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[en] In order to improve the tribology and high-temperature oxidation properties of the Ti-48Al-2Cr-2Nb intermetallic alloy simultaneously, mixed NiCr-Cr3C2 precursor powders had been investigated for laser cladding treatment to modify wear and high-temperature oxidation resistance of the material. The alloy samples were pre-placed with NiCr-80, 50 and 20%Cr3C2 (wt.%), respectively, and laser treated at the same parameters, i.e., laser output power 2.8 kW, beam scanning speed 2.0 mm/s, beam dimension 1 mm x 18 mm. The treated samples underwent tests of microhardness, wear and high-temperature oxidation. The results showed that laser cladding with different constitution of mixed precursor NiCr-Cr3C2 powders improved surface hardness in all cases. Laser cladding with NiCr-50%Cr3C2 resulted in the best modification of tribology and high-temperature oxidation behavior. X-ray diffraction (XRD), optical microscope (OM), scanning electron microscopy (SEM) and energy-dispersive spectrometer (EDS) analyses indicated that the formation of reinforced Cr7C3, TiC and both continuous and dense Al2O3, Cr2O3 oxide scales were supposed to be responsible for the modification of the relevant properties. As a result, the present work had laid beneficial surface engineering foundation for TiAl alloy applied as future light weight and high-temperature structural candidate materials
[en] The effects of constitution of precursor mixed powders and scan speed on microstructure and wear properties were designed and investigated during laser clad γ/Cr7C3/TiC composite coatings on γ-TiAl intermetallic alloy substrates with NiCr-Cr3C2 precursor mixed powders. The results indicate that both the constitution of the precursor mixed powders and the beam scan rate have remarkable influence on microstructure and attendant hardness as well as wear resistance of the formed composite coatings. The wear mechanisms of the original TiAl alloy and laser clad composite coatings were investigated. The composite coating with an optimum compromise between constitution of NiCr-Cr3C2 precursor mixed powders as well as being processed under moderate scan speed exhibits the best wear resistance under dry sliding wear test conditions
[en] The effects of La2O3 addition on the microstructure and wear properties of laser clad γ/Cr7C3/TiC composite coatings on γ-TiAl intermetallic alloy substrates with NiCr-Cr3C2 precursor mixed powders have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive spectrometer (EDS) and block-on-ring wear tests. The responding wear mechanisms are discussed in detail. The results are compared with that for composite coating without La2O3. The comparison indicates that no evident new crystallographic phases are formed except a rapidly solidified microstructure consisting of the primary hard Cr7C3 and TiC carbides and the γ/Cr7C3 eutectics distributed in the tough γ nickel solid solution matrix. Good finishing coatings can be achieved under a proper amount of La2O3-addition and a suitable laser processing parameters. The additions of rare-earth oxide La2O3 can refine and purify the microstructure of coatings, relatively decrease the volume fraction of primary blocky Cr7C3 to Cr7C3/γ eutectics, reduce the dilution of clad material from base alloy and increase the microhardness of the coatings. When the addition of La2O3 is approximately 4 wt.%, the laser clad composite coating possesses the highest hardness and toughness. The composite coating with 4 wt.%La2O3 addition can result the best enhancement of wear resistance of about 30%. However, too less or excessive addition amount of La2O3 have no better influence on wear resistance of the composite coating
[en] Highlights: ► We have designed and fabricated the Ni-based (Ni-3) coating for the substitute of Co-free alloy coating used in the nuclear power sealing surfaces. ► The dry sliding wear volume loss of the Ni-3 coating is half of Fe-based Norem02 coating and close to Co-based Stellite06 coating at 360 °C. ► The mass loss of Norem02 coating is the largest, Ni-3 coating is the second, while Stellite06 coating shows the best hot corrosion resistance at 350 °C. ► The Ni-based (Ni-3) coating and laser cladding technique appears to be the viable alternative material and technique for Co-free alloy. - Abstract: As a further step in obtaining high temperature wear and corrosion performances of a newly designed and laser cladded cobalt-free, nickel-based alloy coating (Ni-3) used for sealing surfaces of nuclear power valves, high temperature dry sliding wear behavior was characterized on a HT-1000 ball-on-disk tribometer at 360 °C, the hot corrosion test was conducted by plating Na2SO4 + K2SO4 mixed salt solution on the coating at 350 °C for 140 h. For comparison, the high temperature wear and corrosion resistance of the laser cladded Co-based Stellite06 and Fe-based Norem02 coatings were also investigated at the same testing conditions. It is found that the microhardness of the Ni-3 coating is about HV500 and close to Stellite06, but less than Norem02, the high temperature wear volume loss of Ni-3 coating is half of Norem02 and close to Stellite06. While during 350 °C hot corrosion test, the mass loss of Norem02 coating is the largest, Ni-3 coating is placed in the middle, Stellite06 coating shows the best hot corrosion resistance. Thus, the newly designed and laser cladded Ni-3 alloy coating appears to be the viable alternative material and technique for Co-free alloy, especially used in the nuclear valve sealing surfaces.
[en] In order to improve the high-temperature wear resistance of austenitic stainless steel, a wear resistant composite coating reinforced with hard (Cr,Fe)7C3 carbide and toughened by ductile γ-(Ni,Fe)/(Cr,Fe)7C3 eutectic matrix was fabricated by a novel central hollow laser cladding technique. The constituent phases and microstructure as well as high-temperature tribological behaviors of the Ni-based coating were investigated, respectively, and the corresponding wear mechanisms were discussed. It has been found that the composite coating exhibits superior wear resistance than substrate either at ambient or high temperatures. The coating shows better sliding wear resistance at 600 deg. C than 300 deg. C owing to high-temperature stability of the reinforced carbide and polishing effect as well as formation of continuous lubricious films, which implied it has large potential industrial applications at relatively higher temperatures.
[en] As a further step in obtaining high performance elevated temperature self-lubrication anti-wear composite coatings on TiAl alloy, a novel Ni-P electroless plating method was adopted to encapsulate the as-received CaF2 in the preparation of precursor NiCr-Cr3C2-CaF2 mixed powders with an aim to decrease its mass loss and increase its compatibility with the metal matrix during a Nd:YAG laser cladding. The microstructure of the coating was examined using X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) and the friction and wear behavior of the composite coatings sliding against the hardened 0.45% C steel ring was evaluated using a block-on-ring wear tester at room temperature. It was found that the coating had a unique microstructure consisting of primary dendrites TiC and block Al4C3 carbides reinforcement as well as fine isolated spherical CaF2 solid lubrication particles uniformly dispersed in the NiCrAlTi (γ) matrix. The good friction-reducing and anti-wear abilities of the laser clad composite coating was suggested to the Ni-P electroless plating and the attendant reduction of mass loss of CaF2 and the increasing of it's wettability with the NiCrAlTi (γ) matrix during the laser cladding process
[en] Microstructure characterization is important for controlling the quality of laser welding. In the present work, a detailed microstructure characterization by transmission electron microscopy was carried out on the laser welding cast Ni-based superalloy K418 turbo disk and alloy steel 42CrMo shaft and an unambiguous identification of phases in the weldment was accomplished. It was found that there are γ-FeCrNiC austenite solid solution dendrites as the matrix, (Nb, Ti) C type MC carbides, fine and dispersed Ni3 Al γ' phase as well as Laves particles in the interdendritic region of the seam zone. A brief discussion was given for their existence based on both kinetic and thermodynamic principles
[en] Exploratory experiments of laser welding cast Ni-based superalloy K418 turbo disk and alloy steel 42CrMo shaft were conducted. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. The corresponding mechanisms were discussed in detail. Results showed that the laser-welded seam had non-equilibrium solidified microstructures consisting of FeCr0.29Ni0.16C0.06 austenite solid solution dendrites as the dominant and some fine and dispersed Ni3Al γ' phase and Laves particles as well as little amount of MC short stick or particle-like carbides distributed in the interdendritic regions. The average microhardness of the welded seam was relatively uniform and lower than that of the base metal due to partial dissolution and suppression of the strengthening phase γ' to some extent. About 88.5% tensile strength of the base metal was achieved in the welded joint because of a non-full penetration welding and the fracture mechanism was a mixture of ductility and brittleness. The existence of some Laves particles in the welded seam also facilitated the initiation and propagation of the microcracks and microvoids and hence, the detrimental effects of the tensile strength of the welded joint. The present results stimulate further investigation on this field
[en] Experiments of autogenous laser full penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 3.5 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser welding velocity, flow rate of side-blow shielding gas, defocusing distance were investigated. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. Results show that high quality full penetration laser-welded joint can be obtained by optimizing the welding velocity, flow rate of shielding gas and defocusing distance. The laser-welded seam have non-equilibrium solidified microstructures consisting of γ-FeCr0.29Ni0.16C0.06 austenite solid solution dendrites as the dominant and very small amount of super-fine dispersed Ni3Al γ' phase and Laves particles as well as MC needle-like carbides distributed in the interdendritic regions. Although the microhardness of the laser-welded seam was lower than that of the base metal, the strength of the joint was equal to that of the base metal and the fracture mechanism showed fine ductility