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Study On Fe-based Amorphous-nanocrystalline Coatings By Laser Cladding

Posted on:2009-01-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q J ZhuFull Text:PDF
GTID:1101360245994531Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
Surface properties of machine parts need to be promoted increasingly because of the rapid development of modern industry. Most of them must work reliably and continuously under a high speed, high pressure, over loading or corrosion circumstance. Composite coatings developed on common metal benefit for improving surface properties of materials, prolonging the working life of machine parts and decreasing environment pollution. Amorphous-nanocrystalline composite materials are promising in the area of surface modification because of its high microhardness, strength and excellent corrosion resistant property. In this paper, alloy powders are pre-coated on 45 steel and Fe-Ni-Si-B-V amorphous-nanocrystalline composite coatings are developed by laser cladding. Microstructures of the coatings, growth mechanism of the amorphous phase and nanocrystalline phase, influence of microalloying on the alloy system and wear properties of the coatings are investigated in detail. At the same time, factors and rules influencing the coatings are studied.Composition and the element existing status of the pre-coated alloy powders are the key role of preparing Fe-based amorphous-nanocrystalline coatings by laser cladding. As far as Fe-Ni-Si-B-V alloy is concerned, it benefit for forming the amorphous phase and depressing the crystalline phase when B is added into the alloy in the status of boron iron instead of B powder. Further more, forming temperature of the molten pool can be decreased and the coatings have good metallic bonding with the substrate. At the same time, it is cheaper and has excellent processing properties. Si severs as not only one component of the alloy, but also the element preventing other component from oxidation. The content of the amorphous phase and nanocrystalline phase in the coatings can be improved by increasing the content of Si properly. However, the alloy deviates from eutectic composition and the amorphous phase and nanocrystalline phase is depressed if Si is over-added. The suitable component of Fe-Ni-Si-B-V alloy in favor of forming amorphous phase and nanocrystalline phase is Fe36Ni32Si16B14V2 by laser cladding.Fe36Ni32Si16B14V2 amorphous-nanocrystalline composite coating fabricated by laser cladding has no cracks and holes. The coatings are studied by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Because of the composition difference in micro-zone and crystalline conditions, the coating is build up by fine dendrites zone, cellular grains and amorphous-nanocrystalline zone and epitaxial growth dendrites zone from the surface to the bottom. The main crystalline phases are identified as Fe2B and y- Fe, Ni. The coatings show a layer-structure obviously. At the same time, content of the amorphous-nanocrystalline phase in it vary along the depth of the coatings. The amorphous-nanocrystalline phase concentrates in the zone beneath 0.40.5mm away from the surface. The amorphous phase and crystalline phase exist side by side in the coating. Moreover, nanocrystalline phase embedded in the amorphous substrate or in the crystalline phase locally. In the bond zone, the grains develop into the coatings in the form of epitaxial growth and the coatings have a good metallic bonding with the substrate. Microhardness of the Fe36Ni32Si16B14V2 amorphous-nanocrystalline composite coatings increases slightly from the surface to the middle of the coatings. The hardest area is in the middle of the coatings. The microhardness of the coatings decreases from the middle zone to the base metal and shows a graded distribution. The wear volume losses of the coatings are one sixth to one tenth of the substrate under the same load and wearing time. Compared to the substrate, the friction coefficients of the coatings are lowered 0.1-0.2 than the substrate and the course of the wear are more gentle.When amorphous-nanocrystalline composite coatings are fabricated by laser cladding, different microalloying elements, such as C, Nb and Ce, have different influence on the forming conditions of amorphous-nanocrystalline phase and crystalline phase. Furthermore, the properties of the coatings vary with their different content.It is shown that the addition of C favorites of the forming of amorphous-nanocrystalline phase and depressing the generation of crystalline phase. Compared to the Fe36Ni32Si16B14V2 coatings, the middle zone of the Fe36Ni31Si16B14V2C1 coatings is composed of complex nanocrystalline phase and amorphous phase. The majority one is nanocrystalline phase. Structurally, amorphous phase distributes among the nanocrystalline phase and is complexity and non-homogeneous. The grains of Fe2B turn to more small and distribute more homogeneously. It is shown that the wear property of the materials is promoted by developing more amorphous-nanocrystalline phase, which alters the wear mechanism of the materials. The wear volume loose of the Fe36Ni31Si16B14V2C1 coatings are only one fifteenth to one twenty second of the substrate under the same load and wearing time. The main wear mechanisms are abrasive wear and cohesive wear.Nb element is unfavour of the developing of amorphous phase. Because of the generation of NbC-VC composite grains, when Nb is added to the Fe36Ni32Si16B14V2 alloy, the high-temperature molten pool has cores for heterogeneity nucleation and the GFA (Glass Forming Ability) of the Fe36Ni32Si16B14V2 alloy is depressed. This causes the decrease of amorphous-nanocrystalline phase and increase of the variation amplitude of microhardness.When Ce is added to the alloy, the crystalline system of Fe36Ni32Si16B14V2 alloy is changed. The crystallization of Fe2B phase is depressed and the crystallization ofγ-Fe, Ni phase is promoted when the addition of Ce is less than 1.0at. %. When the addition of Ce reaches 2.0at. %, both of the crystalline phases are depressed and the amorphous phase is promoted. Compared to the Fe36Ni31Si16B14V2C1 coatings, microstructure of Fe36Ni30Si16B14V2Ce2 coatings are more homogeneous and have much more amorphous phase. However, the microhardness of the coatings is lower. The wear properties of Fe36Ni30Si16B14V2Ce2 coatings lower because of the Fe-based amorphous alloys are hard and brittle materials. The main wear mechanism is peeling and the worn scars are smooth.It is shown that C has the most important influence on the wear property of Fe36Ni32Si16B14V2 coatings among the microalloying elements.
Keywords/Search Tags:Laser cladding, Coatings, Amorphous-nanocrystalline, Wear property
PDF Full Text Request
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