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

由于具有几何灵活性和资源效率潜力，粉末床融合-激光束工艺（PBF-LB）在科学和工业上都引起了极大的兴趣。然而，可用于PBF-LB的铁基材料的种类最终仅限于那些具有低冷裂敏感性的材料，因为在此过程中存在很强的热梯度和相应的残余应力。为了克服标准原料粉末可加工性的限制和扩大可用材料的范围，粉末添加剂可以是一种合适的工具。该项目的主要目标是在生产氮(N)合金、耐腐蚀钢牌号的背景下，对添加氮化硅（Si3N4）颗粒的钢粉有一个基本的了解。通过实验和DEM模拟研究了粉末添加对粉末流变特性和反射行为的影响。PBF-LB和随后的热等静压超高速淬火（HIP-URQ）的可加工性以及微观组织的形成在尺度桥接微观组织表征和扩散模拟中得到了解决。在这种情况下，在PBF-LB和hips - urq过程中，Si3N4添加剂颗粒的溶解和可能的再沉淀作为影响微观结构和材料性能的主要因素而受到特别关注。本方法的目的是避免在PBF-LB过程中Si3N4添加剂颗粒的完全熔化，因为这可能导致显着的脱气和孔隙形成。相反，在后续的HIP-URQ后处理中，需要完全溶解剩余的添加剂颗粒，诱导元素N和Si扩散均匀化到钢基体中，最终得到N合金不锈钢。通过这种“反应性添加”方法，利用添加剂颗粒对粉末原料的潜在有益影响及其PBF-LB的可加工性得到了解决。此外，固相奥氏体相对较高的N溶解度被有意地用于向钢中引入大量的N，以改善材料性能，这些性能将与腐蚀测试以及静态和动态力学测试中的显微组织状态相关。

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.