Jing Jie Guo

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

Relationship between Thickness of Lamellar α+β Phase and Mechanical Properties of Titanium Alloy

Titanium alloy castings are made by means of induction melting technology. The relationships thickness of lamellar α+β phase and tensile strength, yield strength, elongation percentage, and Vickers-hardness, as well as the effect of tensile property on the Vickers-hardness are investigated for Ti-6Al-4V alloy castings. The results show that the relationships between thickness of lamellar α+β phase, and tensile strength, yield strength, specific elongation, and Vickers-hardness meet the Hall-Petch equation. And the tensile property increases linearly with Vickers-hardness.

Microstructure and Interface Of Mg3Zn6Y Quasicrystal Particulate Reinforced Mg-8Gd-3Y Alloy

Mg3Zn6Y quasicrystal particulate reinforced Mg-8Gd-3Y alloy was fabricated by adding the Mg3Zn6Y quasicrystal particles into the Mg-8Gd-3Y matrix alloy melt. The effect of volume fraction of quasicrystal particles on the microstructure of quasicrystal particulate reinforced Mg-8Gd-Y alloy was studied using XRD, SEM, TEM and HRTEM. It was found that Mg3Zn6Y quasicrystal particles were melted partly and only 1/3 of them were added into the matrix alloy. The quasicrystal particles distributed in the grain and grain boundary. At the same time, the intermetallic phase composition and the morphology in Mg-8Gd-3Y matrix alloy were changed. The interface between quasicrystal particles and α-Mg was smooth and clear, they combined closely each other and there is no obvious interface chemical reaction when the direction of electron beam was parallel to the 2-fold and 3-fold axes of icosahedral quasicrystal phase particles.

Investigation on a Fabrication Technique of TiAl Sheet

A TiAl sheet fabrication technique from Ti/Al foils was studied experimentally. Firstly, the reaction sequence between Ti and Al foils were studied with DSC and then a three stage solid transformations processing was carried out. At the first stage, Al diffused into Ti foils and formed Al3Ti. The heating temperature was selected based on a ruler that the pure Al was consumed by solid diffusion with a short time and avoiding the melting of Al. The resulted microstructure consists of pure Ti and Al3Ti. At the second stage, part of Al in Al3Ti diffused into pure Ti and formed the high temperature phase. At the third stage, the sample was heated into the high temperature -phase zone and hold a given time and then cooling with the furnace. A full density γ-TiAl based alloy sheet with lamellar microstructure was successfully fabricated under above processing. The lamellar orientations are aligned around 0-45o compared to the longitudinal direction of the sheet.

The Effect of the Addition of Nano-SiC and Nano-TiN in the ZGMn13Cr2 Alloy on the Microstructure and Mechanical Properties

The modifiers of nano-SiC and nano-TiN were added into the melt of ZGMn13Cr2 alloy, respectively. The effect of the additions on the microstructure and mechanical properties were investigated. Results show that the two kinds of nano modifiers both have refining effect and make the improvement of impact toughness and wear resistance.

Optimization of Tilt-Casting of Aluminum Alloy Automobile Drain Pipes through Numerical Simulation

The effect of major parameters, such as pouring temperature, tilt time and preheating temperature of mold, on the tilt casting of aluminum alloy automobile drain pipes is studied through numerical simulation using single factor and orthogonal test methods. The simulation results of single factor test indicate that the increase in the pouring temperature and mold preheating temperature could effectively reduce the shrinkage tendency and the shortage of tilt time would cause lower temperature gradient of casting at the beginning of solidification, thus would lead the extension of solidification time. And the results of orthogonal test showed that the sequence of factors having effect on the filling and solidification process, are pouring temperature, tilt time and preheating temperature of mold. Finally, combined the result of single factor test with orthogonal test, an optimized tilt-casting process was proposed by investigating the velocity distribution at each stage of the filling process and the isolated area distribution of liquid phase in the solidification process, and by predicting the position of porosity in the casting. The effectiveness of the proposed process parameters in producing high quality castings is also proved through numerical simulation.

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