Page - 76 - in 3D Printing of Metals

Metals 2016,6, 218 definedas thehatchdistance (dH). Basedonparameteroptimizationandmanufacturerdata,dH,dP, and twereset to80μm,65μm,and80μs, respectively. Thethicknessofeachpowder layerwasset to 40μm. Finally, sampleswereproducedusingameander scanning strategyandby rotating the scanningdirectionby67◦ aftereach layer. Figure2.GeometryofSLMprocessedbars. After removal fromthesteelbaseplate, the samplesweresectionedandmachined for further analyses.Microstructuralobservationswerecarriedoutbyopticalmicroscope(LeicaMicrosystems, RichmondHill,ON,Canada), andbyscanningelectronmicroscope (SEM,Oberkochen,Germany) equippedwithenergydispersiveX-rayanalysis (EDX,OxfordInstruments,Abingdon-on-Thames, UK)andelectronback-scattereddiffraction(EBSD,OxfordInstruments,Abingdon-on-Thames,UK) detectors. SampleswerepreparedbystandardgrindingandpolishingfollowedbyetchingwithPicral ormodifiedFry’s reagent. Aging responseof theSLMsampleswasevaluatedstarting fromsamplesboth in theas-built condition and after a standard solution treatment, carried out at 815 ◦C for 30min, followed by waterquenching. Differential scanningcalorimetry (DSC,LabsysSetaram,Caluire,France)analyseswerecarried outontheas-builtandonthesolutiontreatedsamples (weightofabout50mg)bytemperaturescans, at a rate of 20 ◦C/min inAratmosphere to assessprecipitation sequenceof strengtheningphases. Isothermalagingcurves (hardnessvs. agingtimeatconstant temperature)were thencollectedat460, 490, 540, and600 ◦Cfor times ranging from10min,up to14days. Theevolutionofhardnesswas followedbyperformingVickers indentationswitha loadof2kg. X-ray diffraction (XRD) patterns were collected using a X-Pert PRO (PANalitical, Almelo, TheNetherlands) instrument equippedwith aRTMSX′Celerator sensor. CuKα (k=0.15418nm) radiationwas employed. Peaks identification andquantitative analysis of phaseswasperformed byRietveldmethodusingMaudsoftware. Samples forXRDweresectionedbydiamondbladeand polisheddownto1μmgrit size,applyingconventionalmetallographicmethods. Inorder toavoid substantial structuralmodification, sampleswerecarefullypreparedbyaverygentleandprolonged polishingstage. Dog-bone tensile specimens having a gauge length of 20 mm and diameter of 4 mm were machined frombars treated according to different conditions investigated. In the present study only specimens built with their longitudinal axis oriented along the horizontal direction are considered. Tensile testswereperformedat roomtemperaturewithacrossheadspeedof0.5mm/min (corresponding to an initial strain rate of 4.2× 10−4·s−1) using aMTSAllianceRT/100universal testingmachine(MTS,EdenPrairie,MN,USA).At least threespecimensforeachconditionweretested. Finally, fractographicanalyseswerecarriedoutbySEMtoidentify themainfracturemechanisms. 76