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Formation of Iron-Rich Intermetallic Phases in Al-7Si-Mg: Influence of Cooling Rate and Strontium Modification
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0003-0260-5153
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-0101-0062
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-3184-7592
2019 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50, no 9, p. 4148-4165Article in journal (Refereed) Published
Abstract [en]

The influence of cooling rate on the formation of iron-rich intermetallic phases during solidification of unmodified and strontium-modified Al-7Si-0.3Mg alloys has been investigated. The effect of strontium on the intermetallic phases was evaluated in unquenched and quenched samples. Samples were quenched before the start of the Al-Si eutectic reaction, along the Al-Si eutectic reaction and just after the end of solidification. The results show that the addition of strontium increased the size of both β-Al5FeSi and π-Al8FeMg3Si6 at low cooling rates. For unmodified and strontium-modified alloys, an increase of cooling rate resulted in a decrease in size of the intermetallic phases, particularly in the strontium modified alloy. In the strontium modified alloy quenched before the start of the Al-Si eutectic reaction, π-Al8FeMg3Si6 appeared as thin platelets at the eutectic cell boundaries. Chinese script-like π-Al8FeMg3Si6 and platelet-like β-Al5FeSi intermetallic phases were observed uniformly distributed in the eutectic regions in the unmodified alloy quenched before the start of the eutectic reaction. Strontium modified semi-solid Al-7Si-0.3Mg castings were produced and the type of intermetallic phase, morphology, size, area fraction and distribution were similar to that observed in the strontium modified alloy quenched before the start of the Al-Si eutectic reaction.

Place, publisher, year, edition, pages
Springer, 2019. Vol. 50, no 9, p. 4148-4165
Keywords [en]
Aluminum alloys, Cooling, Eutectics, Intermetallics, Iron, Magnesium alloys, Platelets, Solidification, Strontium alloys, Al-7Si-0.3Mg alloys, Al-Si eutectic, Chinese script, Eutectic cells, Eutectic reactions, Intermetallic phase, Intermetallic phasis, Thin platelets, Silicon alloys
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-45391DOI: 10.1007/s11661-019-05343-5ISI: 000477997600021Scopus ID: 2-s2.0-85068741757Local ID: HOA JTH 2019OAI: oai:DiVA.org:hj-45391DiVA, id: diva2:1338528
Funder
Vinnova, 2014-05096Available from: 2019-07-23 Created: 2019-07-23 Last updated: 2020-10-27Bibliographically approved
In thesis
1. Semi-solid Al-7Si-Mg castings – Microstructure and mechanical properties
Open this publication in new window or tab >>Semi-solid Al-7Si-Mg castings – Microstructure and mechanical properties
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The use of lightweight parts by the vehicle industry produces immediate benefits on the reduction of emissions because less energy is consumed during the production, service, and recycle stages of a product life cycle. Therefore, the development of processes that allow high design freedom for topology optimisation and materials with high specific strength is a great need. Semi-solid Al-7Si-Mg castings provide great potential for weight reduction, particularly in critical applications where materials such as steel and cast iron are typically used. However, critical applications have higher requirements in mechanical and fatigue properties compared to conventional aluminium castings applications. Therefore, the control of microstructure and defect formation in all steps of the semi-solidcasting process is essential to produce lightweight, reliable castings for future demands.

In semi-solid aluminium casting, a slurry consisting of primary α-Al crystals dispersed in the liquid is injected into the die-cavity. In this study, the slurry preparation involved the immersion of a cylinder (so-called EEM) while rotating into a superheated alloy. This investigation showed that the α-Al crystals in the slurry are a combination of equiaxed α-Al crystals that nucleate in the thermal undercooled liquid surrounding the EEM, crystal fragments from the columnar dendrites solidified on the EEM surface and undissolved crystals from the original EEM. The addition of grain refiners has no significant effect on the size and shape of the α-Al crystals in the slurry. The dissolution of the EEM duringslurry preparation was studied using a new tag-and-trace method of α-Al crystals. Whenthe EEM disintegrates into large α-Al crystal agglomerates during slurry preparation can result in detrimental effects on the fatigue properties of SSM castings.

Alloy composition, cooling rate, strontium modification, and heat treatment affect the type, size, and shape of the intermetallic phases formed in the Al-7Si-Mg castings. This study showed that high cooling rates and strontium modification are beneficial for the formation of smaller and less detrimental iron-rich intermetallic phases to mechanical and fatigue properties.

The precipitation hardening response of the SSM Al-7Si-Mg castings strongly affects mechanical and fatigue properties. The results in this study showed that the 0.2% offset yield strength increases linearly with the increase of the magnesium concentration in the interior of the α-Al crystals formed during slurry preparation of SSM Al-7Si-Mg castings in the T5 and T6 conditions. Macrosegregation regions surrounded by an oxide layer were preferential sites for fatigue crack initiation in the SSM Al-7Si-Mg castings tested in this study.

Abstract [sv]

Viktminskningen av detaljer som ska användas inom fordonsindustrin ger omedelbara fördelar för reduktion av utsläpp. Detta beror på att det går åt mindre energi att producera, flytta och återvinna dessa lätta komponenter. Utvecklingen av processer som möjliggör hög designfrihet för topologioptimering och material med hög specifik styrka har stor efterfrågan och är därför viktig. Gjutgods framtagning genom semisolidgjutning av Al-7Si-Mg ger stora möjligheter till viktminskning, och då särskilt i kritiska applikationer. Här har traditionella material, som stål och gjutjärn samt processer som smide använts, med större designbegränsningar. Emellertid har kritiska applikationer strängare krav påmekaniska egenskaper och utmattningsegenskaper jämfört med konventionellt gjutgods av aluminium. Därför är styrningen av mikrostruktur och defekter i alla steg i semisolidgjutningsprocessen avgörande för att producera lätt, pålitligt gjutgods för framtida behov.

I semisolidgjutning av aluminium pressas en slurry bestående av primära α-Al-kristaller dispergerade i smältan in i formhåligheten. Slurryberedningen i den studerade processen innebär att cylinder, vid låg temperatur (så kallad EEM), förs ner i en överhettad smälta under rotation. Den fasta fasen i slurryn är en kombination av de likaxliga α-Alkristallerna som kärnbildas i den underkylda smältan närmast EEMet, och kristallfragment från de dendriterna som bildar på EEM cylinderns yta och från EEMet. Tillsats av ympmedel har ingen signifikant effekt på storlek eller formen på α-Alkristallerna i slurryn. När EEMet faller sönder i mer eller mindre stora α-Al-kristaller kan kluster i slurryn resultera i defekter som kan skada utmattningsegenskaperna hos semisolidgjutgods.

Legeringssammansättning, kylhastighet, strontiummodifiering och värmebehandling påverkar typ, storlek och form på de intermetalliska faserna som bildas i Al-7Si-Mg-gjutgods. Höga kylhastigheter och strontiummodifiering är fördelaktiga för bildandet av mindre stora järnrika intermetaller som dessutom är mindre skadliga för mekaniska egenskaper och utmattningsprestandan hos gjutgodset.

Härdningsresponsen för semisolidgjutet Al-7Si-Mg-gods påverkar mekaniska egenskaper och utmattningsegenskaper kraftigt. Resultaten i denna studie visade att den sträckgränsen ökar linjärt med öknande magnesiumhalt i det inre av α-Al-fasen bildad under slurrytillverkningen av semisolitt Al-7Si-Mg-gjutgods i T5- och T6-tillstånd. Makrosegregeringregioner omgivna av ett oxidskikt var primärorsaker för initiering av utmattningssprickor av semisolitt Al-7Si-Mg gjutgods.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2020. p. x, 75
Series
JTH Dissertation Series ; 057
Keywords
Rheometal™ process; semi-solid casting; aluminium alloys; dissolution; grain refinement; segregation; intermetallic phases; heat treatment; mechanical properties; fatigue properties, Rheometal™ processen; semi-solid gjutgarna; aluminiumlegeringar; upplösning; kornförfining; segregering; intermetalliska faser; värmebehandling; mekaniska egenskaper; utmattningsegenskaper
National Category
Materials Engineering
Identifiers
urn:nbn:se:hj:diva-50882 (URN)978-91-87289-61-3 (ISBN)
Public defence
2020-11-27, E1405, Tekniska högskolan, Jönköping, 13:00 (English)
Opponent
Supervisors
Available from: 2020-10-27 Created: 2020-10-27 Last updated: 2020-10-27Bibliographically approved

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Santos, JorgeJarfors, Anders E.W.Dahle, Arne

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