Microstructural evolution in aluminium alloy 7050 during processing

doi:10.1016/j.msea.2004.05.006

Materials Science and Engineering: A

Volume 382, Issues 1–2, 25 September 2004, Pages 112-121

https://doi.org/10.1016/j.msea.2004.05.006 Get rights and content

Abstract

The microstructural evolution in AA7050 during post homogenization cooling, and its influence on behaviour during preheat, hot rolling, and solution treatment, has been investigated. It is shown that slow cooling, at a rate typical of industrial processing, leads to a complex sequence of large, heterogeneously nucleated precipitates. In particular, S-phase particles grow to over 7 $μ$ m in diameter. These particles are retained during preheat to rolling, and are above the critical size required to act as particle stimulated nucleation sites for recrystallization. Furthermore, by removing solute they prevent the formation of fine particles during preheat. This leads to a final recrystallized fraction that is over four times larger in homogenized and slow cooled material than in material quenched rapidly from homogenization.

Introduction

The industrial production of high strength 7xxx (Al–Zn–Mg–Cu) alloys to produce thick plate involves a number of processing stages including casting, homogenization, preheat, hot rolling, solutionizing and ageing. Precipitation and dissolution of second phase particles occur during each of these processing steps and have a major influence on the microstructural evolution in subsequent stages. Therefore, to understand the origins of the microstructure of the final product, it is necessary to understand these interactions. This understanding is also critical in designing optimized processing practices.

Precipitation during intermediate processing steps usually occurs under non-isothermal conditions. The particular case of non-isothermal precipitation in high strength 7xxx alloys during cooling from solution treatment has been the subject of a number of studies [1], [2], [3], [4], [5], [6], [7]. This precipitation is of industrial importance since the removal of solute into coarse precipitates formed during cooling has a detrimental effect on the subsequent age-hardening response. Under these conditions, a sequence of precipitate phases form, nucleated on heterogeneous sites such as grain and subgrain boundaries, dispersoids and dislocations. The most commonly observed precipitate phases are the S (Al₂CuMg) orthorhombic phase, the Mg(Zn₂,AlCu) M (or η) hexagonal phase and the Al₃₂(Mg,Zn)₄₉ T phase [1], [2], [6], [7]. Which of these phases are observed in practice depends sensitively on the local chemistry, prior microstructure, and cooling rate.

Cooling following solution treatment is not the only point in the processing route where high temperature, non-isothermal precipitation can occur. Such precipitation is also likely earlier in the processing sequence, when the ingot is cooled after homogenization. In this case, the homogenized ingots are not usually force quenched (unlike after solution treatment) and this can result in very slow cooling rates (>24 h to cool to room temperature). Such conditions allow a long time for precipitation and coarsening, potentially leading to the formation of large particles that may play a significant role during subsequent processing.

Precipitation during post-homogenization cooling has received far less attention than precipitation during cooling after solution treatment, largely because any particles formed during post-homogenization cooling are assumed to be broken up and redissolved during solutionizing. However, such coarse particles may have an influence on the evolution of microstructure in the intervening processing steps, for example by acting as sites for particle stimulated nucleation of recrystallization (PSN) [8]. They may also have a direct detrimental effect on the final plate if they become large enough during slow cooling that they remain after solution treatment.

Limited studies of post-homogenization cooling have shown that a significant volume fraction of precipitation does occur in 7xxx alloys under normal cooling conditions. For example, Couch et. al. [9] have shown that for AA7010, cooling rates of less than 130 K/h lead to precipitate volume fractions that increase with decreasing cooling rate up to the equilibrium fraction. However, the nature and distribution of the precipitates (i.e the microstructure), as well as the influence on subsequent processing has yet to be reported.

The aim of this work was to characterize precipitation during cooling from homogenization in detail, for cooling rates typical of thick commercial ingots destined to be rolled to plate. Precipitation under these conditions is highly heterogeneous, and this study therefore provides an insight into heterogeneous precipitate distributions in these alloys. In addition, the subsequent microstructural evolution during preheat, rolling, and solution treatment has also been investigated.

Section snippets

Experimental

Specimens approximately 100 mm in length, 40 mm wide and 15 mm in thickness were cut from the quarter thickness (T/4) position of a slice of stress relieved, direct-chill cast 7050 ingot, supplied by Alcoa Flat Rolled Products, Europe. At this thickness position, the ingot is enriched in the major alloying elements as a result of macrosegregation [10], and will therefore be most prone to precipitation on cooling. Stress relief was performed by soaking the ingot for 6 h in a furnace heated to 400 °C.

Equilibrium calculations

To understand which precipitation reactions are likely to occur on slow cooling it is useful to first know how the equilibrium fraction of the precipitate phases changes as a function of temperature. Fig. 2 shows a calculation of the equilibrium precipitate fractions, for the particular 7050 composition used in this study, performed using the Alcoa thermodynamic database [13]. Also marked on this plot are the homogenization temperature (H) and the temperatures at which specimens were quenched

Conclusions

A study has been performed on the effect of slow cooling of 7050 ingot from the homogenization temperature on the microstructural evolution. The influence of the precipitation that occurs during slow cooling on the subsequent microstructural development during preheat, rolling, and solution treatment has also been investigated. The following conclusions may be drawn from this work.

(1)
A complex sequence of precipitate phases forms on slow cooling from homogenization. Above 460 °C fine S’ needles

Acknowledgements

The author is grateful to Alcoa Flat Rolled Products (FRP), Europe and the Alcoa Technical Center (ATC) for funding this work. Thanks to Alex Morris and Sunil Khosla of Alcoa FRP Europe and Larry Lalli, Ralph Shuey and Jaakko Suni of the ATC for valuable discussions and provision of materials.

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