DIRECT LASER METAL DEPOSITION OF TUNGSTEN CARBIDE–COBALT-CHROME (WC/Co/Cr) POWDER

摘要

Direct Laser Metal Deposition (DLMD) is actually onernof the most attractive techniques in the group ofrnMaterial Accretion Manufacturing (MAM) processes.rnIn fact, the DLMD technology is able to realize, tornrepair and to restore, directly from the 3D CAD model,rnin a rapid and economic way, objects, moulds andrntools, also with a very complex geometry, using a greatrnvariety of metals, including those very difficult to workrnwith the conventional techniques. Moreover, thisrntechnique is well suited to produce very hard coatings.rnMetallic parts, with good mechanical properties, can berndirectly obtained through melting coaxially fedrnpowders with a laser.rnThe primary goal to pursue in the DLMD process is tornproduce samples with minimum porosity and with arngood adhesion to the substrate.rnThe objective of this work was to deposit layers ofrntungsten carbide-cobalt-chrome (WC/Co/Cr) powderrnon a substrate of AISI 316 stainless steel. WC/Co/Cr isrna composite widely used as a wear-resistant materialrnfor cutting tools, moulds, coatings and otherrnapplications.rnIn order to achieve good levels of adhesion, thernmetallic material used for the substrate and therndeposited one should have a certain degree of ductilityrnand a similar thermal linear expansion coefficient.rnPrevious experimental studies performed on directrnlaser deposition of tungsten carbide showedrnconsiderable difficulties in processing this type ofrnpowder; in fact, the deposited material has arnconsiderable hardness, but a low ductility. In addition,rnthere is a large difference between values of thernthermal expansion coefficients of the substraterncompared to that of the deposited layers. This leads,rngenerally, to the partial or total lack of adhesion of therndeposit on the substrate, affecting, in a definitive way,rnthe applicability of the process parameters used, asrnthey have been optimized to achieve high values ofrnhardness and relative density.rnIn order to overcome this problem, it was decided tornuse the strategy of Functionally Graded Materialsrn(FGM), which consists in interposing layers ofrnmaterials with variable composition between thernsubstrate and the coating of tungsten carbide. Thernnickel alloy Colmonoy 227, to mix with the powder ofrnWC/Co/Cr, was chosen for this purpose.rnResults of experimentation showed that it is possible tornobtain coatings of WC/Co/Cr with a good adhesion,rnbetween the first layer and the substrate and betweenrnlayers, and with good mechanical properties.