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Al-Si Cast Alloys - Microstructure and Mechanical Properties at Ambient and Elevated Temperatures
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-1190-836X
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Sustainable development
Sustainable Development
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. , 66 p.
Series
JTH Dissertation Series, 21
Keyword [en]
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: urn:nbn:se:hj:diva-35185ISBN: 978-91-87289-22-4 (print)OAI: oai:DiVA.org:hj-35185DiVA: diva2:1080478
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
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 (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 (ScopusID)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. 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 (ScopusID)
Available from: 2015-06-22 Created: 2015-06-22 Last updated: 2017-03-14Bibliographically approved
3. 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 (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 (ScopusID)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
4. 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 (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 (ScopusID)
Conference
TMS 2015, Orlando, Florida, March 15-19, 2015
Available from: 2015-03-27 Created: 2015-03-27 Last updated: 2017-03-14Bibliographically approved
5. Determination of Optimum Sr Level for Eutectic Si Modification in Al–Si Cast Alloys Using Thermal analysis and Tensile Properties
Open this publication in new window or tab >>Determination of Optimum Sr Level for Eutectic Si Modification in Al–Si Cast Alloys Using Thermal analysis and Tensile Properties
2016 (English)In: International Journal of metalcasting, ISSN 1939-5981, E-ISSN 2163-3193, Vol. 10, no 4, 457-465 p.Article in journal (Refereed) Published
Abstract [en]

This study presents a correlation between the depression in eutectic growth temperature as a result of Sr modification and the tensile properties of Al–Si cast alloys. In order to study the role that Sr exerts on the solidification behavior, modification and mechanical properties, controlled solidification experiments including thermal analysis were performed. Using three mold materials for different cooling rates, tensile testing was conducted on Al–Si alloys with various Sr levels (~35–500 ppm). The gradient solidification technique was used to produce directionally solidified tensile test specimens containing low levels of defects. The depression in eutectic Si growth temperature was correlated with the Sr additions and the tensile properties (elongation to failure and tensile strength).

Place, publisher, year, edition, pages
Springer, 2016
Keyword
aluminum cast alloys, eutectic modification, thermal analysis, tensile properties
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-30660 (URN)10.1007/s40962-016-0032-8 (DOI)
Available from: 2016-06-20 Created: 2016-06-20 Last updated: 2017-03-14Bibliographically approved
6. Effect of cooling rate and eutectic modification on texture and grain structure of Al-Si-Cu-Mg die cast alloy
Open this publication in new window or tab >>Effect of cooling rate and eutectic modification on texture and grain structure of Al-Si-Cu-Mg die cast alloy
2016 (English)In: La Metallurgia Italiana, ISSN 0026-0843, Vol. 108, no 6, 5-8 p.Article in journal (Refereed) Published
Abstract [en]

The effect of cooling rate and eutectic modification on texture evolution and grain structure of an Al-Si-Cu-Mg die cast alloy were investigated using optical microscopy (OM) and electron backscatter diffraction (EBSD) techniques. Directional solidification technique was utilized to produce as-cast specimens having low level of casting defects with controlled microstructural scale: specimens with average SDAS of 10 and 25 µm. Mode of solidification, cooling rate and eutectic modification did not induce any significant texture in the microstructure. An increase in cooling rate resulted in reduction grain size. High degree of grains orientation randomness was found in high cooling rate regardless of modification treatment.

Place, publisher, year, edition, pages
Milano: Associazione Italiana di Metallurgia, 2016
Keyword
Al-Si alloy, Anistropy, Microstructure, Directional Solidification, EBSD
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-30662 (URN)000382098100007 ()2-s2.0-84985912317 (ScopusID)
Available from: 2016-06-20 Created: 2016-06-20 Last updated: 2017-03-14Bibliographically approved
7. The role of transition metal additions on the ambient and elevated temperature properties of Al-Si alloys
Open this publication in new window or tab >>The role of transition metal additions on the ambient and elevated temperature properties of Al-Si alloys
(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:hj:diva-35198 (URN)
Available from: 2017-03-14 Created: 2017-03-14 Last updated: 2017-03-14
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, 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)
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: 2017-03-14Bibliographically approved

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