Page - 140 - in 3D Printing of Metals

Metals 2017,7, 113 Figure13.Comparisonofdifferentpathstrategies for lasercladding. (a) straightpathwithparallel offset of the laser beam; (b) straight pathwith parallel offset of the laser beam+ circular outline; (c) spiralpathof the laserbeam. 3.2.2. LOMof theCladding ThecladdingshowninFigure13cwasanalyzedmetallographically. The interfacebetweenbase materialandcladlayer isshowninFigure14. Themicrographsindicateagoodconnectionbetweenthe basematerialandthedepositedlayers. Threedifferentzonescanbedistinguished: thedepositionlayer, a thinheataffectedzoneandthebasematerial. Theformersolid–liquid interfacedeeplypenetrates into thesubstratewhichresults inabout50%dissolutionbetweenbaseandcladmaterial. It canbe seenthat thesolidificationof themelt startsat theboundaryof thebasematerial inacolumnarway. Thehighcoolingrateassociatedwitharapidsolidificationpromotesafinedendriticmicrostructure in thedeposited layer. The transitionbetweencladdingandbasematerial is smooth.Hence,nostiffness jumpandnostresslocalizationaretobeexpected,whichmaybeadvantageouscomparedtopatchwork blanks. Thesamplesshowedsomeporosityof theclad layer.Apore ismagnifiedinFigure14c. Figure14. (a)overviewofclad layer (b)micrographof the interfacecladding—basematerial; (c)apore in thecladding. 3.2.3.Hole-FlangingExperiments Theresultsof theflangingexperimentsshowthat theflangingoperationwithmonolithicsheet metalwasfeasiblewithallholeexpansionratios (cf. Table2). For thespecimenswithacladdingon the insideof theflange(facingthepunch), crackswereobservedin theradiusof theflange,which is stretchedduringforming.Claddingontheoutsideof theflange ledtofailureprimarily in theedge of thecuthole. Examplesof these failuresare showninFigure15. Figure16bshowsamicrograph through a crack thatwas generated in the cladding during forming. The curvature of the crack propagation indicates that thecrackhas formedalongthegrainboundarieswhere thebrittleSiphase is concentrated. Figure 17 shows the results of successful flanging experimentswith andwithout cladding,withmachinedholesofseven-millimeter radius. 140