６０８２アルミニウム合金溶接材の各部の破壊靭性

Abstract

The effects of Mn addition on toughness of welded Al–Mg–Si alloys have been investigated. In-situ SEM observations of fracture toughness tests have also been used to characterize crack initiation and propagation behavior through the weldment. Recrystallization of HAZ is completely suppressed by adding Mn, while a sample without Mn exhibits recrystallized coarse grain structure. Recrystallization in HAZ can lead to drastic decrease of its fracture toughness comparing with the samples containing Mn. Many microcracks are distributed at grain boundaries within several mm ahead of a crack-tip where crack-tip stresses are relatively weak. Fracture-mechanical analysis reveals that the toughness degradation is attributed to such premature damage evolution before the onset of crack extension. However, the microcracking is not attributed to so-called liquation cracks, but degradation of the grain boundaries due to the formation of film-like Al–Mg intermetallic compounds. The microcracks ahead of the crack-tip aligned at an angle of approximately 60° from an initial notch direction. Tension obliquely applied to the HAZ next to welding pool appears to give rise to such inclination of the microcracks. It is suggested that the toughness degradation can be suppressed by utilizing such anisotropy even if the GB films are formed during welding.