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Casting and anodising of Al alloys- Alloy design, manufacturing process and material properties
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-8098-4788
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Casting and semi-solid metal (SSM) casting are widely used to manufacture near-net-shape components of Al-Si alloys in the automotive and electronic industries. In such applications, casting components need to meet the combined requirements of good mechanical properties and corrosion and wear resistance. Hence, a good understanding of the relevant aspects such as material design, manufacturing and surface treatments have a significant impact on the final performance of the component. The objective of this thesis is to understand the interaction among manufacturing and surface treatments and how their combination impacts the microstructure and final properties. The results will accordingly highlight the potential for improving the mechanical and anodising properties of rheocast components.

The influence of the most relevant alloying elements has been investigated in this study. It is found that Si and Fe have a significant influence on anodising. During anodising, Si particles are oxidised at a much lower rate than Al phase and embedded in the oxide layer. Due to the presence of Si particles and their morphology, residual metallic Al phase and cracks are introduced in the oxide layer. A reduced number of residual metallic Al phase, as well as defects, can be obtained by changing the Si particle morphology to disconnected fibrous by Sr modification. On the contrary, Fe-rich intermetallics could be partly dissolved during anodising, leaving vacancies or voids as defects in the oxide layer. So, it was proved that by modifying Si particles and removing Fe-rich intermetallics from the surface, the defects in the oxide layer are reduced, and better corrosion protection is achieved.

The SSM process increases the microstructural inhomogeneity such as transverse macrosegregation and longitudinal macrosegregation in the cast component. The results show that the presence of surface liquid segregation (SLS) layer by transverse macrosegregation does not have a significant impact on the corrosion resistance and hardness of the oxide layer of as-cast surfaces compared to liquid casting. The longitudinal macrosegregation influences the corrosion protection provided by the anodised layer but does not affect the hardness of the rheocast component before or after anodising. In this study, it is also found that, during the casting of Al-Si alloys, the surface of the component can be enriched in Fe-rich intermetallics due to the SLS or interaction with the die material. Despite this affects only the very superficial thickness, it has a big impact on the corrosion resistance and hardness of the oxide layer.

This study has revealed that the high value of the oxide layer thickness, as well as the hydrothermal sealing, is not a guarantee for improving the corrosion resistance of the oxide layer. An increase of the oxide layer thickness by increasing applied voltage or anodising time decreases both the corrosion resistance and hardness of the oxide layer. Moreover, the hydrothermal sealing after anodising significantly decreases the corrosion protection provided by the anodised layer in Al-Si alloys due to cracks formation.

This study has observed that the casting defects such as oxide film, cold shots and the solute-rich layer which are related to the casting process dominate the fatigue behaviours of the SSM cast components. The fractographic examination indicates that the oxide film, cold shots and solute-rich layer act as crack initiation points during fatigue testing. Therefore, it was found that, in these conditions, the anodising does not have an evident impact on fatigue properties, despite the anodising process adds a brittle anodised layer on the surface.

Abstract [sv]

Det blir allt vanligare att använda semi-solid metallgjutning (SSM) för att tillverka komponenter av Al-Si-legeringar för fordons- och telekomindustrin med minimalt behov av mekanisk efterbearbetning. I dessa applikationer måste de gjutna komponenterna leva upp till en kombination av krav på mekaniska egenskaper, samt även korrosions- och nötningsbeständighet. Det är därför viktig att ha förståelse för hur relevanta aspekter som materialdesign, tillverkning och ytbehandling påverkar komponentens slutgiltiga egenskaper. Syftet med denna avhandling är att presentera nya lösningar som kombinerar de kostnadseffektiva processerna gjutning och anodisering genom att anpassa val av legering med processkontroll för både gjutning och anodisering. Resultatet visar att det är möjligt att förbättra både mekaniska och ytegenskaper hos Rheo-gjutna komponenter.

Denna studie har undersökt inverkan av legeringselement som Si och Fe på anodisering. Resultatet visar att både Si och Fe har signifikant påverkan på anodiseringen. Under anodisering oxideras Si-partiklar mycket långsammare än den omgivande Al-fasen och blir därmed inneslutet i oxidskiktet. Som ett resultat av Si-partiklarnas existens och deras morfologi bildas isolerade kvarvarande rester av metallisk Al-fas samt sprickor i oxidskiktet. Genom att ändra Si-partiklarnas morfologi till osammanhängande fibrer med hjälp av Sr-modifiering är det möjligt att minska mängden av metallisk Al-fas samt defekter i oxidskiktet. I motsättning till Si-partiklar lösas Fe-rika intermetaller delvist upp under anodiseringen och lämnar defekter i form av hålrum i oxidskiktet. Studien visar att en betydande förbättring av korrosionsbeständigheten av anodiserade komponenter kan uppnås genom att ändra Si-partiklarnas morfologi till fibrer och minska innehållet av Fe-rika intermetaller.

SSM-processen ökar den mikrostrukturella inhomogeniteten i gjutna komponenter på grund av tvär- och längsgående makrosegregering. Det har visats att uppkomsten av ytsegregering som följd av tvärgående makrosegregering inte inverkar på korrosionsbeständighet och hårdhet av anodiseringsskiktet på obearbetade komponenter vid jämnföring med konventionell smältgjutning som också bildar gjuthud. Däremot påverkar den längsgående makrosegregeringen oxidskiktets korrosionsskyddande effekt, men inte hårdheten. Studien visar också att det sker en uppkoncentrering av Fe-rika intermetaller i ytan på Rheogjutna Al-Si-legering. Detta kan bero på ytsegregering eller växelverkan med gjutformen. Även om detta bara påverkar det yttersta skiktet av ytan på komponenten, kan det ha stor inverkan på korrosionsbeständighet och hårdheten av oxidskiktet.

Det har avslöjats att varken ett tjockt oxidskikt eller hydrotermisk försegling är någon garanti för att förbättra oxidskiktets korrosionsegenskaper. Att växa ett tjockare oxidskikt genom att använda högre elektrisk spänning eller anodisera i längre tid minskar i bägge tillfällen både oxidskiktets korrosionsbeständighet och hårdhet. Ytterligare har det visats att också hydrotermisk försegling av anodiserade Al-Si-legeringar ger sämre korrosionsskydd.

Studien visar också att gjutdefekter som oxidfilmer från smältan, kallflytningar och invers segregering, som alla härstammar från gjutprocess dominerar utmattningsegenskaperna för SSM-gjutna komponenter. Fraktografi visar att defekterna fungerar som sprickinitierare under utmattninglast. Slutligen har det visats att anodisering, i sig själv inte påverkar utmattningsegenskaperna trots att processen skapar ett sprött ytskikt på komponenten.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering , 2019. , p. 52
Series
JTH Dissertation Series ; 41
Keywords [en]
Cast aluminium, Semi-solid casting, Anodising
Keywords [sv]
Gjutning av aluminium, semi-solid gjutning, anodisering, komponent, gjutlegering, kisel, järn
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-43619ISBN: 978-91-87289-43-9 (print)OAI: oai:DiVA.org:hj-43619DiVA, id: diva2:1314611
Public defence
2019-06-14, E1405 Gjuterisalen, School of Engineering, 10:00 (English)
Opponent
Supervisors
Available from: 2019-05-09 Created: 2019-05-09 Last updated: 2019-05-09Bibliographically approved
List of papers
1. Influence of Si and cooling rate on microstructure and mechanical properties of Al–Si–Mg cast alloys
Open this publication in new window or tab >>Influence of Si and cooling rate on microstructure and mechanical properties of Al–Si–Mg cast alloys
2016 (English)In: Surface and Interface Analysis, ISSN 0142-2421, E-ISSN 1096-9918, Vol. 48, no 8, p. 861-869Article in journal (Refereed) Published
Abstract [en]

This paper aims to investigate the effect of Si-content and microstructure on mechanical properties of Al–Si–Mg alloys with thepurpose of to develop high-strength cast aluminum alloys that are anodizable. The Si-content was 2.5, 3.5 and 5.5wt% Si, andboth Sr-modified and unmodified conditions were used. The samples were produced using the gradient solidification furnaceto generate awell-controlled microstructure. The resulting secondary dendrite armspacingwas 10 and 20μm. Themicrostructuralfeatures were evaluated by employing SEM/EDS and optical microscopy. Furthermore, computed tomography (CT) scan technologywas used to provide a 3D view of high-density phases in the microstructure. The mechanical properties of these alloys werestudied by means of tensile and hardness testing where the latter was performed on the macrolevel and microlevel. The resultsdemonstrate clearly how the Si growth in the microstructure is restricted by increased cooling rate and modification and its rolein strength development in Al-Si alloys. Additionally, the CT scan visualized the morphology of intermetallics and supported inidentifying the oxide layer growth as a result of the anodizing process.

Place, publisher, year, edition, pages
John Wiley & Sons, 2016
Keywords
Al-Si alloys; microstructure; mechanical properties; hardness; anodizing
National Category
Manufacturing, Surface and Joining Technology Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-28024 (URN)10.1002/sia.5814 (DOI)000383754600024 ()2-s2.0-84940688989 (Scopus ID)
Funder
Knowledge Foundation, 20100280
Available from: 2015-09-22 Created: 2015-09-22 Last updated: 2019-05-09Bibliographically approved
2. A study of formation and growth of the anodised surface layer on cast Al-Si alloys based on different analytical techniques
Open this publication in new window or tab >>A study of formation and growth of the anodised surface layer on cast Al-Si alloys based on different analytical techniques
Show others...
2016 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 101, p. 254-262Article in journal (Refereed) Published
Abstract [en]

This paper aims to investigate the mechanisms of formation and growth of the anodised surface layer on Al-Si castings by applying different analytical techniques such as optical microscopy, scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and X-ray computer tomography (X-ray CT) scanning. Three different Al alloys with various Si content (2.43%, 3.53% and 5.45%) were investigated. Si particle morphological modification by Sr addition, as well as directional solidification, was used to vary the microstructural coarseness in a controlled manner to study the influence of these parameters on the growth behaviour of the oxide layer. This study observed residual unanodised Al phases trapped beneath or between Si particles in the oxide layer. It was found, depending on the geometry and morphology of Si particles, that Al can be shielded by Si particles and prevented from oxidising.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
Al-Si cast alloys, Anodising, Oxide layers, Silicon
National Category
Engineering and Technology Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-29789 (URN)10.1016/j.matdes.2016.04.013 (DOI)000375413100032 ()2-s2.0-85020846163 (Scopus ID)
Available from: 2016-04-18 Created: 2016-04-18 Last updated: 2019-05-09Bibliographically approved
3. Effect of Si content and morphology on corrosion resistance of anodized cast Al-Si alloys
Open this publication in new window or tab >>Effect of Si content and morphology on corrosion resistance of anodized cast Al-Si alloys
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2017 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 164, no 7, p. C435-C441Article in journal (Refereed) Published
Abstract [en]

This paper investigates the influence of Si content and Si particle morphology on the corrosion protection of anodized oxide layers on Al-Si alloys. Two Al alloys with low Si concentrations (2.43 wt-% and 5.45 wt-%, respectively) were studied and compared with 6082-T6 via electrochemical impedance spectroscopy (EIS) in 3 wt-% NaCl solution prior to oxide layer sealing. Si particles were also modified by the addition of Sr to study the influence of Si particle morphology on the corrosion protection of the oxide layer. The EIS showed that the corrosion protection provided by the oxide layer on Al-Si alloys is significantly affected by the presence of Si particles. Si particles make the oxide layer locally thinner and more defective in the eutectic region, thereby increasing the ease of substrate corrosion attack. However, the addition of Sr can improve the corrosion protection of anodized Al-Si alloys significantly. Furthermore, it was proved that higher Si level influences negatively the anodized oxide corrosion protection due to the higher amount of cracks and defects, but Sr modification is efficient in preventing this negative effect.

Place, publisher, year, edition, pages
The Electrochemical Society, 2017
Keywords
cast Al-Si alloys, anodising, EIS, 3D tomography
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-35095 (URN)10.1149/2.1631707jes (DOI)000404397300077 ()2-s2.0-85020855659 (Scopus ID)JTHMaterialIS (Local ID)JTHMaterialIS (Archive number)JTHMaterialIS (OAI)
Note

Part of licentiate thesis in its submitted form.

Available from: 2017-02-17 Created: 2017-02-17 Last updated: 2019-05-09Bibliographically approved
4. Influence of the Sr modification and post-treatment on corrosion resistance of oxide layer of cast Al-(low)Si alloys
Open this publication in new window or tab >>Influence of the Sr modification and post-treatment on corrosion resistance of oxide layer of cast Al-(low)Si alloys
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2017 (English)In: EUROCORR 2017 - The Annual Congress of the European Federation of Corrosion, 20th International Corrosion Congress and Process Safety Congress 2017, Asociace koroznich inzenyru z.s.- AKI - Czech Association of Corrosion Engineers , 2017, p. 27-39Conference paper, Published paper (Refereed)
Abstract [en]

The current development of casting process makes it possible to cast low Si content aluminium alloys, which open the gate for the application of anodising on cast aluminium components. This paper aims to investigate the influence of Sr modification as well as the post-treatment on corrosion resistance of oxide layer of low Si content cast Al alloys. In the present study, designed Al-Si alloys with 5.45 wt-% Si were studied by performing electrochemical impedance spectroscopy (EIS) in 3 wt-% NaCl solution. The morphology of Si particle was determined by controlling the level of Sr. The microstructure features were evaluated by optical microscopy and scanning electron microscopy (SEM). Furthermore, focus ion beam-SEM (FIB-SEM) equipped with energy-dispersive X-ray spectroscopy (EDXS) was applied to exam the corrosion morphology and the anodised layer with Si rich eutectic phases. The EIS study revealed that the corrosion resistance of the oxide layer on cast Al alloys can be significantly influenced by the morphology of Si particle and the post-treatment. It was found that the change of Si particle morphology from flake-like to fibrous by Sr addition can substantially improve the corrosion resistance of the oxide layer in the unsealed condition due to the formation of the more compacted oxide layer with less defects. Normally, the application of a sealing step such as the hydrothermal sealing can improve the corrosion protection of the oxide layer. However, in the current study, it was found the hydrothermal sealing step leads to a significant reduce of the corrosion protection due to the generation of cracks into the oxide layer, and the hydrothermal sealing step can counteract the improvement of the corrosion protection by Sr addition.

Place, publisher, year, edition, pages
Asociace koroznich inzenyru z.s.- AKI - Czech Association of Corrosion Engineers, 2017
Keywords
Anodising, Cast Al-Si alloys, Corrosion resistance, EIS, FIB-SEM, Aluminum alloys, Corrosion protection, Corrosion resistant alloys, Electrochemical corrosion, Electrochemical impedance spectroscopy, Energy dispersive spectroscopy, Ion beams, Morphology, Scanning electron microscopy, Silicon alloys, Sodium chloride, Strontium, Casting process, Corrosion morphology, Energy dispersive x-ray spectroscopy, FIB SEM, Hydrothermal sealing, Sr modification, Aluminum corrosion
National Category
Corrosion Engineering
Identifiers
urn:nbn:se:hj:diva-41534 (URN)2-s2.0-85052280685 (Scopus ID)
Conference
Joint European Corrosion Congress 2017, EUROCORR 2017 and 20th International Corrosion Congress and Process Safety Congress 2017; Prague; Czech Republic; 3 - 7 September 2017
Available from: 2018-09-24 Created: 2018-09-24 Last updated: 2019-05-09Bibliographically approved
5. A study of anodising behaviour of Al-Si components produced by rheocasting
Open this publication in new window or tab >>A study of anodising behaviour of Al-Si components produced by rheocasting
Show others...
2019 (English)In: Solid State Phenomena, ISSN 1012-0394, E-ISSN 1662-9779, Vol. 285, p. 39-44Article in journal (Refereed) Published
Abstract [en]

This paper aims to investigate the anodising behaviour of Al-Si components produced by rheocasting, to understand the effect of the surface liquid segregation (SLS) on the anodising response. The material investigated was EN AC 42000 Al-alloy with an addition of 150 ppm Sr. The component was rheocast and conventionally liquid cast for benchmarking. The RheoMetalTM process was used to prepare slurry and subsequently cast using a vertical pressure die casting machine. Prior to anodising, mechanical grinding was used as pre-treatment method for selected samples as comparison with components in the as-cast state. Anodising was performed on the components using a constant controlled voltage at 25 V, in 1 M H2SO4, at room temperature. The duration of anodising was varied from 30 mins to 120 mins to examine the relationship between oxide layer thickness and the anodising time. The oxide layer was investigated and characterised. The results demonstrated that the presence of the SLS layer, which was enriched with alloying elements, had a significant influence on the anodising behaviour of the cast component. The oxide layer thickness of the components produced by rheocasting and fully liquid casting was measured and compared. The relations between the oxide layer thickness and anodising time, as well as the casting methods are presented and discussed in this paper.

Place, publisher, year, edition, pages
Trans Tech Publications, 2019
Keywords
Anodising, Oxide Layer, Rheocasting
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-42630 (URN)10.4028/www.scientific.net/SSP.285.39 (DOI)2-s2.0-85059944290 (Scopus ID)
Conference
15th International conference on semi-solid processing of alloys and composites, October 22nd to 24th, 2018, Shenzhen, China
Available from: 2019-01-14 Created: 2019-01-14 Last updated: 2019-05-09Bibliographically approved
6. Hardness and corrosion behaviour of anodised Al-Si produced by rheocasting
Open this publication in new window or tab >>Hardness and corrosion behaviour of anodised Al-Si produced by rheocasting
2019 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 173, article id 107764Article in journal (Refereed) Published
Abstract [en]

The anodised layer of Al-Si alloys produced by rheocasting was studied and compared to anodised traditional liquid casting in this paper. The anodising was performed in 1.0 M H2SO4 solution at room temperature on the as-cast substrates, and anodising voltage and time were optimised as process parameters. This study focuses on understanding the effect of the surface liquid segregation (SLS) layer by rheocasting on the hardness and corrosion protection of the oxide layer. The hardness depends on the anodising parameters and varies along the oxide thickness. The corrosion protection given by the oxide layer was evaluated by electrochemical impedance spectroscopy (EIS) in 3 wt-% NaCl solution, and the results revealed that the longitudinal macrosegregation influences the corrosion protection, with the near-to-vent region showing lower corrosion protection due to a higher eutectic fraction. A comparison between liquid and rheocast samples indicated that the presence of SLS layer by the transverse macrosegregation does not have a significant impact on the corrosion resistance of the oxide layer. Moreover, it was found that an increase of the oxide layer thickness by longer anodising time or higher applied voltage decreases both the hardness and corrosion resistance of the oxide layer.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Rheocasting, Anodising, Al oxide layer, Corrosion protection, Surface layer segregation
National Category
Materials Engineering
Identifiers
urn:nbn:se:hj:diva-43433 (URN)10.1016/j.matdes.2019.107764 (DOI)000465533900004 ()2-s2.0-85063813035 (Scopus ID)GOA JTH 2019 (Local ID)GOA JTH 2019 (Archive number)GOA JTH 2019 (OAI)
Funder
Knowledge Foundation, 20100280; 20170066
Available from: 2019-04-08 Created: 2019-04-08 Last updated: 2019-06-03Bibliographically approved
7. Influence of Fe-rich intermetallics and their segregation on anodising properties of Al-Si-Mg rheocast alloys
Open this publication in new window or tab >>Influence of Fe-rich intermetallics and their segregation on anodising properties of Al-Si-Mg rheocast alloys
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-43617 (URN)
Note

Submitted to journal.

Available from: 2019-05-09 Created: 2019-05-09 Last updated: 2019-05-09
8. Influence of Mg content on the fatigue behaviour of Al-Si-Mg alloys by rheocasting process
Open this publication in new window or tab >>Influence of Mg content on the fatigue behaviour of Al-Si-Mg alloys by rheocasting process
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-43618 (URN)
Available from: 2019-05-09 Created: 2019-05-09 Last updated: 2019-05-09

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