Producing non-dendritic A356 and A380 feedstocks: Evaluation of the effects of cooling slope casting parameters

In this study, low superheat casting and cooling slope casting processes were carried out with A356 and A380 aluminum alloys in order to obtain feedstocks with near globular microstructures. Castings were conducted at 20 °C above of the liquidus temperature for both processes and molten alloys were cast through a copper cooling slope with two different lengths (350 mm and 650 mm) and two different tilt angles (30° and 60°). In order to evaluate the significance of boron nitride coating on cooling slope, castings in short slope were carried out under both coated and uncoated conditions. Microstructural examinations and hardness measurements were carried out. According to obtained results, cooling slope casting caused superior microstructural properties as compared to low superheat casting, especially for A356 alloy. 30° tilt angle was found more efficient in order to obtain a more globular microstructure for A356, while, on the other hand, the tilt angle of 60° was detected more favorable in that manner for A380. Obtained grain size measurements were slightly improved with the employment of short slope and coating was found beneficial especially for A356 alloy. The measured hardness values did not display any significant difference in the same alloy type except in the low superheat cast A356 specimen, which was obtained with coarser microstructure.

Research of Preparing Parameters in the Complex Process on Microstructure in Semisolid A356 Alloy Slurry with Micro-Addition of La

Based on the green and saving concept, a complex process preparing semisolid alloy slurry was developed, which was composed of the low superheat pouring and low frequency electromagnetic stirring. The semisolid A356-La slurry was prepared by the complex process, and the microstructure of the semisolid A356-La alloy was researched under the different preparing parameters in the complex process, such as the pouring temperature, electromagnetic stirring frequency, stirring time and micro-addition of La. The results indicated that it was feasible to reduce the addition of La and consumption of energy during the preparation of semisolid alloy slurry by optimizing the preparing parameters in the complex process. The suitable preparing parameters were obtained by the experiment, in which the pouring temperature was 630 °C, the frequency of electromagnetic stirring was 30 Hz, and the stirring time was 8 s. When semisolid A356 alloy slurry added 0.3 wt% La was prepared by the suitable preparing parameters in the complex process, the refining effects on microstructure in the semisolid A356-0.3La alloy was indistinguishable with that of the conventional addition amount of 0.6 wt% La in semisolid A356 alloy prepared by low superheat pouring.

Experimental and Stochastic Modeling of the Globular Microstructure and the Microsegregation Evolution during the Solidification of Magnesium Alloys Cast at Low Superheat via Containerless Melting

Casting at low superheat has a strong influence on the solidification morphology and macro- and microstructures of cast alloys. This paper shows the microstructure and microsegregation of cast Mg AZ31B alloy at superheat of 5°C, summarizes the multiscale model, and describes the simulation results obtained with a previously developed stochastic mesoscopic solidification model capable of predicting the microstructure of the cast alloy. The simulation results are compared with casting experiments carried out using the Magnetic Suspension Melting process. Casting at this low superheat was found to produce castings with a fine globular grain structure, with an average grain size of about 80 µm, which is about one-third the size of that obtained by conventional casting techniques. The stochastic model was validated against the experimental measured grain structure and Al microsegregation. A parametric study was performed to determine some key material and process parameters related to processing of cast Mg AZ31B alloy.

Water Droplet Vaporization on Superhydrophilic Nanostructured Surfaces at High and Low Superheat

This paper summarizes results of an experimental exploration of heat transfer during vaporization of a water droplet deposited on a superhydrophilic nanostructured surface at high and low superheat conditions. The superhydrophilic surface is composed of a vast array of zinc oxide (ZnO) nanostructures grown by hydrothermal synthesis on a smooth copper substrate. The individual nanostructures are randomly-oriented and have a mean diameter of about 400 nm, a mean length of 2 μm and a mean centerline spacing of about 700 nm. The macroscopic wetting characteristics of the surface were measured and scanning electron microscope imaging was used to document the nanoscale features of the surface before and after the experiments. These surfaces typically exhibited water contact angles less than 5 degrees. In single droplet deposition experiments at atmospheric pressure, a high-speed video camera was used to document the droplet-surface interaction, and the heat transfer coefficients were simultaneously determined from thermal measurements in the test apparatus. At low superheat levels (10–20°C), droplets spread rapidly over the heated surface when deposited. For these conditions, no bubble nucleation was observed, and we nevertheless observed extremely high heat transfer coefficients resulting from rapid evaporation of the thin liquid film formed by the spreading droplet. At high wall superheat levels, the vaporization process exhibited Leidenfrost droplet vaporization. The extreme wetting for these surfaces resulted in extremely high Leidenfrost transition temperatures. The results document a trend of increasing Leidenfrost temperature with decreasing contact angle, which is consistent with earlier studies. The results of this study are compared with early work in this area and the implications for applications are discussed.