Study Of Surface Strengthening And Repairing Technique Of Thin Wall Components By Laser Cladding

Laser cladding technology can fabricate a compact cladding layer of low dilutionwhich has metallurgical bonding with the substrate and has little deformation for thecomponents, which makes it have a wide application prospect in surface strengthening andrepairing of the spares. Up to now, laser cladding is mainly concentrated the largercomponents, for which the most important question is to eliminate the porosity, inclusionand cracks. In aeronautical and space industry, there are many thin wall parts made ofsuper alloys, such as blade. When laser cladding is applied to strengthen and repair thethin wall parts, not only the metallurgical quality and property but also the deformationextent of the parts should be cared. However there has been little work to be reportedabout laser cladding technique on thin wall components. It is significant to conduct somebasic research about this topic.In this dissertation, the laser processing parameters on the deformation and themicrostructures have been studied systematically. Different parameters such as laser mode(low-order mode and multi-mode laser), output method of laser (continuous and pulsedlaser), the repetition rates and the specific energy of the laser beam have been concerned.Especially, the two main super alloys including the cast super alloy and the deformedsuper alloy have been used as the substrate. The main research results are as fellows:Firstly, the interfacial shapes between the cladding coatings and the substrates arestudied by the use of both the low-order mode and multi-mode CO₂ laser. It is found thatthe melted depth and the dilution of the substrate are very big, and the morphology ofmelting zone shows the mushroom shape when the defocused length e is within the focaldepth of the laser beam⊿, i.e. e＜⊿. When e＞⊿, the melted depth of the substrate isthin with low dilution, and the interface is flat-bottomed in shape. Thus the laser claddingcondition for low-order mode CO₂ lasers is that the distance L between the sample andfocused mirror is larger than F+⊿ (F is the focal length). Cladding layers with lowdilution and good metallurgical bonding with the substrates can be obtained by optimizingprocess parameters such as laser power, scanning speed etc.The deformed behaviors of thin plates during laser cladding have also been investigated systematically. The experimental results demonstrate there is a criticalprocessing parameter combination for laser power, scanning speed and line energy. Belowthe critical value, the bending angle of substrate increases with increasing laser power,scanning speed and line energy. But the bending angle of substrate decreases withincreasing laser power, scanning speed and line energy due to the parameters combinationexceeding the critical value. The thin plate becomes saddle due to contraction distortion inthe cross and longitudinal direction under the effect of thermal stress during laser cladding.For L-type thin plates, the contraction distortion mainly occurs in the longitudinaldirection to make the surfaces become paraboloid in shape. The bending extent is decidedby the distribution of yield strength, temperature gradient and molten zone in the substrate.In this dissertation, experiments were conducted by laser cladding of the super alloyGH4133 with pulsed and continuous CO₂ laser. The results show that no cracks are foundin the cladding layers by continuous wave (CW) CO₂ laser cladding. But cracks can befound after ageing treatment when the melted depth is large. In comparison, cracks areeasy to be found in cladding layers by pulsed CO₂ laser cladding. Under the action ofalternating temperature field and thermal stress, cracks occur at the location of melted lineand expand into substrates along chain educts at grain boundary. In addition, experimentswere conducted by laser cladding of the casting super alloy K403 with pulsed andcontinuous CO₂ laser. The results show that hot cracks appear in cladding layers duringCW CO₂ laser cladding when the line energy is higher than 320 J/mm. But for the pulsedCO₂ laser cladding, hot cracks occur in the cladding layers when the line energy is higherthan 562 J/mm. Obviously, the sensitivity of hot cracks is reduced by the use of the pulsedCO₂ laser cladding, which makes parameters vary in wider range to solve crack problem.According to the results above mentioned, it is very important to choose proper CO₂lasers to clad super alloys because of the specific property of super alloys. The basicprinciple is that pulsed lasers should be used to clad casting super alloys and CW CO₂lasers should be used to clad deformed super alloys.On the basis of above researches, the performances of stellite X-40 heterogeneousmetal cladding layers on thin wall of GH4133 alloy by the use of CW CO₂ laser arestudied. The results demonstrate that the corrosion rate of stellite X-40 cladding layer islower than that of GH4133 alloy in artificial seawater. The thermal fatigue cracks are formed at the location of melted line close to the GH4133 alloy and propagate into theGH4133 alloy along the grain boundary under the heat cycle in the range of 800℃to 17℃. Hot corrosion behavior of cladding layer in 75%wt Na₂SO₄+25%wt NaCl in air at 800℃is studied. Owing to formation of low melting point eutectic (Ni₃S₂·Ni), uniformcorrosion occurs in GH4133 alloy. For the stellite X-40 cladding layers, the hot corrosionis slow because of the compact chromium oxide film produced to prevent the diffusion ofsulfur in the cladding layers. All the results show that the performance of stellite X-40cladding layer is better than that of GH4133 cladding layer, which assures the propertiesof cladding layer by laser cladding of stellite X-40 using CW CO₂ lasers.