Alloy- and process design of additively manufactured bulk and lattice structures of austenitic advanced high-strength steels

Laser powder bed fusion (LPBF), with its potential to overcome design deficiencies of conventional manufacturing processes and to print highly complex parts such as lattice structures with unique properties, is one of the key technologies of metal additive manufacturing. Material related challenges include taking advantage of the fast solidification conditions during LPBF and the development of new process adapted alloys with increased mechanical properties. In this work, an alloy- and process design strategy was developed to take advantage of the fast solidification conditions during LPBF and enable microstructure refinement and texture randomization of austenitic advanced high-strength steels (AHSS). In addition to the micro-structural adaptation, alloy-specific deformation mechanisms were used to tailor the work-hardening behavior of LPBF produced bulk and lattice structures. During the process design, LPBF process parameters were adjusted to understand process-microstructure-property linkages. To determine additional process characteristics that are difficult to determine experimentally, the melt pool geometry and the local solidification parameters were simulated by a finite element melt pool model and correlated with the microstructure. AHSS produced with higher laser speeds and an alternative laser scan strategy revealed higher strength and energy absorption potential during plastic deformation, which was attributed to the partial transition from a columnar to equiaxed solidification of grains and the high accumulation of geometrically necessary dislocations. The alloy design was guided by computational thermodynamics and the usage of powder mixtures of AHSS and pure aluminum to generate bulk and lattice structure specimens with varying aluminum content by LPBF. The transition from austenitic to ferritic-austenitic solidification and solid-state ferrite to austenite phase trans-formations allowed to control the microstructure and texture evolution during LPBF. Additionally, the variation of the aluminum ...