It is well known that high strength aluminium 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 weldablilty due to mainly rapid quench sensitivity inducing brittleness and cracking in the unsound heat affected zone after conventional welding.
In this present study, to see if the added Mn content improve the strength of the weldable Al-Zn-Mg alloy, the effects of Mn-dispersoid on the mechanical properties and precipitation characteristics of the Al-Zn-Mg alloy have been investigated. In addition the effects of precipitate free zone(PFZ) width and Mn-dispersoid on low cycle fatigue behaviors have also been examined.
The range of chemical composition of the aluminium alloys used in the study are 4.10-4.70wt% Zn, 2.8-3.0wt% Mg and 0.12-0.18wt% Zr. The ingots were hot-extruded and peak-aged by double aging. the porpose of the addition of Zr in these alloys is to refine the grain size and at the same time to inhibit recrystallization.
Adding manganse in to Al-Zn-Mg alloy in the range of 0.5-1.13wt%, Mn-dispersoid which is composed of Al, Mn and Zn is formed but does not influence the precipitation kinetics in an Al-Zn-Mg alloy, and the density of Mn-dispersoid is increased uniquely with Mn content without changing the morphology and size of the dispersoid. Especially the Mn-dispersoid is found to significantly inhance the strength of the alloys to that of high strength aluminium alloys such as Al 7075 and 7050 without sacrificing elongation.
For the origin of the beneficial effect of Mn-dispersoid on the mechanical properties, it can be concluded that the large increment of strength is probably due to hardening effect of Mn-dispersoid working as the dispersion hardening particle, and simultaneously not much decreasing of elongation is due to the dispersoin of slip behavior of the planar slip and reduction of strain or stress concentration by a homogenization of slip.
In order to study the effect of PFZ width on low cycle fatigue behavior, one Al-Zn-Mg alloy without Mn was prepared. Having appropriate two step-aging, specimens are hear-treated to have two different width of PFZ but almost same precipitates in matrix or on grain boundary and tensile properties.
With increasing width of PFZ, the life of low cycle fatigue is increased, and it is explicitly observed in TEM work that slip bands are developed well in PFZ and blocked on grain boundary For the impovement of the low cycle fatigue life, it is believed that stress or strain concentration at the end of slip band induced by planar slip is relaxed increasingly with increment of PFZ width. This phenomenon can be explained by the double pile-up model in conjunction with Ryum's proposal.
Furthemore, Mn-dispersoid is also observed to improve the properties of low cycle fatigue. Considering the dislocation arrangement formed during cyclic deformation observed by TEM, it is belived that Mn-dispersoids are distributed evenly in an Al-Zn-Mg alloy to transfer the deformation mechanism from planar slip to wavy slip accompanying with cross slip.
