to 0.209 mm). However, the mean depth of intergranular fracture of the welded joint decreases insignificantly, namely, from 0.3 mm to 0.25 mm. In general, the depth of intercrystalline corrosion after HT does not exceed the permissible value (0.35 mm). The influence of HT on the resistance to corrosion-mechanical cracking under constant loading in the case of full immersion in a 3% NaCl solution is ambiguous: for the base metal, we observe an insignificant decrease in the time of fracture (from 73 to 66 h on the average); at the same time, for welded joints, the fracture time increases by about 35% (from 20 to 33 h). This confirms the decrease in heterogeneity of the entire welded joint. The results of comparative studies demonstrate that the welded products subjected to the full cycle of HT can be characterized by their resistance to intercrystalline corrosion and corrosion cracking.