Page - 4 - in 3D Printing of Metals

Metals 2017,7, 2 implants [11–13]. Magnesium based materials have lower Young’s modulus (41–45 GPa) than commonlyusedmetallicbiomaterialssuchas titanium(55–110GPa),316Lstainlesssteel (210GPa), andcobalt chromiumalloys (240GPa)andshownoindicationsof localorsystemic toxicity [14–16]. In addition, they are also osteo-conductive, facilitate bone cell in-growth, andhave a role in cell attachment [17]. Due to these advantages, new types ofmagnesium implantmaterials havebeen developedwhicheffectivelyaid in themitigationof stress-shieldingeffects andhavepotential for useasabioresorbablematerial fordegradablebonereplacement,eliminatingtheneedofsecondary surgicalprocedures [10].Althoughmagnesiumhasmanyadvantages, suitable forhardtissue implant andtissueengineeringscaffoldmaterial,usageofmagnesiumisstill limitedinclinicalapplicationsdue to itspoor formability, rapiddegradation inahighchloridephysiologicalenvironmentandhydrogen evolution[18]. Therefore, continuouseffortsarebeingmadebyresearchers todevelopnewtypesof magnesiumalloysandcomposites tomeetspecificpropertyrequirementsandexplorenewprocessing technologies to fabricatepatient-specific implantcomponents thatcanbeprovidedwithadditional functions to furtherbroadenthehorizonofmagnesiumutilization inbio-medicalapplications. Magnesiumbasedmaterialsareusually fabricatedbyconventionalmanufacturingmethodssuch asdeformationprocessing,casting,andpowdermetallurgy(P/M)techniques.Usually lightweight engineeringpartswithhighperformancecanbeobtainedfromdeformationprocessingofmagnesium basedmaterials. However, due to the hexagonal closed packing (HCP) structure ofmagnesium, magnesiumalloys exhibit poor coldworkability at roomtemperature. Deformationprocessingof magnesiumthereforeneeds tobeperformedatelevatedformingtemperatures toactivatemoreslip systemsandtoallowbetter formability,which leads topoorsurfacequalityandoxidationofparts andlimitsefficiency[19].Asaresult, consumptionofwroughtmagnesiumproductsonlyrepresent a small fraction,merelyabout1.5%of totalmagnesiumconsumption [20]. Presently, casting is the mostconventionalanddominantsynthesis routeusedfor themanufactureofmagnesiumalloysand composites.Although,casting techniquesensuregreatefficiencywithhigherprecision, it isdifficult to fabricatenear-netshapestructuresofcomplexshapesandintricateinternalarchitectures.Moreover, it is oftenthecasethatproductquality isdegradedbythethermodynamicallystablephasesthatareformed duringsolidificationfromthemeltandstrongoxidising tendencyofmagnesium[21]. It isnotpossible tocontrol themorphologyand/ordistributionof thesephasesduringcooling. Therefore, severalP/M routes are being explored to target unique microstructures, novel alloy compositions, and high performance inmagnesiumalloys [22]. Promisingresultswereobtainedbyreinforcingmagnesium withnanocrystallineandamorphousalloypowders. Forexample,Mg-Zn-Yalloyshavingveryhigh tensileyield strengths in the rangeof 480–610MPawithanelongationbetween5%and16%were developedusingarapidlysolidifiedP/Mapproach[23].Also,advancedpowderbasedmanufacturing processessuchasadditivemanufacturing (AM), coldspray,metal injectionmoulding,andfrictionstir processingarebeingdevelopedto fabricatemagnesiumalloyshavingnon-equilibriumcompositions andlimiteddefects [22]. These techniquescanbesuccessfullyemployedtodesign intricateandnear net shapedstructures. Inarecentstudy,Tandonetal. [24] showedthatmagnesiumalloypowderscan bepotentiallyusedtomanufactureandrepair lightweightcomponents foraerospaceapplicationsby usingcoldsprayandlaserassisteddepositionprocesses. Asmagnesiumisexpanding intoamorepromising lightweight regimeandmedical technology applications, there is a great need for intelligent selection ofmanufacturingprocesses to provide uniquefunctionalproperties, crashperformance,andcorrosionresistance.Customisedcomponents andimplantswith improvedmechanicalandphysicalpropertiescanbemanufacturedbyadditive manufacturing(AM)techniques.AMincludesawholehostof“bottomup”approaches,wherein the processes involvecreatingthree-dimensionalobjects fabricateddirectly fromcomputeraideddesign (CAD)modelsbygraduallybuildingthemup, layer-by-layerwithinapowderbed.AmongtheAM methods, laser-basedAMhasanimmensepotential forproducingfullydensemetallicstructuresusing avarietyofavailablemetalpowdersandhasattractedmoreandmoreattention[25]. Selective laser melting(SLM)isonesuchprocess thatuseshighintensity laserasanenergysource todirectly fuse 4