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Mechanical Properties of Semi-Solid Al Castings: Role of Stirring
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-0431-4352
2022 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Semi-solid metal (SSM) casting has been widely used in automotive industries to reduce the weight. In RheoMetalTM process which is one of the variations of SSM, the slurry fabrication can be finished within 30 second and can the slurry making process can be integrated into a high pressure die casting (HPDC) route without significant adjustments, making the process a promising alternative for industrial application. However, the application of SSM is still limited due to the semi-solid deformation-induced casting defects, such as macrosegregation and large pores. Due to the short stirring duration, the inefficient stirring is the leading cause of defects formation. Another critical issue in the RheoMetalTM process is the oxidation during the stirring process, which results in the increase of oxides in the castings, reducing the mechanical properties.

This study aims to investigate the ductility and the fatigue performance of SSM castings. The main focuses were on the role of the stirring and oxides. The quenched slurry was analyzed to evaluate the effect of the stirring on the particle distribution in the slurry, and its effect on the formation of pores and segregation was discussed. To investigate the oxidation during the slurry making process, two alloys with different Mg content were cast. Scanning electron microscopy (JEOL JSM-7001F SEM) equipped with Energy-dispersive X-ray spectroscopy (EDS) was used to identify the oxides on the fracture surface. Tensile test and fatigue test accompanied with direct current potential drop (DCPD) were performed to investigate the ductility and fatigue performance, respectively.

The results suggest that the intensive stirring can avoid the formation of the large clusters, making the particle distribution homogeneous in the slurry. The Mg contents determined the types of the oxides formed in the slurry making process. For alloys with sufficient Mg, the oxides would be MgAl2O4, which exist as small films with numerous cracks, while a large oxides film will be formed in the case of low Mg content.

The combined influence of porosity and oxides was concluded. In the 42000 alloy, because of the low Mg content, the ductility was dominated by the large oxide films. In contrast, in the Magsimal 59 alloy, the presence of small oxides (less than 0.2 mm in majority) leads to the influence of oxides on the elongation negligible. However, a good correlation was obtained between the largest pores and ductility.

The fatigue test shows that the surface liquid segregation (SLS) determined the fatigue strength under cyclic bend loading, due to its higher hardness. The effect of the inner pores on the fatigue performance was negligible, as the maximum stress was applied on the surface.

Abstract [sv]

Halvsolid metall (SSM) gjutning har använts i stor utsträckning inom bilindustrin för att minska vikten. I RheoMetalTM-processen, som är en av varianterna av SSM, kan slurrytillverkningen avslutas inom 30 sekunder och kan slurryframställningsprocessen integreras i en högtrycksgjutningsrutt (HPDC) utan betydande justeringar, vilket gör processen till ett lovande alternativ för industriell tillämpning. Tillämpningen av SSM är dock fortfarande begränsad på grund av de halvfasta deformationsinducerade gjutdefekterna, såsom makrosegregering och stora porer. På grund av den korta omrörningstiden är den ineffektiva omrörningen den främsta orsaken till att defekter bildas. En annan kritisk fråga i RheoMetalTM-processen är oxidationen under omrörningsprocessen, vilket resulterar i ökningen av oxider i gjutgodset, vilket minskar de mekaniska egenskaperna.

Denna studie syftar till att undersöka duktiliteten och utmattningsprestandan hos SSM-gjutgods. Huvudfokus var på omrörningens och oxidernas roll. Den kylda uppslamningen analyserades för att utvärdera effekten av omrörningen på partikelfördelningen i uppslamningen, och dess effekt på bildandet av porer och segregation diskuterades. För att undersöka oxidationen under slurrytillverkningsprocessen göts två legeringar med olika Mg-halt. Svepelektronmikroskopi (JEOL JSM-7001F SEM) utrustad med energidispersiv röntgenspektroskopi (EDS) användes för att identifiera oxiderna på brottytan. Dragtest och utmattningstest tillsammans med likströmspotentialfall (DCPD) utfördes för att undersöka duktiliteten respektive utmattningsprestandan.

Resultaten tyder på att den intensiva omrörningen kan undvika bildningen av de stora klustren, vilket gör partikelfördelningen homogen i slammet. Mg-innehållet bestämde vilka typer av oxider som bildades i slurrytillverkningsprocessen. För legeringar med tillräckligt med Mg skulle oxiderna vara MgAl2O4, som finns som små filmer med många sprickor, medan en stor oxidfilm kommer att bildas vid låg Mg-halt.

Den kombinerade inverkan av porositet och oxider konstaterades. I 42000-legeringen, på grund av det låga Mg-innehållet, dominerades duktiliteten av de stora oxidfilmerna. Däremot, i Magsimal 59-legeringen, leder närvaron av små oxider (mindre än 0,2 mm i majoritet) till att oxidernas inverkan på förlängningen är försumbar. En god korrelation erhölls dock mellan de största porerna och duktiliteten.

Utmattningstestet visar att ytvätskesegregeringen (SLS) bestämde utmattningshållfastheten under cyklisk böjbelastning, på grund av dess högre hårdhet. Effekten av de inre porerna på utmattningsprestandan var försumbar, eftersom den maximala belastningen applicerades på ytan.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering , 2022. , p. 49
Series
JTH Dissertation Series ; 069
Keywords [en]
Rheometal™ process; aluminium alloys; stirring; segregation; porosity; oxides; ductility; fatigue performance
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:hj:diva-56203ISBN: 978-91-87289-75-0 (print)OAI: oai:DiVA.org:hj-56203DiVA, id: diva2:1651995
Presentation
2022-05-12, Tekniska Högskolan, Jönköping, 13:00 (Swedish)
Opponent
Supervisors
Available from: 2022-04-14 Created: 2022-04-14 Last updated: 2023-11-22Bibliographically approved
List of papers
1. Role of Iron-Rich Phases and Porosity on the Ductility of Rheocast Al-Mg-Si-Alloys
Open this publication in new window or tab >>Role of Iron-Rich Phases and Porosity on the Ductility of Rheocast Al-Mg-Si-Alloys
2022 (English)In: Solid State Phenomena / [ed] J. Li & A. Rassili, Trans Tech Publications, 2022, Vol. 327, p. 65-70Chapter in book (Refereed)
Abstract [en]

Treatment of the slurry is important during RheoMetalTM casting. In this work, semi-solid slurries were prepared under different stirring intensities, using two types of stirrers: a naked rod (for regular stirring) and a rod with two blades (for intensive stirring). Tensile tests were performed, investigating fracture surfaces, as well as metallographic samples. The results show that intensive stirring produces castings with finer primary particles and a more homogeneous microstructure. On the other hand, more faceted Fe-rich phases are found along the α-Al grains boundary, due to the dissolution of Fe from the stirrers. Moreover, for intensive stirring castings, the porosity found on the fracture surfaces are smaller, while more second (intermetallic) phases, especially Fe-rich phases, are observed. Consequently, the castings with intensive stirring show worse ductility. Finally, a quantitative analysis was made regarding ductility, affected both by porosity and the presence of Fe-rich phases.

Place, publisher, year, edition, pages
Trans Tech Publications, 2022
Series
Solid State Phenomena, ISSN 0377-6883, E-ISSN 1662-9779 ; 327
Keywords
Elongation, Fe-rich intermetallic, Intensive stirring, Porosity, Segregation, Semi-solid casting
National Category
Materials Engineering
Identifiers
urn:nbn:se:hj:diva-56184 (URN)10.4028/www.scientific.net/SSP.327.65 (DOI)2-s2.0-85127198596 (Scopus ID)9783035717792 (ISBN)
Conference
16th International Conference on Semi-Solid Processing of Alloys and Composites, S2P 2021
Funder
Vinnova, 2018-02831
Available from: 2022-04-12 Created: 2022-04-12 Last updated: 2024-01-09Bibliographically approved
2. On the Role of Stirring on Microstructure and Ductility of Rheocast Al Alloys
Open this publication in new window or tab >>On the Role of Stirring on Microstructure and Ductility of Rheocast Al Alloys
2021 (English)In: JOM: The Member Journal of TMS, ISSN 1047-4838, E-ISSN 1543-1851, Vol. 73, no 12, p. 3848-3857Article in journal (Refereed) Published
Abstract [en]

Two alloys containing different Mg contents have been used to study the combined effect of stirring and oxidation on microstructure and ductility. The results show that intensive stirring can sufficiently disperse the alpha-Al particles and enable better liquid feeding during solidification and consequently reduce the porosity. The morphology of the oxides is determined by the amount of both Mg and stirring. With lower Mg content, the oxides present as oxide films, which can be broken up during stirring. In alloy with higher Mg content, the oxides exist as particles with numerous cracks, and the particle size increases slightly after stirring. In the Magsimal 59 alloy, due to the presence of large clusters of pores in the fracture surface, the influence of the small oxide particles on the ductility is negligible. In contrast, in the 42000 alloy, large oxide films on the fracture surface are correlated with the ductility.

Place, publisher, year, edition, pages
Springer, 2021
National Category
Materials Engineering
Identifiers
urn:nbn:se:hj:diva-54852 (URN)10.1007/s11837-021-04905-6 (DOI)000703371300004 ()2-s2.0-85116319612 (Scopus ID)HOA;;771344 (Local ID)HOA;;771344 (Archive number)HOA;;771344 (OAI)
Funder
Vinnova, 2018-02831
Available from: 2021-10-14 Created: 2021-10-14 Last updated: 2023-11-22Bibliographically approved
3. Four-point bending fatigue behavior of TX630 alloy: role ofthe surface liquid segregation
Open this publication in new window or tab >>Four-point bending fatigue behavior of TX630 alloy: role ofthe surface liquid segregation
(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:hj:diva-56205 (URN)
Note

Included in licentiate thesis in manuscript form.

Available from: 2022-04-14 Created: 2022-04-14 Last updated: 2023-11-22

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