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Title: Modelling of laser cladding of magnesium alloys with pre-placed powders
Authors: Guo, Lifeng
Degree: Ph.D.
Issue Date: 2005
Abstract: As a surface engineering technique, high-power laser cladding, has shown great potential for improving the corrosion resistance of magnesium alloys. Its main advantage over other processes, is its ability to form relatively thick protective coatings on selected areas where improved properties are desired. It is also a 'clean' process. However, previous research studies have found that in laser cladding of magnesium alloys, the problem of a high degree of dilution cannot be easily overcome. Moreover, in-depth studies using analytical or numerical modelling can rarely be found in the literature for addressing laser cladding with pre-placed powders with the aim of predicting the level of dilution. In the first phase of this study, a simplified thermal model based on the finite element method (FEM) was developed to study the phenomenon of dilution in laser cladding of a magnesium alloy. In the model, the powder bed was treated as a continuum, and a high power continuous wave (CW) laser was employed. The results of the simulations of the FEM model together with those of the statistical analyses showed that although, under normal cladding conditions, a process window can be established for achieving good interfacial bonding between the substrate and the clad coating, a low dilution level was extremely difficult to achieve. This was primarily attributed to the low melting point and the high thermal diffusivity of magnesium as well as the relatively long laser-material interaction time. To overcome the dilution problem, the double-layer cladding technique was explored, and was found to be able to produce low dilution clads with improved corrosion resistance. However, the rough surface and the presence of an oxide layer are problems associated with this technique. In view of these, an alternative method was proposed. To overcome the weakness of CW mode, i.e. a relatively long laser-material interaction time, a pulse mode of operation was selected in the second phase of the research. To gain an in-depth understanding of the cladding process under the condition of the pulse mode with a pre-placed powder bed, numerical models based on FEM have been developed. In the first stage of the modelling work, the melting behaviour of a powder bed at the particle level, i.e. individual powders and voids, was considered. The model employed computational fluid dynamics to simulate the temperature and the fluid velocity fields of a partially melted powder bed under laser irradiation, taking into consideration, inter alia, the melting of the powders and the effect of surface tension. In the model, the complication induced by the moving boundaries of solid/liquid and gas/liquid was successfully resolved by using the 'Volume of Fluid (VOF)/Continuum Surface Force (CSF)' capability of ANSYS. With this model, the major features regarding the temperature and velocity fields as well as the shape of the molten pool during the melting of a powder bed were established. Such modelling work has not previously been reported in the literature. Based on these findings, the model was further developed to examine the profile of the clad and the effects of process parameters on dilution level under single pulse irradiation taking into account the thermal contact resistance between the powder bed and the substrate. The model was found to be capable of predicting the process window that yields a low level of dilution, and the prediction was supported by experimental results. However, it was also found that the model cannot be simply extended to the cladding of a surface under the condition of a multi-pulse. Recognising this, a nonlinear, three-dimensional numerical FEM model was developed that took into account the phenomenon of partial re-melting of the track of the clad, the effect of heat accumulation resulted from successive pulses, and the geometries of the clad coating. Using this model, the relationship between the thickness of powder bed (L,), the laser power (P), the pulse duration (t), and the dilution level was established. The limit of the laser pulse duration for achieving a low level of dilution was found mainly, to be controlled by the thickness of the powder bed and, to a lesser extent, by the laser power. The effect of track overlapping was found to be insignificant. The experiment of laser cladding of Al-12 Si alloy on magnesium WE43 was conducted to verify the t-L-P relationship. The results of the microstructural analyses showed that the relationship predicted by the model agreed well with the experimental results. Both the theoretical predictions and experimental results showed that it is possible to obtain a clad coating on magnesium alloys with a relatively low dilution level under the condition of a pulse mode. In this respect, the model provides a sound basis for the determination of the process operation. In considering the improvement of corrosion resistance that can be caused by laser surface modification to magnesium alloys, a comparison was made between the techniques of laser surface melting and laser cladding. The results of the potentiodynamic polarisation tests showed that the improvement obtained from laser surface melting was far less than that could be provided by laser cladding. Although, laser surface melting could effect a rapid solidification, and as a consequence, a homogenised microstructure was obtained, this has not changed the extreme position of magnesium in the electrochemical series nor has it changed the fact that magnesium cannot form self-healing passivating surface films in corrosive environments. On the contrary, the application of laser cladding could completely change the surface chemistry of magnesium alloys and create a more noble surface coating. However, it must be recognised that to benefit most from laser cladding, a low level of dilution is desirable. Accordingly, a careful selection of the laser processing parameters is needed. To achieve this, the 3-D numerical FEM model presented in the present research has shown to be of great value.
Subjects: Hong Kong Polytechnic University -- Dissertations.
Magnesium alloys.
Metal cladding.
Lasers -- Industrial applications.
Pages: xi, 206 leaves : ill. (some col.) ; 30 cm.
Appears in Collections:Thesis

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