Formation of insoluble silver-phases in an iron-manganese matrix for bioresorbable implants using varying laser beam melting strategies

The combination of immiscible metals is targeted to fulfil the requirements for certain technical and biomedical applications. The degradation behavior of iron (Fe) is adjustable by the insertion of silver (Ag) phases with high electrochemical potential causing anodic dissolution of the Fe-based matrix. To process such immiscible materials, powder-based processes like laser beam melting (LBM) can be applied. The continuous fusion process characterized by a small melt pool, strong melt flow, and rapid solidification enables the creation of well-dispersed Ag phases. To enable the adaption of insoluble phases this study aims to understand the mechanism responsible for the incorporation of insoluble phases within the matrix. Based on the characterization of FeMn with 5 wt.-% Ag processed via LBM the surface tension is determined as the essential influencing factor and a model of the melt pool including melt flow is developed. The Ag forms an almost closed layer on the top surface with an increased thickness between adjacent melt tracks. The lower surface tension of Ag compared to FeMn keeps Ag on the top and the outward-directed melt flow of underlying FeMn transports the Ag to the sides of the melt pools. During the generation of the next slice, deep parts of the Ag layer remain in the bulk material. By adaption of process conditions, e. g. the hatch distance, the size and shape of Ag phases are influenced. Moreover, the chemical composition depends on the LBM strategy, since the diffusion of alloying elements is differently pronounced.