Invar processed by selective laser melting – Tailoring the thermal expansion coefficient by process-induced defects and residual stresses

Fe-36%-Ni合金因瓦因其低热膨胀系数（CTE）和在低温环境中良好的机械性能而闻名。由于其独特的性能，Invar通常用作高精度和高可靠性的材料，用于在宽温度范围内需要上级尺寸稳定性的部件。由于因瓦合金是一种强度相对较低的材料，复杂几何形状的传统加工极具挑战性且成本高昂。选择性激光熔化（SLM）作为一种近净成形制造工艺，是克服因瓦合金传统加工所面临挑战的一种有前途的候选方法。对最新技术的分析清楚地表明，SLM工艺对因瓦合金部件的一般加工性能已得到肯定。此外，第一结果表明，基于所选择的一组工艺参数，由于工艺引起的缺陷以及残余应力，对热膨胀系数的影响是可能的。然而，系统的调查缺陷，残余应力，微观结构和CTE之间的相互关系还没有得到解决。因此，本研究项目的主要目标是通过更深入地了解SLM制造的因瓦合金的工艺缺陷、残余应力和热膨胀行为之间的关系来解决这一研究空白。通过选择合适的工艺参数以及分析所产生的缺陷和微观结构（通过扫描电子显微镜和计算机断层扫描）和热膨胀系数，具有最小CTE值的因瓦条件被确定为体积能量的函数。此外，这些条件下的过程引起的残余应力的贡献将定量确定和评估之间的比较建成和应力消除退火样品通过X射线衍射。为了分析缺陷和残余应力对力学行为的影响，CTE最小化条件进一步进行全面的力学表征。由于许多由因瓦合金制成的部件在实践中受到循环载荷，因此除了单调载荷下的行为外，还研究了低周疲劳区域中的疲劳行为。最后，进行断口分析，以合理化的疲劳试验的结果，以铺平道路的CTE最小化SLM因瓦钢条件的结构完整性的安全和可靠的评估。

The Fe-36%-Ni alloy Invar is known for its low coefficient of thermal expansion (CTE) and good mechanical properties in cryogenic environments. Due to its unique properties, Invar it is commonly used as a high precision and highly reliable material in components where superior dimensional stabilities are required in a wide temperature range. Since Invar is a material of relatively low strength, conventional machining of complex geometries is highly challenging and cost-intensive. As a near-net-shape manufacturing process, selective laser melting (SLM) represents a promising candidate to overcome the challenges related to conventional machining of the Invar alloy. An analysis of state-of-the-art clearly reveals that the general processability of Invar components by the SLM process has been affirmed. In addition, first results show that, based on the chosen set of process parameters, an influence on the thermal expansion coefficient is possible due to process-induces defects as well as residual stresses. However, systematic investigations of the interrelationships between defects, residual stresses, microstructure and CTE have not been addressed so far. Thus, the main objective of this research project is to tackle this research gap by gaining a deeper understanding of the relationship between process-induced defects, residual stresses and the thermal expansion behavior of SLM-manufactured Invar. By selecting apt processing parameters as well as analyzing the resulting defect- and microstructures (by scanning electron microscopy and computed tomography) and thermal expansion coefficients, Invar conditions with minimized CTE values are to be determined as a function of volume energy. Furthermore, the contribution of process-induced residual stresses for these conditions will be quantitatively determined and evaluated by a comparison between as-built and stress-relief annealed samples via X-ray diffraction. In order to analyze the influence of defects and residual stresses on the mechanical behavior, the CTE-minimized conditions are further subjected to comprehensive mechanical characterization. Since many components made of Invar are subjected to cyclic loading in practice, the fatigue behavior in the low-cycle-fatigue regime is investigated in addition to the behavior under monotonic load. Finally, fractography is performed allowing to rationalize the results from the fatigue tests in order to pave the way for a safe and reliable evaluation of the structural integrity of the CTE-minimized SLM Invar conditions.