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Recyclability of aluminium and aluminium alloys
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Mathew, James (2020) Recyclability of aluminium and aluminium alloys. PhD thesis, University of Warwick.
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WRAP_Theses_Mathew_2020.pdf - Submitted Version - Requires a PDF viewer. Download (7Mb) | Preview |
Official URL: http://webcat.warwick.ac.uk/record=b3492773~S15
Abstract
Aluminium and aluminium silicon alloys are widely used materials for the automobile, structural and aerospace applications due to its effective weight reduction compared to other materials. The increase in demand of cast aluminium alloys and the surge in aluminium waste scraps leads to the secondary production/recycling of aluminium waste scraps. The total production cost and energy usage required for the production of primary Al from the bauxite ores is high compared to the secondary production of aluminium. One of the major concerns in aluminium recycling is the detrimental effect of iron impurities which generally cannot be removed completely by physical separation techniques. The iron in the scrap combines with other elements to form Fe-rich intermetallic compounds. The iron content should not exceed a particular level/critical iron content (Fecrit) to prevent the weakening effects of Fe-rich intermetallic compounds. This critical iron content (Fecrit) is different for different alloy compositions. Higher percent of iron presence beyond an optimum limit would result in deterioration of mechanical property of the alloys.
In this work, pure aluminum, aluminium-7 wt% silicon and aluminium-12 wt% silicon alloys were studied using 0 wt% , 0.6 wt% (Fecrit of Al-7Si) and 2 wt% Fe cast at 740℃ for understanding the effect of silicon and iron content in the formation of intermetallic particles. The increase in iron content and silicon content increases the thickness and quantity of iron intermetallic particles formed. The microstructure and mechanical properties of these alloys were studied to understand the mechanism behind the failure of these alloys and to mitigate its deteriorating effects by suitable modification methods. Because of its sharp edged platelet morphology, the brittle iron intermetallic compounds act as stress raisers, help in crack propagation and deteriorate the mechanical properties of the cast. Hence the effective methods of modification of these iron intermetallic particles in order to reduce its weakening effects are either by refining them or by changing its morphology. The melt treatment processes such as varying cooling rates and superheat is found to refine these intermetallic particles. The effect of cooling rate is studied with a water cooled copper wedge mold, cast iron mold and a graphite mold. Even though high cooling rate refines the particles and grain size compared to the low cooling rates, the mechanical properties of Al-7Si alloy with high iron content (2%) is still significantly low compared to the low iron content (0.6%). Therefore the superheat effect on Al7Si-2Fe alloys were studied using the alloy samples prepared at 700℃, 800℃ and 900℃. The microstructure studies using optical microscopy, SEM-EDS and XCT confirmed the refinement of intermetallic phases in Al-7Si-2Fe alloys upon superheating at 900℃ compared to the 700℃ and 800℃. But the porosity is more for 900℃ cast compared to 700℃ cast, which results in reduced mechanical properties for 900℃ cast compared to 700℃ cast.
The most commonly used economically and industrially viable modification method of iron intermetallic particles is by the addition of chemicals/elements which can react with the Al, Si and Fe phases. In this work one of the rarely studied rare earth element, lanthanum, is used for the modification of iron intermetallic particles in AlFe and Al-7Si-Fe alloys. Lanthanum is found to form La (Al Si)2 and Al11La3 phases prior to the formation of iron intermetallic phases during solidification. The addition of 1% 10 of La to the Al-2Fe alloy resulted in the refinement of iron intermetallic particles along with the formation of Al-La based particles. The SEM-EDS, TEM-EDS and TEM-SAD patterns identified the particles in Al-2Fe alloy as Al13Fe4 and the particles in Al-2Fe1La alloy as Al5Fe2 and Al11La3. The 3D FIB-SEM images shows the morphological modification of iron intermetallic particles by preventing the formation of platelet shaped Al13Fe4 phase from Al5Fe2 phase after the addition of lanthanum. This helps in improving the mechanical properties of Al-2Fe alloy. Further, the addition of 1% La to the Al-7Si-Fe alloy resulted in the refinement of silicon particles and thereby improves the mechanical properties. The mechanism of modification of iron intermetallic particles on addition of lanthanum is explained based on the intermetallic phase formation sequence in low silicon and high silicon aluminium alloys and found that addition of lanthanum cannot modify the iron intermetallic particles in high silicon Al-Si alloys. The modification of iron intermetallic particles using Mn and Sr addition to Al-Si alloys were studied to understand its limits and found the formation of large sharp edged brittle α-AlFeMnSi particles at higher fractions of iron content (2%) which culminates the modification effect.
Item Type: | Thesis (PhD) | ||||
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Subjects: | T Technology > TD Environmental technology. Sanitary engineering T Technology > TN Mining engineering. Metallurgy |
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Library of Congress Subject Headings (LCSH): | Aluminum -- Recycling, Aluminum-silicon alloys -- Recycling, Intermetallic compounds | ||||
Official Date: | 2020 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Warwick Manufacturing Group | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Srirangam, Prakash ; Williams, Mark Andrew ; Gibbons, Gregory John, 1970- | ||||
Format of File: | |||||
Extent: | 150 leaves : illustrations (some colour) | ||||
Language: | eng |
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