整体高温辅助激光定向能量沉积TiAl4822合金开裂抑制机理及组织性能研究

Suppression of cracking and microstructure-property investigation of TiAl4822 alloy by laser directed energy deposition with integral high-temperature assistance

摘要

激光定向能量沉积（laser directed energy deposition，LDED）凭借高效率与工艺柔性，正成为解决室温高脆性和高活性的TiAl4822（Ti-48Al-2Cr-2Nb）合金传统工艺难加工、难制备大型复杂构件问题的关键途径，以充分发挥其航空发动机等高温轻质部件的理想材料潜力。然而，LDED过程中快速熔融-凝固循环会产生极大的温度梯度和残余应力，从而导致构件开裂，但目前尚无成熟手段能够完全抑制裂纹产生。本工作利用整体高温辅助LDED制备出30 mm×25 mm×6 mm致密无裂纹的TiAl4822合金薄壁构件，并对其宏观形貌、微观组织、孔隙率及显微硬度进行研究。研究结果表明：在常温条件下，LDED制备的TiAl4822合金薄壁样件易发生以解理为主的脆性断裂，显微组织以细小等轴晶为主；引入800 ℃整体高温辅助后，沉积层晶粒定向生长为自下而上倾斜的柱状晶，孔隙率从0.05%降至0.008%，孔径分布更均匀，表面未见宏观裂纹；与此同时，显微硬度由常温样件的390.46HV_0.2降至354.94HV_0.2，这主要归因于在高温辅助条件下晶粒长大、晶界减少及析出相中γ相的含量相对增加。因此，整体高温辅助不仅有效抑制裂纹与大尺寸孔隙的产生，还优化微观组织均匀性，为TiAl4822合金的高致密、高性能制备提供新途径。

Abstract

The TiAl4822 (Ti-48Al-2Cr-2Nb) alloy, renowned for its exceptional high-temperature mechanical properties and low density, stands out as a highly promising candidate for critical aerospace components. However, its high chemical reactivity and inherent room-temperature brittleness pose significant challenges to the conventional manufacturing of large and complex geometries. Laser directed energy deposition (LDED), characterized by its high fabrication efficiency and remarkable process flexibility, has emerged as a crucial approach for preparing TiAl4822 alloy components. Nevertheless, the rapid melting-solidification cycle during LDED induces a substantial temperature gradient and residual stress, which results in component cracking. Currently, there is no well-established method to completely prevent crack formation. In this study, a dense and crack-free thin-walled TiAl4822 alloy component with dimensions of 30 mm×25 mm×6 mm is successfully fabricated using the whole high-temperature-assisted LDED technique. An investigation is conducted on their macro-morphology, microstructure, porosity, and microhardness. The results reveal that the thin-walled TiAl4822 alloy specimen prepared by LDED at room temperature is prone to brittle fracture primarily through cleavage, and its microstructure mainly comprises fine equiaxed grains. After implementing whole high-temperature assistance at an integral temperature of 800 ℃, the grains in the deposited layer transform from bottom to top into inclined columnar grains. The porosity is significantly reduced from 0.05% to 0.008%, accompanied by a more uniform pore-size distribution, and no macroscopic cracks are observed on the surface. Concurrently, the microhardness decreases from 390.46HV_0.2 to 354.94HV_0.2, which can be attributed to grain coarsening, a decrease in grain-boundary density, and precipitate evolution under high-temperature conditions. Overall, the integral high-temperature-assisted LDED effectively inhibits crack initiation and the formation of large pores while homogenizing the microstructure, providing a novel pathway for high-density, high-performance TiAl4822 preparing.

关键词

激光定向能量沉积 / TiAl4822合金 / 裂纹 / 显微组织 / 显微硬度

Key words

laser directed energy deposition / TiAl4822 alloy / crack / microstructure / microhardness

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基金资助

国家自然科学基金项目(52375312)

大连理工大学精密/特种加工及微制造技术教育部重点实验室（B类）开放课题基金资助项目(B202305)