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

Fe-36%-Ni合金Invar以其低热膨胀系数（CTE）和良好的低温环境力学性能而闻名。由于其独特的性能，英瓦尔通常用作高精度和高度可靠的材料，用于需要在宽温度范围内具有优异尺寸稳定性的部件。由于因瓦尔是一种强度相对较低的材料，传统的复杂几何形状的加工是极具挑战性和成本密集型的。选择性激光熔化（SLM）作为一种近净形状制造工艺，有望克服因瓦尔合金传统加工的挑战。对最新技术的分析清楚地表明，通过SLM工艺确定了Invar组件的一般可加工性。此外，初步结果表明，基于所选择的工艺参数集，工艺诱导缺陷和残余应力可能会影响热膨胀系数。然而，到目前为止，对缺陷、残余应力、微观结构和CTE之间相互关系的系统研究尚未得到解决。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Invar的过程诱导缺陷，残余应力和热膨胀行为之间的关系来解决这一研究空白。通过选择合适的加工参数以及分析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，将确定最小CTE值的因瓦尔条件作为体积能量的函数。此外，在这些条件下，过程引起的残余应力的贡献将通过x射线衍射对建成和应力消除退火样品的比较来定量确定和评估。为了分析缺陷和残余应力对力学行为的影响，进一步对cte最小化条件进行了综合力学表征。由于在实际应用中许多由英瓦尔合金制成的构件都受到循环载荷的作用，除了单调载荷作用下的疲劳行为外，还研究了低周疲劳状态下的疲劳行为。最后，进行断口分析，使疲劳测试结果合理化，为安全可靠地评估cte最小化SLM Invar条件下的结构完整性铺平道路。

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.