摘要
TC11钛合金是一种α+β双相钛合金,属难变形材料,广泛应用于航空航天工业领域。基于动态材料模型理论的加工图技术是一种用于金属热变形工艺设计和优化的工具,利用加工图不仅可以避开流动失稳区,而且还可获得优化的可加工温度和应变速率范围。作者以片状和等轴状两种初始组织的TC11钛合金为研究对象,通过热压缩实验对其热态变形行为进行了研究,并利用加工图技术对锻造变形工艺进行了优化。研究结果对合理制定TC11钛合金的锻造变形工艺,确保获得组织和性能稳定一致的无缺陷锻件具有重要的理论指导意义和实际应用价值。
论文综述了动态材料模型及其加工图技术的理论基础及发展过程,对确定加工图中稳定或失稳变形区的各种准则的不可逆热力学和耗散结构基本理论以及推导过程进行了较详细的介绍,从理论角度分析和比较了这些准则的优缺点及其适用范围。理论分析表明,Gegel准则和Malas准则的本质相同,二者既考虑了材料的机械稳定性和热力学稳定性,又可使变形过程对外界的扰动具有较好的自修正性,但当所研究材料的m值不为常数时,Malas准则比Gegel准则更合理些。在应用这两个准则时,由于限制条件较多,可能会缩小可加工的变形热力参数范围。Prasad准则和Murty准则的本质亦相同,但当所研究材料的m值不为常数时,选用Murty准则更为合理。
系统地研究了变形热力参数对两种初始组织TC11钛合金流动应力和变形组织的影响规律。结果表明,这两种组织TC11钛合金的流动应力均随变形温度的升高和应变速率的降低而减小,其应力-应变曲线在应变速率较高时为应变软化型,在应变速率较低时为稳态流动型。从变形抗力角度考虑,这两种组织TC11钛合金宜在较低的应变速率下进行变形,当温度从低向高变化时,应变速率可以适当地提高,即适宜的应变速率范围变宽。两种组织TC11钛合金随变形温度降低,应变速率提高,变形均匀性变差。对于片状组织TC11钛合金的α+β两相区变形,当应变速率≤0.01s~(-1)时,α片层开始球化,故从变形均匀性和获得球化组织角度考虑,应变速率以≤0.01s~(-1)为宜;对于片状组织TC11钛合金的近β和β单相区变形,当应变速率≤0.01s~(-1)时,动态再结晶较完全,故从获得动态再结晶组织角度考虑,应变速率亦以≤0.01s~(-1)为宜。对于等轴组织TC11钛合金的α+β两相区变形,α相的形态总体变化不大,但当应变速率较高时,变形均匀性变差,故从变形均匀性角度考虑,变形宜在较低的应变速率进行;对于等轴组织TC11钛合金的β单相区变形,在应变速率为0.01s~(-1)～0.1s~(-1)时发生较完全的动态再结晶,且晶粒细小,故从获得细小动态再结晶组织角度考虑,适宜的应变速率为0.01s~(-1)～0.1s~(-1),比片状组织要高一个数量级。
对两种初始组织TC11钛合金的本构方程进行了研究。结果表明,Arrhenius型双曲正弦方程和改进的Arrhenius型幂函数方程可分别作为片状组织TC11钛合金在近β和β单相区,以及α+β两相区的本构关系模型;改进的Arrhenius型幂函数方程亦可作为等轴组织TC11钛合金在整个变形温度区间的本构关系模型。通过数理统计方法确定出了模型中系数。误差分析表明,所建立的本构方程具有较高的精度,片状组织TC11钛合金在近β和β单相区,以及α+β两相区的平均误差分别为5.04%和5.57%;等轴组织TC11钛合金的平均误差为5.20%。通过改进Arrhenius型幂函数方程来建立本构关系模型的方法具有普遍适用性,可用于其它材料本构方程的建立。
首次利用加工图技术研究了两种初始组织TC11钛合金的锻造工艺优化,分别采用不同的稳定或失稳变形准则绘制了两种初始组织TC11钛合金的加工图,分析和比较了不同稳定或失稳变形准则的适用性。结果表明,基于Murty准则绘制的加工图总体上比基于Prasad或Malas准则绘制的加工图在预测稳定变形区、失稳变形区和优化锻造热力参数方面更准确。基于Murty准则绘制的加工图预测结果表明,对于片状组织TC11钛合金的α+β两相区变形,其流动失稳区为750℃～875℃、0.005s~(-1)～10.0s~(-1)和875℃～1000℃、0.2s~(-1)～10.0s~(-1),对应的失稳现象为宏观剪切裂纹、绝热剪切带和原始β晶界孔洞;较佳的锻造热力参数为750℃～900℃、0.001s~(-1)～0.005s~(-1)和900℃～1000℃、0.001s~(-1)～0.03s~(-1),对应的变形机制以球化为主;最佳的锻造热力参数位于840℃～980℃、0.001s~(-1)附近。对于片状组织TC11钛合金的近β和β单相区变形,其流动失稳区为1000℃～1100℃、1.0s~(-1)～10.0s~(-1)和1075℃～1100℃、0.001s~(-1)～0.003s~(-1),对应的失稳现象为β晶粒拉长、晶界破碎、“项链”状的混和组织以及晶粒的动态长大;较佳的锻造热力参数为1000℃～1100℃、0.001s~(-1)～0.05s~(-1)(除去1075℃～1100℃、0.001s~(-1)～0.003s~(-1)这个小区域),对应的变形机制为动态再结晶;最佳的锻造热力参数在应变小于0.4时位于1050℃、0.001s~(-1)附近,在应变大于0.4时位于1050℃、0.016s~(-1)附近。对于等轴组织TC11钛合金的α+β两相区变形,其流动失稳区为780℃～850℃、0.008s~(-1)～70.0s~(-1),850℃～927℃、0.01s~(-1)～70.0s~(-1)和927℃～1008℃、0.1s~(-1)～70.0s~(-1),对应的失稳现象为绝热剪切带、局部流动和β相中的裂纹和空洞;较佳的锻造热力参数为780℃～850℃、0.001s~(-1)～0.008s~(-1),850℃～940℃、0.001s~(-1)～0.01s~(-1)和940℃～1008℃、0.001s~(-1)～0.01s~(-1),对应的变形机制以超塑性为主;最佳的锻造热力参数位于900℃、0.001s~(-1)附近。对于等轴组织TC11钛合金的β单相区变形,其流动失稳区为1008℃～1080℃、4.0s~(-1)～70.0s~(-1),对应的失稳现象为β晶界的破碎和晶粒的拉长;较佳的锻造热力参数在应变小于0.7时为1030℃～1080℃、0.001s~(-1)～0.1s~(-1),在应变大于0.7时为1020℃～1060℃、0.004s~(-1)～0.6s~(-1),对应的变形机制为动态再结晶;最佳的锻造热力参数在应变小于0.7时位于1060℃～1080℃、0.001s~(-1)附近,在应变大于0.7时位于1040℃～1050℃、0.016s~(-1)～0.07s~(-1)。
对等轴组织TC11钛合金的超塑性变形行为进行了初步研究。结果表明,在β单相区不能获得超塑性,在α+β两相区可获得超塑性,且最佳超塑性出现在900℃附近,应变速率越低越好,这与用加工图预测的结果相吻合。在900℃、0.0001s~(-1)条件下的延伸率高达1215%。初生α和β相的体积比对超塑性具有较大的影响,初生α相含量在70%时对应着最佳的超塑性。超塑性变形过程中有动态再结晶、扩散蠕变、晶内变形及界面滑移的参与,且界面的滑移以α/β相界面滑移为主。
TC11 is anα+βtitanium alloy and belongs to difficult-to-deform materials, which has been widely used in aerospace industry. Processing map technology based on Dynamic Materials Model (DMM) is a tool for design and optimization of hot deformation processes of metals, with which not only flow instability regimes can be avoided but also optimized ranges of temperature and strain rate can be identified. In this paper, the hot deformation behavior of two kinds of titanium alloy TC11 with different original microstructures, namely equiaxed and lamellar, has been studied by hot compression tests, and the forging processes for these two kinds of titanium alloy TC11 have been optimized by using processing map technology. The studied results are of great guidance significance in theory and practical application value for making reasonable forging processes to manufacture the forgings free from deficiencies with excellent and uniform microstructures as well as properties on a repeatable basis in a manufacturing environment.
The theoretical basis and development of DMM and its processing map technology were here reviewed, and the basis and principle of both irreversible thermodynamics and dissipative structure related to various criteria for identifying the regimes of flow stability or instability as well as the criteria’s deduction processes were introduced in detail. The advantage and disadvantage, including available situations, of the criteria based on DMM were theoretically compared and analyzed. Analysis in theory indicated that the essence of Gegel criterion is identical to that of Malas criterion. These two criteria not only take mechanistic stability and thermodynamic stability of investigated materials into account, but also make the deformation process have self-modification to outside disturbance. If the value of m of investigated material is not a constant, Malas criterion is more reasonable than Gegel criterion. When using these two criteria to identify the flow stability regimes, the feasible ranges of thermomechanical parameters for hot working of material may be reduced because of more limiting conditions. The essence of Prasad criterion is identical to that of Murty criterion, and the latter is more reasonable when the value of m of investigated material is not a constant.
The effects of deformation thermomechanical parameters on flow stresses and deformed microstructures of two kinds of titanium alloy TC11 with different original microstructures have been studied systematically. The results indicated that the flow stresses of these two kinds of titanium alloy TC11 decrease with increasing temperatures and decreasing strain rates. The stress-stain curves are type of strain softening at high strain rates and type of steady-state at low strain rates. As viewed from resistance of deformation, these two kinds of titanium alloy TC11 are feasible to be worked at low strain rates. If the temperatures increase, the strain rates may be increased aptly, thus the feasible range of strain rates will get wider. For these two kinds of titanium alloy TC11, the deformation uniformity will degenerate with decreasing temperatures and increasing strain rates. For titanium alloy TC11 with lamellar microstructure, inα+βphase field, the globularization ofαlamellas will occur when the strain rates are lower than and equal to 0.01s~(-1), thus the feasible strain rates are lower than and equal to 0.01s~(-1) as viewed from deformation uniformity and obtaining globularized microstructures. For titanium alloy TC11 with lamellar microstructure, in near-βandβphase field, theβgrains will undergo relatively full dynamic recrystallization when the strain rates are lower than and equal to 0.01s~(-1), thus the feasible strain rates are lower than and equal to 0.01s~(-1) as viewed from obtaining recrystallized microstructures. For titanium alloy TC11 with equiaxed microstructure, the morphology ofαphase changes just a little generally inα+βphase field. If the strain rates are higher, the deformation uniformity will degenerate, thus this kind of titanium alloy TC11 is feasible to be worked at low strain rates as viewed from deformation uniformity. For titanium alloy TC11 with equiaxed microstructure, the relatively full dynamic recrystallization ofβgrains occurs at 0.01s~(-1)~0.1s~(-1) with fined grains inβphase field. Thus the feasible strain rates are in the strain rate range of 0.01s~(-1)~0.1s~(-1) as viewed from obtaining fined recrystallized microstructures, which are higher than that of titanium alloy TC11 with lamellar microstructure an order of magnitude.
The constitutive equations for these two kinds of titanium alloy TC11 have been studied. The results indicated that the hyperbolic sine equation of Arrhenius type and the modified power function equation of Arrhenius type may be regarded as the constitutive relationship models of titanium alloy TC11 with lamellar microstructure in near-βandβphase field andα+βphase field respectively, and the modified power function equation of Arrhenius type may also be regarded as the constitutive relationship model of titanium alloy TC11 with equiaxed microstructure in entire temperature regime. Error analysis indicated that these constitutive equations constructed have high precision. The average differences between the calculated and experimental flow stresses for titanium alloy TC11 with lamellar microstructure in near-βandβphase field andα+βphase field are 5.04% and 5.57%, respectively. The average difference between the calculated and experimental flow stresses for titanium alloy TC11 with equiaxed microstructure in entire temperature regime is 5.20%. The approach that constructing the constitutive model by modifying power function equation of Arrhenius type has a universal applicability and may be used for construction of constitutive equation for other materials.
The optimization of forging processes for these two kinds of titanium alloy TC11 has been studied by using processing map technology for the first time, and the processing maps of these two kinds of titanium alloy TC11 have been plotted by using various criteria of flow stability or instability, respectively. The applicability of various criteria of flow stability or instability to these two kinds of titanium alloy TC11 has been analyzed and compared. The results indicated that the processing maps based on Murty criterion are in general more reasonable than those based on Prasad criterion or Malas criterion in predicting the regimes of flow stability, predicting the regimes of flow instability and optimizing forging thermomechanical parameters. The predicted results by using the processing maps based on Murty criterion are the following. For titanium alloy TC11 with lamellar microstructure, inα+βphase field, the regimes of flow instability are in the temperature ranges and the strain rate ranges of 750℃~875℃and 0.005s~(-1)~10.0s~(-1), 875℃~1000℃and 0.2s~(-1)~10.0s~(-1) with corresponding manifestations of flow instability including macro cracks, adiabatic shear bands and priorβboundary cavities. The better thermomechanical parameters for forging are in the temperature ranges and the strain rate ranges of 750℃~900℃and 0.001s~(-1)~0.005s~(-1), 900℃~1000℃and 0.001s~(-1)~0.03s~(-1), and the corresponding main deformation mechanism in these two domains is globularization ofαlamella. The optimum thermomechanical parameters for forging lies in the temperature range of 840℃~980℃and near 0.001s~(-1). For titanium alloy TC11 with lamellar microstructure, in near-βandβphase field, the regimes of flow instability are in the temperature ranges and the strain rate ranges of 1000℃~1100℃and 1.0s~(-1)~10.0s~(-1), 1075℃~1100℃and 0.001s~(-1)~0.003s~(-1) with corresponding manifestations of flow instability including elongate ofβgrains, break ofβgrain boundaries, mixed microstructures like“necklace”and dynamic growth ofβgrains. The better thermomechanical regime for forging is in the temperature range of 1000℃~1100℃and the strain rate range of 0.001s~(-1)~0.05s~(-1) (except a little regime of 1075℃~1100℃and 0.001s~(-1)~0.003s~(-1)) with corresponding deformation mechanism of dynamic recrystallization. The optimum thermomechanical parameter for forging lies near 1050℃and 0.001s~(-1) at stains below 0.4 and near 1050℃and 0.016s~(-1) at strains above 0.4, respectively. For titanium alloy TC11 with equiaxed microstructure, inα+βphase field, the regimes of flow instability are in the temperature ranges and the strain rate ranges of 780℃~850℃and 0.008s~(-1)~70.0s~(-1), 850℃~927℃and 0.01s~(-1)~70.0s~(-1), 927℃~1008℃and 0.1s~(-1)~70.0s~(-1) with corresponding manifestations of flow instability including cracks and cavities inβphase, adiabatic shear bands and flow localization. The better thermomechanical parameters for forging are in the temperature ranges and the strain rate ranges of 780℃~850℃and 0.001s~(-1)~0.008s~(-1), 850℃~940℃ and 0.001s~(-1)~0.01s~(-1), 940℃~1008℃and 0.001s~(-1)~0.01s~(-1), and the corresponding main deformation mechanism in these three domains is superplasticity. The optimum thermomechanical parameter for forging lies near 900℃and 0.001s~(-1). For titanium alloy TC11 with equiaxed microstructure, inβphase field, the regime of flow instability is in the temperature range of 1008℃~1080℃and the strain rate range of 4.0s~(-1)~70.0s~(-1) with corresponding manifestations of flow instability including elongate ofβgrains and break ofβgrain boundaries. The better thermomechanical parameters for forging are in the temperature range of 1030℃~1080℃and the strain rate range of 0.001s~(-1)~0.1s~(-1) at strains below 0.7, in the temperature range of 1020℃~1060℃and the strain rate range of 0.004s~(-1)~0.6s~(-1) at strains above 0.7, and the corresponding main deformation mechanism in these two domains is dynamic recrystallization. The optimum thermomechanical parameters for forging lies in the temperature range of 1060℃~1080℃and near 0.001s~(-1) at strains below 0.7, in the temperature range of 1040℃~1050℃and the strain rate range of 0.016s~(-1)~0.07s~(-1) at strains above 0.7.
The superplastic deformation behavior of titanium alloy TC11 with equiaxed microstructure has been investigated preliminarily here. The results indicated that this alloy would not exhibit superplasicity inβphase field, but exhibit superplasicity inα+βphase field. The optimum superplastic deformation temperature is near 900℃and the optimum superplastic strain rate is the lowest in the investigated strain rate range, which is in agreement with that predicted by using processing map. At 900℃and 0.001s~(-1), the elongation reaches 1215%. The volume ratio ofαandβhas a great effect on the superplasticity. When the volume fraction of primaryαphase is about 70%, this alloy exhibits optimum superplasticity. During superplastic deformation, dynamic recrystallization, diffusion creep, intracrystalline deformation and interface sliding operate together, and interface sliding mainly occurs atα/βphase interface.
引文
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