Fabricability of a high alloy tool steel produced with LPBF, with a focus on part geometry

Abstract: Additive manufacturing (AM) is a promising manufacturing process that provides that ability to fabricate components with complex geometries with relatively low lead times compared to other manufacturing processes. This allows for more freedom of design, as prototypes can easily be produced throughout the development process. AM is also especially beneficial in tooling applications, where internal geometries such as cooling channels are required in order to improve the quality of the manufactured parts. These geometries are more difficult to produce with more conventional manufacturing methods such as forging or casting. Currently, Laser Powder Bed Fusion (LPBF) shows the most promise in the field of Additive Manufacturing (AM) of metals, as it offers the freedom to produce complex components with little post processing required. Additionally, post processing with Hot Isostatic Pressing (HIP) can be implemented to significantly enhance the final properties of the material.  The LPBF-process can produce many different defects within the parts such as: part porosity and lack of fusion. This is mainly due to the layer-by-layer configuration of the process. Parts can also experience large thermal fluctuations and rapid cooling rates which can generate large residual stresses. This can result in significant cracking in certain high alloyed materials which can impact part quality and  material properties. If the cracking is severe enough, it will result in failure of the entire component and render the entire parts completely useless. Post processing with HIP may remove some of these defects and reduce the residual stresses in the material and thus produce a material with properties that are satisfactory. The purpose of this thesis is to investigate the processability of a high alloy cold work tool steel with LPBF. The main focus is the influence of the processing parameters and part geometry on the quality of the produced parts. Furthermore, the influence of the processing parameters on defects and microstructure will also be investigated. The aim is to produce parts that can be enhanced with HIP as a post processing treatment. Additionally, the impact of HIP on the properties of the part will also be investigated in order to determine if the there are any improvements in terms ofreduction in part defects and the removal of any undesired microstructural features which are produced from the process. The experimental results showed that the processability of the tool steel is difficult. Several sample volumes were produced with varying processing parameters and scanning strategies, and all the specimens from all sample volumes exhibited some cracking. Parts produced with a combination of contouring and hatching strategy, where there is an internal structure showed the most promise, as these parts exhibited the least amount of severe cracking. However, additional research of the processing parameters and scanning strategies is required in order to reduce the amount of cracking of the external shell structure and thus, achieve proper densification of the parts when post processing with HIP.