12th Tooling Conference & Exhibition - Tooling 2022

The use of Laser Additive Manufacturing (LAM) techniques, such as Laser powder bed fusion (L-PBF) or Laser Directed Energy Deposition (L-DED) for the manufacturing of forming tools has gained increasing interest in both industry and academia due to the processes’ high geometrical flexibility. LAM allows for a layer-wise build-up of parts based on 3D-CAD-data by either the local melting of a metallic powder bed by a laser beam (L-PBF) or a local application and laser beam melting of powder material by a nozzle (L-DED). Owing to the iterative local heat input by the laser beam, a locally and temporally unsteady heat flow as well as rapid heating and cooling occur in the part, resulting in non-equilibrium solidification, phase transformations and the formation of microstructural defects, cold cracks and distortion. In the case of carbon-martensitic tool steels, which are usually employed in tooling applications due to their high hardness and wear resistance, especially their cold crack susceptibility is problematic and is usually counteracted by a build-plate/ substrate preheating. Since preheating can increase the oxygen take-up of the powder and alter the part microstructure, preheating can be disadvantageous for part quality and powder reusability. In this study, a carbon-martensitic hot work tool steel specifically designed for the production of parts with low crack density by LAM without preheating is investigated. The study focuses on the microstructure and mechanical properties of the L-PBF- and L-DED-manufactured steel in as-built as well as heat treated condition. Results show that the steel can be LAM-processed without preheating resulting in specimen with low crack densities and a martensitic microstructure with retained austenite. Both hardness and strength can be increased by quenching and tempering. However, directly tempering the as-built specimen without previous quenching leads to a shift of the secondary hardness peak towards higher hardness and higher temperature.