选区激光熔化成形高Nb-TiAl合金凝固组织的形成与调控机制研究

Due to their superior high-temperature strength and oxidation resistance to common TiAl alloys, high-Nb TiAl alloys are regarded as next-generation high-temperature structural materials with low weight and high strength. However, the addition of Nb makes it difficult to be machined. Selective laser melting (SLM), a new developed additive manufacture technique, owns obvious advantage on the rapid prototyping of difficult-to-machine materials and components with complex shape. Currently, the forming and modulation mechanisms of solidification microstructure are the key issues in the SLM processed high-Nb TiAl components with high dense and high quality. In the present project, we will study the melt track morphology, solidification process, solidification microstructure and segregation, recrystallization induced by residual stress, evolution of solidification microstructure during heat treatment, and the mechanical properties of high-Nb TiAl alloys processed by SLM. We want to resolve some key scientific issues, including the formation mechanism of solidification microstructure in high-Nb TiAl alloys during SLM, the mechanism of recrystallization induced by residual stress, and the mechanism of solidification microstructure evolution during heat treatment. The present work will promote the development of the SLM forming techniques for high-Nb TiAl alloys and the results will definitely accelerate the applications of high-Nb TiAl alloys in the field of aerospace.

高Nb-TiAl合金具有比普通TiAl更优异的高温强度和抗氧化性能，是新一代轻质高强高温结构材料。然而，高含量Nb的加入进一步增加了合金的加工难度和成本。选区激光熔化（SLM）易于实现难加工材料和形状复杂构件的快速成型，将其应用于高Nb-TiAl合金的关键是获得高致密成形构件并实现对组织和性能的控制，核心是揭示凝固组织的形成和调控机制。本项目将通过开展高Nb-TiAl合金SLM成形中的熔道形态、凝固过程、凝固组织特征与偏析、残余应力引起的再结晶、热处理过程中凝固组织的演变以及合金力学性能等内容研究，揭示SLM成形高Nb-TiAl合金的凝固特征和凝固组织形成机制，残余应力导致的再结晶机理，热处理过程中凝固组织的演变规律等关键科学问题，以实现对SLM成形高Nb-TiAl合金组织和性能的调控。项目将为高Nb-TiAl合金SLM成形技术的发展奠定科学基础，并推动其在涡轮叶片等航空航天构件上的应用。

TiAl合金具有低密度（约为镍基高温合金的50%、优异的高温力学性能和抗氧化性能、好的阻燃能力等特点， 可替代传统镍基高温合金，用于航空航天工业的喷气发动机和涡轮、以及超高速飞行器的翼、壳体等部件，其应用所带来的减重效果和燃油效率的提升十分显著。与普通TiAl合金相比，高Nb-TiAl合金的使用温度提高 60-100℃、强度提高 300~500MPa，是TiAl 合金的重要发展方向之一。然而，TiAl 合金都具有室温塑性低、可加工性差的缺点，高含量Nb的加入进一步增加了合金的加工难度。选区激光熔化（SLM ）易于实现难加工材料和形状复杂构件的快速成型，将其应用于高Nb-TiAl合金的关键是获得高致密成形构件并实现对组织和性能的控制，核心是揭示SLM成形高Nb-TiAl合金的凝固特征和凝固组织形成机制，以及热处理过程中凝固组织的演变规律等关键科学问题。本项目在自主研制气雾化制粉设备的基础上，通过开展高Nb-TiAl合金SLM成形中的熔道形态、凝固过程、凝固组织特征与偏析、热处理过程中凝固组织的演变以及力学性能等内容研究，发现SLM成形高Nb-TiAl合金的凝固行为与SLM工艺的快速冷却特性紧密相关，并进一步对其凝固组织特征进行了深入地表征，发现成形态组织晶粒细小、以亚稳α2-Ti3Al相为主，并揭示了其凝固组织形成机制；本项目采用原位高温XRD和原位加热TEM研究揭示了亚稳α2-Ti3Al相到γ-TiAl相的相变过程，揭示了热处理过程中凝固组织的演变规律。本项目实现了高致密度、高质量高Nb-TiAl合金构件的打印成形，并初步实现了对其组织和性能的调控，是TiAl合金成形技术的新发展。本项目研究成果深化了对高Nb-TiAl 合金激光增材制造的认识和理解，将为高Nb-TiAl合金SLM成形技术的发展奠定科学基础，并推动其在涡轮叶片等航空航天构件上的应用。