Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Four-point bending fatigue behavior of Rheocast AlSi7Mg0.3 alloy: Effect of sample thickness
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-0431-4352
CICECO - Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, Aveiro, Portugal.ORCID iD: 0000-0003-0260-5153
KTH Royal Institute of Technology, School of Industrial Engineering and Management, Materials Science and Engineering, Stockholm, Sweden.
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-0101-0062
(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:hj:diva-62910OAI: oai:DiVA.org:hj-62910DiVA, id: diva2:1813792
Note

Included in doctoral thesis in manuscript form.

Available from: 2023-11-22 Created: 2023-11-22 Last updated: 2023-11-22
In thesis
1. Mechanical properties of semi-solid Al castings: Role of stirring
Open this publication in new window or tab >>Mechanical properties of semi-solid Al castings: Role of stirring
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Aluminum castings have been widely used in the automotive industry to reduce the vehicle's weight. However, the existence of casting defects significantly limits its application. The most common and detrimental defects in aluminum castings are porosity and oxides. The formation of the pores comes from the solute hydrogen and volumetric reduction during the solidification process, resulting in the gas and shrinkage pores, respectively. The oxides can be introduced by either the operation during the process or the originally existing oxides in the melt. To reduce these defects, optimizing the casting process and controlling the melt quality is essential.

In this work, the Semisolid Metal (SSM) process was used as it can significantly reduce the formation of shrinkage pores. The main focuses were on the influence of stirring intensity on the formation of casting defects and, thus, the resultant mechanical properties. In addition, to control the original melt quality, particularly the amount of oxides, efforts were made to develop proper methods for the melt quality assessment.

The results show that the slurry-making process, mainly through stirring intensity, can affect the casting defects significantly. On the one hand, the increasing stirring intensity can distribute the primary Al particles more homogeneously, reducing the pores in terms of size and number by increasing the permeability during the solidification process. On the other hand, the increasing stirring intensity can affect the size of oxides differently, depending on the composition, for instance, the Mg content.

For the alloys with sufficient Mg, the oxides would be MgAl2O4, which are small films with numerous cracks. Under intensive stirring, the oxides can break down into small oxide particles. As a result, intensive stirring can improve ductility by reducing the formation of big pores. However, the oxides would mainly be Al2O3 films for alloys with low Mg content. In this case, the current stirring intensity is insufficient to break the oxide films. Instead, the increased stirring has introduced more oxide films into the melt. Consequently, in the casting with intensive stirring, the increasing oxide films dominated the ductility rather than the reduced porosity.

The SSM castings exhibit better bending fatigue properties than the casting using the traditional high-pressure die casting (HPDC) process. This improvement is mainly due to the formation of the harder surface liquid segregation (SLS) layer on the SSM casting surface. Furthermore, compared with the standard SSM process, the castings using intensive stirring (hereinafter referred to as the modified SSM process) show similar but more reliable fatigue properties. This reliable fatigue property can be attributed to eliminating the big internal pores through intensive stirring, which results in local stress concentration and significantly reduces fatigue performance. Besides, due to the gradient stress distribution in the bending loading, the surface defects play a significant role in the fatigue properties. With the increase of the specimens’ thickness, the failure mechanisms changed.

The shrinkage pores in the reduced pressure test (RPT) test play a significant role in the accuracy of melt quality assessment. A good correlation between the bifilm index (BI)/ density index (DI) and hydrogen content is observed for the RPT samples without significant shrinkage pores. In addition, the correlation between the BI and elongation is also strongly affected by the clusters of shrinkage pores due to the conflict between the definition of the BI and the influence of clusters of shrinkage pores on the ductility. Based on this, we proposed an optimized BI where the clusters of shrinkage pores were treated as single pores, increasing the reliability of the correlation between the BI and elongation. 

Abstract [sv]

Aluminiumgjutgods har använts i stor utsträckning inom bilindustrin för att minska fordonets vikt. Förekomsten av gjutningsdefekter begränsar emellertid dess tillämpning avsevärt. De vanligaste och mest skadliga defekterna i aluminiumgjutgods är porositet och oxider. Bildandet av porerna kommer från löst väte och volymetrisk reduktion under stelningsprocessen, vilket resulterar i att gasporerna respektive krymperporerna. Oxiderna kan införas antingen genom operationen under processen eller de ursprungligen existerande oxiderna i smältan. För att minska mängden av dessa defekter måste gjutningsprocessen optimeras och smältkvaliteten måste kontrolleras kritiskt samtidigt.

I detta arbete användes Semisolid Metal (SSM)-processen. Fokus ligger på inverkan av omrörningsintensiteten på bildandet av gjutdefekter och de resulterande mekaniska egenskaperna. För att kontrollera den ursprungliga smältkvaliteten, särskilt mängden oxider, gjordes ansträngningar för att utveckla lämpliga metoder för bedömning av smältkvaliteten.

Resultaten visar att slurrytillverkningsprocessen påverkar gjutdefekterna avsevärt. Å ena sidan kan den ökande omrörningsintensiteten fördela de primära Al-partiklarna mer homogent, vilket minskar porerna vad gäller storlek och antal genom att öka permeabiliteten under stelningsprocessen. Å andra sidan kan den ökande omrörningsintensiteten påverka storleken på oxider olika beroende på sammansättningen, till exempel Mg-halten.

För legeringar med tillräckligt med Mg skulle oxiderna vara MgAl2O4, som existerar som små filmer med många sprickor. Under intensiv omrörning kan oxiderna brytas ner till små oxidpartiklar. Som ett resultat kan intensiv omrörning förbättra duktiliteten genom att minska bildningen av stora porer. För legeringarna med lågt Mg-innehåll är emellertid oxiderna huvudsakligen Al2O3-filmer. I detta fall är den aktuella omrörningsintensiteten inte tillräcklig för att bryta oxidfilmerna, utan fler oxidfilmer kan införas i smältan. Följaktligen, i gjutningen med intensiv omrörning, dominerades duktiliteten av de ökande oxidfilmerna snarare än den minskade porositeten.

På grund av bildandet av det hårdare segrade ytsmältskiktet (SLS) på SSM-gjutytan uppvisar SSM-gjutgodset bättre böjningsutmattningsegenskaper än högtryckspressgjutprover (HPDC). Vidare, i jämförelse med standard SSM-processen, visar gjutgods med intensiv omrörning liknande men mer tillförlitliga utmattningsegenskaper. Detta kan tillskrivas elimineringen av de stora inre porerna genom intensiv omrörning. Dessutom, på grund av gradientspänningsfördelningen i böjbelastningen, spelar ytdefekterna en betydande roll för utmattningsegenskaperna. Med ökningen av provernas tjocklek förändrades felmekanismerna.

Krympporerna i testet med reducerat tryck (RPT) spelar en betydande roll för noggrannheten av smältkvalitetsbedömningen. För RPT-prover utan signifikanta krympningsporer observeras en god korrelation mellan bifilmindex (BI)/densitetsindex (DI) och vätehalt. Dessutom är korrelationen mellan BI och förlängning också starkt påverkad av förekomsten av kluster av krympningsporer. Detta kan tillskrivas konflikten mellan definitionen av BI och påverkan av kluster av krympningsporer på duktiliteten. Baserat på detta föreslog vi en optimerad BI där klustren av krympningsporer behandlades som enstaka porer, vilket ökade tillförlitligheten av korrelationen mellan BI och förlängning. 

 

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2023. p. 75
Series
JTH Dissertation Series ; 085
Keywords
Rheometal™ process, aluminum alloys, stirring, segregation, porosity, oxides, ductility, fatigue performance, melt quality
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-62912 (URN)978-91-89785-03-8 (ISBN)978-91-89785-04-5 (ISBN)
Public defence
2023-12-08, E1405 (Gjuterisalen), Tekniska Högskolan, Jönköping, 09:00 (Swedish)
Opponent
Supervisors
Available from: 2023-11-22 Created: 2023-11-22 Last updated: 2023-11-22Bibliographically approved

Open Access in DiVA

No full text in DiVA

Authority records

Zhang, QingSantos, JorgeJarfors, Anders E.W.

Search in DiVA

By author/editor
Zhang, QingSantos, JorgeJarfors, Anders E.W.
By organisation
JTH, Materials and Manufacturing
Materials Engineering

Search outside of DiVA

GoogleGoogle Scholar

urn-nbn

Altmetric score

urn-nbn
Total: 70 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf