激光定向能量沉积Ti6Al4V合金组织调控及力学性能研究

Microstructure and Mechanical Properties of Ti6Al4V Alloy Fabricated by Laser Directed Energy Deposition

摘要:

固溶时效热处理是优化激光定向能量沉积Ti6Al4V合金显微组织，提升其力学性能的重要途径。研究结果表明，沉积态Ti6Al4V试样由宽度为200~500 μm的初生β柱状晶构成，包含晶界α相、晶界魏氏组织和晶内网篮组织。垂直和平行于打印方向的抗拉强度分别为1010和996 MPa，延伸率分别为8.1%和11.2%。在β相变点以下固溶后并经随后中温时效处理，显微组织演变为不连续的晶界α相、短棒状α相和β转变组织（由残余β相和次生α相组成）。而当固溶温度提升至β相变点以上时，晶粒显著粗化并形成粗大魏氏组织。经850 ℃/2 h+500 ℃/4 h AC处理，Ti6Al4V合金展现出优异的室温力学强塑性匹配，其中垂直于打印方向和平行于打印方向的抗拉强度分别为1033和1007 MPa，延伸率分别为12.1%和15.8%。强塑性协同提升主要源自β相变点以下固溶配合中温时效所构建的多尺度α/β结构，晶界α相由连续转为不连续/球化状态以削弱边界开裂通道，晶内α相适度粗化并由片状向短棒状演化以提升变形协调，β转变组织中弥散析出的次生α相与残余β相共同促进滑移传递与加工硬化。

Abstract:

Solution-aging treatment is a critical approach for optimizing the microstructure and enhancing the mechanical properties of laser directed energy deposition (LDED)-fabricated Ti6Al4V alloys. The results show that as-deposited Ti6Al4V alloys consist of primary β columnar grains with width ranging from 200 μm to 500 μm. The intragranular features include grain-boundary α phase, Widmanst?tten structure at grain boundaries, and basket-weave microstructure. The ultimate tensile strengths in the directions perpendicular and parallel to the printing direction are 1010 and 996 MPa, with corresponding elongations of 8.1% and 11.2%, respectively. After sub-β-transus solution treatment followed by intermediate-temperature aging, the microstructure consists of discontinuous grain-boundary α phase, short rod-like α phase, and transformed β phase (composed of retained β phase and secondary α phase). Solution treatment above the β-transus causes pronounced grain coarsening and coarse Widmanst?tten structure. The treatment of 850 °C/2 h+500 °C/4 h with air cooling (AC) yields the excellent strength-ductility balance at room temperature. The ultimate tensile strengths perpendicular and parallel to the printing directions reach 1033 and 1007 MPa, respectively, with corresponding elongations of 12.1% and 15.8%. The strength-plasticity synergy originates from the multiscale α/β structure formed by sub-β-transus solution combined with intermediate-temperature aging. Specifically, the grain-boundary α phase transforms from a continuous to a discontinuous/spheroidized morphology, thereby weakening the cracking propagation channels along boundaries. The intragranular α phase is moderately coarsened and evolves from acicular to short-rod morphology, which improves deformation compatibility. Meanwhile, the secondary α phase dispersed within the transformed β phase and the residual β phase jointly promote slip transfer and work hardening.