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High performing cast aluminium-silicon alloys
Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-0458-3456
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The need to produce lighter components due to environmental aspects and the development of electrical vehicles represents an opportunity for cast aluminium-silicon alloys. With high specific strength, good castability, high corrosion resistance and recyclability, these alloys offer an attractive combination of properties as an alternative to steel, cast iron and titanium-based components in certain applications. To take advantage of such a combination of properties, there is a need to ensure that they can be reliably achieved. In other words, high performing components need to be produced. For that, the production cycle, from alloy selection and melt preparation, to the casting and heat treatment of the component must be understood and controlled as a whole. The different steps in the production cycle will affect the microstructure of the components and hence the resulting mechanical properties. Understanding the relation between the different steps in the production cycle, its consequences on the microstructural features and on the mechanical properties constitutes the aim of this thesis.

Experiments applying state-of-the-art knowledge regarding effect of casting process, alloying system and post-process variables were performed aimed at achieving properties similar to those of high pressure die casting (HPDC) components. Different melt quality determination tools were evaluated on three different EN AC-46000 melt qualities. The influence of modification, grain refinement and both treatments together was assessed on an Al-10Si alloy solidified under different cooling rates. The tensile behaviour and the impact of features such as secondary dendrite arm spacing (SDAS) or grain sizes was quantified.

It was corroborated that by appropriate selection and control of such alloying system, process and post-process variables it is possible to achieve HPDC EN AC-46000 tensile and fatigue properties through a T5 treated sand cast EN AC-42100 alloy. On the other hand, the available techniques for melt quality assessment are inadequate, requiring further analysis to successfully identify the melt quality. Additionally, it was observed that decreasing the melt quality by additions of 25 wt.% of machining chips did not significantly decrease the tensile properties but slightly increased the variation in them. In relation to the modification and grain refinement of Al-10Si alloys it was concluded that with the slowest cooling rate tested, additions of only grain refiner did not successfully produce equiaxed grains. For cooling rates corresponding to dendrite arm spacings of 15 μm and slower, combined additions of grain refiner and modifier can lead to higher tensile properties compared to the corresponding separate additions. SDAS was observed to describe flow stress through the Hall-Petch equation but grain size did not show a physically meaningful relationship. Furthermore, beginning of cracking was detected in the plastic deformation region at dendrite/eutectic boundaries and propagated in a trans-granular fashion.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering , 2017. , p. 44
Series
JTH Dissertation Series ; 33
Keywords [en]
Aluminium cast alloys, melt quality, eutectic modification, grain refinement, microstructure, tensile properties
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-38005ISBN: 978-91-87289-34-7 (print)OAI: oai:DiVA.org:hj-38005DiVA, id: diva2:1160739
Presentation
2017-12-14, E1405 (Gjuterisalen), Jönköping University, School of Engineering, Jönköping, 10:00 (English)
Opponent
Supervisors
Available from: 2017-11-28 Created: 2017-11-28 Last updated: 2017-11-28Bibliographically approved
List of papers
1. Tailoring Al-7Si-0.3Mg cast alloy properties to represent HPDC tensile and fatigue behaviour in component prototypes
Open this publication in new window or tab >>Tailoring Al-7Si-0.3Mg cast alloy properties to represent HPDC tensile and fatigue behaviour in component prototypes
2016 (English)In: La Metallurgia Italiana, ISSN 0026-0843, Vol. 108, no 6, p. 33-36Article in journal (Refereed) Published
Abstract [en]

To produce prototypes with mechanical properties expectable from EN AC 46000 HPDC components, prototyping related processes such as sand and plaster gravity casting as well as proper alloying and post solidification processes need to be understood and adjusted. Therefore, the influence of process, composition and heat treatment on tensile and fatigue behaviour has been investigated for an EN AC 42100 alloy. Sand cast test samples comprised the base alloy in as-cast condition and T5 treated as well as a 2 wt. % Cu addition in as-cast condition. Plaster cast test samples consisted of the base alloy in ascast condition and T6 treated as well as a 1.7 wt. % Cu addition in as-cast condition. Tensile and fully reversed bending fatigue tests (R=-1) have been performed and the results have been compared to EN AC 46000 HPDC values. Samples in heat treated conditions and with Cu addition exhibited superior tensile properties than the base alloy in as-cast state for both casting processes. Yield strength and elongation values for the sand cast T5 treated and with Cu addition samples were similar to the HPDC ones. In terms of fatigue behaviour, T6 treated and with Cu addition samples exhibited strength improvements for plaster cast samples, while no changes were observed for sand cast samples. Only sand cast samples exhibited similar fatigue behaviour to the HPDC samples. The sand cast T5 treated samples were found to produce the most similar overall mechanical behaviour to EN AC 46000 HPDC.

Place, publisher, year, edition, pages
Milano: Associazione Italiana di Metallurgia, 2016
Keywords
aluminium alloys, prototyping, tensile properties, fatigue properties, sand cast, plaster cast
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-31164 (URN)2-s2.0-85008957398 (Scopus ID)
Available from: 2016-08-03 Created: 2016-08-03 Last updated: 2017-11-28Bibliographically approved
2. Complexities in the assessment of melt quality
Open this publication in new window or tab >>Complexities in the assessment of melt quality
2018 (English)In: International Journal of metalcasting, ISSN 1939-5981, E-ISSN 2163-3193, Vol. 12, no 3, p. 441-448Article in journal (Refereed) Published
Abstract [en]

If high-performance aluminium castings are to be produced, the melt quality needs to be properly assured. Multiple tests for melt quality assessment exist and have previously been analysed. In most studies, the techniques were used separately. In this work, reduced pressure, fluidity, Prefil and tensile tests were evaluated. A commercial EN 46000 alloy was used as the base material with additions of 25 and 50 wt% machining chips to degrade the melt quality. In reduced pressure and fluidity tests, oxides floated to the top of samples, decreasing the reliability. Bifilm index increased with addition level, but not correspondingly. Density index, Prefil and fluidity tests did not present significant variations, and tensile properties only deteriorated with the 50 wt% addition level. The investigated techniques provided information, but measuring the melt quality reliably remains a challenge.

Place, publisher, year, edition, pages
Springer, 2018
Keywords
melt quality; reduced pressure test; fluidity; tensile properties; Prefil; hydrogen content
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-38003 (URN)10.1007/s40962-017-0179-y (DOI)000436927100005 ()2-s2.0-85049321529 (Scopus ID)
Conference
Symposium on Light Metals Alliance - Light Metals Technology (LMT) held during the Conference on Materials Science and Technology (MS and T), October 8, 2017, Pittsburgh, PA, USA
Available from: 2017-11-28 Created: 2017-11-28 Last updated: 2018-08-22Bibliographically approved
3. Interactive effects of grain refinement, eutectic modification and solidification rate on tensile properties of Al-10Si alloy
Open this publication in new window or tab >>Interactive effects of grain refinement, eutectic modification and solidification rate on tensile properties of Al-10Si alloy
2017 (English)In: Journal of materials processing & manufacturing science (Print), ISSN 1062-0656, E-ISSN 1530-8065, Vol. 703, p. 270-279Article in journal (Refereed) Published
Abstract [en]

This study aims to clarify the effect of grain size and Si modification on the microstructure and tensile properties of the Al-10Si cast alloy, solidified under various cooling rates. To replicate the effect of cooling rate, directionally solidified samples were produced by remelting of the as-cast cylindrical bars. Tensile properties, grain sizes, Si modification level and chemical composition profiles were evaluated. Results showed that fast cooling rates alone, without the addition of grain refiners (Al-5Ti-1B master alloy), did not lead to equiaxed grain morphologies. On the other hand, for the slowest cooling rate tested, combined additions of the Al-5Ti-1B and the Al-10Sr master alloys resulted in equiaxed grain structures while addition of only grain refiner resulted in columnar grains. The combined additions effectively produced an equiaxed grain structure at all cooling rates tested, and further improved the tensile properties.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Directional solidification, Grain size, EBSD, Microstructure, Aluminium alloys, Casting
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-36814 (URN)10.1016/j.msea.2017.07.074 (DOI)000412034600031 ()2-s2.0-85026242378 (Scopus ID)
Available from: 2017-08-09 Created: 2017-08-09 Last updated: 2017-12-20Bibliographically approved
4. Hall-Petch Equation in a Hypoeutectic Al-Si Cast Alloy: Grain Size vs. Secondary Dendrite Arm Spacing
Open this publication in new window or tab >>Hall-Petch Equation in a Hypoeutectic Al-Si Cast Alloy: Grain Size vs. Secondary Dendrite Arm Spacing
Show others...
2017 (English)In: Procedia Engineering, Elsevier, 2017, Vol. 207, p. 19-24Conference paper, Published paper (Refereed)
Abstract [en]

The Al-Si cast alloy family is widely used in the production of complex castings for various applications and known for its very good castability and high strength-to-weight ratio. However, early cracking under tensile loading is sometimes a limiting factor. Among other parameters, it is yet controversial whether grain boundaries are dominant strengthening factor in cast alloys, instead of dendrite/eutectic boundaries. This study presents the effect of secondary dendrite arm spacing (SDAS) and grain size on crack initiation and propagation of Al-Si cast alloys under tensile loading. The Al-10Si (wt.%) alloy with modified Si morphology was cast using inoculants (Al-5Ti-B master alloy) under different cooling rates to obtain a range of grain sizes (from below 138 μm to above 300 μm) and SDAS (6, 15 and 35 μm). Conventional tensile test as well as in-situ tensile test in a scanning electron microscope, equipped with an electron backscatter diffraction (EBSD) was carried out to understand the deformation mechanisms of the alloy. Observation of slip bands within the dendrites showed that in modified Si structure, the interdendritic (eutectic) area takes more portion of the strain during plastic deformation. Besides, only a few cracks were initiated at the grain boundaries; they were mostly initiated from dendrite/eutectic interface. All cracks propagated trans-granularly. Hall-Petch calculations also showed a strong relationship between SDAS and flow stress of the cast alloy. Although statistically correct, there was no physically meaningful relationship between the grain size and the flow stress. Nevertheless, formation of identical slip bands in each grain could be an evidence for the marginal effect of the grain size on the overall strength development of the alloy. Consequently, among other effects, the combinational dominant effect of SDAS and modest effect of grain size shall be considered for modification of the Hall-Petch equation for precise prediction of mechanical properties of cast alloys.

Place, publisher, year, edition, pages
Elsevier, 2017
Series
Procedia Engineering, ISSN 1877-7058 ; 207
Keywords
Aluminum, Aluminum alloys, Aluminum compounds, Cracks, Grain boundaries, Grain size and shape, High strength alloys, Plastic flow, Plasticity, Scanning electron microscopy, Silicon, Tensile stress, Tensile testing, Titanium alloys, Titanium compounds, Cast alloys, Crack initiation and propagation, Cracking mechanisms, EBSD, Electron back scatter diffraction, Hall-petch, Prediction of mechanical properties, Secondary dendrite arm spacing, Silicon alloys
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-38004 (URN)10.1016/j.proeng.2017.10.731 (DOI)XYZ ()2-s2.0-85036643961 (Scopus ID)
Conference
International Conference on the Technology of Plasticity, ICTP 2017; Hucisko; United Kingdom; 17 September 2017 through 22 September 2017
Available from: 2017-11-28 Created: 2017-11-28 Last updated: 2018-06-11Bibliographically approved

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