Page - 83 - in 3D Printing of Metals

Metals 2016,6, 218 wereadopted. Fromthepresentresults (DSCscansandevolutionofhardnessduringisothermalaging) it canbe inferredthat thesolidificationandcoolingrates inducedbySLMprocessingare fastenough topreserve the full agingpotential of the 18-Ni 300maraging alloywithout anyneed toperform anadditionalpost-SLMsolution treatment. This result is in full agreementwith the researchwork publishedby Jägle and coauthors [6]. By atomprobe tomography, these researcherswere able to showthat, inaCo-containingmaragingsteel similar to thepresentalloy,noprecipitatesorclustersof atomswerepresent in theSLMas-built state, evenconsideringthateach layerhadbeensubjectedto significant reheatingduringthe layer-by-layermanufacturingprocess. Furtherliteraturereferredtoweldingofawrought18-Ni250grademaragingsteel[18]alsoreports that, intheabsenceofhigh-temperaturehomogenizingtreatments, theas-weldedmicrostructuremight containsegregationofMoandTiatcellboundaries. Especially,Mocanenhance thereversionrateof martensite intoausteniteonagingduetoearly formationofFe2Mo,resulting in localenrichment in Niof thematrix.Molinarietal. [19]alsoclaimedthatagingandaustenite reversion isacceleratedby enhanceddensityofstructuraldefects inmaragingsteelprocessedbysparkplasmasintering.Asimilar effect isexpectedtobeactivealso in therapidlysolidifiedsampleshere investigated.Microstructure observations (Figure9)confirmedthataustenite reversiononagingfirstoccurredatcellboundaries andonlyonover-aging(8hat540 ◦Cand4hat600 ◦C) intracellularaustenitecouldappear. Reversionofausteniteduringaging inwroughtalloyscanbeusedto tuneductilityandfracture toughness. Viswanathanandco-workers [8], by investigatingawrought18-Ni350maragingsteel, stated that reversedaustenitecausesadecrease inyieldstrengthandultimate tensile strengthbut it is beneficial totensileductilityandimpact toughness intheinitialstagesofover-aging. Thesameauthors also investigatedthemorphologyof reversedaustenite formedatdifferentstagesofover-aging. It is to remarkthat in thewroughtalloy,austeniteappearedbothassubmicrometric-sizeprecipitatesat boundariesofmartensiteblocksandasmuchcoarser intragranularpockets, aroundfivemicrometres insize. In the SLMed samples of the present investigation, retained austenitewas already found in as-built samples and further precipitated,mainly at boundaries of cells, in a verydispersed form onaging. It canbe reasonably supposed that their tinysizeandfinedispersion, asopposed to the micrometricpocket-likephases found inwroughtalloys, is such toavoidasubstantial toughening actionascrackarresterphase. Finally, it is toremarkthat theaustenite reversionwasstimulated in thepresent investigationby prolongedagingtreatments (i.e., over-aging).However, theprogressionofaginggeneratesanumber of combined effects that have to be carefully analysed before drawing any conclusion about the present results.Agingmainlypromotes: (i) thereversionofmartensite intosofteraustenite; (ii) the formationofstrengtheningprecipitates; (iii) thecoarseningofprecipitatesandembrittelementofalloy onover-aging. Thecombinedeffectsofagehardeningandsoftening inducedbyformationof reversedaustenite intheinvestigatedSLMedalloycanbebetteranalysedbyreplottingthefulldatasetoftensileproperties, as shown inFigure 13. The inverse relationbetweenyield strengthand tensileductility is readily observed inFigure13awhereclusteringof thedatapoints into twodistinct regions isalsonoticed. Afirstgroupofdata referred toagedsamples isvisibleon the top left cornerof theplot,while the threedatapointspertainingtoasbuilt samplesaredisplayedontherightsideof theplot, showing moderatestrength levels,buthigher tensileductility. 83