铸造高Nb-TiAl合金成分优化及其精密铸造工艺研究

发布时间：2018-06-16 11:14

本文选题：高Nb-TiAl合金 + 成分优化　；参考：《北京科技大学》2016年博士论文

【摘要】：同Nb-TiAl合金使用温度比普通TiAl合金高60-100℃、强度高约300-500MPa,并且具有优异的高温抗氧化性能,成为TiAl合金发展的方向。高Nb-TiAl合金的不足是其室温塑性低,导致制备加工困难,限制了其工业化应用进程。熔模精密铸造法具有近终成型性,可以生产结构复杂、表面质量优异的铸件,是制备高Nb-TiAl合金部件的重要途径。本论文主要通过合金化的方法设计适合铸造用的性能均衡的高Nb-TiAl合金,研究合金元素对高Nb-TiAl合金组织、力学性能、铸造性能的影响。并通过数值模拟与实际试验相结合的方法,制备出质量优异的低压涡轮叶片铸件。主要结论和成果如下：(1)合金元素对高Nb-TiAl合金组织及力学性能的影响研究结果表明,少量B(0.2%)的加入可以有效细化Ti-45Al-8Nb(除特殊标注外,全为原子分数)合金组织。Ti-45Al-8Nb-0.4B合金具有最佳的室温力学性能。0.5%Cr的添加会提高Ti-45Al-8Nb-0.4B合金的室温力学性能。因为Cr的加入有利于降低合金层错能,增加合金变形孪晶,从而起到提高合金室温力学性能的作用。但是当加入2%Cr时,合金力学性能降低。因为β相稳定元素Cr的加入会使合金铸态组织中脆性B2相的体积分数增加,使合金力学性能恶化。β相稳定元素Mn在高Nb-TiAl合金中的随机占位,会使得其它β相稳定元素(Nb或Cr)在B2相中分布增加,导致B2相体积分数显著增加,对合金力学性能起到不利影响。Si的加入会使Ti-45Al-8Nb-0.4B合金片层团内部及晶界处析出大量硅化物,对合金力学性能起到不利影响。Ni加入后,合金中B2相含量显著增加,导致合金室温力学性能明显下降。Y的加入会使得片层晶界处析出大量白色点状钇化物颗粒,导致晶界脆化,对合金室温力学性能不利。(2)高Nb-TiAl合金加入Si合金化后,组织中析出的硅化物为具有D88结构的Nb5Si3(ε):相,而不是普通TiAl合金中的Ti5Si3相。ε相的形成有利于合金组织中β(B2)相偏析的减少,因为ε相析出会导致基体中β(B2)相稳定元素Nb含量降低,使β(B2)相区减小。β(B2)相偏析减弱,有利于提高合金铸态室温拉伸力学性能。但是,当含Si高Nb-TiAl合金经1000℃以上热处理后,ε相的大量析出会导致合金室温拉伸力学性能恶化。因为ε相与γ相界面为半共格界面,会导致界面位错产生。ε相会导致裂纹沿片层产生与增殖,而且应力会导致硅化物进一步析出,进一步加速裂纹的产生与扩展。γ相稳定元素Si的加入会导致退火处理或循环热处理组织内部ε相附近形成γ相,导致片层粗化或溶解。(3)对高Nb-TiAl合金充型性能研究表明,降低合金液相线温度、减小合金固液相线温度区间、降低合金粘度、提高合金抗界面反应能力等因素有利于提高合金的充型性能。对Ti-45Al-8Nb合金充型性能有利的元素为(0.5%)Si、Y、Cr、B、Ni;无明显影响的为W、Mn、Co;而有害的为Mo或V。最终,综合考虑合金力学性能与铸造性能,优化的铸造合金成分为Ti-45Al-8Nb-(0.5-0.7)Cr-0.5Si-0.4B。Cr的加入可以有利于降低合金层错能,增加合金变形孪晶,从而起到提高合金室温力学性能的作用；Si的加入可以提高合金充型性能,减少合金β(B2)相偏析；B的加入有利于细化组织,提高合金综合力学性能。(4)应用有限元铸造模拟软件Procast模拟熔模铸造工艺过程,优化工艺参数结果表明,最优的重力铸造工艺参数为浇注温度1650℃、浇注速度1 m/s、模壳预热温度400℃。最优的离心转速为500r/min。利用模拟得到的最佳工艺参数进行实际浇注实验,成功得到涡轮叶片铸件。模拟结果与实验结果吻合良好,模拟对实际铸造过程具有很好的指导作用。离心铸造叶片表面及内部质量均优于重力铸造叶片。重力铸造叶片表面有浇不足现象,而离心铸造叶片表面质量良好。重力铸造叶片缩孔缩松缺陷较多且分散,离心铸造叶片缩孔缩松较少。重力铸造叶片表面裂纹较多,离心铸造叶片裂纹较少。(5)优化出的Ti-45Al-8Nb-0.7Cr-0.5Si-0.4B合金形成的整个叶片组织比较均匀细小,组织为近片层组织。重力铸造片层团尺寸约70μm,叶片边缘与叶片中心位置组织没有明显变化,均是β凝固的近片层组织。由于离心铸造叶片冷却速度较大,叶片片层团尺寸较小约为50μm,而且B2相数量较少,表明该铸造合金可能具有优异的综合力学性能。
[Abstract]:The use temperature of the same Nb-TiAl alloy is 60-100 C higher than that of ordinary TiAl alloy, the strength is high about 300-500MPa, and it has excellent oxidation resistance at high temperature. It has become the direction of the development of TiAl alloy. The deficiency of high Nb-TiAl alloy is the low plasticity of the alloy at room temperature, which leads to the difficult preparation process and restricts its industrial application process. The precision casting method of the melt mold has the near final end. It is an important way for the preparation of high Nb-TiAl alloy parts. This paper mainly deals with the design of high performance Nb-TiAl alloys suitable for casting, and studies the influence of alloy elements on the high Nb-TiAl alloy fabric, mechanical properties and casting properties by alloying. The main conclusions and results are as follows: (1) the effect of alloy elements on the microstructure and mechanical properties of high Nb-TiAl alloy shows that a small amount of B (0.2%) can effectively refine the Ti-45Al-8Nb (except for the special annotation, all atom fraction) alloy. The addition of.Ti-45Al-8Nb-0.4B alloy with the best mechanical properties at room temperature.0.5%Cr will increase the mechanical properties of Ti-45Al-8Nb-0.4B alloy at room temperature. Because the addition of Cr is beneficial to reducing the dislocation energy of the alloy layer and increasing the deformation twinning of the alloy, thus improving the mechanical properties of the alloy at room temperature. But when 2%Cr is added, the mechanical properties of the alloy have been added to the alloy. The addition of the beta phase stable element Cr increases the volume fraction of the brittle B2 phase in the alloy cast microstructure and worsens the mechanical properties of the alloy. The random occupying of the beta phase stable element Mn in the high Nb-TiAl alloy will increase the distribution of the other beta stable elements (Nb or Cr) in the B2 phase, resulting in a significant increase in the volume fraction of the B2 phase and the force of the alloy force. The addition of.Si can precipitate a large number of silicides in the inner and grain boundaries of the Ti-45Al-8Nb-0.4B alloy lamellar mass. The mechanical properties of the alloy have adverse effects on the mechanical properties of the alloy. The content of the B2 phase in the alloy increases significantly after the addition of.Ni, resulting in a significant decrease in the mechanical properties of the alloy at room temperature and the addition of.Y will precipitate a large number of white points at the grain boundary. Yttrium particles can cause the grain boundary embrittlement and disadvantageous to the mechanical properties of the alloy at room temperature. (2) after adding Si alloy to high Nb-TiAl alloy, the precipitated silicides in the tissues are Nb5Si3 (E) with D88 structure, but not the Ti5Si3 phase in the ordinary TiAl alloy. The formation of the phase is favorable for the decrease of the segregation of beta (B2) phase in the alloy structure, because the phase of the epsilon phase precipitates. The B2 phase stable element Nb content is reduced and the phase region of the beta (B2) phase decreases. The segregation of beta (B2) phase decreases, which is beneficial to the increase of tensile mechanical properties at room temperature at room temperature. However, when the Si high Nb-TiAl alloy is heat-treated over 1000 C, a large amount of precipitation of the epsilon phase will lead to the deterioration of tensile mechanical properties at room temperature. The semi common interface leads to the formation of interface dislocation. The epsilon phase leads to the formation and proliferation of cracks along the lamellar, and the stress causes further precipitation of silicides, which further accelerates the formation and expansion of the cracks. The addition of gamma phase stable element Si leads to the formation of gamma phase in the annealing treatment or in the inner epsilon phase of the heat treatment group, leading to the coarsening of the layers. (3) the study of the filling properties of high Nb-TiAl alloy shows that reducing the temperature of the liquid phase line, reducing the temperature range of the alloy solid liquid phase, reducing the viscosity of the alloy and improving the interfacial reaction ability of the alloy are beneficial to improve the filling properties of the alloy. The favorable elements for the filling properties of Ti-45Al-8Nb alloys are (0.5%) Si, Y, Cr, B, Ni. The effects are W, Mn, and Co; while harmful to Mo or V., considering the mechanical properties and casting properties of the alloy, the addition of the optimized casting alloy composition to Ti-45Al-8Nb- (0.5-0.7) Cr-0.5Si-0.4B.Cr can help to reduce the dislocation energy of the alloy layer and increase the deformation twinning of the alloy to improve the mechanical properties of the alloy at room temperature; the addition of Si It can improve the filling properties of the alloy and reduce the segregation of alloy beta (B2) phase. The addition of B is beneficial to refining the microstructure and improving the comprehensive mechanical properties of the alloy. (4) the simulation software Procast of the finite element casting simulation software is used to simulate the casting process. The optimum parameters of the process parameters show that the optimum casting process parameters are 1650 centigrade, and the casting speed is 1. M/s, the preheating temperature of the mold shell is 400. The optimum centrifugal speed is 500r/min. using the best process parameters obtained by simulation. The turbine blade casting is successfully obtained. The simulation results are in good agreement with the experimental results. The simulation has good guidance for the actual casting process. The surface and internal quality of the centrifugal casting blade are excellent. In gravity casting, the surface of the blade surface of gravity casting is inadequate, while the surface quality of the centrifugal casting blade is good. The shrinkage and shrinkage defects of the gravitational casting blade are more and dispersed, and the shrinkage and shrinkage of the centrifugal casting blade are less. There are more cracks on the surface of the centrifugal casting blade, and the cracks in the centrifugal casting blade are less. (5) the optimized Ti-45Al-8Nb-0.7Cr-0.5S The whole blade tissue formed by i-0.4B alloy is more uniform and finer, and the tissue is near lamellar tissue. The size of the gravity casting lamella is about 70 mu m, and there is no obvious change in the position of the blade edge and the central position of the blade. The smaller number of 2 phases indicates that the casting alloy may have excellent comprehensive mechanical properties.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG27;TG146.23

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