Page - 75 - in 3D Printing of Metals

Metals 2016,6, 218 parameters, but also by surfacemorphologygivenby thepre-deposited layers and the stochastic particledistributionon thepowderbed, aswell as by thephysical surface andbulkproperties of thepowder itself [17]. Thesolidificationprocessandtheresultingmicrostructureassume, therefore, animportantrole thatstillneedsadeeperunderstandingandcontrol. Concurrently,alloyoptimization should consider these issues by deliberately promoting enhanced damage tolerance properties, especiallywhen considering opportunities offered by tuning of alloy chemistry and the selection ofpost-SLMthermal treatments. Inwroughtmaraging parts requiring increased toughness, over-aged temper conditions are preferablyselected inorder toallowtheformationofacontrolledamountofaustenite that remains stableevenatroomtemperature. Suchover-agingpromotessofteningandincreasescrackblunting effects [1].However, thisbeneficial influencemaybecounterbalancedbymatrixembrittlementwhen toocoarseparticlesstartactingascracknucleationsites [8]. Thepresent study is, therefore, aimedat investigating themechanical behaviourof 18-Ni300 maragingalloysamplesproducedbySLMasafunctionofspecificmicrostructuralconditionsobtained bydifferent thermal treatments.Analyseswillbeparticularly focusedonthepossibilityof improving thecombinationofstrengthandductilitybytailoredagingtreatments. 2.MaterialsandMethods An 18-Ni 300 maraging alloy (1.2709) supplied by Sandvik Osprey LTD (Neath, UK) as gas-atomized powder was investigated. The alloy chemical composition is given in Table 1, whileFigure1showsthegeneralmorphologyof thebatchofpowderconsidered. Fromtheparticle sizedistributionobtainedbylaserdiffractionanalysis, anaverageparticle sizeof35μmwasobtained andit couldbestatedthat90%of theparticlesdidnotexceedthesizeof54μm. Table1.Chemicalcomposition(weight fraction,%)of the investigated18-Ni300steelpowder. Ni Mo Co Ti Al Si 17.6 5.3 9.6 0.7 0.09 0.2 ȱ Figure1.Viewof the18-Ni300alloypowder investigated. ARenishawAM250SLMsystem(Wotton-under-Edge,UK)wasusedtoproduceasetof samples consisting of horizontal and vertical bars (10 mm× 10 mm× 75 mm), as depicted in Figure 2. MeltingofpowderwasperformedunderAratmospherebyasinglemodefiber laserwithapowerof 200Wandanestimatedbeamdiameterata focalpointof75μm.Lasermeltingwasperformedby discreteandpartiallyoverlappedspotsexposedto theradiationforafixedtime(t) andtheirdistance is calledpointdistance (dP). At theendof each scan line, the laser shifts to apartiallyoverlapped adjacent line to scan the selected surface of the layer. Thedistance betweenadjacent scan lines is 75