Page - 31 - in 3D Printing of Metals

Metals 2017,7, 2 Figure 18. Scaffold-like structureswith designed inter-connected porosity out ofWE43made by SLM[65]. 9.ConcludingRemarks Theavailablestudiesontheselective lasermelting(SLM)ofmagnesiumandmagnesiumalloy powdersarereviewedin thiswork. Processingparameters involvedduringSLMandtheireffectson formability,densification,microstructureandmechanicalpropertiesofdifferentmagnesiummaterials aredescribed. Itwasalsodemonstrated that theappropriatecontrolofprocessingparameters isof primeimportance forachievingthehighestpossibledensemagnesiumparts,andthesubsequently requiredmechanicalproperties.Analysisof the literaturerevealedthatmicrostructuralevolution in SLMfabricatedmagnesiumalloyshasbeengreatlycontrolledbythespecific laserenergy inputand becauseof thehighcoolingratesachieved, themicrostructuresproducedhavebeenextremelyfine. SLMisable toproducebulkmagnesiumpartswithcomparablemechanicalpropertiesandsuperior corrosionbehaviour to thoseofas-cast andwroughtalloys.Metallurgicaldefectsobservedduring SLMprocessingofmagnesiumalloyssuchasporosity, thermalcracking,oxide inclusions,andlossof alloyingelementswerealsoreviewed. Researchonadditivemanufacturing(AM)ofmagnesiumbasedmaterialshas increased inrecent yearsdueto thecombinationof thegeometrical freedomofferedbyAMalongwith itscapability to manufacture components and implantswith unique functional properties. The potential of SLM to fabricate topology optimized lightweight magnesium components that are tailored to actual loadingcasesaswell as toproduce implantswithdefined, interconnectedpore structures that can substantially improvebone ingrowthhasattractedsignificant interest fromtheautomotive,aerospace, andbiomedical sectors.However, comparedto iron, titanium,nickel, andaluminiumalloys, research onSLMofmagnesiumbasedmaterials is still in its infancyandaconsiderableamountof research isnecessarybefore thesematerials seeawidespreaduse in industrialapplications.Onearea that is crucial toestablish theSLMprocess formagnesiumpowdermaterials is theability toproducefully densepartsdevoidofballingandoxidation.Onlywhenthiscapability is routinelyandeasilyachieved canworkbeginonunderstandingthe influenceofprocessingparametersaswellaspowderproperties onthermalgradients, solidification, residual stresses,andeventually themicrostructure/mechanical properties of the SLM fabricatedmagnesium alloy components. From this, studies can begin to branchout todeterminetheresponseof thesematerials todifferent loadingconditions,aswellas to determine the influenceof temperatureandenvironmental conditionsontheirperformance towards moreunderstandingof thecorrosionbehaviourandbiological response inorder to furtherascertain thereliabilityof theproductsandsuitabilityofSLMtechnology inproducingmagnesiumalloys. A critical reviewshows that theunderstandingof themicrostructural aspects ofmagnesium alloys produced by SLM is well now developed but the mechanical properties are not widely available.Mostavailablestudieshavereportedonlydensityandhardnessvaluesandthemechanical properties suchas tensile, compression,andfatiguehavenotbeenreported. In theabsenceofsuch detailed data, the development of theoreticalmodels is not keeping pacewith the experimental work. Inaddition to theexperimentalwork,modellingandsimulationareneeded tooptimize the propertiesof the fabricatedparts. Particularareas that shouldbefocussedonare theeffectofvarious 31