Papers by Keyword: Globular Microstructure

Abstract: A three phase electromagnetic stirrer was used to agitate aluminum A356 slurry and a dry and oxygen free argon gas was introduced in to the slurry by a porous graphite core at a same time. The prepared semi-solid slurry was then transferred into a metallic mold and was compacted by a drop weight. Results demonstrated a favorable increase in shape factor, decrease in aspect ratio and average diameter size at different intensities of stirring. The intensity of stirring was changed by altering the current passed through the magnetic coil and also bubbling intensity via the porous graphite diffuser. Different time intervals for electromagnetic stirring and gas induction were applied. Agitating the slurry for 90 Sec. separately by electromagnetic stirrer and GISS method, gave better results in terms of shape factor, decrease in average diameter of the globules and aspect ratio.

Abstract: In this paper the effect of globular microstructure on the cavitation erosion resistance was assessed and compared to that of conventional dendritic one. Three different wrought aluminum alloys in as-cast conditions were investigated. The samples were completely characterized by metallographic analyses and microhardness measurements. Cavitation erosion tests were performed according to ASTM G 32 standard. The volume loss was evaluated during the test by periodical interruptions. It was identified the damaging mechanism in case of both dendritic and semisolid microstructure. It was also found that the globular microstructure increases the cavitation erosion resistance only for one of the studied alloys.

Abstract: The aim of this work was to develop understanding of microstructural evolution of the alloy casted in semi-solid condition using a cooling slope and conventional casting followed by ECAP in a 120^o die. Feed materials were prepared by cooling slope casting and conventional casting for ECAP process. The microstructures and Vickers hardness of the worked materials extruded by two routes (A and B_C) were evaluated. The primary α-Al phase tends to be elongated after processing by route A. However, its morphology was similar in nature to the microstructure of the as-cast sample after processing by route B_C. The Si particles become fragmented during ECAP processing and are more nearly globular in shape and uniform in size than in the as-cast sample. The microstructure of the semi-solid cast ECAPed samples was more homogenous than that of the conventional cast ECAPed sample followed by ECAP for both routes. The hardness of semi-solid cast ECAP samples was also higher than that of conventional cast ECAPed samples for both routes.

Abstract: A combination of hot rolling and equal channel angular pressing (ECAP) was explored to generate globular microstructures in the Mg-3%Zn alloy after re-heating to the semisolid state. It was found that the single-step deformation of as-cast alloy via hot rolling at 350°C with a thickness reduction of 50% refined the alloy microstructure by creating deformation bands of the Mg (α) phase with a size of the order of tenths of micrometers. After re-heating to 630 °C, the microstructure transformed into spheroidal morphologies with an average globule size of 82 μm. An additional deformation of the hot-rolled alloy by the ECAP method at 250 °C further refined the alloy microstructure to sub-micrometer grains of lath and equiaxed shapes. After re-heating of this microstructure to 630 °C the average globule size reached 62 μm, which is roughly 25% smaller than that achieved for the hot-rolled precursor. The role of strain-induced melt activation (SIMA) techniques in generation of globular morphologies in Mg-based alloys after partial re-melting is discussed.

Abstract: Equal channel angular pressing (ECAP) could be used to achieve ultra fine grains in bulk aluminum alloy through severe plastic deformation. Typically a feed material of as-cast aluminum alloys is used with a typical hypoeutectic solidification structure, consisting of primary aluminum dendrites and interdendritic network of lamellar eutectic silicon. On the other hand, semi-solid metal casting provides non-dendritic and globular microstructure which is one of a considerable factor in obtaining homogenous microstructure after ECAP. This work is an attempt to produce aluminum alloy with globular microstructure using cooling slope semi-solid casting process which is believed suitable as a feedstock for ECAP. The aim of this work described in this paper was to understand of microstructural evolution of aluminum structure during cooling slope casting process. Two experiments were carried out. A sample was casted via a cooling slope into a vertical cold mild steel mould at pouring temperature of 640°C. Cooling slope length of 250 mm, slope temperature of room temperature and tilt angle of 60^o was applied. Another sample was casted directly into a vertical cold mild steel mould at pouring temperature of 640°C. The primary α-Al phases in the sample that casted without cooling slope was mostly in dendritic throughout the section of sample whilst the primary α-Al phases transformed completely into non-dendritic in the sample that was casted via the cooling slope. Therefore, the transformation is believed resulted from the effect of cooling slope

Abstract: A fusion welding technique to join a semi-solid processed A356 cast plate is explored using Gas Tungsten Arc Welding (GTAW). Semi-solid metal (SSM) billets of non-dendritic microstructure produced by rheocasting in a mould placed inside a linear electromagnetic stirrer were used for this study. GTAW experiments were conducted to simulate different thermal gradients near the fusion zone. The geometries of the weld pool as well as the temperature gradient in the fusion boundary were measured to understand the microstructure evolution. Simulation of the welding process was performed to aid in the analysis. Quantitative metallography provided the shape factor as a measure of globularity of the primary a-Al phase. Based on the studies, a model has been proposed to explain the observation of globular microstructure in the fusion zone of the welds. Conclusions show a positive correlation of thermal gradient with globular microstructure formation in this class of alloys.

Abstract: Thixoforming and related semi-solid processing (SSP) methods require thixotropic materials. One of the many SSP techniques is the cooling slope (CS) casting process, which is simple and has minimal equipment requirements, and which is able to produce feedstock materials for semisolid processing. When the feedstock is reheated to the semisolid temperature range, non-dendritic, spheroidal solid particles in a liquid matrix suitable for thixoforming are obtained. In this study, equipment for the CS technique was first established, and then the effects of the pouring temperature and inclined slope angle on the microstructures of A380 aluminum alloy (ISOAlSi8Cu3Fe) were studied. Optimum parameters for thixoforming experiments were selected, and it was found that the microstructure produced by the inclined plate depended on its angle and the pouring temperature.

Abstract: In this study, A380 aluminum alloy feedstock produced with cooling slope casting was exposed to isothermal treatment to obtain a globular microstructure which is a key feature for semisolid forming. The dendritic primary phase in the conventionally cast A380 alloy has readily transformed into a non-dendritic one in ingots cast over a cooling plate from pouring temperatures between 615, 630 and 650 °C. After the casting process, isothermal treatment was carried out at 565 °C in induction unit. Isothermal treatment yields a globular microstructure. To determine the tribological properties of this alloy, a pin-on-disc tribometer was used to carry out tribological tests under dry sliding conditions. The results showed that both cooling slope casting and isothermal treatment has an effect on tribological properties.

Abstract: In this study, in order to compare effect of unidirectional compression and rolling on final microstructure of strain induced melt activated (SIMA) A356 aluminum alloy, rectangular samples with dimensions of 3cm×5cm in area and 1cm in thickness and cylindrical specimens with 2.5cm in diameter and 1cm in length, have been prepared for rolling and compressing processes, respectively. Then, these samples were plastically deformed at a same strain in ambient temperature. Afterward, the strained samples were cut into equal quarters. In the next stage, to produce globular microstructure, these specimens were partially remelted in 580°C for different times. Results obtained from light microscopy showed that specimen's thickness and so, its strain affected zones influence on the globulization of dendrites. In addition, it was seen that at a given strain and constant diameter, increase of H/D ratio led to increase of needed time for reaching a certain sphericity in cylindrical samples. Also, it was showed that microstructural evolutions during SIMA processing of both rolled and unidirectional compressed samples were relatively identical. However, at a same condition, ultimate size of globulized dendrites in the rolled samples was smaller than those of compressed ones.

Abstract: The main goal of this work is the analysis of rheological properties of steel alloy at high temperatures, just below the solidus point, and in the semi-solid state with low liquid phase content. Data obtained from the analysis can form the basis of numerical simulation for designing and optimizing the thixoforming processes. The rheological properties should be known over a wide temperature range so that the simulations could also predict defects such as incomplete die filling. The analysis concerned M2 tool steel alloy. The paper also discusses development of globular microstructure in partially melted steel.