Title page for ETD etd-10172011-195355


Document Type Doctoral Thesis
Author Moller, Heinrich
Email hmoller@up.ac.za
URN etd-10172011-195355
Document Title Optimisation of the heat treatment cycles of CSIR semi-solid metal processed Al-7Si-Mg alloys A356/7
Degree PhD(Eng)
Department Material Science and Metallurgical Engineering
Supervisor
Advisor Name Title
Prof W E Stumpf Supervisor
Keywords
  • transmission electron microscopy
  • high pressure die casting
  • rheocasting
  • atom probe tomography
  • semi-solid metal
  • temper
  • Al-Si-Mg alloys
  • heat treatment
  • artificial aging
  • natural aging
Date 2011-09-06
Availability unrestricted
Abstract
Conventional casting alloys Al-7Si-Mg A356/7 contain between 6.5 and 7.5% Si, together with 0.25-0.7% Mg and are used for critical castings in the automotive and aerospace industries. These alloys are also the most popular alloys used for semi-solid metal (SSM) forming due to good castability and fluidity imparted by the large volumes of the Al-Si eutectic. Despite their industrial importance, there is a lack of detailed research work revealing precipitate micro- and nanostructural evolution during aging of these alloys compared with the Al-Mg-Si 6000 series wrought alloys. This study characterises the heat treatment response of SSM-processed Al-7Si-Mg alloys in comparison with conventionally liquid cast alloys (investment casting and gravity die casting). It is shown that, provided that the maximum quantity of the alloy’s Mg is placed into solid solution during solution treatment, and that the alloy’s Fe content is within specification, the response to age hardening of Al-7Si-Mg alloys is independent of the processing technique used. The nanostructural evolution of Al- 7Si-Mg alloys after artificial aging with and without natural pre-aging has been characterized using transmission electron microscopy and atom probe tomography and correlated with hardness and mechanical tensile properties. The number densities and Mg:Si ratios of solute clusters, GP zones and β"-needles were determined. The heat treatment response of SSM-processed casting alloys A356/7 alloys are also compared with SSM-processed Al-Mg-Si 6000 series wrought alloys, with the advantage of having similar globular microstructures. The high Si-content of the casting alloys compared to the wrought alloys offers several advantages, including a faster artificial aging response (shorter T6 aging cycles), higher strength for comparable Mg contents and less sensitivity to prior natural aging on peak strength. Finally, an age-hardening model was developed for the Al-7Si-Mg alloys, including a method of incorporating the effects of changes in Mg-content on the aging curves.

© 2011 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria.

Please cite as follows:

Moller, H 2011, Optimisation of the heat treatment cycles of CSIR semi-solid metal processed Al-7Si-Mg alloys A356/7, PhD thesis, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-10172011-195355/ >

D11/9/112/ag

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