Directional Solidification Of TiAl-Based Alloys Billets With Cold Crucible

It is of significance to prepare TiAl-based alloys by cold crucible electromagnetic confinement and directional solidification technique because of its unique features without contamination and continuity during process. A rectangular cold crucible with high width-to-thickness ratio(3:1) was projected in this paper. On the basis of optimal design and production of the related cold crucible, the intensity and distribution of magnetic field that induced by an alternate electrical current with high frequency was measured inside the cold crucible. Then the electromagnetic efficiency of the cold crucible was compared between the new one and the original cold crucible with a width-to-thickness ratio of 2.5:1. Thereafter, the experimental and analysis have been performed on viewpoint of the directional solidification of TiAl-based ingots with rectangular cross-sections.The results which depend on the measurement and analysis of the magnetic characters for the cold crucible with high width-to-thickness ratio show that the magnetic intensity contours the inner periphery of cold crucible was in the larger values at the area of the short rim, and declined gradually and stopped at the center lines of short and long edges of rectangle, respectively. Magnetic intensity was higher enough localized at the slits areas of crucible which indicated that the distribution of magnetic field was no longer shaped to be saddle-back but changed alternatively into single or multiple W-shape depends on the positions and configurations of slits. The magnetic intensity in the middle area of the cold crucible is homogenous along the height direction in the rectangular cold crucible, the magnetic intensity is strong in center of coil longitudinally and become weak gradually to both ends of coil. The magnetic intensity was increased with increasing the input electric power.As the result shown by experiment, the electromagnetic efficiency increased by nearly 40% for cold crucible as the wall thickness was reduced and also the segments were set as half-moon in cross shape and the slits were layout optimizedly as well. So a good match between the limited power supplied by AC plant and the melt overheating necessarily was obtained. The experimental on directional solidification for TiAl-based alloys have been performed with cold crucible of width-to-thickness ratio of 2.5:1 and 3:1 respectively. A number of apparent casting defects have been observed including feeder local melting, feeder/ingot sticking, and hump exploding, as well as surface crinkling, and the formation mechanism was deduced. Effects of withdraw rate, relative position between dummy and coil, also relative position of dummy and feeder and so the forth on the casting ingot's surface quality have been analyzed. Using 2.5:1 cold crucible to directionally solidify Ti43Al3Si(at%), it is found that the power 62-65kW, the withdraw rate 0.8-1.4mm/min, the dummy putting at the center of coil in height, and distance between dummy and feeder 14-18mm is an optimized parameter. And the ingots of slabs are with directional solidification structures and good surface qualities. On the contrary, for casting using 2.5:1 cold crucible, a set of parameters of 35-40kW in power, 1-2mm/min in withdraw rate, also dummy putting at the center of coil in height and 15-20mm in distance between dummy and feeder has been beneficial for directional solidification of Ti50Al(at%) slabs with desirable good surface qualities.The appearance of the skull was reserved by quenching in the liquid Ga-In-Sn, it is benefit to provide information on mode of formation of skull and its influence on directional solidification in cold crucible. The morphology of liquid/solid interface in three-dimension can be described by the skull. Then an U-shape is clearly showed for the interfacial area, in which the bottom of the interface displays water wave in shape with the distribution of the layout of the slits. At the short edge of the cold crucible, the interfacial area is straight, but at the long edge, the area is slightly bending up, the trend is preferential with the increasing of withdraw rate.Finally the microstructure of the directionally solidified Ti43Al3Si and Ti50Al was observed respectively. The tensile properties and microhardness were tested. It is shown that the specimen are in fully lamellar(α2/γ) structure. The tensile strength of Ti43Al3Si is higher than that of Ti50Al.