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Hall-Petch Equation in a Hypoeutectic Al-Si Cast Alloy: Grain Size vs. Secondary Dendrite Arm Spacing
Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-7527-719X
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
Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0001-5753-4052
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
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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. Vol. 207, p. 19-24
Series
Procedia Engineering, ISSN 1877-7058 ; 207
Keywords [en]
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: urn:nbn:se:hj:diva-38004DOI: 10.1016/j.proeng.2017.10.731ISI: XYZScopus ID: 2-s2.0-85036643961OAI: oai:DiVA.org:hj-38004DiVA, id: diva2:1160734
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
In thesis
1. High performing cast aluminium-silicon alloys
Open this publication in new window or tab >>High performing cast aluminium-silicon alloys
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
Aluminium cast alloys, melt quality, eutectic modification, grain refinement, microstructure, tensile properties
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-38005 (URN)978-91-87289-34-7 (ISBN)
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

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Ghassemali, EhsanRiestra, MartinBogdanoff, ToniSeifeddine, Salem

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