N. Wang

Grain selection during casting Ni-base,single-crystal superalloys with spiral grain selector

The behavior of grain selection in a spiral grain selector during investment casting of a Ni-base, single-crystal (SX) superalloy, DD3, has been investigated by electron backscattered diffraction (EBSD) techniques and optical microscopy. The results indicated that the main function of starter block is to optimize the crystal orientation. During the process of grain selection in spiral passage, the grain near the inner wall of spiral passage was usually selected as the final single crystal. It was found that the dendrites near the inner wall could develop new tertiary dendritic arms that paralleled the primary dendrites from the secondary dendritic arms to overgrow the dendrites far away from the inner wall. The crystal orientation that was examined by X-ray diffraction revealed that (1) the crystal orientation did not change obviously with increasing spiral thickness or angle and (2) the crystal orientation could be optimized by increasing the withdrawal rate and ceramic mold temperature. The influence of pouring temperature on crystal orientation was also discussed.

Containerless rapid solidification of Ag28.1Cu41.4Ge30.5 ternary alloy

Abstract Droplets of Ag28.1Cu41.4Ge30.5 ternary alloy with different sizes are solidified during containerless processing in a 3 m drop tube. The experimental result shows that the microstructural evolution depends mainly on droplet size. Generally, droplet undercooling is inverse proportional to its diameter. When the droplet diameter is large, the (Ge) primary phase and intermetallic compound η phase show granular and strip shapes, respectively. In the medium-sized droplets, the (Ge) phase grows in a dendritic mode whereas η phase has little change. Meanwhile, both the grain size and interphase spacing also decrease with the reduction of droplet size. Moreover, it is found that η phase can nucleate directly from alloy melt and does not depend on primary (Ge) phase during the formation of binary eutectic structure. If the droplet diameter is smaller than 250 μm, a spherical anomalous eutectic grain structure forms, which consists of three very fine phases and is surrounded by coarse (Ge) and η phases. There is no evident feature of macrosegregation of semiconducting (Ge) phase and it distributes quite homogeneously, which is ascribed to the reduced gravity condition and rapid cooling in the drop tube.

Growth Characteristics of TiCxNy Ceramics under Laser Melting Conditions

By laser melting technique, TiCxNy ceramics are prepared under the laser powers of 300, 400, 500, 600, and 700 W, respectively. The size of molten pool decreases with the increase of scanning speed from 1 to 4 mm/s at a certain power. TiCxNy microstructure continuously changes from the top to bottom of molten pool. Based on the XRD spectrum, the composition of ceramic can be confirmed as Ti(O0.19C0.53N0.32), C0.7N0.3Ti, C2MoTi, and Ti8C5. Based on the analysis of the growth process, the growth conditions can be confirmed. The maximum depth of molten pool obtained at the laser energy density of 500 W/cm2. The more deviation from this value, the more shallow the pool. The corrosion experiment result shows that TiCxNy ceramic has good corrosion resistance to various corrosive agents except HNO3.