Page - 78 - in 3D Printing of Metals

Metals 2016,6, 218 ȱ ȱ Figure5.Representativemicrographsofthesolutiontreatedsample, (a)revealedbyopticalmicroscopy; (b)EBSBorientation image;and(c)phasemaphighlightingγ-Fephase (coloured inpink). Theagingbehaviourof theSLM-treated18-Ni300steelwasfirst investigatedbyDSCanalyses. A comparative study between samples cut from the as built and from solution treatedmaterials, showninFigure6a,allowedtostate that theagingsequence for these twotemperswassubstantially equivalent. BothDSCcurvesexhibitedfourpeaks. Thefirstexothermicpeak(peak#1) isbelievedtobe producedbythe formationofcarbideorcoherentprecipitationzones,whereas thesecondexothermic peak (peak #2) is usually associated in the literature to the formation of themain strengthening precipitates,namely theNi3Tiphase followedbyFe2MoorFe7Mo6 [1–4,19]. Theendothermicpeaks (peaks#3and#4)are likelydueto theaustenite reversionandto thedissolutionofprecipitates [4,19]. Isothermalagingtreatmentswerecarriedoutatselectedtemperaturesonboththeas-builtandthe solution-treatedsamples toevaluatepossibledifferences in theachievablestrength. InFigure6bthe hardnessevolutionduringagingat490 ◦Cisreported. Thesolutionannealingleadstoahardnessdrop from371HVto279HVwhich isbelievedtobeduetostress relievingeffectsandtoslightcoarsening of themicrostructure. However, thisgap inhardness is readilybridgedafteronly30minofaging. Thereafter, thehardnessof the twosamplesremainscomparable forallof theagingtimesconsidered here, in fullagreementwithobservationsderivedfromtheDSCresults. 78