To reduce the cost of preparing a semi-solid slurry, grain shape must be refined and improved by controlling the pouring temperature or using a low pouring temperature.
Liu Zheng, Material and Chemical Engineering Institute of Jiangxi University of Science and Technology, Ganzhou, China; Mao Weiming, Materials Science and Engineering Institute of Beijing University of Science and Technology, Beijing
Rheocasting was developed as a new casting technique which involves stirring the alloy continuously during solidification or maintaining an isothermal state to produce semi-solid slurry, which is injected directly into the die. To save the cost of preparing the semi-solid slurry, grain shape must be refined and improved by controlling the pouring temperature or using a low pouring temperature. Because of the solidifying characteristic of alloy and the difference in thermo-conduction of the mold, the liquid alloy could have varying cooling rates throughout the mold resulting in different microstructure morphology. At present, a good solution to achieving uniform structure morphology in semi-solid alloys has not yet been found.
To obtain the semi-solid slurry needed for rheocasting, some measurements were taken, such as rigorously controlling pouring temperature and the alloy’s cooling rate, or holding the liquid alloy in the liquid-solid range, to ensure morphology of the primary phase. The measurements taken can ensure the primary phase particles with the proper morphology will be obtained, but the operation becomes difficult and costly. On the basis of low superheat pouring and slightly electromagnetic stirring technology (LSPSEMS), authors applied local chilling to the semi-solid slurry of aluminum alloy prepared by LSPEMS, in which the compound process is formed, to realize the control of microstructure morphology and reduction of cost.
Fig.1 shows the microstructure in the central area at the different positions of an A356 alloy ingot prepared by the compound process. It was seen in Fig. 1 that the microstructure in the central area, whether the position was from top or bottom, consists of particle-like or globular-like primary α-Al, and the grain size of the primary α-Al was finer. The morphology of the primary α-Al was not obviously changed along the direction of height in the ingot. It showed that the microstructure of the central area consisted of particle-like or globular-like primary α-Al, and the grain was fine when A356 alloy was prepared by the compound process.
Fig. 2 shows the microstructure in the edge area at the different position of an A356 alloy ingot prepared by the compound process. It was seen in Fig. 2 that little change occurred in the morphology of the microstructure in this area compared with that in the central and the edge area. Some changes in microstructure in the edge area along the height of the ingot from top to bottom occurred. The microstructure in the edge area of the top consisted of particular-like primary α-Al, rosette- like primary α-Al, and smaller amounts of rosette like primary α-Al (Fig.2a). Moreover, the amount of rosette-like primary α-Al slightly increased in the microstructure from the edge area of the middle position, the fine particular-like primary α-Al in the microstructure resulted from the secondary arms broken off from dendritic crystal, as shown in Fig. 2b. The microstructure on the edge area, near the bottom, still consisted of particle-like and rosette-like α-Al, as shown in Fig. 2c. In addition, no dendritic-like crystals which did not fully change into rosette-like primary α-Al or dendritic-like crystals were observed in the microstructure.
Along the radial direction of the same height in the ingot, the microstructure in the top did not change, and basically consisted of particle-like or globular-like primary α-Al and rosette-like primary α-Al, as shown in Fig. 1a and Fig. 2a. Few changes to the morphology of microstructure along the radical direction in the mid position were noticed from the particle-like or globular-like primary α-Al in the central area and the transition area changing to the microstructure mixed with particle-like or globular-like primary α-Al in the edge area, as shown in Fig. 1b and 2b. Some changes to the morphology of microstructure occurred along the radial direction in the bottom, from the particle-like or globular-like primary α-Al in the central area transforming to microstructure mixed with particle- like or globular-like primary α-Al and rosette-like primary α-Al, up to the microstructure mixed with particle-like and rosette- like primary α-AL in the edge area, as shown in Fig.1c and 2c.
Fig. 3 shows the microstructure morphology of a semi-solid A356 alloy prepared by LSPSEMS with the same processing parameters. The obvious changes for the micro- structure morphology occurred along the radial direction, presenting three typical structural areas. The morphology of primary α-Al in the central area of ingot was particle-like or globular-like, and the grain size of the primary α-Al was finer but coarser than that of the primary α-Al prepared by the compound process, as shown in Fig. 3a. The primary α-Al morphology was changed from the particle-like or globular-like to the rosette-like in the transition area of the ingot. A few coarse dendritic-like crystals which did not fully change into rosette-like primary α-Al still existed, as shown in Fig. 3b. In the edge area of the ingot, the microstructure consisted of dendritic-like crystals which did not fully change into rosette-like primary α-Al, as shown in Fig. 3c.
As seen in Fig.4a, the A356 semi-solid alloy prepared by the compound process had a diameter of 68.8μm in the central area, 77.6μm in the transition and 84.7μm in the edge area. The A356 semi-solid alloy prepared only by LSPSEMS (as shown in Fig. 3) had diameters of 85.6μm in the central area and 112.6μm in the transition area, where there were more rosette-like primary α-Al. The primary α-Al at the edge area consisted of fine and small dendritic grains which could not satisfy the requirement of rheocasting. The shape factor of the semi-solid A356 alloy prepared by the compound process was 0.83 at the center, 0.77 at the transition and 0.59 at the edge area. The shape factor in semi-solid A356 alloy prepared by LSPSEMS was 0.78 at the center, 0.54 at the transition and 0.28 at the edge area (Fig. 4b). The shape factor in the different areas of semi-solid A356 alloy prepared by the different processes was consistent with the observation on the primary α-Al morphology at the different areas. The results from Fig. 4 indicate the compound process could effectively improve the size and primary α-Al morphology at different areas in a semi-solid A356 alloy.
Previous research indicated semi-solid A356 slurry with fine particle-like or globular-like primary α-Al could be prepared by LSPSEMS, but pouring temperature has an important effect on the grain morphology and size of the primary α-Al. The morphology of the primary α-Al presented rosette-like grains that were course with high pouring temperatures; the morphology of the primary α-Al presented globular-like or particle-like microstructure and the size of grain was fine at the low pouring temperature. In the contrast experiment, due to the higher pouring temperature of 650C, even though the power of stirring reached 354W, the microstructure morphology of globular-like or particle-like could be obtained in the central area of the ingot, where there were three obvious structural areas. From another point of view, during preparation of the semi-solid A356 slurry by the compound process, such as using a pure copper rod to produce the local chilling effect, even if the pouring temperature as high as 650C, the amount of globular-like or particle-like primary α-Al with finer microstructure in the semi-solid slurry prepared greatly increase. The transition area in the ingot also disappeared, and the uniformity of microstructure morphology in the ingot was obviously improved. A pouring temperature as high as 650C in the compound process, on the condition of some chilling measurements taken, could be suitably raised to a temperature needed for rheocasting.
It was feasible to prepare semi-solid slurry of A356 alloy by the compound process. The pouring temperature could be suitably raised in this technique to be convenient for real operation.
The primary α-AL could be effectively fined when semi-solid slurry of A356 alloy is prepared by the compound process. The semi-solid primary phase consisted particle-like or globular-like α-AL in the ingot, and the microstructure uniformity was greatly improved .The size of primary α-AL prepared by the compound process was even finer than that prepared by LSPSEMS.
During the preparation of semi-solid A356 alloy slurry by the compound process, the solidified layer contained globular-like finer primary grains was formed on the surface of the pure copper rod through the local chilling produced by the rod. The grains were easy to drift away from the surface of the rod into melt to become the nucleation sites for α-AL under the forced convection.
During the compound process, the mechanism of grain refine was concerned with that the rod quicken up temperature reduction in the center of melt, perhaps the temperature field in which the temperature is gradually decreased from the wall of mold to the center of melt. There was higher surviving rate of nuclei in this temperature field, like as increasing the number of nuclei. Moreover, the rod could rapidly dissipate heat from the melt, or the increase of the local cooling rate in the melt. The crystals as nuclei could not grow up for short time so that the even finer grains were formed in the melt.