Investigation On Microstructure And Properties Of CoCrFeNiSiB Amorphous Coatings Cladded By Laser On H13 Steel

Amorphous alloy coatings could dramatically enhance wear-, corrosionresistance and high-temperature properties of key component parts of engines, critical parts of ships, hot-working dies, nuclear waste storage tank, structural components of quarrying and oil production due to a set of unique properties, including high hardness, excellent anti-wear and anti-corrosion, medium-temperature and high-temperature mechanical performances. Based on laser surface modification and overlay remanufacturing of large sophisticated expensive hot-working dies, an amorphous coating was fabricated on H13 steel successfully by laser cladding Co Cr Fe Ni Si B selffluxed alloy powder. The main contents of the dissertation include optimization of process parameters during laser cladding Co Cr Fe Ni Si B amorphous coating, microstructure and phase constitution of the amorphous coating, thermal stability analysis, high-temperature mechanical property and wear mechanism of the amorphous coating. Finally, the dissertation reveals the influences of the contents of main alloying elements on microstructure and properties of the amorphous coating.With microhardness, dilution rate and high-temperature wear loss as the evaluation criteria, laser cladding process was optimized by using the method of orthogonal experiment and Co₃₄Cr₂₉Fe₈Ni₈Si₇B₁₄ amorphous coating was fabricated successfully. The phase composition and amorphous phase content was systematically investigated by means of field emission scanning electron microscope?FE-SEM?, Xray diffraction?XRD? and transmission electron microscope?TEM?. According to the microstructural morphology characteristics of the amorphous coating, the coating was divided into two parts which were the amorphous zone and the interfacial dendritic zone. It was found that the height of interfacial dendrites was about 10 ?m, the amorphous phase volume fraction of amorphous zone was as high as 85 % while the crystallization phase within amorphous zone was?Co, Fe? solid solution phase.Thermal stability of the Co₃₄Cr₂₉Fe₈Ni₈Si₇B₁₄ amorphous coating was studied by differential scanning calorimetry. The results showed that glass-transition temperature?Tg?, crystallization temperature?Tx? and supercooling liquid zone??Tx? were 298 oC, 342 oC and 44 oC, respectively. The relationship between crystallization temperature?Tx? and activation energy?E? was discussed on the basis of the Kissinger equation, it was finally indicated that the thermal stability of amorphous coating depends on chemical composition of the coating. The test results of mechanical properties evaluation results showed that microhardness of the amorphous zone was five times higher than that of the substrate, frictional coefficient of the coating was 0.15 under the condition of 500 oC. Wear mechanism of the amorphous zone was dominated by abrasive wear, furthermore, there was a combination of a little oxidative wear and brittle spalling.The influences of atomic contents of Co element in(Fe_100-x)₄₂Cr₂₉Ni₈Si₇B₁₄?x=48, 43, 38, 33, 28? powders on phase constitution, microstructure and properties of the coatings were analyzed. It was found that the coatings were divided into two parts including dendritic zone and amorphous-equiaxed zone. The dendritic zone was mainly composed of?Fe, Ni? solid solution phase and?Co, Fe? solid solution phase as the matrix phases. The precipitated phases included chromium compound, Ni-Ni3 B eutectic phase, borides and carbides, etc. The height of interfacial columnar dendrites increased from 15 ?m to 50 ?m with the diminution of Co content. The decrease of Co content could also promote the formation of Co Fe15.7 phase and ??Fe, Ni? phase, decrease the volume fraction of amorphous phase, and make the size of equiaxed grains increase from 1 ?m to 2-3 ?m. The microhardness value of the coating decreased from 850HV0.2 to 650HV0.2 while both high-temperature frictional coefficient and wear rate increased significantly. The size of oxides on wearing surface of the coating were enlarged from 20 ?m to 400 ?m, the distributive location of the oxides extended from the inside to the edge of the wear track. The wear mechanism of all the coatings were found to be the combination of abrasive wear and oxidative wear. Under the mechanical properties differences between amorphous phase and crystallization phase, the paper proposed an analytic method which were impact wear and X-ray diffraction to identify the crystallization phases of the(Fe_100-x)₄₂Cr₂₉Ni₈Si₇B₁₄?x=48, 43, 38, 33, 28? amorphous coatings accurately. After the impact wear experiment ended, the morphology of crystallization phases could be observed clearly by employing scanning electron microscope directly.