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Compared to the traditional Star impeller, the designed impeller (hereinafter termed as the XSR impeller) has the characteristic of pumping which can promote degassing efficiency, and smaller area of the dead region and higher turbulent kinetic energy are obtained within the melt simultaneously. In the range of the discussed immersion depths of impeller (H = 185 mm, 315 mm and 435 mm), both too shallow depth (185 mm) and too deep depth (435 mm) are bad for the distribution of flow field. For the discussed rotational velocities of impeller (V = 390 rpm, 430 rpm, 480 rpm and 550 rpm), as the V increases, the intensity of fluid flow increases accordingly, and the area of lower turbulent kinetic energy reduces gradually. For the XSR impeller, the optimal immersion depth and rotational velocity are 315 mm and 550 rpm, respectively. The experimental data are in well accordance with the simulated results. Under the optimal process parameters, the average volume fraction of voids and inclusions of the A356 melt decreases by 98%. The as-cast A356 alloy with higher cleanliness exhibits an ultimate tensile strength (UTS) of 161 MPa, a yield strength (YS) of 72 MPa and an elongation (EL) of 14%. [ABSTRACT FROM AUTHOR]

Abstract: EPMA and TEM analysis shows that there is a Ti transition layer around TiC particles in the α-Al grain center both in the usually solidified sample and in the rapidly solidified ribbon. Because the rapid solidification restrains the solute of the melt to diffuse before solidification, the rapidly solidified ribbon can conserve the instantaneous structure of the melt mostly, indicating that the Ti transition layer originates from the Ti transition zone in the melt. A new theory named as “Ti transition zone” theory has been proposed as a possible refinement mechanism for pure aluminum by Al–Ti–C refiner, and its formation model is given. The influence of the Ti transition zone on the melt viscosity was also discussed in this paper. [Copyright &y& Elsevier]

It is well known that solidification of molten metals occurs under the influence of transient unsteady phenomena, which are difficult to control in actual casting operation. Examples include buoyancy flows, natural heat convection, repeated remelting and solidification in mushy zone and solid shell at the mold interface. This paper presents the results suggesting that ultrasound waves, irradiated into the hot-top mold offers an attractive way to control the transient phenomena in DC casting of aluminum alloy billets. A novel mathematical model was developed to simulate the DC casting with considering transient melt flow, heat transfer, ultrasound propagation, acoustic streaming and solidification. It was found that ultrasound irradiation alters the solidification behavior of melt and sump evolution especially in the earlier stage of casting, however as the billet length increases, the sump profile becomes almost the same for both the ultrasonic and conventional castings. In this condition, the casting speed becomes the key parameter influencing the sump evolution and mushy zone volume. The results reveal that ultrasound-driven acoustic streaming and turbulent oscillations result in a suppression of buoyancy flow in the sump and lead to an increase of frequency of the melt temperature oscillations at the solidification interface. One of the useful results of these phenomena can be a more frequent repetition of remelting/solidification cycle and improvement of the billet quality, particularly the billet surface morphology. The effect of ultrasound irradiation on the billet morphology was verified experimentally using a pilot DC caster. [ABSTRACT FROM AUTHOR]