Seite - 84 - in 3D Printing of Metals

Metals 2016,6, 218 (a)ȱ (b)ȱ (c)ȱ Figure13.Relationbetween0.2%yieldstrength, tensileelongationat fracture,andaustenitevolume fractionfor theSLMprocessed18-Ni300steel. Both fracture elongation andyield strength can then be independently plotted as a function of austenite volume fraction, as depicted in Figure 13b,c, respectively. These figures highlight an unexpected trend considering that austenite can act, in principle, as a soft and more ductile constituent in steelmicrostructures. These results remark an importantdifferencebrought by the peculiarmicrostructural features generatedbySLMprocessingwith respect to themicrostructure found inwroughtproducts. Indeed, thehighestyieldstrengthand lower tensile elongation found for peak-aged samples (that also feature higher fractions of austenite) over the as-built condition arebelievedtobemainlydueto theoverwhelmingeffectof strengtheningprecipitates,whereas the concurrent reversionof theγ-Fephaseseemstoplayaminorrole. 5.Conclusions Astudywasundertakento investigate theeffectsofselective lasermeltingon18-Ni300maraging steelmicrostructureandmechanicalbehaviour. Themainconclusionscanbesummarizedas follows: A submicrometric cellularmicrostructurewas detected in the as-built samples. Evidence of epitaxialgrowthacrossdifferent laser trackswasrevealed.Afterhigh-temperaturesolutionannealing, the tracesof solidificationwere lostandthecellular structurewasreplacedbyfairlycoarsemartensite blocks. Small fractionofaustenitewerepreferentially locatedatblockboundaries. Acomparisonabout theagingbehaviourof theas-builtandthesolution-treatedalloyrevealed that theprecipitationsequenceduringagingandhardnessevolutionaresubstantiallyequivalent. 84