Seite - 138 - in 3D Printing of Metals

Metals 2017,7, 113 Obviously,amaximumincrease instiffness isachievedbysupporting theedgeofcentralhole with additionalmaterial. However, laser cladding close to theunsupported inner hole leads to a significantly higher risk of distortion than in the case of cladding at the outer edge of the dome. Toevaluate the influenceof thepositionof thecladdedlayeronthedistortionsimulationsof theplastic straincausedbythelasercladdingprocesswereperformed.Tothisend,singlecircularpathsofthelaser claddingprocessweresimulatedandtheoccurrenceofplasticdeformationwasmonitored.According to theexperimental settings, thebeamdiameterwasset to0.6mm,thefeedratewas4000mm/min and the laser powerwas 860W.Themoving laser spot is simulated asmovingheat sourceusing theuser subroutine *DFLUX in theABAQUSfinite element solver. The tophat distributionused in theexperiment is specified throughthis routine. Laser spot size,powerandfeedratematch the experimental values. It is assumed that 60%of the laser energy are absorbed by thematerial in accordancewithearlier investigations. The simulationconsideredonly theheat inputby the laser sourceandnopowderflowandbuild-upvolume. It is thereforeonlyaroughestimationof thestresses andstrainsoccurring. Theresultsgiven inFigure9showthatcircular laserpathsclose to the inner hole leadtosmallplasticdeformation, i.e.,permanentdistortionof thecomponent. Foradistanceof more than15mmfromthecenter,noplasticdeformation isobserved. Figure9. Simulationofheat inputduring lasercladding(a) andplasticdeformationdueto thermal stresses (b). Basedon thesefindings, a seconddesignwas calculatedwhichpenalizes thedistance to the centerhole, i.e., thecloser the thickness increase is situatedto thecentralhole, thehigher thepenalty, Figure8b. This led toasecond,pareto-optimalvariantwhich iscomparedto the initially founddesign andto thereferencecasesofahomogeneous thicknessandapatchworkblank in the following. 3.1.2.Microstructureof theCladding Figure10showsacross sectionof thecladded layers,whichexhibits the typicalfinedendritic microstructure of primary solidified Al crystals (light) and interdendritic solidified Si (dark). Firmbondingto thesubstrate isobserved. Figure10.Microstructureofas-cladAlSi10Mg;Alsolidsolution(light) andSi (dark). 138