Page - 27 - in 3D Printing of Metals

Metals 2017,7, 2 forvariousmagnesiumalloysare in the rangeof 20–35GPa,whichare close to the corresponding onesofhumanbones (3–20GPa), indicatingthat theycanbeconsideredasapotential candidate for biomedical implants [114]. Figure 15. Comparison of hardness and Young’s modulus values of SLM processed parts with conventionallycastandwroughtmagnesiumalloys. Figure16.Grainsizeagainst themicrohardnessvaluesofsingle trackmagnesiumsamples [56]. Figure 17 presents the comparison of tensile properties reported for lasermelted partswith conventionallycastandwroughtmagnesiumalloys in the literature.AscanbeseenfromFigure17, theyieldstrengthsofSLMpartsarecomparableorsuperior to those fabricatedfromcastandwrought materials inmostcases. This isgenerallyattributed to thenatureof theSLMprocess,whereavery smallamountofmaterial ismeltedata timeandrapidsolidification takesplace. This results inamore uniformmicrostructure throughout the part. For alloys, the segregation of the alloying elements takesplaceonamuchsmallerscale. Thechemicalcomposition ismoreuniformthroughout thepart, resulting inhigherstrength thanforcastparts [115].However, theelongation to failure is typically lower forSLMpartsandthiscouldbeattributedtomicro-porosityandoxide inclusionswithin the parts—aresultof thenon-optimizedSLMprocess.Weietal. [61] investigatedthetensileproperties ofSLMprocessedAZ91Dalloyatdifferent laserenergy inputs. Itwasestablishedthat laserenergy inputhada significant influenceon tensileproperties as theaverageultimate tensile strengthand yieldstrengthof theSLMedsampleswere foundtodecreasegradually from296MPaand254MPa at 166.7 J/mm3 to274and237MPaat 83.3 J/mm3. This is as a result ofpoor relativedensities of 27