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Influence of Si and cooling rate on microstructure and mechanical properties of Al–Si–Mg cast alloys
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.ORCID iD: 0000-0002-7095-1907
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0001-6481-5530
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-0101-0062
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. Vol. 48, no 8, p. 861-869
Keywords [en]
Al-Si alloys; microstructure; mechanical properties; hardness; anodizing
National Category
Manufacturing, Surface and Joining Technology Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-28024DOI: 10.1002/sia.5814ISI: 000383754600024Scopus ID: 2-s2.0-84940688989OAI: oai:DiVA.org:hj-28024DiVA, id: diva2:855858
Funder
Knowledge Foundation, 20100280Available from: 2015-09-22 Created: 2015-09-22 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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