Effect Of Reversion Process On Expandingperformance Of An Al-zn-mg-cu Alloy tube In T4 Temper

摘要

Cold workability of Al-Zn-Mg-Cu alloys in T4 temper becomes lower with natural age hardening, while it improves with reversion heat treatment (RHT). But, there are few examples in industrial use. In this study, the effect of RHT on the cold workability, especially the expanding performance of an Al-Zn-Mg-Cu alloy tube in T4 temper was investigated. The most appropriate condition of RHT on which the hardness decreased and the workability improved was at 433K for 45s in this study. In the case of long time RHT at relatively low temperature, e.g. 393K for 3600s, the workability improved although the hardness after RHT became equal to that before RHT. The hardness attained to the lowest at 393K for 600s, so it was considered that the G.P. zones dissolved and afterward the second-phase particles (η' and/or η phases) precipitated at 3600s. In the specimen without RHT, microbands were observed that had developed in one direction, while they were developed in two directions in the reversion heat treated specimen. Because the second-phase particles formed during RHT were semicoherent and/or incoherent to the matrix, it was thought that they acted as the dissolution sites of dislocations and/or recovery sites. As a result, the increase of the activated slip systems resulted in the formation of the sessile dislocations, and the deformation uniformly propagated. It was then suggested that the reversion at relatively lower temperature for longer time could be useful to industrial production.