Influence of trace solutes on the grain refinement and modification of Al-Si based alloys

Research output: ThesisDoctoral Thesis

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Abstract

Al-Si based alloys are widely used in automotive and aerospace applications as components manufactured from these alloys are significantly lighter than cast iron and steel parts and they possess a good strength-to-weight ratio. The properties are affected by several factors. In this work, the effect of solutes on molten metal treatments is investigated using casting experiments, thermal analysis (TA), multi-scale characterisation methods, density functional theory (DFT), differential scanning calorimetry (DSC) and entrained droplet technique. In terms of grain refinement, the investigated solutes are Ti and Ta. In terms of modification, eutectic grain refinement and heterogeneous nucleation of eutectic Si, the investigated solutes are Eu, P and Sr. The controlled addition of solute Ta in combination with stoichiometric Al-2.2Ti-1B grain refiner results in a lower grain size than the addition of conventional Al-5Ti-1B grain refiner. The grain size is further decreased with increasing Ta content. Both Ti and Ta-refined alloys exhibit a negligible undercooling. Atomic investigation using transmission electron microscopy (TEM) reveals Ta at the interface of TiB2 and DFT interface energy shows that Ta is as energetically favourable as Ti at the interface of TiB2. No interaction of Ta with other elements is observed. Ta is concluded to be a suitable remedy for Si-poisoning. Modification is achieved via the addition of Sr and Eu. The shape factor increases with increasing the addition of Eu, however, the shape factor decreases with increasing the addition of P. Al2Si2Sr and Al2Si2Eu intermetallic phases are observed in the microstructure. Lattice mismatch calculation and entrained droplet technique elucidate that Al2Si2Eu is nucleated on EuP, which is favourably formed from increasing Eu and P additions. The new nucleation sequence is proposed: EuP ¿ Mg3P2 ¿ Al2Si2Eu in the presence of Mg. In terms of growth of eutectic Si, the solute Eu atom is calculated to be preferably located along the twin boundary of eutectic Si. Ta shows no mutual interaction with Eu or Sr. Combined additions of Eu and P result in a refined eutectic grain size while maintaining the modified eutectic Si. Ta and TiB2 are found to have no effect on nucleation of eutectic Si. The addition of P into Eu-modified high purity alloy produced using entrained droplet technique shows a decrease in undercooling and displacement of nucleation temperature to higher temperatures. Entrained eutectic Si droplets are consisting of fine fibrous and randomly oriented Si particles. Addition of P results in a transition to fewer number of single oriented Si plates. The peak hardness is achieved 1 h later in high Ta alloy compared to conventional alloy refined with Ti. Interface energy of Al and ß'' precipitate with Ti and Ta-doped interfaces shows a lower interface energy for Ta-doped interface. DSC revealed a very low activation energy of ß precipitate with the addition of Eu, therefore complete precipitation is expected to save economic costs. The highest elongation of 24 % is observed in high Ta alloy in T4 condition. Ta-refined and/or Eu-modified alloys exhibit a comparable strength and a lower elongation than conventional Ti-refined and Sr-modified alloy, presumably due to formation of Al2Si2Eu on the grain boundaries and different porosity distribution in Eu-modified alloys. The present work demonstrates the strong influence of solutes (in hundreds of ppm) on the nucleation, growth, kinetic and thermodynamic processes and mechanical properties.
Translated title of the contributionEinfluss von Spuren gelöster Stoffe auf die Kornverfeinerung und Modifikation von Legierungen auf Al-Si-Basis
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Holec, David, Co-Supervisor (internal)
  • Wu, Menghuai, Assessor A (internal)
  • LI, Jiehua, Supervisor (internal)
  • Pogatscher, Stefan, Assessor B (internal)
DOIs
Publication statusPublished - 2023

Bibliographical note

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Keywords

  • Al-Si-Mg based alloy
  • Grain refinement
  • Modification
  • Eutectic grain refinement
  • Heat treatment
  • DSC
  • SEM
  • EBSD
  • TEM
  • DFT

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