Page - 28 - in 3D Printing of Metals

Metals 2017,7, 2 SLMed samples obtained at lower energy inputs coupledwith lower solid solubility of theα-Mg matrixandasmallerquantityof intermetallicβphase inducedbylowerenergyinputs.Analysisof the fracturebehaviourofSLMpartsshowedfeaturesofductile-brittlehybridfracture.Owingto the layerby layermanufacturingapproach, itwasobservedthat thepart’sbuildingorientationduring SLMwas found to affect the resultant tensile properties of the part. Specimens deposited in the directionalong(e.g.,parallel to) the lengthof the tensilesamples (e.g.,X-direction) typicallyexhibit higher tensile strength than thosewith layers deposited perpendicular to their length (e.g.,Y or Z-direction) [116]. Althoughthereasons for thishavenotbeenwell investigated, itdoesshowthat the effects of thebuildingorientation formagnesiumalloys ismore involvedandrequires further investigation.Also,performingahot isostaticpressing(HIP)procedureafterSLMcansignificantly reduce theanisotropicmechanicalbehaviourof theSLMedpartsbyreducingmanufacturing induced porosity [116]. Figure 17. Comparisonof tensile properties of SLMprocessedpartswith conventionally cast and wroughtmagnesiumalloys. 7.CorrosionBehaviour Magnesiumalloys,generally, revealapoorcorrosionresistance,which ismainlyassociatedwith theirhighchemicalactivityandthe lackofaprotectivepassiveoxidefilm[117,118].Thisdisadvantage hasbeenamajorobstacle restricting their furtherapplication inautomotive,aerospace,andelectronics industries.Also,magnesiumhas ahighnegative standard electrodepotential,which leads to the rapid corrosion of magnesium based alloys in chloride physiological conditions [119]. This has delayed the introduction of magnesium based materials for therapeutic applications to date, as thehydrogengasproducedatahighrate fromcorrosioncannotbedealtwithbythehost tissue [18]. Additionally, shift inalkalinepHin theregionsurrounding thecorrodingsurface isalsoaconcern for biomedical applications [119]. Therefore, development of magnesium alloys with improved corrosion behaviourmay help to resolve the current limitations ofmagnesium alloys for use in the aforementioned industries. Rapid solidification has been identified as an effectivemethod to improve thestrengthandcorrosionresistanceofmagnesiumalloys forstructuralandcorrosivemedia utilization [40]. Lasermelting is one such rapid solidification process involving cooling rates up to 106–8 ◦C/s and is capable ofmodifying surface properties as it canhomogenize and refine the microstructure,anddissolvesecondaryphases [120,121].However, so far, littleefforthasbeenmade toexamine thecorrosionbehaviourof theSLM-producedmagnesiumpartsasmost studieson the SLMofmagnesiumalloypowdershavebeenfocussedonthedensificationandmechanicalproperties of theSLM-producedsamples. Becauseofthepaucityofliteraturedealingwithcorrosionbehaviouranditsassociatedmechanism duringSLMofmagnesiumalloys, itwouldbeuseful toexamine theeffectof laser surfacemelting 28