Page - 15 - in 3D Printing of Metals

Metals 2017,7, 2 andspherical shapecancontribute towards improvementof the thermalconductivityof thepowder bedresulting in increaseddensityof theSLMprocessedpart [89,91].AstudybyHuetal. [59] suggests thatsmallermagnesiumpowderswithameshsizeof400(D50 =25.85μm)requirealowerlaserenergy input thanpowderswith amesh size of 250 (D50 = 43.32μm) to bemelted. Similar observations weremadeduringSLMprocessingof 316L stainless steel powders,whereinpowderswithD50 of 15μmand28μmrequired a lower laser energy input to achieve 99%density thanpowderswith D50 of 38μm[92]. However, severeoxidationandballingphenomenawereobserved for theparts madewithfinerparticlesas themeltpool temperaturewasobservedtobehigher thanthatofcoarser particles for the sameenergy input. Further,Huet al. observed the appearance of small grooves parallel to the spreadingdirection at the starting edgeof specimen fabricatedwith 400mesh size powderduringthespreadingprocessasshowninFigure6.At, thestartingedgeof thespecimen,as thescanspeedincreasedfromzeroto100mm/s, longer interactiontimebetweenpowderandthelaser beamcausedthemoltenpool toexist fora longtimeresulting inabsorptionofpowder fromtheheat affectedzone.Asaresult,groovesnear theprotuberanceedgeare formeddueto insufficientpowder availability. Furthermore,nogrooveswere formedinthecaseofcoarserparticlesas theycannotbe absorbedbythemeltingpoolaseasilyas thefinerparticles.However, theeffectsofpowdersizeor sizedistributionontheprocessingmapsofdifferentmagnesiumalloysarestillunclearas theyhave yet tobe investigated independently. Figure6.Macro-morphologiesof specimens fabricatedusingmagnesiumpowderswithgranularityof 400mesh(a) and250mesh(b) [59]. TheeffectivenessofSLMprocessinghasbeenfoundtobea functionofphysicalpropertiesof the material (Table5) suchas lowabsorptivity to the laserbeam, lowboilingpointelements,highthermal conductivity, high co-efficient of thermal expansion, tendency to form lowmeltingpoint eutectic phases, and lowviscosity [52]. One of the integral aspects of SLM is thedirect interaction of the powderswitha laserbeamandtheabsorptionofenergyby thepowder. Theabsorptance,defined as the ratio of the absorbed radiation to the incident radiation, affects the energy efficiencyof the SLMprocess. Determining theway energy is absorbed is essential to the thermal development since it allows for the determination of a suitable processingwindow, free of superheating and evaporationduetoexcessive laserenergyinputoranon-responseofpowderduetoaninsufficient laserenergy input [93]. Initially, incidentphotonsareabsorbedat theouter surfaceof theparticles in anarrow layerdeterminedby thebulkproperties of thematerial, leading to an increase in the temperatureof theparticle surfacesduring interaction.Until a local steadystateof temperaturewithin thepowder is reached, theheatflowwillbe fromthesurface to thecentreof theparticlesafterwhich the thermaldevelopment takesplace throughheat transferdeterminedbythesurroundingpowder properties [36]. This local no-uniformity in theabsorptance characteristics ofpowders can lead to selectiveareavaporisationduringthe interactionbetweenlaserandpowderparticles [36].Magnesium is highly reflective of the laser energies in the infrared region, having an absorptivity of 8%–20% foraNd:YAGlaserbeamwithawavelengthof1.06μmandanabsorptivityof~3%foraCO2 laser beamwith awavelength of 10.6μmat room temperature [94]. In comparison to the absorptance 15