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Influence of the Sr modification and post-treatment on corrosion resistance of oxide layer of cast Al-(low)Si alloys
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-8098-4788
Department of Industrial Engineering, University of Trento, Trento, Italy.
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0001-6445-6005
Jönköping University, School of Engineering, JTH, Materials and Manufacturing. RISE Research Institute of Sweden, Borås, Sweden.ORCID iD: 0000-0002-7095-1907
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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. p. 27-39
Keywords [en]
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: urn:nbn:se:hj:diva-41534Scopus ID: 2-s2.0-85052280685OAI: oai:DiVA.org:hj-41534DiVA, id: diva2:1250660
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
In thesis
1. Casting and anodising of Al alloys- Alloy design, manufacturing process and material properties
Open this publication in new window or tab >>Casting and anodising of Al alloys- Alloy design, manufacturing process and material properties
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
Cast aluminium, Semi-solid casting, Anodising, Gjutning av aluminium, semi-solid gjutning, anodisering, komponent, gjutlegering, kisel, järn
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-43619 (URN)978-91-87289-43-9 (ISBN)
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

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