A Study of Beta Processing of Ti-6Al-4V: Is it Trivial?
A Study of Beta Processing of Ti-6Al-4V: Is it Trivial?
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DOI：
10.1115/1.1372708
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发表时间：
2001-07
影响因子：
1.2
通讯作者：
Y. Prasad;T. Seshacharyulu;S. Medeiros;W. G. Frazier
中科院分区：
文献类型：
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作者：
Y. Prasad;T. Seshacharyulu;S. Medeiros;W. G. Frazier
Ti-6Al-4V合金在 \beta 相场通常被认为是微不足道的，因为材料在这些温度下是高度可加工的，通常不会在加工过程中造成问题。鉴于此，研究了 \beta 和那些在上 $\alpha- \beta$变形。本文的重点是了解 \beta 研究Ti-6Al-4V的变形特征，以检验这些研究是否有助于优化工艺设计和实现微观组织控制。重点是两个工业等级，即商业纯度（CP）和超低间隙（ELI），以及启动微观结构（转化）的影响 \beta 对等轴 $\alpha+\beta$）. 压缩时得到的应力-应变曲线在应变率低于 $1\hspace{2mm}s^{-1}$ 以及在较高应变速率下的振荡/软化行为。对不同温度和应变速率下的流动应力数据进行动力学分析表明，应力指数在3.6 ～ 3.8之间，表观活化能在150 ～ 287 kJ/mol之间，与自扩散过程相当 \beta 相（150kj /mol）。的动态再结晶(DRX \beta 在所有病例中，除了ELI级转化的 \beta 起始结构。前任 \beta DRX区的晶粒尺寸与Zener-Hollomon参数具有良好的相关性。在ELI级的情况下，已经确定了一种涉及先前群体边界滑动的大晶粒超塑性机制。然而，在ELI级中，靠近横向的变形导致孔洞形核，在变形温度下，在拉伸残余应力的影响下，孔洞可能在浸泡过程中长大。结果清楚地表明，一项研究 \beta 变形机理是Ti-6-4热加工的关键。更重要的是，不应将CP级Ti-6-4的加工流程用于ELI级，以实现微结构控制，避免出现缺陷。
Deformation of Ti-6Al-4V alloy in the \beta phase field is generally considered to be trivial since the material is highly workable at these temperatures and does not normally pose problems during processing. In view of this, studies on the hot deformation behavior of \beta are scanty compared to those on the $\alpha- \beta$deformation. This paper is focussed on understanding the \beta deformation characteristics in Ti-6Al-4V with a view to examine whether such studies help in optimizing the process design and achieving microstructural control. The emphasis has been on the two industrial grades, viz. commercial purity (CP) versus extra-low interstitial (ELI), and also on the effect of starting microstructure (transformed \beta versus equiaxed $\alpha+\beta$). The stress-strain curves obtained in compression exhibited steady-state behavior at strain rates lower than $1\hspace{2mm}s^{-1}$ and oscillatory/softening behavior at higher strain rates. Kinetic analysis of the flow stress data obtained at different temperatures and strain rates has shown that the stress exponent is about 3.6-3.8 and the apparent activation energy is in the range 150-287 kJ/mole, which is comparable to that of self-diffusion in \beta phase (150 kJ/mol). Dynamic recrystallization (DRX) of \beta is identified as the microstructural mechanism in all the cases except in ELI grade with transformed \beta starting structure. The prior \beta grain size in the DRX region exhibits a good correlation with the Zener-Hollomon parameter. In case of ELI grade, a mechanism of large grained superplasticity involving sliding of prior colony boundaries has been identified. However, deformation close to the transus in ELI grade causes nucleation of voids which may grow during soaking at the deformation temperature under the influence of tensile residual stress. The results clearly show that a study of \beta deformation mechanisms holds the key during hot working of Ti-6-4. More importantly, the processing schedule used for CP grade Ti-6-4 should not be used for ELI grade to achieve microstructural control and avoid defects.