Page - 13 - in 3D Printing of Metals

Metals 2017,7, 2 than500mm/s,powderparticleswerenotmeltedcompletelyandcausedvoidsbetweentheunmelted powders, resulting inasharpdropin therelativedensityof thesample to82.25%. Table4.RelativedensitiesofAZ91parts formedwithvaryingscanningspeedandhatchspacingsat laserpower=200W. ScanningSpeed(mm/s) RelativeDensity (%)atDifferentHatchSpacings 70μm 90μm 110μm 130μm 333 99.4 99.5 99.2 98.8 500 99.2 99.3 99 98.4 667 99.1 98.8 93.5 89.1 833 97.4 95.9 84.4 77.2 1000 91.8 89 76.5 73.4 Figure4.X-raydiffraction(XRD)patternsof theSLMedAZ91Dsamples fabricatedatdifferent laser energydensities indicatingabsenceofanysurfaceoxidefilms[61]. Layer thickness isanother importantparameter thathasasignificant impactontheporosityand layerbondinginthefabricatedpartsaffectingthetensilestrength,hardnessanddimensionalaccuracies in thedirectionof thebuild [83]. For instance,ahigherpowder layer thicknesswill result in less fusion betweentheparticlesbecause thesameamountofenergyhas toberadiatedtoagreateramountof materialascomparedtoa lower layer thickness.Consequently, lessdensepartswithmoreporesand voidswillbe formedas the laserenergydensitypenetrating thepowderbedwillbe insufficient to completelymelt thepowderparticles. Therefore, anoptimumlayer thicknessmustbeestablished toachievefiner resolutionandallowforgoodconnectivitybetween thepowder layerswith lesser degreeofspheroidisation leadingto formationofdenserparts.Minimumlayer thicknesscanhelp in improvingthequalityof themetallurgicalbondbetweensuccessive layersas ithelps increasingthe penetrationdepthof themoltenpools resulting inmultipleremeltingof thepreviouslydeposited layer andimprovingthewettingcharacteristics [61]. Layer thicknessalsohasaclose inverserelationship with the total processing time and determines the amount of energy required tomelt a layer of powder. Theeffectof layer thicknesson thedimensionalandmechanical featuresof the fabricated tracks frommagnesiumpowder, suchasmeltingdepth,meltingwidth,oxidation,elasticmodulus, andnanohardnesswasstudiedbySavalanietal. [58]. Intheirstudy,pulsedmodeNd:YAGlaserwitha wavelengthof1.06μm,spotsizeof270μm,pulsedurationof20nsandamaximumpowerof30Wwas usedtoprocess themagnesiumpowder. Scanningspeedswereadjustedbetween10and200mm/s while the layer thickness varied from150 to 300μm. The results demonstrated that there exists a criticalvalueof layer thicknessbeyondwhichsuccessful remeltingof thepreviouslyprocessed layers wasnotpossible leadingto formationof irregularanddisruptedsurfaces,whichwas250μminthat 13