Laser Cladding Repairing Technology And Microstructure And Properties Of Laser Cladding Layer

Laser cladding has been widely used in remanufacturing field because of its refined microstructure, limited heat affected zone and strong metallurgical bonding between the coating and substrate. To repair the 42 CrMo double chain wheel by laser technology, Fe55 alloy powder and Fe60 alloy powder were self-designed referred to MCrAlY alloy and Fe�Cr�C alloy, and then the laser cladding coatings with a continuous and smooth appearance, and an improved microhardness and wear resistance were fabricated by laser cladding. In order to further improve the wear resistance of the cladding layer, the thermite reactants were added to the Fe60 alloy powder based on the principle of thermite reaction of Al and Fe2O3 powders. Finally, the Al2O3 ceramic and M7C3 carbide reinforcing Fe-based composite coatings were fabricated on a 42 CrMo steel substrate by laser controlled reactive synthesis and cladding. The effects of different additions of thermite reactants on the phase transition, microstructure evolution, microhardness and wear resistance of the laser cladding composite coatings were investigated by means of X-ray diffraction(XRD), optical microscope(OM), scanning electron microscope(SEM), transmission electron microscope(TEM), Vickers microhardness test and MM-200 type block-on-ring wear test, respectively.The results indicate that Fe55 alloy coating and Fe60 alloy coating were fabricated by laser cladding with the self-designed Fe55 alloy powder and Fe60 alloy powder. The main phases of Fe55 alloy coating are ?-Fe, Ni-Cr-Fe and ?-(Fe, Ni) solid solution, N iAl intermetallic and a little Al2O3 oxide. Because of the addition of carbon element, M7C3 carbide is in situ synthesized in Fe60 alloy coating. The microstructure of Fe55 alloy coating is mainly composed of polygonal grains and interdendritic areas, and the polygonal grain consists of fine acicular phases which are parallel to each other. The Fe60 alloy coating is mainly composed of equiaxed crystal and M7C3 carbide. Compared with the substrate, the microhardness and wear resistance of Fe55 alloy coating and Fe60 alloy coating are greatly improved. The microhardness and wear resistance of Fe60 alloy coating are further increased due to M7C3 carbide precipitation strengthening.With different proportions of thermite reactants in Fe60 alloy powder, Al2O3 ceramic and M7C3 carbide are in situ synthesized via the laser controlled reactive synthesis and cladding. The Al2O3 ceramic and M7C3 carbides prefer to distribute along the ?-Fe phase boundary continuously, which separates the ?-Fe matrix and is beneficial to the grain refinement. The microstructure is characterized by a cellular dendritic structure at the bottom, a typical cellular microstructure in the middle and an equiaxed crystal at the top of the composite coatings. With the increase of thermite reactants, the amount of Al2O3 ceramic and M7C3 carbide in the composite coatings increases gradually. Moreover the prior austenite grows bigger and martensite grows coarser in the heat affected zone(HAZ). The increased thermite reactants improve the microhardness and wear resistance of the in situ composite coatings obviously. The microhardness of the cladding layer with 0%, 5% and 10% additive of thermite reactants has an average microhardness of 645.0,676.7 and 710.5HV, respectively, as compared with 277.4HV of the substrate. The relative wear resistance of the laser cladding layer with 0%, 5% and 10% additive of thermite reactants is 4.574, 8.075 and 11.639, respectively. The increased microhardness and wear resistance of the cladding layer should be ascribed to the grain refinement, ceramic and carbide precipitation and solid solution strengthening.