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On the influence of imperfections on microstructure and properties of recycled Al-Si casting alloys
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting. Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0003-4005-812X
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

There are great energy savings to be made by recycling aluminium; as little as 5% of the energy needed for primary aluminium production may be required. Striving to produce high quality aluminium castings requires knowledge of microstructural imperfections, which is extra important when casting recycled aluminium that generally contains higher levels of imperfections compared to primary aluminium. Imperfections include amongst others Si, Fe, and Mn as well as oxides. Si is needed for castability, but it may also initiate fracture. There are different types of Fe-rich intermetallics influencing properties of castings, generally in a negative direction. Oxides constitute cracks and they are elusive because they are difficult to quantify.

This thesis aims to increase knowledge about imperfections in recycled aluminium castings originating from alloying elements and the melt. Experiments were performed in advanced laboratory equipment, including X-radiographic imaging during solidification and in-situ tensile testing in a scanning electron microscope. Experiments were also performed at industrial foundry facilities.

The experiments showed that the nucleation temperature of primary α-Fe intermetallics increased with higher Fe, Mn, and Cr contents. Primary α-Fe are strongly suggested to nucleate on oxides and to grow in four basic morphologies. Lower nucleation frequency of α-Fe promoted faster growth and hopper crystals while higher nucleation frequency promoted slower growth rates and massive crystals. Results also showed that a decrease in the size of the eutectic Si and plate-like β-Fe intermetallics improved tensile properties, foremost the elongation to fracture. In β-Fe containing alloys the transversely oriented intermetallics initiated macrocracks that are potential fracture initiation sites. In alloys with primary α-Fe foremost clusters of intermetallics promoted macrocracks. In fatigue testing, a transition from β-Fe to α-Fe shifted the initiation sites from oxides and pores to the α-Fe, resulting in a decrease of fatigue strength. Oxides in Al-Si alloys continue to be elusive; no correlations between efforts to quantify the oxides and tensile properties could be observed.

Abstract [sv]

Genom att återvinna aluminium kan stora energibesparingar göras eftersom återvinning kan förbruka så lite som 5% av den energi som behövs för produktion av primär aluminium. Vid gjutning av högkvalitativa aluminiumprodukter krävs förståelse för defekter i mikrostrukturen och denna kunskap är extra viktig vid användning av återvunnen aluminium, som i regel innehåller mer defekter än primär aluminium. Defekterna består bland annat av Si, Fe och Mn samt oxider. Si behövs för gjutbarhet men kan också initiera brott. Järnrika intermetaller kan ha olika morfologier som generellt påverkar gjutna komponenter negativt. Oxider, som kan utgöra sprickor, är gäckande då de är svåra att kvantifiera.

Denna avhandlings syfte är att öka kunskapen om defekter i gjutna komponenter av återvunnen aluminium. Experiment utfördes med avancerad laborationsutrustning så som röntgenfotografering av prover under stelning och dragprovning i svepelektronmikroskop. Experiment utfördes också i industrimiljö.

Experimenten visade att kärnbildningstemperaturen steg för primära α-Fe intermetaller med ökade andelar av Fe, Mn och Cr. Resultaten tyder starkt på att primär α-Fe kärnbildas på oxider och att de växer i fyra olika morfologier. Lägre kärnbildningstäthet av α-Fe främjade snabbare tillväxt av kristaller med håligheter men högre kärnbildningstäthet främjade långsammare tillväxt av massiva kristaller. Resultaten visade också att minskad storlek av eutektiskt Si och β-Fe intermetaller ledde till förbättring av dragprovsresultaten, främst brottförlängningen. I legeringar med β-Fe ledde transversellt orienterade intermetaller till makrosprickor vilka kan initiera brott. I legeringar med primär α-Fe var det främst kluster av intermetaller som orsakade makrosprickor. I utmattningsprovning orsakade modifiering av β-Fe till α-Fe förflyttning av sprickinitieringen från oxider och porer till α-Fe, vilket resulterade i en reducerad utmattningshållfasthet. Oxiderna i Al-Si-legeringar fortsätter att gäcka; ingen korrelation mellan försök att kvantifiera oxiderna och draghållfasthet kunde påvisas.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering , 2017. , p. 67
Series
JTH Dissertation Series ; 28
Keywords [en]
Imperfections, Recycled Al-Si alloys, Fe-rich intermetallics, Melt quality, Fractography
Keywords [sv]
Defekter, Återvunna Al-Si legeringar, Fe-rika intermetaller, Smältakvalité, Fraktografi
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-36963ISBN: 978-91-87289-29-3 (print)OAI: oai:DiVA.org:hj-36963DiVA, id: diva2:1134894
Public defence
2017-09-15, E1405 (Gjuterisalen), Tekniska Högskolan, Jönköping, 10:00
Opponent
Supervisors
Available from: 2017-08-21 Created: 2017-08-21 Last updated: 2017-08-21Bibliographically approved
List of papers
1. Assessment of Quality when Delivering Molten Aluminium Alloys Instead of Ingots
Open this publication in new window or tab >>Assessment of Quality when Delivering Molten Aluminium Alloys Instead of Ingots
2013 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 765, p. 266-270Article in journal (Refereed) Published
Abstract [en]

Recycled aluminium alloys manufactured at Stena Aluminium, in Älmhult, Sweden, are delivered in special designed transport containers, called thermoses. Thermoses are best described as an insulating layer protected by a steel cover with a heat loss of about 5 degrees/h. Three thermoses are transported by a truck, giving the possibility for a total capacity of about 24 tonnes delivered aluminium just-in-time to the foundry. By delivering a full load of liquid aluminium, about 2 tonnes of carbon dioxide emissions are saved, compared with delivering ingots. The aim of the paper is to assess the quality benefits, in terms of inclusions sedimentation and mechanical properties, assured by delivering aluminiummelts instead of ingots. The results indicate that materials produced by just-in-time melt delivery have slightly improved quality compared to ingots. The trends are explained in terms of quality, density and bifilm indexes, based on microstructural observations as well as tensile test data analysis.

Place, publisher, year, edition, pages
Trans Tech Publications, 2013
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-20256 (URN)10.4028/www.scientific.net/MSF.765.266 (DOI)2-s2.0-84882945035 (Scopus ID)978-3-03785-766-3 (ISBN)
Conference
6th International Light Metals Technology Conference, LMT 2013; Old Windsor; United Kingdom; 24 July 2013 through 26 July 2013
Funder
Knowledge Foundation
Available from: 2013-01-17 Created: 2013-01-17 Last updated: 2018-09-13Bibliographically approved
2. On the formation of sludge intermetallic particles in secondary aluminum alloys
Open this publication in new window or tab >>On the formation of sludge intermetallic particles in secondary aluminum alloys
2015 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 46, no 8, p. 3713-3722Article in journal (Refereed) Published
Abstract [en]

The primary precipitation of Fe-rich intermetallics in AlSi9Cu3(Fe) type alloys is studied for different Fe, Mn, and Cr contents and cooling rates. Differential scanning calorimetry, thermal analysis, and interrupted solidification with a rapid quenching technique were used in combination in order to assess the nucleation temperature of sludge particles, as well as to follow their evolution. The results revealed that the sludge nucleation temperature and the release of latent heat during sludge formation are functions of Fe, Mn, and Cr levels in the molten alloy (i.e., the sludge factor, SF) and cooling rate. Moreover, it can be concluded that sensitivity to sludge formation is not affected by cooling rate; i.e., a decrease in the SF will reduce sludge nucleation temperature to the same extent for a higher cooling rate as for a lower cooling rate. The sludge formation temperature detected will assist foundries in setting the optimal molten metal temperature for preventing sludge formation in holding furnaces and plunger systems.

Place, publisher, year, edition, pages
Springer, 2015
Keywords
aluminium alloys, sludge factor, Fe-rich intermetallics, DSC, thermal analysis
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-22716 (URN)000356728100039 ()2-s2.0-84932195555 (Scopus ID)
Available from: 2013-12-13 Created: 2013-12-13 Last updated: 2018-06-08Bibliographically approved
3. In-situ study of morphology and growth of primary α-Al(FeMnCr)Si intermetallics in an Al-Si alloy
Open this publication in new window or tab >>In-situ study of morphology and growth of primary α-Al(FeMnCr)Si intermetallics in an Al-Si alloy
Show others...
2017 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 130, p. 1-9Article in journal (Refereed) Published
Abstract [en]

Morphology and growth of primary α-Al(FeMnCr)Si intermetallics have been studied in-situ during solidification of a commercial secondary aluminum alloy employing X-radiographic imaging combined with deep-etching. The α-Al(FeMnCr)Si intermetallics were found to nucleate primarily on surface oxides, and the continued growth yielded both rhombic dodecahedrons and elongated rod-like morphologies. Both morphologies were observed as hopper and massive types, where the hopper intermetallics had the higher growth rates. The growth rate, which determines the type, appears to be linked to nucleation frequency; higher nucleation frequency promoted massive types and lower nucleation frequency promoted hopper intermetallics. 

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Aluminum, Casting, Crystal growth, Intermetallics, Primary α-Al(FeMnCr)Si, Aluminum alloys, Hoppers, Morphology, Nucleation, Silicon, Silicon alloys, Deep etching, In-Situ Study, Nucleation frequencies, Rhombic dodecahedron, Rod-like morphology, Secondary aluminum, Surface oxide, X-radiographic imaging
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-35343 (URN)10.1016/j.actamat.2017.03.026 (DOI)000401388700001 ()2-s2.0-85015790499 (Scopus ID)JTHMaterialIS (Local ID)JTHMaterialIS (Archive number)JTHMaterialIS (OAI)
Available from: 2017-04-10 Created: 2017-04-10 Last updated: 2017-08-21Bibliographically approved
4. On the complexity of the relationship between microstructure and tensile properties in cast aluminum
Open this publication in new window or tab >>On the complexity of the relationship between microstructure and tensile properties in cast aluminum
2015 (English)In: International Journal of Modern Physics B, ISSN 0217-9792, Vol. 29, no 10-11, article id 1540011Article in journal (Refereed) Published
Abstract [en]

The relationship between microstructure and mechanical properties in cast aluminium alloys is very complex. This relationship is also strongly affected by the casting process and melt handling. In the current study the mechanical properties were investigated and correlated with microstructural features such as porosity, Fe-rich particles, SDAS, Si-length. Process quality measures such as bifilm index, density index, and sludge factor were also investigated. The aim of the work was to understand the critical interactions between material microstructure and process quality in the development of high performance materials.

Keywords
imperfections, defects, porosity, fe-rich, sdas, silicon, bifilm, density, sludge, mechanical, properties, aluminium, aluminum
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-24687 (URN)10.1142/S0217979215400111 (DOI)000353523600012 ()2-s2.0-84928555789 (Scopus ID)
Conference
Asia Pacific Conference on Materials Processing 2014
Available from: 2014-12-23 Created: 2014-09-09 Last updated: 2017-12-05Bibliographically approved
5. The effects of Fe-particles on the tensile properties of Al-Si-Cu alloys
Open this publication in new window or tab >>The effects of Fe-particles on the tensile properties of Al-Si-Cu alloys
2016 (English)In: Metals, ISSN 2075-4701, Vol. 6, no 12, article id 314Article in journal (Refereed) Published
Abstract [en]

The effect of Fe-rich particles has been a topic for discussion in the aluminum casting industry because of the negative impact they exert on the mechanical properties. However, there are still contradictions on the effects of various morphologies of Fe-particles. In this study, microstructural characterization of tensile tested samples has been performed to reveal how unmodified and modified Fe-rich particles impact on the tensile behavior. Analysis of additions of Fe modifiers such as Mn and Cr, showed higher amounts of primary Fe-rich particles (sludge) with increased porosity and, as result, degraded tensile properties. From the fracture analysis of tensile tested HIPed samples it could be concluded that the mechanical properties were mainly governed by the Fe-rich particles, which were fracturing through cleavage, not by the porosity.

Place, publisher, year, edition, pages
MDPI (Multidisciplinary Digital Publishing Institute), 2016
Keywords
Fe-rich particles, sludge, tensile properties, fracture, casting, solidification, aluminum, silicon, recycling
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-26789 (URN)000389600600020 ()2-s2.0-85007193518 (Scopus ID)
Available from: 2015-05-28 Created: 2015-05-28 Last updated: 2017-12-18Bibliographically approved
6. The effect of Fe-rich intermetallics on crack initiation in cast aluminium: an in-situ tensile study
Open this publication in new window or tab >>The effect of Fe-rich intermetallics on crack initiation in cast aluminium: an in-situ tensile study
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-36962 (URN)
Available from: 2017-08-21 Created: 2017-08-21 Last updated: 2017-08-21
7. Al-Si-Cu alloys for high pressure die casting: Influence of Fe, Mn, and Cr on room temperaturemechanical properties
Open this publication in new window or tab >>Al-Si-Cu alloys for high pressure die casting: Influence of Fe, Mn, and Cr on room temperaturemechanical properties
Show others...
2016 (English)In: La Metallurgia Italiana, ISSN 0026-0843, no 6, p. 77-80Article in journal (Refereed) Published
Abstract [en]

Al-Si-Cu alloys with high Fe content are widely employed in high pressure die casting (HPDC). Even if Feis usually considered an impurity in secondary aluminum alloys, leading to the formation of harmfulintermetallic compounds, it helps in mitigating or eliminating the problem of die soldering. As a result,secondary Al alloys with Fe content of about 1 wt% are commonly employed for the production of HPDCcastings. Aiming to change the morphology of harmful Fe-bearing phases towards less detrimentalmorphologies, proper alloying elements may be added to the alloys. Mn and Cr (both present in thealuminum scrap), as instance, are reported to prevent from the formation of the acicular β-Al5FeSi phase,leading to the formation of more compact and polygonal intermetallics. Such phases are usually referredto as “sludge” particles. The influence of sludge particles on mechanical properties of Al -Si-Cu castings isstill under investigation. The present work aims at evaluating the effect of impurities (Fe, Mn and Cr)typically present in secondary Al alloys on the microstructure and mechanical properties of the A380 (Al -Si-Cu) alloy. Samples with different Fe, Mn and Cr content were produced and processed through adirectional solidification equipment to obtain specimens with controlled SDAS (~10 μm). Hardness androtating bending fatigue tests were carried out at room temperature. Mechanical properties of the alloyswere then related to the microstructure, analyzed by optical and scanning electron microscopy.

Place, publisher, year, edition, pages
Milano: Associazione Italiana di Metallurgia, 2016
Keywords
High pressure die casting, HPDC, Al-Si-Cu alloys, sludge, fatigue, microstructure
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-31423 (URN)2-s2.0-84993960856 (Scopus ID)JTHMaterialIS (Local ID)JTHMaterialIS (Archive number)JTHMaterialIS (OAI)
Conference
High Tech Die Casting 2016, Venice, 22-23 June, 2016.
Available from: 2016-08-22 Created: 2016-08-22 Last updated: 2017-11-28

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