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A dislocation density based constitutive model for as-cast Al-Si alloys: Effect of temperature and microstructure
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-1190-836X
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-9886-9710
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
Division of Mechanics of Solid Materials, Luleå University of Technology, Luleå, Sweden.
Show others and affiliations
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. Vol. 121, p. 164-170
Keyword [en]
Aluminium cast alloy; Dislocation density; Eutectic phase; Si precipitation; Si solute; Vacancy concentration
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:hj:diva-27375DOI: 10.1016/j.ijmecsci.2017.01.003ISI: 000395216300015Scopus ID: 2-s2.0-85008703756OAI: oai:DiVA.org:hj-27375DiVA, id: diva2:824632
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
In thesis
1. Al-Si Cast Alloys - Microstructure and Mechanical Properties at Ambient and Elevated Temperature
Open this publication in new window or tab >>Al-Si Cast Alloys - Microstructure and Mechanical Properties at Ambient and Elevated Temperature
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Aluminium alloys with Si as the major alloying element form a class of material providing the most significant part of all casting manufactured materials. These alloys have a wide range of applications in the automotive and aerospace industries due to an excellent combination of castability and mechanical properties, as well as good corrosion resistance and wear resistivity. Additions of minor alloying elements such as Cu and Mg improve the mechanical properties and make the alloy responsive to heat treatment. The aim of this work is studying the role of size and morphology of microstructural constituents (e.g SDAS, Si-particles and intermetalics) on mechanical properties of Al-Si based casting alloy at room temperatures up to 500 ºC.

The cooling rate controls the secondary dendrite arm spacing (SDAS), size and distribution of secondary phases. As SDAS becomes smaller, porosity and second phase constituents are dispersed more finely and evenly. This refinement of the microstructure leads to substantial improvement in tensile properties (e.g. Rm and εF). Addition of about 280 ppm Sr to EN AC- 46000 alloy yields fully modified Si-particles (from coarse plates to fine fibres) regardless of the cooling conditions. Depression in eutectic growth temperature as a result of Sr addition was found to be strongly correlated to the level of modification irrespective of coarseness of microstructure. Modification treatment can improve elongation to failure to a great extent as long as the intermetallic compounds are refined in size.

Above 300 ºC, tensile strength, Rp0.2 and Rm, of EN AC-46000 alloys are dramatically degraded while the ductility was increased. The fine microstructure (SDAS 10 μm) has superior Rm and ductility compared to the coarse microstructure (SDAS 25 μm) at all test temperature (from room to 500 ºC). Concentration of solutes (e.g. Cu and Mg) in the dendrites increases at 300 ºC and above where Rp0.2 monotonically decreased. The brittleness of the alloy below 300 ºC was related to accumulation of a high volume fraction damaged particles such as Cu- Fe-bearing phases and Si-particles. The initiation rate of damage in the coarse particles was significantly higher, which enhances the probability of failure and decreasing both Rm and εF compared to the fine microstructure. A physically-based model was adapted, improved and validated in order to predict the flow stress behaviour of EN AC- 46000 cast alloys at room temperature up to 400 ºC for various microstructures. The temperature dependant variables of the model were quite well correlated to the underlying physics of the material

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2015. p. 76
Series
JTH Dissertation Series ; 07
Keyword
Al-Si based casting alloys, elevated temperature, microstructural scale effect, Sr modification
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-26805 (URN)978-91-87289-08-8 (ISBN)
Presentation
2015-06-05, E1405, School of Engineering, Gjuterigatan 5, Jönköping, 10:00 (English)
Opponent
Supervisors
Funder
Knowledge Foundation
Available from: 2015-06-22 Created: 2015-05-28 Last updated: 2015-06-22Bibliographically approved
2. Al-Si Cast Alloys - Microstructure and Mechanical Properties at Ambient and Elevated Temperatures
Open this publication in new window or tab >>Al-Si Cast Alloys - Microstructure and Mechanical Properties at Ambient and Elevated Temperatures
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Swedish industry is a global leader in development and manufacture of automotive and aviation components where the usage of aluminium products is remarkable. In addition to manufacturing aluminium components, casting enables low-cost and low-emission production of complex geometry components with a range of sizes. Aluminium with Si as the major alloying element forms a class of alloys representing the most significant fraction of all cast products, for a wide range of applications due to an excellent combination of castability and mechanical properties, as well as good corrosion resistance, wear resistance and recyclability. The microstructure in Al-Si alloys strongly governs their mechanical properties. Several industrial practices such as eutectic modification and alloying are well-known to improve mechanical properties. Al-Si cast alloys generally suffer a lack of ductility and poor high temperature properties due to presence of either brittle or thermally unstable phases. The aim of this work is to study the explicit role of each microstructural constituent on the behaviour of Al-Si cast alloys at room and high temperatures. The results will accordingly highlight the potential for improvement in properties of such alloys.

Casting defects have an immediate and negative effect on the properties of Al-Si alloys and reducing the overall level of defects substantially improves tensile properties. An increased cooling rate refines all microstructural features and reduces volumetric porosity which leads to substantial improvement in tensile properties (e.g. Rm and εF) at any test temperature. Modification of eutectic Si-particles (through Sr-addition) generally has a positive effect on alloy ductility. Depression in eutectic growth temperature as a result of eutectic modification was found to be strongly correlated to the level of modification irrespective of coarseness of the microstructure.

Addition of transition metals (Ni-Ti-Zr-Cr-V) to Al-Si improves tensile strength, particularly at temperatures above 200 ºC caused by formation of thermally stable intermetallic compounds. Below 200 ºC however, a substantial potential for improvement through solute-reinforcement was obtained.

A physically-based constitutive model with a wide validity range was successfully developed to describe the flow behaviour of Al-Si alloys at different temperatures, as a reliable input for finite element simulation. 

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2017. p. 66
Series
JTH Dissertation Series ; 21
Keyword
Aluminium cast alloys, eutectic modification, microstructural scale effect, transition metals, room and elevated temperatures tensile properties, physically-based constitutive model.
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-35185 (URN)978-91-87289-22-4 (ISBN)
Public defence
2017-04-07, E1405, School of Engineering, Gjuterigatan 5, 10:00 (English)
Opponent
Supervisors
Funder
Knowledge Foundation
Available from: 2017-03-13 Created: 2017-03-10 Last updated: 2017-03-14Bibliographically approved
3. 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
Keyword
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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Zamani, MohammadrezaDini, HodaSvoboda, AlesSeifeddine, SalemAndersson, Nils-EricJarfors, Anders

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