高温材料在航空航天、核能和汽车等领域的关键金属构件制造中具有举足轻重的地位，当前快速发展的金属增材制造技术使其复杂结构的直接成形制造成为可能。然而，高温材料构件服役条件的特殊需求决定了其对增材制造工艺过程调控和制件性能的高要求，亟需深入研究高温材料增材制造过程中热-力行为的物理机制和影响成形质量的关键因素。本文以Ti6Al4V、INCONEL718和纯钼等在高温下服役的材料为研究对象，采用数值模拟与试验相结合的方法，系统分析了增材制造成形过程的热-力行为及其对温度场、残余应力与变形、裂纹等的影响机理，为高温材料的增材制造提供了理论和数据支持。增材制造的热过程是决定其它复杂物理现象的来源，也是进行力学过程分析的基础，本文研究了纯钼增材制造过程中的温度分布及熔池在多层沉积中的演变。研究发现，熔池几何的稳定性与热源的能量密度直接相关，电子束粉末床增材制造中高达1200 J/mm3的高能量密度保证了多层扫描过程中熔池的稳定性，可获得无未熔合缺陷的致密成形，其致密度可达到99%以上。增材制造过程的力学行为决定了高温合金的残余应力和最终变形，为提高计算效率，本文采用隐式和显式相结合的有限元计算模型对Ti6Al4V和 INCONEL718的多层沉积过程进行热力耦合分析，实现了对增材制造残余应力的高效预测，使计算时间减少了一半。通过试验和计算研究了多层沉积过程中应力和应变的演化过程和影响机制，发现先前层热积累对后续层的预热作用和后续层对先前层的热处理作用共同导致了应力和变形的减小，最大残余应力和变形可分别减少25%以上。高温材料增材制造过程中容易产生裂纹，本文通过实验和有限元模拟，分析了纯钼超高速熔覆过程中裂纹产生的条件，提出了裂纹敏感指数（CVI），并通过预热抑制了裂纹的产生。研究发现，裂纹的产生是由于材料中微缺陷引起的应力集中和热-力过程产生拉应力共同作用引起的；裂纹敏感指数表征了材料性能、微缺陷、和应力状态的综合影响，可用来表示高温材料增材制造裂纹敏感性，为纯钼裂纹敏感区的预测提供了有效的策略。实验和计算表明，对基体预热150℃可有效降低由于热-力过程引起的最大拉应力，使裂纹的敏感性大大降低，抑制了裂纹的产生，可为纯钼超高速熔覆的应用提供重要参考。
Metallic materials with high service temperatures play a crucial role in the manufacturing of key components in the aerospace, nuclear, and automotive industries. They offer exceptional resistance to thermal wear and erosion at elevated temperatures without significant degradation. Conventional manufacturing for complex and customized fabrications is quite challenging, and processes are handicapped to create complicated and intricate structures. Metal additive manufacturing has recently emerged as a novel manufacturing approach for complex and customized fabrications. Perhaps, materials with high service temperatures are not well explored from an additive manufacturing viewpoint, especially localized thermal interaction, rapid solidification, and uneven material response are considerably challenging and often lead to several process bottlenecks and geometric discrepancies. Experimental investigations are expensive and time-consuming to optimize the process and fabrication quality. Fortunately, numerical simulation has become a prevalent tool to interpret and optimize layer-by-layer material consolidation mechanisms. In this thesis, the research is centered around Ti6Al4V, Inconel718, and pure molybdenum, whose service temperatures reach 400℃, 700℃, and 1300℃ respectively. The thermomechanical behavior during the consolidation process is systematically analyzed by combining numerical simulation and experimentation, including the temperature field, residual stress and deformation, and crack. It provides a theoretical framework and data support to facilitate the three-dimensional printing of components with high-temperature serviceability.The thermomechanical phenomenon in additive manufacturing serves as the driving mechanism of complex process physics and subsequently dictates mechanical behavior. In this thesis, the evolution of temperature distribution is primarily investigated via experimental and numerical simulation in the EBSM? process for pure molybdenum. The results indicate that adequate material modeling is imperative for conducive simulation results. In the EBSM? process, the melt pool width of 1.3 mm is consistent with experiments and subsequent temperature melting up to 1.5 times of layer thickness. Which drives spatial and temporal thermal evolution in multilayer and multi-track consolidation of pure molybdenum. A substantial energy density of 1200 J/mm3 is favorable for achieving consistent temperature in scan tracks of powder layers. Side surface roughness and micropores are substantially controlled with optimized process parameters. Finally, based on simulation guidance and experimental optimization, the density of pure molybdenum printed with EBSM? reaches more than 99%.The mechanical behavior of the additive manufacturing process determines the residual stress and final deformation of alloys with high service temperature. Aiming at the mechanical behavior of additive manufacturing, a thermomechanical analysis of the multilayer deposition process for INCONEL718 and Ti6Al4V was carried out. A hybrid FEM scheme is adopted for efficient prediction of residual stress and distortion in the multilayer process. The implemented hybrid FEM simulation demonstrated almost 50% reduction in elapsed time as compared to the traditional FEM scheme. The simulated melt pool with Gaussian heat distribution is consistent with the consolidated layer geometry. A maximum tensile residual stress of 373 ± 5 MPa is found in the vicinity of the layer right in the middle of the substrate and predicted results are precisely validated with experiments. Similarly, a 0.68 ± 0.01 mm distortion is observed with numerical simulation and showed a precise agreement with experiments. Furthermore, the evolution of residual stress and distortion is investigated in the multilayer DED process of Ti6Al4V. The FEM simulation demonstrated a strain relief emerged with the addition of layers. It is observed that frequent melting warms up the upcoming layer and serves as a post-heat treatment for previously deposited layers. This reduces the accumulation of residual stresses and distortion more than 25% in the final geometry.Excessive crack susceptibility is one of the critical hindrances for additive manufacturing of materials with high service temperatures. Aiming at the problem of crack susceptibility, the mechanism of crack initiation is examined with a series of experiments. This reveals that tensile residual stresses become more critical in the presence of micro voids and further cause cracking in pure molybdenum cladding. Numerical simulation is implemented for thermomechanical analysis for crack vulnerability during the process. Therefore, a so-called CVI (Crack Vulnerability Index) is proposed to predict the crack-susceptible region with FEM simulation. The simulation results accurately validated the experimental investigation. Finally, the substrate is preheated up to 150C° to mitigate crack susceptibility in molybdenum cladding.
