Page - 56 - in 3D Printing of Metals

Metals 2017,7, 91 Figure 4. Samples respectively of the partsmanufactured in the EBMand SLMprocess in both orientations—0◦ and45◦—after removal fromthesubstrateplateandfacecutting the topandbase areaof the test specimenaswellasofcharacteristic cross-sectionalareas. Basedonthedatasetsof theCADsamples,all scaffoldswere fabricatedviaselective lasermelting usingtitaniumpowder (Ti6Al4V)underanultra-pureargonatmosphere. Thepowdercomplieswith therequirementsofASTMF67andhasadeterminedmeanparticlesizeof43.5μm.Thepartswere fabricatedwiththeSLM280bySLMsolutionsGmbH,Lübeck,Germany,usingacontinuous-wave Ytterbiumfiber laser. All partswerebuilt usingan identical energydensity (J/mm3). The energy densityE isdefinedbythe followingequation[41]: E= P v × d× t (1) whereP is laserpower,v is scanspeed,d ishatchspacingand t is layer thickness. Allpartswerebuilt ona substrateplatewitha support structureand in the sameorientation. The sampleswere heat-treatedunder hot isostatic pressing conditions after fabrication andwere subsequentlyremovedfromthesubstrateplate.Theheat treatmentwasperformedat920 ◦Cunder a pressure of 1000 bar for a duration of 120 min under an argon atmosphere by Bodycote Hot Isostating Pressing, Bruxelles, Belgium. The support structures were removed mechanically by hand. Onemanufacturingserieswithoutheat treatmentwas removed fromthesubstrateplate for acomparisonwith theEBM-manufacturedparts. TheelectronbeammeltingsystemA1byArcamAB,Mölndal,Sweden,wasusedtobuildtheparts forcompressiontestingandforcomparingthevalueswith thepartsmadebySLM.Theprocesswas carriedoutusingTi6Al4Vpowder fromArcamABwithameanparticlesizeof70μm.Thechamber, where the powder is applied andmelted, was evacuated until a pressure of 5× 10−4 mbarwas reachedanda layer thicknessof 50μmwaschosen. Theprocess consistedofapreheat themeand amelt theme. Thepreheat theme sinters the powder prior tomelting it for stability reasons [42]. AcomparisonwithSLMof theprocessparameters (Tables2and3) isnoteasysincescanspeedand beamcurrent (i.e., beampower) changeduring theprocessdue tovarious algorithms suchas the thickness equation, turningpoint or speed function. However, an average energydensity canbe calculatedforcomparativereasons. 56