In-situ Microstructure Modification of Duplex Stainless Steels by means of Powder Bed Fusion using Laser Beam Melting of Metals (PBF-LB/M)

双相不锈钢（DSS）的材料性能基于双相显微组织，其可分为相等份额的奥氏体和铁素体相，其同时负责良好的耐腐蚀性和良好的机械性能。当使用基于激光的增材制造技术（如激光束熔化（PBF-LB/M））加工DSS时，形成了以铁素体为主的微观结构。为了保持钢的PBF-LB/M特定性能，需要调整加工策略以原位生成双相结构。因此，将检查处理条件对相比率和相应的材料性质的影响。除了PBF-LB/M特定参数外，还将修改屏蔽气体的氮气浓度。这些处理条件导致所制造的样品内的每个点的不同的热历史。为了评估和理解加工条件对热历史的影响，将开发基于三维热传导方程和适当的替换源的简化模型，用于预测制造的试样内部的温度发展。通过将PBF-LB/M过程中部件内部的空间和时间温度发展与形成的相关联，可以导出调整的处理策略，以实现具有50%的亚铁酸-铁素体比的双重微观结构。通过添加充当成核剂的纳米颗粒提供用于调整微观结构和晶粒尺寸的第二种方法。目标是研究浓度低于0.1重量%的纳米颗粒的添加程度为了满足和解决PBF-LB/M制造的零件中的热致应力，将开发调整的热处理策略，其同时保持相分布和工艺特定的晶粒尺寸。与所有这些研究平行，宏观材料性能如硬度、抗拉强度和耐腐蚀性将被确定，并与获得的微观结构和相形成相关联。基于所提出的研究，对加工条件、微观结构形成和由DSS制成的PBF-LB/M部件的材料性能之间的相关性产生了基本的理解。通过调整PBF-LB/M中的保护气体气氛、纳米颗粒浓度和后处理热处理策略的工艺条件，将获得用于使用PBF-LB/M工艺生成DSS部件的高度控制。

The material properties of duplex stainless steels (DSS) are based on the duplex microstructure, which can be divided into an equal share of the austenite and ferrite phase, which are responsible for a good corrosions resistivity and good mechanical properties at the same time. When processing DSS using laser-based additive manufacturing technologies like laser beam melting (PBF-LB/M), a ferrite-dominant microstructure is formed. For maintaining the PBF-LB/M-specific properties of the steel, adjusted processing strategies are required for the in-situ generation of the duplex structure. Therefore, the effect of processing conditions on the phase ratio and the corresponding material properties will be examined. In addition to PBF-LB/M-specific parameters, the nitrogen concentration of the shield gas will be modified. These processing conditions result in a varying thermal history for every point inside the manufactured specimen. To assess and understand the influence of the processing conditions on this thermal history, a simplified model based on the three-dimensional heat conduction equations and an adequate replacement source will be developed for predicting the temperature development inside the manufactured specimen. By correlating the spatial and temporal temperature development inside the part during PBF-LB/M with the formed phases, adjusted processing strategies can be derived for achieving a duplex microstructure with an austenite-to-ferrite ratio of 50 %.A second approach for adjusting the microstructure and grain size is provided by the addition of nanoparticles that act as nucleation agents. Goal is to investigate to which extend the addition of nanoparticles in concentrations below 0.1 wt.-% affects the microstructure and the solidification behavior of DSS.To meet and resolve the thermally induced stresses in PBF-LB/M-manufactured parts, adjusted heat treatment strategies will be developed which simultaneously maintain the phase distribution and the process-specific grain size. Parallel to all these presented investigations, macroscopic material properties like hardness, tensile strength and corrosion resistivity will be determined and correlated with the obtained microstructure and phase formation.Based on the presented investigations, a fundamental understanding on the correlations between processing conditions, microstructure formation, and the resulting material properties of PBF-LB/M parts made from DSS is generated. Through adjusting process conditions shielding gas atmosphere in PBF-LB/M, nanoparticle concentration, and post-process heat treatment strategy, a high degree of control will be obtained for generating DSS parts using the PBF-LB/M process.