It is well known that high strength aluminum alloys such as 7075, 7050, and 7150 have been widely used in many applications for their good combinations of strength and stress corrosion cracking resistance. However, they are unfortunately limited to utilize in some applications because of their poor weldability. So our laboratory have developed Al-Zn-Mg-Mn-Zr wedable aluminum alloy recently which has improved strength and not much decreased ductility compared with existing weldable aluminum alloys.
In this study, to form a part of mechanical tests of Al-Zn-Mg-Mn-Zr wedable aluminum alloy, the effects of environment on the low cycle fatigue made a comparative study with Al 7039 existing wedable aluminum alloy, with variation of manufacturing processes and solution heat treament conditions of Al-Zn-Mg-Mn-Zr wedable aluminum alloy.
The largest difference in chemical compositions between Al-Zn-Mg-Mn-Zr alloy and Al 7039 is contents of Mn, Zr, Cr, which are called transition elements. Namely, Al-Zn-Mg-Mn-Zr aluminum alloy was added 0.16wt% Zr instead of Cr and increased in Mn contents from 0.18wt% to 0.68wt% relative to Al 7039. This chemical compositional difference made a change in microsturcture by formation of Zr-dispersoids and largely increased Mn-dispersoids density in Al-Zn-Mg-Mn-Zr wedable aluminum alloy. The increase of Mn-dispersoids density is found to significantly enhance the strength of the Al-Zn-Mg-Mn-Zr alloy and have similar low cycle fatigue(LCF) life in various environments compared with Al 7039. For the origin of the beneficial effect of Mn-dispersoids on the mecanical properties, it can be concluded that the large increment of strength is probably due to hardening effect of Mn-dispersoids, and simultaneously no marked decreasing of LCF life is due to the improved dispersion of slip behavior of the planar slip and reduction of strain or stress concentration by the homogenized slip.
Meanwhile, alternation of solution heat treatment condition of Al-Zn-Mg-Mn-Zr alloy from 465℃, 90min to 490℃, 120min, increase in Y.S. and U.T.S. a little together with slightly decreased in elongation, and improved LCF life in various environments in some degree. And extruded Al-Zn-Mg-Mn-Zr alloy, which has fibrous grain structure, largely varied in LCF life with variation of total strain amplitude than rolled Al-Zn-Mg-Mn-Zr alloy does.
Decrease in LCF life of Al-Zn-Mg-Mn-Zr alloy and Al 7039 in corrisive environment is regarded due to acceleratetion of fatigue crack initiation by corrosion at inclusion-matrix interface.
