Research on Laser Remelting of MCrAlY and Ceramic Coatings Fabricated by Plasma Spraying on TiAl Alloy Surface
Wang Dong-sheng
CNK I
Abstract
TiAl base intermetallic alloys (hereafter referred as TiAl alloys) are new types of high-temperature structural materials applied for aerospace and aircraft industry because of their low density, high strength and good creep-resistance. The problem of poor ductility of these materials at room temperature has been mostly solved by alloying and microstructure controlling.
Thus the problems of further improving high-temperature oxidation resistance and wear resistance have become the most important research subjects. As a process commonly used to protect a target material, surface coating technique organically combines advantages of both the substrate and coating material for purpose of some mechanical properties (such as strength, toughness, etc.) and environmental service demands (such as wear and corrosion resistance, etc.). With ultrafine grain size and extremely high specific surface, nanostructured materials exhibit a number of unique physical, chemical as well as mechanical properties, taking on a feasible way to enhance coating’s performance on TiAl alloy surface. In this dissertation, the plasma spraying and plasma spraying-laser remelting technology were performed respectively to obtain MCrAlY (M=Ni, Co, Fe)coating, conventional and nanostructured Al2O3-13%TiO2 (mass fraction) ceramic coatings, as well as conventional and nanostructured ZrO2-7%Y2O3(mass fraction) thermal barrier coatings (TBCs).
The microstructure, phase constitution and properties of these coatings were studied systematically.The main work accomplished and conclusions gotten in this paper are presented as follows:
(1) In view of the characters of laser remelting, finite element models for analyzing the temperature and stress field in single-pass and multi-pass laser remelting MCrAlY coating prepared by plasma spraying were established by using an indirect thermal-mechanical coupling method.The effects of various process parameters on the temperature and stress filed were studied.
(2) The MCrAlY coating fabricated by plasma spraying on TiAl alloy surface was remelted by laser. The phase and microstructure of the plasma-sprayed and laser-remelted coatings were compared. The oxidation and hot corrosion experiments at 850 ℃ were carried out. The influences of laser remelting on high-temperature oxidation resistance and hot corrosion behavior of the coatings were researched.
(3) The microstructure of the conventional and nanosturctured Al2O3-13%TiO2 composite ceramic coatings prepared by plasma spraying and laser remelting were studied in detail. It was indicated that due to the rapid heating and cooling of laser remelting, there were still some nano-particles in the remelted region because of an insufficient time for grains growth. The feasibility of fabrication nanostructured coatings by plasma spraying-laser remelting technology was verified.
(4) A temperature field model of laser remelting Al2O3-13%TiO2 ceramic coating prepared by plasma spraying was developed. The temperature distribution along depth direction during laser remelting process was analyzed. Meanwhile, the effects of temperature distribution on microstructure were discussed.
(5) A trimodal microstructure model of the plasma-sprayed nanosturctured Al2O3-13%TiO2 composite ceramic coatings was brought forward based on the existed bimodal microstructure model. Moreover, the formation mechanism of the different microstructures was discussed.
(6) The microhardness, wear behaviors and erosion resistance of the Al2O3-13%TiO2 coatings with different feedstock and preparation methods were conducted. The wear and erosion mechanisms were also discussed based on the experimental results.
(7) To optimize the processing parameters of nanostructured coating prepared by plasma spraying using agglomerated powders, according to the heat transfer characters of plasma spraying,a computing model for analyzing the melting process of plasma spraying agglomerated ZrO2-7%Y2O3 powders was established with the employment of ANSYS. The influences of the plasma spraying parameters on melting process of powders were investigated.
(8) The morphology, microstructures and phase constitution of the conventional and nanosturctured ZrO2-7%Y2O3 ceramic coatings prepared by plasma spraying and plasma spraying-laser remelting technology were investigated. The oxidation and thermal shock tests of the ZrO2-7%Y2O3 coatings at 850 ℃were carried out. The oxidation and thermal shock performance and mechanism were discussed.
Key words: TiAl alloy, plasma sparying, laser remelting, MCrAlY coatings, ceramic coatings,nanostructured coatings
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