2001 Volume 42 Issue 11 Pages 2386-2391
6082 Al alloys are commercial and medium strength alloys, widely used as materials for welded structures. The purpose of this study is to investigate the effects of Mn addition on toughness of welded Al–Mg–Si alloys. To evaluate microstructural effects quantitatively, in-situ SEM observations of crack initiation and propagation behaviors through weldment are carried out. For the consideration of in-situ observation of fracture toughness test, stress field at crack-tip is analyzed using elasto-plastic finite element method (Hereinafter, FEM.) assuming that a crack is near a boundary between a weld metal and heat affected zone (Hereinafter, HAZ.). When small amount of Mn is added, recrystallization is completely suppressed as compared to specimens to which no Mn is added, thereby fibrous grains are kept. On the other hand, recrystallization of HAZ causes drastic decrease in fracture toughness in the case of no Mn addition. With the extension of a main crack, many microcracks are formed at grain boundaries ahead of a crack-tip despite the fact that the stress is relatively low. Such microcracking is not attributed to so-called liquation cracks, but the degradation is caused by the formation of film like Al–Mg intermetallic compounds at grain boundaries. The microcracks are aligned ahead of the crack-tip at an angle of 60 degrees from an initial notch direction. This is attributable to the experimentally-observed direction of the intermetallic compound film, which is also confirmed by the numerical analysis.