| Summary: | 碩士 === 國立中正大學 === 機械系 === 91 === In this study, the theoretical model of fusing pre-placed powder layer was validated by cladding experiment on nickel-based alloy powder. We successfully deposited cobalt-based alloy on mild carbon steel substrate by using pre-placed powder laser cladding process. During the cladding process, backward blowing of shielding gas significantly improved the morphology of clad layer. Threshold value of laser power for nickel-based alloy was also decreased was also decreased so that a broadened range for successful cladding can be achieved.
However, laser cladding process is always accompanied with thermal strain and metallurgical phase transformation. All these phenomena will result in the residual stresses generated in the work piece after laser cladding which have detrimental effects on clad layer such as crack formation and decrease of bonding strength between clad layer and substrate. In order to overcome these drawbacks, we should understand the ongoing process of residual stress formation and the trends of residual stress distribution. Therefore, a numerical model was developed in this study using FEM code SYSWELD to simulate and analyze the cladding process of pre-placed powder layer.
The Goldak’s heat source model was selected as an internal heat source to simulate the heat flux distribution in the work piece of laser beam. With reasonable heat transfer boundary conditions, a transient non-linear analysis model was applied to calculate the temperature and phase proportion distribution in the work-piece at each time step. For single pass clad model, martensite transformation occurred in the HAZ beneath the clad layer. For multi pass clad model, martensite transformation occurred in the HAZ beneath the clad layer of first several passes during deposition due to rapid cooling. When laser cladding further proceeded, the increasing substrate temperature resulted in a slower cooling rate, and most of the region in the HAZ transformed into bainite. Then, base on the above results and applied the mechanical properties of each phase, the thermal stress and residual stress distribution in the work piece were calculated in the thermal-mechanical computation. The results showed that higher tensile residual stresses and thermal stresses in the order of magnitude of yielding stress occurred in clad layer and fusion zone. Therefore we can predict that cracks maybe generated in the clad layer and in the fusion zone. This prediction agrees well with experimental results.
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