Ductility and solidification issues in Al-Si-Cu-Mg alloys

Sivarupan, Tharmalingam (2014). Ductility and solidification issues in Al-Si-Cu-Mg alloys PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland. doi:10.14264/uql.2015.91

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Author Sivarupan, Tharmalingam
Thesis Title Ductility and solidification issues in Al-Si-Cu-Mg alloys
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
DOI 10.14264/uql.2015.91
Publication date 2014-10-30
Thesis type PhD Thesis
Open Access Status Other
Supervisor Carlos H Caceres
John A Taylor
Total pages 176
Language eng
Subjects 0912 Materials Engineering
Formatted abstract
Earlier research carried out on the ductility of Al-Si-Cu-Mg alloys have showed that the ductility of Al-Si-Cu-Mg alloys with high levels of Fe (0.5 wt. %) and Cu (4.0 wt. %) is increased at high (7~9%) concentrations of Si. Quantitative metallographic analysis performed on these alloys suggested that the refinement of both Fe-rich β-Al5FeSi and Curich θ-Al2Cu intermetallics, arose from the shorter solidification path accounting for the increased ductility. More recent studies on the effect of Si concentration on the size and shape of iron-rich (Al5FeSi) intermetallic phase of ternary Al-Si-0.8Fe alloys showed that the size of β-Al5FeSi plates increased with increasing silicon content. It was then proposed that the presence of Cu was adamant to the refining effect of Si, presumably through the introduction of low melting point pools of Cu-Al eutectic. It was proposed that Fe remained in solution in the Cu-rich liquid, whereas the liquid pools were dispersed by the high volume fraction of Al-Si eutectic, thus resulting in a finely dispersed and highly refined distribution of Fe- and Cu- rich intermetallics.

Hence, this project has conducted a systematic study across a range of Al-Si-Cu-Mg alloys to elucidate the mechanisms by which increased Si refines and disperses the Fe- and Curich phases, increasing the material’s ductility. Limits to the effect have been determined, and the optimum content of the alloy components have been identified. This is supplemented by an optimisation of composition, providing the basis for improved property-processing alloy selection. 

Thermal information of various Al-Si-Cu-Mg-Fe alloys were obtained during solidification, using computer controlled data acquisition device and the LabView-Signal Express® software, and the corresponding SDAS were also measured on optical microscope images followed by intermetallic particle size distributions that were measured using image analysis software, Image-Pro® software, on the BSE SEM images in order to compare the size refining effect of Si and Cu content on the Fe-rich and Cu-rich intermetallic phase particles for different SDAS, i.e. different cooling rate. EBSD and EDS mapping and point analysis were used to identify the Fe-containing intermetallic phases. Sample prepared using FIB was also used to further confirm elements, by EDS in TEM, on the script-like Fecontaining intermetallic phase.

Si and/or Cu content showed a considerable decrease in the size of SDAS for a constant average solidification rate which should only be calculated between the liquidus and  solidus temperatures; not between the liquidus and eutectic temperatures, especially when having many solute elements, for the range of experimental Al-Si-Cu-Mg- Fe) alloys  studied here. The relationship between cooling rate and the SDAS for the alloys was determined in the form of λ2 = a R −n where a and n are composition-  ependent fitting parameters, that also depend on the cooling rate calculation method. The combined presence of high levels of Cu, Si and Fe which produces intermetallics  throughout the entire solidification period, but particularly in the latter stages, hinders the SDAS coarsening, refining the SDAS for given cooling rate.  

Fe intermetallics exist as two different phases depending on the SDAS and Si concentration: (1) script-like α-Al8Fe2Si; (2) plate-like β-Al5FeSi. A preferential formation of  α-phase particles is observed at high cooling rates, leading to an overall decrease in the size of the intermetallics. Modification with Sr increases the size of pre-eutectically (Al-Si)  formed β- l5FeSi intermetallic plates whereas it reduces the co-eutectic and post eutectic β-Al5FeSi plates (observed as curved-shape), especially at low cooling rates. i.e. there are two different distributions of Fe-bearing intermetallics that form in Sr-modified Al-Si-Fe alloys. 

High levels of Si and/or Cu decreased the amount and size of the β-Al5FeSi  platelets in Al-Si-Cu-Mg-Fe casting alloys, especially at small SDAS if the Fe level is below the Si dependent critical level for the formation of the pre-eutectic platelets (for the Fe-bearing  phases precipitation in the post eutectic stage). At small SDAS, in the low Si (4.5%) alloys, Cu leads to decreased size of the intermetallics whereas in the Cu-free, high-Si alloy, the  plates are replaced by a mixture of the irregular alpha-Al8Fe2Si and β-Al5FeSi platelets, i.e. in the Cu containing alloys, plates are  formed even at small SDAS. 

High levels of Si decrease the size of the Fe-bearing intermetallics in Al-Si-Fe alloys at 0.2  and 0.5 Fe level. This effect is accompanied by the formation of script-like phases,  possibly α-Al8Fe2Si or branched β-Al5FeSi plates, isolated or clustered together. Thescript-like Fe-intermetallics are formed, in high Si  alloys, at low and high solidification rates for the 0.2 and 0.5 Fe alloys. In the low Si alloys, this only occurs at high solidification rates. The number density of script-like particles is higher in the high Si and low Fe alloys,  suggesting that the eutectic Si provides additional nucleation sites. A high level of Cu refines the intermetallic particles in  the low Si alloys, but it may have the opposite effect when the Si level is high. The optimal composition for a strong and ductile casting using secondary Al-Si-Cu alloys  appears to be 9Si and 1 Cu, for the alloys Fe<0.5, and for the alloy with Fe >0.5 a high Si (>9 mass %) with a maximum SDAS of 30 μm. The SDAS  should not exceed 30  μm for Cu >1%, for both low and high Si, in order to obtain a wellrefined population of intermetallic particles.
Keyword Casting alloy
Casting
Al-Si alloys
SDAS
Intermetallics
β-plates
α-particle
Solidification
Alloy composition
Dendrite arm spacing
Alpha particle
Beta plate
Ductility
Strength
EBSD
SEM and TEM images
Back Scattered electron diffraction

Document type: Thesis
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Created: Thu, 16 Oct 2014, 13:56:29 EST by Mr Tharmalingam Sivarupan on behalf of Scholarly Communication and Digitisation Service