Page - 77 - in 3D Printing of Metals

Metals 2016,6, 218 3.Results Figure 3 shows low-magnification views of the as built structure. The width and depth of the distinct laser tracks could be clearly identified after etching on lateral sections (Figure 3a). Viewsobtained fromcross-sections takenperpendicularly to thebuilddirection,hereafter referred to as topviews, highlight thediscontinuousnature of themeltingprocess inducedby thepulsed laser beam: distinct pools pertaining to different layers are visible on the same track (Figure 3b). Highmagnificationmicrographs takenbySEMreveal theexpectedcellularsolidificationstructureand evidenceof theepitaxialgrowthacrossdifferent trackboundaries,asshowninFigure4. ȱ ȱ Figure 3. Optical images of the as builtmicrostructure of the steel investigated. (a) Lateral view; and(b) topview. ȱ Figure 4. SEM image of the cellular solidification microstructure and epitaxial growth across a trackboundary. After solution treatment, the traces of solidification completely disappeared and the cellular structurewasreplacedbyamartensitic structure,asdepicted inFigure5a. EBSDanalysesshowedthat the martensitic structure was fairly coarse. According to literature, the detected microstructural features can be interpreted as massive martensite blocks or packets (depicted in Figure 5b by theEBSDorientation image), consistingoffinebundles of parallel, heavilydislocated laths [8,18]. Tracesof reversedaustenite (γ-Fephase)werealsorevealedbyphasemapsbasedoncrystallographic information. It canbe stated that theγ-Fephase ismainly locatedatboundariesof themartensite blocks (Figure5c). 77