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Optimization and validation of a dislocation density based constitutive model for as-cast Mg-9%Al-1%Zn
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-9886-9710
Department of Materials and Manufacturing, School of Engineering, Jönköping University, Jönköping, Sweden.
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0001-6445-6005
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-7527-719X
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2018 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 710, p. 17-26Article in journal (Refereed) Published
Abstract [en]

A dislocation density-based constitutive model, including effects of microstructure scale and temperature, was calibrated to predict flow stress of an as-cast AZ91D (Mg-9%Al-1%Zn) alloy. Tensile stress-strain data, for strain rates from 10-4 up to 10-1 s-1 and temperatures from room temperature up to 190 °C were used for model calibration. The used model accounts for the interaction of various microstructure features with dislocations and thereby on the plastic properties. It was shown that the Secondary Dendrite Arm Spacing (SDAS) size was appropriate as an initial characteristic microstructural scale input to the model. However, as strain increased the influence of subcells size and total dislocation density dominated the flow stress. The calibrated temperature-dependent parameters were validated through a correlation between microstructure and the physics of the deforming alloy. The model was validated by comparison with dislocation density obtained by using Electron Backscattered Diffraction (EBSD) technique.

Place, publisher, year, edition, pages
Elsevier, 2018. Vol. 710, p. 17-26
Keywords [en]
Casting methods, Characterization, Magnesium alloy, Optimization, Physically-based model, Plasticity, Aluminum, Aluminum alloys, Constitutive models, Magnesium castings, Microstructure, Plastic flow, Strain rate, Ternary alloys, Zinc, Zinc alloys, Zirconium alloys, Casting method, Dislocation densities, Electron back-scattered diffraction, Model calibration, Physically based modeling, Secondary dendrite arm spacing, Temperature dependent, Tensile stress strain, Magnesium alloys
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-38144DOI: 10.1016/j.msea.2017.10.081ISI: 000429888200003Scopus ID: 2-s2.0-85032297009OAI: oai:DiVA.org:hj-38144DiVA, id: diva2:1165156
Available from: 2017-12-12 Created: 2017-12-12 Last updated: 2018-09-21Bibliographically approved
In thesis
1. As-cast AZ91D magnesium alloy properties: Effects of microstructure and temperature
Open this publication in new window or tab >>As-cast AZ91D magnesium alloy properties: Effects of microstructure and temperature
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Today, there is an essential need for lightweight, energy-efficient, environmentally benign engineering systems, and this is the driving force behind the development of a wide range of structural and functional materials for energy generation, energy storage, propulsion, and transportation. These challenges have motivated the use of magnesium alloys for lightweight structural systems. Magnesium has a density of 1.74 g/cm3, which is almost 30% less than that of aluminium, one quarter of steel, and almost identicalto polymers. The ease of recycling magnesium alloys as compared to polymers makes them environmentally attractive, but their poor mechanical performance is the primary reason for the limited adoption of these alloys for structural applications.

The Mg-Al-Zn alloy AZ91D exhibits an excellent combination of strength, die-castability, and corrosion resistance. However, its mechanical performance with regard to creep strength, for example, at evaluated temperatures is poor. Moreover, very little is known about the correlation between its mechanical properties and microstructural features. This thesis aims to provide new knowledge regarding the role played by microstructure in the mechanical performance of the magnesium alloy. The properties/performance of the material in relation to process parameters became of great interest during the investigation.

An exhaustive characterisation of the grain size, secondary dendrite arm spacing (SDAS) distribution, and fraction of Mg17Al12 was performed using optical and electron backscatter diffraction (EBSD). These microstructural parameters were correlated to the offset yield point (Rp0.2), fracture strength, and elongation to failure of the material. It was proposed that the intermetallic phase, Mg17Al12, plays an important role in determining the mechanical and physical properties of the alloy in a temperature range of room temperature to 190°C by forming a rigid network of intermetallic. The presence of this network was confirmed by studying the thermal expansion behaviour of samples of the alloy containing different amounts of Mg17Al12.

A physically based constitutive model with a wide validity range was successfully adapted to describe the flow stress behaviour of AZ91D with various microstructures. The temperature-dependent variables of the model correlated quite well with the underlying physics of the material. The model was validated through comparison with dislocation densities obtained using EBSD.

The influence of high-pressure die-cast parameters on the distortion and residual stress of the cast components was studied, as were distortion and residual stress in components after shot peening and painting. Interestingly, it was found that intensification pressure has a major effect on distortion and residual stresses, and that the temperature of the fixed half of the die had a slight influence on the component's distortion and residual stress.

Abstract [sv]

Numera finns det ett väsentligt behov av lätta, energieffektiva och miljövänliga tekniksystem. Detta behov är drivkraften för utveckling av ett brett utbud av material för energigenerering, energilagring, framdrivning och transport. Dessa utmaningar motiverade användningen av magnesiumlegeringar för lättviktskonstruktioner. Magnesium har en densitet på 1,74 g/cm3, vilket är ca 30% lägre än för aluminium, en fjärdedel av densiteten för stål och nästan i nivå med många polymerer. Då magnesiumlegeringar dessutom är lätta att återvinna, jämfört med polymerer, gör det dem miljömässigt attraktiva. Låga mekaniska egenskaper är den främsta orsaken till begränsad användning av dessa legeringar för lastbärande tillämpningar.

Mg-Al-Zn-legeringen AZ91D uppvisar en utmärkt kombination av styrka, gjutbarhet och korrosionsbeständighet. Dess mekaniska egenskaper vid förhöjd temperatur, som tex kryphållfasthet, är låga. Dessutom är korrelationen mellan mikrostruktur och mekaniska egenskaper oklar. Denna avhandling syftade till att ge ny kunskap om mikrostrukturens roll för magnesiumlegeringars mekaniska egenskaper. Slutligen var materialets egenskaper i förhållande till processparametrar vid tillverkningen av stort intresse.

En omfattande karaktärisering av kornstorleks-, sekundära dendritarmavstånds (SDAS)-fördelning och fraktion av Mg17Al12 utfördes med hjälp av optisk mikroskopering och diffraktion av bakåtspridda elektroner (EBSD). Mikrostrukturen korrelerades till sträckgränsen (Rp0.2), brottstyrkan och brottförlängningen. Det föreslogs att den intermetalliska fasen, Mg17Al12, spelar en viktig roll vid bestämning av legeringens mekaniska och fysikaliska egenskaper vid temperaturintervall från rumstemperatur upp till 190°C genom att bilda ett styvt nätverk av intermetaller. Uppkomsten av ett sådant nätverk stöddes genom en studie av den termiska expansionen av legeringen för olika fraktioner av Mg17Al12.

En fysikalisk konstitutiv modell med ett brett giltighetsområde användes framgångsrikt för att beskriva det plastiska flytbeteendet hos AZ91D för olika mikrostrukturer. De temperaturberoende variablerna i modellen korrelerade ganska väl med materialets underliggande fysik. Modellen validerades genom att jämföra dislokationstätheten som predikterades av modellen och den med EBSD uppmätta dislokationstätheten.

Påverkan av pressgjutningsparametrar på geometrisk tolerans och restspänning hos de gjutna komponenterna studerades. Vidare studerades geometrisk tolerans och restspänning av komponenter efter pening och målning. Intressant nog hade eftermatningsfasen en stor effekt på geometrisk tolerans och restspänningar. Dessutom hade temperaturen på den fasta formhalvan av verktyget även ett visst inflytande på komponentens geometriska tolerans och restspänning.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2017. p. 77
Series
JTH Dissertation Series ; 30
Keywords
Magnesium; Magnesium Alloy; AZ91D; High-Pressure Die-Casting; Mechanical Property; Microstructural Characterisation; Physical Modelling; Dislocations; Distortion; Residual Stress, Magnesium; Magnesiumlegering, AZ91D; Pressgjutna, Mekanisk Egenskap, Mikrostrukturkarakterisering, Fysikalisk Modellering; Flytspänning; Dislokationer; Geometrisk Tolerans; Restspänning
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-38148 (URN)978-91-87289-31-6 (ISBN)
Supervisors
Funder
Knowledge Foundation, 20100280
Available from: 2017-12-12 Created: 2017-12-12 Last updated: 2017-12-12Bibliographically approved

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