Powder bed-based laser beam melting (PBF-LB) for the production of tailored microstructures in quenched and tempered steel 42CrMo4

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Quenched and tempered steels form a significant proportion of the world's annual steel production (16-18%) and can now be found in many automotive, aerospace and mechanical engineering components such as shafts, gears and other higher strength structural parts. Nevertheless, the production of optimised components from quenched and tempered steels using additive manufacturing methods (e.g. laser beam-based powder bed fusion - PBF-LB) and research into resulting material and component states has made little progress. The reasons for this are mainly to be found in difficulties in consolidation from the metal powder by the laser exposure process. Quenched and tempered steels are generally considered difficult to weld due to their higher carbon content. Also in the PBF-LB process, similar to microwelding, high cooling rates occur in the weld and heat-affected zone due to the local energy input and the associated self-quenching. The thermal and transformation-related strains induced by the cooling rates lead to strong component distortion. The martensitic transformation in the weld zone also increases the probability of cracking. The probability of crack formation can be reduced during the process by using a heated build plate. However, the mechanical properties in the "as-built" state do not usually correspond to the technical specifications from the application. In addition, the downstream heat treatment is time-consuming and cost-intensive. The core of the project is therefore to control the intrinsic transformation and tempering effects occurring in the PBF-LB process and to use them specifically to integrate the conventional and otherwise downstream heat treatment directly in the PBF-LB process. In addition to the microstructural and mechanical characterisation, the present research proposal will use a phase-specific thermal-mechanical FE simulation to investigate the influence of novel exposure strategies with regard to a stress-appropriate, integrated and local heat treatment in the process. To validate the simulation, an in-situ characterisation of the process at the synchrotron is also planned. In this way, an important research gap in the field of additive manufacturing of heat-treatable steels will be closed.