Performance of humid gas stress corrosion cracking of Al-Mg-Si-Cu alloys

The 6110 aluminum alloys with excess Si and copper addition are considered advantageous in terms of strength, compared to the 6061 alloys. In the present study, performance of humid gas stress corrosion cracking (HG-SCC) was investigated on samples cut from ingots of four Al-Mg-Si-Cu alloys within the standard composition range of the 6110 alloy; Cu was changed from 0.2mass% (the lower limit) to 0.4mass%, and Si was changed from 1.0mass% (slightly lower than the median) to 1.5mass% (the upper limit). The samples were homogenized at 470°C for 7h, solution-treated at 555°C for 3h, water-quenched and then artificially aged at 180°C for 6h. In all the four specimens, crack propagation length was found to exceed the criterion of the HG-SCC standard, and hence they were classified as fail in the test. The HG-SCC region of all the specimens had a tongue shape with granular units approximately 80-90 μ m in size, where micro-dimples and second-phase particles were observed. All the specimens had equi-axed grains consisting of dendrite-cell structures. From these results, the large HG-SCC propagation length was attributable to the solidified microstructures remaining in all the specimens caused by lack of plastic working, resulting in insufficient toughness.