Development and application of titanium alloy casting technology in China

The development and research of titanium cast alloy and its casting technology, especially its application in aeronautical industry in China are presented. The technology of molding, melting and casting of titanium alloy, casting quality control are introduced. The existing problems and development trend in titanium alloy casting technology are also discussed.

Characterization of zirconia-based slurries with different binders for titanium investment casting

The materials and physical properties of primary slurry are crucial to the surface quality of the finished castings,especially for high reactivity titanium alloys.The aim of this study is to investigate the influence of different binders on the physical properties of primary slurry for titanium alloy investment casting.The zirconia-based slurries with different binders were evaluated by comparing the parameters:viscosity,bulk density,plate weight, suspensibility,gel velocity and strength.The results indicate that a higher viscosity of binder leads to a higher viscosity and suspensibility of slurry with the same powder/binder ratio.The retention rate and thickness of primary layer increase with an increase in the viscosity of the slurry,and a higher retention rate is associated with a thicker primary layer.The gel velocity of the slurry is correlated with the gel velocity of the binder.The green strength and the baked strength of the primary layer are determined by the properties of the binder after gel and by the production of the binder after fired,respectively.

Evaluation of Alpha-case in Titanium Castings

The alpha-case formation reactions between Ti and investment molds （Al-2O-3,ZrSiO-4,ZrO-2,CaO stabilized ZrO-2） were evaluated in a plasma arc melting furnace.Regardless of thermodynamic approaches,there were distinct alpha-case formations.The reaction products were characterized by electron probe micro-analysis and transmission electron microscopy.Theα-case generation between Ti and Al-2O-3 mold was not able to be explained by the conventionalα-case formation mechanism,which is known to be formed by the interstitials,especially oxygen dissolved from mold materials.However,from our experimental results and thermodynamic calculations,it was confirmed that theα-case is formed not only by an interstitial element but also by substitutional metallic elements dissolved from mold materials.Our newly establishedα-case formation mechanism will surely lead to a variety of significant applications of theα-case controlled Ti casting.

EFFECTS OF OXIDES IN INVESTMENT MOLD ON SURFACE PROPERTIES OF TITANIUM CASTINGS

Effects of different facing oxides, including including yttria colloidal and powders (Y/Y), yttria stabilized zirconia colloidal and powders (ZY/ZY), zirconia colloidal and powders (Z/Z) and zirconia colloidal and zireonite powders (Z/ZS) on the qualities of investment castings are studied. The outward appearance, microstructures, and microhardness profiles of castings made of commercial pure titanium, Ti--Al--V and Ti--Al--Zr are investigated. Castings made by shells with yttria as face materials have the least contamination and by zirconite shells have the most contamination. Thermodynamic of metal-mold reactions is also taken into consideration.

Effect of SiO_2 concentration in silica sol on interface reaction during titanium alloy investment casting

Using silica sol as a binder for titanium investment casting is very attractive due to its good stability and reasonable cost as compared with yttrium sol and zirconium sol. However, the mechanism of interface reaction in the related system remains unclear. In this investigation, the interface reaction between Y_2O_3-SiO_2(YSi) shell mold and titanium alloys was studied. A group of shell molds were prepared by using Y_2O_3 sand and silica sol with different contents of SiO_2. Ti-6Al-4V alloy was cast under vacuum by gravity casting through cold crucible induction melting(CCIM) method. Scanning electron microscopy(SEM) and energy dispersive x-ray spectroscopy(EDS) were employed to characterize the micromorphology and composition of the reaction area, respectively X-ray photoelectron spectroscopy(XPS) was used to confirm the valence state of relevant elements. White ligh interferometer(WLI) was used to obtain the surface topography of Y-Si shells. The results show that the thickness of reaction layers is below 3 μm when the SiO_2 content of silica sol is below 20 wt.%. Whereas, when the SiO_2 content increases to 25 wt.%, the thickness of the reaction layer increases sharply to about 15 μm. There is a good balance between chemical inertness and mechanical performance when the SiO_2 content is between 15 and 20 wt.%. Moreover, it was found that the distribution of SiO_2 and the roughness at the surface of the shell are the key factors that determine the level of reaction.

Reaction between Ti and boron nitride based investment shell molds used for casting titanium alloys

The aim of this paper was to study the reaction between a Ti-6Al-4V alloy and boron nitride based investment shell molds used for investment casting titanium. In BN based investment shell molds, the face coatings are made of pretreated hexagonal boron nitride （hBN） with a few yttria （Y2O3） and colloidal yttria as binder. The Ti-6Al-4V alloy was melted in a controlled atmosphere induction furnace with a segment water-cooled copper crucible. The cross-section of reaction interface between Ti alloys and shell mold was investigated by electron probe micro-analyzer （EPMA） and microhardness tester. The results show that the reaction is not serious, the thickness of the reacting layer is about 30-50 μm, and the thickness of α-case is about 180-200 pro. Moreover the α-case formation mechanism was also discussed.

Numerical Simulation of Bubble Migration in Liquid Titanium Alloy Melt During Vertical Centrifugal Casting Process

The bubble migration in liquid titanium alloy melt during vertical centrifugal casting process has been predicted. The effects of different parameters, such as the initial bubble location, the mold rotational speed and the mold rotational direction on the bubble in the migration process are investigated. The results show that the bubble migration can be divided into the radial movement to the mould rotation axis and the circular movement to the mould wall opposite to rotational direction of the casting mould. The casting mould wall has an impeditive effect on the circular movement of the bubble during its migration process. And the bubble finally migrates like a straight line along the mould wall located at the opposite direction of the rotational casting mould whether it rotates clockwise or anti-clockwise. The bubbles at the position near the mould wall located at the opposite direction of the rotational casting mould are much easier to migrate in a straight line. The instantaneous speed of the gas bubble increases with the increment of the mould rotational speed. However the mould rotational speed is high or low, the moving speed of the gas bubble increases slightly at the primary stage, and then decreases gradually like a ladder.

Filling mode and regularity of vertical centrifugal casting process of titanium alloy in thin-walled cylinder cavity

The mold filling process of titanium alloy in a thin-walled cylinder cavity under vertical centrifugal casting process was studied by means of the hydraulic simulation experiments. Results show that the filling mode of the melt in the cylinder cavity varies with casting wall-thickness. When the casting wall-thickness is less than or equal to the thickness of the first layer during the filling process, the melts fill the cavity from the bottom to the top.When the casting wall-thickness is greater than the thickness of the first layer during the filling process, the melts first fill the largest radius parts of the cavity with a certain thickness of the first layer from the bottom to the top of the cavity, and then they fill the cavity from the larger radius part to the smaller radius part. The melt filling ability increases with the increment of the mold rotational speed and the pouring temperature. In another aspect, the melt filling ability rises with the decrement of the melt viscosity, and the melt with the better filling ability is prone to fill the cylinder cavity layer by layer.

Computer simulation for centrifugal mold filling of precision titanium castings

Computer simulation codes were developed based on a proposed mathematical model for centrifugal mold filling processes and previous computer software for 3D mold filling and solidification of castings (CASM-3D for Windows). Sample simulations were implemented for mold filling processes of precision titanium castings under gravity and different centrifugal casting techniques. The computation results show that the alloy melt has a much stronger mold filling ability for thin section castings under a centrifugal force field than that only under the gravity. A 'return back' mold filling manner is showed to be a reasonable technique for centrifugal casting processes, especially for thin section precision castings.

基于ProCAST软件的发动机连杆铸造仿真及优化

借助ProCAST软件,对发动机连杆用钛铝合金真空吸铸过程进行仿真分析。探究了浇注温度与速度对铸件充型过程与凝固特性的影响;并获得了钛铝合金连杆吸铸工艺参数的优化图。结果表明,合理提升浇注温度与钛铝合金液的浇注速度,均有利于铸件充型。

Mechanical Properties of V-,Nb-,and Ti-bearing As-cast Microalloyed Steels

The influence of microalloying additions on the mechanical properties of a low-carbon cast steel containing combinations of V, Nb, and Ti in the as-cast condition was evaluated. Tensile and hardness test results indicated that good combinations of strength and ductility could be achieved by V and Nb additions. While the yield strength and UTS （ultimate tensile strength） increased up to the range of 378-435 MPa and 579- 590 MPa, respectively in the microalloyed heats, their total elongation ranged from 18% to 23%. The presence of Ti, however, led to some reduction in the strength. Microstructural studies including scanning electron microscopy （SEM） and optical microscopy revealed that coarse TiN particles were responsible for this behavior. The Charpy impact values of all compositions indicated that microalloying additions significantly decreased the impact energy and led to the dominance of cleavage facets on the fracture surfaces. It seems that the increase in the hardness of coarse ferrite grains due to the precipitation hardening is the main reason for brittle fracture.

Elimination of misrun and gas hole defects of investment casting TiAl alloy turbocharger based on numerical simulation and experimental study

Casting technology of thin-wall TiAl alloy turbochargers was studied by investment casting and numerical simulation.Misruns and gas holes were the main defects observed in preliminary work due to the poor fluidity of alloy,and to gas entrapment.In order to eliminate these defects,cast parameters,such as centrifugal rotation rate and mould preheating temperature,were optimized by numerical simulation,meanwhile,the structure of the shell mould was optimized to improve the filling capacity of TiAl alloy.Pouring experiments were carried out by vacuum induction melting furnace equipped with a water-cooled copper crucible based on the above optimization.The quality of the TiAl alloy casting was analyzed by fluorescent penetrant inspection and X-ray detection.The results show that a centrifugal rotation rate of 200 rpm,mould preheating temperature of 600°C,shell preparation through organic fiber addition can dramatically improve the mould filling capacity,and integrated turbochargers were finally prepared.

基于ProCAST模拟的钛合金环形铸件缩孔预测

使用ProCAST软件对钛合金环形铸件铸造过程中的流场、温度场及缩孔分布进行了数值模拟。为验证模拟结果,根据模拟模型设计并开展了浇注试验。模拟结果显示,金属液流动过程平稳,铸件成形完整,缩孔呈条形分布在铸件中部的厚大环形热节区域。模拟和试验的缩孔结果吻合很好,说明数值模拟能准确预测钛合金铸件的缩孔分布,从而为工艺设计提供指导。

Microstructure and interface reaction of investment casting TiAl alloys

In order to research the microstructure of TiAl alloy and TiAl-mould reaction between TiAl and ceramic mould shells prepared with the low cost binder in investment casting, the ceramic mould shells were prepared with low cost binder and refractory materials. Using two kinds of casting methods (gravity casting and centrifugal casting), the titanium aluminum alloys with rare earth element (Ti-47.5Al-2Cr-2Nb-0.3Y and Ti-45Al-5Nb-0.3Y) were cast into the mould shells. The microstructures of investment casting titanium aluminum alloys were observed by optical microscope (OM). The distributions of elements of topping investment on the surfaces of titanium aluminum alloys castings were analyzed by the means of electron probe micro-analysis (EPMA), and the mechanical properties were studied. The results show that the microstructures of two kinds of titanium aluminum alloys are both lamella shape, and lamella is thin. The thickness of reaction and diffusing layer of Ti-47.5Al-2Cr-2Nb-0.3Y alloy is about 80μm, and that of Ti-45Al-5Nb-0.3Y is less than 30μm.

Numerical simulation and experimental investigation of two filling methods in vertical centrifugal casting

A mathematical model of the centrifugal filling process was established. The calculated results show that the centrifugal field has an important influence on the filling process. Moreover, the process of liquid flow and the location of free surface in sprue were simulated based on the Solution Algorithm-Volume of Fraction (SOLA-VOF) technique. In order to verify the mathematical model and computational results, hydraulic simulation experiment was carried out. The results of experiments and numerical simulation indicate the accuracy of mathematical model. Two kinds of filling methods were investigated and the results show that the bottom filling is better than the top filling that can achieve stable filling and reduce defects.

Reaction procedure of a graphite fiber reinforced Ti-Al composite produced by squeeze casting-in situ reaction

The in situ reaction procedure and microstructure evolution of a graphite fiber reinforced Ti-Al composite （Grf/Ti-Al） was investigated, and the stability of TiAl3 at high temperature was discussed. As-cast material was prepared by pressing molten pure aluminum into a preform, which was composed of titanium particles and graphite fibers. The in situ reaction procedure of the as-cast material was investigated by differential scanning calorimetry （DSC）, and phases in the products were detected by X-ray diffraction （XRD）. Experimental results showed that TiAl3 was formed first. With an increase in temperature, TiC and Al4C3 were observed, but TiAl3 decreased. In the final product, Al2O3 and TiO2 were observed. It was considered that the previous forming TiAl3 decomposed, then TiC precipitated, and subsequently, oxidation resulted in the formation of Al2O3 and TiO2.

A MOLD MATERIAL USED FOR TITANIUM-ALUMINUM ALLOY-CALCIA

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Microstructure and mechanical properties of as-cast Ti-N alloy

In-situ nitride reinforced Ti matrix composites with 0.045%-0.27%N（mass fraction） were produced by a casting process and the microstructure and the compressive mechanical properties of the as-cast composites were investigated in detail. The results show that the addition of N has significantly changed the microstructure of the as-cast Ti-N alloy. When N content ranges from 0.045% to 0.18%, the reinforcement is TiN 0.3 with mainly fine dendrite and small amount of thin flake structure and linear distribution. When the N content is increased to 0.225%, the reinforcement is changed from TiN 0.3 to Ti2N. For all kinds of the composites, the hardness, compressive strength and elastic modulus are higher than those of matrix pure Ti. In addition, the compressive strength, hardness and elastic modulus of the composites increase with the increase of Ti content. The fracture surface analysis results show that the composites fracture in a cleavage brittle model.

Microstructure and high-temperature wear properties of in situ TiC composite coatings by plasma transferred arc surface alloying on gray cast iron

In this work, an in situ synthesized TiC-reinforced metal matrix composite （MMC） coating of approximately 350-400μm thickness was fabricated on a gray cast iron （GCI） substrate by plasma transferred arc （PTA） surface alloying of Ti-Fe alloy powder. Microhard- ness tests showed that the surface hardness increased approximately four-fold after the alloying treatment. The microstructure of the MMC coating was mainly composed of residual austenite, acicular martensite, and eutectic ledeburite. Scanning electron microscopy （SEM） and X-ray diffraction analyzes revealed that the in situ TiC particles, which were formed by direct reaction of Ti with carbon originally contained in the GCI, was uniformly distributed at the boundary of residual anstenite in the alloying zone. Pin-on-disc high-temperature wear tests were performed on samples both with and without the MMC coating at room temperature and at elevated temperatures （473 K and 623 K）, and the wear behavior and mechanism were investigated. The results showed that, after the PTA alloying treatment, the wear resistance of the sam- ples improved significantly. On the basis of our analysis of the composite coatings by optical microscopy, SEM with energy-dispersive X-ray spectroscopy, and microhardness measurements, we attributed this improvement of wear resistance to the transformation of the microstruc- ture and to the presence of TiC particles.

立式离心铸造钛合金薄壁复杂铸件宏观偏析研究

立式离心铸造过程中,由于离心压力在铸件不同半径处有梯度性,会形成严重的宏观偏析。以立式离心铸造TC4钛合金薄壁复杂铸件为研究对象,阐明铸型转速、离心半径及壁厚对铸件宏观偏析的影响规律。结果表明,提高铸型转速会加重钛合金铸件的宏观偏析。当离心半径较小时,钛合金铸件中钒和铝的含量较高;当离心半径较大时,钒和铝的含量较低,形成负偏析。钛元素含量的分布正好与此相反,形成正偏析。离心半径为71 mm时,铸件薄壁部位钛、铝、钒含量为88.1%、8.1%和4.1%;离心半径增加到143 mm时,钛、铝、钒含量为89.9%、7.7%和3.6%(质量分数)。在铸件厚壁部位,其冷却速度比薄壁部位大,合金元素的升高或降低趋势更明显,偏析更严重。整体而言,该薄壁复杂钛合金铸件的偏析不严重。