| In this dissertation, the structure and performance requirements of a space rocket propulsion system valve components, research the diffusion bonding of a kind of Ni-Cr-Mo system of nickel-based superalloy vacuum. Firstly, the addition of the diffusion layer is in the way of ultrasonic electroplating metal Ni layer. To comprehensively consider the effect of coating on the diffusion bonding, we determine the optimum parameters of ultrasonic electroplating. According to the study of the impact of temperature, connection pressure and holding time to interface structure of microcosmic joint, mechanical properties and the influence of the deformation, considering the workpiece service condition, we determine the reasonable range. The theoretical calculation method has been used to characterize the element diffusion coefficient and the concentration distribution of joint.Using three different current density at 1A/dm2, 5A/dm2 and 10A/dm2, and under the low current density,Ni coating morphology is perpendicular to the plating of the surface of the conical crystal. Through the XRD analysis, we can know that the growth direction of coating under low current density tends to be at(200), ultrasound has little effect on preferred orientation; at high current density, preferred direction changes from(220) to(200), which is because hydrogen evolution phenomenon under the high current density is intensified, but applying ultrasonic,micro fluidic and cavitation effect can significantly weaken the hydrogen evolution, and weaken grain preferential growth direction, thus grain grow more disorderly. With the increase of current density, grain size is reduced, but the influence of the apply of ultrasound on grain size is not great. Under the same current density, applying ultrasound under the same current density, we can get smaller surface roughness, lower porosity and better coating roughness. Eventually determining the plating process: the current density of 10A/dm2, ultrasonic power 1.0W/dm2, electroplating time 5 min, getting Ni plating on the thick of 10 microns.Using 10 microns thick Ni coating as an intermediary, high temperature alloy diffusion bonding experiment is carried out on the base of Ni-Cr-Mo, joint interface is mainly composed of Ni, Cr solid solution, there is no obvious interface reaction or harmful compounds generated; the porosity of the joint interface is not closed. With the increase of diffusion bonding temperature, connection pressure and heat preservation time, joint porosity decreases, and elements between parent metal and middle layer diffuse more full, interface elements concentration gradient gradually weakens. Spectrum analysis shows that the spread rate of the Cr and Mo element within the joint distinct greatly. Joint shear fracture surface shows that fracture occurrs in the middle of Ni coating, with the increase of test specification, the mechanical properties of joint gradually increase, but the increase of the deformation will affect the overall dimensions of the fittings. So the most reasonable process : connection temperature T=1050℃, holding time T=60min, connect pressure P=30MPa, The shear strength of joints reached to 355.6MPa.By studying parent metal/Ni connector under the different temperature of 1000℃ and 1100 ℃. According to the spectral data and Matlab simulation, using the Boltzmann-Matano method to calculate the Cr, Mo two elements in the parent metal and average diffusion coefficient of pure Ni, then get the diffusion equation of the Cr, Mo diffusion coefficient under arbitrary temperature. Using numerical diffusion coefficient to establish the equation of the joint element concentration distribution, the calculated results and the spectral line scan results are similar. |
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