Development of Laser Cladding Stellite F Alloy On Valve Face

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Table 2 Chemical composition of bead weldingmaterials(%) Stellite 6, stellite 12 and stellite F alloys are used as beadwelding materils for valve face in diesel engines. Amongthese bead welding materials the stellite F alloy iscommomly used. These materials include more alloyelement contents of such elements as nickel, chromium,cobalt etc in higher content which give greater hardnessand resistance to wear. The hardness at high temperaturefor exhause valve materials and bead welding materials areshown in Table 3.EXAMINATION METHODMetallographic structure of laser cladding layer on valvefave was eximined using MeF3 metallograph.Microhardness distribution was astimated by Micro-Duromat 4000E microhardness tester. The chemicalcomposition and the area distribution of iron elememt wasdetermined using PV9900 energy dispersive x-ray analyzerand WDX-2A wavelenth dispersive x-ray spectrometer.Further, 1000 hours durability engine test for the exhaustvalve with laser cladding stellite F alloy was conducted ina WD615.68 high-performance test engine.RESULTS AND DISCUSSIONMETALLOGRAPHIC STRUCTUREThe metallogphic structures of laser cladding layer andbead welding layer are shown in Fig.1 and Fig.2.Table 3 Hardness at high temperature for exhaust valvematerials and bead welding materials Fig.1 Metallogphic structure of LC layer 100 In experiments, stellite F alloy is selected as laser claddingmaterial, stellite 6 is selected as bead welding material forcompareson experiment.LASER CLADDING PROCESSA 2KW CO 2 laser beam welding apparatus was used forlaser cladding process. The method of pre-laid podwermixture paste over on the surface of valve face wasdetermined as pre-treatment method. The thickness of prelaiedpowder mixture paste over is about 2 mm. The laserpower is 2KW. The diameter of laser beam is 5 mm. Thescanning speed of laser beam is 2.8-7.2 mm/s. Nitrogengas was used during laser cladding process for preventionof oxidation and buring of cladding powder.Fig.2 Metallogphic structure of BW layer 100Fig.1 shows that the metallographic structure of lasercladding layer is a typical branched structure which iscomposed of fine, even rounded crystals and branchedcrystals diffirent in shape after laser cladding stellite Falloy. Three ranges can be divied in cross section of ametallographic sample in the vertical direction of surfaceof laser cladding layer: the melted range, the heat influence2
ange and the substrate. There is a obrious rapidly-coolinglayer near the substrate. The fine branched crystals locatein the surface layer. In the center part there are branchedcrystals unregular in the direction. The pre-laid powderlayer will be quick-melted partly or total as the powerdensity of laser is very high. Through contraling theparameters of laser the substrate can be melted hardly.During rapidly cooling of laser cladding layer the contentsof alloy elements existed in matrix phase will be increased.These alloy elements distribute in the matrix phase r as theclose and complex eutectic carbides which appear in formsof network structure.In bead welding process large scale branched crystalspresent in the matrix phase r. Rapidly-cooling layer nearthe substrate has not be observed clearly. The contents ofcarbon, tungsten etc have be dicreased, then it promotedcoare-grain structure because the melted range near thesubstrate was enlarged in bead welding process. Accordingto the examination results the melted range of lasercladding layer is 0.38mm less than the melted range ofbead welding layer. The process of rapidly melting andcooling in laser cladding reduces evidently the range ofheat influence. In the range of heat influence of beadwelding the austenite and lamellar Cr 2 N structure willweaken the thermal strength and corrosion resistance ofthe steel[5].direction from the surface to the inner, the microhardnessof layer distributes from high to low. In the trasition rangeof laser cladding layer the coare-needle martensite resultsin microhardness drop, a low tide occurred in themicrohardness curve. But the crypto-crystalline martensitein the range of phase change will intensify microhardness.The extent of oxidation and decarburization of the surfacein bead welding process is more serious than that in lasercladding process, increasing the content of hypo-eutecticbainite and reducing microhardness, the peak ofmicrohardness curve is moved to subsurface layer. Themicrohardness droped markedly in comparison with lasercladding process. The microhardness of laser claddinglayer is about HV 0.2 100 higher than that of bead weldinglayer at same depth.COMPOSITION DISTRIBUTIONMICROHARDNESSEquidistance pointFig.4 Element distribution of LC layerDistance to the surface(mm)Fig. 3 Microhardness of LC and BW layersFig.3 shows the microhardness distribution of the lasercladding layer and the bead weding layer. From Fig.3 itcan be observed that the curve amplitude of microhardnessdistribution of the laser cladded layer is gentler and thehardened range is larger. However, the curve amplitude ofmicrohardness distribution of the bead welded layer ismore fluctuant and the hardened range is smaller. Themicrohardness of the laser cladding layer is above 600kg/mm 2 .Because the density of laser power is very high,usually the density of laser power of KW class is 10 5 ~10 7w/cm 2 , and the time for finishing laser cladding process isonly in o.1~1 second, the structure of the laser claddinglayer should be fine and close at so high heating andcooling speed. And the higher microhardness of lasercladding layer should be obtained.As quick melting and solidifying promote fine grainstructure in surface layer, moreover the change of thestructure from the fine to the coarse distributed along theEquidistance pointFig.5 Element distribution of BW layerFig.4 and Fig.5 show the main element distributions in thelaser cladded layer and the bead welded layer by energydispersive analyzer of x-ray. From these compositiondistributions, it is clear that the dilutability in lasercladding layer is much lower than that in bead weldinglayer.3
Page 1: Seoul 2000 FISITA World Automotive

Development of Laser Cladding Stellite F Alloy On Valve Face

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