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Study on Laser Cladding Cobalt Based Alloy and Its Nano-composite Coatings

Time:2013-09-02 09:27:09  From:CNKI  Author:Li Mingxi

  ABSTRACT
  Co-based alloy and its nano-oxide (nano-Y2O3/ Al2O3) composite coatings, deposited on Ni-based superalloy substrate by laser cladding are introduced. Cross-section and longitudinal section of single track, multi-track, single and milti-layer of the coatings have been examined to reveal microstructure and phase composition using optical microscope, scanning electron microscope (SEM, including EDS microanalysis), transmission electron microscope (TEM)and X-ray diffraction instrument (XRD). Microhardness profile, wear and oxidation resistance of the coatings were also tested. The influences of parameters on microstructure and properties of the coatings were also studied, such as laser processing parameters, means of powder supply, multi-track and multi-layer coatings, preheating and nano-materials addition,etc.
  The results showed that there were primary phase (γ-Co) and eutectic (γ-Co+Cr23C6) in the coatings of Stellite 6 and Höganäs (HMSP 2528/2537) and un-equilibrium phase (Co3W9C4) was observed in HMSP 2537 coatings. γ-Co, Ni2Y and Cr3Ni2SiC phases were shown in CoNiCrAlY coatings.
  A thin plane front solidification layer appeared at the interface of the coatings on 45 steel substrate, which could not be seen at the interface of the coatings on Ni-based superalloy substrate. Rapid directional solidification was found at the interface and the growth direction was perpendicular to the interface. Growth direction is found parallel to the scanning direction of laser beam at the surface
  With the increase of laser power more substrate was fused and part of unmelted substrate in the melting pool acted as subsistent surface of solidification that resulted in changes of solidified form near interface. The fine microstructure was found in single layer coatings,whereas coarsened structures were observed at the boundary of multi-tracks or multi-layer coatings.
  The dendrite arm spacing (λ1) was greatly refined by adding nano-Al2O3 or nano-Y2O3 particles. The particles were well distributed. Equilibrium phase (ε-Co) and other new phases transformation appeared by adding nano particles. The sub-microstructure changes from dislocation to stacking fault. It is shown from calculation results that the degree of misfit between Y2O3, Cr23C6 and γ-Co was small. Agglomeration of nanocrystalline and its growth should be avoided by adjusting laser-processing parameters.
  The microhardness and wear resistance of the coating were much higher than that of the substrate, and were further improved by adding nano particles in the coatings. But the wear resistance decreased by adding two much additives (>3%). The oxidation resistance of the coatings was improved and CoCrNiAlY coatings was shown the best oxidation resistance at the temperature of 1 100 ℃.
  The mechanism of effects of parameters on microstructures in Co-based coatings was proposed and the distribution of particles in nano-composite coatings were analyzed and modelled.
  Pads with coatings made of Sellite 6 and HMSP 2537 alloy have been successfully used in billet heating furnace in steel plants.Key words: laser cladding; Cobalt-based alloy; Al2O3/Y2O3 nano particles; composite coatings

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