Influence of Si3N4 powder additivation on PBF-LB processability of stainless steels and microstructural evolution during PBF-LB and a subsequent HIP-URQ densification process

由于具有几何柔性和资源利用潜力，粉末床熔融激光加工技术(PBF-LB)在科学界和工业界都受到了极大的关注。然而，可用于PBF-LB的铁基材料的种类最终仅限于那些对冷裂敏感性较低的材料，因为在此过程中存在强烈的温度梯度和相应的残余应力。为了克服标准原料粉在加工性能方面的局限性，拓宽可用原料的范围，粉末加入法可以是一种合适的手段。该项目的主要目标是在生产氮(N)合金化、耐腐蚀钢种的背景下，实现对钢粉添加氮化硅(Si3N4)颗粒的基本了解。通过实验和DEM模拟研究了粉末添加剂对粉末流变性和反射行为的影响。在比例桥联显微组织表征和扩散模拟中，研究了PBF-LB和随后的热等静压超快速淬火(HIP-URQ)的加工性以及微观组织的形成。在这种背景下，Si3N4添加剂颗粒在PBF-LB和HIP-URQ过程中的溶解和可能的再析出是特别值得关注的，因为它是影响微观结构和材料性能的主要因素。本方法的目的是避免在PBF-LB过程中Si3N4添加剂颗粒完全熔化，因为这可能导致显著的放气和气孔形成。相反，需要在随后的HIP-URQ后处理中完全溶解剩余的添加剂颗粒，导致元素N和Si扩散均匀到钢基质中，最终得到N合金化不锈钢。通过这种“反应加成”的方法，探讨了添加剂颗粒对粉末原料的潜在有益影响及其在PBF-LB中的加工性能。此外，还特意利用固相奥氏体较高的N溶解度向钢中引入了大量的N，以改善材料的性能，这些性能与腐蚀试验以及静态和动态力学试验中的组织状态有关。

Due to the geometric flexibility and resource efficiency potential, the powder bed fusion - laser beam process (PBF-LB) has gained significant interest in both science and industry. However, the variety of available iron-based materials for PBF-LB is ultimately limited to those offering a low susceptibility for cold cracking, because of strong thermal gradients and consequential residual stresses acting during the process. To overcome the limitations in processability of standard feedstock powders and broaden the range of available materials, powder additivation can be a suitable instrument. The main objective of this project is to achieve a fundamental understanding of steel powder additivation with silicon nitride (Si3N4) particles in the context of the production of nitrogen (N) alloyed, corrosion resistant steel grades. The investigation includes the influence of powder additivation on rheological powder properties and reflection behavior, studied by experiments and DEM simulation. The processability in PBF-LB and subsequent hot isostatic pressing with ultra-rapid quenching (HIP-URQ) as well as microstructure formation is addressed in scale-bridging microstructural characterizations and diffusion simulations. In this context, the dissolution and possible re-precipitation of Si3N4 additive particles during PBF-LB and HIP-URQ is of particular interest as the main influencing factor on microstructure and consequentially on material properties. The aim of the present approach is to avoid the complete melting of the Si3N4 additive particles during PBF-LB as this can result in significant outgassing and pore formation. Instead, a complete dissolution of the remaining additive particles is desired upon a subsequent HIP-URQ post-processing, inducing a diffusional homogenization of the elements N and Si into the steel matrix and ultimately resulting in a N alloyed stainless steel. With this approach of “reactive additivation”, the utilization of a potentially beneficial influence of the additive particles on the powder feedstock and its processability by PBF-LB is addressed. Furthermore, a comparatively high N solubility of the solid phase austenite is purposely utilized to introduce considerable amounts of N into the steel for an improvement of the material properties which will be correlated to the microstructural states in corrosion testing as well as static and dynamical mechanical testing.