Page - 11 - in 3D Printing of Metals

Metals 2017,7, 2 affects the coherency of the formed tracks therebyhindering the construction of subsequent layers [70,71]. The balling phenomenon is a complex metallurgical process caused by the instability of themelt pool and theMarangoni effect [51]. Balling is also found to occur at very lowscanning speedsowing to the reduction in the surface energyof the liquidat short lengthscaleand long lifetimeof the liquid. Occurrenceofballingcausesamoredeteriorated surfaceresulting fromthecombinationof thermalstressesandweakinterlayerbondingbetween grainsandlayers. Effectsofballingwillbediscussedfurther insubsequentsections. Theoccurrenceofvarious regions in theprocessingwindowsofmagnesiumandmagnesium alloypowdersaresimilar to theregionsreportedforSLMprocessingmapsstudiesonaluminium[72], stainlesssteel [73],gold[74], andironpowders [67]. Table3.Reported laserenergydensitiesandrelativedensityofSLMedmagnesium-basedalloys. MaterialsSystem SpecificLaserEnergyDensities in theFormationZone RelativeDensity (%) Mg[56,57] * 1.27×10 3–7.84×103 J/mm2 (continuouswave irradiation) 1.13×106–9.8×106 J/mm2 (pulsedwave irradiation) - Mg[59] 200–300 J/mm3 95.3–96.1 Mg-9Al [60] 93.75–250 J/mm3 74.5–82 AZ91D[61] 83–167 J/mm3 73–99.5 ZK60[62] 138–416 J/mm3 82.25–94.05 ZK60[49] 420–750 J/mm3 72.8–97.4 *Onlysingle trackswereused. 3.2. Effects ofLaserProcessingParameterson theProcessingandDensificationofMetalPowders Laser processing parameters such as laser power, scanning speed, hatch spacing, and layer thicknesshavesignificant impactson the resulting relativedensityofSLM-producedsamples [75]. Therefore, it is important tounderstandtheeffectsof laserprocessingparametersduringthebuilding processandobtainsuitablecombinationof theseparameters tosuccessfullybuildnear fullydensity parts. Also, the rangeof suitableprocessingparametersobtained is found tobedependenton the chemical compositionof theprimaryalloys [52]. Generally, ithasbeenreportedthataminimumlaserenergydensity (orspeedbelowacritical value) is required to produce partswithmaximumdensity [76,77]. Zhang et al. [60] studied the influenceof laser energydensitieson thedensificationofMg-9%Alalloypartsby setting the laser powerandlaserscanspeedusingacontinuouswaveNd:YAGlaser. Itwasobservedthat the increase inlaserenergydensityfrom7.5J/mm2to15J/mm2resultedinimprovedrelativedensityofthemelted samples from74.5% to82%. At lower laser energydensities (higher speeds), there is a significant porositycausedbyincompletemeltingleadingtoformationofdiscontinuousscanningtrackswithlarge sizedballs.With increase in laserenergydensity,bettermeltingof thepowderswasachievedenabling more liquidphase toflowandinfiltrate thevoidsbetweentheparticlesandconsequently, thepores dispersed anddiminished to forma relatively smooth surfacewith increaseddensity. Maximum densificationwasachievedundera lower laserpower inputof15Wandascanningspeedof0.02m/s. However, further increment in the laser energydensity to 20 J/mm2 led to a reduction in relative density to76.1%.Thoughasufficientamountof liquidphasewasgeneratedathigher laserpowers, reducedscanspeedcausesreduction in thesurfaceenergyof the liquidatshort lengthscalecausing spheroidisationandbreakdowninthemeltpool [67]. Further. Theyconcludedthatacritical scanning speedof0.02m/swasrequiredtoensurecompletemeltingof thepowderswithoutevaporationand a soundpartwasnot obtainedabovea speedof 0.04m/s for all the laserpowersused. Asimilar phenomenonhasbeenreported inSLM-processedcommerciallypureTi [68] andTi-24Nb-4Zr-8Sn alloy [78]wherein itwas concluded that, once thepowder is fullymolten, therewasnobenefit in increasingthe laserenergydensity furtherduetooccurrenceofsomedetrimentalphenomenasuchas ballinganddross formation in themeltpool, resulting inapoorsurfacefinishandlowerdensity. The 11