快速加热非平衡态双相钛合金冷模热板成形机理及组织演变规律

Cold die/hot blank stamping process is attracting attention from industry due to its great potential to reduce the manufacturing cost of titanium alloys thin-walled components such as air inlet, skin panel, carbin structure component, Ω-shaped stiffener and so on. However, crack failure and obvious springback occurred easily due to the decreasing temperature and high strain rate during the forming process. Therefore, a new cold die/hot blank stamping process for two-phase titanium alloys with nonequilibrium microstructure induced by rapid heating was proposed in this project, where the sheet was rapidly heated to a relatively high temperature in the two-phase zone. In this new forming process, the rapid heating could reduce the α to β transformation and decrease the grain coarsing and oxidation simultaneously, which could ensure the material having a good formability under high heating temperature condition, and therefore reduce the springback after forming. The fraction, morphology and grain size of β phase are in nonequilibrium state due to the short heating time, which would affect the following deformation greatly. In this project, the evolution of the nonequilibrium microstructure and the subsequent tensile deformation will be studied to reveal the interaction mechanisms between the nonequilibrium microstructure and plastic deformation; a microstructure prediction model and a set of unified viscoplastic constitutive equations will be created for the FE simulation purpose based on the tensile test and microstructure characterization; then based on the above results, forming tests of the Ω-shaped component will be performed to optimize the processing window. Some fundamental and theoretical progress will be obtained by this project to help reduce the manufacturing cost of titanium alloys thin-walled components.

为了降低钛合金薄壁单层类构件包括进气道、蒙皮、舱体结构件及Ω型加强筋等的制造成本，钛合金冷模热板冲压成形工艺开始受关注，在该工艺中只加热板材，不加热模具。然而该工艺条件下，材料易开裂且回弹控制难度大。为此本项目提出快速加热非平衡态双相钛合金冷模热板成形新方法，在两相区较高温度区间通过快速加热控制相变、减少晶粒粗化及表面氧化，在提高加热温度的同时，避免组织转变引起的延伸率下降，从而保证成形复杂构件的能力及构件的精度和性能。由于加热时间缩短到几十秒内，β相含量、形貌及尺寸均处于非平衡态，对后续变形影响很大。本项目通过对双相钛合金快速加热非平衡态组织的演化规律及高温变形行为研究，揭示非平衡态组织演化与塑性变形的交互作用机制，建立非平衡态组织预测模型及统一粘塑性本构方程，获得Ω型构件冷模热板成形最佳工艺窗口，为钛合金薄壁构件低成本快速精密成形提供基础理论支撑。

钛合金在航空航天及国防等领域有着广泛的应用，为了降低钛合金薄壁单层类构件包括进气道、蒙皮、舱体结构件及Ω型加强筋等的制造成本，本项目提出快速加热非平衡态双相钛合金冷模热板成形新方法，在两相区较高温度区间通过快速加热控制相变、减少晶粒粗化及表面氧化，在提高加热温度的同时，避免组织转变引起的延伸率下降，从而保证成形复杂构件的能力及构件的精度和性能。研究结果表明，钛合金在加热过程中，在靠近α/β相界的α晶粒区域形成一个过渡区，随着加热温度升高及时间延长，该过渡区逐渐转变为β相，在快速加热过程中，由于元素扩散不充分，该过渡区宽度小于慢速加热条件下的宽度，导致快速加热条件下相变偏少，同时单相区快速加热后可避免β相显著粗化，非平衡态组织相变机制为元素扩散。基于非平衡态组织演变规律提出了钛合金强韧化新方法，通过在单相区快速加热获得尺寸细小的全马氏体组织，在提高钛合金材料强度的同时保留较好的延伸率。当将TC4钛合金以50 ℃/s的速率加热到1000℃然后淬火，可以将其抗拉强度从原始的1057.0MPa提升到1263.0MPa，增幅19.5%，同时延伸率仅从原始的13.6%降到9.6%。建立了双相钛合金非平衡态冷模热板冲压成形实验装置，使得材料热暴露时间大大缩短，钛合金薄壁构件成形效率由传统的平均3小时1件提高到平均1分钟1件，成形后构件性能和原始材料相当，Ω型构件成形精度可达±0.26mm。本项目的研究成果可以大幅度提高钛合金薄壁构件生产效率、降低成本，变革钛合金钣金构件生产方式，具有广阔的应用前景；同时将传统的钛合金相变及组织性能调控相关的基础理论拓展到了非平衡态范围，为钛合金薄壁构件高性能制造提供了新的理论支撑。