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Coupled and competitive eutectic growth

100%

Trepczyńska-Łent M.

Archives of Foundry Engineering

2011

tom Vol. 11, iss. 3 spec.

237--240

A description of the competitive growth is presented in the paper. The description is associated with the competition between eutectic structure and primary phase formation. A coupled zone for the eutectic solidification is drawn in the phase diagrams. The coupled zone is shown as a range of solute concentration versus under-cooling to justify the formation of the eutectic structure, exclusively. Interface growth temperatures of the single and coupled eutectic are illustrated schematically as a function of growth rates at a given composition Co.

Evaluation of Al-5Ti-1B and Al-10Sr in LM6 sand castings

84%

Lim Y. P.

Journal of Achievements in Materials and Manufacturing Engineering

2009

tom Vol. 34, nr 1

71-78

Purpose: of this paper is to study the separate effects of grain refiner and modifier on the metallurgical properties of LM6 sand castings. The research aims to determine how grain refiner and modifier will influence the microstructure of LM6 sand casting and the associated mechanical properties of hardness, ultimate tensile strength and engineering strain. Design/methodology/approach: A grain refiner consists of 0.5wt% Al-5Ti-1B and a modifier consists of 0.5wt% Al-10Sr have been introduced separately into the melt of hypoeutectic LM6 Al-Si alloy to produce slab castings by CO2 sand casting process. The geometry of the slab casting is designed to have different section moduli to induce directional solidification. Temperature profile of cooling process is taken to correlate between the section modulus and cooling rate. This enables the effect of cooling rate on microstructure and mechanical properties to be studied for grain refined and modified LM6 sand castings. Findings: The significant finding is that a nearly full eutectic solidification at 540°C is achieved in the Sr-modified casting with faster cooling rates than those of solely grain-refined and original LM6. The hardness and ultimate tensile strength can be improved respectively between 20~40% and 11~18% by Al-10Sr modifier. It is observed that the Al-5Ti-1B grain refiner and Al-10Sr modifier increase the strain at UTS by 11% and 22% respectively. The metallographic observations show that the eutectic phase is finer and in fibrous form in the Sr-modified LM6. Research limitations/implications: Further research can study a wider range of inoculant percentage. Practical implications: This eutectic solidification at 540°C achieved in Sr-modified LM6 is lower than the solidus temperature of LM6 to reduce superheat and save electrical energy consumption while ensuring complete liquid mould filling. Originality/value: This is the original work of slab casting of LM6 and the effects of Sr modifier and TiB grain refiner on its metallurgical properties.

Eutectic solidification as explained by the thermodynamics of irreversible processes

67%

Wołczyński W. , Guzik E.

Archives of Foundry Engineering

2009

tom Vol. 9, iss. 1

113-116

The grain size diameter is the main parameter characterizing a given metallic alloy. In the case of Al-Si or Fe-C eutectic alloy the average inter-lamellar spacing is a good parameter which seems to be adequate to describe this irregular structure. To define the average inter-lamellar spacing the regular areas within generally irregular structure has been distinguished. It has been postulated that the formation of regular structure could be related to the minimum entropy production criterion. From the other side the maximum destabilization of the non-faceted phase interface could be referred to marginal stability. The criterion of minimum entropy production allows to formulate the growth law for regular lamellar structure solidifying under stationary state. It defines the regular eutectic spacing versus growth rate. The marginal stability concept allows to define the maximum wavelength which can be developed at the solid / liquid interface of non-faceted (Al) phase. It defines the maximum spacing within irregular structure taking into account the wavelength of instability (marginal stability) created at the non-faceted phase interface. An average inter-lamellar spacing results from the relationship formulated on the basis of both spacings. It should be emphasized that both conditions (criteria) are deduced from the thermodynamics of irreversible processes. The simplified scheme of irregular structure incorporates, additionally the intermediate lamella of faceted phase that is also taken into account in the definition of average inter-lamellar spacing, [...] . The intermediate morphology existing between two distinguished distances is treated as being under oscillation between stationary state and marginal stability. The state of marginal stability is defined by a vanishing excess entropy production. Therefore, it is suggested that the structural oscillation takes place between an attractor and point of bifurcation in the system.