Plastic anisotropy of additively manufactured maraging steel : influence of the build orientation and heat treatment

This study investigates the combined effect of three Additive Manufacturing (AM) build orientations (0° , 45° , and 90° ) and an extensive array of heat-treatment plans on the plastic anisotropy of EOS maraging steel 300 (coded by EOS as ’MS1’) fabricated on the EOSINT M280 Direct Metal Laser Sintering (DMLS) system. A comparison between the material microstructural characteristics, mechanical performance, plastic strain behaviour and anisotropy levels of identical parts fabricated on two of these independently operated DMLS systems was also conducted. Considerable plastic anisotropy has been confirmed in the as-built condition, which can be reduced by aging heat-treatment, as verified in this study. However, it has identified that a degree of transverse strain anisotropy is likely to remain due to the AM alloy’s fabrication history, a finding that has not been previously reported in the literature. Significant microstructural discrepancies, affecting mechanical performance, plasticity and anisotropy levels, have also been observed in the as-built samples obtained from the two DMLS systems. A difference in the angle of the laser scan strategy, in conjunction with unfavourable powder feedstock characteristics, are understood to have had a profound influence on the plasticity and anisotropy divergences observed. A comprehensive visual representation of the material’s overall mechanical properties against the various heat-treatment plans is offered through time-temperature contour maps. Moreover, the heat-treatment plan (6h at 490° C) recommended by the DMLS system manufacturer has been found not to be the optimal in terms of achieving high strength, hardness, ductility and low anisotropy for the MS1 material. With the use of the comprehensive experimental data collected and analysed in this study, and presented in the constructed contour maps, the alloy’s heat-treatment parameters (time, temperature) can be tailored to meet the desired strength/ductility/anisotropy design requirements, either for research or part production purposes.