Page - 30 - in 3D Printing of Metals

Metals 2017,7, 2 supportedbytheMgalloy. Inaddition,once thecorrosionwas initiated, therefinedMg7Zn3phases distributedevenlyandmainlyservedasabarrier to impedecorrosionprocess. Similarly,Yangetal., duringSLMprocessingofMg-Mnalloys [66],observedthatcorrosionresistanceofpuremagnesium wasenhancedbytheadditionMnofupto2wt%during immersiontestingcarriedout insimulated bodyfluid (SBF) (pH7.4)at37 ◦Cfor48h. ThehydrogenvolumeevolutionobservedforMg-2Mn alloy (0.017 mL·cm−2·h−1) was significantly lower than that of pure Mg (0.068 mL·cm−2·h−1). The enhancement ofMgcorrosion resistancewas attributed to the increase in corrosionpotential andgrain refinement causedby the solid solutionofMn. Further, they suggestedSLMprocessed Mg-2Mnalloyasapotential candidate for futurebone implants.However, corrosion inbodyfluids is influencedbyvarious factorssuchaspH,concentrationandtypesof ions,proteinadsorptionon orthopedic implant, and influence of the biochemical activities of surrounding tissues [135,136]. Therefore, further investigations are still necessary to develop the reliability of SLM processed magnesiumparts forbiomedicalapplications. 8. PotentialofSLMtoFabricatePorousMagnesiumStructures Human bone has a hierarchical structure with three major anatomic cavities of different sizeswhich are haversian canals (50μm) [137], osteocytic lacunae (fewmicrometres) [15,138,139], and canaliculi (<1 μm) [140,141]. Each of the three cavities has amajor role in remodelling the processesandmechanical integrityof thebone[14].Aporousstructureallowsadequatespaces for transportationofnutrientsandforgrowthof livingtissues [142]. Especially formetallicorthopaedic applications,byadjustingtheporosity levels, themodulusof thematerialscanbegreatlycontrolled whichprovides anopportunity todesignmaterialswith amodulus closer to that of natural bone therebymitigatingproblemsrelated tostress shielding[143].Metal cellular structurescanbeclassified into structureswith stochastic and non-stochastic geometries.Metal stochastic porous structures typicallyhavearandomdistributionofopenorclosedvoids,whereasmetalperiodiccellular lattice structureshaveuniformstructuresthataregeneratedbyrepeatingaunitcell [144].Apart frommedical fields,metalliccellular/porousstructureshavealsobeenbroadlyutilizedintheautomotive,aerospace, andchemical industriesastheypossesslowerweight,goodenergyabsorptioncharacteristics,andgood thermalandacousticproperties[145,146].However, it isdifficulttofabricatestructuresofsuchcomplex externalshapesandintricate internalarchitecturesbyconventionalcastingandpowdermetallurgy methods. Althoughtheshapeandsizeof theporescanbeadjustedbychanging theparametersof thesemanufacturingprocesses,onlyarandomlyorganizedporousstructurecanbeachieved[144]. However,additivemanufacturing(AM)technologiescanfabricateporousmetalswithapredefined external shapeandinternalarchitecture tomatchthemodulusorstiffnessofbone, therebyminimizing oreliminatingstressshielding[147,148]. Selective lasermelting(SLM)offersasignificantadvantage ofproducingsuchveryfineandporousstructureswhileat thesametimeaccommodatingavariety ofshapes thatarenotonly limitedtoprismaticones [26]. Thismakes it thepreferredtechnologyfor producingmetallic scaffoldsandimplants.Also,StudiesonSLMhaveshownthat the internaland surfacefinishof the implantscanbetailoredtohaveselectivelyporousand/or lattice-likestructures to promote osseo-integration (bonding between the bones and the implant) in the implants [42]. SLMtechnologyhasbeenusedtoproducecomplexporous/cellularstructuresdirectly fromdifferent engineeringmaterials suchasstainlesssteel [144]andtitaniumalloys [108,149–151]andthereforehas thepotential toproduceporousstructures inmagnesiumalloys. Inarecentstudy, Jaueretal. [65]were successful in fabricatingscaffold-likestructureswithdesignedinterconnectedporosityoutofWE43 bymeansofSLM(Figure18).However, theprocessingparameters for fabricatingsuchstructuresare currentlyunderdevelopmentanddetailsof researchinthisareaarenotavailable in theopenliterature. 30