Semi-solid Processing of Al-Si7-Mg Alloys

Muhammad Kamran

Research output: ThesisDoctoral Thesis

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Abstract

The cooling slope can be an effective technique for the production of a thixotropic precursor alloy but the control of the microstructure is complex and is dependent upon the processing parameters. This work investigates the semi-solid processing of Al-Si-Mg alloys using the cooling slope technique. A newly determined contact time parameter was used obtaining a correlation with undercooling, inclination angle and flow velocity. For a non-refined non-modified AlSi7Mg alloy, the effect of pouring temperature (680, 660 and 640 °C), contact time (0.04, 0.09 and 0.13 s), inclination angle (20, 40 and 60°), mould material (sand and metal), sample thickness (10 mm to 34 mm in sand mould) and coating on the cooling slope (graphite and boron nitride) were examined with respect to the final microstructure which, for optimal results should be fine and globular. The microstructure and temperature characteristics of the alloy were compared with an as-cast and as-received commercial AlSi7Mg alloy and a commercially produced magnetohydrodynamic stirred (MHD) A356 alloy. The results indicated that the chemical composition of the alloys have a prominent effect on features such as solidification reaction temperatures, solidification sequence and intermetallic phases. From findings it became clear that methods typically used in semi-solid processing studies to analyse globular grains were erroneous. It was found that the anodizing of samples helped in reducing these errors. The change in alloy characteristics has also a profound effect on the change in fraction solid in the semi-solid range which resulted in the variation in both the temperature and fraction solid limit for thixoforming. The cooling slope has a dual role: the extraction of heat thereby affecting the grain size and morphology both on the slope interface and the bulk melt by creating a thermal undercooling necessary for the columnar to equiaxed transition (CET); and, by influencing the dominant nucleation mechanism. The final observed microstructures were additionally influenced by the heat extraction and thermal gradients imposed by the mould material. This explains the complex and different microstructure observed
Translated title of the contributionProduktion von halbflüssigen Al-Si7-Mg Legierungen
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Schumacher, Peter, Assessor A (internal)
  • Kneißl, Albert, Assessor B (internal)
Publication statusPublished - 2008

Bibliographical note

embargoed until null

Keywords

  • Semi-solid processing cooling slope AlSi7Mg alloys AlSi alloys intermetallic phases contact time CET thixotropy anodizing

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