Grain boundary transition associated intergranular failure analysis at TMAZ/SZ interface of dissimilar AA7475-AA2198 joints by friction stir welding

Post-weld heat-treatment (PWHT) of dissimilar AA7475-AA2198 friction stir welds (FSW) at 560 degrees C followed by water-quenching since cracks were observed at the TMAZ/SZ interface in retreating AA7475 upon quenching. Microstructural studies revealed that composition difference at high-temperature solution-treatment was the driving force for Cu diffusion from advancing AA2198 through grain boundary liquid-metal thin-films, and caused intergranular segregation of a sufficient amount of Cu-Zn phase. Weld residual stresses, weak interfacial strength, grain boundary transition (premelting), and wetting along with thermal expansion coefficient discrepancies were found to be the associated phenomena in intergranular failure. (C) 2020 Elsevier B.V. All rights reserved.