| In this thesis,pulsed laser-induction hybrid cladding(PLIC)is proposed for the first time.The purpose of this technology is to realize the efficient preparation of Ni-Cr solid solution toughening Cr13Ni5Si2 metal silicide composite coatings on pure copper with excellent elevated-temperature wear resistance.It provides a feasible and reliable technical scheme for the laser surface strengthening of copper and copper alloys to increase the service life greatly.In this thesis,the influence of the processing parameters on the forming of the composite coatings,the microstructure of the composite layer,the volume fraction of the toughening phase and the strengthening phase are studied.Numerical simulation analysis of temperature field evolution characteristics about PLIC process is carried out.The wear resistance of composite coatings at elevated-temperature is investigated,and the wear mechanism of composite coatings under room-temperature and elevated-temperature was analyzed.The main conclusions are as follows:Our research group sets up a pulse laser-induction hybrid cladding system.Pulse laser has high energy density,however,the periodic output characteristic determines the low powder deposition efficiency and the preparation of coating is easy to crack.The introduction of induction heating coincides with the characteristics of pulse laser to form complementary advantages.Because of the poor wettability between the copper and most metal powders,it is difficult to prepare the coating on copper.We design a gradient coating system for Cu substrate→Cu-Ni alloy transition layer→Ni-Cr-Si metal silicide composite layer.The transition layer plays a role in increasing the laser absorptivity and improving wettability between copper substrate and composite coatings.The microstructure of the four groups of original powders at the Si content of 6 wt.%-7.0 wt.% are prepared under the same process parameters.The results show that the volume fraction of Cr13Ni5Si2 strengthening phase increases with the increase of Si content.The volume fraction of the strengthening phase and the toughening phase determine the hardness and toughness of the composite coatings.The experimental results show that the coating can guarantee the maximum hardness(741HV0.1)and good toughness at the same time when the volume fraction of the Cr13Ni5Si2 metal silicide phase is 82.7%,.The laser pulse width and repetition rate are the most important factors for metallurgical bonding of metal silicide composite coatings and copper substrate,the powder deposition efficiency and coating forming quality.Narrow pulse width with high frequency has high powder deposition efficiency,but the coating and the substrate are hard to achieve good metallurgical bongding for the insufficient of single pulse energy.Coating forming quality,such as coating sheddiing and cracking,is sensitive as the change in laser power.The introduction of induction heating plays an important role in improving the deposition efficiency,reducing cracks and improving the quality of coating forming.When the induction heating temperature is 750°C,the coating not only ensures good deposition efficiency and molding quality,but also completely eliminates cracks,pores and other defects.The pulse width and frequency of the laser pulse and the induction heating temperature have great influence on the microstructures of composite coatings and the volume fractions of the strengthening phase and the toughening phase.This is mainly due to the influence of pulse width and frequency on the energy of single pulse laser.Induction heating also plays an important role in the energy injection during solidification.With the increase of pulse width,the energy of single pulse increases rapidly,and then the volume fraction of primary phase Cr13Ni5Si2 increases.But a exorbitant pulse width(or large single pulse energy)and induction heating temperature make the volume fraction of primary phase Cr13Ni5Si2 too large,and the toughness of composite coatings become worse.Numerical simulation shows that there is a large temperature gradient in the coating substrate,and the maximum temperature gradient is concentrated in the middle and lower regions of the coating.The temperature gradient induction heating assisted pulsed laser cladding coating can obviously reduce the lower,at 750°C induction heating assisted pulsed laser cladding,the temperature gradient is about 1/3 as compared with simple pulse laser cladding,greatly reduce the area of longitudinal thermal stress.Due to the moving and periodic output of laser,there is a severe temperature change in the composite coating along the direction of laser moving path.For pure pulse laser cladding,the laser action region produces a large temperature gradient along the path in the pulse interval after the laser pulse has just been completed.Pulse laser induction hybrid cladding technology can greatly reduce the temperature gradient.The simulation results show that in the 750°C induction heating assisted pulsed laser cladding,the temperature gradient along the direction path is only 49% as compared to the pulse laser cladding.The room temperature wear resistance of Cr13Ni5Si2 based metal silicide coating is close to that of plasma sprayed NiCr/Cr3C2 cemented carbide,which is widely used in mould copper plate.But composite coating has excellent abrasion resistance at evelated-temperature.At 500°C wear test,the wear resistance of NiCr/Cr3C2 cemented carbide coatings decreased by 49%(compared with its room temperature wear resistance),while the Cr13Ni5Si2 composite coatings only decreased by 0.17%.The room temperature wear mechanism of Si3N4 grinding ball-Cr13Ni5Si2 composite coating is mainly two body abrasive wear.In the 500°C wear test,due to the high temperature friction process with high temperature environment,which makes the coating formed on the surface with nickel oxide film which has a low coefficient of friction.The oxide film prevents the Si3N4 counterpart ball from directly micro-ploughing on the coating surface.wear loss in the wear test at 500°C is mainly caused by falling oxide debris in a reciprocating motion between the Si3N4 counterpart ball and the oxide film and the abrasion by microploughing between the oxide debris and the oxide film. |
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