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phase matrix in order to provide high strength and toughness to the cast component.9


to affect the precipitation of Mg2 Prior Art


The maximum solubility of Si in the primary Al phase at the eutectic temperature is about 1.6 wt%. Typically, Al alloys with less than 1.3 wt% Si are termed wrought alloys because of their inability to be shape cast and their ability to attain high strength and toughness by solid state transformations such as rolling, forging and extrusion. The hypoeutectic Al- Si alloys are the most popular Al casting alloys.10


in the primary Al phase during the aging treatment. This precipitation reaction imparts high strength and toughness to the components. The morphology of the eutectic Al and Si phases is modified by the addition of trace levels (about 0.02 wt%) of Sr or Na to increase the tensile and fatigue proper- ties of the cast component.11-16


ed to these alloys to enable the precipitation reactions lead- ing to the formation of the terminal and stable Mg2


Sr additions to the alloy fur-


ther changes the coarse flaky eutectic Si phase to a fibrous morphology and refines the eutectic Al grains as well.11


One of the main effects of the rate of quenching after the solution treatment is the resultant quantity and distribu- tion of vacancies (point defects) in the primary Al matrix; this enables and defines the mobility of Mg and Si atoms during the precipitation reaction in the aging treatment.17,18 Faster rates of quenching retain a higher vacancy concentra- tion enabling higher mobility of the elements in the primary Al phase during aging. An optimum rate of quenching is necessary to maximize retained vacancy concentration and minimize part distortion after quenching.4,19


A slow rate of


quenching would reduce residual stresses and distortion; however, it causes detrimental effects such as precipitation during quenching, localized over-aging, increased tendency for corrosion and a reduced response to aging treatment.20,21 Bates 22


and Zhang23 have defined the quench sensitivity on the mechanical properties.


Much work has been carried out on the mechanism of incubation in Al-Si-Mg alloys and specially the 6xxx Al


The ratio of Si to Mg atoms would be about 1 in 6xxx Al wrought alloys and 4 in the A356.2 casting alloy, thus making the latter alloy rich in Si in the primary Al phase. The ratio of Si to Mg atoms coupled with the rate of va- cancy formation/annihilation in the primary Al phase at the onset of the precipitation reaction all play a critical role in determining the kinetics of the precipitation reaction. How- ever, the thermodynamics dictating the sequence of phase precipitation reactions in both 6xxx and A356 alloys would be similar.


Edwards et al.6


Following is the proposed precipitation sequence by Ed- wards et al:6


ing process for a 6xxx series Al-Si-Mg wrought alloy for the precipitation of the terminal stable Mg2


Super Saturated Solid Solution (SSSS)  Clusters of Si atoms and Clusters of Mg atoms  Dissolution of Mg clusters  Formation of Mg/Si clusters  Small precipitates of unknown structure (Mg:Si≅ 1:1)  β”  β’ + B’  β (Stable Mg2


Si precipitate)


Table 1. Standard Specifications for T6 Heat Treatment of Permanent Mould Cast Components of A356.0 Alloy2,3


summarized the various stages in the ag- Si phase.


Mg is add- Si phase


Si phase in the primary Al


wrought alloys such as 6061.24-26


The effect of incubation


on the mechanical properties of A356.2 was recently stud- ied27,28


a profound effect on the final mechanical properties of the alloy component. Hernandez-Paz et al.28


and it was concluded that the incubation process had concluded that the


tensile properties of A356 cast alloys are significantly af- fected by the duration of the incubation process at room temperature. Möller et al.29


had concluded that the effect


of the incubation process is profound on the precipitation reaction response during artificial aging and mechanical properties in A356 alloy castings. However, there are few detailed publications on the precipitation during incubation and its effect on the precipitation during artificial aging at high temperature and the resultant mechanical properties for the Al-Si-Mg casting alloys, albeit there being several internal publications on this topic in companies such as Al- coa.30


of precipitation during incubation and artificial aging is es- sential to fully understand the effect of aging on properties and this was one of the objectives of this study.


An in-depth understanding of the transient sequence


18


International Journal of Metalcasting/Fall 2011


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