Crystallization of titanium ingots in the course of electron-beam melting

Using mathematical models of the heat transfer processes in the course of ingot formation by electron-beam remelting, we determined the temperature conditions and the corresponding technological parameters under which a homogeneous structure without coarse columnar crystallites is reached in ingots of round and rectangular cross sections. A technology and equipment were developed for the production of large-scale ingots of titanium alloys up to 1200 mm in diameter and 4.5 m in length directly from uncrushed cakes of spongy titanium. The quality of ingots of both commercially pure and alloyed titanium meets the requirements of the ASTM B348-00 standard. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 21–27, May–June, 2008.     

低成本钛合金研究现状与发展趋势

钛合金较高的成本限制了其应用范围,低成本钛合金已成为当前钛合金研究的热点。概述了低成本钛合金国内外的研究现状,着重介绍了宝钛集团研发的低成本钛合金及钛合金的低成本化制备情况,新近研制的BTi31、Ti-3111、BTi-341、BTi-421111、BTi-61111S等低成本钛合金已在汽车、体育休闲等领域获得应用;在钛合金的低成本化制造方面,利用电子束冷床炉熔炼,大量添加返回料,熔炼的扁锭直接轧制板材,显著降低原料成本和加工成本。同时对低成本钛合金的发展方向进行展望,指出今后的研究重点是进一步优化熔炼和加工工艺,提升合金性能水平,提高成材率,拓展应用范围。     

低成本钛合金研究现状与发展趋势

钛合金较高的成本限制了其应用范围,低成本钛合金已成为当前钛合金研究的热点.概述了低成本钛合金国内外的研究现状,着重介绍了宝钛集团研发的低成本钛合金及钛合金的低成本化制备情况,新近研制的BTi31、Ti-3111、BTi-341、BTi-421111、BTi-61111S等低成本钛合金已在汽车、体育休闲等领域获得应用;在钛合金的低成本化制造方面,利用电子束冷床炉熔炼,大量添加返回料,熔炼的扁锭直接轧制板材,显著降低原料成本和加工成本.同时对低成本钛合金的发展方向进行展望,指出今后的研究重点是进一步优化熔炼和加工工艺,提升合金性能水平,提高成材率,拓展应用范围.     

电子束冷炉床熔炼(EBCHM)技术的发展与应用

本文系统总结了电子束冷炉床熔炼(Electron Beam Cold Hearth Melting-EBCHM)技术近四十年的发展脉络。详细介绍了电子束冷床炉熔炼的各主要方面,包括电子枪的改进,冷床炉结构演变和功能扩展以及消除钛合金冶金缺陷的独特优势、电子束冷床炉熔炼科学技术应用现状及当前的主要研究方向等。指出近年来EBCHM技术在钛及钛合金工业生产中的应用持续稳定增长,除了钛的废料回收,消除钛的冶金缺陷提高合金质量,满足航空发动机转动部件要求,是主要的推动因素。可为广大钛冶金、材料应用工作者和设计人员提供有价值的参考。     

TC4钛合金EB铸坯板材轧制组织及性能研究

通过工艺试验,金相组织分析和力学性能测试,对轧制过程中组织演变及成品性能进行了研究,验证了低成本EB锭钛合金加工可获得标准要求的高性能TC4钛合金板材.使用TC4钛合金EB铸坯制备出质量符合GB/T3621-2007《钛及钛合金板材》标准要求的4mm厚度规格热轧板.     

Modeling of the mechanical treatment of a solid reactant under active gas in the high-energy mill on the example of the titanium-gaseous nitrogen system

A mathematical model for calculating the dynamics of product formation (titanium nitride) in the gas (nitrogen) medium during milling and activation of a solid reactant in the high-energy mill was built based on a new macrokinetic theory of mechanochemical synthesis. For the first time, the model uses an approach based on the theory of short-lived active sites to take into account the intensification of chemical processes in the volume of the solid and on the surface formed during destruction. The model can calculate the following macroscopic parameters: temperature, depth of chemical transformations, size of particles, gas pressure, structural defects of particles (excess energy). An analytical formula for determining the mechanochemical reaction on an active surface was derived. The modelling results were compared with the known experimental data.     

Stochastic modeling of columnar-to-equiaxed transition in Ti–(45–48 at%) Al alloy ingots

A two-dimensional (2-D) stochastic model of macro–micro type for predicting the columnar-to-equiaxed transition (CET) during solidification processes of the large size Ti–(45–48 at%) Al alloy ingots is developed in this paper. The macroscopic part is based on a finite differential method (FDM) for modeling of heat transfer. The microscopic part consists of a cellular automaton method (CA) for modeling of nucleation and growth. The formation of a shrinkage cavity at the top of ingot is taken into account. The effects of casting variables on CET are presented and discussed. A quantitative relation between CET position and casting variables is obtained. The columnar zone is found to expand with decreasing alloy composition, mold-preheated temperature and convection, and increasing the thermal conductivity of mold, superheat and heat transfer coefficient.     

钛合金功能梯度构件渐变组织主动调控的新思路

为主动调控高温钛合金大型功能梯度构件的渐变组织,提出了局部控温局部加载成形的新思路。基于实现组织的渐变过渡以避免不同组织之间的"弱连接"这一难题,在钛合金功能梯度构件的成形制造过程中,通过对钛合金组织-性能关系与其功能梯度构件成形方法的国内外的研究现状进行分析,拟协同控制温度梯度分布和材料的定向流动,在保证高性能、低成本制造的基础上,实现高温钛合金大型功能梯度构件的柔性成形。     

Influence of crystal—melt interface shape on self-seeding and single crystalline quality

The growth of Sb-based crystals (InSb, GaSb etc) was undertaken using resistive heater furnace by vertical directional solidification (VDS) technique. Crystal—melt interface shape during the growth was shown to convert from concave to convex along the crystal axis of the ingots. Many antimonide (Sb) crystals of 8 mm to 18 mm diameter were grown by optimized growth parameters. The forced convection and absence of conducting support to ampoule showed improvement in crystal quality of as grown ingots. Crystals showed preferred orientation and self-seeding. Results on interface shape and crystallinity of ingots were found to be in good agreement with the experiments.     

等离子旋转电极雾化法制备高品质Ti-6.5Al-1.4Si-2Zr-0.5Mo-2Sn合金粉末

旨在制备高品质Ti-6.5Al-1.4Si-2Zr-0.5Mo-2Sn粉末,为后续粉末高温钛合金构件的制备奠定基础。首先采用真空自耗电弧熔炼(VAR)技术制备Ti-6.5Al-1.4Si-2Zr-0.5Mo-2Sn合金铸锭,对铸锭进行化学成分检测,并分析其合金元素损耗、成分均匀性以及显微组织和物相组成。利用制得棒料,采用等离子旋转电极雾化法(PREP),选取不同转速制备得到钛合金粉末,将粉末筛分成不同粒度范围。研究了棒料转速与粉末理化性能间的关系。采用X射线衍射分析仪(XRD)、扫描电镜(SEM)、金相显微镜(OM)分别分析了粉末的物相组成、形貌和微观组织。研究表明:通过独特的压制电极设计,可制得成分均匀、元素损耗小的钛合金铸锭,且各合金元素含量满足国标的要求。铸锭微观组织为层片状结构,基体中存在少量大小不均的Ti5Si3硅化物相。PREP法制得的钛合金粉末呈正态分布,且球形度好,无空心球和卫星球。随着转速增加,小颗粒粉末占比增加,大颗粒粉末占比大幅度降低。粉末颗粒以胞状组织为主,存在少量的枝晶。合金粉末主要由α′马氏体相组成。相比合金铸锭,粉末中各合金元素略有损耗,O元素质量分数小于0.1%,有利于制得高性能的粉末钛合金。     

Development of highly porous titanium scaffolds by selective laser melting

The selective laser melting (SLM) of the TiH₂-Ti blended powder was performed in the present work. Porous titanium scaffolds characterized by high porosity (∼ 70%), interconnected Ti walls and open porous structures with macroscopic pores (in a range of ∼ 200 to ∼ 500 μm) were successfully prepared at a laser power of 1000 W and a scan speed of 0.02 m/s. The effects of componential and processing conditions in terms of TiH₂ content and scan speed on the microstructural development of porous titanium (porosity and pores size) were investigated. Reasonable mechanisms for pores formation during SLM apart from microstructural evolutions were proposed.     

Microstructural modelling of dynamic recrystallisation using an extended cellular automaton approach

Dynamic recrystallisation (DRX) governs the plastic flow behaviour and the final microstructure of many crystalline materials during thermomechanical processing. Understanding the recrystallisation process is the key to linking dislocation activities at the mesoscopic scale to mechanical properties at the macroscopic scale. A modelling methodology coupling fundamental metallurgical principles with the cellular automaton (CA) technique is here derived to simulate the dynamic recrystallisation process. Experimental findings of a titanium alloy are considered for comparison with theory. The model takes into account practical experimental parameters and predicts the nucleation and the growth kinetics of dynamically recrystallised grains. Hence it can simulate different stages of microstructural evolution during thermomechanical processing. The effects of hot working temperature and strain rate on microstructure were studied, and the results compared with experimental findings.     

Influence of mould design on the solidification of heavy forging ingots of low alloy steels by numerical simulation

Ingot casting of a 6-ton, heat-treatable Cr–Mo low alloy steel was simulated using finite element method in three dimensions. Effects of casting parameters including bottom pouring rate, mould slenderness ratio, mould slope, and height and shape of the hot top isolate on solidification behaviour and crack susceptibility during subsequent hot forging of the ingot were investigated. The simulation model was validated against experimental data of two different ingot mould designs. Influences of the casting parameters on the riser efficiency and possible crack formation in the intersection of hot top and ingot body during subsequent open-die forging of the cast steel ingots were discussed. Results showed that pouring the melt under a constant rate, reducing the mould slenderness ratio, and using a proper design for the hot top isolate would all improve the riser efficiency and thereby possibly reduce crack susceptibility during subsequent hot forging.     

Ti-V-Cr系阻燃钛合金厚板组织与力学性能对比研究

研究了WSTi3515S合金和TB12合金的大型铸锭成分和组织均匀性,对比分析了两种Ti-V-Cr系阻燃钛合金厚板的力学性能。结果表明:采用真空自耗电弧熔炼技术制备的Ti-V-Cr系阻燃钛合金3t级大型铸锭成分均匀;利用大型挤压机和大型快锻机制备的阻燃钛合金大尺寸厚板组织和性能均匀;两种合金的室温拉伸、硬度、冲击和540℃高温拉伸等性能相当,WSTi3515S合金在540℃条件下的热强性能较好;V和C元素有利于提高合金的热强性能,且C元素还能细化组织。     

Influence of substrate properties on the growth of titanium films: part III

The growth of thin Ti-oxide films (12 nm) on alumina substrate films formed by reactive evaporation of Ti in an oxygen atmosphere was studied by in situ internal stress measurements under ultra high vacuum conditions and transmission electron microscopy. Oxygen pressure and substrate temperatures were the varied parameters of the reactive evaporation. These Ti-oxide-films with different oxygen content (O₂/Ti-films) were then used as substrate films for the deposition of a clean titanium film. The growth stress of the titanium film on the as-deposited O₂/Ti-substrate films is comparable with that previously found for H₂O/Ti-substrates and indicates island growth and the formation of polycrystalline titanium films. Annealing (400°C, 20 min) of the as-deposited – amorphous – O₂/Ti-films gives rise to the formation of crystalline TiO₂. The amount of TiO₂ formed during annealing is strongly dependent on the oxygen content of the O₂/Ti-film. The oxygen content, in return, is dependent on oxygen partial pressure and substrate temperature during O₂/Ti-film deposition. The corresponding changes in the substrate film properties (oxygen content, crystallinity, etc.) are reflected in significant changes in the growth stress of the titanium film. The stress vs. thickness curve of these titanium films appears to indicate a superposition of the growth stress of two different growth modes, i.e. growth of a polycrystalline film with island growth on the as-deposited, amorphous oxide substrate and epitaxial growth of a quasi single crystalline film on the crystalline TiO₂-substrate.     

Microstructural characteristics of TiC and (TiW)C iron matrix composites

The present work investigated microstructures of in situ synthesized 10 vol% TiC-Fe and 10 vol% (TiW)C-Fe composites. The results show that in the TiC-Fe composite, as only second phase, TiC is in two kinds of morphologies, i.e., spherulic and rod-like ones. It is thought that the spherulic TiC is a proeutectic phase and the rod-like one is a eutectic phase. In situ synthesis of the 10 vol% (TiW)C-Fe composite in liquid iron is feasible. In the composite, the (TiW)C reinforcements as only one second phase are more homogeneously distributed in iron matrix, and most of them are spherulic and the few rod-like. Within the spherulic (TiW)C phase, there is an inhomogeneous distribution of titanium and tungsten. Its core is rich in titanium, while the periphery rich in tungsten. Such the characteristic microstructure is closely related with its formation during solidification. Comparing with the TiC, the (TiW)C phase has a density matchable for iron melt, which makes it more suitable as in situ synthesized reinforcements in large size ingots of iron matrix composites.     

Numerical simulation of the casting process of titanium removable partial denture frameworks

The objective of this work was to study the filling incompleteness and porosity defects in titanium removal partial denture frameworks by means of numerical simulation. Two frameworks, one for lower jaw and one for upper jaw, were chosen according to dentists recommendation to be simulated. Geometry of the frameworks were laser-digitized and converted into a simulation software (MAGMASOFT). Both mold filling and solidification of the castings with different sprue designs (e.g. tree, ball, and runner-bar) were numerically calculated. The shrinkage porosity was quantitatively predicted by a feeding criterion, the potential filling defect and gas pore sensitivity were estimated based on the filling and solidification results. A satisfactory sprue design with process parameters was finally recommended for real casting trials (four replica for each frameworks). All the frameworks were successfully cast. Through X-ray radiographic inspections it was found that all the castings were acceptably sound except for only one case in which gas bubbles were detected in the grasp region of the frame. It is concluded that numerical simulation aids to achieve understanding of the casting process and defect formation in titanium frameworks, hence to minimize the risk of producing defect casting by improving the sprue design and process parameters.     

Comparative study on the densification process of different titanium powders

In the powder metallurgy of titanium and titanium alloys, titanium powders produced through hydrogenation/dehydrogenation (HDH) approach and titanium hydride powder are extensively used. The choice of initial powder greatly influences the properties and performance of as-sintered materials. In the present study, comparative experiments were performed on two powders of various sizes to elucidate the peculiarities of their densification process and the characteristics (as-sintered density, impurity content, and tensile properties) of the processed materials. As expected, the sintering performance of both powder-type compacts were greatly affected by the specific surface and contact areas, so finer powders and higher compaction pressures were used to achieve higher densities upon sintering. However, the systematic results clearly indicated the advantage of using titanium hydride powder as a starting material in titanium powder metallurgy. At equal size, compaction, and sintering parameters, materials processed from titanium hydride powder had higher density and lower impurity content, thereby providing better balance of tensile properties compared with materials processed from HDH titanium powder. This advantage is explained by the higher relative density of green compacts made of brittle titanium hydride powder and by the higher sintering ability of such compacts activated by powder-released hydrogen.     

Formation of Porous Structures in Metal–Hydrogen Systems

Crystallization of hydrogen-saturated melts under controlled conditions enables one to obtain a new class of porous materials (gasars). The technological parameters of the process such as the partial hydrogen pressure, the total gas pressure, the temperature of the melt, and the crystallization rate affect the porosity, size and form of pores, which enables one to control the structure of gasars. Using methods of quantitative metallography, we determine conditions of its formation. The most homogeneous nickel ingots of gasars are obtained with increase in the total gas pressure in the system during crystallization of the melt and the decrease in the partial hydrogen pressure. The gas gap between the chill and the ingot appearing in the process of directional crystallization of hydrogen-saturated melts limits heat exchange and leads to a disturbance of the cooperative growth of gas bubbles and crystals. However, the negative influence of this process considerably decreases with increase in the partial hydrogen pressure, which also favors the production of homogeneous ingots of gasars.     

钛合金/钢异种连接接头组织与性能研究进展

本文对钛合金/钢的异种接头连接技术的研究现状进行了总结和综述，分析了钛合金/钢直接连接和含中间层连接界面的显微组织特征，重点阐述了添加不同中间层(铜、铜基合金及其他)的钛合金/钢界面产物的形成和演变过程，并归纳了不同中间层与制备工艺的钛合金/钢力学性能，最后总结了制备良好的钛合金/钢接头可以采取的方法与设计思路，并指出该领域未来发展除使用传统方法继续深化现有研究外，可与模拟仿真结合，达成更具深度的认识与实验预测。