Notch Toughness and Fracture Characteristics of Al-Zn-Mg Alloy and its Welds

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In this paper, notch toughness and fracture characteristics of Al-4% Zn-2%Mg alloy and its welds are investigated. The notch toughness is evaluated by the ratio of tensile strength between notched and unnotched specimens and the Charpy impact values. Notch toughness of base metals is considerably influenced by the state of precipitates, though the notch toughness evaluated by tensile tests is not always similar to that by impact test. Notch toughness of welds is remarkably influenced by the filler metals and it is also reduced with aging after welding. Particularly, the Charpy impact values of welds tend to be hyperbolically decreased with the increase of Vickers hardness. Granular ruggedness and parallel lamellar are likely to appear in the optical fractography of welds with excellent notch toughness and with poor notch toughness, respectively. By electron fractography, the former is shown to be a dimple patterns associated with ductile rupture and the latter to be a cleavage pattern associated with brittle rupture. It is considered that the notch toughness and the fracture characteristics of Al-Zn-Mg alloy and its welds microscopically depend upon the state of precipitates and the dendritic structures such as dendrite cell size, distribution of second phase and solute concentration in matrix.
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