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A study of formation and growth of the anodised surface layer on Al-Si casting alloys based on different analytical techniques
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-8098-4788
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing - Surface technology. SP-Technical Research Institute of Sweden.
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing - Surface technology. SP-Technical Research Institute of Sweden; University of Trento, Dept of Industrial Engineering.ORCID iD: 0000-0003-2924-137X
Thin Film Physics Division, Department of Physics, Chemistry and Biology, Linköping University.
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2015 (English)Conference paper, Published paper (Refereed)
Abstract [en]

One of the major limitations in the application of anodizing of aluminum casting alloys is the non-uniform thickness of the oxide layer. Previous researches have studied the formation of the oxide film during anodizing in detail. These investigations have mainly been limited to aluminum and wrought aluminum alloys, and only a few papers have given some insights about the formation of the oxide layer on cast aluminum alloys. The majority of cast aluminum alloys contains relatively higher amounts of Si and other elements (e.g. Cu and Fe) than wrought alloys. This paper aims to investigate the mechanisms of formation and growth of the anodized surface layer on Al-Si casting alloys by applying different analytical techniques such as optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray computer tomography (X-ray CT) scanning. Three different Al alloys with various Si content (2.4%, 3.5% and 5.5%) were investigated. Si morphological modification by Sr addition, as well as gradient solidification was used to vary the microstructural coarseness in a controlled manner to study the influence of these parameters on the growth of the oxide layer. The microstructure features of the anodized layer and the interface between matrix and the oxide layer were studied, employing optical microscopy (figure 1), and SEM/EDS (figure 2). X-Ray CT-scanning (figure 3) was used to examine the high density phases (Fe and Cu bearing phases) and also to give a 3-D view of the anodized oxide surface. Furthermore, TEM was used to examine the general filming behavior of anodized layer and the local effects associated with Si particles in the matrix.

It was found that: (i) during anodizing, the oxide front grew around the Si particles and tends to engulf them; (ii) the oxide front grew inwards but did expand in other directions in the eutectic areas; (iii) a scalloped interface between substrate and oxide indicated different anodizing rates in dendrites and eutectic phases. The oxide front was moving faster in the primary dendrite Al-phase than in the eutectic, and the growth rate was particularly slow in larger eutectic areas; (iv) different anodizing rates in dendrite and eutectic phases resulted in thinner layers in specimens with lower SDAS (secondary dendrite arm spacing); and (v) a more uniform layer thickness in Sr-modified specimens illustrates that oxide growth speed was faster in modified eutectic phase. However, diffusion of Si or long distance between Si particles after modification could even results in the adverse situation with a bad anodized surface appearance. 

Place, publisher, year, edition, pages
2015.
National Category
Engineering and Technology Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-27128OAI: oai:DiVA.org:hj-27128DiVA: diva2:820027
Conference
EastForum 2015 : Progress in Functional and Sustainable Surface Technology, Lund, June 25-26, 2015.
Available from: 2015-06-11 Created: 2015-06-11 Last updated: 2017-02-17Bibliographically approved
In thesis
1. On the influence of Si on anodising and mechanical properties of cast aluminium alloys
Open this publication in new window or tab >>On the influence of Si on anodising and mechanical properties of cast aluminium alloys
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The combination of two cost-effective processes, i.e. casting and anodising, would be an interest for the aluminium component applications. However, there are some obstacles in the application of anodising on cast Al alloys. The challenges mostly relate to the alloying elements especially Si and the surface quality. With the development of casting process, cast aluminium alloys with low Si content can be casted, and a complex geometry component with reasonably good surface finish can be achieved. This study aims to identify the influence of Si on anodising and mechanical properties of Al-Si alloys.

In this study, six Al-Si alloys with three different Si level and two different Sr level were investigated. Sr acts as a modifier to change the morphology of Si particles. The directional solidification technology was used to vary the microstructure coarseness by controlling the cooling rate to study the influence of Si level, Si particle morphology and cooling rate on mechanical properties, oxide layer formation and corrosion protection performance in cast Al-Si alloys.

This study has observed that Si has a significant influence on anodising. During anodising, Si particles are anodised at a lower rate than the Al phase. The presence of Si particles in eutectic phase make the oxide layer locally thinner and more defected due to the low oxide growth rate in eutectic phase. This study observed the presence of residual metallic Al phase beneath or between Si particles. Due to their presence and their geometry, Al can be shielded by Si particles and prevented from oxidation. Si particles also act as a key role in the corrosion protection of oxide layer in Al-Si alloys. The corrosion attack propagates along Si particles as well as oxide defects to the Al substrate.

It is found that the morphology of Si particles has a significant influence on the oxide layer formation and corrosion protection performance of the oxide layer on cast Al-Si alloys. A substantially improvement the corrosion resistance of anodised layer on Al-Si alloys is attributed to the morphology change from interconnected flakes to disconnected Si fibres when Sr is added, with less oxide defects and better oxide distribution.

The Si level governs the mechanical properties of Al-Si based alloys. An increase of Si content in Al alloys improves the mechanical properties such as ultimate tensile and yield strength as well as hardness of the materials, but decreases the ductility. However, an increase of Si level in Al alloys decreases the thickness of oxide layer, and thereby, the corrosion protection of the oxide layer is deteriorated.

Abstract [sv]

Kombinationen av två kostnadseffektiva processer, gjutning och anodisering, är av intresse för tillämpning på aluminiumkomponenter. Det finns dock hinder för tillämpning av anodisering på gjutna aluminiumlegeringar. Utmaningarna relaterar till effekten av legeringselement, i synnerhet Si, och komponentens ytkvalité. Med utvecklingen av gjutprocesser kan aluminiumlegeringar med lågt Si-innehåll gjutas, och komponenter med komplex geometri med förbättrad ytkvalité kan uppnås. Denna studie syftar till att identifiera påverkan av Si på anodisering och mekaniska egenskaper hos Al-Si-baserade legeringar.

I denna studie undersöktes sex Al-Si-legeringar med tre nivåer av Si och två nivåer av Sr. Tillsatser av Sr leder till modifikation av morfologin hos Si-partiklar. Med hjälp av tekniken ”riktad stelning” varierades stelningshastigheten för att studera sambanden mellan halten och morfologin av Si, mikrostrukturens grovlek och dess inverkan på mekaniska egenskaper, samt oxidskiktets bildning och korrosionsbeständighet hos gjutna Al-Si-legeringar.

Denna studie visar att Si har ett betydande inflytande på anodisering r. Under anodisering, anodiseras Si-partiklar i en lägre takt än Al-fasen. Närvaron av Si-partiklar i eutektisk fas bidrar till ett lokalt förtunnat oxidskikt med fler defekter på grund av den låga oxidtillväxthastigheten i eutektisk fas. Denna studie observerade förekomsten av kvarvarande metalliska Al fasen under eller mellan Si-partiklar. På grund av Si-partiklarnas närvaro och geometri, kan Al skärmas från oxidation. Si-partiklar har även en nyckelroll i korrosionsskydd av oxidskiktet i Al-Si-legeringar. Korrosionsangreppet propagerar längs Si-partiklar samt oxiddefekter till Al-substrat.

Morfologin av Si-partiklar har funnits ha en betydande inverkan på oxidskiktets bildning och korrosionsskydd hos gjutna Al-Si-legeringar. En väsentlig förbättring av korrosionsbeständigheten i anodiserat skikt hos Al-Si-legeringar tillskrivs förändring av kiselns morfologi från sammankopplade flingor till osammanhängande fibrer när Sr tillsätts, med mindre oxiddefekter och bättre fördelning av oxid.

Si halten påverkar de mekaniska egenskaperna hos Al-Si-baserade legeringar. En ökning av Si innehålleti Al-legeringar förbättrar de mekaniska egenskaperna såsom brott och sträckgräns samt hårdhet hos materialen, men minskar duktiliteten. En ökning med Si halt i Al-legeringar minskar dessvärre tjockleken hos oxidskiktet, och därigenom, försämrar oxidskiktets korrosionsskydd.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2017. 44 p.
Series
JTH Dissertation Series, 19
Keyword
Al-Si based casting alloys, anodising, corrosion protection, mechanical properties, Al-Si-baserade gjutningslegeringar, anodisering, korrosionsskydd, mekaniska egenskaper
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-35096 (URN)978-91-87289-20-0 (ISBN)
Supervisors
Available from: 2017-02-17 Created: 2017-02-17 Last updated: 2017-02-17Bibliographically approved

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