Seite - 82 - in 3D Printing of Metals

Metals 2016,6, 218 Analysis of the fracture surfaces of broken tensile specimens clearly showed that fracture of the as-built sampleswas basedon aductile-type failuremechanismconsisting in voidnucleation, growth and their coalescence. Larger voids mainly originated from pre-existing defects, such as unmeltedpowderparticles(seesphericalparticlesinFigure11a),splats(largerremeltedparticlesthatare blownawayfromthemoltenpoolandarethenengulfedintothefollowinglayers),cavitiesoriginatedby incompletemeltingbetweenadjacent tracks. Peakagedsamples (Figure11b) featuredsimilar fracture nucleation sites but the development of crackswasmainly ruled by a quasi-cleavage decohesion mechanism,presumablyacross themartensiteblocks (compareFigures5and11b). Figure 11. Fracture surfaces of broken tensile specimens: (a) as-built sample; and (b) 460 ◦C peak-agedsample. Additional informationaboutdevelopmentofcracksunder tensioncanbedrawnbyobserving longitudinally-sectioned specimens close to the fracture line, as shown in Figure 12. The low magnification image(Figure12a)allowsonetohighlight thegrowthof relatively largecracksstarting fromboundaries of adjacent laser tracks,whereas, fromhighermagnification views (Figure 12b), it canbesuggestedthat thepresenceof reversedaustenite locatedatboundariesofcellsdoesnotplay acrucial roleonfractureprocess. Figure12.Low-magnificationviewsof longitudinally-sectionedbrokentensile specimensclose to the fracture line. (a)As-built sample,optical image;and(b) 460 ◦Cpeak-agedsample,SEMimage. 4.Discussion Wrought maraging steels are usually aged after a solution treatment carried out at high temperature to homogenize the microstructure inherited from previous operations and to fully transform it intomartensite on cooling. Even though it is stated thatmartensite can form even aftermoderatecoolingrates, inpublishedresearchworkseitherwatercooling[2,4]orair cooling[5,8] 82