| Due to the excellent thermal conductivity and anticorrosion property, copper and its alloy are widely used as crystallizer in iron continuous casting. However, the poor wear resistance of copper significantly reduces its service life and limits its application in industries. In order to solve this problem, nowadays, surface modification methods such as electroplating and thermal spraying are generally used. However, these methods have a series of problems such as becoming invalid easily because the combination between coating and copper substrate is mechanical combination. And the greatest advantage of laser cladding is that the fully dense coating with an excellent metallurgical bonding at the interface can be achieved, which could improve the wear resistance of copper substrate. Therefore, it is of great practical significance and scientific value to prepare a new kind of dense coating without cracks, porosities and other defects, bonding with the substrate well and having good wear resistant on the surface of copper alloy by laser cladding technology.In this paper, the Ni60alloy is selected as cladding material, under the optimal single-channel and multi-track laser process parameters, a large area coating with good quality is successfully prepared on crystallizer copper alloy using laser cladding technique by Nd:YAG laser. In order to solve the problem of the emergence of cracks and segregation during cladding process and improve the quality and performance of cladding layer, measures such as adding Y2O3to optimize the cladding material and remelting the coating by laser to optimize the process are carried out. Then the surface morphology, micro-structure, micro-hardness, phase compositing, wear resistance and thermal vibration performance of coating at high temperature are characterized systematically. The results show that:The surface of Ni60coating has the unique "ripple" morphology of pulsed laser. There is dense structure with no crack, pore or other defects inside of the coating, and the interface between the coating and the substrate is metallurgical combination. The microstructure of single-channel coating is compoed of fine, cellular dendritic and equiaxed with granular eutectic existence, which is the typical microstructure of laser rapid solidification; the surface microstructure of multi-track coating is cylindrical arc-shaped dendritic structure within the grain as a result of the overlapping effects of adjacent single-channel cladding, while the inside microstructure of multi-track coating is equiaxed with granular eutectic existence.XRD analysis shows that there are Cr、(Fe, NiFeSi and Cr7C3phases generated in the Ni60coating. Due to grain refinement, solid solution strengthening and dispersion strengthening, the maximum micro-hardness of the single-channel and multi-track coating respectively reach1156HV and917HV, which are almost11.6times and9.2times of copper substrate (100HV).The thermal conductivity of the copper substrate sample with laser cladding Ni60treatment is318.85W/(m-k), explaining it has high thermal conductivity; During the same wear time, the wear loss of Ni60coating is only1/3times of the Cu substrate, and its friction coefficient between the coating and the friction disk is much smaller and more stable than that of copper substrate. The tests prove that the wear resistance of Ni60coating prepared by laser cladding is much better than copper substrate. In addition, thermal shock at250℃and750℃test results show that, the surface of Ni60coatings don’t show any cracks, drum package or peeling, and still combine with copper substrate closely; XRD analysis after thermal shock at750℃shows that there are new phases formed such as CuO, Feo.9740and Cuo.83Sio.17preventing the further oxidation, proving that Ni60coating has both good thermal shock toughness and high temperature oxidation resistance.Optimization of cladding powder by adding Y2O3makes the micro-morphology of coating much smaller and more compact, and reduces the tendency of defects such as cracks, component segregation, which proves that the quality of Ni60coating is improved. Y2O3changes the style of cladding solidification, promoting the formation of new phases such as Fe2B, FeNi and making them more evenly dispersed. Grain refinement and dispersion strengthening play an important role in micro-hardness increasing and the maximum micro-hardness reaches956HV, which is almost9.6times of copper substrate. These changes have further improved the high temperature wear resistance of copper alloys, the wear loss of the coating is only about1/4times of the copper substrate, and its friction coefficient is much smaller and more stable.The micro-morphology of Ni60coating after laser remelting processing is more uniform and compact than before. Because of releasing the internal stress, reducing composition segregation and making the dispersed hard phase more uniform by the second melting and the scanning velocity reduction, its maximum micro-hardness reaches987HV, which is almost9.9times of copper substrate. The wear loss of the Ni60remelted coating is only nearly1/6times of the copper substrate, indicating that laser remelting processing significantly improves wear resistance at high temperature. |
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