THE INFLUENCE OF PRIOR NATURAL AGING ON ARTIFICIAL AGE HARDENING IN AL-MG-SI ALLOYS

Abstract

Aluminum alloys of the 6000 series (Al-Mg-Si) are among the most prevalent and established structural materials in the world. Their use dates back decades, and currently they are crucial to the construction and transportation industries. One of the primary reasons for this is heat-treatability, i.e. the ability to control strength and ductility relationships with a relatively low temperature artificial aging treatment. Although multiple aluminum alloys can be employed for this purpose (e.g. 2000, 7000 series), Al-Mg-Si alloys are capable of achieving a highly attractive combination of moderate to high strength (sometimes \u3e 350 MPa UTS), high ductility (\u3e20% elongation), and excellent corrosion resistance, extrudability, weldability, and surface finish. The largest unknown in the world of 6000 series alloys today is a result of natural aging (NA) prior to artificial aging (AA), and the rather mysterious impacts on strengthening precipitation and hardenability that are produced. The origins of these impacts involve complex interactions between solute atoms, solute-clusters, and vacancies, which ultimately influence the kinetics and thermodynamics of precipitation. Although this has been the topic of extensive research in recent years, the actual mechanisms responsible are still very controversial. This dissertation will focus on the effects of prior natural aging on artificial age hardening response under a wide range of compositional and heat treatment conditions. Ultimately it will be shown that the behavior of naturally aged solute-clusters during artificial aging is dependent on alloy composition, and furthermore that these clusters are directly responsible for the changes in strengthening precipitation and performance. General background to the topic and an overview of the experimental methods are described in Chapters 1-2. The most important chapters of this dissertation are Chapters 3-4, which focus on the influence of prior NA in high strength Al-Mg-Si alloys after a water quench from the solutionization temperature. In Chapter 3, a compositional (Mg/Si) dependence of the NA effect is identified, and proposed to due to a corresponding dependence of NA cluster stability during artificial aging. In Chapter 4, the actions of NA clusters at AA temperatures are investigated with atom probe tomography, and the relative thermal stabilities suggested in Chapter 3 are proven. The rest of the dissertation includes studies focused on the NA effect under different compositional and heat-treatment conditions, using the previous cluster thermal stability findings to aid in analysis (Chapter 5). Mechanical property characterization of key experimental conditions is performed in Chapter 6, and good agreement with the previous results is found. Finally, key conclusions, implications, and suggested future work is summarized in Chapter 7 & 8

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This paper was published in Michigan Technological University.

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