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On the Residual Stresses and Microstructure Formation of Aluminum-Silicon Cast Alloys
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Cast aluminum-silicon alloys are being increasingly used in automotive and aerospace industries for critical structure applications because of their excellent castability, low density, acceptable mechanical properties and low cost. Different casting and heat treatment parameters largely affect the microstructure and residual stress of the components, which in turn, has a great impact on their mechanical properties. In cast components, residual stresses are those stresses which may remain in the casting after it has been removed from the mould. The magnitude and distribution of the residual stresses can be critical to performance and should be considered in the design of a component. This work has been devoted to study the microstructure formation, residual stresses, and mechanical properties of aluminum-silicon castings.

The effects of casting parameters on the microstructure of the selected alloys were investigated by means of optical and scanning electron microscopes. With the help of EDS, WDS, EBSD, and quantitative analysis techniques, it was found that solidification rate, modification, superheat, casting temperature can significantly affect the nucleation and growth, morphology and chemical composition of different phases. Based on the performed microsegregation analysis, a clear correlation between the concentration of silicon in the primary phase and cooling rate was found.

Tensile tests at room and elevated temperatures in addition to microhardness experiments were performed to analyse the behaviour of the alloys under mechanical loads. The results showed that elongation to fracture, modulus of elasticity, and ultimate tensile strength of the tested alloys are temperature dependent. Moreover, the obtained information was used to establish a good model for simulating the behaviour of the cast alloys, as well as the mechanical properties and residual stresses.

The type and magnitude of residual stresses were mainly evaluated by strain gauge, sectioning, thermal analysis methods, and was also simulated by finite element analysis using Abaqus software. It was found that casting parameters such as superheat, mould hardness, casting temperature, modification, and the casting geometry, can influence the accumulated residual stress in the component. The thermal treatment experiments also indicated that the base temperature of the cast part before fast cooling, maximum temperature difference within the component, and cooling water flow can influence the residual stress. Extensive simulation work done by Abaqus showed that the results obtained by simulation are in a reasonable relationship with the experimental measurements, considering the linearly elastic/linearly isotropically hardening plastic model.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2011. , 48 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1369
Keyword [en]
Al-Si alloys, residual stresses, microstructure, microsegregation, nucleation behavior, modeling
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-70117ISBN: 978-91-7393-176-2 (print)OAI: oai:DiVA.org:liu-70117DiVA: diva2:435565
Public defence
2011-09-07, A35, A-huset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2011-08-19 Created: 2011-08-19 Last updated: 2012-01-19Bibliographically approved
List of papers
1. EBSD investigation of the effect of the solidification rate on the nucleation behavior of eutectic components in a hypoeutectic Al-Si-Cu alloy
Open this publication in new window or tab >>EBSD investigation of the effect of the solidification rate on the nucleation behavior of eutectic components in a hypoeutectic Al-Si-Cu alloy
2012 (English)In: Metals and Materials International, ISSN 1598-9623, E-ISSN 2005-4149, Vol. 18, no 3, 405-411 p.Article in journal (Refereed) Published
Abstract [en]

This article is devoted to a study of the influence of solidification rate on the crystallographic orientation of eutectic components with respect to primary α-Al in the tested hypoeutectic alloy. Electron backscattering diffraction (EBSD) patterns were produced from Al-Si cast specimens, solidified with different cooling rates, and prepared by ion etch polishing as a complementary method after mechanical polishing. Results indicated a strong orientation relationship between the primary α-Al and eutectic aluminum phase at all cooling rates. It is also found that the silicon eutectic flakes are nucleated heterogeneously in the interdendritic eutectic liquid. Increasing the cooling rate from 2 to 80 mm/min is observed to be effective in lowering the intensity of relationship between the primary α-Al and eutectic aluminum phase and changing the misorientation angle clustering between primary α-Al and eutectic silicon phase in the interval 41-60 to lower angle intervals.

Keyword
EBSD, Pole figure analysis, Al-Si alloy, Nucleation, Orientation relationship
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-70096 (URN)10.1007/s12540-012-3004-4 (DOI)000305689900004 ()
Available from: 2011-08-18 Created: 2011-08-18 Last updated: 2017-12-08Bibliographically approved
2. Elemental Distribution, Morphological Analysis, and Hardness Measurement of an Al-Si Alloy, as Affected by Cooling Rate
Open this publication in new window or tab >>Elemental Distribution, Morphological Analysis, and Hardness Measurement of an Al-Si Alloy, as Affected by Cooling Rate
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The effects of cooling rate on the as-cast microstructure and hardness of an Al - Si alloy was studied. To obtain different cooling rates, a special gradient casting furnace was designed and built. The microstructural analysis was carried out on as-cast samples to investigate the morphology (size, shape, and distribution) of the present phases. By using an analytical scanning electron microscope, the microsegregation of different alloying elements in the centre of aluminium dendrites was investigated. To study the effect of cooling rate on the mechanical properties, hardness of the samples and microhardness of the primary α-Al dendrites were measured. The results showed that the morphology of the silicon eutectic, refinement of the intermetallics, and the size of primary dendrites are cooling rate dependent. Moreover, a clear correlation between the concentration of silicon concentration in the centre of the aluminium dendrites and cooling rate was found.

Keyword
Elemental distribution, morphological analysis, hardness, Al-Si alloy
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-70097 (URN)
Available from: 2011-08-18 Created: 2011-08-18 Last updated: 2011-08-19Bibliographically approved
3. Experimental realization and finite element simulation of residual stresses in Al-Si water sprayed cast components
Open this publication in new window or tab >>Experimental realization and finite element simulation of residual stresses in Al-Si water sprayed cast components
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this study, experiments were carried out to develop insights into  and understanding of the residual stresses that can arise during different thermal treatments of Al-Si components. The study consisted of two steps: experimental measurement of the residual stresses accumulated in the components and simulation of the results by finite element analysis. A special cooling apparatus was designed and built in order to perform selective cooling of the component. In the simulation step, the tensile behavior and parameters such as Young’s modulus values determined by tensile tests at different temperatures and the recorded cooling curves in the first step were used as input for the Abaqus software. The experimental results obtained by thermal analysis and residual stress measurement showed that by choosing a specific holding temperature before quenching, the value of residual stress increases linearly with flow rate of water spray coolant. On the other hand, for a constant value of cooling water flow rate, the maximum temperature difference between the middle and side bars of the test specimen (ΔTmax) and residual stress level decrease when the value of starting temperature of cooling decreases. It was shown that the strain gage method and thermal analysis are reliable techniques for measuring thermal residual stresses and prediction of residual strains in Al-Si components, respectively. It was also shown that the results obtained by simulation are reasonably in acceptable agreement with the experimentally measurements.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-70098 (URN)
Available from: 2011-08-18 Created: 2011-08-18 Last updated: 2011-08-19Bibliographically approved
4. The effects of casting parameters on residual stresses and microstructure variations of an Al-Si cast alloy
Open this publication in new window or tab >>The effects of casting parameters on residual stresses and microstructure variations of an Al-Si cast alloy
2009 (English)In: Advances in X-Ray Analysis (CD-ROM), ISSN 1097-0002, Vol. 52, 553-560 p.Article in journal (Refereed) Published
Abstract [en]

Different casting parameters can change the microstructure and residual stresses of castings. The microstructure of Al-Si cast alloys is influenced by the morphology of silicon particles (shape, size and distribution), aluminum grain size and dendrite parameters. Dimensional changes resulting from casting caused by residual stresses can particularly affect the quality of near net shape castings. In this research, the influence of casting process parameters such as modification, superheat temperature, mould hardness and mould design on residual stresses and microstructure of an Al-Si-Mn alloy have been investigated. Experiments were conducted with different superheat, mould hardness, modification with Al-10Sr and two different casting designs. The micro structural changes associated with these parameters have been studied by optical microscopy, scanning electron microscopy and image analysis. The type and extent of residual stresses of all samples were determined using the sectioning method. The results show that all of the mentioned casting parameters have clear effects on residual stresses. The residual stresses decrease with lowered superheat, temperature and mould hardness. It was found that the residual stress increases both with adding a eutectic modifier and with change of casting design. It wasalso found that microstructure and mechanical properties are  influenced significantly by the mentioned parameter.

Place, publisher, year, edition, pages
Newton Square, PA, United States: International Centre for Diffraction Data (ICCD), 2009
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:liu:diva-60908 (URN)
Conference
57th Annual Conference on Applications of X-ray Analysis Denver X-ray Conference (DXC), Denver, Colorado, USA, 4–8 August 2008
Available from: 2010-10-29 Created: 2010-10-29 Last updated: 2016-02-17Bibliographically approved
5. The effects of solidification conditions and heat treatment on the microstructure and mechanical properties of EN-AC 44400 alloy
Open this publication in new window or tab >>The effects of solidification conditions and heat treatment on the microstructure and mechanical properties of EN-AC 44400 alloy
2010 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 649, 505-510 p.Article in journal (Refereed) Published
Abstract [en]

Improved mechanical and physical properties of an Al-Si alloy as a well-known casting alloy is strongly dependent upon the morphology of silicon particles, Al grains and also type of intermetallics which are in turn a function of alloy composition, solidification rate and heat treatment. This study aims at investigating the influence of the different solidification conditions (high pressure die, gradient and sand cast) and heat treatment on the microstructure (dendrite parameters, silicon particle morphology, intermetallic compounds), mechanical properties and fracture surface appearance of Al- 9Si- 4Mn alloy. To identify the features of microstructure and fracture surface analysis, a combination of optical metallography, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) have been employed. The results show that the cooling rate has a strong effect on the evolution of intermetallics, morphology of the silicon and dendrite parameters.

Place, publisher, year, edition, pages
Switzerland: Trans Tech Publications, 2010
Keyword
solidfication conditions, microstructure, mechanical properties, intermetallics, heat treatment
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-54536 (URN)10.4028/www.scientific.net/MSF.649.505 (DOI)
Available from: 2010-03-22 Created: 2010-03-22 Last updated: 2017-12-12Bibliographically approved
6. The influence of casting geometry on the tensile properties and residual stresses in aluminium castings
Open this publication in new window or tab >>The influence of casting geometry on the tensile properties and residual stresses in aluminium castings
2010 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 652, 174-179 p.Article in journal (Refereed) Published
Abstract [en]

The soundness and the performance of castings are significantly affected by generation and accumulation of residual stresses. This has been proven that mechanical properties and residual stresses level of the casting components are affected by thermal gradient across the casting component during cooling and some intrinsic physical properties of the material. In the present work, microstructural development, mechanical properties, residual stress evolution and cooling curves associated with different legs of a mixed–section grid castings have been investigated employing scanning electron microscope, optical microscope, 3D measurement equipment, computerized thermal data acquisition instrument and tensile testing machine. Experimental results show that the accumulated residual stress in circular, triangular and rectangular grid shapes is increasing respectively. From the results it can be seen that there are clear influences of grid’s geometry on the microstructure and mechanical properties. The geometrical stiffness can affect a lot the residual stress level and the casting modulus has a big influence on the microstructure and mechanical properties.

Place, publisher, year, edition, pages
Switzerland: Trans Tech Publications, 2010
National Category
Engineering and Technology Other Materials Engineering
Identifiers
urn:nbn:se:liu:diva-60910 (URN)10.4028/www.scientific.net/MSF.652.174 (DOI)
Available from: 2010-10-29 Created: 2010-10-29 Last updated: 2017-12-12Bibliographically approved
7. The effect of heat treatment on the mechanical properties and residual stresses in a HPDC component of Al-Si-Mn alloy
Open this publication in new window or tab >>The effect of heat treatment on the mechanical properties and residual stresses in a HPDC component of Al-Si-Mn alloy
2007 (English)Conference paper, Published paper (Refereed)
Abstract [en]

This study aims at investigating the influence of the heat treatment process on microstructure, mechanical properties and residual stresses in a high pressure die cast (HPDC) of an aluminum alloy component. Two series of high pressure die cast components containing mainly Al -Si 8.4- Mn 0.43 were tested. The first group of components was as cast and the second heat treated. X-ray diffraction, quantitative metallography and mechanical testing methods were used to determine the residual stresses, microstructural features and mechanical properties respectively. Results indicate that the amount of residual stresses varies depending on heat treatment process. The selected heat treatment process can also change the microstructure and mechanical properties.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-39193 (URN)47126 (Local ID)47126 (Archive number)47126 (OAI)
Conference
5th International Conference on Mechanical Stress Evaluation by Neutrons and Synchrotron Radiation, September 24-26, Vienna, Austria
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2011-08-24Bibliographically approved

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