Metal Additive Manufacturing Conference 2019

Hot isostatic pressing (HIP) is a well-established technology for reduction of residual porosity in cast materials as well as production of powder metallurgy tool steels. With the advent of additive manufacturing, especially for the medical and aerospace industry, HIP has become a standard treatment to close residual porosity or lack of fusion defects in order to increase the performance of 3d printed components. One of the traditional draw-backs of HIP is the long cycle time, notably due to the slow cooling phase. Thus, a 3d printed component typically required an additional heat treatment after HIP. The recent development of Uniform Rapid Quenching (URQ) techniques, however, allows for (at least) the same cooling rates within HIP furnaces as in conventional heat-treatment practices. The possibility to combine HIP, austenitizing/solution annealing and tempering/ageing treatment is therefore an attractive opportunity also for the tooling industry, especially with growing interest in additive manufacturing and near-net shaping of tool inserts. However, there is currently lack of knowledge on the effects of HIP-heat treatment on microstructure and properties of additively manufactured tool steels and to what degree a URQ HIP treatment influences the final properties. In this work, we present cases of components produced by powder bed fusion techniques using different types of tool steel materials. Analysis of microstructure in as-printed state and after (URQ) HIP together with mechanical property evaluation allows to identify the type of printing defects which are solvable by HIP treatment. PM versions of the tested materials were also produced to compare the printed properties with bulk “defect free” materials. Results show that the contouring strategy is of primary importance in order to obtain the necessary closed porosity in the near-surface, otherwise HIP treatment proves to be ineffective. On the other hand, larger defects produced during printing at the center of the specimen can be effectively closed. Results show also that PM materials cannot always be used as benchmark to evaluate the “low limit”-properties of additively manufactured components.