Weldability of High Strength Al-Zn-Mg Alloys (Report 2)

Aging Characteristics of Pure Al-Zn-Mg Ternary Alloys

Abstract

In the previous report, the recovery of hardness in H.A.Z. of commercial Al-Zn-Mg alloy containing Cr and Mn was examined in details. In the case of welds on T6 treated materials, the region where partial reversion had occurred during welding was the weakest part in H.A.Z.. This phenomenon seems to be due to the existence of Cr and Mn in specimen. From this point of view, this paper deals with pure Al-Zn-Mg alloys. X-ray small angle scattering method and thin foil electron microscopy have also been used to make further investigation about the problem in H.A.Z..
The results are summarized as follows;
(1) T4 treated materials consist of G.P. zone (10-15 Å radius). T6 materials consist of G.P. zone (about 30 Å radius) and η' phase at boundaries or sub-boundaries.
(2) In the region where perfect reversion had occurred, the recovery rate of hardness became larger as Zn content increased. Mg enhanced the value of hardness immediately after welding. Inculaticn time was recognized in the material containing Cr and Mn.
(3) In the weakest part in H.A.Z. of welds on T6 treated material, partial reversion occurred and G.P. zone had a tendency to resolve as peak temperature increased.
The coarsening of η' phase could not be ascertained, but the distribution of partial reversed G.P. zone was sparse.
(4) The width of H.A.Z. of welds on T4 treated material was larger than that of welds on T6 treated material in the case of constant heat input. This phenomenon is related to the fact that the size of G.P. zone in T4 treated material is smaller than that of T6 treated materials