Microstructure Evolution of EBM Gamma Titanium Alumiide (TNM-B1)

在过去的十年里，加法制造已经引起了科学和工业的极大兴趣。这使得量身定做的部件制造能够应用于广泛的领域，例如航空航天和医药行业。这些行业往往依赖于难以加工的材料。基于粉末床的方法，如电子束熔化工艺(EBM)，由于工艺温度高、冷却速度慢而成为此类应用的最佳选择。钛铝化物因其优异的高温行为和低密度而被认为是航空航天工业中潜在的替代材料。然而，这些材料在700°C左右表现出从韧性到脆性的转变。因此，选择性激光熔化(SLM)的铸造或加工(也是一种生成过程)很容易出现热诱发裂纹。EBM工艺可以为由这些材料组成的复杂零件的近净成形制造提供一条替代的加工路线。本项目致力于实验研究EBM制造的TNM-B1零件的组织演变--TNM合金的进一步发展--沿着共同的EBM制造链。TNM合金是钛铝化物，与传统的伽马-TiAl合金相比，由于减少了偏析和织构敏感性，其加工性得到了改善。TNM合金的显微组织对成分性能有很大影响，并受EBM加工后的热处理控制。因此，研究这两种工艺对TNM合金组织演变的影响是本课题研究的重点。因此，应处理两个主要方面。第一，元素可能蒸发，导致化学和物相组成的变化；第二，与铸造的TNM-B1组分相比，可能的微观结构变化。以前的研究已经强调了其他钛铝化物在EBM加工过程中铝的蒸发。对于TNM系统，这可能导致脆性α-TiAl相的数量增加，从而影响组织和残余应力状态。这可能会增加热致开裂的风险，特别是在淬火步骤期间(例如，在固溶退火热处理之后)。考虑到这一点，应调查EBM制造链上残余应力的演化和稳定性。残余应力检查的重点将放在EBM工艺之后的多步热处理上。根据所有结果，将评估EBM工艺在用TNM-B1或TNM合金制造部件方面的适用性。在那里，还将使用微观结构和残余应力状态调查的结果来确定EBM制造链上的相关关键工艺和工艺参数。

Over the past decade additive manufacturing has become of great scientific and industrial interest. This enables tailored component manufacturing for a wide range of applications, for example in aerospace and medicine industry. These industries often rely on difficult to process materials. Powder-bed based methods like the electron beam melting process (EBM) emerged as an excellent choice for such use cases due to high process temperatures and slow cooling rates.Titan aluminides are known to be a potential candidate for material substitution in aerospace industry due to their excellent high temperature behavior as well as low density. However these materials exhibit a ductile-to-brittle transition at around 700 °C. Thus casting or processing by selective laser melting (SLM), a generative process as well, are susceptible to thermal induced cracks. The EBM process could establish as an alternative processing route for near net shape manufacturing of complex parts consisting of those materials.This project focusses on the experimental investigation of the microstructural evolution of EBM-manufactured TNM-B1 parts – a further development of TNM-alloy – along a common EBM manufacturing chain. TNM alloys are titanium aluminides with improved workability due to decreased segregation and texture susceptibility compared to conventional gamma-TiAl alloys. The microstructure of TNM-alloys significantly influences the component properties and is controlled by a thermal treatment subsequent to the EBM-processing. Thus a key point of this project is to investigate the influence of those two processes on the microstructural evolution of TNM alloys. Thereby two main aspects shall be addressed. First a possible evaporation of elements which lead to changes of the chemical and phase composition and second a possible microstructural change in comparison to casted TNM-B1 components. Former researches already highlighted the evaporation of aluminum during EBM-processing for other titanium aluminides. For the TNM system this could lead to an increased amount of brittle alpha2-TiAl-phase and thus influence microstructure and residual stress state. An increased risk of thermal induced cracking especially during quenching steps (e.g. subsequent to the solution annealing heat treatment) might be the consequence. Taking this into account the evolution as well as the stability of residual stresses along the EBM-manufacturing chain shall be investigated. The emphasis of the residual stress examinations will be on the multi-step heat treatment subsequent to the EBM-process.Based on all results the applicability of the EBM-process for component manufacturing from TNM-B1 or TNM-alloys will be evaluated. There the outcomes of the microstructural and residual stress state investigations will also be used to identify relevant and critical processes and process parameters along the EBM manufacturing chain.