Page - 21 - in 3D Printing of Metals

Metals 2017,7, 2 Figure10.Microstructureof (a–c)AZ91.[61]and(d)ZK60[62]alloysafterselective lasermelting: in(a), half-moonshapedmeltpoolsareclearlyvisible, (b)vertical section indicatingmultipleremeltingof each layer,while (d) showscolumnarα-Mggrainsdominating themarginzoneof themoltenpooland equiaxedα-Mggrains in thecentrezoneof themoltenpool. Microstructural featuresofSLMprocessedmagnesiumalloyscanbesignificantly influencedby theprocessingparametersused. Thecombinationofhigherscanningspeedsandlower laserpower results ina lower incidentenergyat the topof thepart, typically resulting infinermicrostructuresdue tohighercoolingrates. Incontrast, lowercoolingratesandcoarsermicrostructurescanbeobtainedby decreasingscanningspeedandincreasing laserpower.Atrelatively lowerscanningspeeds,prolonged interactionof the laserbeamwithpowders results in therestrainingofheatdissipation in themelt pool.Asaresult, relativelyequivalentcoolingratesduringsolidificationcanbeachieveddueto larger heat accumulation and thusproviding enhancedkinetic qualifications for epitaxial growthof the grains [49].With the increaseof laserenergydensity, thecrystallinestructureofmagnesiumalloys experiencesuccessivechanges in theorderofclusteredfinerdendrites,uniformequi-axedgrains to coarsenedequi-axedgrains.AscanbeseenfromthemicrostructureofSLMprocessedZK60alloys, extremelyfinedendrites (~2μm)which clustered severely together,were observedat a relatively lower laserenergy inputof420 J/mm3 (Figure11a). Thedendritescoarsenedtosomeextent (~4μm) and changed to a column shaped structurewith an increase in laser energy input to 500 J/mm3 (Figure11b),butstill exhibitedadisordereddistribution. Further increase in the laserenergy input to600J/mm3and750J/mm3 resulted inorderlydispersed,equi-axedgrainsof~6μm(Figure11c) and~8μm(Figure 11d), respectively. Thedendritic crystalline structurewas formed through the heterogenousnucleationofα-Mgandsubsequentdendritegrowth,whereas, theequi-axedcrystalline structurewas formedthroughthehomogenousnucleationofα-Mgandsubsequentequi-axedgrowth ofgrains [49]. Similar resultswereobservedinthe investigationofSLMofMg-9%Alalloypowders byZhanget al. [60]wherein significant grain refinement in the laser-melted regionwasobserved withgrainsizes in therangeof10–20μm.Themicrostructure theMg-Alalloyconsistedofequi-axed grains, transformedfromdendriticgrainsunderahightemperaturegradient.AnXRDanalysisof the laser-meltedsamples indicatedthepresenceofphases likeα-Mg,Mg17Al12,MgO,Al2O3. TheAl2O3 phasewas formedasa resultof incomplete reactionbetweenMgandAl, onlyundera lowenergy density inputof93.75 J/mm3. Further, itwasalsoobservedthat thecontentofMgdecreasedinthe laser-meltedregionbecauseofselectiveevaporationwith the increase in laserenergydensity. 21