| 1. |
- Bogdanoff, Toni
(author)
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The influence of microstructure on the crack initiation and propagation in Al-Si casting alloys
- 2021
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Licentiate thesis (other academic/artistic)abstract
- For reducing the CO2 footprint in many industrial fields, the goal is to produce lighter components. The aluminium-silicon (Al-Si) cast alloys are promising candidates to fulfill these goals with a high weight-to-strength ratio, good corrosion properties, excellent castability, and recyclable material. However, the variations within these components need to be understood to produce high-performance components for critical applications. The main reason for the rejection in these applications is defects and microstructural features that reduce the mechanical properties. The addition of copper (Cu) is one way of increasing the mechanical properties in Al-Si alloys and is commonly used in the automotive industry. Casting defects harm the mechanical properties, and these defects can be reduced by improving the melt quality, the correct design of the component, and the gating system.The study aims to investigate the static mechanical properties and the crack initiation and propagation under cyclic loading in an Al-7Si-Mg cast alloy with state-of-the-art experiments. The main focuses were on the effect of the HIP process and the role of Cu addition. In-situ cyclic testing using a scanning electron microscope coupled with electron back-scattered diffraction, digital image correlation, focused ion beam (FIB) slicing, and computed tomography scanning was used to evaluate the complex interaction between the crack path and the microstructural features.The amount of Cu retained in the α-Al matrix in as-cast and heat-treated conditions significantly influenced the static mechanical properties by increasing yield strength and ultimate tensile strength with a decrease in elongation. The three-nearest-neighbor distance of eutectic Si and Cu-rich particles and crack tortuosity were new tools to describe the crack propagation in the alloys, showing that a reduced distance between the Cu-rich phases is detrimental for the mechanical properties. Three dimensional tomography using a FIB revealed that the alloy with 3.2 wt.% Cu had a significantly increased quantity of cracked Si particles and intermetallic phases ahead of the crack tip than the Cu-free alloy. The effect of Cu and HIP process in this work shows the complex interaction between the microstructural features and the mechanical properties, and this needs to be considered to produce high-performance components.
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| 2. |
- Sainis, Salil
(author)
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An Insight into the Critical Role of Microstructure and Surface Preparation on Localized Conversion Coating Deposition on Cast Al Alloys
- 2022
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Doctoral thesis (other academic/artistic)abstract
- The spontaneous cerium conversion coating formation over aluminium alloys is affected by the chemical process conditions, the surface preparation prior to conversion treatment and the microstructure of the underlying alloy. Most research performed until now focus only on the former whereas the influence of the latter two is poorly understood. The microstructure of aluminium alloys contains cathodic intermetallic particles that have a crucial role in conversion coating formation as they are responsible for the increase in pH to a critical level where chemical precipitation reaction of conversion coating compounds becomes possible. There are many different types of intermetallics in the aluminium alloys’ microstructure whose cathodic potential depends on their chemical composition, but no systematic study exists on the influence of their geometric properties on the reactivity of intermetallics. The surface preparation also critically affects reactivity of intermetallics, and a consensus exists regarding which preparation procedure makes an alloy surface most conducive for conversion coating deposition, but little is known about the topographical and volta potential changes occurring around the intermetallics from the surface preparations. An insight into both the role of microstructure and surface preparation is thus crucial as they can help design better treatments.To address this knowledge gap, model microstructures of hypoeutectic cast Al-7Si alloys have been created in the study with the desired intermetallic composition and geometry. By addition of Cu and Fe to the hypoeutectic cast alloy, two types of intermetallics, namely θ-Al2Cu and β-Al5FeSi form and are the main objects of investigation in the study. The geometrical dimensions of the intermetallics were modified by directionally solidifying the cast alloys at different rates. Different surface preparation procedures, namely mechanical polishing, NaOH, NaOH-HNO3 and NaOH-H2SO4 have been tried in the study. Furthermore, parameters such as etching time have been varied to understand their influence. The microstructural features, particularly intermetallic geometry and cathodic potential have been systematically compared with localized deposition on them through conversion coating treatment. Experimentally derived data-based analyses have been conducted to come to conclusions in the study.Firstly, the study found differences in the initiation of localized deposits on θ-Al2Cu and β-Al5FeSi attributed this to the conductivity variation due to compositional difference. Among the three differently sized θ-IM investigated in the study, namely fine, coarse and bulky θ, increasing the size from fine to coarse led to increase in reactivity of the IM for localized deposition. But increasing the size even further from coarse to bulky θ decreased the reactivity. Such a decrease in reactivity was found to be due to a combination of factors such as volta potential difference relative to the matrix and geometry.Surface preparation affected localized deposition and the choice of procedure applied critically depends on the intermetallics present in the microstructure. The β-Al5FeSi is most reactive for conversion coating deposition when prepared with NaOH etching solution but becomes passive when further pickled with HNO3 solution. The θ-Al2Cu, on the other hand, becomes most active when subjected to multi-step NaOH- HNO3 procedure. Such observations make the choice of a surface preparation procedure difficult when the alloy microstructure contains both θ-Al2Cu and β-Al5FeSi intermetallics. The localized deposition of cerium conversion compounds after surface preparation are triggered due to a combination of surface factors like including volta potential and presence of Al(OH)3 smut, with varying degrees of dominance depending on the type of IM. Al(OH)3 smut content was found sensitive to the NaOH etching time. A further HNO3 pickling step cleans the surface. A “cleaner” surface was found to be associated with more consistent coverage numbers, while the presence of Al(OH)3 smut, on the one hand provides additional source of alkalinity, does not consistently result in good coverage.
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| 3. |
- Sainis, Salil
(author)
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The influence of Al alloy microstructure on conversion coating formation
- 2021
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Licentiate thesis (other academic/artistic)abstract
- The formation of conversion coatings based on Ce, trivalent Cr and Ti/Zr is triggered by the local pH increase at cathodic IM sites of the aluminium alloy microstructure. The pH gradient is created over the cathodic intermetallic (IM) sites of the microstructure and its intensity is influenced by their activity which depends on their chemical composition, their size, and spatial distribution. Furthermore, the pre-treatment applied also affects the surface reactivity. The role of each of the microstructural features on the increased pH gradient and the subsequent triggering of local conversion coating deposition remains to be understood. To address the knowledge gap, model cast Al-Si-Fe and Al-Si-Cu-Fe alloys have been designed. Cerium based conversion coating treatment with standard parameters is applied to investigate the microstructure’s influence. Furthermore, four different surface pre-treatments’ effect on the topographical and electrochemical properties have been investigated by localized techniques and have been correlated with deposition experiment observations to prove surface reactivity.In this study, it was found that the four surface pre-treatments – polishing, NaOH, NaOH-HNO3, NaOH-H2SO4 activate the surface of alloys containing Fe-rich IM and Cu-rich IM differently. The surface pre-treatment NaOH-HNO3 was found most detrimental to the surface reactivity as the pre-treatment resulted in passivation of the IM and a drastic reduction in its volta potential. The best pre-treatment for the alloy Al-Si-Fe was found to be one with NaOH etching. In the case of Al-Si-Cu-Fe alloy, pre-treatments where a pickling step (with either H2SO4 or HNO3) was applied followed a NaOH etching step, the surface of the IM was activated more than other pre-treatments due to selective Al dealloying and Cu-redeposition. The extent of Cu-redeposition was observed to be the most when surfaces were pickled with HNO3 solution and with the NaOH- HNO3 pre-treatment, fastest deposition kinetics were observed.In the cast Al-Si-Cu-Fe alloy, the localized deposits were preferentially observed to form on only strong cathodic Cu-rich IM. The size (surface area) of the Cu-rich IM correlated linearly with the lateral deposition area as well as z-direction spread. It was found that the pH gradient resulting from the oxygen reduction reaction near an IM is very local and does not affect pH gradients of a neighbouring Cu-rich IM. The size did not have a profound impact on the extent of deposition occurring on a Cu-rich IM, but it was found that big Cu-rich IM activated faster for deposition reaction than small Cu-rich IM. When the progression of deposition on both coarse and fine microstructure cast Al-Si-Cu-Fe was quantitatively monitored at increasing conversion coating times 0.5h, 1h and 2h, it was observed that big Cu-rich IM in the coarse alloy triggered deposition faster than small Cu-rich IM.Deposition mechanism on Fe-rich IM was found to be composition specific. In the cast of big Fe-rich β-Al5FeSi IM, localized deposition initiated at the border on the IM and is explained based on Si content in the composition of the IM, which has very high resistivity. In another Fe-rich IM, although of a much smaller size, which had lower Si content and was richer in Fe, a localized deposition was observed on the entire IM.
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