Page - 16 - in 3D Printing of Metals

Metals 2017,7, 2 of bulkmaterialswith smooth surfaces, powdermaterials have significantly higher absorptance regardlessof thewavelengthof irradiation. Forexample,atλ=1.06μm,theabsorptanceofTipowder wasobservedtobe77%,which isnearly2.5 timesgreater thanforTibulkmaterial,whichwas just 30%[36,95].Althoughpowdermaterialswillhavesignificantlyhigherabsorptance incomparisonto bulkmaterialsowingtomultipleabsorptionandreflections in thepowderbed,higher thanexpected laserenergiesarerequiredtoovercomeissuessuchasstrongevaporationandreflectionof the laser radiation [95]. Also, temperaturegradients likely tobe formedduring theuseofoverlappingscan trackscausedbydifference in theabsorptivityofalreadymeltedmagnesiumandtheneighbouring powder,may lead to balling. SLMprocessing ofmetal powderswas reported to be significantly betterwith theNd:YAGlaser compared toCO2 laserdue to its shorterwavelength,which in turn reducedthe threshold irradiancerequiredformeltingofpowdersatequivalentpenetrationdepthand scanningspeed. Also, aNd:YAGlaserbeamproducesa slightly larger andmore stable scan track thanaCO2 laserbeamduringSLMprocessing[96].However, it isnoteworthythat theabsorptivity ofmagnesiumoxide (MgO) forNd:YAGlaserbeamis~20%whereas ithasanenergyabsorptivity of93%–98%foraCO2 laserbeamimplyingthatMgOsurface layersonmeltedpartscaneffectively increase theabsorptivityof laserbeams [97].Therefore, itwouldbean interestingandworthwhile taskto investigate ifapplicationofaCO2 laserbeamcanmeltorevenevaporate theoxidespresent in themoltenpoolowing toenhancedenergyabsorption, therebyreducing theoxide inclusions in fabricatedparts. Table5.Propertiesofpureandalloyedmagnesiumat itsmeltingpoint [94]. Properties (Unit) PureMg Ionisationenergy(Ev) 7.6 Specificheat (J/(kg·K)) 1360 Specificheatof fusion(J/kg) 3.7×105 Meltingpoint (K) 650 Boilingpoint (K) 1090 Viscosity (Pa-s) 0.00125 Surface tension(N/m) 0.559 Thermalconductivity (W/(m·K)) 78 Thermaldiffusivity (M2/s) 3.73×10−5 Expansionco-efficient (1/K) 25×10−6 Density (kg/m3) 1590 Elasticmodulus (N/m3) 4.47×1010 Vapourpressure (Pa) 360 Magnesiumalloysalsopossessveryhigh thermal conductivity inaddition tohighreflectivity. However, deposited powders have relatively poor thermal conductivity and can be considered thermally insulating compared to the solid part of the substrate [86]. As heat is applied, it flows moreslowly throughthepowder,whichcan leadtooverheatingof themelt surface located, impacting thesizeof themeltpool formedandcausingdensitydifferencesbetweenthesolidandthepowder[98]. Meanwhile,duetohighreflectivityandhighthermalconductivityofmagnesiumpowder, theSLM process isdifficult tocontrol comparedwithproducingothermetalpowderssuchasstainless steelsor titaniumalloys.Also,more laserenergy is requiredtomelt themagnesiumpowders thanformaterials with lowerconductivity. During SLM processing of Mg-9%Al powders, Zhang et al. [60], witnessed formation of cauliflower likegrainswith interconnectedporeson the surfaceof themetal as shown inFigure7 causedbyametal evaporation–solidificationprocess. Thoughthemajorityof the laserenergywas reflectedbecauseof lowabsorptivityofbothmagnesiumandaluminium,adequateamountofMg-Al powder could bemelted and amolten poolwas formed even at lower energy densities. Due to differences in the thermalproperties, themoltenpoolwouldabsorbmuchmore laserenergythanthe surroundingpowderlayer, leadingtoincreaseinthemoltenpool temperaturebeyondtheboilingpoint ofmagnesium(1093 ◦C).Subsequently,an increase invapourpressureofmagnesiumfrom0.36KPaat 16