Page - 20 - in 3D Printing of Metals

Metals 2017,7, 2 Figure 9. SEM micrographs of the microstructure of AZ91D alloys: (a) as-received alloy [81]; (b) laser surface melting (LSM) [81]; (c) Selective Laser Surface Melting (SLSM) of the β-phase (arrowmarked) [81]and(d)Selective lasermelting(SLM)[61]. Underanopticalmicroscopewith lowermagnifications, theSLMformationfeaturesareshown clearly. For example, the curve-like regular lasermelted trackson the cross-section correspond to laser scanning strategy, and the laminarmaterial structure and columnar architecture throughout thevertical sectionaredeterminedbythespecimenbuildingstrategy, suchas thescanningpattern, hatchspacing, and thicknessof layers. Thecutendsofmelted tracks in the formofaseriesofarcs onthevertical sectionwhicharealignedlayerby layer,are inducedbytheGaussenergydistribution of laserasshowninFigure10a [61]. Thepenetrationdepthof themoltenpoolswasobservedtobe upto312μm,whichwasapproximatelyeight times the layer thickness (40μm)usedin thestudyfor AZ91Dalloys (fabricatedata laserenergydensityof166.7 J/mm3) indicatingthateach layerof the asdepositedsampleundergoesaremeltingprocessmore thanonce. Themultipleremeltingprocess plays a significant role in determining themicrostructure of SLMed samples as different thermal histories experiencedbydifferent layers of the part, led to variation ofmicrostructures along the heightdirection,as theconduction,convection,andradiationconditionschange[61]. Theasprocessed microstructurecontainsat least twodistinct regions: onesignificantlyfiner thantheother,asshown inFigure10c,dforAZ91DandZK60alloys, respectively.At theedgesof themeltpool, thematerial experiencesmorecyclesof the remeltingprocess causedbybothoverlappingof thescan linesand creationofsubsequent layers to inducerelatively lowercoolingrates, resulting in localizedcoarsening of themicrostructure. Thisdifference in thermalhistorybetweentheedgesandcentreof themeltpool can inducenon-uniformdistributionofmicrostructure in thescaleof severalmicrons.Ascanbeseen fromFigure10c in thecaseofAZ91Dalloy, thegrainsonthecentreof thescanningtracks (CST)were finer (~1μm)thanthosenear theoverlappingedges (OLR)becauseof thedecreasingcoolingrateand multiple remeltingcyclesexperiencedat theedgesof themeltpool.Also,decreasingthe temperature gradient inside themelt pool, can lead to occurrence of columnar-to-equiaxed transition towards the centre of themelt pool. As canbe seen fromFigure 10d, columnarα-Mggrainsdominate the marginzoneof themoltenpoolwhereasα-Mggrains in thecentrezoneof themoltenpoolpresented anequiaxedmorphology in thecaseofZK60alloys. 20