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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.
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Buch 3D Printing of Metals"
3D Printing of Metals
- Titel
- 3D Printing of Metals
- Autor
- Manoj Gupta
- Herausgeber
- MDPI
- Ort
- Basel
- Datum
- 2017
- Sprache
- englisch
- Lizenz
- CC BY-NC-ND 4.0
- ISBN
- 978-3-03842-592-2
- Abmessungen
- 17.0 x 24.4 cm
- Seiten
- 170
- Schlagwörter
- 3D printing, additive manufacturing, electron beam melting, selective laser melting, laser metal deposition, aluminum, titanium, magnesium, composites
- Kategorien
- Naturwissenschaften Chemie