Potential to influence microstructure and properties by applying a defined high porosity in the shell-core concept with PBF-LB and HIP

• Applicant: Dr.-Ing. Anke Kaletsch
• Subject Area: Materials in Sintering Processes and Generative Manufacturing Processes
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 431968665

Powder bed fusion with laser beam (PBF-LB) is one of the most widely used additive manufacturing processes. Nevertheless, PBF-LB manufactured parts are not yet used in many fields and their suitability for series production is often limited because the build rate for high volumes or large components is not yet economical and, at the same time, the reliability of the manufactured components is often insufficient due to internal defects and anisotropic microstructures. A common method to improve the mechanical properties of additively manufactured components is hot isostatic pressing. Some recent research aims to densify PBF-LB samples with high porosity when using HIP post-densification, as the PBF-LB process can be accelerated when lower quality/density samples are fabricated. In order to densify high porosity samples with HIP, a dense shell must be built around the porous core in the process; therefore, these approaches are usually referred to as the shell-core concept. This research project is based on the idea that the shell-core concept followed by HIP can not only accelerate the PBF-LB process but also can specifically influence the material microstructure and mechanical properties by a defined porosity before HIP. In general, grain refinement can be observed in high porosity PBF-LB specimens after HIP compared to fully dense built specimens. The reasons are not comprehensively understood so far. Since different types of porosity occur depending on the PBF-LB scanning strategy, different mechanisms that may be responsible for grain refinement are discussed in literature. This project aims to understand the different mechanisms in order to be able to use this knowledge to tailor the microstructure. At the same time, the influence of high amounts of compressed argon on microstructure and properties will be investigated. It is known that argon has no solubility in the metallic crystal lattice and thus remains in highly compressed pores in the material after HIP. Heat treatment following HIP densification results in expansion of the argon, leading either to growth of densified pores or even direct damage to the material. Both have been shown to have a negative effect on fatigue strength. New HIP systems are equipped with rapid cooling systems and are therefore able to perform the heat treatment directly under pressure. This prevents expansion of the argon and the gas remains highly compressed in the material. Whether these highly compressed Ar pores in very high quantity influence the microstructure locally or lead to residual stresses in the material is also to be investigated within the project. In addition, the influence of oxides in interaction with porosity will be given special consideration.

• DFG Programme: Research Grants