Hong Sheng Ding

Continuous Casting and Directional Solidification of Titanium Alloys with Cold Crucible

Titanium alloy billets were continuously cast and directionally solidified under vacuum and compelling cooling conditions by using round or rectangular cross-section cold crucible. It is found that hot crack is caused by friction between the skull and the inner wall of crucible, a coating of CaO and CaF2 flux can eliminate the hot cracks. The experimental results show that the withdrawal velocity, where the changes was setting as 5, 3, 1, 0.5mm/min respectively, is the significant factor that affects the macrostructure and solidification front shape from concave to flat and then to convex. Directional columnar grains and a single crystal can be obtained when the velocity is controlled to either 1 or 0.5mm/min under a round cross-section crucible, and columnar grains can be also obtained at the speed of 2mm/min under a rectangular cross-section crucible. Finally, the temperature fields during the process are calculated and the trend of solidification front is in good agreement with the experiment. Specimens composed of columnar grains or single crystal exhibit excellent tensile properties. Introduction Titanium alloys are promising engineering materials, which are applied to aerospace and shipbuilding industries owning to their high specific strength, hot resistance and corrosion resistance. However, there are some difficulties in controlling the chemical elements and temperature in melting titanium alloys. Recent directional solidification of titanium alloys experiments were performed in optical floating furnace [1-3] or in resistance furnaces using Al2O3[5], Y2O3[4] or CaO [5] crucible. But titanium will react with all commercial ceramic mould materials, which in turn affect the microstructure and mechanical properties. Water-cooling cold crucible provides an effective method for melting titanium alloys because of its high speed in melting, non-contamination and continuous casting [6,7]. Cold crucible continuous casting and directional solidification includes induction melting, soft contact, continuous casting and directional solidification. Not only aluminum, steel and titanium alloys [8], but also photovoltaic multi-crystalline and single silicon are continuously cast by cold crucible [9]. But directional solidification of titanium alloys with cold crucible has not been reported elsewhere. This paper investigates the methodology of continuous casting and directional solidification with cold crucible when it was applied to Ti-6Al-4V alloy. Experimental method The experimental apparatus is schematically shown in Fig.1. This apparatus comprises: -a furnace chamber made of steel, in which a vacuum can be established by an exhaust pump -a water-cooling segment copper crucible with 8 pieces of vertical slits and 30mm in the internal diameter and 130mm in height. -a four-turn water cooling coil surrounding the crucible and supplying 50 kHz frequency alter current generated by a transistor generator, of which the maximum output power is 100kW -a withdrawal system and a feeder system, both of them can be controlled respectively. The procedure is as follows. A Ti-6Al-4V alloy billet is fixed on feeder and a primer is placed in the crucible. A vacuum of 1Pa is established, then Argon gas is introduced to 200 Pa. Power is gradually Materials Science Forum Online: 2005-01-15 ISSN: 1662-9752, Vols. 475-479, pp 2575-2578 doi:10.4028/www.scientific.net/MSF.475-479.2575

Electromagnetic Cold Crucible Technology Applied for Producing Big-Sized γ-TiAl Based Ingots with Directional Growth Structure

Developing high performance aero-engines usually depends on the development of new candidate materials with charming properties such as relatively light-weight, good high temperature strength and environmental resistance. Fortunately, γ-TiAl alloys were followed into this catalogue that they offer a significant potentiality for weight savings. Recently, in order to improve their low room temperature plasticity, a motive technology called electromagnetic cold crucible directional solidification technology for preparation γ-TiAl alloy ingots with directional growth structure was put forward by our group. By the approach, TiAl ingots with controllable lamellar structures and less impurity contaminations are obtained. Typically, Ti-46Al-0.5W-0.5Si and Ti-47Al-2Cr-2Nb were performed with respect to compositions and solidification parameters. As a result, Ti-46Al-0.5W-0.5Si DS-samples exhibit good combination of mechanical properties at room temperature in TS of 500MPa and EL of 2% in average. Meanwhile TS of DS Ti-47Al-2Cr-2Nb reaches 650 MPa and EL exceeds 3%.

Effects of Technical Parameters on Initial Silicon Melting in Round Cold Crucible Continuous Casting

The effects of technical parameters on initial silicon melting in cold crucible continuous casting were studied. These parameters include the materials, the shape and the position of the base, the mass of the silicon that set on the top of the base. Through experimental and theoretical analysis, the optimized parameters were finally given: the base graphite with obconical shaped should be put at the level of the second turn of the coil, and the initial silicon with 10g should be put on the base. The mechanism of these parameters affecting on the initial melting are discussed and revealed.

Study of Solid/Liquid Interface of Ti46Al0.5W0.5Si Ingot Directionally Solidified by a Near-Rectangular Cold Crucible

In this study, Ti46Al0.5W0.5Si ingots were directionally solidified by a near-rectangular cold crucible under different process parameters. These process parameters include the electromagnetic stirring, the crucible configuration and the molten drop, all of them have important effect on the S/L interface. The effects of the parameters on the solid/liquid (S/L) interface morphology were investigated, and the mechanisms of the parameters influencing the S/L interface were discussed and revealed. Results showed that the typical S/L interface of the ingots was presented as a curved ‘W-type’ shape. The uneven temperature distribution in the front of the solidified interface is the main reason for a curved S/L interface. Further, the requirements for obtaining a planar S/L interface in the process of cold crucible directional solidification were given, which provided a guide for the future work.

Effects of Parameters on the Stability of Silicon Melt Pool in Cold Crucible Continuous Casting

Effects of parameters on the stability of silicon melt pool in cold crucible continuous casting were discussed. The results indicate that the extinguishing of induction heating is mainly caused by the side wall feeding, low superheat degree and volume of the pool. The hot melt splashes are caused by the higher input power and uncompletely melted silicon of the dome top surface. The pool was shown to be stabilized as the raw materials were fed in the center area of the dome surface, the superheat degree and volume of the pool were properly increased, the pool was controlled in the effective heating range of the coil and the input power was set properly

Temperature Distribution and Operating Efficiency in Rectangular Cold Crucible Directional Solidifying Ti46Al0.5W0.5Si Alloys

This paper gives a detailed study of temperature distribution on Ti46Al0.5W0.5Si alloy directionally solidified by cold crucible. Based on experiment and numerical calculation, the heating process under different power was investigated, and the operating efficiency of cold crucible was roughly calculated. Further, the temperature field of Ti46Al0.5W0.5Si alloy under high power was numerically calculated, the finite element model predicted the temperature distribution at different moment. The results give a significance guide on cold crucible directional solidification.

Microstructure and Mechanical Properties of a Rapid Directionally Solidified Ti47Al Alloy

The ingot of Ti47Al alloy was prepared by a newly developed rapid directional solidification, the microstructure and compressive properties of the ingot was observed and tested. The results show that the macrostructure consisted mainly of coarse columnar grains parallel to the axial direction, with the size of 0.5mm wide and 10mm in length. The direction of lamellar is almost perpendicular to the growth direction in the longitudinal section and no dendritic core is found. The average ultimate compressive strength of the specimens with grain growth parallel/perpendicular to the compressive direction is 1233.3 and 861.7 MPa respectively. The fracture mode for specimens with grain growth parallel to the compressive direction exhibits predominantly translamellar fracture, however, that for specimens with grain growth perpendicular to the compressive direction exhibits predominantly delamenation or interlamellar fracture.

Grain Growth Behaviors of Multicrystalline Silicon during Cold Crucible Continuous Casting

A square shaped silicon ingot was cast by cold crucible continuous casting. The grain growth behaviors during different growth stages were investigated. In the first, grains nucleate at the base surface in the centre and in the three phase junction at periphery, and then grow upwards and inclined upwards separately. In the second, grain grow competitively, two distinguished zones are formed. In the final, a cap is formed on the liquid free surface, and precipitates are present.

Columnar Crystal Growth of Ti46Al0.5W0.5Si Alloys Directional Solidified by Rectangular Cold Crucible

This paper has systematically studied the evolving process of column crystals of Ti46Al0.5W0.5Si alloys that is directionally solidified by rectangular cold crucible. The results showed that: the solid/liquid interface presented convex to the liquid metal due to the complicated influences by cold crucible. Overheating temperature and new crystal nucleus had great effects on the growth continuity and the stability of column crystals. Furthermore, the precipitation phases have been studied. As a result, the methods for controlling growing process of columnar crystals have been submitted, which are playing a critical role in optimizing experimental parameters and devising the construction-elements of cold crucible.

Experimental Study of the Silicon Start-up Heating and Melting Process in an Inductive Cold Crucible

The electromagnetic continuous pulling is a newly growth technology as a promising process for silicon preparation, start-up heating is necessary for the semiconductivity of silicon at room temperature. Investigations were carried out to study this process in a square crucible with the frequency of 50kHz, details of the experimental procedure were given, the affecting factors, electrical performance and heat explosion problems during the process were measured and discussed. The results indicated that the best conditions for the preheating were the central position of the base in the coil, the higher power and the proper primary mass of silicon setting. The electrical performance indirectly showed the pool conditions and so it can be effectively used to control the melting operation.