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As-cast AZ91D magnesium alloy properties: Effects of microstructure and temperature
Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-9886-9710
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 [en]
Magnesium; Magnesium Alloy; AZ91D; High-Pressure Die-Casting; Mechanical Property; Microstructural Characterisation; Physical Modelling; Dislocations; Distortion; Residual Stress
Keywords [sv]
Magnesium; Magnesiumlegering, AZ91D; Pressgjutna, Mekanisk Egenskap, Mikrostrukturkarakterisering, Fysikalisk Modellering; Flytspänning; Dislokationer; Geometrisk Tolerans; Restspänning
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-38148ISBN: 978-91-87289-31-6 (print)OAI: oai:DiVA.org:hj-38148DiVA, id: diva2:1165186
Supervisors
Funder
Knowledge Foundation, 20100280Available from: 2017-12-12 Created: 2017-12-12 Last updated: 2017-12-12Bibliographically approved
List of papers
1. Effects of microstructure on deformation behaviour of AZ91D cast alloy
Open this publication in new window or tab >>Effects of microstructure on deformation behaviour of AZ91D cast alloy
2014 (English)In: TMS 2014 Annual Meeting Supplemental Proceedings TMS, The Minerals, Metals & Materials Society, San Diego, February 16-20, 2014, John Wiley & Sons, 2014, p. 565-572Conference paper, Published paper (Refereed)
Abstract [en]

The deformation behavior of AZ91D cast magnesium alloy was investigated using uniaxial tensile tests from room temperature up to 190°C and strain rates from 0.0001 up to 0.1 1/s. In present work gradient solidification in a Bridgeman furnace was employed to study the effect of initial microstructure on the tensile mechanical response of the AZ91D alloy. The furnace drawing rate was varied from minimum 0.3 to maximum 6 mm/s, which yielded a variation of SDAS from 4.2 up to 25 as well as a variation of the fraction of the γ -phase (Mg17Al12),. The effects of microstructural parameters such as SDAS and γ -phase morphology on the yield strength (YS), ultimate tensile strength (UTS) and hardening were investigated

Place, publisher, year, edition, pages
John Wiley & Sons, 2014
Series
TMS Annual Meeting
Keywords
AZ91D, Microstructure, Mechanical properties, Temperature, Strain rate, Phase fraction
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-28468 (URN)000354941300068 ()2-s2.0-84899708224 (Scopus ID)978-1-118-88972-5 (ISBN)
Conference
TMS 2014
Available from: 2015-12-02 Created: 2015-12-02 Last updated: 2017-12-12Bibliographically approved
2. Microstructural Scale Effects on Thermal Expansion Behaviour of Cast AZ91D
Open this publication in new window or tab >>Microstructural Scale Effects on Thermal Expansion Behaviour of Cast AZ91D
2015 (English)In: Magnesium Technology 2015 - TMS 2015 144th Annual Meeting and Exhibition, Orlando, March 15-19, 2015, Hoboken: John Wiley & Sons, 2015, p. 361-365Conference paper, Published paper (Refereed)
Abstract [en]

The effect of microstructure on thermal expansion of AZ91D cast alloy was studied. Samples with equiaxed grains and a controlled secondary dendrite arm spacing (SDAS) were fabricated using gradient solidification. SDAS was chosen to represent the range ofmicrostructural scale found in sand castings down to that of high pressure die casting. Optical microscopy and electron backscatter diffraction (EBSD) were used for microstructural characterization. The relation between thermal expansion and microstructuralscale of existing phases precipitated, in particular grain size, SDAS and fraction of Mg17Al12 was analyzed.

Place, publisher, year, edition, pages
Hoboken: John Wiley & Sons, 2015
Keywords
AZ91D, Magnesium, Mg17Al12, Microstructure, Thermal expansion
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-28486 (URN)10.1002/9781119093428.ch67 (DOI)2-s2.0-84942124208 (Scopus ID)978-111908243-9 (ISBN)
Conference
Magnesium Technology 2015 - TMS 2015
Available from: 2015-12-07 Created: 2015-12-07 Last updated: 2017-12-12Bibliographically approved
3. Effect of Mg17Al12 Fraction on Mechanical Properties of Mg-9%Al-1%Zn Cast Alloy
Open this publication in new window or tab >>Effect of Mg17Al12 Fraction on Mechanical Properties of Mg-9%Al-1%Zn Cast Alloy
2016 (English)In: Metals, ISSN 2075-4701, Vol. 6, no 10, article id 251Article in journal (Refereed) Published
Abstract [en]

In the current study it was observed that the offset yield point of Mg-9%Al-1%Zn alloy was strongly influenced by the connectivity of Mg17Al12. It was suggested that an increase in the fraction of Mg17Al12 from 8% to 11% could lead to the formation of a Mg17Al12 network which resulted in a higher offset yield point. In addition, it was observed that elongation to failure of the Mg-9%Al-1%Zn alloy strongly depended on the fraction of Mg17Al12. Moreover, the apparent toughness showed a strong inverse relation to the secondary dendrite arm spacing. This approach might be extended to forecast the behavior in other magnesium alloys forming a network of the Mg-Al phase.

Keywords
casting, microstructure, tension test, magnesium alloys, mechanical properties
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-34778 (URN)10.3390/met6100251 (DOI)000388888100024 ()2-s2.0-84995680440 (Scopus ID)
Available from: 2017-01-18 Created: 2017-01-18 Last updated: 2017-12-12Bibliographically approved
4. Modeling the Deformation Behavior of As-Cast AZ1D Including the Effect of The Cast Microstructure
Open this publication in new window or tab >>Modeling the Deformation Behavior of As-Cast AZ1D Including the Effect of The Cast Microstructure
Show others...
2017 (English)In: Proceedings of PLASTICITY ’17: The Twenty Third International Conference on Plasticity, Damage, and Fracture, 2017, p. 37-39Conference paper, Published paper (Refereed)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-38142 (URN)
Conference
The Twenty Third International Conference on Plasticity, Damage, and Fracture, January 3-9, 2017, Puerto Vallarta, Mexico
Available from: 2017-12-12 Created: 2017-12-12 Last updated: 2017-12-12Bibliographically approved
5. Optimization and validation of a dislocation density based constitutive model for as-cast Mg-9%Al-1%Zn
Open this publication in new window or tab >>Optimization and validation of a dislocation density based constitutive model for as-cast Mg-9%Al-1%Zn
Show others...
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
Keywords
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:nbn:se:hj:diva-38144 (URN)10.1016/j.msea.2017.10.081 (DOI)XYZ ()2-s2.0-85032297009 (Scopus ID)
Available from: 2017-12-12 Created: 2017-12-12 Last updated: 2018-01-19Bibliographically approved
6. Effect of process parameters on distortion and residual stress of high-pressure die-cast AZ91D components
Open this publication in new window or tab >>Effect of process parameters on distortion and residual stress of high-pressure die-cast AZ91D components
2017 (English)In: International Journal of metalcasting, ISSN 1939-5981, E-ISSN 2163-3193Article in journal (Refereed) Epub ahead of print
Abstract [en]

This paper presents a study of distortion and residual stress within a high-pressure die-cast AZ91D component, cast under different processing conditions. The influence of process parameters, i.e., die temperature, cooling time, intensification pressure and first-phase injection speeds, was examined. Distortions were measured using the in-house standard analog quality control fixture. Residual stress depth profiles were measured using a prism hole-drilling method. It was found that the most important process parameter affecting the distortion was intensification pressure and the second most important was temperature difference between the two die halves (fixed and moving side). Tensile residual stresses were found very near the surface. Increasing the intensification pressure resulted in an increased level of tensile residual stresses.

Place, publisher, year, edition, pages
Springer, 2017
Keywords
magnesium alloy; high-pressure die cast; distortion; residual stress
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-38146 (URN)10.1007/s40962-017-0186-z (DOI)
Available from: 2017-12-12 Created: 2017-12-12 Last updated: 2017-12-12
7. Effect of process parameters on distortion and residual stress in high pressure die cast AZ91D components after shot peening and painting
Open this publication in new window or tab >>Effect of process parameters on distortion and residual stress in high pressure die cast AZ91D components after shot peening and painting
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-38147 (URN)
Available from: 2017-12-12 Created: 2017-12-12 Last updated: 2017-12-12
8. A dislocation density based constitutive model for as-cast Al-Si alloys: Effect of temperature and microstructure
Open this publication in new window or tab >>A dislocation density based constitutive model for as-cast Al-Si alloys: Effect of temperature and microstructure
Show others...
2017 (English)In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 121, p. 164-170Article in journal (Refereed) Published
Abstract [en]

The flow stress of an as-cast Al-Si based alloy was modeled using a dislocation density based model. The developed dislocation density-based constitutive model describes the flow curve of the alloy with various microstructures at quite wide temperature range. Experimental data in the form of stress-strain curves for different strain rates ranging from 10−4 to 10−1 s−1 and temperatures ranging from ambient temperature up to 400 °C were used for model calibration. In order to model precisely the hardening and recovery process at elevated temperature, the interaction between vacancies and dissolved Si was included. The calibrated temperature dependent parameters for different microstructure were correlated to the metallurgical event of the material and validated. For the first time, a dislocation density based model was successfully developed for Al-Si cast alloys. The findings of this work expanded the knowledge on short strain tensile deformation behaviour of these type of alloys at different temperature, which is a critical element for conducting a reliable microstructural FE-simulation.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Aluminium cast alloy; Dislocation density; Eutectic phase; Si precipitation; Si solute; Vacancy concentration
National Category
Materials Engineering
Identifiers
urn:nbn:se:hj:diva-27375 (URN)10.1016/j.ijmecsci.2017.01.003 (DOI)000395216300015 ()2-s2.0-85008703756 (Scopus ID)
Note

Finns som manuskript med titeln A Dislocation Density Based Yield Stress Model for as-cast EN AC-46000 Alloy i licentiatuppsats. 

Available from: 2015-06-22 Created: 2015-06-22 Last updated: 2017-12-12Bibliographically approved

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