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Al-Si Cast Alloys - Microstructure and Mechanical Properties at Ambient and Elevated Temperature
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting. Jönköping University, School of Engineering, JTH, Mechanical Engineering.ORCID iD: 0000-0002-1190-836X
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. , 76 p.
Series
JTH Dissertation Series, 07
Keyword [en]
Al-Si based casting alloys, elevated temperature, microstructural scale effect, Sr modification
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-26805ISBN: 978-91-87289-08-8 (print)OAI: oai:DiVA.org:hj-26805DiVA: diva2:814885
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
List of papers
1. Effects of microstructure and defects on tensile and fracture behaviour of a HPDC component: Potential properties and actual outcome of En AC-44300 alloy
Open this publication in new window or tab >>Effects of microstructure and defects on tensile and fracture behaviour of a HPDC component: Potential properties and actual outcome of En AC-44300 alloy
2014 (English)In: Light Metals 2014 / [ed] J. F. Grandfield, John Wiley & Sons, 2014Conference paper, Published paper (Refereed)
Abstract [en]

The aim of present work is to study the influence of microstructure and defects on the mechanical properties of a structural high pressure die cast (HPDC) component of a commercial Al-Si based foundry alloy, EN AC-44300. The alloy which contains mainly 12% Si and 0.7% Fe, is a successful application of a die-casting alloy for the automotive market. Tensile test specimens were extracted from both high pressure die cast components and from ones with comparable microstructures produced through gradient solidification technique, which offers specimens with low levels of defects. The microstructure and defects available in the component were well mapped via X-ray inspection system, optical and scanning electron microscopy.  The results clearly confirmed the components’ performance dependency to configuration of defects and Si morphology as well as revealed the potential of the alloy in terms of ultimate tensile strength and ductility.

Place, publisher, year, edition, pages
John Wiley & Sons, 2014
Keyword
Microstructure, Casting defects, Porosity, HPDC component, Aluminum alloy
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-22814 (URN)2-s2.0-84899859397 (Scopus ID)978-1-118-88908-4 (ISBN)
Conference
TMS 2014, 143rd Annual Meeting & Exhibition, February 16-20, 2014: San Diego, CA, USA
Projects
CompCAST
Funder
Knowledge Foundation
Available from: 2013-12-18 Created: 2013-12-18 Last updated: 2017-03-14Bibliographically approved
2. The role of Sr on microstructure formation and mechanical properties of Al-Si-Cu-Mg cast alloy
Open this publication in new window or tab >>The role of Sr on microstructure formation and mechanical properties of Al-Si-Cu-Mg cast alloy
2013 (English)In: TMS Light Metals, 2013, 297-302 p.Conference paper, Published paper (Refereed)
Abstract [en]

The aim of this paper is to assess the role of Sr modification on eutectic Si, Fe-rich intermetallic phases and porosity and their responses to the mechanical properties of a commercial high pressure die cast alloy Al-Si-Cu-Mg with Fe level up to 1%. Tensile test samples with a variety of coarsenesses, containing different Sr levels were cast using the gradient solidification technique, that enables a study of the solely influence of Sr on microstructure and tensile properties. The modification altered the morphology and size of eutectic silicon, while did not make a significant change on morphology of Fe-rich intermetallic and volume fraction distribution of porosity. The tensile test results indicate that eutectic Si modification is not a guarantee for improved mechanical properties due to the presence of a variety of intermetallics that tend to have a larger role on initiating and propagating cracks leading to premature failures in these commercial alloys.

Keyword
Al-Si-Cu-Mg cast alloy; Fe-rich intermetallics; Mechanical properties; Porosity; Sr modification
National Category
Materials Engineering
Identifiers
urn:nbn:se:hj:diva-27373 (URN)2-s2.0-84876068368 (Scopus ID)978-111860572-1 (ISBN)
Conference
TMS 2013 Annual Meeting and Exhibition; San Antonio, TX; United States; 3 March 2013 through 7 March 2013
Available from: 2015-06-22 Created: 2015-06-22 Last updated: 2017-03-14Bibliographically approved
3. Assessment of modification level in En AC-46000 aluminum casting alloy using thermal analysis and microscopic evaluation
Open this publication in new window or tab >>Assessment of modification level in En AC-46000 aluminum casting alloy using thermal analysis and microscopic evaluation
2015 (English)In: Light Metals 2015, 2015, 955-960 p.Conference paper, Published paper (Refereed)
Abstract [en]

The quantitative methods for controlling and predicting the level of Si modification in EN AC-46000 aluminum cast alloys were examined using thermocouples (thermal analysis) and optical microscopy (image analysis). A wide range of Sr, from 37 to 486 ppm, was added to the alloy. The alloys were cast using three different molds providing different cooling rate and consequently varied microstructure coarseness. Large difference in nucleation and growth temperature of unmodified and modified alloy was found irrespective of coarseness of microstructure. The depression in growth temperature of eutectic Si found to be strongly correlated to content of modification agent as well as modification level. Thermal analysis technique was realized as a non-biased, accurate and inexpensive approach for on-line prediction of Si modification level in the EN AC-46000 alloy cast under different cooling rate.

Keyword
Si modification level, Al-Si-Cu-Mg casting alloys, Thermal analysis, Cooling rates
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-26294 (URN)2-s2.0-84942894290 (Scopus ID)
Conference
TMS 2015, Orlando, Florida, March 15-19, 2015
Available from: 2015-03-27 Created: 2015-03-27 Last updated: 2017-03-14Bibliographically approved
4. High Temperature Tensile Deformation Behaviour and Failure Process of an Al-Si-Cu-Mg Cast Alloy: The Microstructural Scale Effect
Open this publication in new window or tab >>High Temperature Tensile Deformation Behaviour and Failure Process of an Al-Si-Cu-Mg Cast Alloy: The Microstructural Scale Effect
2015 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 86, 361-370 p.Article in journal (Refereed) Published
Abstract [en]

In this study the high temperature tensile deformation behavior of a commercial Al–Si–Cu–Mg cast alloy was investigated. The alloy was cast with two different cooling rates which resulted in average secondary dendrite arm spacing of 10 and 25 μm, which is typical of the microstructure scale obtained from high pressure die casting and gravity die casting. Tensile tests were performed at different strain rates (10− 4 s− 1 to 10− 1 s− 1) and over a wide temperature range from ambient temperature to 500 °C. The fine microstructure had superior tensile strength and ductility compared to the coarse microstructure at any given temperature. The coarse microstructure showed brittle fracture up to 300 °C; the fracture mode in the fine microstructure was fully ductile above 200 °C. The fraction of damaged particles was increased by raising the temperature and/or by microstructure coarsening. Cracks arising from damaged particles in the coarse microstructure were linked in a transgranular-dominated fashion even at 500 °C. However, in the fine microstructure alloy the inter-dendritic fracture path was more prevalent. When the temperature was raised to 300 °C, the concentration of alloying elements in the dendrites changed. The dissolution rates of Cu- and Mg-bearing phases were higher in the fine microstructure.

Keyword
EN AC-46000 cast alloy; Elevated temperatures; Deformation behavior; Microstructure
National Category
Materials Engineering Metallurgy and Metallic Materials Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:hj:diva-27374 (URN)10.1016/j.matdes.2015.07.084 (DOI)000362862700047 ()2-s2.0-84942244775 (Scopus ID)
Available from: 2015-06-22 Created: 2015-06-22 Last updated: 2017-12-04Bibliographically approved
5. 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, 164-170 p.Article 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
Keyword
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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Zamani, Mohammadreza

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