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Zyska A.,Katedra Odlewnictwa Politechniki Czestochowskiei | Konopka Z.,Katedra Odlewnictwa Politechniki Czestochowskiei | Lagiewka M.,Katedra Odlewnictwa Politechniki Czestochowskiei | Nadolski M.,Katedra Odlewnictwa Politechniki Czestochowskiei
Archives of Metallurgy and Materials | Year: 2010

The paper presents the results of investigations concerning the solidification kinetics, the mechanical properties, and the porosity of AlZn5Mg alloy castings produced by squeeze casting method and, for the purpose of comparison, by gravity casting. The examinations have been performed for 200x100x25 mm plate castings squeeze cast under the pressure values of 30, 60, or 90 MPa, at the constant die temperature of 250°C. The influence of the squeeze pressure value on the liquidus temperature of the considered alloy has been assessed on the basis of Clapeyron-Clausius equation. The characteristic phase transition points have been determined by means of derivative differential thermal analysis (DDTA) and then the maximum supercooling of the alloy in the initial stage of solidification has been calculated taking into account the influence of pressure on the equilibrium temperature value. Further the change of linear solidification rate of a casting has been determined depending on the squeeze pressure. It has been found that the supercooling of the alloy increases in proportion to the increase in squeeze pressure and reaches the highest value of 14°C for the 90 MPa pressure. For this pressure value the linear solidification rate is almost six times greater than for gravity castings. Metallographic examination has allowed for observing that the external pressure exerted on the solidifying casting results in the distinct refining of the alloy structure, the change in morphology of primary crystals, and the decrease of the gradient structure of the alloy. Squeeze casting advantageously influences the plastic properties of the alloy. The unit elongation of the squeeze cast alloy reaches the value of 16% and is twice the A 5 value determined for the gravity castings. The optimum squeeze pressure for achieving the highest mechanical properties is 30 MPa. The porosity of castings has been determined by the hydrostatic weighing method. The measurements have been performed for the specimens cut out of the middle parts of castings as well as of their edges. It has been found that the porosity of gravity castings reaches 4% and that the central part of the plate in particularly susceptible to the occurring of microporosity defects. The porosity distribution in squeeze castings is virtually uniform and its value is less than 1% for the entire examined range of squeeze pressure values.

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