Mechanisms controlling natural aging and its effect on artificial aging in Al-Mg-Si alloys with and without trace element additions

Marion Werinos

Research output: ThesisDoctoral Thesis

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Abstract

Al-Mg-Si alloys (6xxx series) represent the commercially most important group of age hardenable aluminum alloys. The wish to control natural aging kinetics and to find a solution to the negative effect of room temperature storage on artificial aging is of major importance for the aluminum manufacturing industry. The main objectives of this thesis were the minimization of natural aging and maximization of the artificial aging potential of Al-Mg-Si alloys with and without trace element additions to commercial Al-Mg-Si alloys. A general understanding could be acquired how trace Sn additions to the alloy AA6061 are able to temporarily suppress natural aging and simultaneously enhance artificial aging kinetics: The strong Sn-vacancy binding energy results in trapping of most quenched-in excess vacancies in predominantly Sn-vacancy pairs. This generates a reduced number of untrapped vacancies which can control diffusional processes of other alloying atoms (Mg, Si or Cu) and thus significantly retards all natural aging clustering processes, whereas the maximum suppression of natural aging is obtained for Sn contents above the solubility limit of ~100 at.ppm. While suppression of natural aging prevails, the release of vacancies allows diffusion on demand for the precipitation of the peak hardening phase. It could be shown that the effect of Sn is phenomenological similar to the positive effect of natural pre-aging at high temperatures > 210 °C in the commercial alloy. Additionally trace Sn addition generates both ultrafast artificial aging kinetics and superior peak hardness. Usually processing parameters and compositional limits of different Al-Mg-Si alloys and products vary. Lower solution treatment temperatures than 570 °C yield a decreasing delay of natural aging hardening, which is explained by a lower maximum quenchable Sn solubility. The significant decrease in the suppressive effect of Sn with Si addition is attributed to Si-related clustering that controls the beginning of natural aging and its kinetics, while Si additionally lowers the quenchable Sn solubility. Cu does not influence the Sn solubility and is therefore believed to show a similar, but weaker effect as Sn, whereas Mg lowers the quenchable Sn solubility only. For an industrial implementation of the trace element effect, the knowledge that material produced in laboratory behaves comparably as industrially produced wrought sheets or plates is important. Industrially produced material only shows a higher as-quenched hardness. Finally, a design strategy for a maximum suppression of natural aging in Al-Mg-Si alloys was developed and led to a new Sn-added Al-Mg-Si alloy that shows natural aging stability of > 6 months and a significant artificial aging potential. Further, combined Sn and indium (In) addition show comparable natural aging kinetics after quenching from different solution treatment temperatures. A strong dependence of natural aging kinetics on the storage temperature was measured for various AA6061 alloys. Applying this knowledge the developed alloy should show up to ~7 years of stability at 5 °C. Based on an in-depth analysis of recent literature on natural aging in Al-Mg-Si alloys and its effect on artificial aging and the connections we have drawn, the picture about underlying mechanisms could be refined. Sn and/or In could be interpreted to decrease the cluster number density during natural aging while increasing the cluster size. Also the effect of natural aging temperature and of prolonged natural aging (up to years) on artificial aging could be interpreted. Furthermore, preliminary results about the influence of stretching and the quenching rate were discussed. In this view, the project succeeded in the development of an alloy that shows maximum suppression of natural aging and simultaneous high artificial aging potential which may pave the way for a new class of Al-Mg-Si all
Translated title of the contributionKontrollmechanismen der Kaltaushärtung in Al-Mg-Si-Legierungen und ihr Einfluss auf die Warmaushärtung mit und ohne Zugabe von Spurenelementen
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Pogatscher, Stefan, Co-Supervisor (internal)
  • Uggowitzer, Peter, Assessor B (external)
  • Antrekowitsch, Helmut, Assessor A (internal)
Publication statusPublished - 2016

Bibliographical note

embargoed until null

Keywords

  • aluminum alloys
  • aging
  • precipitation
  • phase transformation kinetics
  • vacancies

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