γ—TiAl增压涡轮熔模铸造过程数值模拟研究

根据熔模型壳离心浇注的铸造工艺特点，建立了γ－TiAl增压涡轮凝固传热过程的数值模型，推导了离心压力下γ－TiAl金属间化合物的凝固收缩和补缩过程数学模型，模拟计算了γ－TiAl增压涡轮铸件的温度场和收缩缺陷，结果表明，模型能可靠计算γ－TiAl增压涡轮铸件凝固过程的温度分布和准确预测铸件的收缩缺陷，数值模拟结果与实验结果吻合良好，数值模拟结果证实了前期工作所提出的进一步减少及消除收缩缺陷的优化工艺措施的合理性。     

γ—TiAl增压涡轮近净形铸造过程实验研究

γ－TiAl金属间化合物的成形工艺是材料成形领域的前沿领域和研究热点，本研究采用水冷铜坩埚真空感应凝壳技术和熔模型壳离心浇注的铸造方法制备γ－TiAl增压涡轮，通过改善冒口工艺获得了健全的γ－TiAl涡轮铸件，分析了γ－TiAl增压涡轮的凝固过程和收缩缺陷产生原因，结果表明，增大冒口与然件的模数之差以及冒口与铸件的体积之比有利于减少及消除涡轮铸件的收缩缺陷，为给数值模拟研究提供热物性参数，针对涡轮铸件用钛铝合金，实验测试了合金的热膨胀系数，比热和热导率等物性参数，其与温度的关系分别为：α1=8.10651 0.0073T-2.97619E-6T^2,Cp=668.28158-0.013T 1.11905E-4T^2;λ=19.82252-0.02781T 6.5197E-5T^2-3.21096E-8T^3.     

TiAl合金涡轮压缩损伤与拉伸断裂行为

为研究TiAl合金涡轮经压力作用后的组织损伤机制及性能弱化规律,设计了对TiAl合金涡轮先压缩再拉伸的实验方法。利用扫描电镜(SEM)对压缩后的涡轮轴颈表面及内部的滑移和微裂纹进行了分析,并观察了拉伸断口形貌。实验结果表明：随着前期涡轮所受的压力的增大,压缩后的TiAl涡轮剩余抗拉强度逐渐降低,当压力为610 MPa时,剩余抗拉强度仅为86 MPa,强度损失率高达70%。TiAl合金压缩过程中形成了以沿层裂纹为主、穿层裂纹为辅的变形损伤特征。与压缩轴成45°的最大剪应力方向上的沿层裂纹是TiAl合金压缩损伤的主要形式。压缩损伤后的TiAl合金涡轮拉伸断裂均发生在靠近涡轮浇铸冒口侧的细轴颈部位。受压变形后的片层组织中的微小裂纹在随后拉应力作用下继续扩展直至韧带桥被贯穿,小裂纹合并成大裂纹,在断口上表现出沿层和穿层的混合断裂形貌。     

TiAl合金增压涡轮断裂分析

某TiAl合金增压器涡轮在超速试验转速达8.6×104r/min时发生断裂失效。通过对失效的TiAl合金增压涡轮及涡轮叶片断口进行宏、微观观察及分析,以确定其失效原因。结果表明：涡轮和涡轮叶片断口的断裂特征主要由沿层断裂及穿层断裂组成;涡轮断裂是从增压器涡轮和涡轮轴颈相连接的圆弧过渡区域处的铸造疏松起源并发生断裂;涡轮断裂失效与层取向、铸造缺陷、圆弧过渡及离心力有关。     

TiAl基合金涡轮熔模型壳离心精密铸造

对TiAl合金涡轮的立式离心熔模陶瓷型壳精密铸造进行了研究，合金熔配设备为水冷铜坩埚真空感应熔化炉。首先研究了TiAl合金铸造时的尺寸收缩问题，结果表明1600℃浇注时，合金的收缩率为1．32％，随着浇注温度的提高凝固收缩率显著提高。从合金熔体立式离心力场下充型模式方面考虑，涡轮浇注工艺应采用类似于重力充型的底注式较好。TiAl合金涡轮离心浇注内部组织致密、无气孔，存在的主要问题是欠浇缺陷，迎流面充型较弱。可以从提高离心转速和浇注温度两方面解决欠浇缺陷，但离心转速提高带来的安全隐患以及提高浇注温度带来的粘砂问题还需进一步研究。     

具有优良高温性能的涡轮用TiAl合金——DAT-TA3的开发

日本大同特殊钢公司自1994年于世界上首次用TiAl合金制造了商用汽车涡轮增压器的涡轮以来,已经生产了超过12000个TiAl合金涡轮。但随着燃耗要求的不断提高,汽车涡轮发动机必须承受更高的排气温度,特别是承受直接排气的涡轮及壳体的耐用温度要求高达1500℃,因此又开发了具有更高耐热性的柴油发动机及低温汽油发动机用的DAT-TA1合金和高温汽油发动机用的DAT-TA2合金。     

TiAl基合金在汽车发动机上的应用

以金属间化合物TiAl为基的合金密度只有铁基或镍基高温合金的一半，且高温比强度高，作为轻质耐热材料非常引人注目。在飞机和汽车发动机中都期待着TiAl基合金的实用化，以降低燃料费，减少排放量。但是，实际上，由于TiAl基合金室温塑性低，高温强度、抗氧化性不足，制造困难等，使实用化相当困难。日本大同特殊钢公司自1985年开展TiAl基合金的实用化研究以来，开发了TiAl基合金近净形加工的精密铸造和接合等相关技术，于1998年实现了TiAl基合金的赛车发动机增压器涡轮的实用化。 1 合金开发 要进行TiAl基合金的实用化，必须要改…     

TiAl合金与异种材料的摩擦焊接

TiAl合金与42CrMo直接摩擦焊接性较差,为此分别引入高温合金GH3039、K418、N80A和纯镍N6作为中间材料,对TiAl-GH3039/K418/N80A/N6-42CrMo异种材料的摩擦焊接工艺进行了研究。采用硬度计、扫描电镜和电子万能试验机对焊后接头区域的硬度、组织和焊合区成分变化以及接头力学性能进行了分析。研究表明,TiAl合金与异种材料焊后接头中形成了复杂的多层状金属间化合物;TiAl合金与GH3039、N80A的摩擦焊接性较好,与K418、N6的摩擦焊接性较差;根据不同材料线膨胀系数随温度的变化规律、与TiAl合金摩擦焊接后接头的性能及其与42CrMo的摩擦焊接性,最终选择GH3039作为中间材料。通过引入中间材料,摩擦焊制备了TiAl合金涡轮-42CrMo转轴的异种材料整体转子,使得TiAl合金在涡轮增压器领域的应用成为可能。     

复合阻隔法的阻隔效应原理及其在扩散连接中的应用

以TiAl金属间化合物增压涡轮与 4 0Cr钢轴的扩散连接为背景 ,提出了复合阻隔法扩散连接工艺 ,并探讨了阻隔效应原理 ,建立了从材料的扩散连接性角度出发的原子半径、原子电负性阻隔层选择原则。利用本文的扩散连接阻隔效应原理 ,确定了TiAl金属间化合物增压涡轮与 4 0Cr钢轴的扩散连接复合阻隔层为Ti/V/Cu ,由此得到的扩散连接接头在V/Cu及Cu/ 4 0Cr的连接界面处出现了对连接性能有利的无限固溶体层 ,在TiAl/Ti的接触面上生成了能够强化接头强度的Ti3 Al TiAl双相层和Ti的固溶体层 ,与TiAl/ 4 0Cr直接扩散连接相比 ,Ti/V/Cu复合阻隔层的加入 ,避免了在TiAl/4 0Cr的接触面上TiC、Ti3 Al、FeAl、FeAl2 金属间化合物脆性相的产生 ,接头强度高达4 2 0MPa ,因此利用本文的阻隔效应原理可以很好地进行复合阻隔层的选择     

TiAl合金铸造工艺数值模拟及分析

利用数值模拟方法对比研究了重力铸造、离心铸造和真空吸铸等工艺铸造TiAl合金涡轮铸件的工艺过程,分析了TiAl合金熔体在型腔中的充填规律及凝固特性.结果表明,传统的重力铸造及离心铸造容易产生浇不足、缩松、缩孔等缺陷,真空吸铸技术能大幅提高钛合全熔体的充填能力,且充填过程十分平稳,凝固过程中易于补缩,是小型TiAl合金铸件理想的成型方法.     

Assessment of Titanium Aluminide Alloys for High-Temperature Nuclear Structural Applications

Titanium aluminide (TiAl) alloys exhibit high specific strength, low density, good oxidation, corrosion, and creep resistance at elevated temperatures, making them good candidate materials for aerospace and automotive applications. TiAl alloys also show excellent radiation resistance and low neutron activation, and they can be developed to have various microstructures, allowing different combinations of properties for various extreme environments. Hence, TiAl alloys may be used in advanced nuclear systems as high-temperature structural materials. Moreover, TiAl alloys are good materials to be used for fundamental studies on microstructural effects on irradiation behavior of advanced nuclear structural materials. This article reviews the microstructure, creep, radiation, and oxidation properties of TiAl alloys in comparison with other nuclear structural materials to assess the potential of TiAl alloys as candidate structural materials for future nuclear applications.     

TiAl金属间化合物及其连接技术的研究进展

近年来 ,TiAl金属间化合物由于其密度小、硬度大、耐高温、具有优良的抗氧化能力等独特的性能 ,因此越来越引起广泛的关注并得到了迅猛的发展。根据目前TiAl金属间化合物被认为是在航空、航天飞行器等军事和民用两者都具有广泛应用前景的高温结构材料 ,文中介绍了世界范围内TiAl金属间化合物研究发展现状。TiAl金属间化合物有效的运用必须要有可靠的连接技术 ,因此TiAl金属间化合物的连接问题是其实用化所要面临的问题之一。固态焊接是实现TiAl金属间化合物连接十分有效的方法。文中介绍了TiAl金属间化合物连接技术的发展现状 ,重点评述了TiAl金属间化合物固态焊接的研究状况 ,指出了需要深入研究的问题     

TiAl金属间化合物及其连接技术的研究进展

近年来 ,TiAl金属间化合物由于其密度小、硬度大、耐高温、具有优良的抗氧化能力等独特的性能 ,因此越来越引起广泛的关注并得到了迅猛的发展。根据目前TiAl金属间化合物被认为是在航空、航天飞行器等军事和民用两者都具有广泛应用前景的高温结构材料 ,文中介绍了世界范围内TiAl金属间化合物研究发展现状。TiAl金属间化合物有效的运用必须要有可靠的连接技术 ,因此TiAl金属间化合物的连接问题是其实用化所要面临的问题之一。固态焊接是实现TiAl金属间化合物连接十分有效的方法。文中介绍了TiAl金属间化合物连接技术的发展现状 ,重点评述了TiAl金属间化合物固态焊接的研究状况 ,指出了需要深入研究的问题     

Modeling of titanium aluminides turbo-charger casting

The castability of titanium aluminide (TiAl) turbo-charger was investigated with various casting parameters. The fluidity of TiAl alloys increased with mold preheating temperature and centrifugal force. The thermo-physical variables estimated by casting process were applied to the modeling of TiAl turbo-charger casting using the Magmasoft^®. In the case of real casting process, there were many misruns and cold-shuts due to low fluidity of molten TiAl alloy. However, in the modeling, molten TiAl alloys never stop even though the turbo-charger has both the curvature and thin wall parts. Therefore, there were no major defects such as misruns and cold-shuts in the modeling process. The gap between casting and modeling remains in TiAl alloy turbo-charger casting since the modeling process cannot explain the microstructural evolution during solidification and metal–mold reaction between mold materials and TiAl alloy.     

Economic net-shape forming of TiAl alloys for automotive parts

Alpha-case formations between investment molds and TiAl alloys were investigated for the economic TiAl alloys net-shape forming. In the case of TiAl alloys, there were no α-case formation reactions. There were neither interstitial nor substitutional α-case formations since TiAl alloys have both negligible solubility of oxygen and low activity in molten states. The fluidity of TiAl alloys is increased with mold preheating temperature since they have a peritectic reaction that appears in the form of envelopment, surrounding each particles of the primary constituent. The results of the investment casting of TiAl alloys confirm that the casting route in our study can be an effective approach for the economic net-shape forming of TiAl alloys.     

提高TiAl基合金室温塑性的方法

TiAl基合金具有密度低、高温性能好等优点，但室温塑性低一直是阻碍TiAl基合金应用的重要原因。本文总结了TiAl基合金的室温塑性的主要影响因素，以及通过添加合金化元素、改善加工工艺等方法来控制显微组织、提高TiAl基金合金的室温塑性的研究进展。     

Microstructure and mechanical properties of TiAl castings produced by zirconia ceramic mould

Owing to their low density and attractive high-temperature properties, gamma titanium aluminide alloys (TiAl alloys, hereafter) have significant potential application in the aerospace and automobile industries, in which these materials may replace the heavier nickel-based superalloys at service temperatures of 600-900℃. Investment casting of TiAl alloys has become the most promising cost-effective technique for the manufacturing of TiAl components. Ceramic moulds are fundamental to fabricating the TiAl casting components. In the present work, ceramic mould with a zirconia primary coat was designed and fabricated successfully. Investment casting of TiAl blades and tensile test of specimens was carried out to verify the correctness and feasibility of the proposed method. The tensile test results indicate that, at room temperature, the tensile strength and the elongation are about 450 MPa and 0.8%, respectively. At 700℃, the tensile strength decreases to about 410 MPa and the elongation increases to 2.7%. Microstructure and mechanical properties of investment cast TiAl alloy are discussed.     

Mechanical properties of TiAl fabricated by electron beam melting—A review

As a typical intermetallic material, TiAl is inevitably difficult to process by conventional methods. Additive manufacturing (AM) has recently become a new option for making net-shape TiAl components. Among all AM methods, electron beam melting (EBM) shows the potential to make TiAl components with good mechanical properties and is used for low pressure turbine blades. The mechanical properties, including tensile and compression properties, fracture toughness, fatigue and creep properties of EBM TiAl are reviewed and compared to the conventionally fabricated alloys. Results show that the tensile strength of EBM alloys is higher than cast alloys, and other properties are comparable to the cast/forged alloys. The sensitivity of mechanical properties and microstructure to EBM processing parameters is presented. Issues including layered microstructure, anisotropy in mechanical properties, and fatigue failure from defects are also reviewed. Finally, some opportunities and challenges of EBM TiAl are identified.     

The effect of Ruthenium addition on the microstructure and mechanical properties of TiAl alloys

The high temperature compression behavior of TiAl–Ru alloys was studied at different temperatures and strain rates. Ru was found to have a strong strengthening effect on TiAl alloys. However the Ru addition amount was limited by its low solubility in γ-TiAl and α₂-Ti₃Al, and the detrimental effect of excessive ternary phase precipitation. Furthermore, the melting temperature decreases when Ru ≥0.6 at.% as the alloy composition approaches a ternary eutectic point. The strengthening mechanism is discussed and two separate mechanisms are proposed, viz. solid solution strengthening and refined colony strengthening. Intergranular cracks were found in the alloys with low Ru or no Ru addition, but were barely detected as Ru content increased to above 0.6 at.%. It was suggested that Ru showed a beneficial effect on both strength and ductility of TiAl alloys due to the refined colony size. Three-point bend test results showed that the Ru addition can also improve the room temperature ductility of TiAl alloys. Hot workability was increased according to the compression tests. Thermal-mechanically treated TiAl–Ru had much smaller grain size than the heat-treated samples due to dynamic recrystallization. But it did not show superior strength in the compression test compared to the heat-treated samples. The Zener–Hollomon parameter was calculated from the compression strength of heat-treated TiAl–Ru alloys. Its relationship with dynamic recrystallization and hot work is discussed. The mechanical properties of TiAl–Ru alloys are compared with TiAl–Nb samples and demonstrate a promising combination of strength and ductility.     

TiAl金属间化合物制备技术的研究进展

TiAl金属间化合物以其低的密度、高的比强度和比模量,具有较好的抗氧化性能以及优异的抗疲劳性能成为一种新型轻质高温结构材料,在航空航天工业和汽车等民用工业领域引起了广泛的关注。本研究着重介绍了TiAl金属间化合物的几种制备方法及其应用,并展望了TiAl金属间化合物的发展前景。